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Ultrasonic testing is the use of high frequency sound to inspect materials. The sound frequency normally ranges from .5 MHz to 25 MHz, which is well above what a human can hear. High frequency sound is used because it is able to penetrate most materials without difficulty, which makes it possible to conduct non-destructive testing. It is commonly used to look for cracks, measure material depth, and check for corrosion and imperfections.
An ultrasonic test works by bouncing sound off of an object and interpreting the resulting echo. It passes through the material until the sound waves encounter an irregularity. The operators usually note this as a “discontinuity.” By analyzing the discontinuity, the operator can determine if there is a flaw in the material.
The thickness of materials such as metals, ceramics and plastics can be measured with ultrasonic testing. Ultrasonic thickness testing is mainly done by calculating the time it takes for sound to bounce off the bottom of the material. Different materials typically reflect sound at different rates. By measuring the change in the time it takes for the sound to be reflected, the operator is able to measure the thickness of each material in a multilayer surface.
There are several types of ultrasonic testing equipment, depending on the required application. The choice of equipment is generally dictated by the material’s temperature, thickness, geometry and phase reversal. Ultrasonic test equipment typically has three components: a transducer, a couplant and an imaging system. A technician operates the ultrasonic testing equipment by manually moving the probe across the surface of the object being tested and interpreting the resulting data.
A transducer or probe produces and receives sound. Normally, a transducer sends sound in either a straight beam or in an angle beam. Straight beam transducers are more widely used than angle beam transducers, which are often used for ultrasonic weld testing.
A couplant is applied to the surface being tested to act as a medium between the transducer and the surface. Its main purpose is to transmit sound to the surface. It also increases accuracy by preventing loss of sound waves. Couplants are often made of gel or paste. Water, however, is often used as a couplant when using an immersion transducer.
Data from the test can be read with an imaging system. The imaging system normally contains the controls and processor. In some portable equipment, the transducer is also integrated into the imaging system.