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Title: Failure of laminates containing embedded wrinkle defects : numerical and experimental study
Author: Mukhopadhyay, Supratik
ISNI:       0000 0004 5992 585X
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2016
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The work presented in this PhD project has investigated the effect of out-of-plane fibre waviness or 'wrinkling' defects on the failure of fibre reinforced composite laminates both experimentally and numerically. A quasi-isotropic lay-up ([+45/90/-45/0]3s) was chosen for this study because it is sufficiently representative of industrial components and from a failure perspective, presents a general scenario where both interlaminar failure (delamination), and intra-laminar failure (transverse cracking and fibre breakage/kinking) occur in a progressive and interactive manner. The initial focus was to assess the quasi-static tension and compression failure response of specimens with an artificially introduced embedded wrinkle of controllable severity. Specimens with three different grades of wrinkle severities were manufactured and tested, with high-speed video being used to monitor the damage initiation and propagation. Following the test findings, finite element models of wrinkle specimens were developed using the actual specimen geometry. Continuum damage models and cohesive zone models were implemented in Abaqus/Explicit to simulate failure. Excellent predictions were obtained both for failure stress levels and matrix crack/delamination interactions. Under axial compression, a switch of failure mode from fibre kinking to delamination with increasing ~le severity was also captured well. The study was then expanded to investigate the fatigue performance of laminates with embedded wrinkles. For the experimental programme, a number of moderate and lowest severity wrinkle specimens were manufactured and tested at 40, 50, 60 and 70% of the quasi-static tensile strength in tension-tension fatigue loading with R ratio 0.1. In addition pristine specimens (without any wrinkle defect) were also tested. The damage progression was continually monitored by a camera focused on the wrinkle, capturing images at regular intervals throughout the test. Some tests were interrupted partway through and x-ray CT scanned to identify the damage location and state inside the laminate. Numerical models for fatigue delamination were implemented in LS-Dyna. A crack-tip tracking based fatigue degradation law was implemented in a cohesive element framework. To improve its accuracy a phenomenological fatigue initiation model was integrated into the code. This m9del showed excellent qualitative and quantitative agreement with the tests. Finally, a parallel stream of work was undertaken to develop and implement a numerical tool to model mesh-independent discrete matrix cracking. This was applied to the problem of quasi-static tensile failure in a wrinkle specimen and successfully shown to overcome the limitation of mesh-orientation bias of crack growth otherwise seen in continuum damage models.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available