U.S. DEPARTMENT OF TRANSPORTATION
ULTRASONIC FOR FAA AERO CENTER RECEIVED APR 2 5 :?‘:
LIBRARY '
AIRCRAFT
FEDERAL AVIATION ADMINISTRATION
ULTRASONIC NONDESTRUCTIVE
TESTING FOR AIRCRAFT 1975 U.S. DEPARTMENT OF TRANSPORTATION
FEDERAL AVIATION ADMINISTRATION
FLIGHT STANDARDS SERVICE For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 2@M2 - Price $1
TABLE OF CONTENTS Chapter 1. INTRODUCTION Page
1. General ___-___--________------------------------------------------------- 1 , 2. Use of Ultrasonics ___________-____ -__ -----_---_--_-___---------- ______ ---- 1
3. Limits on Application of Ultrasonics _---_--_--_---------__________ _________ 1
4. Ultrasonic Inspection for Corrosion Detection - ____________-_-_ -_-- _________ 1
5.-9. Reserved ________________-_-_____________________------------------------- 1 Chapter 2. TRANSDUCERS (SEARCH UNITS, PROBES, CRYSTALS)
10. General ~-_--_-~-~----___------~--~---~-------~~---~~--~----------~~~~---~ 3
11. Transducer Materials ______________--_--_____________________------------- 3
12. Piezoelectric Effect ---~___--_--_-__----------~-~-~-~~-~~--------~~~-~~~~~- 3
13. Transducer Type Search Units __-___-------_--_--_____________________----- 3
14. Transducer Groups ---___----------~~_~------~~~----~--~~~-~-----~~~~~~~~~ 4
15.-20. Reserved ______----______________________________------------------------- 5 Chapter 3. WAVE PROPAGATION
21. Ultrasonic Waves -----_-_---------_-_------------------------------------- 7
7.2. Longitudinal Waves ______ ----_-__-__- _________ -_--_-_-__--------- _________ 7
23. Shear Waves --_------_-----------------~-~~~~----------~~~~~-~~-~~~~~~~~~ 7
24. Surface (Rayleigh) Waves ----_--_-- __________ --_--_--_---------- _________ 8
25. Plate (Lamb) Waves --------------_--_-_-------------- _-__ -_--_- ____-____ 8
26.-30. Reserved ---_---~~--_____----____________________~~~-~~~----~------------- 8 Chapter 4. ULTRASONIC VIBRATIONS
31. Testing Methods ~~~-----__~__~~~-----------------------~~~~~~~~~~~--~-~~~~ 9
32. Reflection of Ultrasonic Waves -------------_--_---______ _____-_-____--_____ 9
33. Refraction and Mode Conversion of Ultrasonic Waves ---------------_________ 10
34. Beam Divergence ----_-__-----_--_-______________________----------------- 10
35.-40. Reserved ~-~--_--------__-------~--~~-~~~~~~~~------~---~~--~----~~------- 10 Chapter 5. ULTRASONIC SYSTEMS
41. General -_-____------_--_------------------------------------------------- 11
42. Pulsed ~-------__~_~-~--__~____________________~~~-~--~~~~~~~~~~~~-~~~~~~~ 11
43. Reasonance ---------------~~-------------~~~~~---------~~~~~~~~~~~~~~~~--~ 12
44.-50. Reserved ------~~~~---_--~_~~____________________------~~~~~~~~~~~~~~~~~-~ 13 Chapter 6. PRESENTATION
51. Observing and Recording Response Patterns _____---_--_--_-----_____________ 15
52. Cathode-Ray Tube (CRT) __-_ -_---------_---- ______-_- - --_-_____ ----- ---- 15
53. Scan Presentation --~-----~~-~------~-____________________---------~~~~~~~~ 16
54. Indications ____------_-__------------------------------------------------- 16
55.-60. Reserved ______-------_-_------------------------------------------------- 19 Chapter 7. RECORDERS
61. Types ---------~---------~____________________-~--------~--~~~----~~~~---~
62. Deflection Modulation, Conventional Pen-Chart Recorders ---~~-~--~--________
63. Intensity Modulation, C-Scan Mehod of Facsimile Recording --------________
64.-70. Reserved --------~-------~~~-____________________----~--~~~~~~~~--~~~~~~~- 21 21 21 21 I
TABLE OF CONTENTS-Continued Chapter 8. ELECTRONIC GATING
71. Functions ~___~-__-~_-_-__~--_____________________~~~-~~---~~--~---~---~-~
72. Basic Operating Principles of Gates --------------------____________________
73.-75. Reserved --~-----------_--___-~-~---~---~~-~~~-~~~-~~~-~._~-___--__-__~~__ Page 23 23 24 Chapter 9. DELAY LINE
76. Description -_~--~~--_~--_-~--~---~---~~----------------~---~----~------~--
77.~80. Reserved _~~___-___-__--~__~-____________________-~~---~---~~--~~--~---~-- 25 25 Chapter 10. COUPLANT
81. General ~---~---~--_~~~--~---~---~~------~----~---~---~-----~--~--~~--~~-- 27
82. Description ---------~----~------~---~~---~---~----~--~~-~~~--------~~~~~-~ 27
83.-85. Reserved ---~-~-~~~-_~---~~--____________________----------------~~~-~~~-~ 27 Chapter 11. ULTRASONIC REFERENCE BLOCKS
86. Introduction _~--~~-----------_~------~~--~---~~---~--~~--~~----~--~--~~~~~
87. Distance-Amplitude Comparison Blocks -_---_--------_---_-_________________
88. Area-Amplitude Comparison Blocks (Alcoa Blocks) __-___- __________________
89. I.I.W. Welding Blocks -------------------_----------------------- _________
90. ASME Weld Reference Plate ---------------------------------------------
91. Special Reference Standards ---_-----------_-------------------------------
92.-95. Reserved _~-__~~-~---~~_-~~-~-~---~---~~--~--------~---~~--~~~--~--~---~~- 29 29 30 30 30 30 31 Chapter 12. TESTING METHODS
96. Description ~__---_~--_~~-__~--_____________________~~--~-----~~-~~-----~~-
97. Contact Testing --~-~-~---~----~-------~---~---~~--------~---~-~----~-~~~~~
98. Immersion Testing ---------------_-------~~-------~--------~--~~--~~-~~~~~
99. Immersed Scanning Technique -_------------------____________________-----
100. Water Column Technique (Bubbler, Squirter) -------_-------__---__________
101. Wheel Search Unit ----------_--------_-----------~--~-~--~~--~----~~-~~~~
102. Ultrasonic Image Converter ~~--~~--_~- -_-- --_---_---__---_----_____________
103.-110. Reserved ~------_---_--------____________________-----~~----------~~~-~~~- 33 33 33 34 36 36 36 37 Chapter 13. INSPECTION OF INTEGRAL FUEL TANKS FOR CORRISON
111. General _---~_---~----~--~-------~~~~~~~~~-~~~-~-~-~~~~-~~--~~~~-~--~~---~
112. Method ~---_--__---_------_-~--~~-~--~~~--~~--~~--~~---~-~-~~--~---~---~~
113. Equipment __--__-____-__-_____-------------------------------------------
114. Indications --------------_---_~----~~~-~~~--~~-~~~-~~~~-~~--~~~~~~~~~~~--~
115.-120. Reserved -_-_-___-___--__________________________------------------------- Chapter 14. INSPECTION OF REPRESENTATIVE AIRCRAFT PARTS
121. General -_~--_~--~-~--~---~----~--~~-------~-----~~-~---~---~~------~---~-
122. Main and Nose Landing Gear Wheels --_----_---_--------------------------
123. Main Landing Gear Torsion Link -__---_---_--_-_---_----------------------
124. Main Landing Gear Torsion Link 1,ugs __-___- ____ --_.-_-_--___-___--__--___
125. Main Landing Gear Oleo Outrr Cylinder --------_--------_-----------------
126. Main Landing Gear Trunnion Support Strnvtow __---_--_----__-_-_-________
127. Nose I,anding Gear Outer Cylinder ----------__---_---_____________________-
128. Inboard and Outboard Nacelle Strut Front Spar Fittings ---_-------_---__---
129. Nose Landing Gear Outer Cylinder _-------__------------------------------
130.-l%. Reserved _~~-_~--~---~---~---____________________--~------------~~~~~--~- .
39 39 39 39 39 41 41 41 41 42 42 42 43 43 43 II
LIST OF ILLUSTRATIONS Figure
1. Angle Beam Search Unit ________________________________________---------------
2. Generation of Ultrasonic Vibration ________________________________________------
3. Diagram of an Ultrasonic Search Unit ________________________________________--
4. Natural Quartz Crystal (X and Y Cut) _____ - ____________________________________
5. Common Transmitter-Receiver Search Unit __________ -_-- _____ - __________________
6. Combined Transmitter-Receiver Search Unit ____________________________________
7. Separate Transmitter-Receiver Search Unit ______________________________________
8. Mechanical Analogy of Wave Propagation --__----_-_--__-__- _______ - ____________
9. Longitudinal Waves ________________________________________--------------------
10. Shear Waves ________________________________________---------------------------
11. Transmitting A Wave at a Small Angle __________ - _______________________________
12. Surface Waves ____-_____-_____________________________-------------------------
13. Surface Wave Technique ________________________________________---------------
14. Plate Waves ________________________________________---------------------------
15. Reflection of Ultrasound ________________________________________---------------
16. Refraction of the Ultrasonic Beam ______ - _______________________________________
17. Generating Shear Waves __-_______--_--____-____________________----------------
18. Generating Surface Waves ___-_____-_-__--_-______________________--------------
19. Beam Divergence of Sound Waves in Steel _______________________________________
20. Diagram of Longitudinal Sound Striking a Surface ___- ___________________________
21. Block Diagram of Basic Pulse-Echo System _-__- _____ - ___________________________
22. Oscilloscope Display in Relationship to Flaw Detection ____________________________
23. Through-Transmission Technique -_- ________________________________________-----
24. Conditions of Ultrasonic Resonance in Metal Plate _______________ - _______________
25. Block Diagram of Resonance Thickness Measuring System ________________________
26. RadieFrequency Trace and Video Trace ________________________________________
27. Types of Marker Systems ________________________________________---------------
28. A-Scan Presentation ________________________________________--------------------
29. B-Scan Presentation ______-_________________________________--------------------
30. C-Scan Presentation _-__-____-__-__-________________________--------------------
31. Immersion Crystal Focused on Test Block and Indications to he Expected ___________
32. Immersion Crystal Focused on Block with Defect and Non-parallel Surface ________
33. Immersion Crystal Focused on Shaft too Long for Back Reflecion of Return _________
34. Angle Beam Penetrating a Weld Bead ________________________________________---
35. Angle Beam Penetrating a Weld Bead ________________________________________---
36. Angle Beam Penetrating a Flat Plate ________________________________________----
37. Results of a Rough Front and Back Surface ___--_-- ______________________________
38. Evaluating Braze of Carbide Tip to Steel __- ______ -__- ___________________________
39. Indication Received from Porous Material _____ - ______ -___- ____ - _________________
40. Irregular Part ________________________________________-------------------------
41. Various Recording Charts ________________________________________---------------
42. Ultrasonic Recording of Brazed Honeycomb Panel ________________________________
43. Controllable Gated Zone in Test Piece ___- ___________ - _____ - ____________________
44. Threshold Gate Circuit Diagram ________________________________________---------
45. Threshold Gate Wave Form ________________________________________-------------
46. Ultrasonic Delay Time ____ - _____ -__- _____ - _____________________________________
47. Couplant (Contact Testing) ________________________________________------------
48. Couplant (Immersion Testing) ________________________________________----------
49. Distance-Amplitude Comparison _-_-- ________________________________________----
50. Step Block ________________________________________----------------------------
51. Area Amplitude Comparison Blocks ________________________________________------
52. I.I.W. Weld Block _________-_______---____________________---------------------
53. ASME Weld Reference Plate ________________________________________----------- Page 3 3 4 4 4 4 5 7 7 7 8 8 8 8 9 10 10 10 10 10 11 11 12 12 12 15 16 16 16 16 17 17 17 18 18 18 18 19 19 19 21 21 23 23 24 25 27 27 29 30 30 31 31 III
Figure LIST OF ILLUSTRATIONS-Continued Page
54. Contact Angle-Beam Test into Hidden Weld Region of a Landing Gear Oleo Strut ---_ 33
55. Ultrasonic Inspection of a Wing Front Spar --------~--~~-~~- --__- - __-____________ 33
56. Ultrasonic Inspection of a Pylon Structural Member --------------~~-~~-__________ 33
57. Principle of Ultrasonic Testing (Contact) ----_--------------- _--------~-~~--~~-- 34
58. Principle of Ultrasonic Testing (Immersion) --------~~-~~- ~~~~~~~~~_~~__~________ 34
59. Bubbler Angle-Beam Testing (Pipe) ----_--_---------------~~--~~~~-~~-~~-~~-~~~ 34
60. Wheel Scanning Method ~-------------------------~~-~~-~~~~~~~~~~~~~~~~~~~~~~~ 34
61. Immersed Angle-Beam Technique (Pipe or Tube) --------------------___________ 34
62. Immersed Angle-Beam Technique (Plate or Sheet) --_--_------------------------ 35
63. Immersed Through-Transmission Technique -----------------_--______ __-_____ -___ 35
64. Removing First Multiple of Interface Indication From Test Area --------~-----_~-~~ 35
65. Focused Search Unit ----_--_--____-__-__--------------------------------------- 35
66. Beam Collimator -------------------~---------------------~--~--~~-~~-~~~~------ 35
67. Bubbler Scanning Method -----------------------------------~-----~~--~~~~~~-- 36
68. Bubbler Angle-Beam Technique (Plate) ---_-__--_------------------------------ 36
69. Wheel Search Unit in a Fixed Position ------------------------- ----- -- _---_-_ -_- 36
70. Wheel Search Unit over the Material ----_-------------------------------------- 36
71. Schematic-Ultrasonic Image Converter System ---------------_----_______________ 37
72. Main and Nose Landing Gear Wheels ------------------------------------------ 41
73. Main Landing Gear Torsion Link ----_--_--_--_--_--_--------------------------- 41
74. Main Landing Gear Torsion Link Lugs -----------------_-- _-__- ----_---_-_--_-_- 41
75. Main Landing Gear Oleo Outer Cyclinder ---------------------------------------- 42
76. Main Landing Gear Trunnion Support Structure ------------------_______________ 42
77. Nose Landing Gear Outer Cylinder -----_-----_--------------------------------- 42
78. Inboard and Outboard Nacelle Strut Front Spar Fitting --------------------------- 43
79. Nose Landing Gear Outer Cylinder -------------------_-------------------------- 43
Chapter 1. INTRODUCTION
1. GENERAL. Ultrasonic inspection of aircraft plays an important role from both a safety and economic aspect. In order to accomplish the prescribed ultrasonic inspection, only minor aircraft open-up for accessibility is usually required. For example, in the case of the aircraft engine, inspections are performed “on the wing” with the engine still installed in the aircraft. For the airframe, ultrasonics required a minimum of disassembly and removal of interfering equipment. Aircraft inspections with ultrasonics can be performed “at the ramp” or “on the line” under circumstances entirely different from total disassembly. It is an extremely sensitive method of nondestructing testing. The limitations are few and when direct coupling (or contact) with the part being checked can be established, the resolving capabilities are excellent. However, interpretation by trained personnel is required.
2. USE OF ULTRASONICS. Ultrasonics employ electronically produced, high-frequency sound waves that will penetrate metals, liquids, composites, and other materials at speeds of several thousand feet per second. This technique can be used to:
a. Detect laps, seams, laminations, inclusions, cracks, corrosion, and other defects in installed parts.
6. Locate porosity, cupping, and nonmetallic inclusions in bar stock.
c. Locate cracks, blow holes, insufficient penetration, lack of fusion, and other discontinuities in welds.
d. Evaluate bond quality in brazed joints and honeycomb composites assemblies.
e. Inspect forgings such as turbine engine shafts, turbine engine discs, and landing gear structural members.
3. LIMITS ON APPLICATION OF ULTRASONIC TESTING. Among the factors which may limit the application of ultrasonic testing are:
a. Sensitivity. The ability of the instrument to detect the small amount of energy reflected from a discontinuity.
b. Resolution. The ability of the instrumentation to detect flaws lying close to the test surface or to separate and distinguish the indications from several defects occurring close together in the specimen.
c. Noise discrimination. The capacity of the instrumentation for differentiating between the signals from defects and the unwanted noise of either electrical or acoustics nature.
d. These factors are affected by others, such as frequency and pulse energy. For example, when frequency is increased, the sensitivity increases.
With the increase in sensitivity, smaller inhomogeneties within the material will become detectable.
This will increase the noise level, thus hindering signal discrimination. With an increase in pulse energy, material noise will increase and resolution will decrease.
4. ULTRASONIC INSPECTION FOR CORROSION DETECTION. Although ultrasonic inspections have been employed by the aviation industry for several years, it was not until recently that ultrasonics have been used as a means of corrosion ,detection. Presently, this method of corrosion detection is still in the early stages and certainly not infallible; but it has been demonstrated that, within limitations, ultrasonics can provide a fairly reliable indication of corrosion attack. Highly trained personnel must conduct the examination if any useful information is to be derived from the indicating devices. This is compounded by the fact that the results obtained vary, depending on the model and make of equipment used, and on the techniques used by the individual performing the examination.
5.-9. RESERVED.
Chapter 2. TRANSDUCERS (SEARCH UNITS, PROBES, CRYSTALS) .
10. GENERAL. To understand how inaudible sound is used to reveal certain conditions which are not perceptible in the normal hearing range, it is first necessary to know how ultrasound is transmitted and received.
11. TRANSDUCER MATERIALS. Three transducer materials which can be used in the manufacture of ultrasonic search units are natural quartz crystals, lithium sulfate, and polarized crystalline ceramics.
a. Quartz. Principal advantages are : electrical and thermal stability, insolubility in most liquids, high mechanical strength, wear resistance, excellent uniformity and resistance to aging. A limitation of quartz is its comparatively low electromechanical conversion efficiency.
b. Lithium Sulfate. Principal advantages are :
ease of obtaining optimum acoustic damping for best resolution, intermediate conversion efficiency, and negligible mode interaction.
c. Polarized Ceramics. Principal advantages are: high conversion efficiency which yields high search unit sensitivity. Because of lower mechanical strength and relatively high electrical capacitance, their use is generally restricted to frequencies below 15 Mega-Hertz (MHz). Another limitation is some interaction between various modes of vibration.
In order for a crystal to utilize its piezoelectric characteristics, it is placed in a circuit much like a condenser. That is, both faces are coated with a conducting material with no contact between the two faces. (See Figure 1.) Coatings for crystals
COAXIAL CABLE CONNECTION k
PLATED BACK
CRYSTAL
PLATED FACE FIGURE l.-Angle Beam Search Unit 3 may be of any conducting material such as aluminum, silver, gold, or chromium. However, coatings are difficult to deposit on lithium sulphate crystals so thin metallic foils are often cemented to the crystal.
12. PIEZOELECTRIC EFFECT. Ultrasonic testing may use the piezoelectric effect to generate ultrasonic vibrations. Crystals, when subjected to an alternating electric charge, expand and contract under the influence of these charges. Conversely, it was found that these materials when subjected to alternating compression and tension developed alternating electric charges on their faces. (See Figure 2.) This was named the piezoelectric effect.
The heart of an ultrasonic testing system is this method of converting electrical energy into mechanical vibrations, and converting the mechancial vibrations back into electrical energy.
ALTERNATING VOLTAGE APPLIED *
TO AN X-CUT CRYSTAL r=- 4.J
CRYSTAL EXPANDS CRYSTAL CONTRACTS FIGURF. 2.-Generation of Ultrasonic Vibration Generation of the ultrasonic pulse is usually accomplished by producing a radio-frequency wave train of the desired frequency at a precise time and converting this into vibrations by means of piezoelectric transducers. Some ultraconsic instruments do not use a radio-frequency wave train, but instead, use a shock pulse and allow the search unit to select the frequency of operation.
13. TRANSDUCER TYPE SEARCH UNITS. The search unit consists of a shell for mechanical protection, a means to conveniently handle or mount the unit for use, the transducer element, electrical connections, and a backing material to dampen the backward directed energy that is transmitted by the crystal. (See Figure 3.)
ELECT.
CONNECTION , SHELL
BACKING ‘MATERIAL
I SPECIMES PIEZOELECTRIC CRYSTAL FIGURE 3.-Diagram of an Ultrasonic Search Unit Transducers are available in a wide variety of types which include :
a.
b.
X-Cut crystals for longitudinal-wave generation. (See Figure 4.)
Y-Cut crystals for shear-wave generation.
(See Figure 4.)
C. Dual crystals with common holder.
d. Mosaics-three or more crystals.
e. High frequency, 50 MHz or more.
f. Alternate crystal materials.
9. Sandwich and tandem arrangements.
h. Curved crysals to fit the specimen.
i. Wheel search units.
i. Focused search units.
k. Temperature search units (for measuring wall thickness at temperature up to l,lOO°F).
% AXIS X-CUT ‘X /Y Y-CL’T AXIS AXIS z AXIS FIGURE 4.-Natural Quartz Crystal (X and Y Cut) Transducers are available that are smaller than l/a” diameter and larger than 1” x 4”. However, for most ultrasonic testing, standard diameters of l/b”, l/2”, and 1.0” are used.
14. TRANSDUCER GROUPS. There are three general groups of transmitter-receiver search units:
a. Common Transmitter-Receiver IT-RI. These search units employ a single crystal and have common connections to the transmitter and receiver amplifier units. (See Figure 5.)
I \ \ I PECIMFN \ I I I I i \ I FIGURF. 5.-Common Transmitter-Receiver Search Unit Since the search unit acts as both transmitter and receiver, it transmits a pulse of 1 to 4 microseconds duration ; then acts as a receiver for a period up to several thousand microseconds. This cycle of transmitting and receiving is repeated at a rate of 50 to 5,000 times per second, or higher if required for high-speed automatic scanning.
b. C o m b i n e d Transmitter-Receiver IT-RI.
These search units have two transducers mounted on a single head and insulated acoustically from each other. One transducer is connected to the pulser and the other is connected to the receiver.
The combined T-R search unit is used for testing close to the entry surface and for thickness measurements from .040” to 2.0” when the opposite side is rough or corroded. The transmitting search unit projects a beam of vibrations into the material ; the - - vibrations travel through the material and are re- FIGURE 6.-Combined Transmitter-Receiver Search Unit 4
fleeted back to the receiving search unit from any discontinuities or from the opposite boundary if Ly parallel to the entrant surface. --(See Figure 6.)
- c. Separate Transmitter-Receiver U-RI or Pitch-Catch, Search Units. Two heads are employed in these units having separate electrical connections to the transmitter and the receiving units.
One head is used as the transmitting unit while the other head is the receiving unit. (See Figure 7.)
. Materials which are coarse grained tend to scatter the ultrasonic sound beam; these materials can be effectively inspected using separate T-R search units that are mounted on individual wedges of a suitable plastic solid. When separate wedges are used, the angle of incidence may be varied according to the section thickness to be examined.
FIGURE 7.--Separate Transmitter-Receiver Search Unit
15.-20. RESERVED.
Chapter 3. WAVE PROPAGATION
21. ULTRASONIC WAVES. Ultrasonic waves can s be propagated to some extent in any elastic material. This propagation occurs as a displacement of the successive elements of the material. If one part of a solid is distributed or displaced in some manner, molecules in other parts of the solid will be affected, but not instantaneously. As successive molecules in the medium are displaced, the disturbance is propagated away from its point of origin. (See Figure 8.) Since the lattice structure of all materials is elastic, a restoring force exists which tends to return each molecule to its original position. Because of inertia, these particles will tend to oscillate about their original undisturbed position until they come to rest. This molecule-tomolecule propagation results in a continuous train of disturbances called a compression-rarefaction wave. If the frequency of motion is above 20,000 cycles per second (cps) the waves are referred to as ultrasonic compressional waves. A small group of these waves, which occur together and are not preceded or followed by other waves, is called a wave train or a pulse. A pulse may have one of several different forms depending upon the individual wave amplitude, and the way the waves build up and decay. The most used modes of vibration are longitudinal, shear, surface, and plate waves.
FIGURE 8.-Mechanical Analogy of Wave Propagation
22. LONGITUDINAL WAVES. The wave is said to be longitudinal (compressional) when the movement of the particles is parallel to the direction of the wave motion.
* (See Figure 9.)
The longitudinal mode of wave transmission is probably the most widely used in ultrasonic testing and is also the easiest to see with respect to the method of propagation. This wave is easily generated, detected, and has a high velocity of travel in most media.
Longitudinal waves are used for the detection and location of defects that present a reasonably large frontal area to the surface from which the test is being made.
PARTICLE VIBRATION
- DIRECTION OF PROPAGATION ft FIGURE 9.-Longitudinal Waves
23. SHEAR WAVES. The wave is said to be shear (transverse) when the movement of the particles are perpendicular to the direction of the wave motion. (See Figure 10.) These waves have a lower velocity than do longitudinal waves (in steel and other metals, about half). Because of their slower speed, shear waves have shorter wave lengths than those of longitudinal waves of the same frequency. This shorter wave length makes shear waves more sensitive to small inclusions, and consequently they are more easily scattered within the specimen.
PARTICLE VIBRATION
DIRECTION OF PROPAGATION FIGURE IO.-Shear Waves The principal advantage of these waves is in applications that require an ultrasonic beam to be transmitted into the test object at a small angle to the surface. (See Figure 11.)
PLASTIC \VEDCE SPECIhlEN , makes surface waves so useful in surface flaw detection. (See Figure 13.)
FIGURE 13.~Surface Wave Technique I” DIRECTION OF SOUND 25. PLATE (LAMB) WAVES. When ultrasonic vi- VIBRATION DIRECTIDS brations are introduced into a relatively thin sheet, FIGURE Il.-Transmitting A Wave at a Small Angle the energy propagates in the form of plate waves.
24. SURFACE (RAYLEIGH) WAVES. Surface waves travel with little attenuation in the direction of propagation, but their energy decreases rapidly as the wave penetrates below the surface. The particle displacement of the wave motion follows an elliptical orbit consisting of both the longitudinal and shear wave motion. (See Figure 12.)
PARTICLE VIBRATION FOLLOWS
IF AN ELLIPTICAL ORBIT
I DIRECTION OF PROPAGATION FIGURE 12.-Surface Waves Velocity of surface waves depends upon the material and is about nine-tenths of the shear-wave velocity. Surface waves are likely to be affected, in their propagation, by variations in hardness, plated coatings, shot peening, or surface cracks, and are easily dampened by dirt or grease on the surface of the specimen. Surface waves can often be produced as an unwanted effect, especially when the contact surface is rough.
The waves are not limited to flat surfaces. They will travel around curves and surface contours.
Sharp corners, such as the boundaries of plates or flaws, will reflect these waves. In fact, it is this characteristic or traveling around contours which Unlike the longitudinal, shear, and surface modes, the plate wave velocity is dependent both on frequency and plate thickness. A complex particle motion exists somewhat like the elliptical orbits described for surface waves. Greatly simplified, they can be divided into two basic types: The symmetrical type are known as dilational waves and the asymmetrical type as bending waves.
Examples of these two modes are illustrated in Figure 14.
ASiMMETRIG SYMMETRIC FIGURE 14.-Plate Waves Plate waves can be used for detecting nonbonded areas in laminated structures such as sandwich panels. By sending waves along the outer surface, any areas of the top sheet that are not jsecurely bonded in place can be made to vibrate in one of these modes. By sensing such local areas of vibration, lack of proper bonding can be detected.