UT uses complex electronic equipment. Any material which transmits mechanical vibrations can be tested. UT detects both linear and non-linear flaws and permits three dimensional interpretations. Evaluation is often difficult. The UT instrument converts electrical pulses into mechanical vibrations or waves. These waves travel across the tested specimen and reflect from flaws because of their different acoustic nature. The returning reflected waves are reconverted to electric energy and displayed as signals on a cathode ray tube (CRT). The position and size of these signals correspond to the position and size of the flaws.
Material density and its thickness set the limits of RT’s usefulness. Internal flaws are RT’s forte; its two dimensional views sometimes its drawback. RT uses penetrating radiation and works on the principle that denser or thicker materials will absorb more radiation and a sheet of radiographic film or image amplifier. A flaw presents anywhere within the specimen will absorb less radiation than the specimen itself. The flaw’s presence and location will be indicated by an area of higher or darker exposure.
The surface defect is difficult to be inspected using technology of RT or UT. Most of UT equipment have a dead zone area that could not inspect a defect that close to the surface. MPI & DPI give the solution to find more exact the surface defect.