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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
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23
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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 _~-__~~-~---~~_-~~-~-~---~---~~--~--------~---~~--~~~--~--~---~~-
29
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30
30
30
30
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.
27
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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
29
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FIGURE 52.- I. I .W. Weld Block
FIGURE 53.-ASME Weld Reference Plate
92,95. RESERVED,
31
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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)
34
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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
41
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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
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