The detection of damage induced acoustic emission in advanced composite materials using embedded optical fibre sensors

A fibre optic Michelson interferometric sensor was developed for the purpose of monitoring damage-induced acoustic emission (AE) in composite materials. The sensor used an active homodyne quadrature-maintaining feedback loop to permit operation with the sensitivity needed for the detection of low amplitude acoustic signals, and a series of high-pass filters and amplifiers to optimize a passband extending from 100 kHz to above 1 MHz.;Two additional NDE techniques (both products of previous developments in the FOSS laboratory) were used to independently establish the extent of internal damage sustained by Kevlar/epoxy samples exposed to transverse, quasi-static loading conditions. Image-enhanced backlighting (IEBL) was employed to visually identify delamination, thereby permitting the categorization of collected AE signals as either pre- or post-delamination events. A second damage detection system, based on the fracture of structurally weakened, embedded optical fibres, was used to verify in situ the connection between collected AE data and damage growth within a composite laminate.;Rudimentary signal analysis was performed on acquired AE data, using both time and frequency domain parameters. The frequency domain analysis revealed a trend with regard to the amount of damage present in a laminate; specifically, the concentration of signal energy showed a tendency to shift from higher (300-600 kHz) to lower (below 300 kHz) frequencies under conditions of increasing load and damage.;Preliminary characterization of the sensor was accomplished by an examination of its response to continuous and transient acoustic excitation sources, and by simultaneous comparisons with the output of a PZT acoustic emission transducer. The sensor's potential for non-destructive evaluation (NDE) was investigated by placing Kevlar/epoxy coupons with embedded fibre sensors under quasi-static, transverse loading conditions sufficient to cause damage.