Acoustic Emission

The term "acoustic emission" is actually somewhat of a misnomer. Acoustic emission, as regards the testing, is not actually sound, but the transient elastic stress waves produced when a material, such as fiberglass, is placed under stress. In AE testing, this stress loading is done by filling the tank or vessel being tested with either process fluid or water. If a defect exists within the vessel structural material, it will generate stress
waves that are detected by piezoelectric sensors mounted on the structure. The sensors convert the mechanical stress wave into an electrical impulse that is then transmitted to instrumentation measuring the characteristics of the wave including: amplitude, duration, counts (i.e., oscillations above a threshold amplitude - a "hit" - that strike a given sensor), signal strength, and rise time (the time it takes a "hit" to reach its maximum amplitude).

It was discovered early on that when a defect-free material is stressed a second time to a given load equal to the first loading, little significant acoustic emission will be produced. This is called the Kaiser Effect. If a material contains a defect, however, significant emissions will be produced during the second loading prior to reaching the magnitude of the first loading.

The ratio of the onset of significant emission during the second loading divided by the magnitude of the first loading is called the Felicity Ratio. This ratio is considered to be an indication of previously induced damage within a structure. Other relationships that have found application include:

• Emissions during load hold - A measure of continuing damage

• Total count - A measure damage

• High-amplitude hits - A measure of the number of high-energy microstructural failures, e.g., fiber breaks

These measures, how to use them, then incorporated into evaluation criteria, and formalized in a document of the Committee on Acoustic Emission from Reinforced Plastic (CARP) of the Soc. of the Plastics Industry (Washington, D.C.) [1].

An additional technique, Intensity Analysis, uses the measure of energy released from a growing defect or imperfection to determine "How bad is bad?" Intensity Analysis gives defects rankings of N, A, B, C and D, ranging from "N - Emission source is structurally insignificant" through "D - Major structural defect. Immediate shutdown and nondestructive examination." This method gives the inspector the ability to confirm defect indications and to weed out false positives.

The Intensity Analysis method will be incorporated into the next revision of the CARP document. Some companies, including NDEI, are already relying heavily on this technique.

 
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