LPT

Liquid Penetrant Inspection

1

Department Of Mechanical & Manufacturing Engg. ~ 1 ~

MIT,Manipal

1. Introduction

1.1 Non- Destructive Evaluation

Non-destructive testing (NDT) is a wide group of analysis techniques

used to evaluate the properties of a material, component or system

without causing damage. The terms Non-destructive examination (NDE),

Non-destructive inspection (NDI) and Non-destructive evaluation (NDE)

are also commonly used to describe this technology. Flaws/cracks present

a material plays a major role in determination of its fracture strength and

hence life. Hence techniques of crack detection became more popular.

The present report is all about one such non-destructive technique, ie.

Liquid Penetrant Testing.

1.2 Liquid Penetrant Test (LPT)

Indian Standard Institution (IS: 3658-1981) (2)

defines liquid penetrant

test as a non-destructive testing method for detecting discontinuities that

are open to surface.

LPT’s may be effectively used in the inspection of both ferrous and non-

ferrous metals and on non-porous, non-metallic materials, such as

ceramics, plastics and glass. Surface discontinuities such as cracks, cold

shunts are indicated by this method. Flaw detection using the help of

liquid penetrant is being increasingly used in various industries.(2)(3)

and hence it is a relevant NDT process in the nation.

LPT utilizes the natural accumulation of a fluid (penetrant) around the

discontinuity to create a recognizable indication of a crack or other

surface anomaly.


2


MIT,Manipal

Capillary action attracts the fluid to the discontinuity in a concentration

heavier than in the surroundings. In order for the fluid concentration to be

recognized, the background (developer) area must be of sufficient

contrast to distinctly reveal the defect on the surface. The complete

penetrant flaw detection system, therefore, consists of the fluid mechanics

on the surface, as well as the recognition system that is used to detect the

indication.

Eg. The testing method can be well explained using the example of a

concrete slab. First the slab is initially wetted with a fluid that flows

readily. Second, the fluid is drawn into the crack by a capillary action.

And, third, the excess fluid is removed from the surface creating a good

contrast between the still moist areas and dried. Finally it is important to

note that the penetrants are useful only for the surface breaking defects.

The crack that did not reach the surface would not be detected.(5)

.

Fig 1.Turbine blade with the application of penetrant


3


MIT,Manipal

Fig.2 Turbine blade with the application of developer.


4


MIT,Manipal

2. Scientific Principles Involved In LPT

The complete penetrant flaw detection system is based on the fluid

mechanics of the penetrant on the surface. Three major areas of fluid

mechanics that play a significant role in LPT are as follows,

Surface Tension

Capillarity

Viscosity

Surface tension is a fluid property that affects the flow of the fluid and

wettability of the fluid penetrant of the surface to be inspected. Capillarity

is the driving force in the movement of the penetrant the solid surface,

into the crack and out of the crack into the developer. Viscosity is the

property of a fluid, affecting the flow of the fluid on the surface to be

inspected.

2.1 Surface Tension

Surface tension is a property of the surface of a liquid that allows it to

resist external force. The cohesive forces between molecules of a liquid

cause surface tension. An example of the influence of surface tension is

the tendency of free liquid, such as a droplet of water, to contract into a

sphere. In such a droplet, surface tension is counter balanced by the

internal hydrostatic pressure of the liquid.

When the liquid comes into contact with a solid surface, the cohesive

force responsible for surface tension competes with the adhesive force

between the molecules of the liquid and the solid surface. These forces

jointly determine the contact angle, θ, between the liquid and the surface.

Contact angle lesser than 90 degree gives good wetting on the surface,

which is desirable.

http://en.wikipedia.org/wiki/Liquid


5


MIT,Manipal

Fig 3.Fluid-solid contact angles (<90, 90,>90) respectively

Fig 4.Surface Tension

The contact angle is a function of the properties of both the fluid and the

material being inspected. The cleanliness of the surface also affects the

contact angle.

The ability of the fluid to enter the surface cavities is directly proportional

to the surface tension, and inversely proportional to the contact angle, the

density and the size of opening.

Table 1: Surface Tension of liquids at 20o c in contact with air

Liquid Surface Tension (m N/m)

Benzene 28.9

Hexane 18.4

Kerosene 26.8

Lube Oil 25-35


6


MIT,Manipal

Methanol 22.6

Octane 21.8

Water 72.8

2.2 Capillarity

It is the ability of a liquid to flow in narrow spaces without the assistance

of, and in opposition to external forces like gravity. But recent studies

made by MIT (US) research group on Non Newtanian Fluids shows that

the liquid drops may lose their spherical shape under the influence of

gravity.

There exists a particular length, denoted κ-1

, beyond which gravity

becomes important. It is referred to as the capillary length. It can be

estimated by comparing the Laplace pressure γ/κ-1

to the hydrostatic

pressure ρgκ-1

at a depth κ-1

in a liquid of density ρ submitted to earth's

gravity g=9.8 m/s2. Equating these two pressures defines the capillary

length.(w 1)

(I)

For water, the capillary length = 2.7 mm

It occurs because of inter-molecular attractive forces between the liquid

and solid surrounding surfaces; If the diameter of the tube is sufficiently

small, then the combination of surface tension (which is caused

by cohesion within the liquid) and adhesive forces between the liquid and

container act to lift the liquid as shown in fig 5.

Capillary rise/fall is a function of surface tension, contact angle & density

of the liquid as shown in the equation (I

http://en.wikipedia.org/wiki/Surface_tension

http://en.wikipedia.org/wiki/Cohesion_(chemistry)

http://en.wikipedia.org/wiki/Adhesion


7


MIT,Manipal

(II)

Fig 5.Capillary rise

Even though flaws are not exactly capillary tubes, the phenomenon of

penetration is by capillary action.


8


MIT,Manipal

Table 2:Contact angles of Various Liquid-solid Interfaces

2.3Viscosity

Viscosity is a measure of the resistance of a fluid which is being

deformed by either shear or tensile stress. Viscosity describes a fluid's

Internal resistance to flow and may be thought of as a measure of

fluid friction.The viscosity of the liquid is not a factor in the basic

equation of capillary rise. Viscosity is related to the rate at which a liquid

will flow under some applied unbalanced stress; in itself, viscosity has a

negligible effect on penetrating ability. In general, however, very viscous

liquids are unsuitable as penetrants because they do not flow rapidly

enough over the surface of the work piece; consequently, they require

excessively long periods of time to migrate into fine flaws.

Liquid Solid Contact angle

water soda-lime

glass

lead glass

fused quartz

0°

ethanol

diethyl ether

carbon

tetrachloride

glycerol

acetic acid

water paraffin wax 107°

silver 90°

methyl iodide soda-lime

glass

29°

lead glass 30°

fused quartz 33°

http://en.wikipedia.org/wiki/Drag_(physics)

http://en.wikipedia.org/wiki/Fluid

http://en.wikipedia.org/wiki/Shear

http://en.wikipedia.org/wiki/Tensile_stress

http://en.wikipedia.org/wiki/Friction

http://en.wikipedia.org/wiki/Water

http://en.wikipedia.org/wiki/Ethanol

http://en.wikipedia.org/wiki/Diethyl_ether

http://en.wikipedia.org/wiki/Carbon_tetrachloride

http://en.wikipedia.org/wiki/Carbon_tetrachloride

http://en.wikipedia.org/wiki/Glycerol

http://en.wikipedia.org/wiki/Acetic_acid

http://en.wikipedia.org/wiki/Water

http://en.wikipedia.org/wiki/Iodomethane


9


MIT,Manipal

3. History and Evolution Of LPT.

The exact origin of liquid penetrant inspection is not known, but it has

been assumed that the method evolved from the observation that the rust

on a crack in a steel plate in outdoor storage was somewhat heavier than

the rust on the adjacent surfaces as a result of water seeping into the crack

and forcing out the oxide it had helped to produce. The obvious

conclusion was that a liquid purposely introduced into surface cracks and

then brought out again would reveal the locations of those cracks.

The only material that fulfilled the known criteria of low viscosity, good

wettability, and ready availability was kerosene. It was found, however,

that although wider cracks showed up easily, finer ones were sometimes

missed because of the difficulty of detecting, by purely visual means, the

small amounts of kerosene exuding from them. The solution was to

provide a contrasting surface that would reveal smaller seepages. The

properties and availability of whitewash made it the logical choice. This

method, known as the kerosene-and-whiting test, was the standard for

many years. The sensitivity of the kerosene-and-whiting test could be

increased by hitting the object being tested with a hammer during testing.

The resulting vibration brought more of the kerosene out of the cracks

and onto the whitewash.

Although this test was not as sensitive as those derived from it, it was

quick, inexpensive, and reasonably accurate. Thus, it provided a vast

improvement over ordinary visual examination. The first step leading to

the methods now available was the development of the fluorescent

penetrant process by R.C. Switzer. This liquid, used jointly with a powder


10


MIT,Manipal

developer, brought penetrant inspection from a relatively crude procedure

to a more scientific operation. With fluorescent penetrant, minute flaws

could be readily detected when exposed to ultraviolet light (commonly

called black light). This development represented a major breakthrough in

the detection of surface flaws.

Switzer's work also included the development of the visible-colour

contrast method, which allowed for inspection under white light

conditions. Although not as sensitive as fluorescent penetrant inspection,

it is widely used in industry for noncritical inspection. Through the

developments described above, liquid penetrant inspection has become a

major non-destructive inspection method.


11


MIT,Manipal

4. Terminologies Related To LPT As In

IS: 3658- 1981.(2)

Penetrant:- A liquid possessing the property of entering into small

openings and to remain there, making it suitable for liquid

penetrant test.

Developer:-A substance ,which is usually white in colour and

powdered in the form and having the property of blotting out a

penetrant retained in defects, thus intensifying the indications and

providing a contrasting background for the penetrant.

Dwell Time: - The period of time wherein an inspection penetrant

is in contact with the surface of the part.

Emulsifiable Penetrants:-Penetrants capable of being converted

into a water washable condition by the addition of emulsifier.

Post-emulsifiable Penetrant:-A Penetrant which must be treated

with an emulsifying agent to render it water washable.

Pre-emulsifiable Penetrant:-A penetrant with suitable

emulsifiable agent, added to render it directly water washable.

Self-emulsifiable Penetrant:-The property of a liquid penetrant

to combine satisfactorily with water, thus facilitating its removal

by washing with water.

Vapour Degreasing:-The removal of oils, greases, and organic

oils by the use of suitable vapour.


12


MIT,Manipal

Ultraviolet Light:-Monochromatic blue light of approximately

4300 Angstrom wavelength, used to cause certain liquid penetrant

s to fluorescence.

Bleed Out:-The action by which the penetrant comes out of the

discontinuities on to the surface of materials due to

blotting/soaking effect of the developer.

Fluorescent Penetrants:-penetrating liquids containing additives

to fluorescence under black light.

Solvent Remove:-A liquid employed for removal of surface

penetrant from parts or for removal of unwanted background

porosity indications.

5. LPT Process Description

LPT process consists of 5 basic steps.viz

1. Surface Preparation. All surfaces to be inspected, whether

localized or the entire work piece must be thoroughly cleaned and

completely dried before being subjected to penetrant inspection.

Flaws exposed to the surface must be free from oil, water, or other

contaminants if they are to be detected.

2. Penetration. After the work piece has been cleaned, penetrant is

applied in a suitable manner so as to form a film of the penetrant

over the surface. This film should remain on the surface long

enough to allow maximum penetration of the penetrant into any

surface openings that are present.


13


MIT,Manipal

3. Removal of Excess Penetrant. Excess penetrant must be removed

from the surface. The removal method is determined by the type of

penetrant used. Some penetrants can be simply washed away with

water; others require the use of emulsifiers (lipophilic or

hydrophilic) or solvent/remover. Uniform removal of excess

surface penetrant is necessary for effective inspection, but over

removal must be avoided.

4. Development. Depending on the form of developing agent to be

used, the work piece is dried either before or directly after

application of the developer. The developer forms a film over the

surface. It acts as a blotter to assist the natural seepage of the

penetrant out of surface openings and to spread it at the edges so as

to enhance the penetrant indication.

5. Inspection. After it is sufficiently developed, the surface is

visually examined for indications of penetrant bleed back from

surface openings. This examination must be performed in a

suitable inspection environment. Visible penetrant inspection is

performed in good white light. When fluorescent penetrant is used,

inspection is performed in a suitably darkened area using black

(ultraviolet) light, which causes the penetrant to fluoresce

brilliantly.


14


MIT,Manipal

Fig. 6 LPT Process Description

Surface Preparation

Key Recommendations for surface preparation by ISI code 3658

are as follows,

In general satisfactory results can be obtained when the

surface is in the as welded, as rolled, as cast/forged

condition, but surface preparation by grinding/machining or

other suitable method may be necessary, when surface

irregularities could otherwise mask indications of

unacceptable discontinuities.


15


MIT,Manipal

Heavy grinding should be avoided to avoid the masking of

fine defects.

The surface to be examined and all adjacent areas within at

least 25 mm should be also dry and clean.

The method of cleaning depends on the nature of material

and contamination. Cleaning using detergents, organic

solvents, alkali solutions, paint remover, descaling solutions,

vapour degreasing, ultrasonic cleaning, abrasive blasting are

in practice.

Penetration

o Classification Of Penetrants

Type I fluorescent penetrant utilizes penetrants that are usually green in

colour and fluorescent brilliantly under ultraviolet light. The sensitivity of

a fluorescent penetrant depends on its ability to form indications that

appear as small sources of light in an otherwise dark area. Type I

penetrants are available in different sensitivity levels classified as

follows:

· Level: Ultralow

· Level 1: Low

· Level 2: Medium

· Level 3: High

· Level 4: Ultrahigh

Type II visible penetrant employs a penetrant that is usually red in

colour and produces vivid red indications in contrast to the light

background of the applied developer under visible light. The visible

penetrant indications must be viewed under adequate white light. The


16


MIT,Manipal

sensitivity of visible penetrants is regarded as Level 1 and adequate for

many applications.

Visible light level-Recommended minimum light intensity at

examination site=100 lux/m2.

(1)

Penetrant selection and use depend on the criticality of the

inspection, the condition of the work piece surface, the

type of processing, and the desired sensitivity.

Fig 7.Application of red liquid penetrant

Penetrant Selection Method

Method A, water-washable penetrants are designed for the removal of

excess surface penetrant by water rinsing directly after a suitable

penetration (dwell) time. The emulsifier is incorporated into the water-

washable penetrant. When this type of penetrant is used, it is extremely

important that the removal of excess surface penetrant be properly

controlled to prevent over washing, which can cause the penetrant to be

washed out of the flaws.

Methods B and D, lipophilic and hydrophilic post emulsifiable

penetrants are insoluble in water and therefore not removable by water

rinsing alone. They are designed to be selectively removed from the


17


MIT,Manipal

surface of the work piece by the use of a separate emulsifier. The

emulsifier, properly applied and left for a suitable emulsification time,

combines with the excess surface penetrant to form a water-washable

surface mixture that can be rinsed from the surface of the workpiece. The

penetrant that remains within the flaw is not subject to over washing.

However, proper emulsification time must be established experimentally

and maintained to ensure that over emulsification, which results in the

loss of flaws, does not occur.

Method C, solvent-removable penetrants are removed by wiping with

clean, lint-free material until most traces of the penetrant have been

removed. The remaining traces are removed by wiping with clean, lint-

free material lightly moistened with solvent. This type of penetrant is

primarily used where portability is required and for the inspection of

localized areas. To minimize the possibility of removing the penetrant

from discontinuities, the use of excessive amounts of solvent must be

avoided.

Emulsifiers

Emulsifiers are liquids used to render excess penetrant on the surface of a

work piece water washable. There are two methods used in the post

emulsifiable method: method B, lipophilic, and method D, hydrophilic.

Both can act over a range of durations from a few seconds to several

minutes, depending on the viscosity, concentration, method of

application, and chemical composition of the emulsifier, as well as on the

roughness of the work piece surface.


18


MIT,Manipal

The length of time an emulsifier should remain in contact with the

penetrant depends on the type of emulsifier employed and the roughness

of the work piece surface.

Method B, lipophilic emulsifiers are oil based, are used as supplied, and

function by diffusion (Fig. 11). The emulsifier diffuses into the penetrant

film and renders it spontaneously emulsifiable in water. The rate at which

it diffuses into the penetrant establishes the emulsification time.

The emulsifier is fast acting, thus making the emulsification operation

very critical. The emulsifier continues to act as long as it is in contact

with the work piece; therefore, the rinse operation should take place

quickly to avoid over emulsification

Fig8. Functioning of Lipophilic Emulsifier


19


MIT,Manipal

Method D, hydrophilic emulsifiers are water based and are usually

supplied as concentrates that are diluted in water to concentrations of 5 to

30% for dip applications and 0.05 to 5% for spray applications.

Hydrophilic emulsifiers function by displacing excess penetrant from the

surface of the part by detergent action. The force of the water spray or the

air agitation of dip tanks provides a scrubbing action. Hydrophilic

emulsifier is slower acting than the lipophilic emulsifier; therefore, it is

easier to control the cleaning action. In addition to the emulsifier

application, method D also requires a pre rinse. Utilizing a coarse water

spray, the pre rinse helps remove the excess penetrant to minimize

contamination of the emulsifier. Of greater significance, only a very thin

and uniform layer of penetrant will remain on the surface, thus allowing

easy removal of the surface layer with minimum opportunity of removing

penetrant from the flaws. This step is required because the penetrant is

not miscible with the hydrophilic emulsifier.

Fig 9.Functioning Of Hydrophilic Emulsifier


20


MIT,Manipal

Recommendations (2)

Penetration Time

When high viscosity florescent penetrant is used, the penetration

time is longer than normal penetration time. In such a case, time

shall be subjected to agreement between the manufacturer and the

purchaser.

If the testing is to be done out of the range of 15-500 c, the

penetration time should be suitably increased, and the whole

process and the materials is to be checked using a comparator

block.

If water washable penetrants are used, the penetration time shall be

about 1.5 to 2.0 times of 10 minutes.

Rinsing

Using water washable penetrant rinsing should be done with water

supply nozzle. Temperature of water should not exceed 43oC and

pressure of 3 kg /cm2 should be maintained.

On smooth surfaces, it may be possible sometimes to remove

excess penetrant merely by wiping the surface with dry rag.

Using fluorescent type of penetrant, it is helpful to use a portable

black light source, while rinsing, so as to ensure that rinsing operation

is complete.

Developing

Generally half of the penetration time is considered as developing

time. Developers may either of dry type or wet type. A good

practice is to start observation as soon as developer applied.


21


MIT,Manipal

Fig 10 Application Of Developer

Inspection

With visible dye penetrants, surface defects are

indicated by bleeding out of penetrant, normally

deep red in colour.

Crack/small openings show a line, tight crack shows

broken lines. Fine porosity will be indicated by

random dots.(see fig 11)

Depth of surface discontinuity can be correlated to

richness of colour and speed of bleeding.

A minimum light intensity of 1000 lx/ m2

is to be

maintained.


22


MIT,Manipal

Fig 11 Random dots indicating fine porosity in the material

Fig 12 Crack Inspection using fluorescent penetrant in black light


23


MIT,Manipal

Fig 12 Crack Inspection using fluorescent penetrant in black light

Table 3.Crack Detection Limits for Dye Penetrant Test (4)

Type of Inspection Crack length (mm)

Production Parts (lab use) 0.127

Production Parts (inspection) 0.76

Structure (Service Inspection) 1.27

Sensitivity of LPT(5)

The sensitivity of LPT is defined as the ratio of number of detected

cracks per scan of UV laser to the total number of actual craks.It is

expressed in percentage. Higher values of sensitivity refer to higher

efficiency of LPT in the crack detection process.


24


MIT,Manipal

6. Process Flow Diagrams of LPT(8)

They are the flow diagrams/charts referring the preferences of LPT and

direct the path of conduction of LPT.

Fig 13 Process flow diagram of postemulsifiable, method B, lipophilic

liquid penetrant system

Fig 14. Processing flow diagram for the water-washable liquid

penetrant system


25


MIT,Manipal

Fig 15 Processing flow diagram for the solvent-removable liquid

penetrant system


26


MIT,Manipal

7. List of ASTM standards for LPT

ASTM E 165- Standard Practices for Liquid-Penetrant Inspection

Method

ASTM E 270- Standard Definitions of Terms Relating to Liquid-

Penetrant Inspection

ASTM E 1208- Standard Method for Fluorescent Liquid-Penetrant

Examination Using the Lipophilic Post-Emulsification Process

ASTM E 1209- Standard Method for Fluorescent-Penetrant

Examination Using the Water-Washable Process


Examination Using the Hydrophilic Post-Emulsification Process


Examination Using the Solvent-Removable Process

ASTM E 1220- Standard Method for Visible-Penetrant

Examination Using the Solvent-Removable Process

ASTM E 1135- Standard Test Method for Comparing the

Brightness of Fluorescent Penetrants

ASTM D 2512- Compatibility of Materials with Liquid Oxygen

(Impact-Sensitivity Threshold Technique)


27


MIT,Manipal

8. Safety Considerations In LPT(4,7)

Changes in environmental protection standards have placed

restrictions on many chemicals used as liquid penetrants.

Methyl Chloroform (MCF), a liquid penetrant was banned after

Montreal Protocol (2002) since it was a CFC causing ozone

depletion. Then a Ozone friendly penetrant HCFC-123 replaced

MCF. (7)

Certain chemicals which are used as a fluorescent penetrants were

irritants and even carcinogenic.

Gloves, aprons, masks should be used when working with many

chemicals.

UV rays from an arc lamp may burn skin/eyes if the filters are

broken.

Many of the volatile solvents are highly inflammable. So cleaning

area should be ventilated to avoid fire accidents.

Special care must be taken in the inspection of liquid oxygen

tanks.eg. Selection of penetrants/cleansers that will not cause

explosive reactions with liquid oxygen residues.


28


MIT,Manipal

Summary of LPT

Advantages of LPT

1. It is portable, well suited for fieldwork.

2. Relatively inexpensive.

3. Applicable to many non-porous materials of wide

range of size and irregular shapes.

4. Flaw orientation will not pose a problem.

5. Large objects can be checked on spot (in situ).

6. It can be designed for high volume production.

Problems with LPT

Only surface anomalies which can absorb the liquid

can be detected.

Rough and porous (low density powder metallurgical

products) surfaces can’t be used.

Significant surface preparation is required for good

results.

Chemically compatible component and penetrant must

be considered.

Sharp corners and complex edges give false indications.


29


MIT,Manipal

9. Conclusion

Of the thousands of parts made for the space shuttle, hundreds are

inspected by penetrant testing. Items such as valves, pipelines, tanks and

structural membranes used in space vehicles’ liquid oxygen systems are

penetrant inspected for flaws. Turbine engine blades are usually inspected

using radiography, eddy current and penetrant testing to ensure there is no

premature failure of the blades. (3)

During maintenance of honeycomb and adhesively bonded aircraft

structures, radiography can detect the presence of corrosion or water, but

it cannot detect the origin of the water. Penetrant testing can find these

areas before water and corrosion happen.

The automotive industry uses penetrant testing to ensure flaw-free parts

such as steering spindles, aluminum suspension parts and aluminum rims.

Fluorescent dye is used in locating leaks in air conditioning systems in

automobiles. The fluorescent dye is added to the system and run for

several minutes; a high-intensity black light locates leaks in the system.

Back in the 1960s it was thought by nondestructive testing engineers that

penetrant testing days were numbered and new technology would replace

it.

Penetrant testing may not be used as much as it was in the past, but it still

has a place in a wide range of industries and applications. Penetrant

testing is going to be around for many years to come.

Documents

LPT