Page 266 - CITS - Welder - Trade Theory

WELDER - CITS

Penetration, or maximum depth (range in a material from which useful indications can be detected is reduced
by the use of high frequencies. This effect is most pronounced in the inspection of metal that has course grain
structure or minute in homogeneities, because of the scattering of the ultrasonic waves. With the decrease in
frequency, the shape of an ultrasonic beam increasingly departs from the ideal of zero beams spread.
Acoustic-Impedance: The acoustic impedance is Zap x V where V-Velocity and p-, density of material
Laws Of Ultrasound : At any angle other than normal the angle of incidence is equal to angle of reflection the
refraction angle is known as Snell’s law and is
Sin A/Sin B=V1/V2]
A-Angle of incidence, Bangle of refraction,
VI-velocity of sound in the medium!
V2. velocity of sound in Medium2
Additionally, mode conversion takes place at the interface when the longitudinal wave is refracted as. longitudinal
and transverse waves.
Attenuation : This intensity of an ultrasonic beam that is received by a transducer is considerably less than the
intensity of the initial transmission. There is a loss in the energy due to absorption of sound in the material. This
coos is known as attenuation.
Probes : Generation as well detection of ultrasonic waves for inspection is accomplished by means of a transducer
element which is contained with a device most often referred to as a search unit or a probe. The active element in
a search unit is a piezo electric crystal. This crystal works on the principle an electrical charge when pressure is
applied to it. Conversely, when an electrical deforms. The most common types of piezo-electric materials used for
Ultrasonic search units are quartz, lithium sulphate, polarized ceramics such as barium titanate, lend zirconated
titanate and lead metaniobate.
Probe Types : Search units are of many types and shapes. Variation in search unit construction includes
transducer element material, transducer element thickness, surface area and shape; and type of backing material
and degree of loading. The basic types are normal beam, angle beam, and Transmitter and receiver probes.
Reference Blocks : Ultrasonic inspection is basically a comparison process and requires the use of reference
blocks for calibration of the equipment and probe set up before put to use on the actual job. The calibration is done
with standard reference blocks. The most widely used reference blocks is Sulzer’s block, ASTM block, IIW block,
miniature angle beam block etc. The calibration of the set up is done to check the following.
Time base linearity, Resolution, Sensitivity, Beam spread, Beam angle. Dead zone probe index and Penetration
power.

Inspection Methods :
A Ultrasonic inspection can be performed by Pulse Echo Method
B Through Transmission Method
Ultrasonic technique in which flaws are detected by measuring intensity and time of flight of reflected sound waves
having single frequency is called pulse echo method of ultrasonic testing which is most widely used perpendicular
to their main dimension. Orbiting the probe from the position which ensures maximum echo height will result in
a sudden amplitude drop. Noticeable difference in echo height may be obtained by successive use of different
probe angles. When the defect orientation is perpendicular to the plate surface about the same echo height will be
noticed with scanning from both sides of the weld. An inclined orientation may result in an echo height difference.
The echo height obtained from slag inclusions does not necessarily differ from those received from cracks or
lack of fusion but may be distinguished by different echo shapes and directional sensitivity. Because of the rough
surface of this type of discontinuity the defect intercepts successive regions in the beam and gives rise to a
succession of echo signals An orbital movement with the prove will cause little change in this echo appearance.
The individual; signals of which the echo is composed however, will change and consequently you there will be
a variation of echo height. A scan from the opposite side of the weld is likely to give a similar echo appearance.
Spherically shaped discontinuities such as gas pores reflect only a small part of the incident beams in the direction
of the receiver. Moreover, the dimensions of this type of defect are small. Low amplitude signals of narrow echo
shape characterize this type of defect. The echo will remain almost constant when the prove is moved around the
defect and when scanning from the opposite side. In the event of porosity a number of these small echoes may
appear close together depending on the number and distribution of the pores.

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CITS : C G & M - Welder - Lesson 83 - 97