Damage in Non-Crimp Fabric carbon fiber reinforced epoxy composites under various mechanical loading conditions

Aaditya Pradeep Suratkar, The University of Western Ontario

Abstract

Composite materials have emerged in the recent years as a frontrunning lightweight replacement for metals for structural applications in lightweight automobiles. However, the key adoption criteria for composites in lightweight vehicles is their crashworthiness, i.e., the amount of energy a structure made with composites would absorb in a crash event. The energy is absorbed in composites through various damage modes such as matrix cracks, delamination, fiber breakage, etc. Therefore, adoption of a certain composite material system requires thorough understanding of the initiation and propagation of damage until failure in that system under various mechanical loading conditions, which is also the central theme of this study.

The response of Non-Crimp Fabric (NCF) carbon fiber reinforced epoxy composites, manufactured using High Pressure-Resin Transfer Molding (HP-RTM), under quasi-static mechanical loads was thoroughly investigated in this study. The behavior of unidirectional lamina and four laminates, [0/±45/90]_s, [90/±45/0]_s, [±45/0₂]_s and [±45/90₂]_s, was characterized in this study. The engineering properties and interlaminar fracture toughness of the lamina and the laminates are first measured using standard test methods. An in-situ damage monitoring technique called as ‘edge replication’ and post-failure fractography were employed to understand the damage initiation and growth in the materials under quasi-static tension and compression. The role of stitching sites, which are the compliant areas formed due to stitching threads in the NCF architecture, in the initiation of damage was clearly understood in this study. The damage generally initiated in the form of localized cracks in the stitching sites at low strains, which then nucleated other damage modes in the laminates at higher strains. Influence of ply thickness and stacking sequence was also investigated. It was concluded that, although the stacking sequence had negligible influence on the global stress-strain response under tension, the rates and extents of damage were strongly dependent on the stacking sequence. The global compression response, measured engineering properties and the observed damage modes showed higher sensitivity to the laminate stacking sequence. Finally, the damage observed in the composites was correlated to the energy dissipated in the creation of damage in this study.