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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Deformation and fracture of non-crimp fabric composites

Author: BIBO, Gary

Year: 1997

Supervisor(s): Paul Hogg

The particular genre of textiles investigated in this document was the warp knitted (non-crimp) multiaxial fabric for the purpose of identifying damage mechanism and quantifying their effect on subsequent mechanical performance. Cost savings through automated high volume production may be passed onto the user through reduced labour time (higher deposition rates) and near net manufacturing making this material attractive as reinforcement in high performance composites. Coupled with lean and agile processing routes, non-crimp fabrics provide an alternative to the benchmark (unidirectional prepreg tape) system.
Basic mechanical properties, toughness, impact resistance and subsequent residual compressive strength measurements were made against equivalent unidirectional prepreg tape laminates for both glass and carbon fibre reinforced materials. Generally, the presence of stitching or the process of warp knitting may be attributed to the global or macroscopic behaviour of the fabric.

Stress concentrations led to lower basic mechanical properties compared with the unidirectional prepreg tape equivalents, however, the fabric architecture causing this had a positive effect on the measured mode II interlaminar toughness. The higher toughness of the non-crimp fabric laminates over the unidirectional prepreg tape laminates, however, did not result in a commensurate decrease in measured impact induced delamination damage. Despite the non-crimp fabrics' (both glass and carbon fibre reinforced materials) lower fundamental compression strength, their respective compression after impact strengths were found to be comparable, and in some cases superior, to their equivalent unidirectional prepreg tape constructions. The reduced rate of residual strength less in the non-crimp fabric laminates was attributed to a crack bridging mechanism, effectively inhibiting delamination propagation.