Laser-Raman Measurement and Analytical Evaluation of Matrix Creep Induced Stress Relaxation in Broken Fibers

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Unidirectional metal-matrix and polymer-matrix composites reinforced forced with long brittle fibers may suffer from creep rupture under constant applied stress, since the stress in broken fibers is relaxed with time. In the present work, laser-Raman stress measurement and analytical evaluation were performed to investigate such stress relaxation, resulting in the following findings :1. Fiber pull-out tests were done using single-fiber model composites for the purpose of clarifying the effect of matrix creep on the stress profile of fiber stress. The stress of carbon fibers embedded in polymers was measured by means of Raman spectroscopy. Three kinds of polymer matrices, i.e., normal epoxy, flexible epoxy and UV-curable acrylic, were employed. It turned out that the profile of fiber stress changes with time more markedly if the matrix creeps more significantly.2. Moreover, the stress relaxation in broken fibers was measured using single-fiber model composites and Raman spectroscopy. It was found that the stress relaxation in broken fibers also occurs more markedly if the matrix creeps more significantly.3. A functional based on complementary energy was presented in an incremental form. It was shown that the functional has a stationary function satisfying a shear lag differential equation. Then, by employing the bilinear approximation of fiber stress profiles, analytical solutions for axisymmetric fiber breakage and fiber pull-out models were obtained from the stationary condition.4. The analytical solutions mentioned above were applied to the present experimental results. The creep constants were identified from creep tests of the polymers employed. It was thus shown that the analytical solutions represent well the tendencies observed in the experiments.