X-Ray and Ultrasonic Testing are the two most common methods of non-destructive testing (NDT) used to detect discontinuities within the internal structure of welds. The obvious advantage of both these methods of testing is their ability to help establish the weld's internal integrity without destroying the welded component. It used as follow:
Detect surface flaws, such as cracks, seams, and 
Internal flaws such as voids or inclusions of foreign material.
Measure wall thickness in tubes and diameters of bars.