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10.3 Measuring of laminate materials
Laminate materials are unique in that their density (and thus the speed of sound
propagation) can vary considerably from one piece to another. Some laminate
materials can even show significant variations in the speed of sound on a single
surface. The only way to reliably measure such materials is to perform a calibration
procedure on a sample of known thickness. Ideally, this sample material should be part
of the same DUT, or at least come from the same batch. By calibrating each test piece
individually, the effects of sound velocity deviations can be minimized.
Another important factor to consider when measuring laminate materials is the fact that
trapped air gaps or pockets cause early reflection of the ultrasonic beam. This effect is
noticeable by a sudden decrease in thickness on an otherwise homogeneous surface.
Although this can affect an accurate measurement of the overall thickness of the
material, it does indicate air gaps in the laminate material.
10.4 Measurement through paint layers and coating
Measuring through paint layers and coating is also unique in that the speed of sound
propagation for paint layers and coating differs significantly from the speed of sound
propagation for the actual measured material. A perfect example would be a mild steel
pipe with about 0.6 mm coating on the surface, where the speed of sound propagation
is 5920 m/s for the pipe and 2300 m/s for the coating. If the calibration procedure for
mild steel pipes and the measurement is performed through both materials, the actual
coating thickness appears to be 2.5 times greater than it really is due to the speed
differences. This error can be corrected by using a special echo-echo mode to perform
measurements in such cases. In the echo-echo mode, the paint or coating thickness
is completely eliminated and the only material measured is steel.
10.5 Suitability of materials
The ultrasonic thickness measurements require that a sound wave is passed through
the material to be measured. Not all materials have good sound conducting properties.
Ultrasonic thickness measurement is applicable to a wide range of materials including
metals, plastics and glass. Some cast materials, concrete, wood, fiberglass, and some
rubbers are considered difficult cases to measure.
