Page 595 - Instrumentation Reference Book 3E

Ultrasonics 577

Scanning frame

T
Specimen

voltages II I Flaw-detector unit
X and Y
indexing
probe
positiorl

tracking unit I Selector

Pen dropping signal
Figure! 24.18 C-scan equipment. Courtesy thewelding Institute.

transmitted into the workpiece. Again a twin 24.4.4 Ultrasonic spectroscopy techniques
crystal. form is available as for compression Ultrasonic spectroscopy is a technique used to
probes, as shown in Figure 24.19(b). analyze the frequency components of ultrasonic
Special probes of the focused, variable-angle,
signals. The origins of spectroscopy date back to
crystall mosaic: or angled compression wave type Newton, who showed that white light contained a
are also available (Figure 24.20). By placing an number of different colors. Each color corres-
acoustic lens in front of a transducer crystal it is ponds to a different frequency, hence white light
possible to focus the emitted beam in a similar may be regarded as the sum of a number of
manner to an optical lens. If the lens is given a different radiation frequencies, Le., it contains a
cylindrical curvature, it is possible to arrange spectrum of frequencies. Short pulses of ultraso-
that, when testing objects in immersion testing, nic energy have characteristics similar to white
the sound beam enters normal to a cylindrical light, i.e., they carry a spectrum of frequencies.
surface. Variable-angle probes can be adjusted The passage of white light through a material
to give a varying angle of emitted sound wave, and subsequent examination of its spectrum can
these being of value when testing at various yield useful information regarding the atomic
angles on the same workpiece. In the mosaic type composition of the material. Likewise the passage
of probe a number of crystals are laid side by side. of ultrasound through a material and subsequent
The crystals are excited in phase such that the
mosaic acts as a probe of large crystal size equal examination of the spectrum can yield informa-
to that of the mosaic. Finally, probes of the tion about defect geometry, thickness, transducer
frequency response, and differences in microstruc-
angled compression type have been used in the ture. The difference in the type of information is
testing of austenitic materials where, due to large related to the fact that ultrasonic signals are elastic
grain structure, shear waves are rapidly attenu- instead of electromagnetic waves.
ated. Such probes can be made by angling the Interpretation of ultrasonic spectra requires a
incident compressional beam until it is smaller knowledge of how the ultrasonic energy is intro-
than the initial angle for emission of shear waves duced into the specimen. Two main factors deter-
only. Although a shear wave also occurs in the mine this:
test piece it is rapidly lost due to attenuation.