Page 590 - Instrumentation Reference Book 3E

572 Non-destructive testing

24.4 Ultrasonics where E is the modulus of elasticity, p the density,
,LL Poisson’s ratio, and G the modulus of shear.
24.4.1 General principles of ultrasonics Other properties of ultrasonic waves relate to
the results of ultrasound meeting an interface, Le.,
Ultrasonics, when applied to the non-destructive a boundary wall between different media. When
testing of an engineering component, relies on a this occurs, some of the wave is reflected, the
probing beam of energy directed into the compon- amount depending on the acoustic properties of
ent interacting in an interpretable q7ay with the the two media and the direction governed by the
component’s structural features. If a flaw is pres- same laws as for light waves. If the ultrasound
ent within the metal, the progression of the beam meets a boundary at an angle, the part of the wave
of energy is locally modified and the modification that is not reflected is refracted, suffering a change
is detected and conveniently displayed to enable of direction for its progression through the second
the flaw to be diagnosed. The diagnosis largely medium. Energy may be lost or attenuated during
depends on a knowledge of the nature of the the propagation of the ultrasound due to energy
probing energy beam, its interaction with the absorption within the medium and to scatter
structural features of the component under test, which results from interaction of the waves with
and the manufacturing history of the component. microstructural features of size comparable with
The ultrasonic energy is directed into the mater- the wavelength. This is an important factor, as it
ial under test in the form of mechanical waves or counteracts the sensitivity to flaw location on the
vibrations of very high frequency. Although its basis of frequency selection. Hence high frequency
frequency may be anything in excess of the upper gives sensitivity to small flaws but may be limited by
limit of audibility of the human ear, or 20 kHz, scatter and absorption to short-range detection.
ultrasonic non-destructive testing frequencies The compression or longitudinal wave is the
normally lie in the range 0.5-10MHz. The equa- most common mode of propagation in ultrason-
tion ics. In this form, particle displacement at each
V point in a material is parallel to the direction of
A=- propagation. The propagating wavefront pro-
f gresses by a series of alternate compressions and
where X is the wavelength, Vthe velocity, andfthe rarefactions, the total distance occupied by one
frequency, highlights this by relating wavelength compression and one rarefaction being the wave-
and frequency to the velocity in the material. length. Also commonly used are shear or trans-
The wavelength determines the defect sensitiv- verse waves, which are characterized by the
ity in that any defect dimensionally less than half particle displacement at each point in a material
the wavelength will not be detected. Conse- being at right angles to the direction of propaga-
quently the ability to detect small defects tion. In comparing these wave motions it should
increases with decreasing wavelength of vibration be appreciated that for a given material the shear
and, since the velocity of sound is characteristic waves have a velocity approximately five-ninths
of a particular material, increasing the frequency of that of compressional waves. It follows that for
of vibration ~7ill provide the possibility of any frequency, the lower velocity of shear waves
increased sensitivity. Frequency selection is thus corresponds to a shorter wavelength. Hence, for a
a significant variable in the ability to detect small given frequency, the minimum size of defect
flaws. detectable will be less in the case of shear waves.
The nature of ultrasonic waves is such that Other forms of shear motion may be produced.
propagation involves particle motion in the me- Where there is a free surface a Rayleigh or surface
dium through which they travel. The propagation wave may be generated. This type of shear wave
may be by way of volume change, the compres- propagates on the surface of a body with effective
sion wave form, or by a distortion process, the penetration of less than a wavelength. In thin
shear wave form. The speed of propagation thus sections bounded by two free surfaces a Lamb
depends on the elastic properties and the density wave may be produced. This is a form of com-
of the particular medium. pressional wave which propagates in sheet mater-
Compression wave velocity ial, its velocity depending not only on the elastic
constant of the material but also on plate thick-
ness and frequency. Such waveforms can be used
in ultrasonic testing. A wave of a given mode of
propagation may generate or transform to waves
Shear wave velocity of other modes of propagation at refraction or
reflection, and this may give rise to practical dif-
ficulties in the correct interpretation of test sig-
nals from the material.