15
15
This is useful, for example, in gaging remaining thickness over internal passages in turbine
blades. Procedures for using immersion transducers are outlined in Section 4.3.2.4, below.
Metals with Thicknesses Ranging from 0.030 inch (0.76mm) Upward.
Single Element Contact Transducers Hard-Faced Wear Plates.
Depending upon active element size and frequency, highly damped (broadband) transducers of
this type can seldom be used on steel or aluminum much below 0.030 inch (0.8mm). The ultimate
minimum thickness limitation must be determined experimentally.
The advantage of using this type of transducer is that much thicker sections can be gaged, and on
materials that have less than smooth surfaces or are relatively attenuative. Again, some
experimentation will indicate the limitations and advantages among transducers of the same type,
but having different sizes, frequencies and spectral characteristics.
When trying to achieve best results on relatively thin materials, the most important considerations
are frequency and spectral characteristics. Foremost is the need to use a transducer having the
greatest bandwidth, all other things being equal. Such transducers have relatively large, high
density masses bonded to the internal face of the active element. This mass minimizes the
amount of "ringing" of the active element following excitation by a short duration, high voltage
electrical pulse. This kind of high mechanical damping minimizes the length of the ultrasonic
wavetrain associated with the high energy initial pulse as well as the subsequent back-wall echos.
Also necessary to achieve best thickness resolution is the need to use the highest frequency
consistent with external factors that affect coupling efficiency, and to take material attenuation into
consideration. IF a transducer is expected to produce minimum thickness resolution and be used
over a broad range of thicknesses, test material attenuation may dictate a compromise. While
higher frequencies produce thinner thickness resolution, attenuation is more pronounced at higher
frequencies.
For relatively flat, smooth-surfaced steel or aluminum test objects, highly damped contact
transducers in the frequency range from 5 to 10 MHZ generally will produce acceptable results in
the thickness range from 0.040 inch (1.0mm) up to 5 inches (125mm) more or less. With
QFT-100, and under ideal external conditions, transducers of 5, 7.5 or 10 MHZ having active
elements of 0.25 to 0.50 inch (6.4 to 12.7mm) diameter will cover the range from 0.030 to 10
inches (0.76 to 250mm) or more.
For procedures outlining the use of broad banded contact transducers, see Section 4.3.2.1.
Single Element Contact Transducers with Membrane and Other Protective Devices.
So-called protected element transducers are used in relatively rugged applications involving
rough, abrasive test object surfaces or surfaces at elevated temperatures. Usual designs of
protected element transducers include reduced internal damping, lower frequencies and larger
diameters. As a result, their application typically ranges from a few tenths of an inch (10mm plus
or minus) upwards.
Often used on castings, forgings and mill-finished piping, protected element transducers are
infrequently used for thickness gaging -- more often used for flaw detection. However, if the
protective element is a relatively thin elastomer or flexible polymer, the procedures for thickness
gaging are similar to those for single element contact-type transducers. For thicker wear-caps or
temperature-protective delay lines, procedures are more like those for delay line transducers.
Dual Element Contact-Type Transducers.
This category of transducers is frequently used in thickness gaging applications, combining some
of the advantages of both single element contact and delay line models. They have two elements,
each with a separate delay line, mounted side-by-side in a single fixed housing. The elements are
