Ultrasonic testing is an NDT method in which beams of high-frequency sound waves are introduced into materials for the detection of surface and subsurface flaws in the material. The sound waves travel through the material with some attendant loss of energy (attenuation) and are reflected at interfaces. The reflected beam is displayed and then analyzed to define the presence and location of flaws or discontinuities.
Most ultrasonic inspection is done at frequencies between 0.1 and 25 MHz--well above the range of human hearing, which is about 20 Hz to 20 kHz. Ultrasonic waves are mechanical vibrations; the amplitudes of vibrations in metal parts being ultrasonically inspected impose stresses well below the elastic limit, thus preventing permanent effects on the parts.
The degree of reflection depends largely on the physical state of the materials forming the interface and to a lesser extent on the specific physical properties of the material. For example, sound 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 material on opposing sides of the interface.
Cracks, laminations, shrinkage cavities, bursts, flakes, pores, disbonds, and other discontinuities that produce reflective interfaces can be easily detected. Inclusions and other inhomogeneities can also be detected by causing partial reflection or scattering of the ultrasonic waves or by producing some other detectable effect on the ultrasonic waves.
Most ultrasonic inspection instruments detect flaws by monitoring one or more of the following:
- Reflection of sound from interfaces consisting of material boundaries or discontinuities within the metal itself
- Time of transit of a sound wave through the testpiece from the entrance point at the transducer to the exit point at the transducer
- Attenuation of sound waves by absorption and scattering within the testpiece
- Features in the spectral response for either a transmitted or a reflected signal
Ultrasonic inspection is one of the most widely used methods of NDT inspection. It can be used for:
- detection and characterization of internal flaws,
- detection of surface flaws,
- definition of bond characteristics,
- measurement of the thickness and extent of corrosion,
- detection of find inclusions in products
- determination of physical properties such as structure, grain size and elastic constants