New Methodology for Determining in situ Fiber, Matrix and Interface Stresses in Damaged Multifiber Composites

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Two recent developments, experimental Micro-Raman spectroscopy (MRS) and theoretical quadratic influence superposition (QIS) analysis, have greatly improved micromechanical measurement and realistic numerical modeling of fiber, matrix and interface stresses around fiber breaks in multifiber, polymer matrix composites. In this study, these, two techniques are combined to develop a methodology for determining the in situ interfacial strength parameters, such as the yield stress and frictional sliding resistance, which are major determinants of local deformation and damage propagation around fiber breaks. With a spatial resolution of 2 mu m, MRS is used to measure fiber axial strain profiles produced by naturally occurring fiber breaks in multifiber, high modulus graphite/epoxy model composites under uniaxial tension. For the same matrix material and surface-treated fibers, several fiber spacings and different interfacial conditions are tested including: sized fibers, unsized fibers and fibers with a PMMA coating applied using RF-Plasma grafting. The QIS micromechanical technique is then used to interpret the MRS data to quantify the complex, local in situ matrix and interface stresses and deformations, which can be accurately described by an elastic-plastic-debond (multiparameter) micromechanical model. These quantities are found to depend highly on interface condition and local fiber spacing, thus motivating questions about the usefulness of traditional single-fiber-composite tests in forecasting localized failure in large composites. Comparing the stress concentration profiles on intact neighboring fibers measured experimentally and predicted by QIS reveals the complex dependence on fiber spacing and the extent and type of interfacial damage. In a few examples, it is shown how the multi-parameter description of the interface, calibrated at the single fiber break level, can be used as input for analysis and prediction of activity around more complex fiber break arrangements in a much larger composite. Based on the present results, recommendations for further investigations; using MRS-QIS, are given.



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