Ultrasonic Inspection is a nondestructive method in which beams  of high frequency sound waves are introduced into materials.  By measuring the time of flight of the sound beam the thickness of a material can be calculated.  Surface and subsurface flaws can also be detected with this method.  Cracks, laminations, shrinkage cavities, bursts, flakes, pores, disbonds and other discontinuities that produce reflective interfaces can be easily detected.
The advantages of ultrasonic thickness gauging include;
Only one side of the work piece needs to be accessible.

High sensitivity to depth and flaw identification.

Great accuracy in determining the exact location of flaws.

Nonhazardous to operators and work pieces.

Extremely portable