TechSolutions 16: Selecting a Nondestructive Testing Method, Part VII: Acoustic Emission Testing
Acoustic emission testing (AET) is a nondestructive technique that monitors defect formation and failure within a material through the detection and analysis of acoustic emission (AE) signals – stress waves generated by the rapid release of strain energy.[1, 2] External stimuli, such as a temperature gradient or an in-service mechanical load, can cause changes to the internal structure of a material. Crack initiation and propagation (release of stored elastic energy), the opening and closing of cracks, twinning*, dislocation movement (i.e., slip), reorientation of grain boundaries, plastic deformation, and phase transformations (release of stored chemical and/or free energy) all lead to the release of AE signals.[1-6] AET is fundamentally different from other nondestructive testing (NDT) methods because it passively detects energy that is released from a material, whereas other methods require an energy input for the defect to be detected (i.e., x-rays, gamma rays, ultrasound, thermal energy, and microwaves).
AET is effective for detecting fatigue and fracture behaviors in metals, composites, plastics, fiberglass, ceramics, concrete, and wood and is commonly used for the detection of faults and/or leaks in pressure vessels, piping systems, and tanks and to monitor welding and corrosion progress.[1, 2, 7] As welds solidify, acoustic emissions are released and detected as a result of internal structural changes; these emissions stop when the welded joint is completely hardened. Corrosion, on the other hand, is detectable since it is a chemical reaction that involves the release of energy. This energy can be detected by AE sensors and as a result, corrosion progress can be tracked.