Electronic Thesis and Dissertation Repository

Thesis Format

Monograph

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Wood, Jeff

2nd Supervisor

Montesano, John

Affiliation

University of Waterloo

Co-Supervisor

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.

Summary for Lay Audience

Non-Crimp Fabric (NCF) composites have emerged as a new class of composite materials in the recent years which combine the advantages of unidirectional prepreg and woven textile materials. They are made by stacking several unidirectional fabrics, made by stitching the fibers together, on each other and impregnating them with the resin material. These materials have limited out-of-plane folding, or crimp, in their architecture, which translates into superior in-plane mechanical properties (like prepreg materials), and are easier to handle (like woven textile materials). The key distinguishing features of NCF composites are the relatively weaker resin-rich zones created due to the stitching fibers pushing two adjacent fibers apart locally and the local fiber misalignment induced by it. The damage behavior, from initiation until failure, in the NCF carbon fiber reinforced epoxy composites under quasi-static tension and compression was thoroughly investigated in this study using in-situ damage monitoring technique called ‘edge replication’ and post-failure microscopic observations.

Damage initiation was found in the form of cracks in the stitching sites in the tested laminates under both, tension and compression. This damage was then found to nucleate other forms of damage such as ply cracks and delamination at higher loads. The rates of multiplication and extents of the distinct damage mechanisms depended on the laminate stacking sequence, geometric factors such as ply thickness and loading conditions. While the influence of stacking sequence on the measured engineering properties was minimal under tensile loads, it had a significant effect on the damage evolution process. For example, higher rates of crack multiplication were observed in the weak transverse plies, which were constrained by adjacent stiff plies. The final failure under tension was, however, governed by the failure of the load-bearing fibers, aligned along the loading direction. Under compression, on the other hand, the effect of laminate stacking sequence was more pronounced on both, the engineering properties and the damage evolution process. The failure of laminates under compression was instantaneous and a combination of different damage mechanisms. Finally, upon connection to energy dissipation, delamination was found to be the most critical damage mechanism, absorbing maximum energy in its creation.

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