UT Testing-Study Notes

Preparatory Notes for ASNT NDT Level III Examination- Ultrasonic Testing, UT

2014-JuneFacilitators: Fion Zhang/ Charliechong

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

http://www.smt.sandvik.com/en/search/?q=stress+corrosion+cracking

Speaker: Fion Zhang2014/6/19

Contents:

1. ASNT Level III Exam Topical Outline2. AE Codes and Standards

ASTM ASME V

3. Reading 01 Introduction to UT by ndt-ed.org

4. Others reading.

ASNT UT Level III Examination Topical Outline

This examination is 4 hours in length, having 135 questions of equal value.

1. Principles/Theory2. Equipment/Materials3. Techniques/Calibrations

Contact Immersion Comparison of contact and immersion methods Remote monitoring Calibration (electronic and functional)

4. Interpretation/Evaluations Evaluation of base metal product forms Evaluation of weldments Evaluation of bonded structures Variables affecting test results Evaluation (general)

5. Procedures Specific applications Codes/Standards/Specifications

6. Safety and Health

References

1. Level III Study Guide: Ultrasonic Testing (2261)2. NDT Handbook: Volume 7, Ultrasonic Testing (147)3. Supplement to Recommended Practice No. SNT-TC-1A (Q&A Book) -

Ultrasonic Testing Method (2028)4. Ultrasonics: Fundamentals, Technology, Applications (341)5. Refresher Course: ASNT offers a UT Refresher Course based on the Body

of Knowledge outlined above.

The number in parentheses following each reference is the ASNT catalog number.

UT - Ultrasonic TestingLength: 4 hours Questions: 135

1. Principles/Theory

Nature of sound waves Modes of sound wave generation Velocity, frequency, and wavelength of sound waves Attenuation of sound waves Acoustic impedance Reflection Refraction and mode conversion Snells law and critical angles Fresnel and Fraunhofer effects

2. Equipment/Materials

Pulse/echo instrumentation Digital thickness instrumentation Transducer operation and theory Transducer operation/manipulations Resonance testing equipment Couplants Calibration blocks Cables/connectors Test specimen Miscellaneous materials

3. Techniques/Calibrations

Contact Immersion Comparison of contact and immersion methods Remote monitoring Calibration (electronic and functional)

4. Interpretation/Evaluations

Evaluation of base metal product forms Evaluation of weldments Evaluation of bonded structures Variables affecting test results Evaluation (general)

5. Procedures

Specific applications Codes/Standards/Specifications

Reference Catalog NumberNDT Handbook, Second Edition: Volume 7,Ultrasonic Testing 132ASNT Level III Study Guide: Ultrasonic Testing 2261AUltrasonics: Fundamentals, Technology,Applications 341

ASME V Article Numbers:Gen Article 1RT Article 2Nil Article 3 UT Article 4 for weldsUT Article 5 for materialsPT Article 6MT Article 7ET Article 8Visual Article 9LT Article 10AE Article 11 (FRP) /Article 12 (Metallic) / Article 13 (Continuous)Qualif. Article 14ACFM Article 15

ASTM/ AWS Standards ASTM E494 10: Practice for Measuring Ultrasonic Velocity in Materials. ASTM standard E-164, "Standard Practice for Contact Examination of

Weldments. AWS Structural Welding Code, section 6. Annual Book of the American Society of Testing and Materials,

ASTM. Volume 03.03, Nondestructive Testing

Other Reading http://techcorr.com/services/Inspection-and-Testing/Ultrasonic-Shear-Wave.cfm http://www.cnde.iastate.edu/faa-

casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Ultrasonic%20Pages/Ultra-principles.html

http://www.ndt.net/article/v05n09/berke/berke1.htm#0 http://www.mie.utoronto.ca/labs/undel/index.php?menu_path=menu_pages/projects_menu.htm

l&content_path=content_pages/fac2_2.html&main_menu=projects&side_menu=page1&sub_side_menu=s2

http://www.olympus-ims.com/en/ndt-tutorials/flaw-detection/ https://www.nde-ed.org/GeneralResources/Glossary/letter/d.htm http://www.olympus-ims.com/en/knowledge/ http://wenku.baidu.com/view/3cf257781711cc7931b716e0.html

Study Note 1:Ultrasonic Testing

Source: http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/cc_ut_index.htm

Content:

Section 1: Introduction

1.1: Basic Principles of Ultrasonic Testing1.2: History of Ultrasonics1.3: Present State of Ultrasonics1.4: Future Direction of Ultrasonic Inspection

Content: Section 2: Physics of Ultrasound

2.1: Wave Propagation2.2: Modes of Sound Wave Propagation2.3: Properties of Acoustic Plane Wave2.4: Wavelength and Defect Detection2.5: Sound Propagation in Elastic Materials2.6: Attenuation of Sound Waves2.7: Acoustic Impedance2.8: Reflection and Transmission Coefficients (Pressure)2.9: Refraction and Snell's Law2.10: Mode Conversion2.11: Signal-to-Noise Ratio2.12: Wave Interaction or Interference2.13: Inverse Square Rule/ Inverse Rule2.14: Resonance2.15 Measurement of Sound2.16 Practice Makes Perfect

Content: Section 3: Equipment & Transducers

3.1: Piezoelectric Transducers3.2: Characteristics of Piezoelectric Transducers3.3: Radiated Fields of Ultrasonic Transducers3.4: Transducer Beam Spread3.5: Transducer Types3.6: Transducer Testing I3.7: Transducer Testing II3.8: Transducer Modeling3.9: Couplants3.10: Electromagnetic Acoustic Transducers (EMATs)

Continues Next Page

3.11: Pulser-Receivers3.12: Tone Burst Generators In Research3.13: Arbitrary Function Generators3.14: Electrical Impedance Matching and Termination3.15: Data Presentation3.16 Error Analysis3.17 Transducer Quality Factor Q3.18 Testing Techniques3.19 Further Reading on Sub-Section 3

Content: Section 4: Measurement Techniques

4.1: Normal Beam Inspection4.2: Angle Beams 4.3: Reflector Sizing4.4: Automated Scanning4.5: Precision Velocity Measurements4.6: Attenuation Measurements4.7: Spread Spectrum Ultrasonics4.8: Signal Processing Techniques4.9: Flaw Reconstruction Techniques4.10: Scanning Methods4.11: Scanning Patterns4.12: Pulse Repetition Rate and Penetration4.13: Interferences & Non Relevant Indications4.14: Exercises

Content: Section 5: Calibration Methods

5.1: Calibration Methods5.2: The Calibrations5.3: Curvature Correction 5.4: Calibration References & Standards5.5: Exercises5.6: Video Time

Content: Section 6: Selected Applications & Techniques

6.1: Defects & Discontinuities6.2: Rail Inspection 6.3: Weldments (Welded Joints)6.4: Pipe & Tube6.5: Echo Dynamic6.6: Technique Sheets6.7: Material Properties-Elastic Modulus Measurements6.8: High Temperature Ultrasonic Testing6.9: TOFD Introduction

Content: Section 7: Reference Material

7.1: UT Material Properties7.2: General References & Resources7.3: Video Time

Content: Section 8: Ultrasonic Inspection Quizzes

8.1: Ultrasonic Inspection Quizzes8.2: Online UT Quizzes

Content:


1.1: Basic Principles of Ultrasonic Testing1.2: History of Ultrasonics1.3: Present State of Ultrasonics1.4: Future Direction of Ultrasonic Inspection

1.1: Basic Principles of Ultrasonic TestingUltrasonic Testing (UT) uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for (1) flaw detection/evaluation, (2) dimensional measurements, (3) material characterization, and (4) more. To illustrate the general inspection principle, a typical pulse/echo inspection configuration as illustrated below will be used.A typical UT inspection system consists of several functional units, such as the pulser/receiver, transducer, and display devices. A pulser/receiver is an electronic device that can produce high voltage electrical pulses. Driven by the pulser, the transducer generates high frequency ultrasonic energy. The sound energy is introduced and propagates through the materials in the form of waves. When there is a discontinuity (such as a crack) in the wave path, part of the energy will be reflected back from the flaw surface.

The reflected wave signal is transformed into an electrical signal by the transducer and is displayed on a screen. In the applet below, the reflected signal strength is displayed versus the time from signal generation to when a echo was received. Signal travel time can be directly related to the distance that the signal traveled. From the signal, information about the reflector location, size, orientation and other features can sometimes be gained.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/ultrasoundInspection.swf

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/ultrasoundInspection.swf

http://www.cnde.iastate.edu/faa-casr/engineers/Supporting%20Info/Supporting%20Info%20Pages/Ultrasonic%20Pages/Ultra-principles.html

Figure below: Immersion UT setup with CRT or computer screen display. IP indicates the initial pulse while FW and BW indicate the front and back wall of the specimen, respectively.

Basics of Ultrasonic Test

Ultrasonic Inspection is a very useful and versatile NDT method. Some of the advantages of ultrasonic inspection that are often cited include:

It is sensitive to both surface and subsurface discontinuities. The depth of penetration for flaw detection or measurement is superior to

other NDT methods. Only single-sided access is needed when the pulse-echo technique is

used. It is highly accurate in determining reflector position and estimating size

and shape. Minimal part preparation is required. Electronic equipment provides instantaneous results. Detailed images can be produced with automated systems. It has other uses, such as thickness measurement, in addition to flaw

detection.

As with all NDT methods, ultrasonic inspection also has its limitations, which include:

Surface must be accessible to transmit ultrasound. Skill and training is more extensive than with some other methods. It normally requires a coupling medium to promote the transfer of sound

energy into the test specimen. Materials that are rough, irregular in shape, very small, exceptionally thin

or not homogeneous are difficult to inspect. Cast iron and other coarse grained materials are difficult to inspect due to

low sound transmission and high signal noise. Linear defects oriented parallel to the sound beam may go undetected. Reference standards are required for both equipment calibration and the

characterization of flaws.The above introduction provides a simplified introduction to the NDT method of ultrasonic testing. However, to effectively perform an inspection using ultrasonics, much more about the method needs to be known. The following pages present information on the science involved in ultrasonic inspection, the equipment that is commonly used, some of the measurement techniques used, as well as other information.

1.2: History of UltrasonicsPrior to World War II, sonar, the technique of sending sound waves through water and observing the returning echoes to characterize submerged objects, inspired early ultrasound investigators to explore ways to apply the concept to medical diagnosis. In 1929 and 1935, Sokolov studied the use of ultrasonic waves in detecting metal objects. Mulhauser, in 1931, obtained a patent for using ultrasonic waves, using two transducers to detect flaws in solids. Firestone (1940) and Simons (1945) developed pulsed ultrasonic testing using a pulse-echo technique.

Shortly after the close of World War II, researchers in Japan began to explore the medical diagnostic capabilities of ultrasound. The first ultrasonic instruments used an A-mode presentation with blips on an oscilloscope screen. That was followed by a B-mode presentation with a two dimensional, gray scale image.

Japan's work in ultrasound was relatively unknown in the United States and Europe until the 1950s. Researchers then presented their findings on the use of ultrasound to detect gallstones, breast masses, and tumors to the international medical community. Japan was also the first country to apply Doppler ultrasound, an application of ultrasound that detects internal moving objects such as blood coursing through the heart for cardiovascular investigation.

Ultrasound pioneers working in the United States contributed many innovations and important discoveries to the field during the following decades. Researchers learned to use ultrasound to detect potential cancer and to visualize tumors in living subjects and in excised tissue. Real-time imaging, another significant diagnostic tool for physicians, presented ultrasound images directly on the system's CRT screen at the time of scanning.

The introduction of spectral Doppler and later color Doppler depicted blood flow in various colors to indicate the speed and direction of the flow..The United States also produced the earliest hand held "contact" scanner for clinical use, the second generation of B-mode equipment, and the prototype for the first articulated-arm hand held scanner, with 2-D images.

Beginnings of Nondestructive Evaluation (NDE)

Nondestructive testing has been practiced for many decades, with initial rapid developments in instrumentation spurred by the technological advances that occurred during World War II and the subsequent defense effort. During the earlier days, the primary purpose was the detection of defects. As a part of "safe life" design, it was intended that a structure should not develop macroscopic defects during its life, with the detection of such defects being a cause for removal of the component from service. In response to this need, increasingly sophisticated techniques using ultrasonics, eddy currents, x-rays, dye penetrants, magnetic particles, and other forms of interrogating energy emerged.

In the early 1970's, two events occurred which caused a major change in the NDT field. First, improvements in the technology led to the ability to detect small flaws, which caused more parts to be rejected even though the probability of component failure had not changed. However, the discipline of fracture mechanics emerged, which enabled one to predict whether a crack of a given size will fail under a particular load when a material's fracture toughness properties are known. Other laws were developed to predict the growth rate of cracks under cyclic loading (fatigue). With the advent of these tools, it became possible to accept structures containing defects if the sizes of those defects were known. This formed the basis for the new philosophy of "damage tolerant" design. Components having known defects could continue in service as long as it could be established that those defects would not grow to a critical, failure producing size.

A new challenge was thus presented to the nondestructive testing community. Detection was not enough. One needed to also obtain quantitative information about flaw size to serve as an input to fracture mechanics based predictions of remaining life. The need for quantitative information was particularly strongly in the defense and nuclear power industries and led to the emergence of quantitative nondestructive evaluation (QNDE) as a new engineering/research discipline. A number of research programs around the world were started, such as the Center for Nondestructive Evaluation at Iowa State University (growing out of a major research effort at the Rockwell International Science Center); the Electric Power Research Institute in Charlotte, North Carolina; the Fraunhofer Institute for Nondestructive Testing in Saarbrucken, Germany; and the Nondestructive Testing Centre in Harwell, England.

1.3: Present State of UltrasonicsUltrasonic testing (UT) has been practiced for many decades. Initial rapid developments in instrumentation spurred by the technological advances from the 1950's continue today. Through the 1980's and continuing through the present, computers have provided technicians with smaller and more rugged instruments with greater capabilities.

Thickness gauging is an example application where instruments have been refined make data collection easier and better. Built-in data logging capabilities allow thousands of measurements to be recorded and eliminate the need for a "scribe." Some instruments have the capability to capture waveforms as well as thickness readings. The waveform option allows an operator to view or review the A-scan signal of thickness measurement long after the completion of an inspection. Also, some instruments are capable of modifying the measurement based on the surface conditions of thematerial. For example, the signal from a pitted or eroded inner surface of a pipe would be treated differently than a smooth surface. This has led to more accurate and repeatable field measurements.

Many ultrasonic flaw detectors have a trigonometric function that allows for fast and accurate location determination of flaws when performing shear wave inspections. Cathode ray tubes, for the most part, have been replaced with LED or LCD screens. These screens, in most cases, are extremely easy to view in a wide range of ambient lighting.

Bright or low light working conditions encountered by technicians have little effect on the technician's ability to view the screen. Screens can be adjusted for brightness, contrast, and on some instruments even the color of the screen and signal can be selected. Transducers can be programmed with predetermined instrument settings. The operator only has to connect the transducer and the instrument will set variables such as frequency and probe drive.

Along with computers, motion control and robotics have contributed to the advancement of ultrasonic inspections. Early on, the advantage of a stationary platform was recognized and used in industry. Computers can be programmed to inspect large, complex shaped components, with one or multiple transducers collecting information. Automated systems typically consisted of an immersion tank, scanning system, and recording system for a printout of the scan. The immersion tank can be replaced with a squirtersystems, which allows the sound to be transmitted through a water column. The resultant C-scan provides a plan or top view of the component. Scanning of components is considerably faster than contact hand scanning, the coupling is much more consistent. The scan information is collected by a computer for evaluation, transmission to a customer, and archiving.

Squirter systems

http://www.ultrasonic-sciences.co.uk/squirter_systems.htm

Today, quantitative theories have been developed to describe the interaction of the interrogating fields with flaws. Models incorporating the results have been integrated with solid model descriptions of real-part geometries to simulate practical inspections. Related tools allow NDE to be considered during the design process on an equal footing with other failure-related engineering disciplines. Quantitative descriptions of NDE performance, such as the probability of detection (POD), have become an integral part of statistical risk assessment. Measurement procedures initially developed for metals have been extended to engineered materials such as composites, where anisotropy and inhomogeneity have become important issues. The rapid advances in digitization and computing capabilities have totally changed the faces of many instruments and the type of algorithms that are used in processing the resulting data. High-resolution imaging systems and multiple measurement modalities for characterizing a flaw have emerged.

Interest is increasing not only in detecting, characterizing, and sizing defects, but also in characterizing the materials. Goals range from the determination of fundamental microstructural characteristics such as grain size, porosity, and texture (preferred grain orientation), to material properties related to such failure mechanisms as fatigue, creep, and fracture toughness. As technology continues to advance, applications of ultrasound also advance. The high-resolution imaging systems in the laboratory today will be tools of the technician tomorrow.

1.4: Future Direction of Ultrasonic InspectionLooking to the future, those in the field of NDE see an exciting new set of opportunities. The defense and nuclear power industries have played a major role in the emergence of NDE. Increasing global competition has led to dramatic changes in product development and business cycles. At the same time, aging infrastructure, from roads to buildings and aircraft, present a new set of measurement and monitoring challenges for engineers as well as technicians.

Among the new applications of NDE spawned by these changes is the increased emphasis on the use of NDE to improve the productivity of manufacturing processes. Quantitative nondestructive evaluation (QNDE) both increases the amount of information about failure modes and the speed with which information can be obtained and facilitates the development of in-line measurements for process control.

The phrase, "you cannot inspect in quality, you must build it in," exemplifies the industry's focus on avoiding the formation of flaws. Nevertheless, manufacturing flaws will never be completely eliminated and material damage will continue to occur in-service so continual development of flaw detection and characterization techniques is necessary.

Advanced simulation tools that are designed for inspectability and their integration into quantitative strategies for life management will contribute to increase the number and types of engineering applications of NDE. With growth in engineering applications for NDE, there will be a need to expand the knowledge base of technicians performing the evaluations. Advanced simulation tools used in the design for inspectability may be used to provide technical students with a greater understanding of sound behavior in materials. UTSIM, developed at Iowa State University, provides a glimpse into what may be used in the technical classroom as an interactive laboratory tool.

As globalization continues, companies will seek to develop, with ever increasing frequency, uniform international practices. In the area of NDE, this trend will drive the emphasis on standards, enhanced educational offerings, and simulations that can be communicated electronically. The coming years will be exciting as NDE will continue to emerge as a full-fledged engineering discipline.

Section 2: Physics of Ultrasound

Content: Section 2: Physics of Ultrasound

2.1: Wave Propagation2.2: Modes of Sound Wave Propagation2.3: Properties of Acoustic Plane Wave2.4: Wavelength and Defect Detection2.5: Sound Propagation in Elastic Materials2.6: Attenuation of Sound Waves2.7: Acoustic Impedance2.8: Reflection and Transmission Coefficients (Pressure)2.9: Refraction and Snell's Law2.10: Mode Conversion2.11: Signal-to-Noise Ratio2.12: Wave Interaction or Interference2.13: Inverse Square Rule/ Inverse Rule2.14: Resonance2.15 Measurement of Sound2.16 Practice Makes Perfect

Ultrasonic Formula

http://www.ndt-ed.org/GeneralResources/Calculator/calculator.htm

Ultrasonic Formula

2.1: Wave PropagationUltrasonic testing is based on time-varying deformations or vibrations in materials, which is generally referred to as acoustics. All material substances are comprised of atoms, which may be forced into vibration motion about their equilibrium positions. Many different patterns of vibration motion exist at the atomic level, however, most are irrelevant to acoustics and ultrasonic testing. Acoustics is focused on particles that contain many atoms that move in unison to produce a mechanical wave. When a material is not stressed in tension or compression beyond its elastic limit, its individual particles perform elastic oscillations. When the particles of a medium are displaced from their equilibrium positions, internal (electrostatic) restoration forces arise. It is these elastic restoring forces between particles, combined with inertia of the particles, that leads to the oscillatory motions of the medium.

Keywords: internal (electrostatic) restoration forces

inertia of the particles

Acoustic Spectrum

Acoustic Wave Node and Anti-Node

http://www.physicsclassroom.com/Class/waves/u10l4c.cfmhttp://www.physicsclassroom.com/Class/waves/h4.gif

http://hyperphysics.phy-astr.gsu.edu/hbase/waves/standw.html

Q151 A point, line or surface of a vibration body marked by absolute or relative freedom from vibratory motion (momentarily?) is referred to as:

a) a nodeb) an antinodec) rarefactiond) compression

In solids, sound waves can propagate in four principle modes that are based on the way the particles oscillate. Sound can propagate as;

longitudinal waves, shear waves, surface waves, and in thin materials as plate waves.

Longitudinal and shear waves are the two modes of propagation most widely used in ultrasonic testing. The particle movement responsible for the propagation of longitudinal and shear waves is illustrated below.

Longitudinal and shear waves

In longitudinal waves, the oscillations occur in the longitudinal direction or the direction of wave propagation. Since compressional and dilational forces are active in these waves, they are also called pressure or compressional waves. They are also sometimes called density waves because their particle density fluctuates as they move. Compression waves can be generated in liquids, as well as solids because the energy travels through the atomic structure by a series of compressions and expansion (rarefaction) movements.

Longitudinal wave: longitudinal and shear. Longitudinal waves (L-Waves) compress and decompress the material in the direction of motion, much like sound waves in air.

Also Knows as:

longitudinal waves, pressure wave compressional waves. density waves

can be generated in (1) liquids, as well as (2) solids

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/longitudinal.swf

Shear waves (S-Waves) vibrate particles at right angles compared to the motion of the ultrasonic wave. The velocity of shear waves through a material is approximately half that of the longitudinal waves. The angle in which the ultrasonic wave enters the material determines whether longitudinal, shear, or both waves are produced.

In the transverse or shear wave, the particles oscillate at a right angle or transverse to the direction of propagation. Shear waves require an acoustically solid material for effective propagation, and therefore, are not effectively propagated in materials such as liquids or gasses. Shear waves are relatively weak when compared to longitudinal waves. In fact, shear waves are usually generated in materials using some of the energy from longitudinal waves.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/transverse.swf

10. For a shear wave travelling from steel to water incident on the boundary at 10 degrees will give a refracted shear wave in water with an angle of:

a) 0 degreesb) 5 degreesc) 20 degreesd) none of the above

2.2: Modes of Sound Wave PropagationIn air, sound travels by the compression and rarefaction of air molecules in the direction of travel. However, in solids, molecules can support vibrations in other directions, hence, a number of different types of sound waves are possible. Waves can be characterized in space by oscillatory patterns that are capable of maintaining their shape and propagating in a stable manner. The propagation of waves is often described in terms of what are called wave modes.

As mentioned previously, longitudinal and transverse (shear) waves are most often used in ultrasonic inspection. However, at surfaces and interfaces, various types of elliptical or complex vibrations of the particles make other waves possible. Some of these wave modes such as (1) Rayleigh and (2) Lamb waves are also useful for ultrasonic inspection.

Keywords:CompressionRarefaction

Rayleigh waves are a type of surface acoustic wave that travel on solids. They can be produced in materials in many ways, such as by a localized impact or by piezo-electric transduction, and are frequently used in non-destructive testing for detecting defects. They are part of the seismic waves that are produced on the Earth by earthquakes. When guided in layers they are referred to as Lamb waves, RayleighLamb waves, or generalized Rayleigh waves.

Rayleigh Characteristics

Rayleigh waves are a type of surface wave that travel near the surface of solids. Rayleigh waves include both longitudinal and transverse motions that decrease exponentially in amplitude as distance from the surface increases. There is a phase difference between these component motions. In isotropic solids these waves cause the surface particles to move in ellipses in planes normal to the surface and parallel to the direction of propagation the major axis of the ellipse is vertical. At the surface and at shallow depths this motion is retrograde , that is the in-plane motion of a particle is counterclockwise when the wave travels from left to right.

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

29. The longitudinal wave incident angle which results in formation of a Rayleigh wave is called:

(a) Normal incidence(b) The first critical angle(c) The second critical angle(d) Any angle above the first critical angle

Lamb Wave:When guided in layers they are referred to as Lamb waves, RayleighLamb waves, or generalized Rayleigh waves.

Lamb waves 2 modes

Lamb waves propagate in solid plates. They are elastic waves whose particle motion lies in the plane that contains the direction of wave propagation and the plate normal (the direction perpendicular to the plate). In 1917, the english mathematician horace lamb published his classic analysis and description of acoustic waves of this type. Their properties turned out to be quite complex. An infinite medium supports just two wave modes traveling at unique velocities; but plates support two infinite sets of lamb wave modes, whose velocities depend on the relationship between wavelength and plate thickness.Since the 1990s, the understanding and utilization of lamb waves has advanced greatly, thanks to the rapid increase in the availability of computing power. Lamb's theoretical formulations have found substantial practical application, especially in the field of nondestructive testing.

The term rayleighlamb waves embraces the rayleigh wave, a type of wave that propagates along a single surface.

Both rayleigh and lamb waves are constrained by the elastic properties of the surface(s) that guide them.

http://en.wikipedia.org/wiki/Lamb_wavehttp://pediaview.com/openpedia/Lamb_waves

Waves

New! Plate wave- Love Stoneley wave Sezawa

Longitudinal and transverse waves were discussed on the previous page, so let's touch on surface and plate waves here.Surface (or Rayleigh) waves travel the surface of a relatively thick solid material penetrating to a depth of one wavelength.

Surface waves combine both (1) a longitudinal and (2) transverse motion to create an elliptic orbit motion as shown in the image and animation below.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/rayleigh.swf

The major axis of the ellipse is perpendicular to the surface of the solid. As the depth of an individual atom from the surface increases the width of its elliptical motion decreases. Surface waves are generated when a longitudinal wave intersects a surface near the second critical angle and they travel at a velocity between .87 and .95 of a shear wave. Rayleigh waves are useful because they are very sensitive to surface defects (and other surface features) and they follow the surface around curves. Because of this, Rayleigh waves can be used to inspect areas that other waves might have difficulty reaching.

Wave velocity:

Longitudinal wave velocity =1v, The velocity of shear waves through a material is approximately half that

of the longitudinal waves, (0.5v) Surface waves are generated when a longitudinal wave intersects a

surface near the second critical angle and they travel at a velocity between .87 and .95 of a shear wave. (0.87~0.95)x0.5v

The major axis of the ellipse is perpendicular to the surface of the solid.

Surface wave

Surface wave or Rayleigh wave are formed when shear waves refract to 90. The whip-like particle vibration of the shear wave is converted into elliptical motion by the particle changing direction at the interface with the surface. The wave are not often used in industrial NDT although they do have some application in aerospace industry. Their mode of propagation is elliptical along the surface of material, penetrating to a depth of one wavelength. They will follow the contour of the surface and they travel at approximately 90% of the velocity of the shear waves.

Depth of penetration of about one wavelength

Direction of wave propagation

Surface wave has the ability to follow surface contour, until it meet a sharpchange i.e. a surface crack/seam/lap. However the surface waves could be easily completely absorbed by excess couplant of simply touching the part ahead of the waves.

TransducerWedge Surface discontinuity

Specimen

Surface wave One wavelength deep

Rayleigh Wave

http://web.ics.purdue.edu/~braile/edumod/waves/Rwave_files/image001.gif

Love Wave

http://web.ics.purdue.edu/~braile/edumod/waves/Lwave_files/image001.gif

Q110: What kind of wave mode travel at a velocity slightly below the shear wave and their modes of propagation are both longitudinal and transverse with respect to the surface?

a) Rayleigh waveb) Transverse wavec) L-waved) Longitudinal wave

Q: Which of the following modes of vibration exhibits the shortest wavelength at a given frequency and in a given material?

A. longitudinal wave B. compression waveC. shear waveD. surface wave

Plate waves

Plate or Lamb waves are the most commonly used plate waves in NDT. Lamb waves are complex vibrational waves that propagate parallel to the test surface throughout the thickness of the material. Propagation of Lamb waves depends on the density and the elastic material properties of a component. They are also influenced a great deal by the test frequency andmaterial thickness. Lamb waves are generated at an incident angle in which the parallel component of the velocity of the wave in the source is equal to the velocity of the wave in the test material. Lamb waves will travel several meters in steel and so are useful to scan plate, wire, and tubes.

Lamb wave influenced by: (Dispersive Wave)

Density Elastic material properties Frequencies Material thickness

Plate or Lamb waves are similar to surface waves except they can only be generated in materials a few wavelengths thick.

http://www.ndt.net/ndtaz/files/lamb_a.gif

Plate wave or Lamb wave are formed by the introduction of surface wave into a thin material. They are a combination of (1) compression and surface or (2) shear and surface waves causing the plate material to flex by totally saturating the material. The two types of plate waves:

Plate or Lamb waves are generated at an incident angle in which the parallel component of the velocity of the wave in the source is equal to the velocity of the wave in the test material.

Q1: The wave mode that has multiple or varying wave velocities is:

A. Longitudinal wavesB. Shear wavesC. Transverse wavesD. Lamb waves

With Lamb waves, a number of modes of particle vibration are possible, but the two most common are symmetrical and asymmetrical. The complex motion of the particles is similar to the elliptical orbits for surface waves. Symmetrical Lamb waves move in a symmetrical fashion about the median plane of the plate. This is sometimes called the extensional mode because the wave is stretching and compressing the plate in the wave motion direction. Wave motion in the symmetrical mode is most efficiently produced when the exciting force is parallel to the plate. The asymmetrical Lamb wave mode is often called the flexural mode because a large portion of the motion moves in a normal direction to the plate, and a little motion occurs in the direction parallel to the plate. In this mode, the body of the plate bends as the two surfaces move in the same direction.The generation of waves using both piezoelectric transducers andelectromagnetic acoustic transducers (EMATs) are discussed in later sections.

Keywords:Symmetrical = extensional modeAsymmetrical = flexural mode

Symmetrical = extensional modeAsymmetrical = flexural mode

Symmetrical = extensional mode

Dispersive Wave:Wave modes such as those found in Lamb wave have a velocity of propagation dependent upon the operating frequency, sample thickness and elastic moduli. They are dispersive (velocity change with frequency) in that pulses transmitted in these mode tend to become stretched or dispersed.

Dispersion refers to the fact that in a real medium such as water, air, or glass, a wave traveling through that medium will have a velocity that depends upon its frequency. Dispersion occurs for any form of wave, acoustic, electromagnetic, electronic, even quantum mechanical. Dispersion is responsible for a prism being able to resolve light into colors and defines the maximum frequency of broadband pulses one can send down an optical fiber or through a copper wire. Dispersion affects wave and swell forecasts at sea and influences the design of sound equipment. Dispersion is a physical property of the medium and can combine with other properties to yield very strange results. For example, in the propagation of light in an optical fiber, the glass introduces dispersion and separates the wavelengths of light according to frequency, however if the light is intense enough, it can interact with the electrons in the material changing its refractive index. The combination of dispersion and index change can cancel each other leading to a wave that can propagate indefinitely maintaining a constant shape. Such a wave has been termed a soliton.http://www.rpi.edu/dept/chem-eng/WWW/faculty/plawsky/Comsol%20Modules/DispersiveWave/DispersiveWave.html

Thickness Limitation:

One can not generate shear / surface (or Lamb?) wave on a plate that is thinner than the wavelength.

2.3: Properties of Acoustic Plane Wave Wavelength, Frequency and Velocity

Among the properties of waves propagating in isotropic solid materials are wavelength, frequency, and velocity. The wavelength is directly proportional to the velocity of the wave and inversely proportional to the frequency of the wave. This relationship is shown by the following equation.

The applet below shows a longitudinal and transverse wave. The direction of wave propagation is from left to right and the movement of the lines indicate the direction of particle oscillation. The equation relating ultrasonic wavelength, frequency, and propagation velocity is included at the bottom of the applet in a reorganized form. The values for the wavelength, frequency, and wave velocity can be adjusted in the dialog boxes to see their effects on the wave. Note that the frequency value must be kept between 0.1 to 1 MHz (one million cycles per second) and the wave velocity must be between 0.1 and 0.7 cm/us.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/applet_2_4/applet_2_4.htm

Java dont work? Uninstalled Reinstalled Then http://jingyan.baidu.com/article/9f63fb91d0eab8c8400f0e08.html

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As can be noted by the equation, a change in frequency will result in a change in wavelength. Change the frequency in the applet and view the resultant wavelength. At a frequency of .2 and a material velocity of 0.585 (longitudinal wave in steel) note the resulting wavelength. Adjust the material velocity to 0.480 (longitudinal wave in cast iron) and note the resulting wavelength. Increase the frequency to 0.8 and note the shortened wavelength in each material.

In ultrasonic testing, the shorter wavelength resulting from an increase in frequency will usually provide for the detection of smaller discontinuities. This will be discussed more in following sections.

Keywords:the shorter wavelength resulting from an increase in frequency will usually provide for the detection of smaller discontinuities

2.4: Wavelength and Defect DetectionIn ultrasonic testing, the inspector must make a decision about the frequency of the transducer that will be used. As we learned on the previous page, changing the frequency when the sound velocity is fixed will result in a change in the wavelength of the sound.

The wavelength of the ultrasound used has a significant effect on the probability of detecting a discontinuity. A general rule of thumb is that a discontinuity must be larger than one-half the wavelength to stand a reasonable chance of being detected.

Sensitivity and resolution are two terms that are often used in ultrasonic inspection to describe a technique's ability to locate flaws. Sensitivity is the ability to locate small discontinuities. Sensitivity generally increases with higher frequency (shorter wavelengths). Resolution is the ability of the system to locate discontinuities that are close together within the material or located near the part surface. Resolution also generally increases as the frequency increases.

The wave frequency can also affect the capability of an inspection in adverse ways. Therefore, selecting the optimal inspection frequency often involves maintaining a balance between the favorable and unfavorable results of the selection. Before selecting an inspection frequency, the material's grain structure and thickness, and the discontinuity's type, size, and probable location should be considered.

As frequency increases, sound tends to scatter from large or course grain structure and from small imperfections within a material. Cast materials often have coarse grains and other sound scatters that require lower frequencies to be used for evaluations of these products.

(1) Wrought and (2) forged products with directional and refined grain structure can usually be inspected with higher frequency transducers.

Keywords:Coarse grains Lower frequency to avoid scattering and noise,Fine grains Higher frequency to increase sensitivity & resolution.

Since more things in a material are likely to scatter a portion of the sound energy at higher frequencies, the penetrating power (or the maximum depth in a material that flaws can be located) is also reduced. Frequency also has an effect on the shape of the ultrasonic beam. Beam spread, or the divergence of the beam from the center axis of the transducer, and how it is affected by frequency will be discussed later.

It should be mentioned, so as not to be misleading, that a number of other variables will also affect the ability of ultrasound to locate defects. These include the pulse length, type and voltage applied to the crystal, properties of the crystal, backing material, transducer diameter, and the receiver circuitry of the instrument. These are discussed in more detail in the material on signal-to-noise ratio.

Since more things in a material are likely to scatter a portion of the sound energy at higher frequencies, the penetrating power (or the maximum depth in a material that flaws can be located) is also reduced.

Coarse grains Lower frequency to avoid scattering and noise,Fine grains Higher frequency to increase sensitivity & resolution.

http://www.cnde.iastate.edu/ultrasonics/grain-noise

Higher the frequency, greater the scattering, thus less penetrating.

Detectability variable:

pulse length, type and voltage applied to the crystal, properties of the crystal, backing material, transducer diameter, and the receiver circuitry of the instrument.

Investigating further on:

Detectability variable:

pulse length, type and voltage applied to the crystal, properties of the crystal, backing material, transducer diameter (focal length Cross sectional area), and the receiver circuitry of the instrument.

Investigating on: Sonic pulse volume pulse length, transducer

Pulse Length:A sound pulse traveling through a metal occupies a physical volume. This volume changes with depth, being smallest in the focal zone. The pulse volume, a product of a pulse length L and a cross-sectional area A, can be fairly easily measured by combining ultrasonic A-scans and C-scans, as will be seen shortly.

For many cases of practical interest, the inspection simulation models predict that S/N (signal to noise ratio) is inversely proportional to the square root of the pulse volume at the depth of the defect. This is known as the pulse volume rule-of-thumb and has become a guiding principle for designing inspections. Generally speaking, it applies when both the grain size and thelateral size of the defect are smaller than the sound pulse diameter.http://www.cnde.iastate.edu/ultrasonics/grain-noise

Determining cross sectional area using reflector- A Scan (6db drop)

Determining cross sectional area using reflector- C Scan

Sonic pulse volume and S/N (defect resolution)

Pulse volume rule-of-thumb:Competing grain noise (pulse volume)

2.5: Sound Propagation in Elastic MaterialsIn the previous pages, it was pointed out that sound waves propagate due to the vibrations or oscillatory motions of particles within a material. An ultrasonic wave may be visualized as an infinite number of oscillating masses or particles connected by means of elastic springs. Each individual particle is influenced by the motion of its nearest neighbor and both (1) inertial and (2) elastic restoring forces act upon each particle.

A mass on a spring has a single resonant frequency determined by its spring constant k and its mass m. The spring constant is the restoring force of a spring per unit of length. Within the elastic limit of any material, there is a linear relationship between the displacement of a particle and the force attempting to restore the particle to its equilibrium position. This linear dependency is described by Hooke's Law.

Spring model- A mass on a spring has a single resonant frequency determined by its spring constant k and its mass m.

In terms of the spring model, Hooke's Law says that the restoring force due to a spring is proportional to the length that the spring is stretched, and acts in the opposite direction. Mathematically, Hooke's Law is written as F =-kx, where F is the force, k is the spring constant, and x is the amount of particle displacement. Hooke's law is represented graphically it the bottom. Please note that the spring is applying a force to the particle that is equal and opposite to the force pulling down on the particle.

Elastic Model / Longitudinal Wave

Shear Wave

The Speed of Sound

Hooke's Law, when used along with Newton's Second Law, can explain a few things about the speed of sound. The speed of sound within a material is a function of the properties of the material and is independent of the amplitude of the sound wave. Newton's Second Law says that the force applied to a particle will be balanced by the particle's mass and the acceleration of the particle. Mathematically, Newton's Second Law is written as F = ma. Hooke's Law then says that this force will be balanced by a force in the opposite direction that is dependent on the amount of displacement and the spring constant (F = -kx). Therefore, since the applied force and the restoring force are equal, ma = -kx can be written. The negative sign indicates that the force is in the opposite direction.

F= ma = -kx

Since the mass m and the spring constant k are constants for any given material, it can be seen that the acceleration a and the displacement x are the only variables. It can also be seen that they are directly proportional. For instance, if the displacement of the particle increases, so does its acceleration. It turns out that the time that it takes a particle to move and return to its equilibrium position is independent of the force applied. So, within a given material, sound always travels at the same speed no matter how much force is applied when other variables, such as temperature, are held constant.

a x

What properties of material affect its speed of sound?

Of course, sound does travel at different speeds in different materials. This is because the (1) mass of the atomic particles and the (2) spring constants are different for different materials. The mass of the particles is related to the density of the material, and the spring constant is related to the elastic constants of a material. The general relationship between the speed of sound in a solid and its density and elastic constants is given by the following equation:

Density mass of the atomic particles

Elastic constant spring constants

Where V is the speed of sound, C is the elastic constant, and p is the material density. This equation may take a number of different forms depending on the type of wave (longitudinal or shear) and which of the elastic constants that are used. The typical elastic constants of a materials include:

Young's Modulus, E: a proportionality constant between uniaxial stress and strain.

Poisson's Ratio, n: the ratio of radial strain to axial strain Bulk modulus, K: a measure of the incompressibility of a body subjected to

hydrostatic pressure. Shear Modulus, G: also called rigidity, a measure of a substance's

resistance to shear. Lame's Constants, l and m: material constants that are derived from

Young's Modulus and Poisson's Ratio.

Q163 Acoustic velocity of materials are primary due to the material's:

a) densityb) elasticityc) both a and bd) acoustic impedance

When calculating the velocity of a longitudinal wave, Young's Modulus and Poisson's Ratio are commonly used.

When calculating the velocity of a shear wave, the shear modulus is used. It is often most convenient to make the calculations using

Lame's Constants, which are derived from Young's Modulus and Poisson's Ratio.

It must also be mentioned that the subscript ij attached to C (Cij) in the above equation is used to indicate the directionality of the elastic constants with respect to the wave type and direction of wave travel. In isotropic materials, the elastic constants are the same for all directions within the material. However, most materials are anisotropic and the elastic constants differ with each direction. For example, in a piece of rolled aluminum plate, the grains are elongated in one direction and compressed in the others and the elastic constants for the longitudinal direction are different than those for the transverse or short transverse directions.

V longitudinal

V transverse

Examples of approximate compressional sound velocities in materials are:

Aluminum - 0.632 cm/microsecond1020 steel - 0.589 cm/microsecondCast iron - 0.480 cm/microsecond.

Examples of approximate shear sound velocities in materials are:

Aluminum - 0.313 cm/microsecond1020 steel - 0.324 cm/microsecondCast iron - 0.240 cm/microsecond.

When comparing compressional and shear velocities, it can be noted that shear velocity is approximately one half that of compressional velocity. The sound velocities for a variety of materials can be found in the ultrasonic properties tables in the general resources section of this site.

Longitudinal Wave Velocity: VLThe velocity of a longitudinal wave is described by the following equation:

VL = Longitudinal bulk wave velocityE = Youngs modulus of elasticity = Poisson ratioP = Material density

Shear Wave Velocity: VSThe velocity of a shear wave is described by the following equation:

Vs = Shear wave velocityE = Youngs modulus of elasticity = Poisson ratioP = Material densityG = Shear modulus

2.6: Attenuation of Sound WavesWhen sound travels through a medium, its intensity diminishes with distance. In idealized materials, sound pressure (signal amplitude) is only reduced by the (1) spreading of the wave. Natural materials, however, all produce an effect which further weakens the sound. This further weakening results from (2) scattering and (3) absorption. Scattering is the reflection of the sound in directions other than its original direction of propagation. Absorption is the conversion of the sound energy to other forms of energy. The combined effect of scattering and absorption (spreading?) is called attenuation.Ultrasonic attenuation is the decay rate of the wave as it propagates through material.

Attenuation of sound within a material itself is often not of intrinsic interest. However, natural properties and loading conditions can be related to attenuation. Attenuation often serves as a measurement tool that leads to the formation of theories to explain physical or chemical phenomenon that decreases the ultrasonic intensity.

Absorption:Sound attenuations are affected by; (1) Geometric beam spread, (2) Absorption,(3) Scattering.

Absorption processes1. Mechanical hysteresis2. Internal friction3. Others (?)

For relatively non-elastic material, these soft and pliable material include lead, plastid, rubbers and non-rigid coupling materials; much of the energy is loss as heat during sound propagation and absorption is the main reason that the testing of these material are limit to relatively thin section/

Scattering: Grain Size and Wave Frequency

Sound attenuations are affected by; (1) Geometric beam spread, (2) Absorption, (3) Scattering.

The relative impact of scattering source of a material depends upon their grain sizes in comparison with the Ultrasonic sound wave length. As the scattering size approaches that of a wavelength, scattering by the grain is a concern. The effects from such scattering could be compensated with the use of increasing wavelength ultrasound at the cost of decreasing sensitivity and resolution to detection of discontinuities.

Other effect are anisotropic columnar grain with different elastic behavior at different grain direction. In this case the internal incident wave front becomes distorted and often appear to change direction (propagate better in certain preferred direction) in respond to material anisotropy.

Anisotropic Columnar Grainswith different elastic behavior at different grain direction.

Spreading/ Scattering / adsorption (reflection is a form of scattering)

Scattering

Scatterbrain

Adsorption

Spreading

The amplitude change of a decaying plane wave can be expressed as:

In this expression Ao is the unattenuated amplitude of the propagating wave at some location. The amplitude A is the reduced amplitude after the wave has traveled a distance z from that initial location. The quantity is the attenuation coefficient of the wave traveling in the z-direction. The dimensions of are nepers/length, where a neper is a dimensionless quantity. The term e is the exponential (or Napier's constant) which is equal to approximately 2.71828.

The units of the attenuation value in Nepers per meter (Np/m) can be converted to decibels/length by dividing by 0.1151. Decibels is a more common unit when relating the amplitudes of two signals.

Attenuation is generally proportional to the square of sound frequency.Quoted values of attenuation are often given for a single frequency, or an attenuation value averaged over many frequencies may be given. Also, the actual value of the attenuation coefficient for a given material is highly dependent on the way in which the material was manufactured. Thus, quoted values of attenuation only give a rough indication of the attenuation and should not be automatically trusted. Generally, a reliable value of attenuation can only be obtained by determining the attenuation experimentally for the particular material being used.

Attenuation Frequency (f )2

Attenuation can be determined by evaluating the multiple back wall reflections seen in a typical A-scan display like the one shown in the image at the bottom. The number of decibels between two adjacent signals is measured and this value is divided by the time interval between them. This calculation produces a attenuation coefficient in decibels per unit time Ut. This value can be converted to nepers/length by the following equation.

Where v is the velocity of sound in meters per second and Ut is in decibels per second.

Amplitude at distance Z

where:

Where v is the velocity of sound in meters per second and Ut is in decibels per second.

Ut

Ao

A

Factors Affecting Attenuation:

1. Testing Factors

Testing frequency Boundary conditions Wave form geometry

2. Base Material Factors

Material type Chemistry Integral constituents (fiber, voids, water content, inclusion, anisotropy) Forms (casting, wrought) Heat treatment history Mechanical processes(Hot or cold working; forging, rolling, extruding,

TMCP, directional working)

Frequency selection

There is no ideal frequency; therefore, frequency selection must be made with consideration of several factors. Frequency determines the wavelength of the sound energy traveling through the material. Low frequency has longer wavelengths and will penetrate deeper than higher frequencies. To penetrate a thick piece, low frequencies should be used. Another factor is the size of the grain structure in the material. High frequencies with shorter wavelengths tend to reflect off grain boundaries and become lost or result in ultrasonic noise that can mask flaw signals. Low frequencies must be used with coarse grain structures. However, test resolution decreases when frequency is decreased. Small defects detectable at high frequencies may be missed at lower frequencies. In addition, variations in instrument characteristics and settings as well as material properties and coupling conditions play a major role in system performance. It is critical that approved testing procedures be followed.

Q94: In general, which of the following mode of vibration would have the greatest penetrating power in a coarse grain material if the frequency of the wave are the same?

a) Longitudinal waveb) Shear wavec) Transverse waved) All the above modes would have the same penetrating power

Q: The random distribution of crystallographic direction in alloys with large crystalline structures is a factor in determining:

A. Acoustic noise levelsB. Selection of test frequencyC. Scattering of soundD. All of the above

2.7: Acoustic ImpedanceAcoustic impedance is a measured of resistance of sound propagation through a part.

From the table air has lower acoustic impedance than steel and for a given energy Aluminum would travel a longer distance than steel before the same amount of energy is attenuated.

Transmission & Reflection Animation:http://upload.wikimedia.org/wikipedia/commons/3/30/Partial_transmittance.gif

Sound travels through materials under the influence of sound pressure. Because molecules or atoms of a solid are bound elastically to one another, the excess pressure results in a wave propagating through the solid. The acoustic impedance (Z) of a material is defined as the product of its density (p) and acoustic velocity (V).

Z = pV

Acoustic impedance is important in:

1. the determination of acoustic transmission and reflection at the boundary of two materials having different acoustic impedances.

2. the design of ultrasonic transducers.3. assessing absorption of sound in a medium.

The following applet can be used to calculate the acoustic impedance for any material, so long as its density (p) and acoustic velocity (V) are known. The applet also shows how a change in the impedance affects the amount of acoustic energy that is reflected and transmitted. The values of the reflected and transmitted energy are the fractional amounts of the total energy incident on the interface. Note that the fractional amount of transmitted sound energy plus the fractional amount of reflected sound energy equals one. The calculation used to arrive at these values will be discussed on the next page.


Reflection/Transmission Energy as a function of Z

2.8: Reflection and Transmission Coefficients (Pressure)Ultrasonic waves are reflected at boundaries where there is a difference in acoustic impedances (Z) of the materials on each side of the boundary. (See preceding page for more information on acoustic impedance.) This difference in Z is commonly referred to as the impedance mismatch. The greater the impedance mismatch, the greater the percentage of energy that will be reflected at the interface or boundary between one medium and another. The fraction of the incident wave intensity that is reflected can be derived because particle velocity and local particle pressures must be continuous across the boundary.

When the acoustic impedances of the materials on both sides of the boundary are known, the fraction of the incident wave intensity that is reflected can be calculated with the equation below. The value produced is known as the reflection coefficient. Multiplying the reflection coefficient by 100 yields the amount of energy reflected as a percentage of the original energy.

Since the amount of reflected energy plus the transmitted energy must equal the total amount of incident energy, the transmission coefficient is calculated by simply subtracting the reflection coefficient from one.

Formulations for acoustic reflection and transmission coefficients (pressure) are shown in the interactive applet below. Different materials may be selected or the material velocity and density may be altered to change the acoustic impedance of one or both materials. The red arrow represents reflected sound and the blue arrow represents transmitted sound.


Reflection Coefficient:

Note that the reflection and transmission coefficients are often expressed in decibels (dB) to allow for large changes in signal strength to be more easily compared. To convert the intensity or power of the wave to dB units, take the log of the reflection or transmission coefficient and multiply this value times 10. However, 20 is the multiplier used in the applet since the power of sound is not measured directly in ultrasonic testing. The transducers produce a voltage that is approximately proportionally to the sound pressure. The power carried by a traveling wave is proportional to the square of the pressure amplitude. Therefore, to estimate the signal amplitude change, the log of the reflection or transmission coefficient is multiplied by 20.

Using the above applet, note that the energy reflected at a water-stainless steel interface is 0.88 or 88%. The amount of energy transmitted into the second material is 0.12 or 12%. The amount of reflection and transmission energy in dB terms are -1.1 dB and -18.2 dB respectively. The negative sign indicates that individually, the amount of reflected and transmitted energy is smaller than the incident energy.

If reflection and transmission at interfaces is followed through the component, only a small percentage of the original energy makes it back to the transducer, even when loss by attenuation is ignored. For example, consider an immersion inspection of a steel block. The sound energy leaves the transducer, travels through the water, encounters the front surface of the steel, encounters the back surface of the steel and reflects back through the front surface on its way back to the transducer. At the water steel interface (front surface), 12% of the energy is transmitted. At the back surface, 88% of the 12% that made it through the front surface is reflected. This is 10.6% of the intensity of the initial incident wave. As the wave exits the part back through the front surface, only 12% of 10.6 or 1.3% of the original energy is transmitted back to the transducer.

Incident Wave other than Normal?

Sample Question:

The figure above shown the partition of incident and reflected wave at water-Aluminum interface at an incident angle of 20, the reflected and transmitted wave are:

A. 60% and 40%B. 40% and 60%C. 1/3 and 2/3D. 80% and 20%

Note: if normal incident the reflected 70% Transmitted 30%

Other Reading (Olympus Technical Note)The boundary between two materials of different acoustic impedances iscalled an acoustic interface. When sound strikes an acoustic interface atnormal incidence, some amount of sound energy is reflected and someamount is transmitted across the boundary. The dB loss of energy ontransmitting a signal from medium 1 into medium 2 is given by:

dB loss of transmission = 10 log10 [ 4Z1Z2 / (Z1+Z2)2]

The dB loss of energy of the echo signal in medium 1 reflecting from aninterface boundary with medium 2 is given by:

dB loss of Reflection = 10 log10 [ (Z1-Z2)2 / (Z1+Z2)2]

For example: The dB loss on transmitting from water (Z = 1.48) into 1020steel (Z = 45.41) is -9.13 dB; this also is the loss transmitting from 1020 steelinto water. The dB loss of the backwall echo in 1020 steel in water is -0.57dB; this also is the dB loss of the echo off 1020 steel in water. The waveformof the echo is inverted when Z2

6. For an ultrasonic beam with normal incidence the transmission coefficient is given by:

http://webpages.ursinus.edu/lriley/courses/p212/lectures/node19.html#eq:acousticRhttp://sepwww.stanford.edu/sep/prof/waves/fgdp8/paper_html/node2.html

2.9: Refraction and Snell's Law

Refraction and Snell's Law

When an ultrasonic wave passes through an interface between two materials at an oblique angle, and the materials have different indices of refraction, both reflected and refracted waves are produced. This also occurs with light, which is why objects seen across an interface appear to be shifted relative to where they really are. For example, if you look straight down at an object at the bottom of a glass of water, it looks closer than it really is. A good way to visualize how light and sound refract is to shine a flashlight into a bowl of slightly cloudy water noting the refraction angle with respect to the incident angle.

Vs1 Only If this medium support shear wave i.e. Solid

VL1VL1

VL2VS2

Refraction takes place at an interface due to the different velocities of the acoustic waves within the two materials. The velocity of sound in each material is determined by the material properties (elastic modulus and density) for that material. In the animation below, a series of plane waves are shown traveling in one material and entering a second material that has a higher acoustic velocity. Therefore, when the wave encounters the interface between these two materials, the portion of the wave in the second material is moving faster than the portion of the wave in the first material. It can be seen that this causes the wave to bend.

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/waveRefraction.swf

http://www.ni.com/white-paper/3368/en/

Snell's Law describes the relationship between the angles and the velocities of the waves. Snell's law equates the ratio of material velocities V1 and V2 to the ratio of the sine's of incident (1) and refracted (2) angles, as shown in the following equation.

Where:VL1 is the longitudinal wave velocity in material 1.VL2 is the longitudinal wave velocity in material 2.

Note that in the diagram, there is a reflected longitudinal wave (VL1' ) shown. This wave is reflected at the same angle as the incident wave because the two waves are traveling in the same material, and hence have the same velocities. This reflected wave is unimportant in our explanation of Snell's Law, but it should be remembered that some of the wave energy is reflected at the interface. In the applet below, only the incident and refracted longitudinal waves are shown. The angle of either wave can be adjusted by clicking and dragging the mouse in the region of the arrows. Values for the angles or acoustic velocities can also be entered in the dialog boxes so the that applet can be used as a Snell's Law calculator.

Snell Law


When a longitudinal wave moves from a slower to a faster material, there is an incident angle that makes the angle of refraction for the wave 90o. This is know as the first critical angle. The first critical angle can be found from Snell's law by putting in an angle of 90 for the angle of the refracted ray. At the critical angle of incidence, much of the acoustic energy is in the form of an inhomogeneous compression wave, which travels along the interface and decays exponentially with depth from the interface. This wave is sometimes referred to as a "creep wave." Because of their inhomogeneous nature and the fact that they decay rapidly, creep waves are not used as extensively as Rayleigh surface waves in NDT. However, creep waves are sometimes more useful than Rayleigh waves because they suffer less from surfaceirregularities and coarse material microstructure due to their longer wavelengths.

Snell Law

Refraction and mode conversion occur because of the change in L-wave velocity as it passes the boundary from one medium to another. The higher the difference in the velocity of sound between two materials, the larger the resulting angle of refraction. L-waves and S-waves have different angles of refraction because they have dissimilar velocities within the same material.s the angle of the ultrasonic transducer continues to increase, L-waves move closer to the surface of the UUT.

The angle at which the L-wave is parallel with the surface of the UUT is referred to as the first critical angle. This angle is useful for two reasons. Only one wave mode is echoed back to the transducer, making it easy to interpret the data. Also, this angle gives the test system the ability to look at surfaces that are not parallel to the front surface, such as welds.

Example: Snells Law L-wave and S-wave refraction angles are calculated using Snells law. You also can use this law to determine the first critical angle for any combination of materials.

Where:2 = angle of the refracted beam in the UUT1 = incident angle from normal of beam in the wedge or liquidV1 = velocity of incident beam in the liquid or wedgeV2 = velocity of refracted beam in the UUT

For example, calculate the first critical angle for a transducer on a plastic wedge that is examining aluminum.

V1 = 0.267 cm/s (for L-waves in plastic)V2 = 0.625 cm/s (for L-waves in aluminum)2 = 90 degree (angle of L-wave for first critical angle)1 = unknown

The plastic wedge must have a minimum angle of 25.29 to transmit only S-waves into the UUT. When the S-wave angle of refraction is greater than 90, all ultrasonic energy is reflected by the UUT.

Snell Law: First critical angle

Snell Law: 1st / 2nd Critical Angles

Q155 Which of the following can occur when an ultrasound beam reaches the interface of 2 dissimilar materials?

a) Reflection b) refraction c) mode conversion d) all of the above

Q. Both longitudinal and shear waves may be simultaneously generated in a second medium when the angle of incidence is:

a) between the normal and the 1st critical angleb) between the 1st and 2nd critical anglec) past the second critical angled) only at the second critical angle

Q: When angle beam contact testing a test piece, increasing the incident angle until the second critical angle is reached results in:

A. Total reflection of a surface waveB. 45 degree refraction of the shear waveC. Production of a surface waveD. None of the above

Typical angle beam assemblies make use of mode conversion and Snell's Law to generate a shear wave at a selected angle (most commonly 30, 45, 60, or 70) in the test piece. As the angle of an incident longitudinal wave with respect to a surface increases, an increasing portion of the sound energy is converted to a shear wave in the second material, and if the angle is high enough, all of the energy in the second material will be in the form of shear waves. There are two advantages to designing common angle beams to take advantage of this mode conversion phenomenon.

First, energy transfer is more efficient at the incident angles that generate shear waves in steel and similar materials.

Second, minimum flaw size resolution is improved through the use of shear waves, since at a given frequency, the wavelength of a shear wave is approximately 60% the wavelength of a comparable longitudinal wave.

Snell Law:

http://techcorr.com/services/Inspection-and-Testing/Ultrasonic-Shear-Wave.cfm

Depth & Skip

More on Snell Law

Like light, when an incident ultrasonic wave encounters an interface to an adjacent material of a different velocity, at an angle other than normal to the surface, then both reflected and refracted waves are produced.

Understanding refraction and how ultrasonic energy is refracted is especially important when using angle probes or the immersion technique. It is also the foundation formula behind the calculations used to determine a materials first and second critical angles.

First Critical Angle

Before the angle of incidence reaches the first critical angle, both longitudinal and shear waves exist in the part being inspected. The first critical angle is said to have been reached when the longitudinal wave no longer exists within the part, that is, when the longitudinal wave is refracted to greater or equal than 90, leaving only a shear wave remaining in the part.

Second Critical Angle

The second critical angle occurs when the angle of incidence is at such an angle that the remaining shear wave within the part is refracted out of the part. At this angle, when the refracted shear wave is at 90 a surface wave is created on the part surface

Beam angles should always be plotted using the appropriate industry standard, however, knowing the effect of velocity and angle on refraction will always benefit an NDT technician when working with angle inspection or the immersion technique.

The above calculator uses the following equation:ultrasonic snells law formula Where:A1 = The angle of incidence.V1 = The incident material velocityA2 = The angle of refractionV2 = The refracted material velocity

http://www.ndtcalc.com/calculators.html

2.10: Mode ConversionWhen sound travels in a solid material, one form of wave energy can be transformed into another form. For example, when a longitudinal waves hits an interface at an angle, some of the energy can cause particle movement in the transverse direction to start a shear (transverse) wave. Mode conversion occurs when a wave encounters an interface between materials of different acoustic impedances and the incident angle is not normal to the interface. From the ray tracing movie below, it can be seen that since mode conversion occurs every time a wave encounters an interface at an angle, ultrasonic signals can become confusing at times.

Mode Conversion

http://www.ndt-ed.org/EducationResources/CommunityCollege/Ultrasonics/Graphics/Flash/ModeConversion/ModeConv.swf

In the previous section, it was pointed out that when sound waves pass through an interface between materials having different acoustic velocities, refraction takes place at the interface. The larger the difference in acoustic velocities between the two materials, the more the sound is refracted. Notice that the shear wave is not refracted as much as the longitudinal wave. This occurs because shear waves travel slower than longitudinal waves. Therefore, the velocity difference between the incident longitudinal wave and the shear wave is not as great as it is between the incident and refracted longitudinal waves. Also note that when a longitudinal wave is reflected inside the material, the reflected shear wave is reflected at a smaller angle than the reflected longitudinal wave. This is also due to the fact that the shear velocity is less than the longitudinal velocity within a given material.

Snell's Law holds true for shear waves as well as longitudinal waves and can be written as follows

=

Where:

VL1 is the longitudinal wave velocity in material 1.VL2 is the longitudinal wave velocity in material 2.VS1 is the shear wave velocity in material 1.VS2 is the shear wave velocity in material 2.

Snell's Law

In the applet below, the shear (transverse) wave ray path has been added. The ray paths of the waves can be adjusted by clicking and dragging in the vicinity of the arrows. Values for the angles or the wave velocities can also be entered into the dialog boxes. It can be seen from the applet that when a wave moves from a slower to a faster material, there is an incident angle which makes the angle of refraction for the longitudinal wave 90 degrees. As mentioned on the previous page, this is known as the first critical angle and all of the energy from the refracted longitudinal wave is now converted to a surface following longitudinal wave. This surface following wave is sometime referred to as a creep wave and it is not very useful in NDT because it dampens out very rapidly.

Reflections

Creep wave

VS1

VS2

Beyond the first critical angle, only the shear wave propagates into the material. For this reason, most angle beam transducers use a shear wave so that the signal is not complicated by having two waves present. In many cases there is also an incident angle that makes the angle of refraction for the shear wave 90 degrees. This is known as the second critical angle and at this point, all of the wave energy is reflected or refracted into a surface following shear wave or shear creep wave. Slightly beyond the second critical angle, surface waves will be generated.

Keywords:

Longitudinal creep wave Shear creep wave

Snell Law- 1st & 2nd Critical Angles

Note that the applet defaults to compressional velocity in the second material. The refracted compressional wave angle will be generated for given materials and angles. To find the angle of incidence required to generate a shear wave at a given angle complete the following:

1. Set V1 to the longitudinal wave velocity of material 1. This material could be the transducer wedge or the immersion liquid.

2. Set V2 to the shear wave velocity (approximately one-half its compressional velocity) of the material to be inspected.

3. Set Q2 to the desired shear wave angle.4. Read Q1, the correct angle of incidence.

Transverse wave can be introduced into the test material by various methods:

1. Inclining the incident L-wave at an angle beyond the first critical angle, yet short of second critical angle using a wedge.

2. In immersion method, changing the angle of the normal search unit manipulator,

3. Off-setting the normal transducer from the center-line for round bar or pipe.

for 45 refracted transverse wave, the rule of thumb is the offset d= 1/6 of rod diameter

Offset of Normal probe above circular object

1

21

R

Calculate the offset for following conditions:Aluminum rod being examined is 6" diameter, what is the off set needed for (a) 45 refracted shear wave (b) Logitudinal wave to be generated?(L-wave velocity for AL=6.3x105cm/s, T-wave velocity for AL=3.1x105 cm/s, Wave velocity in water=1.5X105 cm/s)

Question (a)

Refraction and mode conversion at non-perpendicular boundaries


http://static4.olympus-ims.com/data/Flash/HTML5/incident_angle/IncidentAngle.html?rev=5E62

Q1. From the above figures, if the incident angle is 50 Degree, what are the sound wave in the steel?

Answer: 65 Degree Shear wave in steel.

Q2. If 50 Degree longitudinal wave in steel is used what is the possible problem?

Answer: If 50 degree Longitudinal wave is generated in steel, shear wave at 28 degree is also generated and this may cause fault indications.

Calculation:Incident angle= 7Refracted longitudinal wave = 29.11Refracted shear wave = 15.49

Q72. In a water immersion test, ultrasonic energy is transmitted into steel at an incident angle of 14. What is the angle of refracted shear wave within the material?Vs = 3.2 x 105 cm/sVw = 1.5 x 105 cm/s

a) 45b) 23c) 31d) 13

Q1. If you were requested to design a plastid shoe to generate Rayleigh wave in aluminum, what would be the incident angle of the ultrasonic energy?VA = 3.1 x 105 cm/sVp = 2.6 x 105 cm/s

a) 37b) 57c) 75d) 48

2.11: Signal-to-Noise RatioIn a previous page, the effect that frequency and wavelength have on flaw detectability was discussed. However, the detection of a defect involves many factors other than the relationship of wavelength and flaw size. For example, the amount of sound that reflects from a defect is also dependent on the acoustic impedance mismatch between the flaw and the surrounding material. A void is generally a better reflector than a metallic inclusion because the impedance mismatch is greater between air and metal than between two metals. Often, the surrounding material has competing reflections. Microstructure grains in metals and the aggregate of concrete are a couple of examples. A good measure of detectability of a flaw is its signal-to-noise ratio (S/N). The signal-to-noise ratio is a measure of how the signal from the defect compares to other background reflections (categorized as "noise"). A signal-to-noise ratio of 3 to 1 is often required as a minimum.

The absolute noise level and the absolute strength of an echo from a "small" defect depends on a number of factors, which include:

1. The probe size and focal properties. 2. The probe frequency, bandwidth and efficiency.3. The inspection path and distance (water and/or solid). 4. The interface (surface curvature and roughness). 5. The flaw location with respect to the incident beam. 6. The inherent noisiness of the metal microstructure. 7. The inherent reflectivity of the flaw, which is dependent on its acoustic

impedance, size, shape, and orientation.8. Cracks and volumetric defects can reflect ultrasonic waves quite differently.

Many cracks are "invisible" from one direction and strong reflectors from another.

9. Multifaceted flaws will tend to scatter sound away from the transducer.

The following formula relates some of the variables affecting the signal-to-noise ratio (S/N) of a defect:

Flaw geometry: Figure of merit FOM and amplitudes responds

Sound Volume: Area x pulse length

Material properties

Rather than go into the details of this formulation, a few fundamental relationships can be pointed out. The signal-to-noise ratio (S/N), and therefore, the detectability of a defect:

1. Increases with increasing flaw size (scattering amplitude). The detectability of a defect is directly proportional to its size.

2. Increases with a more focused beam. In other words, flaw detectability is inversely proportional to the transducer beam width.

3. Increases with decreasing pulse width (delta-t). In other words, flaw detectability is inversely proportional to the duration of the pulse (t) produced by an ultrasonic transducer. The shorter the pulse (often higher frequency), the better the detection of the defect. Shorter pulses correspond to broader bandwidth frequency response. See the figure below showing the waveform of a transducer and its correspondingfrequency spectrum.

Acoustic Volume: wxwyt

Determining cross sectional area using reflector- A Scan (6db drop)

Determining cross sectional area using reflector- C Scan

Sonic pulse volume and S/N (defect resolution)

4. Decreases in materials with high density and/or a high ultrasonic velocity. The signal-to-noise ratio (S/N) is inversely proportional to material density and acoustic velocity.

5. Generally increases with frequency. However, in some materials, such as titanium alloys, both the "Aflaw" and the "Figure of Merit (FOM)" terms in the equation change at about the same rate with changing frequency. So, in some cases, the signal-to-noise ratio (S/N) can be somewhat independent of frequency.

Pulse Length

Pulse Length Affect Resolution

2.12: Wave Interaction or InterferenceBefore we move into the next section, the subject of wave interaction must be covered since it is important when trying to understand the performance of an ultrasonic transducer. On the previous pages, wave propagation was discussed as if a single sinusoidal wave was propagating through the material. However, the sound that emanates from an ultrasonic transducer does not originate from a single point, but instead originates from many points along the surface of the piezoelectric element. This results in a sound field with many waves interacting or interfering with each other.

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