Acoustic emission (AE) produced during thermo-mechanical loading of composite materials is known as thermo-acoustic emission (thermo-AE). When thermally cycled, coupons of various composite materials have been shown to produce quantities of AE which are directly proportional to the damage level. This work demonstrates the applicability of thermo-AE monitoring to an engineering structure.
Papers presented at the symposium held in Charlotte, NC, March 1989, examine the phenomenon in which elastic or stress waves are emitted from a rapid, localized change of strain energy in a material. The first section focuses on AE sensors and systems. The second deals with fundamental investigation
The results of a study of the acoustic emission (AE) behavior of impact-damaged, spherical, composite test specimens subjected to thermal cycling and biaxial mechanical loading are presented. Seven Kevlar{reg_sign}-epoxy, filament-wound, spherical composite test specimens were subjected to different levels of impact damage. The seven specimens were a subset of a group of 77 specimens made with simulated fabrication-induced flaws. The specimens were subjected to two or three cycles of elevated temperature and then hydraulically pressurized to failure. The pressurization regime consisted of two cycles to different intermediate levels with a hold at each peak pressure level; a final pressurization to failure followed. The thermal and pressurization cycles were carefully designed to stimulate AE production under defined conditions. Both impacted and nonimpacted specimens produced thermo-AE (the term given to emission stimulated by thermal loading), but impacted specimens produced significantly more. Thermo-AE was produced primarily by damaged composite material. Damaged material produced emission as a function of both rising and falling temperature, but the effect was not repeatable. More seriously damaged specimens produced very large quantities of emission. Emission recorded during the static portion of the hydraulic loading cycles varied with load, time, and degree of damage. Static load AE behavior was quantified using a newly developed concept, the event-rate moment, and various correlations with residual strength were attempted. Correlations between residual strength, long-duration events, and even-rate moments were developed with varying degrees of success.
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