Transverse Failure Mechanisms of Composite Laminates with Yarn-level Fibre-hybridisation

UoM administered thesis: Phd

  • Authors:
  • Chengguang Liu

Abstract

Poor damage tolerance under impact loading is a critical challenge for composite material, especially for safety-critical structural components. In recent years, to address the issue of damage tolerance in composite laminates, several toughening routes were investigated at different length scales—yarn-level fibre-hybridisation is one of such approaches where high strength brittle fibres are combined with toughening fibres to introduce conductive failure mechanisms. This research focuses on intra-yarn hybrid fibre composite laminates. The objective is to understand computationally the role of such fibre-hybridisation on transverse failure mechanisms. This research studied composite laminates reinforced by commingled S-glass and polypropylene continuous fibres. The role of intra-yarn fibre-hybridisation on damage mechanisms are studied using a three-phase two-dimensional Representative Volume Element (RVE) approach. For the micro-mechanical model, statistically equivalent fibre distributions are generated using the Nearest Neighbour Algorithm (NNA). Damage modes were predicted using a Mohr-Coulomb yield criterion and Cohesive Zone Model (CZM). Both thermal and mechanical loading was considered in this study. In-situ 3 point bending test was conducted to validate the proposed modelling approach. The results of numerical investigations showed that the toughening fibres play an important role in transverse damage mechanisms by introducing new damage modes. Under transverse loading and thermal loading, impact energy was dissipated via PP/epoxy interface debonding before S-glass/epoxy interface debond. Consequently, progressive and ductile fracture behaviour was observed, showing the toughening effect of fibre hybridisation at yarn-level. Low PP/epoxy interface properties would ensure such damage mechanism being valid, while high PP/epoxy interface properties could constrain such toughening mechanism. A parameter α is defined to describe the quality of PP/epoxy interface. Parametric studies illustrated that there are threshold α_T above which damage mechanism will change. To ensure the toughening mechanism being valid, a high α_T is desired. In a parametric study, it was found that by increasing the PP fibre content, the threshold value could be increased lightly. Moreover, α_T appears to be sensitive to S-glass fibre distribution rather than the PP content. The thermal residual stress would increase α_T significantly, at the cost of reduced transverse tensile strength. Above phenomenon were observed in both tensile and shear loading. However, two loading cases have different α_T value. Moreover, under shear loading, the fracture path would change if α exceed α_T while fracture path remain the same under tension. This finding illustrated that the damage mechanism are independent of loading case but the local stress/strain field could be influenced by loading conditions. In summary, the proposed modelling approach proved that PP fibre could alter damage initiation and propagation mechanisms, and could potentially be used for introducing conductive damage modes, and thus tailoring energy absorption and damage tolerance of composite laminates.

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Original languageEnglish
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Award date1 Aug 2020