Page 265 - CITS - Welder - Trade Theory

WELDER - CITS

In Ultrasonic testing : very short ultrasonic pulse-waves with centre frequencies nagging from 0.5-15 Mitz and
occasionally up to 50 MHz are launched into materials to detect internal flaws or to characterise materials. The
technique is also commonly used to determine the thickness of the test object, for example, to monitor pipework
corrosion. Ultrasonic testing is often performed on steel and other metals and alloys, though it can also be used
on concrete, wood and composites, albeit with less resolution. It is a form of non-destructive testing sued in many
industries including aerospace, automotive and Most common modes of propagation of ultrasonic waves are: i)
Longitudinal waves ii) Transverse wave Surface or Rayleigh waves.
Longitudinal Waves : These waves travel through the medium as a series of alternate compressions and
rarefaction in which the particles transmitting the wave vibrate back and forth in the direction of travel of the
waves. Longitudinal ultrasonic waves are readily propagated in liquids and gases as well as in elastic solids. The
velocity of longitudinal waves is about 5920/sec. In steel.

Transverse Waves : In transverse or shear waves, each particle vibrates up and down in a plane perpendicular
to the direction of propagation. The velocity of transverse waves is about 50% of the longitudinal wave velocity
for the same material.
Surface or Rayleigh Waves And Plate or Lamb Waves:
These are another type of waves used in the inspection of surface soundness of metals. These waves travel along
with flat or curved surfaces of relatively thick solid parts. Lamb waves, also know, as plate waves are another type
of ultrasonic waves used in the non-destructive inspection of metals. These are propagated in metals that arte
only few wavelengths in thickness.

Major Variables : Sensitivity or the ability to detect a very small discontinuity is generally increases by using
relatively high frequencies (short wave lengths). Resolution, is the ability of the systems to give simultaneous,
separate indications from discontinuities that are close together in depth. Resolution is directly preoperational so
probe bandwidth and inversely related to pulse length but is not affected by frequency.

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