Dynamic stress/strain response of the interphase in polymer matrix composites

The interphases of various sized E-glass-fiber/epoxy-amine systems were tested at displacement rates in the range of 230 to 2450 μm/sec using a new experimental technique (dynamic micro-debonding technique). The fiber systems include unsized, epoxy-amine compatible sized, and epoxy-amine incompatible sized glass fibers. A data reduction scheme was developed to relate the force vs. displacement response obtained from the dynamic micro-debonding technique to interphase shear stress/strain response. The stress/strain curves and interphase shear modulus values were obtained from these composite systems under average shear strain rates (ASSR) in the range of 215-3278 (1/s). The results showed that the magnitude of the interphase shear modulus was sizing and strain rate dependent. In all cases, the shear modulus was found to be more compliant than the bulk matrix. The two sized fiber systems exhibited the highest strain rate sensitivity, with modulus increasing about threefold over the range studied. In addition, the rate dependent behavior of the model interphase materials were determined using the dynamic mechanical analysis (DMA) technique. The model interphase materials closely resemble the interphase that forms on unsized and compatible sized fibers. Master curves relating the flexural storage modulus to strain rate were constructed based on the time-temperature superposition principle from DMA frequency sweep measurements. The DMA measured results are consistent with the dynamic micro-debonding test results, providing confidence in the test method as a reliable technique for characterizing the high strain rate properties of the interphase in composites.