Influence of damage accumulation under fatigue loading on the AE-based health assessment of composite materials: Wave distortion and AE-features evolution as a function of damage level

Pattern recognition of Acoustic Emission (AE) data is generally performed based on prior knowledge about the acoustic signatures of material damage mechanisms. Traditionally, these signatures are implicitly assumed to be unaffected by damage accumulation during the fatigue life of the material. This study investigates the influence of cumulated damage under cyclic loading on the acoustic signatures of local fracture mechanisms in composites. Artificial AE sources are created using an ultrasonic transmitter and a Pencil Lead Break in order to reproduce various real-like AEs, such that the waveforms, measured at the sensor level, are characterized as similar to real acoustic signatures of local fractures in carbon/epoxy composites. It is shown that these waveforms are distorted with damage accumulation in both time and frequency domains, leading to important changes in the AE-features used in data classification. Consequently, this can engender unreliable statistical representation of the AE sources in such non stationary media. (C) 2016 Elsevier Ltd. All rights reserved.