The energy absorption of damaged braided and non-crimp fibre composite material structures
The effects of pre-existing damage on the mode of failure and energy absorption characteristics of Non-Crimp Fabric (NCF) and biaxially braided tubular sections under axial loading were considered. Loading rate effects were incorporated by testing at quasi-static rates and impact rates up to 7ms-1 and the pre-existing damage was simulated through stress concentrations and out-of-plane impact damage. Circular and square geometries were tested, and a range of NCF and braided fibre architectures were investigated. A number of failure modes were exhibited. NCF tubes were seen to splay at static and impact rates; at impact rates a reduction in Specific Energy Absorption (SEA) was recorded. Braided tubes failed in a combination of buckling and splaying at static rates. Under dynamic conditions all braided samples splayed and where a change in failure mode was seen, SEA was increased. Both NCF and biaxially braided tubes have been shown to offer a much lower SEA than Continuous Filament Random Mat (CoFRM) samples. A threshold size of damage was observed, where, typically, below that threshold the SEA was unaffected by the damage, and above that size the tube would fail globally. The NCF tubes exhibited an improved damage tolerance over CoFRM and braided samples. The braided tubes showed a poor damage tolerance at quasi-static rates although results suggest that the architectures with high axial fibre content will have a higher damage tolerance. It has been shown that modelling damaged areas of tubes as a cut-out is a conservative approach to finding failure levels. Peak stress was seen to be the dominant factor in determining loading at global failure as samples could fail even though the crushing stress is less than the failure stress. Previous work upon Interlaminar Shear Strength (ILSS) and SEA has been investigated and shown to overestimate energy absorption for tubular specimens and found to apply only to samples that fail by progressive crush or fragmentation; for those that do fail in this mode a link between ILSS and SEA has been stated. A technique for determining SEA and for predicting the effect of a SCF on failure mode of composite tubes has been proposed using UCS (Ultimate Compressive Strength) data and SCF (Stress Concentration Factor) data.