It is a non-destructive testing (NDT) method in which beams of high frequency sound waves that are introduced into the material being tested are used to detect surface and sub-surface flaws. The sound waves travel through the materials with some attenuation of energy and are reflected at interfaces. The reflected beam is detected and analyzed to define the presence and location of flaws. Ultrasonic waves are almost completely reflected at metal gas interfaces. Partial reflection occurs at metal liquid or metal solid interfaces, with the specific percentage of reflected energy depending mainly on the ratios of certain properties of the matter on opposite sides of the interface.

Cracks, laminations, shrinkage, cavities, bursts, flakes, pores, bonding faults and other discontinuities that can act as metal-gas interfaces can be easily detected. Inclusions and other inhomogenities in the metal being inspected can also detected by causing partial reflection or scattering of the ultrasonic waves, or by producing some other detectable effect on the ultrasonic waves.

Most of the ultrasonic inspection instruments detect flaws by monitoring one or more of the following:

1. Reflection of energy from metal-gas interfaces, metal-liquid interfaces or discontinuities within the metal itself
2. Time required for the beam to travel to the flaw and back
3. Time of transit of a sound wave through the test piece from the entrance point at the sending (transmitting) transducer to the exit point at the receiving transducer, and
4. Attenuation of the beam of sound waves by absorption and scattering within the test piece.

Applicability:

Ultrasonic testing or inspection (UT) is used for quality control and materials testing in all major industries. This includes Ultrasonic testing of castings, forgings, plates, extruded components, weld joints, electrical and electronic component manufacturing, production of steel, aluminum and titanium, fabrication of structures such as air frames, pressure vessels, ships, bridges, motor vehicles, machinery and jet engines. In service ultrasonic testing for preventive maintenance is used for detecting impending failure of rail road rolling stock axles, press columns, earth-moving equipment, mill rolls, mining equipment and other machines and compo nets. The flaws to be detected include voids, cracks, inclusions, pipe, laminations, bursts and flakes. They may be inherent in the raw materials, may result from fabrication and heat treatment, or may occur in service from fatigue, corrosion or other causes. Ultrasonic testing can also be used to measure thickness of metal sections during manufacturing and maintenance inspections.

Limitations:

1. Manual Ultrasonic Flaw detection requires careful attention by experienced technicians
2. Extensive technical knowledge is required for the development of Ultrasonic testing procedures.
3. Parts that are rough, irregular in shape, very small or thin or not homogenous are difficult to be tested
4. Discontinuities that are preset in a shallow layer immediately beneath the surface may not be detectable.
5. Couplants are needed to provide effective transfer of ultrasonic wave energy between transducers and parts being tested.
6. Reference standards are needed, both for calibrating the equipment and for characterizing flaws.

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