On the tensile failure of 3D composites

Tensile tests are reported for some graphite/epoxy composites with three-dimensional woven interlock reinforcement. Composite failure consists of the accumulation of discrete tow rupture events distributed over a band of damage typically lo-20mm wide. Load-displacement data for gauges spanning the band indicate work of fracture values ranging from 0.4 to 1.1 MJ m-2. Most of these unusually high values derives from the ability of the composite to sustain loads near peak load (-1 GPa)for displacements significantly beyond those at which tows have all failed. The key mechanism is very strong friction or lockup that couples sliding, broken tows to the surrounding composite. Lockup is the product of the geometrical irregularity of nominally straight tows and clamping compressive stresses generated by the through-thickness reinforcement. Lesser contributions to the work of fracture arise from plastic straightening of tows prior to their rupture and the relatively easy but prolonged pull-out of tows following failure of the lockup mechanism.