Acoustic emission

Acoustic emission (AE) is the phenomenon of radiation of acoustic (elastic) waves in solids that occurs when a material undergoes irreversible changes in its internal structure, for example as a result of crack formation or plastic deformation due to aging, temperature gradients or external mechanical forces. In particular, AE is occurring during the processes of mechanical loading of materials and structures accompanied by structural changes that generate local sources of elastic waves. This results in small surface displacements of a material produced by elastic or stress waves generated when the accumulated elastic energy in a material or on its surface is released rapidly. The waves generated by sources of AE are of practical interest in the field of structural health monitoring (SHM), quality control, system feedback, process monitoring and others. In SHM applications, AE is typically used to detect, locate and characterise damage.

AE is commonly defined as transient elastic waves within a material, caused by the rapid release of localized stress energy. Hence, an event source is the phenomenon which releases elastic energy into the material, which then propagates as an elastic wave. Acoustic emissions can be detected in frequency ranges under 1 kHz, and have been reported at frequencies up to 100 MHz, but most of the released energy is within the 1 kHz to 1 MHz range. Rapid stress-releasing events generate a spectrum of stress waves starting at 0 Hz, and typically falling off at several MHz.

The three major applications of AE techniques are: 1) source location - determine the locations where an event source occurred; 2) material mechanical performance - evaluate and characterize materials/structures; and 3) health monitoring - monitor the safety operation of a structure, i.e. bridges, pressure containers, and pipe lines, etc.

More recent research has focused on using AE to not only locate but also to characterise the source mechanisms i.e. crack growth, friction, delamination, matrix cracking, etc. This would give AE the ability to tell the end user what source mechanism is present and hence allow them to determine whether or not structural repairs are necessary.

AE can be related to an irreversible release of energy. It can also be generated from sources not involving material failure including friction, cavitation and impact.

The application of acoustic emission to non-destructive testing of materials, typically takes place between 100 kHz and 1 MHz. Unlike conventional ultrasonic testing, AE tools are designed for monitoring acoustic emissions produced within the material during failure or stress, rather than actively transmitting waves, then collecting them after they have traveled through the material. Part failure can be documented during unattended monitoring. The monitoring of the level of AE activity during multiple load cycles forms the basis for many AE safety inspection methods, that allow the parts undergoing inspection to remain in service.

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