Acoustic Emission Testing – Physical Principles

Sonic impulses are emitted by a flaw in a ductile material subjected to a gradually increasing applied stress, due to the release of strain energy in the intensified stress field associated with the flaw. The release of strain energy creates stress waves that propagate through the material at the speed of sound. These stress waves behave like sonic impulses, and are, therefore, commonly referred to as acoustic emissions. However, there may be no significant release of strain energy in a brittle material to cause acoustic emissions prior to the occurrence of fracture.

Acoustic emissions attributable to flaws in metals are usually inaudible, but they can be thought of as being similar to the familiar sounds emitted by a piece of wood as it is gradually bent to the breaking point. The sounds not only reveal that the wood is cracking, but an individual hearing the sound can also sense the direction that it is coming from to help locate the piece of wood. It is useful to draw another analogy for brittle materials, such as a glass rod that is bent to the breaking point. No warning sounds will be heard as the glass rod is bent before it breaks.

The nominal applied stress in a component of a pressure vessel manufactured from a ductile metal can be well below the yield strength of the material, but the local stress concentrations developed at flaws can be high enough to cause highly localized plastic deformation or enlargement of the flaw by microcrack propagation. Both plastic deformation and crack propagation produce acoustic emissions that can be detected by acoustic sensors to disclose the existence of the flaw.

A single sensor can usually detect acoustic emissions originating anywhere in a relatively large vessel, because attenuation of the sonic impulses in a metal shell is very low. However, several sensors are usually employed to obtain confirming indications, and, more importantly, to locate the origin of the emissions in the shell by triangulation.

Categories: In-Service Inspection | Tags: | Leave a comment