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9/8/98 AC 43.13-1B Par 5-1 Page 5-1 CHAPTER 5. NONDESTRUCTIVE INSPECTION (NDI) SECTION 1. GENERAL 5-1. GENERAL. The field of NDI is too varied to be covered in detail in this Advisory Circular (AC). This chapter provides a brief description of the various Nondestructive Testing (NDT) used for inspection of aircraft, powerplant, and components in aircraft in- spection. The effectiveness of any particular method of NDI depends upon the skill, experi- ence, and training of the person(s) performing the inspection process. Each process is limited in its usefulness by its adaptability to the par- ticular component to be inspected. Consult the aircraft or product manufacturer’s manuals for specific instructions regarding NDI of their products. (Reference AC 43-3, Nondestructive Testing in Aircraft, for additional information on NDI. The product manufacturer or the Federal Aviation Administration (FAA) generally specifies the particular NDI method and pro- cedure to be used in inspection. These NDI requirements will be specified in the manu- facturer’s inspection, maintenance, or overhaul manual; FAA Airworthiness Directives (AD); Supplemental Structural Inspection Documents (SSID); or manufacturer’s service bulletins (SB). However, in some conditions an alter- nate NDI method and procedure can be used. This includes procedures and data developed by FAA certificated repair stations under Ti- tle 14 of the Code of Federal Regulations, (14 CFR), part 145. 5-2. APPROVED PROCEDURES. Ti- tle 14 CFR, part 43 requires that all mainte- nance be performed using methods, tech- niques, and practices prescribed in the current manufacturer’s maintenance manual or in- structions for continued airworthiness prepared by its manufacturer, or other methods, techniques, and practices acceptable to the administrator. If the maintenance instructions include materials, parts, tools, equipment, or test apparatus necessary to comply with indus- try practices then those items are required to be available and used as per part 43. 5-3. NDT LEVELS. Reference Air Trans- port Association (ATA) Specification 105- Guidelines For Training and Qualifying Per- sonnel In Nondestructive Testing Methods. a. Level I Special. Initial classroom hours and on-the-job training shall be sufficient to qualify an individual for certification for a specific task. The individual must be able to pass a vision and color percep- tion examination, a general exam dealing with standards and NDT procedures, and a practical exam conducted by a qualified Level II or Level III certificated person. b. Level I/Level II. The individual shall have an FAA Airframe and Powerplant Mechanic Certificate, com- plete the required number of formal classroom hours, and complete an examination. c. Level III. (1) The individual must have graduated from a 4 year college or university with a de- gree in engineering or science, plus 1 year of minimum experience in NDT in an assignment comparable to that of a Level II in the applica- ble NDT methods: or (2) The individual must have 2 years of engineering or science study at a university,

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9/8/98 AC 43.13-1B

Par 5-1 Page 5-1

CHAPTER 5. NONDESTRUCTIVE INSPECTION (NDI)

SECTION 1. GENERAL

5-1. GENERAL. The field of NDI is toovaried to be covered in detail in this AdvisoryCircular (AC). This chapter provides a briefdescription of the various NondestructiveTesting (NDT) used for inspection of aircraft,powerplant, and components in aircraft in-spection. The effectiveness of any particularmethod of NDI depends upon the skill, experi-ence, and training of the person(s) performingthe inspection process. Each process is limitedin its usefulness by its adaptability to the par-ticular component to be inspected. Consult theaircraft or product manufacturer’s manuals forspecific instructions regarding NDI of theirproducts. (Reference AC 43-3, NondestructiveTesting in Aircraft, for additional informationon NDI.

The product manufacturer or the FederalAviation Administration (FAA) generallyspecifies the particular NDI method and pro-cedure to be used in inspection. These NDIrequirements will be specified in the manu-facturer’s inspection, maintenance, or overhaulmanual; FAA Airworthiness Directives (AD);Supplemental Structural Inspection Documents(SSID); or manufacturer’s service bulletins(SB). However, in some conditions an alter-nate NDI method and procedure can be used.This includes procedures and data developedby FAA certificated repair stations under Ti-tle 14 of the Code of Federal Regulations,(14 CFR), part 145.

5-2. APPROVED PROCEDURES. Ti-tle 14 CFR, part 43 requires that all mainte-nance be performed using methods, tech-niques, and practices prescribed in the currentmanufacturer’s maintenance manual or in-structions for continued airworthiness preparedby its manufacturer, or other methods,

techniques, and practices acceptable to theadministrator. If the maintenance instructionsinclude materials, parts, tools, equipment, ortest apparatus necessary to comply with indus-try practices then those items are required to beavailable and used as per part 43.

5-3. NDT LEVELS. Reference Air Trans-port Association (ATA) Specification 105-Guidelines For Training and Qualifying Per-sonnel In Nondestructive Testing Methods.

a. Level I Special.

Initial classroom hours and on-the-job trainingshall be sufficient to qualify an individual forcertification for a specific task. The individualmust be able to pass a vision and color percep-tion examination, a general exam dealing withstandards and NDT procedures, and a practicalexam conducted by a qualified Level II orLevel III certificated person.

b. Level I/Level II.

The individual shall have an FAA Airframeand Powerplant Mechanic Certificate, com-plete the required number of formal classroomhours, and complete an examination.

c. Level III.

(1) The individual must have graduatedfrom a 4 year college or university with a de-gree in engineering or science, plus 1 year ofminimum experience in NDT in an assignmentcomparable to that of a Level II in the applica-ble NDT methods: or

(2) The individual must have 2 years ofengineering or science study at a university,

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college, or technical school, plus 2 years ofexperience as a Level II in the applicable NDTmethods: or

(3) The individual must have 4 years ofexperience working as a Level II in the appli-cable NDT methods and complete an exami-nation.

5-4. TRAINING, QUALIFICATION, ANDCERTIFICATION. The success of any NDImethod and procedure depends upon theknowledge, skill, and experience of the NDIpersonnel involved. The person(s) responsiblefor detecting and interpreting indications, suchas eddy current, X-ray, or ultrasonic NDI, mustbe qualified and certified to specific FAA, orother acceptable government or industry stan-dards, such as MIL-STD-410, NondestructiveTesting Personnel Qualification and Certifica-tion, or Air Transport Association (ATA)Specification 105-Guidelines for Training andQualifying Personnel in Nondestructive Test-ing Methods. The person should be familiarwith the test method, know the potential typesof discontinuities peculiar to the material, andbe familiar with their effect on the structuralintegrity of the part.

5-5. FLAWS. Although a specific discus-sion of flaws and processes will not be givenin this AC, the importance of this area shouldnot be minimized. Inspection personnelshould know where flaws occur or can be ex-pected to exist and what effect they can have ineach of the NDI test methods. Misinterpreta-tion and/or improper evaluation of flaws orimproper performance of NDI can result inserviceable parts being rejected and defectiveparts being accepted.

All NDI personnel should be familiar with thedetection of flaws such as: corrosion, inherentflaws, primary processing flaws, secondaryprocessing or finishing flaws, and in-service

flaws. The following paragraphs classify anddiscuss the types of flaws or anomalies thatmay be detected by NDI.

a. Corrosion. This is the electrochemicaldeterioration of a metal resulting from chemi-cal reaction with the surrounding environment.Corrosion is very common and can be an ex-tremely critical defect. Therefore, NDI per-sonnel may devote a significant amount oftheir inspection time to corrosion detection.

b. Inherent Flaws. This group of flaws ispresent in metal as the result of its initial so-lidification from the molten state, before anyof the operations to forge or roll it into usefulsizes and shapes have begun. The followingare brief descriptions of some inherent flaws.

(1) Primary pipe is a shrinkage cavitythat forms at the top of an ingot during metalsolidification, which can extend deep into theingot. Failure to cut away all of the ingotshrinkage cavity can result in unsound metal,called pipe, that shows up as irregular voids infinished products.

(2) Blowholes are secondary pipe holesin metal that can occur when gas bubbles aretrapped as the molten metal in an ingot moldsolidifies. Many of these blowholes are cleanon the interior and are welded shut into soundmetal during the first rolling or forging of theingot. However, some do not weld and canappear as seams or laminations in finishedproducts.

(3) Segregation is a nonuniform distri-bution of various chemical constituents thatcan occur in a metal when an ingot or castingsolidifies. Segregation can occur anywhere inthe metal and is normally irregular in shape.However, there is a tendency for some con-stituents in the metal to concentrate in the liq-uid that solidifies last.

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(4) Porosity is holes in a material’s sur-face or scattered throughout the material,caused by gases being liberated and trapped asthe material solidifies.

(5) Inclusions are impurities, such asslag, oxides, sulfides, etc., that occur in ingotsand castings. Inclusions are commonly causedby incomplete refining of the metal ore or theincomplete mixing of deoxidizing materialsadded to the molten metal in the furnace.

(6) Shrinkage cracks can occur in cast-ings due to stresses caused by the metal con-tracting as it cools and solidifies.

c. Primary Processing Flaws. Flawswhich occur while working the metal down byhot or cold deformation into useful shapessuch as bars, rods, wires, and forged shapes areprimary processing flaws. Casting and weld-ing are also considered primary processes al-though they involve molten metal, since theyresult in a semi-finished product. The follow-ing are brief descriptions of some primaryprocessing flaws:

(1) Seams are surface flaws, generallylong, straight, and parallel to the longitudinalaxis of the material, which can originate fromingot blowholes and cracks, or be introducedby drawing or rolling processes.

(2) Laminations are formed in rolledplate, sheet, or strip when blowholes or inter-nal fissures are not welded tight during therolling process and are enlarged and flattenedinto areas of horizontal discontinuities.

(3) Cupping is a series of internal metalruptures created when the interior metal doesnot flow as rapidly as the surface metal duringdrawing or extruding processes. Segregationin the center of a bar usually contributes to theoccurrence.

(4) Cooling cracks can occur in castingdue to stresses resulting from cooling, and areoften associated with changes in cross sectionsof the part. Cooling cracks can also occurwhen alloy and tool steel bars are rolled andsubsequently cooled. Also, stresses can occurfrom uneven cooling which can be severeenough to crack the bars. Such cracks are gen-erally longitudinal, but not necessarily straight.They can be quite long, and usually vary indepth along their length.

(5) Flakes are internal ruptures that canoccur in metal as a result of cooling too rap-idly. Flaking generally occurs deep in a heavysection of metal. Certain alloys are more sus-ceptible to flaking than others.

(6) Forging laps are the result of metalbeing folded over and forced into the surface,but not welded to form a single piece. Theycan be caused by faulty dies, oversized dies,oversized blanks, or improper handling of themetal in the die. They can occur on any areaof the forging.

(7) Forging bursts are internal or exter-nal ruptures that occur when forging opera-tions are started before the material to beforged reaches the proper temperaturethroughout. Hotter sections of the forgingblank tend to flow around the colder sectionscausing internal bursts or cracks on the sur-face. Too rapid or too severe a reduction in asection can also cause forging bursts or cracks.

(8) A hot tear is a pulling apart of themetal that can occur in castings when the metalcontracts as it solidifies.

(9) A cold shut is a failure of metal tofuse. It can occur in castings when part of themetal being poured into the mold cools anddoes not fuse with the rest of the metal into asolid piece.

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(10) Incomplete weld penetration is afailure of the weld metal to penetrate com-pletely through a joint before solidifying.

(11) Incomplete weld fusion occurs inwelds where the temperature has not been highenough to melt the parent metal adjacent to theweld.

(12) Weld undercutting is a decrease inthe thickness of the parent material at the toeof the weld caused by welding at too high atemperature.

(13) Cracks in the weld metal can becaused by the contraction of a thin section ofthe metal cooling faster than a heavier sectionor by incorrect heat or type of filler rod. Theyare one of the more common types of flawsfound in welds.

(14) Weld crater cracks are star shapedcracks that can occur at the end of a weld run.

(15) Cracks in the weld heat-affectedzone can occur because of stress induced in thematerial adjacent to the weld by its expansionand contraction from thermal changes.

(16) A slag inclusion is a nonmetallicsolid material that becomes trapped in the weldmetal or between the weld metal and the basemetal.

(17) Scale is an oxide formed on metalby the chemical action of the surface metalwith oxygen from the air.

d. Secondary Processing or FinishingFlaws. This category includes those flaws as-sociated with the various finishing operations,after the part has been rough-formed by roll-ing, forging, casting or welding. Flaws may beintroduced by heat treating, grinding, and

similar processes. The following are brief de-scriptions of some secondary processing orfinishing flaws.

(1) Machining tears can occur whenworking a part with a dull cutting tool or bycutting to a depth that is too great for the mate-rial being worked. The metal does not breakaway clean, and the tool leaves a rough, tornsurface which contains numerous short dis-continuities that can be classified as cracks.

(2) Heat treating cracks are caused bystresses setup by unequal heating or cooling ofportions of a part during heat treating opera-tions. Generally, they occur where a part has asudden change of section that could cause anuneven cooling rate, or at fillets and notchesthat act as stress concentration points.

(3) Grinding cracks are thermal typecracks similar to heat treating cracks and canoccur when hardened surfaces are ground. Theoverheating created by the grinding can becaused by the wheel becoming glazed so that itrubs instead of cutting the surface; by usingtoo little coolant; by making too heavy a cut;or by feeding the material too rapidly. Gener-ally, the cracks are at right angles to the direc-tion of grinding and in severe cases a completenetwork of cracks can appear. Grinding cracksare usually shallow and very sharp at theirroots, which makes them potential sources offatigue failure.

(4) Etching cracks can occur whenhardened surfaces containing internal residualstresses are etched in acid.

(5) Plating cracks can occur when hard-ened surfaces are electroplated. Generally,they are found in areas where high residualstresses remain from some previous operationinvolving the part.

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e. In-Service Flaws. These flaws areformed after all fabrication has been completedand the aircraft, engine, or related componenthas gone into service. These flaws are attrib-utable to aging effects caused by either time,flight cycles, service operating conditions, orcombinations of these effects. The followingare brief descriptions of some in-service flaws.

(1) Stress corrosion cracks can developon the surface of parts that are under tensionstress in service and are also exposed to a cor-rosive environment, such as the inside of wingskins, sump areas, and areas between twometal parts of faying surfaces.

(2) Overstress cracks can occur when apart is stressed beyond the level for which itwas designed. Such overstressing can occur asthe result of a hard landing, turbulence, acci-dent, or related damage due to some unusual oremergency condition not anticipated by the de-signer, or because of the failure of some re-lated structural member.

(3) Fatigue cracks can occur in partsthat have been subjected to repeated orchanging loads while in service, such as riv-eted lap joints in aircraft fuselages. The crackusually starts at a highly-stressed area andpropagates through the section until failure oc-curs. A fatigue crack will start more readilywhere the design or surface condition providesa point of stress concentration. Commonstress concentration points are: fillets; sharpradii; or poor surface finish, seams, or grindingcracks.

(4) Unbonds, or disbonds, are flawswhere adhesive attaches to only one surface inan adhesive-bonded assembly. They can be theresult of crushed, broken, or corroded cores inadhesive-bonded structures. Areas of unbondshave no strength and place additional stress onthe surrounding areas making failure morelikely.

(5) Delamination is the term used to de-fine the separation of composite material lay-ers within a monolithic structure. Ultrasonic isthe primary method used for the detection ofdelamination in composite structures.

5-6. SELECTING THE NDI METHOD.The NDI method and procedure to be used forany specific part or component will generallybe specified in the aircraft or componentmanufacturer’s maintenance or overhaulmanuals, SSID’s, SB’s, or in AD’s.

NOTE: Some AD’s refer to SB’swhich may, in turn, refer to manufac-turer’s overhaul or maintenancemanuals.

a. Appropriate Method. The appropriateNDI method may consist of several separateinspections. An initial inspection may indicatethe presence of a possible flaw, but other in-spections may be required to confirm theoriginal indication. Making the correct NDImethod selection requires an understanding ofthe basic principles, limitations, and advan-tages and disadvantages of the available NDImethods and an understanding of their com-parative effectiveness and cost.

b. Other Factors. Other factors affectingthe inspection are:

(1) The critical nature of thecomponent;

(2) The material, size, shape, andweight of the part;

(3) The type of defect sought;

(4) Maximum acceptable defect limitsin size and distribution;

(5) Possible locations and orientationsof defects;

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(6) Part accessibility or portability; and

(7) The number of parts to be inspected.

c. Degree of Inspection. The degree ofinspection sensitivity required is an importantfactor in selecting the NDI method. Criticalparts that cannot withstand small defects andcould cause catastrophic failure require the useof the more sensitive NDI methods. Less criti-cal parts and general hardware generally re-quire less-sensitive NDI methods.

d. Material Safety Data Sheets (MSDS).The various materials used in NDI may containchemicals, that if improperly used, can be haz-ardous to the health and safety of operators andthe safety of the environment, aircraft, and en-gines. Information on safe handling of materi-als is provided in MSDS. MSDS, conformingto Title 29 of the Code of Federal Regulations(29 CFR), part 1910, section 1200, or itsequivalent, must be provided by the materialsupplier to any user and must be prepared ac-cording to FED-STD-313.

e. Advantages and Disadvantages. Ta-ble 5-1 provides a list of the advantages anddisadvantages of common NDI methods. Ta-ble 5-1, in conjunction with other informationin the AC, may be used as a guide for evaluat-ing the most appropriate NDI method when

the manufacturer or the FAA has not specifieda particular NDI method to be used.

5-7. TYPES OF INSPECTIONS. Nonde-structive testing methods are techniques usedboth in the production and in-service environ-ments without damage or destruction of theitem under investigation. Examples of NDImethods are as follows:

a. Visual inspection

b. Magnetic particle

c. Penetrants

d. Eddy current

e. Radiography

f. Ultrasonic

g. Acoustic emission

h. Thermography

i. Holography

j. Shearography

k. Tap testing

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TABLE 5-1. Advantages and disadvantages of NDI methods.

METHOD ADVANTAGES DISADVANTAGES

VISUAL InexpensiveHighly portableImmediate resultsMinimum trainingMinimum part preparation

Surface discontinuities onlyGenerally only large discontinuitiesMisinterpretation of scratches

DYE PENETRANT PortableInexpensiveSensitive to very small discontinuities30 min. or less to accomplishMinimum skill required

Locate surface defects onlyRough or porous surfaces interfere with testPart preparation required (removal of finishes and sealant, etc.)High degree of cleanliness requiredDirect visual detection of results required

MAGNETICPARTICLE

Can be portableInexpensiveSensitive to small discontinuitiesImmediate resultsModerate skill requiredDetects surface and subsurface discontinuitiesRelatively fast

Surface must be accessibleRough surfaces interfere with testPart preparation required (removal of finishes and sealant, etc.)Semi-directional requiring general orientation of field to discontinuityFerro-magnetic materials onlyPart must be demagnetized after test.

EDDYCURRENT

PortableDetects surface and subsurface discontinuitiesModerate speedImmediate resultsSensitive to small discontinuitiesThickness sensitiveCan detect many variables

Surface must be accessible to probeRough surfaces interfere with testElectrically conductive materialsSkill and training requiredTime consuming for large areas

ULTRASONIC PortableInexpensiveSensitive to very small discontinuitiesImmediate resultsLittle part preparationWide range of materials and thickness can be inspected

Surface must be accessible to probeRough surfaces interfere with testHighly sensitive to sound beam - discontinuity orientationHigh degree of skill required to set up and interpretCouplant usually required

X-RAYRADIOGRAPHY

Detects surface and internal flawsCan inspect hidden areasPermanent test record obtainedMinimum part preparation

Safety hazardVery expensive (slow process)Highly directional, sensitive to flaw orientationHigh degree of skill and experience required for exposure and interpretationDepth of discontinuity not indicated

ISOTOPERADIOGRAPHY

PortableLess expensive than X-rayDetects surface and internal flawsCan inspect hidden areasPermanent test record obtainedMinimum part preparation

Safety hazardMust conform to Federal and State regulations for handling and useHighly directional, sensitive to flaw orientationHigh degree of skill and experience required for exposure and interpretationDepth of discontinuity not indicated

5-8. 5-14. [RESERVED.]

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SECTION 2. VISUAL INSPECTION

5-15. GENERAL. Visual inspection is theoldest and most common form of NDI for air-craft. Approximately 80 percent of all NDIprocedures are accomplished by the direct vis-ual methods. This inspection procedure maybe greatly enhanced by the use of appropriatecombinations of magnifying instruments,borescopes, light sources, video scanners, andother devices discussed in this AC. Visual in-spection provides a means of detecting and ex-amining a wide variety of component and ma-terial surface discontinuities, such as cracks,corrosion, contamination, surface finish, weldjoints, solder connections, and adhesive dis-bonds. Visual inspection is widely used fordetecting and examining aircraft surfacecracks, which are particularly important be-cause of their relationship to structural failures.Visual inspection is frequently used to provideverification when defects are found initiallyusing other NDI techniques. The use of opti-cal aids for visual inspection is beneficial andrecommended. Optical aids magnify defectsthat cannot be seen by the unaided eye and alsopermit visual inspection in inaccessible areas.

5-16. SIMPLE VISUAL INSPECTIONAIDS. It should be emphasized that the eye-mirror-flashlight is a critical visual inspectionprocess. Aircraft structure and componentsthat must be routinely inspected are frequentlylocated beneath skin, cables, tubing, controlrods, pumps, actuators, etc. Visual inspectionaids such as a powerful flashlight, a mirrorwith a ball joint, and a 2 to 10 power magni-fying glass are essential in the inspection proc-ess.

a. Flashlights. Flashlights used for air-craft inspection should be suitable for indus-trial use and, where applicable, safety ap-proved by the Underwriters Laboratory or

equivalent agency as suitable for use in haz-ardous atmospheres such as aircraft fuel tanks.Military Specification MIL-F-3747E, flash-lights: plastic case, tubular (regular, explosion-proof, explosion-proof heat resistant, traffic di-recting, and inspection-light), provides re-quirements for flashlights suitable for use inaircraft inspection. However, at the presenttime, the flashlights covered by this specifica-tion use standard incandescent lamps and thereare no standardized performance tests forflashlights with the brighter bulbs: Krypton,Halogen, and Xenon. Each flashlight manu-facturer currently develops its tests and pro-vides information on its products in its adver-tising literature. Therefore, when selecting aflashlight for use in visual inspection, it issometimes difficult to directly compare prod-ucts. The following characteristics should beconsidered when selecting a flashlight: foot-candle rating; explosive atmosphere rating;beam spread (adjustable, spot, or flood); effi-ciency (battery usage rate); brightness afterextended use; and rechargeable or standardbatteries. (If rechargeable, how many hours ofcontinuous use and how long is required forrecharging?) If possible, it would be best totake it apart and inspect for quality of con-struction and to actually use the flashlight likeit would be used in the field. Inspection flash-lights are available in several different bulbbrightness levels:

(1) Standard incandescent (for long-battery life).

(2) Krypton (for 70 percent more lightthan standard bulbs).

(3) Halogen (for up to 100 percent morelight than standard bulbs).

(4) Xenon (for over 100 percent morelight than standard bulbs).

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b. Inspection Mirrors. An inspectionmirror is used to view an area that is not in thenormal line of sight. The mirror should be ofthe appropriate size to easily view the compo-nent, with the reflecting surface free of dirt,cracks, worn coating, etc., and a swivel jointtight enough to maintain its setting.

c. Simple Magnifiers. A single con-verging lens, the simplest form of a micro-scope, is often referred to as a simple magni-fier. Magnification of a single lens is deter-mined by the equation M = 10/f. In this equa-tion, “M” is the magnification, “f” is the focallength of the lens in inches, and “10” is a con-stant that represents the average minimumdistance at which objects can be distinctly seenby the unaided eye. Using the equation, a lenswith a focal length of 5 inches has a magnifi-cation of 2, or is said to be a two-power lens.

5-17. BORESCOPES. These instrumentsare long, tubular, precision optical instrumentswith built-in illumination, designed to allowremote visual inspection of internal surfaces orotherwise inaccessible areas. The tube, whichcan be rigid or flexible with a wide variety oflengths and diameters, provides the necessaryoptical connection between the viewing endand an objective lens at the distant, or distal tipof the borescope. Rigid and flexible bores-copes are available in different designs for avariety of standard applications and manufac-turers also provide custom designs for spe-cialized applications. Figure 5-1 shows threetypical designs of borescopes.

a. Borescopes Uses. Borescopes are usedin aircraft and engine maintenance programs toreduce or eliminate the need for costly tear-downs. Aircraft turbine engines have accessports that are specifically designed for bores-copes. Borescopes are also used extensively ina variety of aviation maintenance programs todetermine the airworthiness of difficult-to-reach components. Borescopes

typically are used to inspect interiors of hy-draulic cylinders and valves for pitting, scor-ing, porosity, and tool marks; inspect forcracked cylinders in aircraft reciprocating en-gines; inspect turbojet engine turbine bladesand combustion cans; verify the proper place-ment and fit of seals, bonds, gaskets, and sub-assemblies in difficult to reach areas; and as-sess Foreign Object Damage (FOD) in aircraft,airframe, and powerplants. Borescopes mayalso be used to locate and retrieve foreign ob-jects in engines and airframes.

b. Optical Designs. Typical designs forthe optical connection between the borescopeviewing end and the distal tip are:

(1) A rigid tube with a series of relaylenses;

(2) A flexible or rigid tube with a bun-dle of optical fibers; and

(3) A flexible or rigid tube with wiringthat carries the image signal from a ChargeCouple Device (CCD) imaging sensor at thedistal tip.

These designs can have either fixed or adjust-able focusing of the objective lens at the distaltip. The distal tip may also have prisms andmirrors that define the direction and field ofview. A fiber optic light guide with whitelight is generally used in the illumination sys-tem, but ultraviolet light can also be used toinspect surfaces treated with liquid fluorescentpenetrant or to inspect for contaminants thatfluoresce. Some borescopes with long work-ing lengths use light-emitting diodes at thedistal tip for illumination.

5-18. VISUAL INSPECTION PROCE-DURES. Corrosion can be an extremely criti-cal defect. Therefore, NDI personnel shouldbe familiar with the appearance of commontypes of corrosion and have training and

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FIGURE 5-1. Typical borescope designs.

experience on corrosion detection on aircraftstructure and engine materials. (Reference:AC 43-4A, Corrosion Control for Aircraft, foradditional information on corrosion.

a. Preliminary Inspection. Perform apreliminary inspection of the overall generalarea for cleanliness, presence of foreign ob-jects, deformed or missing fasteners, security

of parts, corrosion, and damage. If the con-figuration or location of the part conceals thearea to be inspected, use visual aids such as amirror or borescope.

b. Corrosion Treatment. Treat any cor-rosion found during preliminary inspectionafter completing a visual inspection of any se-lected part or area.

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NOTE: Eddy current, radiography,or ultrasonic inspection can deter-mine the loss of metal to corrosion.

c. Lighting. Provide adequate lighting toilluminate the selected part or area.

d. Personal Comfort. Personal comfort(temperature, wind, rain, etc.) of the inspectorcan be a factor in visual inspection reliability.

e. Noise. Noise levels while conducting avisual inspection are important. Excessivenoise reduces concentration, creates tension,and prevents effective communication. Allthese factors will increase the likelihood of er-rors.

f. Inspection Area Access. Ease of ac-cess to the inspection area has been found tobe of major importance in obtaining reliablevisual inspection results. Access consists ofthe act of getting into an inspection position(primary access) and doing the visual inspec-tion (secondary access). Poor access can affectthe inspector’s interpretation of discontinuities,decision making, motivation, and attitude.

g. Precleaning. Clean the areas or surfaceof the parts to be inspected. Remove any con-taminates that might hinder the discovery ofexisting surface indications. Do not removethe protective finish from the part or area priorto inspection. Removal of the finish may berequired at a later time if other NDI techniquesare required to verify any visual indications offlaws that are found.

h. Inspection. Carefully inspect the areafor discontinuities, using optical aids as

required. An inspector normally should haveavailable suitable measuring devices, a flash-light, and a mirror.

(1) Surface cracks. When searching forsurface cracks with a flashlight, direct the lightbeam at a 5 to 45 degree angle to the inspec-tion surface, towards the face. (See fig-ure 5-2.) Do not direct the light beam at suchan angle that the reflected light beam shines di-rectly into the eyes. Keep the eyes above thereflected light beam during the inspection.Determine the extent of any cracks found bydirecting the light beam at right angles to thecrack and tracing its length. Use a 10-powermagnifying glass to confirm the existence of asuspected crack. If this is not adequate, useother NDI techniques, such as penetrant, mag-netic particle, or eddy current to verify cracks.

(2) Other surface discontinuities. In-spect for other surface discontinuities, such as:discoloration from overheating; buckled,bulging, or dented skin; cracked, chafed, split,or dented tubing; chafed electrical wiring; de-laminations of composites; and damaged pro-tective finishes.

i. Recordkeeping. Document all discrep-ancies by written report, photograph, and/orvideo recording for appropriate evaluation.The full value of visual inspection can be re-alized only if records are kept of the discrep-ancies found on parts inspected. The size andshape of the discontinuity and its location onthe part should be recorded along with otherpertinent information, such as rework per-formed or disposition. The inclusion on a re-port of some visible record of the discontinuitymakes the report more complete.

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FIGURE 5-2. Using a flashlight to inspect for cracks.

5-19. 5-24. [RESERVED.]

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SECTION 3. EDDY CURRENT INSPECTION

5-25. EDDY CURRENT INSPECTION.Eddy current is used to detect surface cracks,pits, subsurface cracks, corrosion on inner sur-faces, and to determine alloy and heat-treatcondition.

a. Eddy Current Instruments. A widevariety of eddy current test instruments areavailable. The eddy current test instrumentperforms three basic functions: generating, re-ceiving, and displaying. The generating por-tion of the unit provides an alternating currentto the test coil. The receiving section proc-esses the signal from the test coil to the re-quired form and amplitude for display. In-strument outputs or displays consist of a vari-ety of visual, audible, storage, or transfer tech-niques utilizing meters, video displays, chartrecorders, alarms, magnetic tape, computers,and electrical or electronic relays.

b. Principles of Operations. Eddy cur-rents are induced in a test article when an al-ternating current is applied to a test coil(probe). The alternating current in the coil in-duces an alternating magnetic field in the arti-cle which causes eddy currents to flow in thearticle. (See figure 5-3.)

(1) Flaws in or thickness changes of thetest-piece influence the flow of eddy currentsand change the impedance of the coil accord-ingly. (See figure 5-4.) Instruments displaythe impedance changes either by impedanceplane plots or by needle deflection.

(2) Figure 5-5 shows typical impedanceplane display and meter display instrument re-sponses for aluminum surface cracks, subsur-face cracks, and thickness.

FIGURE 5-3. Generating an eddy current.

FIGURE 5-4. Detecting an eddy current.

5-26. EDDY CURRENT COILS ANDPROBES. A wide variety of eddy currentcoils and probes is available. Coils and probesare not always interchangeable between vari-ous types of instruments and, for best results,should be matched to a specific instrument andfrequency range. Special probe holders can befabricated to facilitate eddy current inspectionof contoured or shaped parts including partedges.

5-27. FIELD APPLICATION OF EDDYCURRENT INSPECTION. Eddy currenttechniques are particularly well-suited for de-tection of service-induced cracks in the field.

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FIGURE 5-5. Typical instrument displays.

Service-induced cracks in aircraft structuresare generally caused by fatigue or stress corro-sion. Both types of cracks initiate at the sur-face of a part. If this surface is accessible, ahigh-frequency eddy current inspection can beperformed with a minimum of part preparationand a high degree of sensitivity. If the surfaceis less accessible, such as in a subsurface layerof structure, low-frequency eddy current in-spection can usually be performed. Eddy cur-rent inspection can usually be performed with-out removing surface coatings such as primer,paint, and anodic films. Eddy current inspec-tion has the greatest application for inspectingsmall localized areas where possible crack ini-tiation is suspected rather than for scanningbroad areas for randomly-oriented cracks.However, in some instances it is more eco-nomical to scan relatively large areas witheddy current rather than strip surface coatings,inspect by other methods, and then refinish.

5-28. SURFACE INSPECTION. Eddycurrent inspection techniques are used to in-spect for surface cracks such as those shown infigure 5-6.

FIGURE 5-6. Typical surface cracks.

a. Equipment Requirements. The fol-lowing are typical eddy current equipment re-quirements for surface crack inspections.

(1) Instruments must meet the liftoffand sensitivity requirements of the applicableNDI procedures. The frequency requirement isgenerally 100 Hz to 200 kHz.

(2) Many types of probes are availablesuch as: flat-surface; spring-loaded; pencil;shielded pencil; right-angle pencil; or fastenerhole probes.

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(3) A reference standard is required forthe calibration of Eddy Current test equipment.A reference standard is made from the samematerial as that which is to be tested. A refer-ence standard contains known flaws or cracksand could include items such as: a flat surfacenotch, a fastener head, a fastener hole, or acountersink hole.

5-29. SUBSURFACE INSPECTION.Eddy current inspection techniques are used toinspect for subsurface cracks such as thoseshown in figure 5-7. The following are typicaleddy current equipment requirements for sub-surface crack inspections.

a. Use a variable frequency instrumentwith frequency capability from 100 Hz to500 MHz.

b. The probe used would be a low-frequency; spot, ring, or sliding probe.

c. Use a reference standard appropriatefor the inspection being performed.

5-30. CORROSION INSPECTION.Eddy current inspection is used to detect theloss of metal as a result of corrosion. An esti-mation of material loss due to corrosion can bemade by comparison with thickness standards.Figure 5-8 shows typical structural corrosionthat may be detected by the use of eddy currentinspection. Remove all surface corrosion

before performing the eddy current corrosioninspection. The following are typical eddycurrent equipment requirements for corrosioninspection.

a. Use a variable frequency instrumentwith frequency capability from 100 Hz to40 kHz.

b. Use a shielded probe with coil diame-ter between 0.15 and 0.5 inch and designed tooperate at the lower frequencies.

c. A reference standard made from thesame alloy, heat treatment, and thickness asthe test structure will be required.

5-31. ESTABLISHING EDDY CURRENTINSPECTION PROCEDURES. When es-tablishing eddy current inspection procedures,where no written procedures are available, thefollowing factors must be considered: type ofmaterial to be inspected; accessibility of theinspection area; material or part geometry, thesignal-to-noise ratio, test system; lift-off ef-fects, location and size of flaws to be detected;scanning pattern; scanning speed; and refer-ence standards. All of these factors are inter-related. Therefore, a change in one of the fac-tors may require changes in other factors tomaintain the same level of sensitivity and reli-ability of the eddy current inspection proce-dure. Written procedures should elaborate onthese factors and place them in their proper or-der.

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FIGURE 5-7. Typical subsurface cracks.

FIGURE 5-8. Typical structural corrosion.

5-32. 5-39. [RESERVED.]

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SECTION 4. MAGNETIC PARTICLE INSPECTION

5-40. GENERAL. Magnetic particle in-spection is a method for detecting cracks, laps,seams, voids, pits, subsurface holes, and othersurface, or slightly subsurface, discontinuitiesin ferro-magnetic materials. Magnetic particleinspection can be used only on ferro-magneticmaterials (iron and steel). It can be performedon raw material, billets, finished and semi-finished materials, welds, and in-service as-sembled or disassembled parts. Magnetic par-ticles are applied over a surface either dry, as apowder, or wet, as particles in a liquid carriersuch as oil or water.

Common uses for magnetic particle inspectionare; final inspection, receiving inspection, in-process inspection; and quality control, main-tenance, and overhaul.

5-41. PRINCIPLES OF OPERATION.Magnetic particle inspection uses the tendencyof magnetic lines of force, or flux, of an ap-plied field to pass through the metal ratherthan through the air. A defect at or near themetal’s surface distorts the distribution of themagnetic flux and some of the flux is forced topass out through the surface. (See figure 5-9.)The field strength is increased in the area ofthe defect and opposite magnetic poles formon either side of the defect. Fine magneticparticles applied to the part are attracted tothese regions and form a pattern around the de-fect. The pattern of particles provides a visualindication of a defect. (See figure 5-10.)

FIGURE 5-9. Magnetic field disrupted.

FIGURE 5-10. Crack detection by magnetic particle in-spection.

a. To locate a defect, it is necessary tocontrol the direction of magnetization, and fluxlines must be perpendicular to the longitudinalaxes of expected defects. Examination ofcritical areas for defects may require completedisassembly. Two methods of magnetization,circular and longitudinal, are used to magnet-ize the part and induce perpendicular fluxpaths. Parts of complex configuration may re-quire local magnetization to ensure propermagnetic field direction and adequate removalof surface coatings, sealants, and other similarcompounds. Possible adverse influence of theapplied or residual magnetic fields on delicateparts such as instruments, bearings, andmechanisms may require removal of theseparts before performing the inspection.

b. Certain characteristics inherent inthe magnetic particle method may introduceerrors in examination results. Nonrelevant er-rors are caused by magnetic field distortionsdue to intentional design features, such as:

(1) Sharp radii, less than 0.10 inch ra-dius, in fillets;

(2) Thread roots, keyways, and drilledholes; and

(3) Abrupt changes in geometry or inmagnetic properties within the part.

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c. Operators must understand nonrele-vant error indications and recognize themduring examination. Proper analysis of indi-cations in these regions will require consider-able skill and experience, and supplementalmethods may be required before a finalevaluation can be made. Special techniquesfor examination of these areas are given insubsequent paragraphs.

5-42. APPLICATIONS. Use magnetic par-ticle inspection on any well-cleaned surfacethat is accessible for close visual examination.Typical parts deserving magnetic particle ex-amination are: steel fasteners and pins; criticalstructural elements; linkages; landing gearcomponents; splice and attach fittings; andactuating mechanisms.

a. During field repair operations, disas-sembly is often not necessary, except when theparts have critical areas or delicate installedcomponents. However, for overhaul opera-tions, a more thorough and critical examina-tion may be obtained with stationary equip-ment in a shop environment with completelydisassembled, and thoroughly cleaned andstripped parts.

b. Magnetic rubber examination mate-rial is useful for in-field service examinationsof fastener holes in areas where the accessibil-ity is limited or restricted, where particle sus-pensions may cause unwanted contamination,when a permanent record is desired, and whenthe examination area cannot be observed visu-ally.

5-43. ELECTRICAL MAGNETIZINGEQUIPMENT. Stationary equipment in therange of 100 to 6000 amperes is normal for usewithin the aerospace industry for overhaul op-erations. Mobile equipment with similar am-perage outputs is available for field examina-tion of heavy structures, such as landing gearcylinders and axles. Small parts and local

areas of large components can be adequatelychecked with the use of small, inexpensivepermanent magnets or electromagnetic yokes.In procuring magnetizing equipment, themaximum rated output should be greater thanthe required examination amperage. Actualcurrent flow through a complex part may bereduced as much as 20 percent by the resis-tance load of the rated output.

5-44. MATERIALS USED IN MAG-NETIC PARTICLE INSPECTION. Theparticles used in magnetic particle inspectionare finely divided ferro-magnetic materials thathave been treated with color or fluorescentdyes to improve visibility against the varioussurface backgrounds of the parts under inspec-tion. Magnetic particles, particle-suspensionvehicles, and cleaners are required for con-ducting magnetic particle inspection. Re-quirements for magnetic particle inspec-tion materials, other than cleaners, are con-tained in the aerospace industry standard,ASTM-E1444, Inspection, Magnetic Particle(as revised). A certification statement whichwill certify that the material meets applicablespecification requirements will generally bereceived when a magnetic particle inspectionmaterial is purchased. Magnetic particle in-spection materials for use on a specific part orcomponent will generally be specified by theaircraft or component manufacturer or theFAA in documents such as; maintenance oroverhaul manuals, AD’s, SSID’s, or manufac-turer’s SB’s. However, if the magnetic parti-cle inspection materials are not specified forthe specific part or component to be inspected,it is recommended that personnel use materialsmeeting the aircraft or component manufactur-ers’ specifications or materials meeting the re-quirements of ASTM-E1444. Other FAA en-gineering-approved materials may also beused. Table 5-2 provides a partial listing ofcommonly accepted standards and specifica-tions for magnetic particle inspection.

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TABLE 5-2. Listing of commonly accepted standards and specifications for magnetic particle inspection.

NUMBER TITLEASTM STANDARDS

ASTM A275/A275 M-96

ASTM A456/A456 M Rev. A.

ASTM D96

ASTM E125-63 (1993)

ASTM E1316-95C

Standard Test Method for Magnetic Particle Examination of Steel Forgings.1995

Standard Specification for Magnetic Particle Examination of Large CrankshaftForgings. 1995

Standard Test Methods for Water and Sediment in Crude Oils by CentrifugeMethod (Field Procedure). 1988

Standard Reference Photographs for Magnetic Particle Indications on FerrousCastings. (Revised 1993) 1963

Standard Terminology for Nondestructive Examination. 1995 (Replaces ASTME269).

SAE-AMSSPECIFICATIONS

AMS 2300G

MAM 2300A

AMS 2303C

MAM 2303A

AMS 2641AMS 3040BAMS 3041B

AMS 3042BAMS 3043A

AMS 3044CAMS 3045BAMS 3046B

Premium Aircraft-Quality Steel Cleanliness Magnetic Particle Inspection Proce-dure. 1991 (Revised 1995)

Premium Aircraft Quality Steel Cleanliness Magnetic Particle Inspection Proce-dure Metric (SI) Measurement. 1992

Aircraft Quality Steel Cleanliness Martensitic Corrosion Resistant Steels Mag-netic Particle Inspection Procedure. 1993

Aircraft Quality Steel Cleanliness Martensitic Corrosion Resistant Steels Mag-netic Particle Inspection Procedure Metric (SI) Measurement. 1993

Vehicle, Magnetic Particle Inspection Petroleum Base. 1988Magnetic Particles, Nonfluorescent, Dry Method. 1995Magnetic Particles, Nonfluorescent, Wet Method, Oil Vehicle, Ready-To-Use.

1988Magnetic Particles, Nonfluorescent, Wet Method, Dry Powder. 1988Magnetic Particles, Nonfluorescent, Wet Method, Oil Vehicle, Aerosol Packaged.

1988Magnetic Particles, Fluorescent, Wet Method, Dry Powder. 1989Magnetic Particles, Fluorescent, Wet Method, Oil Vehicle Ready-to-Use. 1989Magnetic Particles, Fluorescent, Wet Method, Oil Vehicle, Aerosol Packaged.

1989U.S. GOVERNMENTSPECIFICATIONS

DOD-F-87935Mil-Std-271FMil-Std-410EMIL-HDBK-728/1MIL-HDBK-728/4A

Fluid, Magnetic Particle Inspection, Suspension. 1993Requirements for Nondestructive Testing Methods. 1993Nondestructive Testing Personnel Qualifications and Certifications. 1991Nondestructive Testing. 1985Magnetic Particle Testing. 1993

OTHER PUBLICATIONS

SNT-TC-1A

ATA No. 105ASM Handbook,Volume 17

American Society for Nondestructive Testing. Recommended Practice . 1992(Personnel Qualification and Certification in Nondestructive Testing and Rec-ommended Training Courses) Note: Updated every 4 years - 1996 editiondue in early 1997.

Air Transport Association of America. Guidelines for Training and QualifyingPersonnel in Nondestructive Testing Methods, (Revision 4 1993)Nondestructive Evaluation and Quality Control. 1989

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5-45. PREPARATION OF SURFACE.

a. Remove protective coatings accordingto the manufacturer’s instructions if necessary.Unless otherwise specified, magnetic particleexamination should not be performed withcoatings in place that could prevent the detec-tion of surface defects in the ferro-magneticsubstrate. Such coatings include paint orchromeplate thicker than 0.003 inch, or ferro-magnetic coatings such as electroplated nickelthicker than 0.001 inch.

b. Parts should be free of grease, oil,rust, scale, or other substances which will in-terfere with the examination process. If re-quired, clean by vapor degrease, solvent, orabrasive means per the manufacturer’s in-structions. Use abrasive cleaning only as nec-essary to completely remove scale or rust. Ex-cessive blasting of parts can affect examinationresults.

c. Exercise extreme care to prevent anycleaning material or magnetic particles frombecoming entrapped where they cannot be re-moved. This may require extracting compo-nents such as bushings, bearings, or insertsfrom assemblies before cleaning and magneticparticle examination.

d. A water-break-free surface is re-quired for parts to be examined by water sus-pension methods. If the suspension com-pletely wets the surface, this requirement ismet.

e. Magnetic particle examination of as-sembled bearings is not recommended becausethe bearings are difficult to demagnetize. If abearing cannot be removed, it should be pro-tected from the magnetic particle examinationmaterials and locally magnetized with a mag-netic yoke to limit the magnetic field acrossthe bearing.

5-46. METHODS OF EXAMINATION.Magnetic particle examination generally con-sists of: the application of magnetic particles;magnetization; determination of field strength;special examination techniques; and demag-netization and post-examination cleaning.Each of these steps will be described in thefollowing paragraphs.

5-47. APPLICATION OF MAGNETICPARTICLES. The magnetic particles usedcan be nonfluorescent or fluorescent (depend-ent on the examination required) and are ap-plied suspended in a suitable substance. Fluo-rescent particles are preferred due to theirhigher sensitivity.

a. Wet Continuous Method. Unless oth-erwise specified, use only the wet continuousmethod. In the wet continuous method, theparticle suspension is liberally applied to wetall surfaces of the part. The magnetizing cur-rent is applied at the instant the suspension isdiverted from the part. Apply two shots ofmagnetizing current, each at least 1/2 secondlong.

(1) Wet suspensions of fluorescent par-ticles, either in water or oil, should be used formost overhaul and in-service examinations ex-cept where the material, size, or shape of thepart prohibits its use.

(2) Water, with a suitable rust inhibitorand wetting agent, may be used as a liquid ve-hicle, provided that magnetic examinationequipment is designed for use or is satisfacto-rily converted for use with water.

b. Dry Continuous Method. Thismethod is not recommended for use on aero-space components because of its lower sensi-tivity level.

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c. Residual Magnetization Method. Inthis method, the part is magnetized and themagnetizing current is then cut off. If the am-perage has been correctly calculated and qual-ity indicator has verified the technique, thenone shot will correctly magnetize the part. Themagnetic particles are applied to the part afterthe magnetization. This method is dependentupon the retentiveness of the part, the strengthof the applied field, the direction of magneti-zation, and the shape of the part.

5-48. MAGNETIZATION.

a. Circular. Circular magnetization is in-duced in the part by the central-conductormethod or the direct-contact method. (See fig-ure 5-11.)

(1) Indirect Induction (central-conduc-tor method). Pass the current through a centralconductor that passes through the part. Whenseveral small parts are examined at one time,provide sufficient space between each

piece to permit satisfactory coverage (withparticles), magnetization, and examination.

(2) Direct Induction (contact method).Pass current through the part mounted hori-zontally between contact plates. As an exam-ple, circular magnetization of a round steel barwould be produced by placing the ends of thesteel bar between the heads of the magnetic in-spection machine and passing a currentthrough the bars. Magnetic particles appliedeither during or after passage of the current, orafter passage of the current in magnetically-retentive steels, would disclose discontinuitiesparallel to the axis of the bar.

NOTE: Exercise extreme caution toprevent burning of the part at theelectrode contact areas. Some causesof overheating and arcing are: insuffi-cient contact area, insufficient contactpressure, dirty or coated contact ar-eas, electrode removal during currentflow, and too high an amperage set-ting.

FIGURE 5-11. Circular magnetization.

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b. Longitudinal. Longitudinal magneti-zation is induced in a part by placing the partin a strong magnetic field, such as the center ofa coil or between the poles of an electromag-netic yoke. (See figure 5-12.) When using acoil, optimum results are obtained when thefollowing conditions are met.

(1) The part to be examined is at leasttwice as long as it is wide.

(2) The long axis of the part is parallelto the axis of the coil opening.

(3) The area of the coil opening is atleast 10 times the cross-sectional areas of thepart.

(4) The part is positioned against theinner wall of the coil.

(5) Three to five turns are employed forhand-held coils formed with cables.

(6) For the 10-to-1 fill factor, the effec-tive region of inspection is 1 coil radius on ei-ther side of the coil with 10 percent overlap.(Refer to ASTME-1444.)

FIGURE 5-12. Longitudinal magnetization.

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(7) The intensity of the longitudinalshots is kept just below the level at whichleakage fields develop across sharp changes ofsection, such as radii under bolt heads, threads,and other sharp angles in parts. This does notapply when checking chrome-plated parts forgrinding cracks.

(a) For example, longitudinal mag-netization of a round steel bar would be pro-duced by placing the DC coil around the bar.After application of the magnetic particles, ei-ther during or subsequent to magnetization,discontinuities perpendicular to the longitudi-nal axis of the bar would be disclosed.

(b) When a yoke is used, the portionof the part between the ends of the yoke com-pletes the path of the magnetic lines of force.This results in a magnetic field between thepoints of contact.

c. Permanent Magnets and Electro-magnetic Yoke. The stability of the magneticfield generated by permanent magnets requiressome agitation of the oxide particles within thefield. The wet method is considered most sat-isfactory. Use a well-agitated plastic squirtbottle for the most effective application of themagnetic particle suspension. When the di-rection of possible cracks in a suspect area isnot known, or would not necessarily be normalto the lines of force between the poles of themagnet, reposition the magnet to the best ad-vantage and recheck. Usually, two shots,90 degrees apart are required. The part mustbe demagnetized between each magnetizationwhen the field direction is changed unless thenext shot is at least 10 percent stronger thanthe previous shot, if this is the case demagneti-zation is not necessary.

5-49. DETERMINATION OF FIELDSTRENGTH. Factors such as part size,shape, magnetic properties of the material, andthe method of magnetization will affect the

field strength induced within a part by a givenapplied magnetizing force. The factors varyconsiderably, making it difficult to establishrules for magnetizing during examination.Technique requirements are best determinedon actual parts having known defects.

a. A magnetization indicator, such as aQuantitative Quality Indicator (QQI), shouldbe used to verify that adequate magnetic fluxstrength is being used. It effectively indicatesthe internally-induced field, the field direction,and the quality of particle suspension duringmagnetization.

b. The level of magnetization requiredfor detection of service-related defects in mostcases can be lower than that required for mate-rial and manufacturing control. Contact themanufacturer for correct specifications.

NOTE: If the examination must beperformed with less current than isdesired because of part size or equip-ment limitations, the lower fieldstrength can be partially accommo-dated by reducing the area of exami-nation for each magnetization, or theexamination can be supplemented byusing electromagnetic yokes. Examineonly 4 inches on either side of a coilinstead of 6, or apply additional mag-netization around the periphery of ahollow cylinder when using an inter-nal conductor.

5-50. SPECIAL EXAMINATION TECH-NIQUES.

a. Magnetic Rubber. Magnetic rubberformulations using finely divided magneticparticles in a silicone rubber base are used forthe inspection of screw, bolt, or other boreholes, which are not easily accessible. Theliquid silicone rubber mixture is poured intoholes in magnetic parts to be inspected.

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Curing time for silicone rubbers varies fromabout 30 minutes and up depending upon theparticular silicone rubber, the catalyst, and theamount of catalyst used to produce the curingreaction.

b. Curing. While curing is taking place,the insides of the hole must be maintained inthe required magnetized state. This can be ac-complished using a permanent magnet, a DCyoke, an electromagnet, or some other suitablemeans. Whatever method of magnetization isused, the leakage fields at any discontinuitiesinside the holes must be maintained longenough to attract and hold in position the mag-netic particles until a partial cure takes place.A two-step magnetizing procedure has beendeveloped.

(1) The first magnetization is accom-plished for a short time in one direction fol-lowed by a second at 90 degrees to the first forthe same length of time. This procedure mustbe repeated for whatever period of time isneeded until the cure prevents particle mobil-ity. Magnetization in two directions90 degrees apart ensures formation of indica-tions at discontinuities in all directions insidethe holes.

(2) After curing, the rubber plugs,which are exact replicas of the holes, are re-moved and visibly examined for indicationswhich will appear as colored lines against thelighter colored background of the silicone rub-ber. Location of any discontinuities or othersurface imperfections in the holes can be de-termined from the location of the indicationson the plugs. The magnetic rubber inspectionmethod is covered in detail in Air Force Tech-nical Order 33B-1-1, section XI.

c. Critical Examination for Sharp RadiiParts. A critical examination is required forcracks in sharp radii; such as threaded parts,splines, gear teeth roots, and abrupt changes in

sections, that cause obscuring and nonrelevantindications during normal examination prac-tices. The procedure provided herein is themost sensitive method for detecting the earlybeginnings of in-service fatigue cracks in thesharp, internal radii of ferro-magnetic parts.Magnetic particle examination equipment maybe used; however, alternating fields are not re-liable to provide the necessary high level of re-sidual magnetism. Optical aids are necessaryto realize the maximum sensitivity provided bythis magnetic particle procedure. Low-power(10x-30x) binocular microscopes are recom-mended. As a minimum, pocket magnifiers of7 to 10 power may be used with the followingprocedure.

(1) Thoroughly clean the part at thesharp radii and fillets where soils, greases, andother contaminants tend to accumulate and atother places where they might be overlookedduring a casual or hasty examination.

(2) The residual method should be usedas an aid in particular problem areas, eventhough it is not considered the best practice inmost of the instances. The conventional wetcontinuous methods should be used initiallyfor overall examination and the residual tech-nique should be applied only for supplemental,local examination of the sharp radii. It shouldnot be applied except in those cases wherenonrelevant indications have proven to be aproblem in the initial examination.

(3) Methods of magnetization should bedone according to standard procedures; how-ever, alternating fields should not be used, andthe level of magnetizing force imposed shouldusually be increased above the normal levels toensure a higher residual field within the part.

(4) Following magnetization, applyparticles in liquid suspension. The applicationshould be liberal and in a manner to causemaximum particle buildup. Immersion of

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small parts such as rod end fittings in a con-tainer of suspension, which has just beenstirred for about 30 seconds, is an excellentmethod.

(5) Check for the presence of particleaccumulation in the sharp radii. It is necessarythat the level of magnetization and the particleapplication result in the formation of nonrele-vant indications. Lack of indications will re-quire remagnetization to a higher level, morecare in applying the particles, or both.

(6) Wash the parts in a clean suspensionvehicle only enough to remove the weakly-held particle accumulations causing the non-relevant indications. Particles at true crackswill be more strongly held and will persist ifthe washing is gently done. This can be ac-complished by flowing or directing a stream ofliquid vehicle over the part, or for a smallcomponent, by gently stirring in a container ofthe vehicle. Closely observe the removal ofthe nonrelevant particle accumulations in theregion to be examined to avoid excessivewashing. If washing is prolonged beyond theminimum needed to remove the nonrelevantindications, the small defect indications mayalso be washed away. A few trials will help todevelop the best method and time required forwashing.

(7) Check for crack indications withoptical magnification and ample lighting. Thesmaller indications that are attainable by thisprocedure cannot be reliably seen or evaluatedwith the unaided eye.

5-51. DEMAGNETIZATION AND POST-EXAMINATION CLEANING. Parts shouldbe magnetized longitudinally last before de-magnetizing.

NOTE: Circular magnetism cannotbe read with a field meter since it is aninternal magnetic field. However, ifthe last shot, was a coil shot the metercan read it if a magnetic field is pres-ent.

a. Demagnetization. Demagnetize be-tween successive magnetization of the samepart, to allow finding defects in all directions,and whenever the residual magnetism inter-feres with the interpretation of the indications.Also, demagnetize all parts and materials aftercompletion of magnetic particle examination.Test all parts at several locations and parts forresidual magnetism of complex configurationat all significant changes in geometry. Repeatdemagnetization if there is any appreciable de-flection of the field indicator needle.

(1) AC method. Hold the part in theAC demagnetizing coils and then move thepart slowly and steadily through the coils andapproximately 3 to 4 feet past the coils. Re-peat this process until the part loses its residualmagnetism. Rotate and tumble parts of com-plex configuration as they are passed throughthe coils.

(2) DC method. Place the part in thesame relative position as when magnetized andapply reversing DC current. Gradually reducethe current to zero and repeat the process untilthe residual magnetic field is depleted.

b. Post-Examination Cleaning.

(1) When oil suspensions are used, sol-vent clean or remove the part until all magneticparticles and traces of oil are removed.

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(2) When parts or materials have beenexamined using water suspension methods,completely remove the water by any suitablemeans, such as an air blast, to ensure that theparts are dried immediately after cleaning.Thoroughly rinse the part with a detergent-base cleaner until all magnetic particles areremoved. Then rinse in a solution of waterand rust inhibitor.

(3) For cadmium-plated parts an air-water vapor blast may be used to remove anyremaining magnetic particle residue.

(4) After final cleaning and drying, usetemporary protective coatings, when necessary,to prevent corrosion.

(5) After magnetic particle examinationhas been completed, restore any removed fin-ishes according to the manufacturer’s repairmanual.

NOTE: Visible penetrant is oftenused interchangeably by NDI person-nel with fluorescent penetrant. How-ever, the chemical within most com-mon red dye penetrants will neutralizethe fluorescence of the chemicals usedin that method. Therefore, a thoroughcleaning of all magnetic particles ismandatory.

5-52. 5-59. [RESERVED.]

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SECTION 5. PENETRANT INSPECTION

5-60. GENERAL. Penetrant inspection isused on nonporous metal and nonmetal com-ponents to find material discontinuities that areopen to the surface and may not be evident tonormal visual inspection. The part must beclean before performing a penetrant inspection.The basic purpose of penetrant inspection is toincrease the visible contrast between a discon-tinuity and its background. This is accom-plished by applying a liquid of high penetrat-ing power that enters the surface opening of adiscontinuity. Excess penetrant is removedand a developer material is then applied thatdraws the liquid from the suspected defect toreveal the discontinuity. The visual evidenceof the suspected defect can then be seen eitherby a color contrast in normal visible whitelight or by fluorescence under black ultravioletlight. (See figure 5-13.)

a. The penetrant method does not de-pend upon ferro-magnetism like magnetic par-ticle inspection, and the arrangement of thediscontinuities is not a factor. The penetrantmethod is effective for detecting surface de-fects in nonmagnetic metals and in a variety ofnonmetallic materials. Penetrant inspection isalso used to inspect items made from ferro-magnetic steels and its sensitivity is generallygreater than that of magnetic particle inspec-tion. Penetrant inspection is superior to visualinspection but not as sensitive as other ad-vanced forms of tests for detection ofin-service surface cracks.

b. The major limitations of the penetrantinspection is that it can detect only those dis-continuities that are open to the surface; someother method must be used for detecting sub-surface defects. Surface roughness or porositycan limit the use of liquid penetrants. Such

surfaces can produce excessive backgroundindications and interfere with the inspection.Penetrant inspection can be used on most air-frame parts and assemblies accessible to itsapplication. The basic steps to perform pene-trant inspections are briefly described in thefollowing paragraphs.

5-61. EQUIPMENT USED IN THEPENETRANT INSPECTION PROCESS.Equipment varies from simple aerosol cansused in portable systems to fully automatedcomputer-controlled systems. Whether fluo-rescent or visible penetrants are used, differentpenetrant bases are available but may requiredifferent cleaning methods. Water-washablepenetrants can often be removed by a simplewater washing process, whereas oil-basepenetrants may require special solvents for re-moval. Some oil-base penetrants have emulsi-fiers, either added to the penetrant before it isapplied or added afterwards, that allow waterwashing to be used. Developers used, can beapplied either by a wet or dry bath. Therefore,each penetrant inspection process may requiredifferent cleaning facilities and procedures.(See table 5-3.)

5-62. BASIC STEPS TO PERFORMPENETRATION INSPECTION. Table 5-4shows a general process, in the proceduresflow sheet, for commonly used penetrant in-spection processes. It is important to ensurethat parts are thoroughly cleaned and dried be-fore doing penetrant inspection. All surfacesto be inspected should be free of contaminants,paint, and other coatings that could preventpenetrant from entering discontinuities. Ta-ble 5-5 shows applications of various methodsof precleaning for penetrant inspection.

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FIGURE 5-13. Penetrant and developer action.

TABLE 5-3. Classification of penetrant inspection materials covered by MIL-I-25135E.

PENETRANT SYSTEMS DEVELOPERS SOLVENT REMOVERS

Type IType IIType III

Fluorescent DyeVisible DyeVisible and FluorescentDye(dual mode)

Form aForm bForm cForm dForm e

Dry PowderWater SolubleWater SuspendibleNonaqueousSpecific Application

Class (1)

Class (2)

Class (3)

Halogenated(chlorinated)Nonhalogenated(nonchlorinated)Specific Application

Method AMethod B

Method CMethod D

Water WashablePost emulsifiable,LipophilicSolvent RemovablePost Emulsifiable,Hydrophilic

Sensitivity Level 1/2Sensitivity Level 1Sensitivity Level 2Sensitivity Level 3Sensitivity Level 4

UltralowLowMediumHighUltrahigh

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TABLE 5-4. Fluorescent and visible penetrant inspection general processing procedures flowsheet.

Penetrant Inspection-General Processing

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TABLE 5-5. Pre-cleaning methods for penetrant inspection.

METHOD USEMechanical Methods

Abrasive tumbling Removing light scale, burrs, welding flux, braze stopoff, rust, casting mold, andcore material; should not be used on soft metals such as aluminum, magnesium,or titanium.

Dry abrasive gritblasting

Removing light or heavy scale, flux, stopoff, rust, casting mold and core material,sprayed coatings, carbon deposits: In general, any brittle deposit. Can be fixedor portable (may peen metal over defect).

Wet abrasive gritblast

Same as dry except, where deposits are light, better surface and better control ofdimensions are required.

Wire brushing Removing light deposits of scale, flux, and stopoff (may mask defect by displac-ing metal).

High pressure waterand steam

Ordinarily used with an alkaline cleaner or detergent; removing typical machineshop contamination, such as cutting oils, polishing compounds, grease, chips,and deposits from electrical discharge machining; used when surface finish mustbe maintained.

Ultrasonic cleaning Ordinarily used with detergent and water or with a solvent; removing adheringshop contamination from large quantities of small parts.

Chemical Methods

Alkaline cleaning Removing braze stopoff, rust, scale, oils, greases, polishing material, and carbondeposits; ordinarily used on large articles where hand methods are too laborious;also used on aluminum for gross metal removal.

Acid cleaning Strong solutions for removing heavy scale; mild solutions for light scale; weak(etching) solutions for removing lightly smeared metal.

Molten salt bathcleaning

Conditioning and removing heavy scale; not suitable for aluminum, magnesium,or titanium.

Solvent Methods

Solvent wiping Same as for vapor degreasing except a hand operation; may employ nonhalo-genated (nonchlorinated) solvents; used for localized low-volume cleaning.

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CAUTION: Improper cleaning meth-ods can cause severe damage or deg-radation of the item being cleaned.Personnel must select and applycleaning processes in accordance withaircraft, engine, propeller, or appli-ance manufacturer’s recommenda-tions.

5-63. CLEANERS AND APPLICATIONS.Use a cleaner to remove contaminants fromparts prior to the application of penetrant in-spection materials. After the inspection iscompleted, penetrant inspection material resi-dues are removed. The following cleaners arecommonly used during the penetrant inspec-tion process.

a. Detergents. Detergent cleaners arewater-based chemicals called surfactants,which surround and attach themselves to parti-cles of contaminants allowing them to bewashed away.

b. Solvents. Solvents dissolve contami-nants such as oils, greases, waxes, sealants,paints, and general organic matter so they caneasily be wiped away or absorbed on a cloth.They are also used to remove Method C pene-trant material prior to developer application.

c. Alkalies. Alkaline cleaners are watersolutions of chemicals that remove contami-nants by chemical action or displacementrather than dissolving the contaminant.

d. Paint Removers. The general types ofremovers used for conventional paint coatingsare solvent, bond release, and disintegrating.

e. Salt Baths. Molten salt baths are usedin removing heavy, tightly-held scale and ox-ide from low alloy steels, nickel, and cobalt-base alloys, and some types of stainless steel.They cannot be used on aluminum, magne-sium, or titanium alloys.

f. Acids. Solutions of acids or their saltsare used to remove rust, scale, corrosion prod-ucts, and dry shop contamination. The type ofacid used and its concentration depends on thepart material and contaminants to be removed.

g. Etching Chemicals. Etching chemicalscontain a mixture of acids or alkalies plus in-hibitors. They are used to remove a thin layerof surface material, usually caused by a me-chanical process, that may seal or reduce theopening of any discontinuities. The type ofetching solution used depends on the part ma-terial and condition.

h. Penetrant Application. Apply thepenetrant by spraying, brushing, or by com-pletely submerging the part in a container ofpenetrant. Wait the recommended amount oftime after the penetrant has been applied toallow it to enter any discontinuities

(1) Removal of Excess Penetrant. Ex-cess penetrant must be removed from thepart’s surface to prevent a loss of contrast be-tween indications of discontinuities and thebackground during the inspection. Removalmay require actually washing or spraying thepart with a cleansing liquid, or may simply re-quire wiping the part clean with a solvent-moistened cloth. The removal method is de-termined by the type of penetrant used.

(2) Drying. If removal of the excesspenetrant involves water or other cleaning liq-uids, drying of the part may be required priorto developer application. When drying is re-quired, the time can be decreased by using ov-ens or ventilation systems.

i. Developer Application. Apply devel-oper after excess penetrant is removed and,where required, the surface is dried. Apply thedeveloper in a thin uniform layer over the sur-face to be inspected. Developer acts like a

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blotter to assist the natural capillary actionbleed-out of the penetrant from discontinuitiesand to spread the penetrant at discontinuitysurface edges to enhance bleed-out indications.After the developer is applied, allow sufficienttime for the penetrant to be drawn out of anydiscontinuities. Follow the manufacturer’srecommendations.

j. Inspection for Discontinuities. Afterthe penetrant has sufficiently developed, visu-ally inspect the surface for indications fromdiscontinuities. Evaluate each indication ob-served to determine if it is within acceptablelimits. Visible penetrant inspection is per-formed in normal visible white light, whereasfluorescent penetrant inspection is performedin black (ultraviolet) light.

k. Post-Cleaning. Remove inspectionmaterial residues from parts after completion

of penetrant inspection. This residue couldinterfere with subsequent part processing, or ifleft on some alloys, it could increase their sus-ceptibility to hydrogen embrittlement, inter-granular corrosion, and stress corrosion duringservice.

5-64. TECHNICAL STANDARDS. Twoof the more generally accepted aerospace in-dustry standards are the MIL-I-25135E, In-spection Materials, Penetrants (see table 5-6)and ASTM-E-1417. The penetrant materialsspecification (MIL-I-25135E) is used to pro-cure penetrant materials and the process con-trol specification (MIL-STD-6866) is used toestablish minimum requirements for conduct-ing a penetrant inspection. Table 5-6 providesa partial listing of commonly-accepted stan-dards and specifications for penetrant inspec-tion.

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TABLE 5-6. Listing of commonly accepted standards and specifications for penetrant inspection.

NUMBER TITLE

ASTM STANDARDSASTM-E-165 Standard Practice for Liquid Penetrant Inspection MethodASTM -E-270 Standard Definitions of Terms Relating to Liquid Penetrant InspectionASTM -E-1135 Standard Method for Comparing the Brightness of Fluorescent PenetrantsASTM -E-1208 Standard Method for Fluorescent Liquid Penetrant Examination Using the Lipophilic Post-Emulsification

ProcessASTM -E-1209 Standard Method for Fluorescent Penetrant Examination Using the Water Washable ProcessASTM -E-1210 Standard Method for Fluorescent Penetrant Examination Using the Hydrophilic Post-Emulsification ProcessASTM -E-1219 Standard Method for Fluorescent Penetrant Examination Using the Solvent Removable ProcessASTM -E-1220 Standard Method for Visible Penetrant Examination Using the Solvent Removable MethodASTM -E-2512 Compatibility of Materials with Liquid Oxygen (Impact-Sensitivity Threshold Technique)

SAE-AMSSPECIFICATIONSAMS-2647 Fluorescent Penetrant Inspection - Aircraft and Engine Component Maintenance

US GOVERNMENTSPECIFICATIONSMIL-STD-271 Requirements for Nondestructive TestingMIL-STD-410 Nondestructive Testing Personnel Qualifications and CertificationsMIL-STD-1907 Inspection, Liquid Penetrant and Magnetic Particle, Soundness Requirements for Materials, Parts and WeldsMIL-STD-6866 Inspection, Liquid PenetrantMIL-STD-728/1 Nondestructive TestingMIL-STD-728/3 Liquid Penetrant TestingMIL-STD-25135E Inspection Materials, PenetrantsQPL-25135 Qualified Products List of Materials Qualified Under MIL-I-25135T.O. 33B-1-1 U.S. Air Force/Navy Technical Manual, Nondestructive Testing Methods

OTHERPUBLICATIONSSNT-TC-1A Personnel Qualification and Certification in Nondestructive Testing and Recommended Training Courses

ATA No. 105Metals Handbook,Ninth Edition,Volume 17

Guidelines for Training and Qualifying Personnel in Nondestructive Methods,Nondestructive Evaluation, and Quality Control

5-65. 5-72. [RESERVED.]

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SECTION 6. RADIOGRAPHY (X-RAY) INSPECTION

5-73. GENERAL. Radiography (x-ray) isan NDI method used to inspect material andcomponents, using the concept of differentialadsorption of penetrating radiation. Eachspecimen under evaluation will have differ-ences in density, thickness, shapes, sizes, orabsorption characteristics, thus absorbing dif-ferent amounts of radiation. The unabsorbedradiation that passes through the part is re-corded on film, fluorescent screens, or otherradiation monitors. Indications of internal andexternal conditions will appear as variants ofblack/white/gray contrasts on exposed film, orvariants of color on fluorescent screens. (Seefigure 5-14.) 5-74. LIMITATIONS. Compared to othernondestructive methods of inspection, radiog-raphy is expensive. Relatively large costs andspace allocations are required for a radio-graphic laboratory. Costs can be reduced con-siderably when portable x-ray or gamma-raysources are used in film radiography and spaceis required only for film processing and inter-pretation. Operating costs can be high becausesometimes as much as 60 percent of the totalinspection time is spent in setting up for radi-ography. With real-time radiography, operat-ing costs are usually much lower, becausesetup times are shorter and there are no extracosts for processing or interpretation of film. 5-75. FILM OR PAPER RADIOGRA-PHY. In film or paper radiography, a two-dimensional latent image from the projectedradiation is produced on a sheet of film or pa-per that has been exposed to the unabsorbedradiation passing through the test piece. Thistechnique requires subsequent development ofthe exposed film or paper so that the latent im-age becomes visible for viewing.

5-76. REAL-TIME RADIOGRAPHY. A two-dimensional image that can be immedi-ately displayed on a viewing screen or televi-sion monitor. This technique converts unab-sorbed radiation into an optical or electronicsignal which can be viewed immediately orcan be processed with electronic or videoequipment. 5-77. ADVANTAGE OF REAL-TIMERADIOGRAPHY OVER FILM RADIOG-RAPHY. The principal advantage of real-timeradiography over film radiography is the op-portunity to manipulate the test piece duringradiographic inspection. This capability al-lows the inspection of internal mechanismsand enhances the detection of cracks and pla-nar defects by allowing manipulation of thepart to achieve the best orientation for flawdetection. Part manipulation during inspectionalso simplifies three-dimensional dynamic im-aging for the determination of flaw locationand size. In film radiography, depth parallel tothe radiation beam is not recorded. Conse-quently, the position of a flaw within the vol-ume of a test piece cannot be determined ex-actly with a single radiograph. To determineflaw location and size more exactly with filmradiography, other film techniques; such as ste-reo-radiography, triangulation, or simplymaking two or more film exposures with theradiation beam being directed at the test piecefrom a different angle for each exposure, mustbe used. 5-78. COMPUTED TOMOGRAPHY(CT). CT is another important radiologicaltechnique with enhanced flaw detection andlocation capabilities. Unlike film and real-time radiography, CT involves the generationof cross-sectional views instead of a planar

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FIGURE 5-14. Radiography.

projection. The CT image is comparable tothat obtained by making a radiograph of aphysically sectioned thin planar slab from anobject. This cross-sectional image is not ob-scured by overlying and underlying structuresand is highly sensitive to small differences inrelative density. Computed tomography im-ages are also easier to interpret than radio-graphs. 5-79. USES OF RADIOGRAPHY. Radi-ography is used to detect the features of acomponent or assembly that exhibit a differ-ence in thickness or density as compared tosurrounding material. Large differences aremore easily detected than small ones. In gen-eral, radiography can detect only those featuresthat have an appreciable thickness in a direc-tion along the axis of the radiation beam.Therefore, the ability of radiography to detectplanar discontinuities, such as cracks, dependson proper orientation of the test piece duringinspection. Discontinuities which have

measurable thickness in all directions, such asvoids and inclusions, can be detected as longas they are not too small in relation to sectionthickness. In general, features that exhibit a2 percent or more difference in radiation ad-sorption compared to the surrounding materialcan be detected. 5-80. COMPARISON WITH OTHERNDI METHODS. Radiography and ultra-sonic are the two generally-used, nondestruc-tive inspection methods that can satisfactorilydetect flaws that are completely internal andlocated well below the surface of the test part.Neither method is limited to the detection ofspecific types of internal flaws. However, ra-diography is more effective when the flaws arenot planar, while ultrasonic is more effectivewhen flaws are planar. In comparison to othergenerally-used NDI methods (e.g., magneticparticle, liquid penetrant, and eddy current in-spection), radiography has the following ad-vantages.

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a. The ability to inspect for both internaland external flaws.

b. The ability to inspect covered or hid-den parts or structures.

c. The ability to detect significant varia-tions in composition.

d. Provides a permanent recording ofraw inspection data.

5-81. FLAWS. Certain types of flaws aredifficult to detect by radiography. Cracks can-not be detected unless they are essentiallyalong the axis of the radiation beam. Tightcracks in thick sections may not be detected atall, even when properly oriented. Minute dis-continuities such as: inclusions in wroughtmaterial, flakes, microporosity, and microfis-sures may not be detected unless they are suf-ficiently segregated to yield a detectable grosseffect. Delaminations are nearly impossible todetect with radiography. Because of their un-favorable orientation, delaminations do notyield differences in adsorption that enablelaminated areas to be distinguished from de-laminated areas.

5-82. FIELD INSPECTION. The field in-spection of thick sections can be a time-consuming process, because the effective

radiation output of portable sources may re-quire long exposure times of the radiographicfilm. This limits field usage to sources oflower activity that can be transported. Theoutput of portable x-ray sources may also limitfield inspection of thick sections, particularlyif a portable x-ray tube is used. Portable x-raytubes emit relatively low-energy (300 kev) ra-diation and are limited in the radiation output.Both of these characteristics of portable x-raytubes combine to limit their application to theinspection of sections having the adsorptionequivalent of 75 mm (3 inches) of steel maxi-mum. Portable linear accelerators and be-tatrons that provide high-energy (> 1 MeV)x-rays can be used for the radiographic fieldinspection of thicker sections.

5-83. SAFETY. Radiographic safety re-quirements can be obtained from; the OEM’smanual, FAA requirements, cognizant FAAACO engineers, and radiation safety organiza-tions such as the Nuclear Regulatory Commis-sion (NRC). Information in radiation safetypublications can be used as a guide to ensurethat radiation exposure of personnel involvedin radiographic operations is limited to safelevels, and to afford protection for the generalpublic.

5-84. 5-88. [RESERVED.]

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SECTION 7. ULTRASONIC INSPECTION

5-89. GENERAL. Ultrasonic inspection isan NDI technique that uses sound energymoving through the test specimen to detectflaws. The sound energy passing through thespecimen will be displayed on a Cathode RayTube (CRT), a Liquid Crystal Display (LCD)computer data program, or video/camera me-dium. Indications of the front and back sur-face and internal/external conditions will ap-pear as vertical signals on the CRT screen ornodes of data in the computer test program.There are three types of display patterns; “A”scan, “B” scan, and “C” scan. Each scan pro-vides a different picture or view of the speci-men being tested. (See figure 5-15.) 5-90. SOUND REFLECTION. Theamount of reflection that occurs when a soundwave strikes an interface depends largely onthe physical state of the materials forming theinterface and to a lesser extent on the specificphysical properties of the material. For exam-ple: sound waves are almost completely re-flected at metal/gas interfaces; and partial re-flection occurs at metal/liquid or metal/solidinterfaces.

5-91. ULTRASONIC INSPECTIONTECHNIQUES. Two basic ultrasonic in-spection techniques are employed: pulse-echoand through-transmission. (See figure 5-16.)

a. Pulse-Echo Inspection. This processuses a transducer to both transmit and receivethe ultrasonic pulse. The received ultrasonicpulses are separated by the time it takes thesound to reach the different surfaces fromwhich it is reflected. The size (amplitude) of areflection is related to the size of the reflectingsurface. The pulse-echo ultrasonic responsepattern is analyzed on the basis of signal am-plitude and separation.

b. Through-Transmission Inspection.This inspection employs two transducers, oneto generate and a second to receive the ultra-sound. A defect in the sound path between thetwo transducers will interrupt the sound trans-mission. The magnitude (the change in thesound pulse amplitude) of the interruption isused to evaluate test results. Through-transmission inspection is less sensitive tosmall defects than is pulse-echo inspection. 5-92. FLAW DETECTION. Ultrasonic in-spection can easily detect flaws that producereflective interfaces. Ultrasonic inspection isused to detect surface and subsurface disconti-nuities, such as: cracks, shrinkage cavities,bursts, flakes, pores, delaminations, and po-rosity. It is also used to measure materialthickness and to inspect bonded structure forbonding voids. Ultrasonic inspection can beperformed on raw material, billets, finished,and semi-finished materials, welds, and in-service assembled or disassembled parts. In-clusions and other nonhomogeneous areas canalso be detected if they cause partial reflectionor scattering of the ultrasonic sound waves orproduce some other detectable effect on theultrasonic sound waves. Ultrasonic inspectionis one of the more widely-used methods ofNDI. 5-93. BASIC EQUIPMENT. Most ultra-sonic inspection systems include the followingbasic equipment; portable instruments (fre-quency range 0.5 to 15 MHz), transducers(longitudinal and shear wave), positioners, ref-erence standards, and couplant.

a. Ultrasonic Instruments. A portable,battery-powered ultrasonic instrument is usedfor field inspection of airplane structure. (Seefigure 5-17.) The instrument generates an

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FIGURE 5-15. Ultrasound.

FIGURE 5-16. Pulse-echo and through-transmission ultrasonic inspection techniques.

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FIGURE 5-17. Typical portable ultrasonic inspection in-strument.

ultrasonic pulse, detects and amplifies the re-turning echo, and displays the detected signalon a cathode ray tube or similar display. Pie-zoelectric transducers produce longitudinal orshear waves, which are the most commonlyused wave forms for aircraft structural inspec-tion.

b. Positioning Fixtures. To direct ultra-sound at a particular angle, or to couple it intoan irregular surface, transducer positioningfixtures and sound-coupling shoes are em-ployed. (See figure 5-18.) Shoes are made ofa plastic material that has the necessary sound-transmitting characteristics. Positioning fix-tures are used to locate the transducer at a pre-scribed point and can increase the sensitivityof the inspection. (See figure 5-19.) If atransducer shoe or positioning fixture is re-quired, the inspection procedure will give adetailed description of the shoe or fixture.

c. Reference Standards. Reference stan-dards are used to calibrate the ultrasonic in-strument (see figure 5-20), reference standardsserve two purposes to provide an ultrasonic re-sponse pattern that is related to the part beinginspected, and to establish the required inspec-tion sensitivity. To obtain a representative re-sponse pattern, the reference standard configu-ration is the same as that of the test structure,

or is a configuration that provides an ultrasonicresponse pattern representative of the teststructure. The reference standard contains asimulated defect (notch) that is positioned toprovide a calibration signal representative ofthe expected defect. The notch size is chosento establish inspection sensitivity (response tothe expected defect size). The inspection pro-cedure gives a detailed description of the re-quired reference standard.

d. Couplants. Inspection with ultrasonicsis limited to the part in contact with the trans-ducer. A layer of couplant is required to cou-ple the transducer to the test piece because ul-trasonic energy will not travel through air.Some typical couplants used are: water, glyc-erin, motor oils, and grease.

5-94. INSPECTION OF BONDEDSTRUCTURES. Ultrasonic inspection isfinding increasing application in aircraftbonded construction and repair. Detailed tech-niques for specific bonded structures should beobtained from the OEM’s manuals, or FAA re-quirements. In addition, further informationon the operation of specific instruments shouldbe obtained from the applicable equipmentmanufacturer manuals.

a. Types of Bonded Structures. Manyconfigurations and types of bonded structuresare in use in aircraft. All of these variationscomplicate the application of ultrasonic in-spections. An inspection method that workswell on one part or one area of the part maynot be applicable for different parts or areas ofthe same part. Some of the variables in thetypes of bonded structures are as follows.

(1) Top skin material is made from dif-ferent materials and thickness.

(2) Different types and thickness of ad-hesives are used in bonded structures.

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Note:All dimensions in inches followed by centimeters in parentheses.

FIGURE 5-18. Example of position fixture and shoe.

FIGURE 5-19. Example of the use if a transducer positioning fixture.

FIGURE 5-20. Example of a typical reference standard.

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(3) Underlying structures contain dif-ferences in; core material, cell size, thickness,and height, back skin material and thickness,doublers (material and thickness), closuremember attachments, foam adhesive, steps inskins, internal ribs, and laminates (number oflayers, layer thickness, and layer material).

(4) The top only or top and bottom skinof a bonded structure may be accessible.

b. Application of Ultrasonic Inspection.Application to bonded structures must be ex-amined in detail because of the many inspec-tion methods and structural configurations.The advantages and limitations of each in-spection method should be considered, andreference standards (representative of thestructure to be inspected) should be ultrasoni-cally inspected to verify proposed techniques.

c. Internal Configuration. Complete in-formation on the internal configuration of thebonded test part must be obtained by the in-spector. Drawings should be reviewed, andwhen necessary, radiographs of the test partshould be taken. Knowledge of details such asthe location and boundaries of doublers, ribs,etc., is required for valid interpretation of ul-trasonic inspection results. The boundaries ofinternal details should be marked on the testpart using a grease pen or other easily remov-able marking.

d. Reference Standards. Standards canbe a duplicate of the test part except for thecontrolled areas of unbond. As an option,simple test specimens, which represent thevaried areas of the test part and contain con-trolled areas of unbond, can be used. Refer-ence standards must meet the following re-quirements.

(1) The reference standard must besimilar to the test part regarding material, ge-ometry, and thickness. This includes

containing: closure members, core splices,stepped skins, and internal ribs similar to thetest part if bonded areas over or surroundingthese details are to be inspected.

(2) The reference standard must containbonds of good quality except for controlled ar-eas of unbond fabricated as explained below.

(3) The reference standard must bebonded using the adhesive and cure cycle pre-scribed for the test part.

e. Types of Defects. Defects can be sepa-rated into five general types to represent thevarious areas of bonded sandwich and laminatestructures as follows:

(1) Type I. Unbonds or voids in anouter skin-to-adhesive interface.

(2) Type II. Unbonds or voids at theadhesive-to-core interface.

(3) Type III. Voids between layers of alaminate.

(4) Type IV. Voids in foam adhesive orunbonds between the adhesive and a closuremember at core-to-closure member joints.

(5) Type V. Water in the core.

f. Fabrication of NDI Reference Stan-dards. Every ultrasonic test unit should havesample materials that contain unbonds equal tothe sizes of the minimum rejectable unbondsfor the test parts. Information on minimumrejectable unbond sizes for test parts should beobtained from the OEM’s manuals, FAA re-quirements, or the cognizant FAA AircraftCertification Office (ACO) engineer. One ormore of the following techniques can be usedin fabricating reference defects; however, sincebonding materials vary, some of the methodsmay not work with certain materials.

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(1) Standards for Types I, II, III, and IVunbonds can be prepared by placing discsof 0.006 inch thick (maximum) Teflon sheetsover the adhesive in the areas selected for un-bonds. For Type II unbonds, the Teflon isplaced between the core and adhesive. Thecomponents of the standard are assembled andthe assembly is then cured.

(2) Types I, II, and III standards canalso be produced by cutting flat-bottomedholes of a diameter equal to the diameter of theunbonds to be produced. The holes are cutfrom the back sides of bonded specimens, andthe depths are controlled to produce air gaps atthe applicable interfaces. When using thismethod, patch plates can be bonded to the rearof the reference standard to cover and sealeach hole.

(3) Type II standards can be producedby locally undercutting (before assembly) thesurface of the core to the desired size unbond.The depth of the undercut should be sufficientto prevent adhesive flow causing bonds be-tween the undercut core and the skin.

(4) Type IV standards can be producedby removing the adhesive in selected areasprior to assembly.

(5) Type V standards can be producedby drilling small holes in the back of the stan-dard and injecting varying amounts of waterinto the cells with a hypodermic needle. Thesmall holes can then be sealed with a smallamount of water-resistant adhesive.

g. Inspection Coverage. Examples ofseveral different configurations of bondedstructure along with suggested inspection cov-erages with standard ultrasonic test instru-ments are shown in figure 5-21. In manycases, access limitations will not permit appli-cation of the suggested inspections in all of theareas shown. The inspection coverages and

suggested methods contained in figure 5-21and table 5-7 are for reference only. Details ofthe inspection coverage and inspections for aparticular assembly should be obtained fromthe OEM’s manuals, or other FAA-approvedrequirements.

h. Inspection Methods. Table 5-8 liststhe various inspection methods for bondedstructures along with advantages and disad-vantages of each inspection method.

5-95. BOND TESTING INSTRUMENTS.Standard ultrasonic inspection instruments canbe used for bond testing as previously noted;however, a wide variety of bond testing in-struments are available for adaptation to spe-cific bonded structure inspection problems.

a. General Principle. Two basic operat-ing principles are used by a variety of bondtesters for single-sided bond inspection.

(1) Ultrasonic resonance. Sound wavesfrom a resonant transducer are transmitted intoand received from a structure. A disbond inthe structure will alter the sound wave charac-teristics, which in turn affect the transducerimpedance.

(2) Mechanical impedance. Low-frequency, pulsed ultrasonic energy is gener-ated into a structure. Through ultrasonic me-chanical vibration of the structure, the imped-ance or stiffness of the structure is measured,analyzed, and displayed by the instrumenta-tion.

b. Operation. In general, operation of theadhesive bond test instruments noted is simi-lar. The test probe is moved over the surfacein smooth overlapping strokes. The directionof the stroke with regard to the surface is gen-erally immaterial; however, when using theSondicator models, the direction of the strokebecomes critical when the test probe is

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FIGURE 5-21. Examples of bonded structure configurations and suggested inspection coverage.

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TABLE 5-7. Acceptable ultrasonic inspection methods associated with the example bonded structure configurationsshown in figure 5-21.

NUMBER ACCEPTABLE METHODS

1 Either(a) Pulse-echo straight or angle beam, on each side

or(b) Through-transmission.

2 Pulse-echo, straight beam, on each skin.

3 Refer to 1 for methods. If all these methods fail to have sufficient penetrationpower to detect reference defects in the reference standard, then the ringingmethod, applied from both sides, should be used. Otherwise, the ringing methodis unacceptable.

4 Either(a) Ringing, on each skin of the doubler.

or(b) Through-transmission

or(c) Damping.

5 Ringing.

6 Through-transmission. Dotted Line represents beam direction.

7 Either(a) Through-transmission

or(b) Damping.

NOTE

A variety of ultrasonic testing methods and instruments are available for adaptation to specific inspectionproblems. Other bond inspection instruments can be used if detailed procedures are developed andproven on the applicable reference standards for each configuration of interest. Some representative in-struments, which can be used for the inspection of bonded structures are; the Sondicator, Harmonic BondTester, Acoustic Flaw Detector, Audible Bond Tester, Fokker Bond Tester, 210 Bond Tester, and Bon-dascope 2100.

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TABLE 5-8. Ultrasonic inspection methods for bonded structures.

INSPECTION METHOD

THROUGH-TRANSMISSION

PULSE-ECHO RINGING DAMPING

Advantages Applicable tostructures with ei-ther thick or thinfacing sheets.

Applicable tostructures withmultiple layersbonded over hon-eycomb core.

Detects unbondson either side

Detects broken,crushed, and cor-roded core.

In some caseswater in core canbe detected.

Applicable to struc-tures with either thickor thin facing sheets.

Determines whichside is unbonded.

Detects small un-bonds.

Detects broken,crushed, and cor-roded core.

In some cases waterin core can be de-tected.

Applicable to com-plex shapes.

Detects small nearsurface unbonds(larger than di-ameter of searchunit).

Applicable tostructures with ei-ther thick or thinfacing sheets.

Applicable to mul-tiple-layered (morethan two layers)structures.

Detects unbondson either side.

Detects small un-bonds (larger thandiameter of re-ceiving searchunit).

Disadvantages Access to bothsides of part is re-quired.

Does not deter-mine layer positionof unbonds.

Alignment ofsearch units iscritical.

Couplant is re-quired.

Inspection rate isslow.

Inspection from bothsides required, doesnot detect far sideunbonds.

Applicable only toskin-to-honeycombcore structures.

Reduced effective-ness on structureswith multiple skinsover honeycombcore.

Couplant required.

Applicable only tonear surface un-bonds, works beston unbonds be-tween top sheetand adhesive, maymiss other un-bonds.

Reduced effective-ness on thick skins.

Couplant required.

Applicable only todoublers and lami-nate-type struc-tures.

Access to bothsides required.

Does not deter-mine layer posi-tions of unbonds.

Couplant required.

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operated near a surface edge. Edge effects onvibration paths give a test reading that may bemisinterpreted. To avoid edge effects, the testprobe should be moved so that the inspectionpath follows the surface edge, giving a con-stant edge for the test probe to inspect. Edgeeffects are more pronounced in thicker mate-rial. To interpret meter readings correctly, theoperator should determine whether there arevariations in the thickness of the material.

c. Probe Sending Signal. With the ex-ception of the Sondicator models, the testprobes of the testers emit a sending signal thatradiates in a full circle. The sending signal ofthe Sondicator probe travels from one trans-ducer tip to the other. For this reason, the testprobe should be held so that the transducer tipsare at right angles to the inspection path.When inspecting honeycomb panels with aSondicator model, the transducer tips shouldbe moved consistently in the direction of theribbon of the honeycomb or at right angles tothe ribbon so that a constant subsurface is pre-sented.

5-96. THICKNESS MEASUREMENTS.Ultrasonic inspection methods can be used formeasurement of material thickness in aircraftparts and structures.

a. Applications. Ultrasonic thicknessmeasurements are used for many applications,such as: checking part thickness when accessto the back side is not available; checking largepanels in interior areas where a conventionalmicrometer cannot reach; and in maintenanceinspections for checking thickness loss due towear and/or corrosion.

b. Pulse-Echo Method. The most com-monly used ultrasonic thickness measurementmethod. The ultrasonic instrument measurestime between the initial front and back surfacesignals or subsequent multiple back reflectionsignals. Since the velocity for a given material

is a constant, the time between these signals isdirectly proportional to the thickness. Cali-bration procedures are used to obtain directreadout of test part thickness.

c. Thickness Measurement InstrumentTypes. Pulse-echo instruments designed ex-clusively for thickness measurements are gen-erally used in lieu of conventional pulse-echoinstruments; however, some conventionalpulse-echo instruments also have direct thick-ness measurement capabilities. Conventionalpulse-echo instruments without direct thick-ness measuring capabilities can also be usedfor measuring thickness by using special pro-cedures.

d. Thickness Measurement Ranges.Dependent upon the instrument used and thematerial under test, material thicknessfrom 0.005 inches to 20 inches (or more) canbe measured with pulse-echo instruments de-signed specifically for thickness measuring.

5-97. LEAK TESTING. The flow of apressurized gas through a leak produces soundof both sonic and ultrasonic frequencies. If thegas leak is large, the sonic frequency sound itproduces can probably be detected with the earor with such instruments as stethoscopes ormicrophones; however, the ear and these in-struments have limited ability to detect and lo-cate small leaks. Ultrasonic leak detectors arefrequently used to detect leaks that cannot bedetected by the above methods, because theyare very sensitive to ultrasonic energy and, un-der most conditions, background noise at otherfrequencies does not affect them.

a. Standard Method. A standard methodof testing for leaks using ultrasonics is pro-vided in ASTM E 1002. The method coversprocedures for calibration, location, and esti-mated measurements of leakage by the ultra-sonic technique (sometimes called ultrasonictranslation).

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b. Detection Distance. Ultrasonic energyin the relatively low-frequency range of30-50 KHz travels easily through air; there-fore, an ultrasonic leak detector can detectleakage with the probe located away from theleak. The maximum detection distance de-pends on the leakage rate.

c. Typical Applications. Some typicalapplications for the ultrasonic leak detector onaircraft are: fuel system pressurization tests, airducts and air conditioning systems, emergencyevacuation slides, tire pressure retention, elec-trical discharge, oxygen lines and valves, in-ternal leaks in hydraulic valves and actuators,fuel cell testing, identifying cavitation in hy-draulic pumps, arcing in wave guides, cabinand cockpit window and door seals, and cabinpressurization testing.

5-98. 5-104. [RESERVED.]

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SECTION 8. TAP TESTING

5-105. GENERAL. Tap testing is widelyused for a quick evaluation of any accessibleaircraft surface to detect the presence of de-lamination or debonding.

a. The tap testing procedure consists oflightly tapping the surface of the part with acoin, light special hammer with a maximum of2 ounces (see figure 5-22), or any other suit-able object. The acoustic response is com-pared with that of a known good area.

b. A “flat” or “dead” response is consid-ered unacceptable. The acoustic response of agood part can vary dramatically with

changes in geometry, in which case a standardof some sort is required. The entire area ofinterest must be tapped. The surface should bedry and free of oil, grease, and dirt. Tap test-ing is limited to finding relatively shallow de-fects in skins with a thickness less than.080 inch. In a honeycomb structure, for ex-ample, the far side bondline cannot be evalu-ated, requiring two-side access for a completeinspection. This method is portable, but no re-cords are produced. The accuracy of this testdepends on the inspector’s subjective inter-pretation of the test response; therefore, onlyqualified personnel should perform this test.

FIGURE 5-22. Sample of special tap hammer.

5-106. 5-111. [RESERVED.]

SECTION 9. ACOUSTIC-EMISSION

5-112. GENERAL. Acoustic-Emission is anNDI technique that involves the placing ofacoustical-emission sensors at various loca-tions on the aircraft structure and then applyinga load or stress. The level of stress appliedneed not reach general yielding, nor does thestress generally need to be of a specific type.Bending stress can be applied to beamedstructures, torsional stress can be applied torotary shafts, thermal stresses can be appliedwith heat lamps or blankets, and pres-sure-induced stress can be applied to pres-sure-containment systems such as the aircraftfuselage. The materials emit sound and stresswaves that take the form of ultrasonic pulses

that can be picked up by sensors. Cracks andareas of corrosion in the stressed airframestructure emit sound waves (different frequen-cies for different size defects) which are regis-tered by the sensors. These acoustic-emissionbursts can be used both to locate flaws and toevaluate their rate of growth as a function ofapplied stress. Acoustic-emission testing hasan advantage over other NDI methods in that itcan detect and locate all of the activated flawsin a structure in one test. Acoustic-emissiontesting does not now provide the capability tosize flaws, but it can greatly reduce the arearequired to be scanned by other NDI methods.

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5-113. APPLICATIONS. A wide variety ofstructures and materials, such as: wood, plas-tic, fiberglass, and metals can be inspected bythe acoustic-emission technique by applyingstress on the test material. The emis-sion-producing mechanism in each type ofmaterial may differ, but characteristic acous-tic-emissions are produced and can be corre-lated to the integrity of the material. Acous-tic-emission technology has been applied quitesuccessfully in monitoring proof tests of pres-sure vessels and tests of fiber-reinforced plas-tic structures of all kinds. There are nowASTM standards and ASME codes applying toits use in testing gas cylinders and both metaland fiber-reinforced plastic vessels, tanks, andpiping.

For a welded structure such as a pressure ves-sel, acoustic-emission testing works well with

relatively simple instrumentation. However,slight movement of bolted or riveted joints canalso generate acoustic signals. Thus a complexstructure may have many acoustic sources be-sides flaws in its components. These un-wanted emission sources greatly complicateacoustic-emission tests of complex structures.The difficulties are not prohibitive, but theyput a premium on the intelligent use of signalprocessing and interpretation. Therefore, be-cause of the complexity of aircraft structures,application of acoustic-emission testing to air-craft has required a new level of sophistication,both in testing techniques and data interpreta-tion. Research and testing programs are cur-rently in progress to determine the feasibilityof acoustic-emission testing on several differ-ent types of aircraft.

5-114. 5-119. [RESERVED.]

SECTION 10. THERMOGRAPHY

5-120. GENERAL. Thermography is an NDItechnique that uses radiant electromagneticthermal energy to detect flaws. The presenceof a flaw is indicated by an abnormal tem-perature variant when the item is subjected tonormal heating and cooling conditions inherentto the in-service life, and/or when artificially

heated or cooled. The greater the material’sresistance to heat flow, the more readily theflow can be identified due to temperature dif-ferences caused by the flaw.

5-121. 5-126. [RESERVED.]

SECTION 11. HOLOGRAPHY

5-127. GENERAL. Holography is an NDItechnique that uses visible light waves cou-pled with photographic equipment to create athree-dimensional image. The process usestwo laser beams, one called a reference beamand the other called an object beam. The twolaser beams are directed to an object, betweenbeam applications the component is stressed.The beams are then compared and recorded

on film, or other electronic recording medium,creating a double image. Indications of ap-plied stresses or defects are shown as virtualimages with a system of fringe lines overlay-ing the part. Holography is most commonlyused for rapid assessment of surface flaws incomposite structures. 5-128. 5-133. [RESERVED.]

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SECTION 12. SHEAROGRAPHY

5-134. GENERAL. Shearography was de-veloped for strain measurements. The processnow provides a full-field video strain gauge,in real time, over large areas. It is an en-hanced form of holography, which requiresthe part to be under stress. A laser is used forillumination of the part while under stress.The output takes the form of an image proc-essed video display. This technique has beenused effectively in locating defects, such asdisbonds and delaminations, through multiplebondlines. It is capable of showing the sizeand shape of subsurface anomalies when the

test part is properly stressed. Shearographyhas been developed into a useful tool for NDI.It can be used easily in a hangar environment,while meeting all laser safety concerns. Otherapplications include the testing of honeycombstructures, such as flaps and control surfaces.Shearography offers a great increase in thespeed of inspection by allowing on-aircraft in-spections of structures without their removal,as well as inspections of large areas in justseconds. 5-135. 5-140. [RESERVED.]

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