Acoustic emission monitoring of rate of damage growth in composite structures (Conference Presentation)

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Damage development in composite structural members under quasi-statically load include distributed matrix cracks, delaminations, and random fiber breaks which start at relatively low loads and at these levels do not pose threat to the load carrying ability of the structure. But at higher load levels the damage growth accelerates and leads to final failure. The existence of undetected impact or other types of damage can severely accelerate this gradual damage growth. Acoustic emission technique can provide a real time assessment of the rate of damage growth. This paper examines the results from test specimens with and without impact damage for which acoustic emission data was collected using wide band sensors capable to recording signals in excess of 2 MHz. Characteristics of signals obtained at different load levels are compared. In these tests unidirectional, cross-ply, and quasi-isotropic carbon-epoxy composite tensile specimens were monitored while they were statically loaded to failure. Huge volume of AE data is obtained during these tests was analyzed in detail to understand the nature of damage growth and identify the source of acoustic emission signals. The difference in the acoustic behavior between undamaged specimens and impact damaged specimens are presented. In addition, the pattern of acoustic emission signals in which a sequence of near identical waveforms appearing in clusters were noted in both undamaged specimens as well as impact damaged specimens. The formation of clusters of such AE signals appears to indicate the approaching failure of the specimen. These clusters of AE signals seem to parallel the formation of clusters of fiber breaks that form near the ultimate failure of composite specimens in CT examinations.