Ultrasonic examination (UT) is a type of Nondestructive Testing commonly used to find flaws in materials and to measure the thickness of objects. An inspection may be manual or automated and is an essential part of the modern manufacturing processes.
Most metals can be inspected, as well as plastics and aerospace composites. Lower frequency ultrasound (50 kHz to 500 kHz) also can be used to inspect less dense materials such as wood, concrete, and cement. When a piezoelectric crystal is driven by high-voltage electrical pulses, the crystal rings at its resonant frequency and produces short bursts of high-frequency vibrations. These sound wave trains generated by the ultrasonic transducer or search unit are transmitted into the material being tested. When the search unit is in direct contact with the test material, the technique is known as contact testing.
How UT Works
If flaws or discontinuities are present, an acoustic mismatch occurs, and some or all of the ultrasonic energy is reflected back to the search unit. The piezoelectric crystal in the search unit converts the reflected sound wave or "echo" back into electric pulses whose amplitudes are related to flaw characteristics and whose time of travel or time of flight through the material is proportional to the distance of the flaw from the entrance surface.
Ultrasonic pulses also are reflected from the back surface to the material, and this signal represents the total distance traveled. The pulse received from the back surface also can represent the width, length, or thickness of the material, depending on its orientations. Ultrasonic thickness testing measures the wall thickness of pipes and vessels by measuring the total distance traveled by the ultrasonic pulses, represented by the distance from the initial pulse or front surface to the back reflection from the back surface.
Ultrasonic flaws and thickness indications are frequently displayed on an instrument or computer display screen.