Section 6: Selected Applications & Techniques

6.7.2 The attenuation method
The attenuation method is based on the decay of multiple echoes from test
piece surfaces. Once a standard is established, other test pieces can be
compared to it by comparing the decay of these echoes to an exponential
curve. This test is especially suited for the microstructural control of
production parts, in which all that is necessary is to determine whether or not
the parts conform to a standard. An example of the use of ultrasonic
attenuation in the determination of differences in microstructure is the control
of graphite-flake size in gray iron castings, which in turn controls tensile
strength. In one application, a water-column search unit that produced a
pulsed beam with a frequency of 2.25 MHz was used to test each casting
across an area of the casting wall having uniform thickness and parallel front
and back surfaces.