Time dependent micromechanics in continuous graphite fiber/epoxy composites with fiber breaks

Time dependent micromechanics in graphite fiber/epoxy composites around fiber breaks was investigated with micro Raman spectroscopy (MRS) and two shear-lag based composite models, a multi-fiber model (VBI) and a single fiber model (SFM), which aim at predicting the strain/stress evolutions in the composite from the matrix creep behavior and fiber strength statistics. This work is motivated by the need to understand the micromechanics and predict the creep-rupture of the composites. Creep of the unfilled epoxy was characterized under different stress levels and at temperatures up to 80°C, with two power law functions, which provided the modeling parameters used as input for the composite models. Both the VBI and the SFM models showed good agreement with the experimental data obtained with MRS, when inelasticity (interfacial debonding and/or matrix yielding) was not significant. The maximum shear stress near a fiber break relaxed at t−α/2 (or as (1+ tα)−1/2) and the load recovery length increased at tα/2(or (1+ tα)1/2) following the model predictions. When the inelastic zone became non-negligible, the viscoelastic VBI model lost its competence, while the SFM with inelasticity showed good agreement with the MRS measurements. Instead of using the real fiber spacing, an effective fiber spacing was used in model predictions, taking into account of the radial decay of the interfacial shear stress from the fiber surface. The comparisons between MRS data and the SFM showed that inelastic zone would initiate when the shear strain at the fiber end exceeds a critical value γ_c which was determined to be 5% for this composite system at room temperature and possibly a smaller value at elevated temperatures.; The stress concentrations in neighboring intact fibers played important roles in the subsequent fiber failure and damage growth. The VBI model predicts a constant stress concentration factor, 1.33, for the 1st nearest intact fiber, which is in good agreement with MRS measurements for most cases except for those with severely debonded interfaces. However, the VBI model usually gives a stress concentration profile narrower than the measured one due to the inelasticity near the fiber break. The low average fiber volume fraction in the model composites caused small relaxation in the stress concentration, which became more obvious at elevated temperatures, especially for large fiber spacing cases. When new break(s) occurred in the original intact neighboring fibers within an effective distance from the original break, the inelastic zones grew at a faster rate due to the strong interactions. Results on the creep-rupture of the bulk composites showed that the failure probability depends on the stress level and the loading time. The time dependent failure probability data could be fitted to a power law function, which suggested a link between the matrix creep, composite short-term strength and the composite creep-rupture.