The application of finite element analysis to holographic interferometry for composite delamination detection
This thesis demonstrates the application of Finite Element Analysis (FEA) to investigate the use of pulse laser holographic interferometry for full-field non-destructive testing of delamination in composite materials. Three specimen composites are investigated: a three-layered composite beam, a fibre-reinforced composite (FRC) Nomex honeycomb panel and an aluminium honeycomb XAS/914 sandwich composite panel. The dynamic and modal response of a beam with a 14.7% delamination was investigated and analysed. The eigenmodes and excitation frequency required to excite the delamination was determined and the ability to detect damage location and extent was investigated. Double-pulsed and time-averaged interferograms were obtained of a simulated "kiss-contact" delaminated composite beam during simple harmonic excitation. Different excitation methods were investigated including simple harmonic excitation and acoustic impulsive excitation from a 25 J spark gap transducer (0.5 N m-2 at 1 m from the composite). To the authors knowledge no attempt has been made to simulate the response of the acoustic transducer described herein, or model with FEA the behaviour of acoustic excitation of a sandwich honeycomb panel for the purposes of holographic interferometry. The propagation of the acoustic shock wave from the acoustic transducer and its impact upon a panel was modelled. Double-pulsed and time-averaged interferograms were recorded using a typical off-axis transmission holography and conventional photographic materials were employed. These interferograms were compared with dynamic implicit finite element models of the damaged composites. The correlation between FEA and holographic interferometry was investigated to detect and quantify the extent of delamination, and to improve the ability to excite delaminated composites at the most appropriate frequencies.