Murat, BIS; (2015) Propagation and Scattering of Guided Waves in Composite Plates with Defects. Doctoral thesis , UCL (University College London).

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Abstract

Failure in composite structures due to low-velocity impact damage raises a significant maintenance concern because it can lead to a barely visible and difficult-to-detect damage. Depending on the severity of the impact, fiber and matrix breakage or delaminations can occur, reducing the load carrying capacity of the structure. Efficient structural health monitoring (SHM) of composite structures can be achieved by using low- frequency guided ultrasonic waves as they have advantages of propagating over large structure and being sensitive to defects located at any thickness position. This work focuses on the use of first antisymmetric guided wave mode (A0) for health monitoring in laminated composite plates. The first part of this work is to investigate the propagation of A0 mode in undamaged composite plates experimentally and compare the results to Finite Element simulations and semi-analytical analysis. This study is essential in order to improve understanding of the guided waves behavior in composite plates and would benefit the interpretation of received signals particularly for defect characterization. To gain a good understanding of the A0 mode interaction with defects in composites, a full three- dimensional (3D) Finite Element (FE) analysis is used. A systematic study of the influence of defect geometry and range of situations on guided wave scattering is demonstrated. Combined delamination with material degradation to simulate mixed- modes defect is shown. Two dimensional FE simulations used for analysis of large delamination are also presented. The final part of this thesis presents the scattering of guided waves at the impact damage using a non-contact laser interferometer. In this study, the results were quantified and compared to baseline measurements on undamaged composite panels. Significant scattering activities were observed, allowing for the detection of impact damage in composite plates. The impact damage was further characterized using standard ultrasonic C-scans. Good agreement between experiments and predictions was found.

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