Acoustic Emission Monitoring of Interlaminar Micro-Cracking and Strength

Abstract

An experimental study was performed to investigate the structural properties of curved composite beams controlled by interlaminar tension and interlaminar tension/shear strength. Two distinct specimen types were fabricated from carbon/epoxy using the VARTM process and pulled in tension to produce failure due to large interlaminar (through thickness) tension and shear stresses. Acoustic emission monitoring was used to experimentally study failure progression from initial matrix cracking to final failure. Initially, the material properties of fiber, matrix and interlaminar shear strengths were found following ASTM tests while acoustic emissions were recorded to establish baselines and thresholds of acoustic emission counts and energy. Later, these thresholds were applied during the tests of the actual specimens. Several loading histories were explored to investigate the Felicity ratio. It was found that the loading history critically affects and lowers the interlaminar tension and shear strengths of the specimen. Furthermore, there was no observed hysteresis or nonlinearities in any of the strain gauge recordings, thus making it difficult to detect an upcoming failure.