Experimental and Numerical Studies of Defect Topology on Delamination Growth in Glass/Epoxy Composites under Flexure
Creators: name: Dixit, Abhishek
Publisher: name: University of Surrey
Year: 2023

description: Composite materials display complex mechanical behaviour, and their failure prediction remains a challenging task, particularly the delamination failure resulting from impact damage in composite laminates. This research investigates the effect of delamination defect topology on the onset and growth of delaminations in both unidirectional glass and woven fabric laminates under flexural loading. As delaminations can take any shape after the impact loading depending on several parameters such as laminate thickness, stacking sequence, and impact energy, the actual delamination shapes were simplified in the current work using a range of primitives which were artificially embedded using Teflon film during the manufacture of the laminates. The defect configurations for unidirectional glass laminates included circular delaminations of various sizes, elliptical and peanut-shaped delaminations in longitudinal and transverse orientations with respect to the loading axis, and finally, the star-shaped delamination. Besides this, specimens with a limited range of idealised delamination shapes, including circular and elliptical shapes, were investigated for 8-Harness Satin (HS) woven glass fabric laminates. It was observed that the biggest delamination growth for all single delamination configurations occurred due to local buckling of the sublaminate and the subsequent delamination growth was very small or negligible until complete failure of the specimen. <br>
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description: For woven specimens, the Digital Image Correlation (DIC) technique was also used during flexural testing to measure the outer surface displacement fields. The work has shown that delamination onset and growth from artificially embedded delamination can be successfully captured using the out-of-plane displacements measured using the DIC technique. The effect of delamination location through the thickness of the laminate was also investigated for unidirectional glass cross-ply laminates which suggested that deeper delaminations (depth greater than 19% of the total laminate thickness) had no significant effect on the failure mode of the laminate. The specimens with multiple delamination configurations arranged in different patterns through the laminate thickness were investigated for unidirectional glass specimens, and it was concluded that a near-surface bigger circular delamination (inverted-triangle pattern) is the most severe among all configurations considered. <br>
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description: Finally, the experimental results of 8HS woven specimens were compared with the results of numerical modelling based on the Virtual Crack Closure Technique (VCCT) technique. The finite element models predicted the delamination growth in the transverse direction for all configurations considered for 8HS woven laminates; this was consistent with the experimental work. However, the delamination growth in the numerical modelling was incremental from the initial delamination compared to the instantaneous delamination growth in the experiment followed by some incremental growth. The out-of-plane displacements calculated from the finite element model showed good agreement with the DIC measurements. <br>