Ultrasonic Testing
By
Eng. Ibrahim
b hi Eldesoky
ld k
1
ULTRASONIC INSPECTION
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ULTRASONIC INSPECTION
is a nondestructive
method in which beams of high
high--frequency sound waves are
introduced into materials for the detection of surface and
subsurface flaws in the material.
material. The sound waves travel through
the material with some attendant loss of energy (attenuation) and
are reflected at interfaces.
interfaces. The reflected beam is displayed and
then analyzed to define the presence and location of flaws or
discontinuities..
discontinuities
Most ultrasonic inspection is done at frequencies between 0.1 and
25 MHz well above the range of human hearing,
hearing which is about
20 Hz to 20 kHz
kHz.. Ultrasonic waves are mechanical vibrations;
vibrations; the
amplitudes of vibrations in metal parts being ultrasonically
inspected
i
d impose
i
stresses well
ll below
b l
the
h elastic
l i limit,
li i thus
h
preventing permanent effects on the parts.
parts.
2
Advantages and Disadvantages
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Superior penetrating power, which allows the detection of flaws
deep in the part.
part. Ultrasonic inspection is done routinely to
thicknesses of a few meters on many types of parts and to
thicknesses of about 6 m
(20 ft) in the axial inspection of parts such as long steel shafts or
rotor forgings
High sensitivity, permitting the detection of extremely small flaws
Greater accuracy than other nondestructive methods in
determining the position of internal flaws, estimating their size,
and characterizing their orientation, shape, and nature
Only one surface needs to be accessible
3
Advantages and Disadvantages
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Operation
p
is electronic,, which p
provides almost instantaneous
indications of flaws
flaws.. This makes the method suitable for
immediate interpretation, automation, rapid scanning, in
in--line
production monitoring,
monitoring and process control
control..
With most systems, a permanent record of inspection results can
be made for future reference
Volumetric scanning ability, enabling the inspection of a volume
of metal extending from front surface to back surface of a part
N h
Nonhazardous
d
t operations
to
ti
or to
t nearby
b personnell and
d has
h no
effect on equipment and materials in the vicinity Portability
Provides an output that can be processed digitally by a computer
to characterize defects and to determine material properties
4
The disadvantages
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Manual operation requires careful attention by experienced
technicians
Extensive technical knowledge is required for the development of
inspection procedures
Parts that are rough, irregular in shape, very small or thin, or not
homogeneous are difficult to inspect
Discontinuities that are present in a shallow layer immediately
beneath the surface may not be detectable
C
Couplants
l t are needed
d d to
t provide
id effective
ff ti transfer
t
f off ultrasonic
lt
i
wave energy between transducers and parts being inspected
·Reference
Reference standards are needed, both for calibrating the
equipment and for characterizing flaws
5
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Some of the major types of equipment that are ultrasonically
inspected for the presence of flaws are:
are:
Mill components
components:: Rolls,
Rolls shafts,
shafts drives,
drives and press columns
Power equipment:
equipment: Turbine forgings, generator rotors, pressure
piping, weldments
weldments,, pressure vessels, nuclear fuel elements, and
other reactor components
Jet engine parts:
parts: Turbine and compressor forgings, and gear
blanks
Aircraft components:
components: Forging stock, frame sections, and
honeycomb
y
sandwich assemblies
Machinery materials:
materials: Die blocks, tool steels, and drill pipe
Railroad parts
parts:: Axles, wheels, track, and welded rail
Automotive parts
parts:: Forgings, ductile castings, and brazed and/or
6
welded components
General Characteristics of Ultrasonic
Waves
„
„
Ultrasonic waves are mechanical waves ((in contrast to, for example,
p
light or x-rays, which are electromagnetic waves) that consist of
oscillations or vibrations of the atomic or molecular particles of a
substance about the equilibrium
q
positions of these p
p
particles..
particles
Ultrasonic waves behave essentially the same as audible sound
waves.. They can propagate in an elastic medium, which can be
waves
solid,, liquid,
q , or gaseous,
g
, but not in a vacuum.
vacuum.
Analogy With Waves in Water. The general characteristics of sonic
or ultrasonic waves are conveniently illustrated by analogy with the
behavior of waves produced in a body of water when a stone is
dropped into it. Casual observation might lead to the erroneous
conclusion that the resulting outward radial travel of alternate
crests and troughs represents the movement of water
7
General Characteristics of Ultrasonic
Waves
„
„
away from the point of impact. The fact that water is not thus
transported
d iis readily
dil deduced
d d d ffrom the
h observation
b
i that
h a small
ll
object floating on the water does not move away from the point of
impact, but instead
I many respects,
In
t a b
beam off ultrasound
lt
d iis similar
i il to
t a beam
b
off
light; both are waves and obey a general wave equation. Each
travels at a characteristic velocity in a given homogeneous medium
a velocity that depends on the properties of the medium
medium, not on the
properties of the wave. Like beams of light, ultrasonic beams are
reflected from surfaces, refracted when they cross a boundary
between two substances that have different characteristic sound
velocities, and diffracted at edges or around obstacles. Scattering
by rough surfaces or particles reduces the energy of an ultrasonic
beam, comparable to the manner in which scattering reduces the
i t it off a light
intensity
li ht beam.
b
8
M j Variables
Major
V i bl in
i
Ult
Ultrasonic
i Inspection
I
ti
9
Major Variables in Ultrasonic Inspection
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The frequency of the ultrasonic waves used affects inspection
capability in several ways. Generally, a compromise must be
made between favorable and adverse effects to achieve an
optimum balance and to overcome the limitations imposed by
equipment and test material.
Sensitivity the ability of an ultrasonic inspection system to
detect a very small discontinuity, is generally increased by using
relatively high frequencies (short wave-lengths).
Resol tion, or the
Resolution
th ability
bilit off the
th system
t
to
t give
i simultaneous,
i lt
separate indications from discontinuities that are close together
both in depth below the front surface of the test piece and in
lateral position
position, is directly proportional to frequency
band-width and inversely related to pulse length. Resolution
generally improves with an increase of frequency.
10
Major Variables in Ultrasonic Inspection
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„
Penetration,,
or the maximum depth
p ((range)
g ) in a material
from which useful indications can be detected, is reduced by the
use of high frequencies.
frequencies. This effect is most pronounced in the
inspection of metal that has coarse grain structure or minute
i h
inhomogeneities,
ii
b
because
off the
h resultant
l
scattering
i
off the
h
ultrasonic waves;
waves; it is of little consequence in the inspection of
fine--grain, homogeneous metal.
fine
metal.
B
Beam
spread,
d or the divergence of an ultrasonic beam from
the central axis of the beam, is also affected by frequency.
frequency. As
frequency decreases, the shape of an ultrasonic beam
i
increasingly
i l departs
d
t from
f
th ideal
the
id l off zero beam
b
spread.
spread
d. This
Thi
characteristic is so pronounced as to be observed at almost all
frequencies used in inspection.
inspection.
11
Major Variables in Ultrasonic Inspection
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Acoustic Impedance
When ultrasonic waves traveling
through one medium impinge on the boundary of a second
medium, a portion of the incident acoustic energy is reflected
back from the boundary while the remaining energy is
transmitted into the second medium. The characteristic that
determines the amount of reflection is the acoustic impedance
p
of
the two materials on either side of the boundary.
If the impedances of the two materials are equal, there will be
no reflection;
fl ti
if the
th impedances
i
d
differ
diff greatly
tl (as
( between
b t
a
metal and air, for example), there will be virtually complete
reflection.
12
Major Variables in Ultrasonic Inspection
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The acoustic impedance
p
for a longitudinal
g
wave, Z, ggiven in
grams per square centimetercentimeter-second, is defined as the product
of material density, p, given in grams per cubic centimeter, and
longitudinal
g
wave velocity,
y, V,, given
g
in centimeters p
per second:
Z = p V,
The acoustic properties of several metals and nonmetals are
listed in following table. The acoustic properties of metals and
alloys are influenced by variations in structure and
metallurgical condition. The percentage of incident energy
reflected from the interface between two materials depends on
the ratio of acoustic impedances (Z2/Z1) and the angle of
incidence. When the angle of incidence is 0°
0° (normal incidence)
13
Major Variables in Ultrasonic Inspection
„
the reflection coefficient, R, which is the ratio of reflected beam
i
intensity,
i Ir, to incident
i id
beam
b
iintensity,
i Ii, is
i given
i
by
b R = Ir/Ii =
[(Z2 – Z1)/(Z2 + Z1)]2 where Z1, is the acoustic impedance of
medium 1, Z2 is the acoustic impedance of medium 2, With T
designating the transmission R + T = 100%,
100% because all the
energy is either reflected or transmitted, and T is simply
obtained from this relation. The transmission coefficient, T, can
aalso
so be ca
calculated
cu ated as tthee ratio
at o oof tthee intensity
te s ty oof tthee transmitted
t a s tted
beam, It, to that of the incident beam, Ii from:
T = It/Ii = 4Z2Z1/(Z2 + Z1)2
14
Major Variables in Ultrasonic Inspection
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Angle of Incidence
At any other angle of incidence, the
phenomena of mode conversion (a change in the nature of the wave
motion)
ti ) and
d refraction
f ti (a
( change
h
iin di
direction
ti off wave propagation)
ti )
must be considered. These phenomena may affect the entire beam or
only a portion of the beam, and the sum total of the changes that
occur att th
the iinterface
t f
d
depends
d on th
the angle
l off iincidence
id
and
d th
the
velocity of the ultrasonic waves leaving the point of impingement on
the interface.
Al possible ultrasonic waves leaving this point are shown for an
incident longitudinal ultrasonic wave in Fig. 5. Not all the waves
g 5 will be p
produced in any
y specific
p
instance of oblique
q
shown in Fig.
impingement of an ultrasonic wave on the interface between two
materials. The waves that propagate in a given instance depend on
g of
the abilityy of a waveform to exist in a ggiven material,, the angle
incidence of the initial beam, and the velocities of the waveforms in
15
both materials.
Major Variables in Ultrasonic Inspection
Diagram showing relationship (by vectors) of all possible
reflected and refracted waves to an incident longitudinal wave
of velocity Vl(1) impinging on an interface at angle α, relative to
normal to the interface.
16
Major Variables in Ultrasonic Inspection
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The general law that describes wave behavior at an interface is
known as Snell’s law.
sin α/sin β= V1 / V2
Critical Angles
If the angle of incidence (α, Fig. ) is small, sound waves
propagating in a given medium may undergo mode conversion
at a boundary, resulting in the simultaneous propagation of
longitudinal and transverse (shear) waves in a second medium
medium.
If the angle is increased, the direction of the refracted
longitudinal wave will approach the plane of the boundary (β =>
90°°). At some specific value of α, β will exactly equal 90
90
90°°, above
which the refracted longitudinal wave will no longer propagate
in the material, leaving only a refracted (mode(mode-converted) shear
wave to propagate in the second medium. This value of α, is
k
known
as the
h fi
first critical
i i l angle.
l
17
Major Variables in Ultrasonic Inspection
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If α, is increased beyond the first critical angle, the direction of
the
h refracted
f
d shear
h
wave will
ill approach
h the
h plane
l
off the
h
boundary (β => 90°
90°). At a second specific value of α, β will
exactly equal 90°
90°, above which the refracted transverse wave
will no longer propagate in the material.
material This second value of aa,
is called the second critical angle.
Critical angles are of special importance in ultrasonic
inspection
inspection. Values of α
α, between the first and second critical
angles are required for most angleangle-beam inspections. Surface
wave inspection is accomplished by adjusting the incident angle
of a contactcontact-type
yp search unit so that it is a few tenths of a degree
g
greater than the second critical angle. At this value, the
refracted shear wave in the bulk material is replaced by a
Rayleigh wave traveling along the surface of the test piece.
18
Attenuation of Ultrasonic
Beams
Beams
19
Attenuation of Ultrasonic Beams
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The intensity
i
i off an ultrasonic
i beam that is
i sensed by a receiving
i i
transducer is considerably less than the intensity of the initial
transmission. The factors that are primarily responsible for the
loss in beam intensity can be classified as transmission losses,
losses
interference effects, and beam spreading
Acoustic impedance
effects can be used to calculate the
amount off sound
d that
h reflects
fl
d
during
i the
h ultrasonic
l
i inspection
i
i off
a test piece immersed in water. For example, when an ultrasonic
wave im-pinges at normal incidence (α = 00°°) to the surface of the
flaw--free section of aluminum alloy 1100 plate during straightflaw
straightbeam inspection, the amount of sound that returns to the search
unit (known as the back reflection) has only 6% of its original
intensity,1.4%
y, % for stainless steel and 1.3%
% for carbon steel. This
reduction in intensity occurs because of energy partition when
waves are only partly reflected at the aluminum/water
interfaces.
20
Attenuation of Ultrasonic Beams
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The sound intensity of contact techniques is usually greater than
that
h off iimmersion
i techniques;
h i
that
h is,
i smaller
ll discontinuities
di
i i i will
ill
result in higher amplitude signals. Two factors are mainly
responsible for this difference, as follows:
Fi t the
First,
th back
b k surface
f
off the
th ttestt piece
i
i a metal/air
is
t l/ i interface,
i t f
which can be considered a total reflector. Compared to a metal/
water interface, this results in an approximately 30% increase
in back reflection intensity at the receiving search unit for an
aluminum test piece coupled to the search unit through a layer
of water.
Second, if a couplant whose acoustic impedance more nearly
matches that of the test piece is substituted for the water, more
energy is transmitted across the interface for both the incident
and returningg beams.
21
Attenuation of Ultrasonic Beams
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The absorption
p
The absorption of ultrasonic energy occurs mainly by the
conversion of mechanical energy into heat. Elastic motion within
a substance as a sound wave p
propagates
p g
through
g it alternately
y
heats the substance during compression and cools it during
rare-faction. Because heat flows so much more slowly than an
ultrasonic wave, thermal losses are incurred, and this
progressively
i l reduces
d
energy in
i the
th propagating
ti wave
Absorption can be thought of as a braking action on the motion
of oscillating particles. This braking action is more pronounced
when
hen oscillations are more rapid,
rapid that is,
is at high frequencies.
freq encies
For most materials, ab-sorption losses increase directly with
frequency.
22
Attenuation of Ultrasonic Beams
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Scattering
Scatteringg of an ultrasonic wave occurs because most materials
are not truly homogeneous. Crystal discontinuities, such as
grain boundaries, twin boundaries, and minute nonmetallic
inclusions, tend to deflect small amounts of ultrasonic energy
outt off the
th main
i ultrasonic
lt
i beam.
b
IIn addition,
dditi
especially
i ll in
i mixed
i d
microstructures or anisotropic materials, mode conversion at
crystalline boundaries tends to occur be-cause of slight
differences in acoustic veloc-ity and acoustic impedance across
the boundaries.
Scattering is highly dependent on the relation of crystallite size
(mainly grain size) to ultrasonic wavelength.
wavelength When grain size is
less than 0.01 times the wavelength, scatter is negligible.
Scattering effects vary approximately with the third power of
ggrain size,, and when the ggrain size is 0.1 times the wavelength
g or
larger, excessive scattering may make it impossible to conduct
valid ultrasonic inspections.
23
Attenuation of Ultrasonic Beams
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In some cases, determination of the degree of scattering can be
used as a basis for acceptance or rejection of parts. Some cast
irons can be inspected for the size and distribution of graphite
flakes, the size and distribution of microscopic voids in some
powder metallurgy parts, or of strengthened in some fiberfiberreinforced or dispersionp
-strengthened
g
materials, can be evaluated
dispersion
by measuring attenuation (scattering) of an ultrasonic beam.
Diffraction.
A sound beam propagating in a homogeneous
medium is coherent; that is, all particles that lie along any given
plane parallel to the wave front vibrate in identical patterns.
When a wave front passes the edge of a reflecting surface, the
front bends around the edge in a manner similar to that in which
light bends around the edge of an opaque object. When the
reflector is very small compared to the sound beam, as is usual for
a pore or an inclusion, wave bending (forward scattering) around
the edges of the reflector produces an interference pattern in a
zone immediately behind the reflector because of phase
24
differences among different portions of the forwardforward-scattered
beam.
Attenuation of Ultrasonic Beams
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Near-Field and Far
NearFar--Field Effects
The face of an ultrasonicultrasonic-transducer crystal does not vibrate
uniformlyy under the influence of an impressed
p
electrical voltage.
g
Rather, the crystal face vibrates in a com-plex manner that can be
most easily de-scribed as a mosaic of tiny, individual crystals, each
vibrating in the same direction but slightly out of phase with its
neighbors.
i hb
E
Each
h element
l
t iin the
th mosaic
i acts
t like
lik a point
i t (Huygens)
(H
)
source and radiates a spherical wave outward from the plane of the
crystal face. Near the face of the crystal, the composite sound beam
propagates chiefly as a plane wave,
wave although spherical waves
emanating from the periphery of the crystal face produce shortshortrange ultrasonic beams referred to as side lobes.
Because of interference effects, as these spherical waves encounter
one another in the region near the crystal face, a spatial pattern of
acoustic pressure maximums and minimums is set up in the
composite sound beam. The region in which these maximums and
minimums oc-cur is known as the near field (Fresnel field) of the
sound beam.
25
Attenuation of Ultrasonic Beams
„
„
Along the central axis of the composite sound beam, the series of
acoustic
i pressure maximums
i
and
d minimums
i i
becomes
b
broader
b d
and more widely spaced as the distance from the crystal face, d,
increases. Where d becomes equal to N (with N denoting the
length of the near field),
field) the acoustic pres-sure
pres sure reaches a final
maximum and decreases approximately exponentially with
increasing distance, as shown in Fig. 8.
The length of the near field is determined by the size of the
radiating crystal and the wave-length, λ, of the ultrasonic wave.
For a circular radiator of diameter D, the length of the near
field can be calculated from:
D2
N= —
4λ
26
Attenuation of Ultrasonic Beams
27
Attenuation of Ultrasonic Beams
„
Near-field and far
Nearfar--field effects also occur when ultrasonic waves
are reflected from interfaces. The reasons are similar to those
for near
near--field and far
far--field effects for transducer crystals; that
is reflecting interfaces do not vibrate uniformly in response to
is,
the acoustic pressure of an impinging sound wave.
28
Search Unit
„
Transducer Elements
The generation and detection of ultrasonic waves for inspection
are accomplished
p
by
y means of a transducer element acting
g
through a couplant. The transducer element is contained within a
device most often referred to as a search unit (or sometimes as a
probe). Piezoelectric elements are the most commonly used
t
transducer
d
iin ultra­
ultra
lt ­sonic
i inspection
i
ti
„
Piezoelectric Transducers
Piezoelectricity is pressure
pressure--induced electricity; this property is
characteristic of certain naturally occurring crystalline
compounds and some manmade materials. As the name
piezoelectric implies,
p
p
an electrical charge
g is developed
p
by
y the
crystal when pressure is applied to it. Conversely, when an
electrical field is applied, the crystal mechanically deforms
(changes shape).
29
Search Unit
„
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„
Piezoelectric crystals exhibit various deformation modes;
thickness expansion is the principal mode used in transducers
for ultrasonic inspection.
The most common types of piezoelectric materials used for
ultrasonic search units are quartz, lithium sulfate, and polarized
ceramics such as barium titanate, lead zirconate titanate, and
lead metaniobate.
Quartz crystals were initially the only piezoelectric elements
used in commercial ultrasonic transducers.. Principal
p
advantages of quartz
quartz--crystal transducer elements are electrical
and thermal stability, insolubility in most liquids, high
mechanical strength, wear resistance, excellent uniformity, and
resistance
it
to
t aging.
i
A limitation
li it ti off quartz
t is
i its
it comparatively
ti l
low electromechanical conversion efficiency
efficiency
30
Search Unit
„
„
Lithi
Lithium
Sulfate
S lf t
The principal advantages of lithium
sulfate transducer elements are ease of obtaining optimum
receivingg characteristics,, intermediate conversion efficiency,
y,
and negligible mode interaction. The main disadvantages of
lithium sulfate elements are fragility and a maximum service
temperature
p
of about 75 °C ((165 °F).
)
Polarized ceramics (Barium titanate) generally have
high electromechanical conversion efficiency, which results in
good
d searchsearch
h-unit
it sensitivity
iti it and
d is
i good
d as a transmitter
t
itt
.
Barium titanate is mechanically rugged. However, its efficiency
changes with temperature, and it tends to depolarize with age,
which
hi h makes
k barium
b i
titanate
tit
t less
l suitable
it bl for
f some applications
li ti
31
Search Unit
32
Search Unit
33
Search Unit
„
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„
Search units with piezoelectric transducers are available in
many types and
d shapes.
h
V
Variations
i i
iin searchsearch
h-unit
i construction
i
include transducer
transducer--element material; transducertransducer-element
thickness, surface area, and shape; and type of backing material
and degree of loading
loading.
The four basic types of search units are the straightstraight-beam
contact type, the angle
angle--beam contact type, the dualdual-element
contact type
type, and the immersion type
Contact--Type Units
Contact
Theyy are usually
y hand held and manuallyy scanned in direct
contact with the surface of a test piece. A thin layer of an
appropriate couplant is almost always required for obtaining
transmission of sound energy across the interface between the
search unit and the entry surface.
surface
34
Search Unit
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Straight-Beam Units
StraightThis unit is hand held and manually scanned in direct contact with
the surface of the test p
piece This type
yp of search unit p
projects
j
a
beam of ultrasonic vibrations perpendicular to the entry surface. It
can be used for either the reflection (echo) method or the through
transmission method.
Angle--Beam Units
Angle
A plastic wedge between the piezoelectric element and the contact
surface establishes a fixed angle of incidence for the search unit,
A l -beam
AngleAngle
b
search
h units
i are used
d ffor the
h iinspection
i
off sheet
h
or
plate, pipe welds or tubing, and test pieces having shapes that
prevent access for straight beam.
DualD l-Element
Dual
El
tU
Units
it
Dual--element units provide a method of increasing the directivity
Dual
and resolution capabilities (especially nearnear-surface resolution) in
contact inspection
inspection. By splitting the transmitting and receiving
functions, two
two--transducer, send and receive inspection can be done
35
with a single search unit
Search Unit
„
„
„
Delay-Tip Units
DelayThe delay shoe allows the indication from the front surface of the
test piece to be delayed by the transmission time through the delay
shoe.
Paintbrush Transducers
Paintbrush transducers are usually constructed of a mosaic or
series of matched crystal elements, it has a wide beam pattern that,
when scanned, covers a relatively wide swath in the manner of a
p
paintbrush
Immersion--Type Units
Immersion
The advantages of immersion inspection include speed of inspection,
ability to control and direct sound beams, and adaptability for
automated scanning. Angulations is used in immersion inspection to
identify more accurately the orientation of flaws below the surface
of the test piece A rule of thumb is to make the water path equal to
one--fourth the test piece thickness plus 6 mm (1/4 in.).
one
36
Search Unit
Focused Units
Sound can be focused by acoustic lenses in a manner similar to that in
which light is focused by optic lenses.
lenses Most acoustic lenses are
designed to concentrate sound energy, which increases beam
intensity in the zone between the lens and the focal point.
„ The advantages of focused search units are listed below; these
advantages apply mainly to the useful thickness range of 0.25 to 250
mm (0.010 to 10 in.) below the front surface:
High sensitivity to small flaws
High resolving power
Low effects of surface roughness
L effects
Low
ff t off ffrontfrontt-surface
f
contour
t
Low metal noise (background)
„
37
Search Unit
38
Search Unit
39
Pulse--Echo Methods
Pulse
„
In p
pulse-echo inspection,
pulsep
short bursts of ultrasonic energy
gy
(pulses) are introduced into a test piece at regular intervals of
time. If the pulses encounter a reflecting surface, some or all of
the energy
gy is reflected. The p
proportion
p
of energy
gy that is reflected
is highly dependent on the size of the reflecting surface in
relation to the size of the incident ultrasonic beam. The direction
of the reflected beam ((echo)) depends
p
on the orientation of the
reflecting surface with respect to the incident beam. Reflected
energy is monitored; both the amount of energy reflected in a
specific
p
direction and the time delay
y between transmission of the
initial pulse and receipt of the echo are measured
40
Pulse--Echo Methods
Pulse
Principles of PulsePulse-Echo Methods
Most pulse
pulse--echo systems consist of:
„ An electronic clock
„ An electronic signal generator, or pulser
„ A sending
di ttransducer
d
„ A receiving transducer
„ An echo
echo--signal amplifier
„ A display device
„
41
Pulse--Echo Methods
Pulse
„
„
At regular intervals
intervals, the electronic clock triggers the signal
generator, which imposes a short interval of highhigh-frequency
alternating voltage on the transducer. Simultaneously, the clock
activates a time
time--measuring circuit connected to the display device.
The operator can preselect a constant interval between pulses by
means of a pulsepulse-repetition rate control on the instrument; pulses
are usually repeated 60 to 2000 times per second. In most
commercially available flaw detectors, the pulsepulse-repetition rate is
controlled automatically.
The transducer then converts the pulse of voltage into a pulse of
mechanical
h i l vibration
ib ti having
h i essentially
ti ll the
th same frequency
f
as the
th
imposed alternating voltage. The mechanical vibration (ultrasound)
is introduced into a test piece through a couplant and travels by
wave motion through the test piece at the velocity of sound
sound, which
depends on the material. When the pulse of ultrasound encounters
a reflecting surface that is perpendicular to the direction of travel,
ultrasonic energy
gy is reflected and returns to the transducer. The
returning pulse travels along the same path and at the same speed
as the transmitted pulse, but in the opposite direction.
42
Pulse--Echo Methods
Pulse
„
„
Upon reaching the transducer through the couplant, the
returning
i pulse
l causes the
h transducer
d
element
l
to vibrate,
ib
which
hi h
induces an alternating electrical voltage across the transducer.
The induced voltage is instantaneously amplified, then fed into
the display device.
device This process of alternately sending and
receiving pulses of ultrasonic energy is repeated for each
successive pulse, with the display device recording any echoes
each
eac time.
t e.
Pulse--echo inspection can be accomplished with longitudinal,
Pulse
shear, surface, or Lamb waves. StraightStraight-beam or angle
angle--beam
techniques
q
can be used,, depending
p
g on test p
piece shape
p and
inspection objectives. Data can be analyzed in terms of type,
size, location, and orientation of flaws, or any combination of
these factors
43
Pulse--Echo Methods
Pulse
„
„
Presentation of Pulse
Pulse--Echo Data
Information from pulsepulse-echo inspection can be
displayed in different forms. The basic data formats
include:
A-scans: This format provides a quantita-tive display
of signal amplitudes and timetime-ofof-flight data obtained at
a single point on the surface of the test piece. The AAscan display, which is the most widely used format, can
be used to analyze the type, size, and location (chiefly
depth) of flaws
44
Pulse--Echo Methods
Pulse
45
Pulse--Echo Methods
Pulse
„
B-scans: This format provides a quantitative display
of timetime-ofof-flight data obtained along a line of the test
piece. The B
p
B--scan display
p y shows the relative depth
p of
reflectors and is used mainly to determine size (length
in one direction), location (both position and depth),
and to a certain degree the shape and orientation of
large flaws
46
Pulse--Echo Methods
Pulse
47
Pulse--Echo Methods
Pulse
„
C-scans: This format provides a semisemi-quantitative or
quantitative display of signal amplitudes obtained over
an area of the test p
piece surface. This information can
be used to map out the position of flaws on a plan view
of the test piece. A CC-scan format also records time
time--ofofflight data, which can be converted and dis-played by
image processing equipment to provide an indication
of flaw depth
48
Pulse--Echo Methods
Pulse
49
Electronic Equipment
„
„
Although the electronic equipment used for ultrasonic
i
inspection
i
can vary greatly
l in
i detail
d il among equipment
i
manufacturers, all generalgeneral-purpose units consist of a power
supply, a pulser circuit, a search unit, a receiver
receiver--amplifier
circuit an oscilloscope
circuit,
oscilloscope, and an electronic clock
clock. Many systems
also include electronic equipment for signal conditioning, gating,
automatic interpretation, and integration with a mechanical or
eelectronic
ect o c sca
scanning
g syste
system.. Moreover,
o eove , advances
adva ces in
microprocessor technology have extended the data acquisition
and signal processing capabilities of ultrasonic inspection
systems.
Power Supply.
Circuits that supply current for all
functions of the instrument constitute the power supply, which
g
byy conventional 115
115--V or 230230-V alternatingg
is usuallyy energized
current. There are, however, many types and sizes of portable
instru-ments for which the power is supplied by batteries
50
contained in the unit.
Electronic Equipment
„
Pulser Circuit.
„
Search Units. The transducer is the basic part of any search
„
When electronically trig-gered, the pulser
circuit generates a burst of alternating voltage. The principal
frequency of this burst, its duration, the profile of the envelope
of the burst, and the burst repeti-tion rate may be either fixed or
adjustable, depending on the flexibility of the unit.
unit. A sending transducer is one to which the voltage burst is
applied, and it mechanically vibrates in response to the applied
voltage. When appropriately
i
cou-pled to an elastic
i medium,
i
the
transducer thus serves to launch ultrasonic waves into the
material being inspected.
A receiving
i i
transducer
converts the ultrasonic waves
that impinge on it into a corre-sponding alternating voltage. In
the pitchpitch-catch mode, the transmitting and receiving
transducers
d
are separate units;
i in
i the
h pulsepulse
l -echo
h mode,
d a single
i l
transducer alternately serves both functions
51
Electronic Equipment
„
„
„
Receiver--amplifier
Receiver
circuits
electronically amplify
g
from the receivingg transducer and modify
y the
return signals
signals into a form suitable for display. The output is fed into an
oscilloscope or other display device.
Oscilloscope
Oscilloscope.
Data received are usually displayed on an
oscilloscope in either video mode or radio frequency mode. In
video--mode display, only peak intensities are visible on the
video
trace;; in the RF mode,, it is p
possible to observe the waveform of
signal voltages. Some instruments have a selector switch so that
the operator can choose the display mode, but others are
designed for single
single--mode operation only.
Clock.
The electronic clock, or timer, serves as a source of
logic pulses, reference voltage, and reference waveform. The
clock coordinates operation
p
of the entire electronic system.
y
52
Electronic Equipment
„
Signal--conditioning and gating circuits are included in
Signal
many commercial ultrasonic instruments. One common example
of a signalsignal-conditioning feature is a circuit that electronically
compensates for the signalsignal-amplitude loss caused by attenuation
of the ultrasonic pulse in the test piece. Electronic gates, which
monitor returning signals for pulses of selected amplitudes that
occur within selected timetime-delay ranges
ranges, provide automatic
interpretation. The set point of a gate corresponds to a flaw of a
certain size that is located within a prescribed depth range.
Gates are often used to trigger alarms or to operate automatic
systems that sort test pieces or identify rejectable pieces.
53
Electronic Equipment
54
Couplants
p
Air is a poor transmitter of sound waves at megahertz frequencies,
and the impedance mismatch between air and most solids is great
enough that even a very thin layer of air will severely retard the
t
transmission
i i off sound
d waves from
f
the
th ttransducer
d
tto th
the ttestt piece.
i
To perform satisfactory contact inspection with piezoelectric
transducers, it is necessary to eliminate air between the transducer
and the test piece by the use of a couplant
couplant.
„ Couplants normally used for contact inspection include water, oils,
glycerin, petroleum greases, silicone grease, wallpaper paste, and
various commercial pastepaste-like substances.
substances Certain soft rubbers
that transmit sound waves may be used where adequate coupling
can be achieved by applying hand pressure to the search unit.
„ The following should be considered in selecting a couplant:
• Surface finish of test piece
• Temperature of test surface
• Possibility of chemical reactions between test surface and couplant
• Cleaning requirements (some couplants are difficult to remove) 55
„
Couplants
„
„
„
„
Water is a suitable couplant for use on a relatively smooth
surface;; however,, a wettingg agent
g
should be added. It is
sometimes appropriate to add glycerin to increase viscosity;
however, glycerin tends to induce corrosion in aluminum and
therefore is not recommended in aerospace applications.
Heavy oil or grease should be used on hot or vertical surfaces or
on rough surfaces where irregularities need to be filled.
Wallpaper paste is especially useful on rough surfaces when
good coupling is needed to minimize background noise and
yield an adequate signalsignal-toto-noise ratio.
Water is not a good couplant to use with carbon steel test
pieces,
i
because
b
it can promote
t surface
f
corrosion.
i
Oils,
Oil greases,
and proprietary pastes of a noncorrosive nature can be used
56
Couplants
„
Couplants
p
used in contact inspection
p
should be applied
pp
as a uniform, thin coating to obtain uniform and
consistent inspection results.
results. The necessity for a
couplant
p
is one of the drawbacks of ultrasonic
inspection and may be a limitation, such as with high
high-temperature surfaces
surfaces.. When the size and shape of the
part being inspected permit, immersion inspection is
often done
done.. This practice satisfies the requirement for
uniform coupling
57
Inspection Standards
The standardization of ultrasonic inspection allows the same test
procedure to be conducted at various times and locations, and by
both customer and supplier, with reasonable assurance that
consistent
i t t results
lt will
ill be
b obtained.
bt i d St
Standardization
d di ti also
l provides
id a
basis for estimating the sizes of any flaws that are found.
„ An ultrasonic inspection system includes several controls that can be
adjusted
dj t d tto di
display
l
as much
h iinformation
f
ti
as is
i needed
d d on the
th
oscilloscope screen or other display device. Inspection or reference
standards are used as a guide for adjusting instrument controls to
reveal the presence of flaws that may be considered harmful to the
end use of the product and for determining which indica-tions come
from flaws that are insignificant, so that needless reworking or
scrapping
pp g of satisfactory
yp
parts is avoided.
„ The inspection or reference standards for pulsepulse-echo testing include
test blocks concon-tuning natural flaws, test blocks containing artificial
flaws, and the technique of evaluating the percentage of back
reflection. Inspection standards for thickness testing can be plates of
various known thicknesses or can be stepped or tapered wedges. 58
Inspection Standards
„
Test blocks containing
g natural flaws
„
Test blocks containing artificial flaws
are metal
sections similar to those parts being inspected. Sections known
to contain natural flaws can be selected for test blocks.
• It is difficult to obtain several test blocks that ggive identical
responses. Natural flaws vary in shape, surface characteristics,
and orientation, and echoes from natural flaws vary accordingly
p
to determine the exact nature of a natural
• It is often impossible
flaw existing in the test block without destructive sectioning
consist of metal sections containing notches,
notches slots,
slots or drilled holes.
holes
These test blocks are more widely accepted as standards than
are test blocks that contain natural flaws.
59
Inspection Standards
„
Test blocks containing drilled holes are widely used for
longitudinal wave, straightstraight-beam inspection. The hole in the
block can be positioned so that ultrasonic energy from the
search unit is reflected either from the side of the hole or from
the bottom of the hole. In the inspection of sheet, strip, welds,
tubing,
tubing and pipe
pipe, angleangle-beam inspection can be used
used. This type
of inspection generally requires a reference standard in the form
of a block that has a notch (or more than one notch) machined
into the block.
block The sides of the notch can be straight and at right
angles to the surface of the test block, or they can be at an angle
(for example, a 60°
60° included angle). The width, length, and
depth of the notch are usually defined by the applicable
specification.
60
Standard Reference Blocks
„
„
„
„
„
„
Many of the standards and specifications for ultrasonic
inspection require the use of standard reference blocks, which
can be prepared from various alloys
alloys, may contain holes
holes, slots
slots, or
notches of several sizes, and may be of different sizes or shapes.
The characteristics of an ultrasonic beam in a test piece are
affected
ec ed by thee following
o ow g variables,
v
b es, w
which
c sshould
ou d be considered
co s de ed
when selecting standard reference blocks:
Nature of the test piece
Alloy type
Grain size
Effects of thermal or mechanical processing
Di t
DistanceDistance
-amplitude
lit d effects
ff t ((attenuation)
tt
ti ) Flaw
Fl
size
i Direction
Di ti off
the ultrasonic beam
61
Standard Reference Blocks
„
„
Three types of standard blocks are ordinarily used for
calibration or reference: area
area--amplitude blocks, distance
distance-amplitude
lit d blocks,
bl k and
d bl
blocks
k off th
the ttype sanctioned
ti d b
by th
the
International Institute of Welding (IIW).
These blocks must be prepared from material having the same
or similar alloy content, heat treatment, and amount of hot or
cold working as the material to be inspected to ensure equal
sonic velocity, attenuation, and acoustic impedance in both the
reference standard and the test piece. If blocks of identical
material are not available, the difference between the material
in the test piece and the material used in the standard reference
blocks must be determined experimentally.
62
Standard Reference Blocks
63
Standard Reference Blocks
64
„
„
Area--amplitude blocks provide artificial flaws of different sizes at
Area
the same depth.
p Eight
g blocks made from the same 50 mm ((2 in.)) diam.
round stock, each 95 mm (3 ¾ in.) high, constitute a set of areaareaamplitude blocks. The block material must have the same acoustic
properties as the test piece material. Each block has a 20 mm (¾ in.)
d
deep
fl
flatt-bottom
flatb tt
h
hole
l d
drilled
ill d iin th
the center
t off th
the b
bottom
tt
surface
f
(Fi
(Fig.
46); the hole diameters vary from 0.4 to 3.2 mm (1/64 to 8/64 in.). The
blocks are numbered to correspond with the diameter of the holes;
that is,
is block No
No. 1 has a 00.4
4 mm (1/64 in
in.)) diam hole,
hole No.
No 2 has a 00.8
8
mm (2/64 in.) diam hole, and so on, up to No. 8, which has a 3.2 mm
(8/64 in.) diam hole.
The amplitude of the echo from a flat
flat--bottom hole in the far field of a
straight--beam search unit is proportional to the area of the bottom of
straight
the hole. Therefore, these blocks can be used to check the linearity of a
pulse--echo inspection
pulse
p
p
system
y
and to relate signal
g
amplitude
p
to the area
(or, in other words, the size) of the flaw
65
66
Standard Reference Blocks
„
Distance--amplitude
Distance
blocks provide arti-ficial flaws of a
given size at various depths. From ultrasonic wave theory, it is
known that the decrease in echo amplitude from a flatflat-bottom
hole using a circular search unit is inversely proportional to the
square of the distance to the hole bottom. Distance
Distance--amplitude
blocks can be used to check actual variations of amplitude with
distance for straightstraight-beam inspection in a given material. They
also serve as a reference for setting or standardizing the
sensitivity (gain) of the inspection system so that readable
indications will be displayed on the oscilloscope screen for flaws
of a given size and larger, but the screen will not be flooded with
indications of smaller discontinuities that are of no interest. On
i
instruments
so equipped,
i
d these
h
bl
blocks
k are used
d to set the
h
sensitivity--time control or distance
sensitivity
distance--amplitude correction so that a
flaw of a given size will produce an indication on the oscillo-scope
screen that is of a predetermined height regardless of distance
from the entry surface.
67
Standard Reference Blocks
„
„
There are 19 blocks in a SeriesSeries-B set. All are 50 mm (2 in.) diam
bl k off the
blocks
h same material
i l as that
h b
being
i iinspected,
d and
d all
ll h
have a
20 mm (3/4 in.) deep flatflat-bottom hole drilled in the center of the
bottom surface (Fig. 46). The hole diameter is the same in all the
blocks of a set; sets can be made with hole diameters of 11.2,
2 22.0,
0
and 3.2 mm (3/64, 5/64, and 8/64 in.). The blocks vary in length to
provide metal distances of 1.6 to 145 mm (1/16 to 5 3/4 in.) from
tthee top (entry)
(e t y) surface
su ace to tthee hole
o e botto
bottom.. Metal
eta distances
d sta ces aare:
e:
• 1.6 mm 1/16 in.)
• 3.2 mm through 25 mm (2/16 in. through 1 in.) in increments of
3.2 mm (2/16 in.)
• 32 mm through 150 mm (1 ¼ in. through 5 ¾ in.) in increments
of 13 mm (1/2 in.)
ASTM blocks can be combined into various sets of area
area--amplitude
and distance
distance--amplitude blocks.
68
Standard Reference Blocks
IIW blocks are mainly used to calibrate instruments prior to contact
inspection using an angle
angle--beam search unit; these blocks are also
useful for:
• Checking the performance of both angleangle-beam and straightstraight-beam
search units
• Determining the sound beam exit point of an angle
angle--beam search unit
• Determining refracted angle produced
• Calibrating sound path distance
• Evaluating instrument performance
„
69
Standard Reference Blocks
„
„
„
„
The material from which a block is prepared is specified by the
IIW as killed, open hearth or electric furnace, low
low--carbon steel in
th normalized
the
li d condition
diti and
d with
ith a grain
i size
i off McQuaidMcQuaid
M Q id-Ehn
Eh
No. 8.
All IIW standard reference blocks are of the same size and shape;
official
ffi i l IIW bl
blocks
k are di
dimensioned
i d iin th
the metric
t i system
t
off units.
it
One of the standard EnglishEnglish-unit designs is shown in Fig.
The miniature angle
angle--beam block is based on the same design
concepts as the IIW block
block, but is smaller and lighter
lighter. The
miniature angle
angle--beam block is primarily used in the field for
checking the characteristics of angle
angle--beam search units and for
calibrating the time base of ultrasonic instruments.
With the miniature block, beam angle and index point can be
checked for an angle
angle--beam search unit, and metalmetal-distance
calibration can be made for either angle
angle--beam or straightstraight-beam
search units
70
Standard Reference Blocks
71
Calibration
„
In many cases, the determination of areaarea-amplitude and distancedistanceamplitude curves constitutes the calibration of a straightstraight-beam
pulse--echo system for the detection of flaws
pulse
flaws. Calibration of an angle
angle-beam pulse
pulse--echo ultrasonic test system for flaw detection can be
accomplished using an IIW standard calibration block. The large
(100 mm, or 4 in., radius) curved surface at one end of the block is
used to determine the index point of angleangle-beam search units (the
point where the beam leaves the unit). The mark labeled "beam exit
point" in Fig. is at the center of this 100 mm (4 in.) radius.
Regardless off the angle off the search unit,
i a maximum
i
echo is
i
received from the curved surface when the index point of the search
unit is at the "beam exit point." To determine the index point of an
angle--beam search unit
angle
unit, the search unit is placed on surface A in the
position shown in Fig. 50(a) and is moved along the surface of the
block until the echo is at maximum amplitude. The point on the
search
sea
c u
unitt tthat
at iss d
directly
ect y ove
over tthee bea
beam exit
e t point
po t of
o the
t e block
b oc can
ca
then be marked as the index point of the search unit.
72
Calibration
„
„
Beam Angle. Once the index point of the search unit is marked, the
50 mm (2 in.) diam hole is used to determine the angle of the beam in
the lowlow-carbon steel from which the block is prepared. The search
unit is placed on surface A or surface B and is aimed toward the 50
mm (2 in.) diam hole. Then, the search unit is moved along the
surface
until
a
maximum-amplitude
maximumecho
is
received. At this position, the index point on the search unit indicates
the beam angle, which is read from one of the degree scales marked
along the sides of the block at the edges of surfaces A and B
Beam spread can be determined by mov
mov-ing
ing the search unit in one
direction (toward either higher or lower beam angles) from the point
of maximummaximum-amplitude echo until the echo disappears and noting
the beam angle at the index point.
point The search unit is then moved in
the opposite direction, past the point of maximummaximum-amplitude echo to
the point where the echo again disappears. The beam spread is the
difference between the angles indicated by the index point at these
two extreme positions.
73
Calibration
74
Calibration
75
Calibration
„
Time Base. The 25 mm (1 in.) radius curved slot (Fig. 47a) is
used
d iin conjunction
j
i with
i h the
h 100 mm (4 iin.)) radius
di surface
f
ffor
calibrating the time base of the ultrasonic instrument being used
with the angle
angle--beam search unit. A direct reflection from the
100 mm (4 in.)
in ) radius surface represents a metal distance of 200
mm (8 in.). Similarly, a beam traveling from the beam exit point
to the 100 mm (4 in.) radius, then reflecting in turn to surface A,
tthen
e to the
t e 255 mm (1
( in.)
.) radius,
ad us, back
bac to"
to surface
su ace A,, oonce
ce aga
again
to the 100 mm (4 in.) radius, and finally back to the search unit,
will give an echo indication at 450 mm (18 in.) metal distance.
The time base band marker spacing can be adjusted until the
echoes
h
corre-sponding
di tto 200
200, 250
250, 450
450, 500
500, 700 mm (8
(8, 10,
10 18,
18
20, 28 in.) and so on are aligned with appropriate grid lines on
the screen or with a convenient number of marker signals.
76
Calibration
„
„
Linearity. Calibration in terms of metal distance or reflector
depth assumes a linear oscilloscope sweep for the instrument,
which can be checked using a straight
straight--beam search unit
unit. The
search unit is placed on either surface C or D to obtain multiple
echoes from the 25 mm (1 in.) thickness. These echo indications
will be aligned with evenly spaced grid lines or scale marks if
the time base is linear. Linearity within ±1% (or less) of the fullfullscale value of thickness is usually obtainable.
Resolution. A straightstraight
g -beam search unit,, as well as the
instrument, can be checked for backback-surface resolution by
placing the search unit on surface A and reflecting the beam
from the bottom of the 2 mm (0.080in.) wide notch and from
surfaces
f
B and
d E on either
ith side
id off it,
it With proper resolution,
l ti
the
th
indications from these three surfaces should be clearly
separated and not overlapped so as to appear as one broad,
jagged indication
indication. Because resolution is affected by test
conditions and by characteristics of the search unit and
77
instrument amplifier, this degree of resolution sometimes may
not be obtainable.
Calibration
„
The dead zone
is the depth
p below the entry
y surface that
cannot be inspected because the initial pulse interferes with echo
signals. An indication of the length of the dead zone of a
straight--beam search unit can be obtained by placing the search
straight
unit
i on surface
f
A or F in
i line
li with
i h the
h 50 mm (2 in.)
i ) diam
di
hole
h l .
When the searchsearch- unit is placed on surface A, a discernible echo
from the 50 mm (2 in.) diam hole indicates a dead zone of less
than 5 mm (0.2
(0 2 in.).
in ) Similarly,
Similarly when the search unit is placed on
surface F, a discernible echo from the 50 mm (2 in.) diam hole
indicates a dead zone of less than 10 mm (0.4 in.). Alternatively,
the length
g of the dead zone can be measured byy calibrating
g the
time base of the instrument, then measuring the width of the
initial--pulse indication at the base of the signal, as illustrated
initial
schematically in Fig.
78
Calibration
79
Calibration
„
Sensitivity.
y The relative sensitivityy of an angleangle
g -beam search
„
R
Range
S
Setting.
i
The range for a searchsearch-unit and instrument
unit in combination with a given instrument can be defined by
placing the unit on either surface A or B and reflecting the beam
from the side of the 1.5 mm (0.060 in.) diam hole . The position
off the
h search
h unit
i iis adjusted
dj
d until
il the
h echo
h ffrom the
h h
hole
l iis
maximum, then the gain of the instrument is adjusted to give the
desired indication height.
system for straight
straight--beam inspection can be set for various
distances by use of the IIW block. From surface F to the 2.0 mm
(0 080 iin.)) wide
(0.080
id notch
t h is
i 200 mm (8 in.),
i ) from
f
surface
f
A to
t
surface B is 100 mm (4 in.), from surface E to surface A is 91.4
mm (3.60 in.), and from surface C to surface D is 25 mm (1 in.).
80
Calibration
„
Miniature--Block Application. The min-iature angleMiniature
anglebeam block is used in a somewhat similar manner as the
larger
g IIW block. Both the 25 and 50 mm ((1 and 2 in.))
radius surfaces provide ways for checking the location of
the index mark of a search unit and for calibrating the time
base of the instrument in terms of metal distance. The
small hole provides a reflector for checking beam angle
and for setting the instrument gain.
gain
81