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U.S. DEPARTMENT OF TRANSPORTATION

ULTRASONI

FOR

FAAAEROCENTER

RECEIVED

APR 2 5 :?:

LIBRARY '

AIRCRAFT

FEDERAL AVIATION ADMINISTRATION

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ULTRASONIC NONDESTRUCTIVE

TESTING FOR AIRCRAFT

1975

U.S. DEPARTMENT OF TRANSPORTATION

FEDERAL AVIATION ADMINISTRATION

FLIGHT STANDARDS SERVICE

For sale by the Superin tendent of Documents, U.S. Government Printi ng Office

Washington, D.C. 2@M2 - Price $1

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TABLE OF CONTENTS

Chapter 1.

IN T R OD U C T ION

Page

1. Gen eral ___-___--___ _____------------------------------------------------- 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. TRANS DUCER S (SEARCH UNITS, PROBES, CRYSTALS)

10. Gen eral ~-_--_-~-~----___------~--~---~-------~~---~~--~----------~~~~---~ 3

11. Transducer Materials ______________--_--_____________________------------- 3

12. Piezoe lectric Effect ---~___--_--_-__----------~-~-~-~~-~~--------~~~-~~~~~- 3

13. Transducer Type Search Units __-___-------_--_--_______ _______ _______ ----- 3

14. Transducer Groups ---___----------~~_~------~~~----~--~~~-~-----~~~~~~~~~ 4

15.-20. Reserved ______----_______ _________ __________ ____------------------------- 5

Chapter 3. WAVE PROPAGATION

21. Ultra son ic Waves -----_-_---------_-_------------------------------------- 7

7.2. Longitud inal Waves ______ ----_-__-__- _______ __ -_--_-_-__--------- _______ __ 7

23. She ar Waves --_------_-----------------~-~~~~----------~~~~~-~~-~~~~~~ ~~~ 7

24. Surface (Rayleig h) Waves ----_--_-- ___ ____ ___ --_--_--_---------- ___ ____ __ 8

25. Plat e (Lamb) Waves --------------_--_-_-------------- _-__ -_--_- ____ -____

8

26.-30. Reserved ---_---~~--_____----___________ _________~~~-~~~ ----~------------- 8

Chapter 4. ULTRA SONIC VIBRATIONS

31. Testing Metho ds ~~~-----__~__~~~-----------------------~~~~~~~~~~~--~-~~~~ 9

32. Reflec tion of Ultrasonic Waves -------------_--_---______ _____ -_-____ --_____ 9

33. Refraction and Mod e Conversio n of Ultrasonic Waves ---------------_________ 10

34. Bea m Divergenc e ----_-__-----_--_-_____________________ _----------------- 10

35. -40. Rese rved ~-~--_--------__-------~--~~-~~~~~~~~------~---~~--~----~~------- 10

Chapter 5.

ULTRASONIC SYSTEMS

41. Gen eral -_-____------_--_------------------------------------------------- 11

42. Pulsed ~-------__~_~-~--__~____________________~~~-~--~~~~~~~~~~~~-~~~~~~~ 11

43. Rea son anc e ---------------~~-------------~~~~~---------~~~~~~~~~~~~~~ ~~--~ 12

44.-50. Reserved ------~~~~---_--~_~~________ ________ ____------~~~~~~~~~~ ~~~~~~~-~ 13

Chapter 6. PRESENTATION

51. Observing and Recording Response Patterns _____---_--_--_-----_____________ 15

52. Cathode -Ray Tube (CRT) __-_ -_---------_---- _____ _-_- - --_-_____---- ---- 15

53. Scan Prese ntation --~-----~~-~------~-____________________ ---------~~~~~~~~ 16

54. Indi cat ion s ____------_-__------------------------------------------------- 16

55.- 60. Rese rved ______-------_-_------------------------------------------------- 19

Chapter 7. RECOR DERS

61. Types ---------~---------~____________________-~--------~--~~~----~~~~---~

62. Deflection Mo dulation, Conventional Pen-Chart Recorders ---~~-~--~--________

63. Intensity Modula tion, C-Scan Me hod of Facsimile Recording --------________

64.-70. Reserved --------~-------~~~-____________________ ----~--~~~~~~~~--~~~~~~~-

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TABLE OF CONTENTS-Continued

Chapter 8. ELECTRONIC GATING

71. Functions ~___~-__-~_-_-__~--_____________________~~~-~~---~~--~---~---~-~

72. Basic Opera ting Princ iples of Gates --------------------____________ ________

73.-75 . Reserved --~-----------_--___-~-~---~---~~-~~~-~~~-~~~-~._~-___--__-__~~__

Page

23

23

24

Chapter 9. DELAY LINE

76 . Des crip tion -_~--~~--_~--_-~--~---~---~~----------------~---~----~------~--

77.~80. Reserved _~~___-___-__--~__~-____________________-~~---~---~~--~~--~---~--

25

25

Chapter 10. COUPLANT

81. Gen eral ~---~---~--_~~~--~---~---~~------~----~---~---~-----~--~--~~--~~-- 27

82. Des crip tion ---------~----~------~---~~---~---~----~--~~-~~~--------~~~~~-~ 27

83.-85 . Reserved ---~-~-~~~-_~---~~--_________________ ___----------------~~~-~~~-~ 27

Chapter 11. ULTRASONIC REFERENCE BLOCKS

86. Intro duc tion _~--~~-----------_~------~~--~---~~---~--~~--~~----~--~--~~~ ~~

87. Distance-Amplitude Comparison Blocks -_---_--------_---_-_________________

88. Area-Amp litude Comparison Blocks (Alcoa Blocks) __-___- ______ _______ _____

89 . I.I.W. We ldi ng Bloc ks -------------------_----------------------- __ __ __ __ _

90 . AS ME Wel d R efer enc e Pl ate ---------------------------------------------

91 . Sp ec ial Re fere nce Sta nda rds ---_-----------_-------------------------------

92.-95 . Res erved _~-__~~-~---~~_-~~-~-~---~---~~--~--------~---~~--~~~--~--~---~~-

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30

30

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31

Chapter 12.

TESTING METHODS

96. Descrip tion ~__---_~--_~~-__~ --_________ _______ _____~~--~-----~~-~~-----~~-

97. Con tact Tes ting --~-~-~---~----~-------~---~---~~--------~---~-~----~-~~~~~

98 . Imm ers ion Tes ting ---------------_-------~~-------~--------~--~~--~~-~~~~~

99. Immers ed Sca nnin g Tec hniqu e -_------------------_______________ _____-----

100. Water Column Technique (Bubbler, Squirter) -------_-------__---__________

101 . Wh eel Sea rch Un it ----------_--------_-----------~--~-~--~~--~----~~-~~~~

102. Ultrasonic Imag e Converter ~~--~~--_~- -_-- --_---_---__---_----_____________

103.-1 10. Reserved ~------_---_--------___________________ _-----~~----------~~~-~~~-

33

33

33

34

36

36

36

37

Chapter 13. INSPECTION OF INTEGRAL FUEL TANKS FOR CORRISON

111. Genera l _---~_---~----~--~-------~~~~~~~~~-~~~-~-~-~~~~-~~--~~~~-~--~~---~

112 . Meth od ~---_--__---_------_-~--~~-~--~~~--~~--~~--~~---~-~-~~--~---~---~~

11 3. Eq uip me nt __--__-___ _-__-____ _-------------------------------------------

114. Indica tions --------------_---_~----~~~-~~~--~~-~~~-~~~~-~~--~~~~~~~~~~~--~

115.-120. Reserved -_-_-___-___--__________________________-------------------------

Chapter 14. INSPECTION OF REPRESENTATIVE AIRCRAFT PARTS

12 1. Gen eral -_~--_~--~-~--~---~----~--~~-------~-----~~-~---~---~~------~---~-

122. Mai n and Nose Lan ding Gear Wh eels --_----_---_--------------------------

123. Mai n Land ing Gear Torsion Link -__---_---_--_-_---_----------------------

124. Main Landing Gear Torsion Link 1,ugs __-___- ____ --_.-_-_--___-___--__--___

125. Mai n La ndin g Gear Oleo Outrr Cylind er --------_--------_-----------------

126. Main Landing Gear Trunnion Support Strnvtow __---_--_----__-_-_-________

127. Nose I,anding Gear Outer Cylinder ----------__---_---_____________________-

128. Inboa rd and Outboa rd Nac elle Strut Front Spar Fittings ---_-------_---__---

129. Nose Lan ding Gear Outer Cylind er _-------__------------------------------

130.-l% . Reserved _~~-_~--~---~---~---____________________ --~------------~~~~~--~-

.

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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. Comm on Transmitter-Receiver Search Unit ______ ____ -_-- _____ - ______ ______ ______

6. Comb ined Transmitter-Receiver Search Unit ______ _______ ______ ______ ______ _____

7. Separate Transmitter-Receiver Search Unit ______ _______ ______ ______ ______ ______ _

8. Mechanical Analogy of Wave Propagation --__----_-_--__-__- _______ - ______ ______

9. Longitu dinal Waves ________________________________________--------------------

10. Shea r Waves _______ _______ ______ _______ _______ ______ ---------------------------

11. Transmit t ing A Wave at a Small Angle __________ - _______________________________

12. Surface Waves ____ -_____-___ _______ _______ _______ _____-------------------------

13. Surface Wave Technique ______________________ __________________---------------

14. Pla te Waves _______ _______ ______ _______ _______ ______ ---------------------------

15. Reflection of Ultrasound ______________________ __________________---------------

16. Refract ion 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 Str iking a Surface ___- ___________________________

21. Block Diagram of Basic Pulse-Echo System _-__- _____ - ______ _______ ______ ______ __

22. Oscilloscope Display in Relationsh ip to Flaw Detection ______ ______ ______ ______ ____

23. Through-Transmission Technique -_- ________ _________ _________ _________ _____-----

24. Condit ions 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 Penetrat ing a Weld Bead ________________________________________---

35. Angle Beam Penetrat ing a Weld Bead ________________________________________---

36. Angle Beam Penetrat ing a Flat Plate ________________________________________----

37. Results of a Rough Front and Back Surface ___--_-- _______ ______ ______ ______ _____

38. Evaluat ing 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. Control lable Gated Zone in Test Piece ___- ___________ - _____ - ____________________

44. Threshold Gate Circuit D iagram ____________ _____________ ____________ ___---------

45. Threshold Gate Wave Form _____________ ___________________ ________-------------

46. Ultrasonic Delay Time ____ - _____ -__- _____ - _____________________________________

47. Couplan t (Contact Testing) ________________ _________________ _______------------

48. Couplan t (Immersion Testing) ______________ ______________ ____________----------

49. Distance-Amplitude Comparison _-_-- ________ ________ ________ _________ _______----

50. Step Block _______ _______ _______ _______ _______ _____----------------------------

51. Area Amplitu de Comparison Blocks __________ _________ __________ __________ _------

52. I.I.W. Weld Block _______ __-_____ __---_______ ______ _______ ---------------------

53. ASM E Weld Reference Plate ______________ ________________ __________-----------

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Figure

LIST OF ILLUSTRATIONS-Continued

Page

54. Contact Angle-B eam Test into Hidden Weld Region of a Landin g Gear Oleo Strut ---_ 33

55. Ultrasonic Inspection of a Wing Front Spar --------~--~~-~~- --__- - __-_____ _______ 33

56. Ultrasonic Inspection of a Pylon Structural Member --------------~~-~~-__________

33

57. Princ iple of Ultrason ic Testing (Contact) ----_--------------- _--------~-~~--~~-- 34

58.

Principle of Ultrasonic Testing (Immersion) --------~~-~~- ~~~~~~~~~_~ ~__~________ 34

59. Bub bler A ngle -Be am Testing (Pipe ) ----_--_---------------~~--~~~~-~~-~~-~~-~~~

34

60. Whee l Sca nnin g Meth od ~-------------------------~~-~~-~~~~~~~~~~~~~~~~~~~~~~~ 34

61. Immersed Angle-B eam Technique (Pipe or Tube) --------------------___________

34

62. Immers ed Ang le-B eam Te chniq ue (Plat e or She et) --_--_------------------------ 35

63. Immers ed Throug h-Transm ission Tech nique -----------------_--______ __-__ ___ -___

35

64. Removing First Multiple of Interface Indication From Test Area --------~-----_~-~~ 35

65.

Focu sed Sea rch Unit ----_--_--____-__-__---------------------------------------

35

66 . Be am Co llim ato r -------------------~---------------------~--~--~~-~~-~~~~------ 35

67. Bub bler Sca nnin g Meth od -----------------------------------~-----~~--~~~~~~--

36

68. Bub bler Angle -Be am Tech nique (Plate) ---_-__--_------------------------------

36

69. Whee l Search Unit in a Fixed Pos ition ------------------------- ----- -- _---_-_ -_- 36

70 . Whe el Sea rch Un it over the Ma ter ial ----_--------------------------------------

36

71. Schem atic-Ultras onic Ima ge Converter Sy stem ---------------_----_____________ __

37

72 . Ma in an d Nos e La nd ing Gear Wheels ------------------------------------------

41

73. Mai n Lan ding Gear Torsion Link ----_--_--_--_--_--_--------------------------- 41

74. Mai n Lan ding Gear Torsion Link Lugs -----------------_-- _-__- ----_---_-_--_-_- 41

75. Mai n Lan ding Gear Oleo Outer Cyclind er ----------------------------------------

42

76. Main Landin g Gear Trunnion Support Structure ------------------_______________ 42

77 . Nos e La nd ing Gear Outer C ylin der -----_-----_--------------------------------- 42

78. Inboa rd and Outboa rd Nac elle Strut Front Spa r Fittin g --------------------------- 43

79 . Nos e La nd ing Gear Outer Cy lind er -------------------_--------------------------

43

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Chapter 1. INTRODUCTION

1. GENERAL.

Ultrasonic inspection of aircraft

plays an important role from both a safety and

economic aspect. In order to accompl ish the pre-

scribed ultrasonic inspec tion, only minor aircraft

open-up for acce ssibil ity is usually required. For

example, in the case of the aircraft engine, inspec-

tions are performed on the wing with the engine

stil l installed in the aircraft. For the airframe ,

ul trasonics required a minimum of disassembly and

remov al of interfering equipm ent. Aircraft insp ec-

t ions with ul trasonics can be performed at the

ramp or on the line unde r circum stanc es entirely

di fferent from total disassemb ly. I t is an extremely

sensi t ive method of nondestructing testing. The

limitations are few and when direct coupling (or

contact) wi th the part being checked can be estab-

lished, the resolving capab ilities are excellen t. How -

ever, interpretation by trained personnel is required.

2. USE OF ULTRASON ICS. Ultrasonics employ

electronical ly produced, high-frequency sound waves

that wi l l penetrate metals, l iquids, compo sites, and

other mater ials at speeds of several thousand feet

per second. This technique can be used to:

a. Detect laps, seam s, laminations, inclusions,

cracks, corrosion, and other defects in installed

parts.

6. Locate porosi ty, cupping, and nonmetal l ic in-

clusions in bar stock .

c. Locate cracks, blow holes, insuff ic ient penetra-

tion, lack of fusion, and other discontinu ities in

welds.

d. Evaluate bond quality

in brazed joints and

honeycomb compo sites assembl ies.

e. Inspect forgings

such as turbine engine

shafts, turbine engine discs, and landing gear struc-

tural members.

3. LIMITS ON APPLICAT ION OF ULTRASONIC

TESTING.

Among the factors which may limit the

application of ultrasonic testing are:

a.

Sensitivity.

The abi l ity of the instrument to

detect the smal l amount of energy reflected from a

discontinui ty.

b. Resolution.

The abi l i ty of the instrumenta-

t ion to detect f laws lying close to the test surface or

to separate and dist inguish the indications from

several defects occurr ing close together in the

specimen.

c. Noise discrimination.

The capaci ty of the

instrume ntation for differentiating betwee n the sig-

nals from defects and the unwanted noise of ei ther

electr ical or acoustics nature.

d. These factors are affected by others,

such

as frequency and pulse energy. For example, when

frequency is increased, the sensi t iv i ty increases.

With the increase in sensi t iv i ty, smal ler inhomo-

geneties within the mater ial wi l l become detectable.

This w ill increase the noise level, thus hindering

signal discrimina tion. With an increase in pulse

energy, material noise will increase and resolution

will decrease.

4. ULTRASONIC INSPECTION FOR CORROSION

DETECTION.

Although ul trasonic inspections have

been employed by the aviation industry for several

years, it was not unti l recently that ul trasonics have

been used as a means of corrosion ,detection. Pres-

ently, this method of corrosion detection is st i l l in

the early sta ges and certainly not infall ible; but it

has been demonstrated that, wi thin l imitat ions,

ultrasonics can provide a fairly reliable indication

of corrosion attack. Highly trained personnel mu st

conduct the examination i f any useful information

is to be der ived from the indicating devices. This

is compounded by the fact that the resul ts obtained

vary, depending on the model and make of equip-

ment used, and on the techniques used by the in-

dividual pe rforming the exam ination.

5.-9. RESERVED.

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Chapter 2. TRANSDUCERS (SEARCH UNITS, PROBES, CRYSTALS)

.

10. GENERA L. To understand how inaudible

sound is used to reveal certain condit ions which are

not perceptible in the normal hearing range, it is

f i rst necessary to know how ul trasound is transmitted

and received.

11. TRAN SDUC ER MATE RIALS. Three transducer

mater ials which can be used in the manufacture

of ul trasonic search units are natural quartz crystals,

lithium s ulfate, and polarized crystall ine ceram ics.

a. Quartz.

Principal advan tages are : electrical

and therma l sta bility, insolubility in mo st l iquids,

high mechanical strength, wear resistance, e xcel lent

uniform ity and resistance to aging. A limitation

of quartz is i ts comparatively low electromechanical

conversion eff ic iency.

b. Lithium Sulfate. Principal advan tages are :

ease of obtaining optimum acoustic damping for

best resolution, intermediate conversion eff ic iency,

and negligible mode interaction.

c. Polar ized Ceramics. Pr incipal advantages

are: high conversion efficienc y which yields high

search uni t sensi t iv i ty.

Because of lower mechan-

ical strength and relatively high electrical capaci-

tance , their use is generally restricted to frequencies

below 15 Mega-Hertz (MHz). Another l imitat ion

is some interaction between var ious modes of vibra-

tion.

In order for a crysta l to util ize its piezoelec tric

chara cteristics, it is placed in a circuit mu ch like a

condenser. That is, both faces are coated with a

conducting mater ial wi th no contact between the

two faces. (See Figure 1.) Coatings for crystals

COAXIAL CABLE CONNECTION

k

PLATED BACK

CRYSTAL

PLATED FACE

FIGUR E l .-Angle Beam Search Unit

3

may be of any conducting mater ial such as alu-

minum , si lver, gold, or chromium. However, coat-

ings are difficult to depo sit on lithium sulphate

crystals so thin metal l ic foi ls are often cemented to

the crystal .

12. PIEZOE LECTR IC EFFE CT. Ultrasonic testing

may use the piezoelectric effect to generate ul tra-

sonic vibrations. Crystals, when subjected to an

alternating electric charge, expand and contra ct

under the inf luence of these charges. Conversely,

i t was found that these mater ials when subjected

to al ternating compression and tension developed

alternating electric ch arges on their face s. (See

Figure 2.) This was named the piezoelectric effect.

The heart of an ul trasonic testing system is this

method of convert ing electr ical energy into me-

chanical vibrations, and convert ing 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 Ultrason ic Vibration

Generation of the ultrasonic pulse is usually ac-

compl ished by producing a radio-frequency wave

train of the desired frequency at a precise t ime and

convert ing this into vibrations by means of piezo-

electr ic transducers. Some ul traconsic instruments

do not use a radio-frequency wave train, but in-

stead, use a shock pulse and al low the search un i t

to select the frequency of operation.

13. TRANSDUCE R TYPE SEARCH UNITS. The

search uni t consists of a shel l for mechanical pro-

tection, a means to conveniently handle or mount

the uni t for use, the transducer element, electr ical

connections, and a backing mater ial to dampen

the backward directed energy tha t is transmitted

by the crystal . (See Figure 3.)

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ELECT.

CONNECTION

, SHELL

BACKING

MATERIAL

I

SPECIMES

PIEZOELECTRIC

CRYSTAL

FIGURE 3.-Diagram of an Ultrasonic Search Unit

Transd ucers are available in a wide variety of

types which include :

a.

b.

X-Cut crysta ls for longitudinal-wave gen-

eration. (See Figure 4.)

Y-Cut crystals for shear-wave generation.

(See Figure 4.)

C.

Dual crystals wi th common holder.

d.

Mosaics-three or more crystals.

e.

High frequency, 50 MH z or more.

f.

Alternate crystal mater ials.

9.

Sandwich and tandem arrangements.

h.

Curved crysals to f i t the specimen.

i.

Wheel search uni ts.

i . Focused search uni ts.

k.

Temperature search uni ts (for measur ing

wal l thickness at temperature up to

l,lOOF).

% AXIS

X-CUT

X

/Y

Y-CLT

AXIS

AXIS

z AXIS

FIGURE 4.-Natural Quartz Crystal (X and Y Cut)

Transd ucers are available that are smaller than

l/a diameter and larger than 1 x 4. How ever,

for most ul trasonic testing, standard diameters of

l/b, l/2, and 1 .0 a re used .

14. TRANSDUCER GROUPS.

There are three gen-

eral groups of transm itter-receiver search units:

a. Commo n Transmitter-Receiver IT-RI.

These

search uni ts employ a single crystal and have com-

mon 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 uni t acts as both transmitter

and receiver, i t transmits a pulse of 1 to 4 micro-

seconds duration ; then acts as a receiver for a per iod

up to several thousand microseconds.

This cycle

of transm itting and receiving is repeated at a rate

of 50 to 5,000 time s per seco nd, or higher if re-

quired for high-speed autom atic s canning .

b. C o m b i n e d Transmitter-Receiver IT-RI.

These search uni ts have two transducers mounted

on a single head and insulated acou stically from

each other. One transducer is connected to the

pulser and the other is connected to the receiver.

The combined T-R search uni t is used for testing

close to the entry surface and for thickness measure-

ments from .040 to 2.0 when the opposi te side is

rough or corroded.

The transmitt ing search uni t

projects a beam o f vibrations into the mater ial ; the

- -

vibrations travel through

the material and are re-

F IGURE

6.-Combined Transmitter-Receiver Search Unit

4

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f leeted back to the receiving search uni t from any

discontinu ities 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 em-

ployed in these units having separate electrical con-

nections to the transmitter and the receiving uni ts.

One head is used as the transmitt ing uni t whi le the

other he ad is the receiving unit.

(See Figure 7.)

. Materials which are coarse grained tend to scat-

ter the ul trasonic sound beam; these mater ials can

be effectively inspected using separate T-R search

units that are mounted on individual wedges of a

suitable pla stic solid. When sep arate wedge s are

used, the angle of incidence may be var ied accord-

ing to the section thickness to be examined.

FIGURE 7.--Separate Transmitter-Receiver Search Unit

15.-20. RESERVED.

5

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Chapter 3. WA VE PROPAGATION

21. ULTRASONIC WAVES. Ultrasonic waves can

be propagated to some extent in any elasti c ma-

terial. This propagation occurs as a displacement

of the success ive 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 dis-

turbance 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 oscill ate about their origina l undisturbed

position until they come to rest. This molecule-to-

molecule 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 indi-

vidual wave amplitude, and the way the waves build

up and decay. The most used modes of vibration

are longitudina l, shear, surface, and plate waves.

FI G URE 8.-Mechanical Analogy of Wave Propagation

22. LONGITUDINAL WAVES. The wave is said

to be longitudinal (compressional) when the move-

ment 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 gen-

erated, detected, and has a high ve locity 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

F I G URE 9.-Longitudinal Waves

23. SHEAR WAVES. The wave is said to be

shear (transverse) when the movement of the par-

ticles 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 fre-

quency. This shorter wave length makes shear

waves more sensitive to small inclusions, and con-

sequently they are more easily scattered within the

specimen.

PARTICLE VIBRATION

DIRECTION OF PROPAGATION

FI G URE

IO.-Shear Waves

The principal advantage of these waves is in ap-

plicat ions that require an ultrasonic beam to be

transmitted into the test object at a small angle to

the surface. (See Figure 11.)

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PLASTIC \VEDCE

SPECIhlEN ,

makes surface waves so useful in surface f law de-

tection . (See Figure 13.)

FIGURE 13.~Surface Wave Technique

I

DIRECTION OF SOUND

25. PLATE (LAMB) WA VES. When ul trasonic vi-

VIBRATIO N DIRE CTIDS brations are introduced into a relatively thin sheet,

FIGURE Il.-Transmitting A Wave at a Sma ll Angle

the energy propagates in the form of plate wave s.

24. SURFA CE (RAYLEIGH) WAVES . Surface waves

travel with little attenuation in the direction of

propagation, but their energy decrea ses rapidly as

the wave penetrates below the surface.

The particle

displacement of the wave motion fol lows an el l ipt ical

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

Veloci ty of surface waves depends upon the ma-

terial an d is about nine-tenths of the shear-wa ve

veloci ty. Surface waves are l ikely to be affected,

in their propagation, by variations in hardne ss,

plated coatings, shot peening, or surface cracks, and

are easily dampen ed by dirt or grease on the sur-

face of the specimen. Surface waves can often be

produced as an unwanted effect, especial ly when

the contact surface is rough.

The wave s are not limited to f lat surfaces.

They

wi l l travel around curves and surface contours.

Sharp corners, such as the boundar ies of plates or

f laws, wi l l ref lect these waves. In fact, i t is this

charac teristic or traveling around contours which

Unl ike the longi tudinal, shear, and surface modes,

the plate w ave veloci ty is dependent both on fre-

quency and plate thickness. A complex part ic le

motion exists som ewh at like the elliptical orbits de-

scr ibed for surface waves. Greatly simpl i f ied, they

can be divided into two basic types: The sym-

metr ical type are known as di lat ional waves and the

asymm etr ical type as bending waves.

Examples of these two modes are i l lustrated in

Figure 14.

ASiMMETRIG

SYMMET R IC

FIGURE 14.-Plate Waves

Plate wave s 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 vibra-

t ion, lack of proper bonding can be detected.

26.-30. RESERVED.

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be reflected, and the rest will be allowed to pass

into the second medium, depending upon the im-

pedance ratio between the two materials. Also ,

the path traveled by the vibrations and whether

they return to their source depends upon the angles

at which the beam impinges upon the reflecting

surfaces, as well as the number and location of

these surfaces. (See Figure 15.)

33. REFRACTION AND MODE CONVERSION OF

ULTRASONIC WAVES.

Ultrasonic beams intro-

duced at an angle into a specimen are refracted.

The veloci ties in the wedge material and the metal

are different, therefore, the longitudinal vibrations

wil l be refracted when passing into the metal. (See

Figure 16.) At certain angles, conversion to other

ANGLE OF BEAM IN

WEDGE MATERIAL

- WEDGE MATERIAL

,TL ANGLE OF BEAM

cl,

IN TEST MATERIAL

FIGURE 16.-Refraction of the Ultrasonic Beam

modes of vibration, such as shear (see Figure 17)

and surface waves (see Figure 18) occurs.

WHERE : (p= angle normal to the beam in

the wedge.

/3= angle of refracted beam in

specimen.

cl = velocity of incident vibra-

tions in the wedge (usually

the longitudinal velocity).

c,= veloci ty of vibration in the

material under inspection

for the desired wave mode.

ANGLE OF BEAM IN

WEDGE

zc LDSCITUDINAL WAVE S

SHEAR WAVES

, CRYSTAL

a/ ANCLE OF BEAM IN

WAVES

I

I

ANGLE OF BEAJt

IN METAL

FIGURE 18.-Generating Surface Waves

34. BEAM DIVERGENCE. Beam divergence varies

with frequency and crysta l diameter. The higher

frequencies give more directivity to the sound beam.

Also , a large diameter crysta l is more directive

than one of a smaller diameter when operated at

the same frequency. (See Figure 19 and 20.)

QI?ARTZ

x

i

I

%/FT

I H/FT

B/FT 3/FT

FIGURE 19.-Beam Divergence of Sound Waves in Steel

FIGURE 20.-Diagram of Longitu dinal Sound

Striking a Surface

35&D. RESERVED.

ANGLE OF BEAM

IN hlETAL

FIGURE 17.-Generating Shear Waves

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-

41. GENERA L. There are two basic ul trasonic

systems: pulsed and resonance.

42. PULSED . The pulsed system may be ei ther

echo or through transmission. The pulse-echo is

the most versati le of the two pulse syste ms and

suitable for service and overhaul use.

a. Echo. Flaws are detected by measur ing the

amplitude of signals reflected and the time required

for these signals to travel between speci f ic surfaces

and discon tinuity. (See Figure 21).

TI.\IIW

CIttCt'IT -

I \/ I

CATIIOI)E RAY OSCILLOSC OPE

FIGURE 21,Block Diagram of Basic PuleEch o System

The time base, which is tr iggered simultaneously

with each transmission pulse, ca uses a spot to sweep

across the screen of the CRT . The spot sweeps

from left to r ight across the face of the scope 50 to

5,000 time s per secon d, or higher if required for

high-speed automated scanning. Due to the speed

of the cycle of transmitt ing and receiving, the pic-

ture on the osci l loscope appears to be stationary.

A few microseconds after the sweep is ini t iated,

the rate generator electrically excite s the pulser

and the pulser in turn em its an electrical pulse.

The transducer converts this pulse into a short train

of ul trasonic sound waves. If the inter faces of the

transduce r and the specim en a re properly orien-

11

Chapter 5. ULTRASONIC SYS TEM S

tated, the ul trasound wi l l be reflected back to the

transducer when i t reaches the internal f law and the

opposi te surface of the specimen. The t ime interval

between the transmission of the ini tial impulse and

the reception of the signals from within the speci-

men is measured by the t iming circui ts. The re-

f lected pulse that is received by the transducer is

ampl i f ied, then transmitted to the osci l loscope where

the pulse received from the f law is displayed on the

CR T screen in the same relationship to the front

and back pulse as the f law is in relation to the front

and back surface of the specimen. (See Figure

22.)

T R AN SD U C ER

TUBE

FIGURE 22.-Oscilloscope Display in Relationsh ip

to Flaw Detection

b. Through Transmissions. This system uses

only ampl i tude information and operates on the

pr inciple that certain speci f ic changes in the sample

will produce significant change s in the intens ity of

an ul trasonic beam passing through i t . This sys-

tem requires two transducers placed on opposi te

sides of the specimen. (See Figure 23.)

One

transducer transmits the wave through the piece and

the other picks up the signal . I f there is a defect

in the path of the wave, the received indication is

reduced in size to the degree that the signal is

blocked. This system is used extensively for bond

testing of laminated and clad mater ials where the

area of a laminar-type defect is of interest, and i ts

probable depth is ei ther known or of no importance.

This system is also useful in testing for metal-

lurgical changes due to heat, pressure, stress, and

fatigue.

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/ TRANSMITTER

I

Ci.T

I

C TRANShIITTER

3

RECEIVER /

RECEIVER

DEFECT

FIGURE

23.-Through-Transmission Technique

43. RESON ANCE. This system di ffers from the

pulse method in that the frequency of transmission

is, or can be, continuously var ied. The resonance

method is pr incipal ly used for thickness measure-

ments when the two sides of the mater ial under test

are smo oth and parallel. The point at which the

frequency matche s the resonance point of the ma-

ter ial under te st is the thickness determining factor.

TRANSDUCER

~R~~~c

F = F, ( FI~NDAMENT iI, FREQUENCi)

I I

FIGURE

24.-Conditions of Ultrasonic Resonance

in Metal Plate

12

It is necessary that the frequency of the ul trasonic

wav es, corresponding to a particular dial setting ,

should be accurately known. Checks should be

made with standard test blocks to guard against

4

t

possible dr i f t of frequency. If the frequency of

an ul trasonic wave is such that i ts wave length is

just twice the thickness of a specimen, then the

reflected wave wi l l arr ive back at the transducer

in the same phase as the or iginal transmission so

that strengthening of the signal , or a resonance,

wi l l occur. I f the frequency is increased so that

three t imes the wave length equals four t imes the

thickness, then the reflected signal wi ll return com-

pletely out of phase with the transmitted signal and

cancellation will occur. Further increase of the

frequency, so that the wave length is equal to the

thickness again, gives a reflected signal in phase

with the transmitted signal and resonance occurs

once more. By star t ing at the fundamental fre-

quency, where the wave length equals twice the

thickness and gradually increasing the frequency,

the successive cancel lations and reinforcements can

be noted and the readings used to chec k the original

or fundam ental, frequenc y reading. (See Figure

24.)

In some instruments, the osci llator ci rcui t con-

tains a motor-driven capaci tor which changes the

frequen cy of the oscillator. (See Figure 25.) In

other instruments, the frequency is changed by

electronic means.

t

c s

GENERATOR

FIGURE

25.-Block Diagram of Resonance Thickness

Measuring System

The change in frequency is synchronized with

the hor izontal sweep of a CRT . The horizontal

axis thus represents a frequency range. If the fre-

quency range contains resonances, the circui try is

arranged to present these vertically.

Calibrated

transparen t scales are then placed in front of the

tube and the thickness can be read directly. The

instruments normal ly operate between 0.25 MH z

and 10 MH z in four or f ive bands.

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Chapter 6. PRESENTATION

.

51. OBSERVING AND RECORDING RESPONSE

PATTE RNS. There are several methods o f observ-

ing and recording ul trasonic response patterns such

as a CRT , indicating l ights, alarm l ights, alarm

devices (bel ls, buzzers, etc.) , paint-spray markers,

str ip-chart and facsimi le recorders, photographic

representations, go/no-go monitors, and others.

These methods may be used in combination to sui t

a particular need.

52. CATHODE-RAY TUBE (CRT) .

a. Size: The screen sizes may vary from 3 to

12, h owev er, there is usually no need for providing

a signal larger than that which can be presented

by a 5 tube. The large screens do not provide

any more picture information. Usual ly a smal l

tube wi l l have better contrast and defini tion. The

primary purpose in using a large screen is in auto-

mated systems where the scanning transducer mus t

be posi t ioned some distance from the viewing screen

of the test instrument.

b. Signal Trace. Figure 26 shows the two mos t

comm on trace presentations. The RadioFrequency

(RF) presentation exhibi ts signals both above and

below the sweep line. This type of presentation

provides maximum resolution for locating defects

close to the surface or for separating signals follow-

ing closely upon one another. The video trace

presenta tion is a cleaner and less cluttered signal

than the RF presentation, however, i t provides for

less signal resolution becau se of its broader pulse

character ist ics. The video presentation is, in fact,

an RF presentation with the bottom hal f elec-

tronically off and only the outline of the original

pulses, added on to the top hal f, is shown. There-

fore, larger signals are indicated, but flaw definition

is more l imited. Of the two traces, the video is

mos t comm only used because i t is easier to read.

c. Range Markers. To provide a means of

measur ing the depth of a f law indication, square

waves are electronical ly super imposed on, or

beneath, the regular sweep l ine of the test presenta-

t ion. The lengths of these waves can be adjusted

to represent inches or feet of the mater ial being

tested. Figure 27 shows three of the most com-

monly used types of wave m arkers.

VIDEO

FIGURE 26.-Radio-Frequency Trace and Video Trace

The square-wave markers shown at the top are

easy to ,dist inguish from the spike- like echo signals.

They general ly provide the mos t precise means of

measur ing f law depth. The pyramidal markers

(center) are espec ially useful in an angle-beam

test where the sonic energy is sent into a piece at

an angle and bounces back and for th between the

wal ls as i t travels on through the part under test.

These m arkers simulate the actual path of the search

beam passing through the piece, making i t easier

to determine the f law depth with respect to the top

and bottom surfaces of he specimen. The bottom

marker i l lustrated di ffers from the other two types

in that i t does not have equal posi t ive and negative

values. These markers are a compressed negative

wave and deflect in the opposi te direction to the

echo signals.

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FIGURE 27.-Types of Marker System s

53. SCAN PRESE NTATIONS. The ul trasonic echoes

are electronically translated into visual presenta-

tions on the CR T. There are three different pres-

entations available ; the A-scan ; B-scan ; and C-

scan.

a. A-scan. In the A-scan presentation, the

horizontal base line on the screen indicates elapsed

time (from left to right) and represents the depth

of the test specimen. The vert ical deflection shows

response amplitude. The signal amplitude repre-

sents the intensi ties of transmitted or reflected

beams. This m ay be related to f law size, sample

attenuation , or other facto rs. (See Figure 28.)

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FIGURE

34.-Angle Beam Penetrat ing a Weld Bead

may be obtained representing both the outer and

inner surfaces of the tube together with pips from

any f laws which may be in the tube. The di ffer-

ences between the resul ts received in Figures 34 and

35 si tuations are due to the beam angles.

F IGURE

35.-Angle Beam Penetrat ing a Weld Bead

f . Figure 36 represents the inspection of a weld

in a heavy plate. The rough surface of the weld

bead prevents a direct overhead inspection of the

weld zone because the ul trasound beam cannot enter

near the surface of the rough bead. Therefore, the

transducer must be directed so the sound beam wi l l

be refracted to str ike the f law at r ight angles. The

ultrasound, when i t leaves the couplant and enters

the metal , wi l l refract in such a way that the beam

wil l be bouncing between the surfaces o f the

material.

When an ul trasonic beam enters metal at some

angle othe r than no rmal to the surfac e, the internal

angle will change rapidly with sm all change s in the

external angle. If the internal deviation angle ex-

ceeds approximately lS, both shear and longitu-

dinal waves will result.

Above 33O, longitudinal

waves wi l l disappear. Shear waves wi l l produce

the same indications on the screen a s longitudinal

waves . To produce shear waves in steel , set the

transducer to some angle above 33 .

*- ,

In shear-wave inspection, a rather sma l l and

poorly defined echo is obtained from the front s ur-

face of the specimen. However, a strong echo wi l l

be returned from vert ical f issures, or cracks, in the

mater ial being inspected. If the searching tube is

moved hor izontal ly wi thout changing i ts angle, the

flaw pip wil l move toward or away from the sur-

face pip. This is because the acoustic path between

the front surface and the f law is made stronger or

longer by the motion of the screening tube.

F IGURE

36.-A&e Beam Penetrat ing a Flat Plate

g. Figure 37 represents a situation in which the

front surface of the metal is so rough that a com-

plete scatter ing of the sound waves resul ts. In such

a situation , no appreciable penetration into the meta l

will occur and any echoe s that ordinarily appear

on the screen will also be dissipated by the rough-

ness and wil l not be received. The only possible

solution to such a problem is ei ther to have the

._ /

F IGU R E

37.-Results of a Rough Front an d Back Surface

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surface smoothed sufficiently, or to conduct the test

through a smoother surface from another angle. It

is also suggested that the lowest frequency possible

be used.

h. Figure 38 represents a typica l reading from

two different materials that have been brazed or

welded faceto-face. Where a good uniform bond-

ing has been obtained, a clear reading wil l be re-

ceived with normal front and back pips, and a

series of very small ridges. These ridges represent

the uneveness of the bonded surfaces. If, however,

the bonding is faulty and a void exists between the

two surfaces, the sound waves wil l be unable to

penetrate the void, and the pip from the bottom

plate will be lost entirely, and a series of pips like

tall grass wil l appear after the front surface in-

dication, which represents reverberations of the

sound waves within the top plate.

TOOL STEEL

+---.-L

FROST SlRFACELFLAM

FIGURE

38.-Evaluating Braze of Carbide Tip to Steel

i . Figure 39 represents an inspection of rela-

tive ly porous material. The front pip wil l be clear,

but the back pip will be either very small or non-

existent because the ultrasonic waves have either

been absorbed or dissipated by the porous nature

of the material. The clear pip of the front surface

wil l probably be followed by a long series of bumps,

or very short grass, which represents tiny echoes

received from the porous structure itself and do not

actually represent flaws.

If a significant flaw is

located within the material, it should create a pip

19

of sufficient size to be recognized. Ultrasound wil l

not, however, penetrate deeply into exceptionally

porous material.

As shown in Figure 37, lower

frequencies penetrate more readily than higher

frequencies.

As shown in Figure 32, distance

measuring wil l ascertain that pips are not actually

an echo from the back surface.

It may be necessary to inspect a sample involving

complex curved surfaces which do not fit any of the

preceding illustrations.

In such cases , the sound

should be directed to hit the flash-line zone or grain

flow at right angles since flaws would be oriented

along the flash line or the grain flow direction. As

shown in Figure 40, the probes are improperly

positioned, therefore, any indication received

would be erroneous.

-II)\5 OF HAc:h

SIIIFACE

IzIMc.4~rIos

41HFAt:E

I.ll(:E E\ol(;II

TO BE 5llO\\S

AS 4 \lAJOH FLAM

FIGURE

39.-Indication Received from Porous Material

I?

IO55IHll.lTl OF I;I..A\\b

FIGURE 40.-Irregular Part

55&O. RESERVED.

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OUTPUT

The gate of Figure 44 is suitable for a positive-

going input signal. The gate signal (also called a

control pulse, a selector pulse, or an enabling pulse)

is a rectangular wave form which makes abrupt

transitions between the negative levels -E, and

-E,.

-, ,

In Figure 45, -E, is assigned a value of minus

5 volts, and for a lo-volt input pulse, a 5-volt out-

put pulse appears. When used in this mann er, the

circuit is referred to as a threshold gate.

FIGURE 45.-Thresho ld Gate Wave Form 73.-75. RESERVED.

24

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Chapter 9. DELAY LINE

76. DESCR IPTION . By using a delay uni t to in-

crease resolution power, c lose-t-surface defects,

such as those normal ly found in spot welds, may be

investigated and evaluated.

The basic operation of a delay l ine depends on

the fact that acoust ic s ignals travel m uch s lower

than electrical signals. At the input end of the

line, the electrical signal to be delayed is converted

to an acoustica l signal by a transdu cer.

The signal travels along the delay l ine as an

acoust ic wave, requiring a speci f ic t ime to travel to

INPUT

r(&JJfJ,

DAhlPER DAhlPER

FI G URE %.-Ultrasonic Delay Time

25

the output end. At the output end, the signal is

converted back to i ts electrical form by another

transduc er. Figure, 46 i l lustrates one of a number

of methods which may .be used to delay or s low

down the electrical signal by converting it to the

slower traveling sound waves.

The ultrasonic delay line show n employs an input

and an output transduce r coil with a magn etostric-

t ive core w hich is attached to a sonic wave guide.

At the input end, the flux chang es in the coil caused

by the electrical input signal set up mecha nical

stress (v ibrations) in the score. These v ibrat ions

travel do wn the l ine to the output coil. Acou stic

absorbers (dampers) are used at both ends of the

line to prevent reflections along the wav e guide,

which would introduce a form of distort ion. A

given amount of delay can be accompl ished by

using different lengths of l ine.

77.-80. RESERVED.

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Chapter 10.

81. G ENERAL . A couplant is a l iquid having good

wett ing propert ies to transmit ul trasonic vibrations

from the transducer to test surfaces.

82. DESC RIPTIO N. Ultrasound generated by the

piezoe lectric effe ct, for all practical purpo ses, will

propagate in air due to its high freq uency and

short wave length. The actual movem ent of a piezo-

electrical crysta l vibration is in the micron range;

i t is often descr ibed as acceleration without motion.

To transmit such energy into a mater ial , use a f luid

couplant between the search uni t face and the ma-

FILM OF OIL

FIGURE

47.-Couplant (Contact Testing)

COUPLANT

terial s urface . A fi lm of oil, glycerine, or wate r is

general ly used. (See Figures 47 and 48.) When

water is used as the couplant, a wett ing agent should

be used to el iminate surface tension and faci l itate

wett ing of the surface. When i t is ei ther im-

practicable or undesirable to use oil or wate r, a

couplant paste is used.

I4

/v SEARCH VNIT

& COIIPLASTCOIIPLAST

II n-l /I

b

FIGURE

48.-C&plant (Immersion Tesing)

83.-85. RESERVED.

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Chapter 11. ULTRASONIC REFERENC EBLOCKS

86. INTRO DUCT ION. Ultrasonic testing should

not be accom plished without first calibrating the

equipme nt to the proper reference standa rd. Stand-

ard Reference Blocks are used to standardize the

ul trasonic equipment, set the sensi t iv i ty, and evaluate

the discontinuities in the material being inspec ted.

These blocks provide the comparison for any com-

bination of crysta l size, frequen cy, or test instru-

ment used to inspect mater ials. The evaluation of

the discontinuities within the material is accom -

plished by comparing the ultrasonic response from

the discontinu ity with the known artificial defec ts

(flat-bottome d holes or notche s) in the Standard

Reference Blocks. The test instrument should be

adjusted to indicate the hole representing the

smal lest defect i t is desired to pick up.

When

making a test set-up, care should be taken to assure

that the test block is of the same formulation al loy

with similar surface conditions as the material under

test. This precaution is necessary to assure that

distance cal ibration is correct. I f any adjustments

are made on the apparatus, or if the probes are

changed during the test , the probe and apparatus

should be checked again on the reference blocks

for sens itivity and proper functioning before re-

suming test.

87. D ISTANCE-AMPLITUDE

C 0 M P A R I S 0 N

BLOCKS.

a. Hi tt Blocks. Ul trasonic Standard Reference

Blocks designed to be used for distance amplitude

comparisons have a speci f ic size f lat-bottomed hole

placed at varying depths below the surface of the

material. The three sets have y&, 5h4, or 8h4

size f lat-bottomed holes. The metal distance of the

blocks normally used varies from $i6 or 6.0.

(See Figure 49.)

b. Calibration of

ultrasonic

thickness meas-

urement equipment may be accompl ished

a step-block. For exac t calibration, it

best to have a block whose thickness wi l l

by using

is always

give five

FIGURE

49.-Distance-Amplitude Comparison

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FIGURE 52.- I. I .W. Weld Block

FIGURE 53.-ASME Weld Reference Plate

92,95. RESERVED,

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SEARCH UNIT CRYSTAL

< TE ST PIECE

e

R ON T

t

ACK SURFA&

SURFACE

DISCONTINUITY

CRT PRESENTATION

BACK REFLECTION

-

DISCONTINUITY JNDICATION

c JNIT IAL PULSE (FRONT SURFACE)

FIGURE

57.-Principle of Ultrasonic Testing (Contact)

WATER PATH

L

B A C K & B F A C E

SEARCH UNIT

DISCONTINUITY

FRONT SURFACE

ENCTH OF WATER PATH

CRT PRESENTAnON

FLECTION

Y ,INDICATION

L

L INTERFACE S IGNAL

INIT IAL PULSE

FIGURE

58.-Principle of Ultrasonic Testing (Immersion)

a. Immersing both the search uni t and the ma-

terial in a liquid couplan t, norma lly wate r.

(See Figure 58.)

b. By a squirter or bubbler which directs a

column of f lowing water to form a couplant

between the face of the transducer and the

surface of the specimen. (See Figure 59.)

c. By a wheel-search uni t in which the transducer

is moun ted in a liquid-fi l led tire. The tire is

operated in direct contact wi th the mater ial .

(See Figure 60.)

SEARCH UNIT

COUPLANT SUPPLY

ULTRASONIC INSTR.

FIGURE 59.-Bubbler Angle-Be am Testing (Pipe)

LIQUID-FILLED TIRE

AXLE STATIONARY

CRYSTAL

FIGURE

60.-Wheel Scanning Method

99. IMMERSED SCANN ING TECHNIQUE. Both

the search unit and the part to be tested are totally

immersed in water. The sound beam is directed

into the material norma lly by a straight-beam

search uni t. Angle-beam techniques such as shear

or plate waves are accompl ished through control

and direction of the sound beam. (See Figures

61, 62, and 63.)

F IGURE

61.-Immersed Angle-Be am Technique

(Pipe or Tube)

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SEAR C H U N IT

FIGURE 62.-Immersed Angle-B eam Technique

(Plate or Sheet)

T R AN SMIT T IN G S .U .

RECEIVING S.U

focused through acoustic lens structures in a manner

simi lar to that achieved by glass lenses in optics.

The acou stic lens is an integral of the focused

search uni t a ssembly and is designed to provide a

wel l -defined and directed sound beam pattern. The

energy propagated from the search un i t can be con-

centrated through spherical lens structu re into cone-

shaped beams with high intensi ty. (See Figure 65 .)

I

\ -,

X

3

THIN PLATE

FIGURE

65.-Focused Search Unit

DEFECT

/ ENERGY REDUCED

BY DEFECT IN PATH OF BEAM

FIGURE

63.--Immersed Through-Transmission Technique

In immersion testing by the straight-beam tech-

nique, the water path distance (search uni t to front

surface of the test piece) is general ly set longer in

t ime than the length of scan ( front surface to back

-9

surface) so that the f i rst mult iple of the inter face

signal wi l l appear fur ther along the CRT sweep

than the back reflection. This is done to clear the

test area of signals which may cau se misinterpreta-

t ions. This is part icular ly important when the test

area is gated for autom atic signalling and recording

operation s. (See Figure 64.)

Q. Focused Search Unit.

The high-frequency

mechanical vibrations of the sound beam can be

I

WATER PATH

\ LENGTH OF WATER PATH

INTERFACE (ENTRANT

SIGNAL SURFACE 1

BACK REFLECTION

DEFECT SIGNAL

INITIAL PULSE

FIGURE

64.-Removing First Multip le of Interface

Indication From Test Area

35

b. Beam Col l imator.

Beam divergence var ies

with frequency and crystal diameter. The higher

frequencies give more directiv i ty to the sound beam

and a 1 diameter crystal at any frequency is more

directive than a I/ diameter crystal operating at

the same frequency. Therefore, to obtain more

directivity (which results in a better definition of

the defect) the frequency mus t be increased or a

larger diameter crystal mu st be used.

Increasing the frequency to obtain a better defi -

ni t ion of the defect often defeats the purpose of the

test, since the increase in frequency is also an in-

crease in sensi t iv i ty. The higher sensi t iv i ty discloses

irrelevant factors such as surface scratches, nicks,

and dents which make the defect evaluation di ff icul t.

Therefore, to obtain more directiv i ty of the sound

beam without increasing the frequency, a larger

diameter crystal is used. Since the defect under

examination may occupy only a smal l port ion of the

TRANSDUCER

/c BEAM

COLLl.\IATOR

TEST

d SPECI\IES

I

I /

-DEFECT

FIGURE

66.-Beam Col l imator

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IMAGE OF FLAW

TO ULTRASONIC

T.V. MON

FOCUSING

DEFLECI

CIRCUI

ELECTRON GUN

11;

PIEZOELECTRIC QUARTZ

F IGU R E

71.~Schemaic-Ultrasonic Image Converter System

103.- l 10. RESERVED.

k

ANK

WINDOW

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Chapter 13. INSPECTION OF INTEGRAL FUEL TANK S FOR CORROSION

111. GENERAL.

The existence of micro organ-

ism s in the fuel used in turbine-powered aircraft,

results in failure of integral fuel tank coatings and

corrosion of the wing skins.

Ultrasonic n ondestructive testing provides a

method of inspection for the ,detection and degree

of corrosion in integral wing fuel tank s.

112. METHOD.

Ultrasonic inspection techniques

are used for determining and recording the condi-

t ion of the interior surfaces of wet-wings. Longi-

tudinal waves should be used since shear and plate

waves wi l l not accurately establ ish the definit ion or

extent of corrosion pi ts.

113. EQUIPMENT.

To scan the underneath sur-

face of an aircraft wing, the fol lowing equipment is

util ized :

a. A wheel search unit.

To assure a satis-

factory coupl ing, the surface to be scanned is

moistened.

b. An oscilloscope

with a fast pulse repetit ion

rate. This is necessary to conduct a rapid

scan of 500 square inches or more in I5

minutes.

c.

A C-scan

facsimile recorder.

d.

An

automatic and manual ly

controlled scan-

ning bridge and carriage.

e. A positioning mechanism,

scanner support

structure, and l i f t platform. This equipment

places the sheel search unit in a position to

scan the lower surface of the wing and pro-

vides the apparatus necessary to relate the

motion of the search uni t to the facsimi le re-

corder. This equipment also permits a rapid

engagement of the scanner with the aircraft

and requires no jacking or other handling of

the aircraft, nor is i t necessary to drain the

fuel tanks.

114. INDICATIONS.

Intergranular corrosion ap-

pears on the C-sca n recording as a solid area

projecting outward from fastener holes or other

locations where there is a transverse cut exposing

the edge of the plate. Large or smal l areas of

pi t type corrosion may appear anywhere on the

recording with high-den sity areas giving a mottled

appearance to the recording. Thus, the extent and

the kind of corrosion can be determined from the

recording.

115.- l 20. RESERVED.

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Chapter 14. INSPECTION OF REPRESENTATIVE AIRCRAFT PARTS

121. GENERA L. A few representative inspections

by the contact method are discussed.

122. MAIN AND NOSE LANDING GEAR WHEELS.

Figure 72 il lustrates the part to be exam ined. A

surface-wave probe is used to scan around the

wheel web for cracks occurr ing adjacent to the

bosses of the t ie-bol t holes. The surface wave is

able to detect cracks which are not always shown

up by other methods of examination.

The fre-

quency used is 2.5 MH z and the depth of penetra-

tion is approxim ately 0.060.

For cracks occurr ing in the zone of the t i re a reas,

a second probe, having an angle o f 30 refraction,

is sometime s employed for testing the bead seat

radius where the test is carried out without dis-

assembly of the wheel. The angle is necessary to

direct the beam away from an y reflecting surfaces,

such as changes in contour, into the zone where de-

fects occur.

SECTION OF WHEEL

THIS AREA

CLEAN AND ZNSPECT

THIS AREA

FIGURE

72.-Main and Nose Landin g Gear Wheels

123. MAIN LANDING GEAR TORSION L INK. A

2.5 MH z surface-wave probe is appl ied to the

member on i ts surface with the beam directed

towards suspected zones. It is normal to scan most

of the surface of the torsion l ink for fat igue cracks

in any position occurring from the highly stresse d

areas. (See Figures 72 and 73).

F IGURE

73.-Main Landin g Gear Torsion Link

124. MAIN LANDING GEAR TORSION LINK

LUGS. A surface-wave probe is used on the lug

surfaces and on the thickened boss section for fa-

tigue crac ks occurring in random directions. It is

necessary to direct the ul trasonic sound beam

toward s the crack at an angle o f 90 in order to

F IGURE

74.-Main Landing Gear Torsion Link Lugs

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scanning is undertaken in two directions for the

,detect ion of cracks which normal ly occur on the

b

inner surface but which can start on the outer

surface. The two addi tional diagrams on the

sketch show the method scanning.

128. INBOARD AND OUTBOARD NACELLE

STRU T FRON T SPAR FITTINGS. Mos t o f the ex -

aminat ion of these components is undertaken on the

curved sect ions of the lugs. The angle of the beam

is approxim ately 20 from the tangent of the circle.

To obtain this angle, th e probe mu st have an inci-

dent angle of approxim ately 9O. Inspection is un-

dertaken around the whole of the curved surface

which gives more or less complete coverage of the

bolt hole by moving a probe in two directions. In

order to di rect the beam towards any posi t ion of

the hole in the lug, the probe has to be move d

acros s the lug section from side to side while being

rotated around the curved surface.

This leaves a

PROBES

FIGURE

78.4nboard and Outboard Nac elle Strut

Front Spar Fitting

smal l sect ion of the hole at the back of the lug

which is not inspected. This is undertaken by a

probe having a smaller angle and a slight taper on

the lug surface. Both upper and lower parts of the

lug have to be inspected by this method so that the

entire bolt-hole section is exam ined. To facil i tate

examination from the curved surface, the probe

surface is also curved. On some ai rcraft , this radius

di f fers so that two probes would normal ly be neces-

sary. Practice has indicated that a probe having

a larger radius can be used on lugs of the smaller

radius. (See Figure 78.)

129. NOSE LANDING GEAR OUTER CYLINDER.

Figure 79 shows the area of test undertaken using

surface-wave probe wi th a frequency of 2.5 MH z.

The probe is applied to the curved surface of the

cyl inder and the beam directed towards the lug.

The sound wave fol lows the surface of the material

and pene trates the flat surface of the lug for detec-

t ion of defects from the corner of the sect ions into

the lug zones marked on the sketch.

\PROBE

FIGURE

79.-Nose Landing Gear Outer Cylinder

130-135. RESERVED.

43

,? . S. GOVERNMENT PRINTING OFFICE : 1975 0 - 569- 931