The graph of peaks using the immersion method is slightly different. Between the initial pulse and the back wall peaks there will be an additional peak caused by the sound wave going from the water to the test material. This additional peak is called the front wall peak. The ultrasonic tester can be adjusted to ignore the initial pulse peak, so the first peak it will show is the front wall peak. Some energy is lost when the waves hit the test material, so the front wall peak is slightly lower than the peak of the initial pulse.
Ultrasonic testing is an NDT test technique that interrogates components and structures to detect internal and surface breaking defects, and measures wall thickness on hard (typically metallic or ceramic) components and structures.
How does ultrasonic testing work?
Ultrasonic operates on the principle of injecting a very short pulse of ultrasound (typically between 0.1 MHz and 100 Mhz) into a component or structure, and then receiving and analyzing any reflected sound pulses.
Conventionally, an operator scans a transducer over the surface of the component in such a way that he inspects all the area that is required to be tested by means of a scanning motion. The inspection relies on the training and integrity of the operator to ensure that he has inspected all that is necessary.
Sound pulses reflected from features within the component or structures are conventionally displayed on a screen. The operator also has to interpret these signals and report if the component or structure is defective or acceptable according to the test specification that he is given.
Typical detection limits for fine grained steel structures or components (hand scanning) are single millimeter sized defects. Smaller defects can be detected by immersion testing and a programmed scan pattern with higher frequency ultrasound (slower testing). Detection limits are in the order of 0.1 to 0.2 mm, although smaller defects (typically 0.04mm diameter) can be detected under laboratory conditions.
In my experience, I have done the experiment to detect the clad material that has thickness about 1.5 mm, and the void about 10 micrometer. The experiment was conducted in Yokohama National University, Sekine Laboratory.