Determination of Interfacial Fracture Toughness in High Temperature Composites

Understanding the behavior of the fiber/matrix interface region over a range of temperatures is essential for designing composites that will have a high service temperature. In the current work, the interface failure sequence was observed during fiber pushout tests on two model composites (steel/epoxy and polyester/epoxy) with different Young’s moduli ratio and residual stress values. Novel photoelastic experiments were conducted on the model composites to measure the interfacial crack length versus load during the fiber push-out test. The data were used to better understand the failure mechanisms during the test and to determine the range of applicability of analytical and computational models of the test. Debonding was observed to occur from either the top or the bottom of the sample depending on the ratio of the elastic moduli of the fiber and matrix and the residual stress state. The pushout data from a polyester/epoxy system which debonded from the top was fit to a shear lag solution to obtain the fiber-matrix interfacial toughness (GIIc). The resulting interfacial toughness was then used to check the predicted debond length as a function of pushout force. The debond length calculated from the shear lag model was less than the measured debond length by a nearly constant 1.5 fiber radii which may correspond to the thickness of the surface effects region for polyester/epoxy. In the future, the results of the model experiments will be used to understand the interfacial properties of two representative high temperature composites, SiC/Ti-Al-V and Al2O3/Ti-Al-V. A special high temperature apparatus was constructed for performing the push-out test at temperatures ranging from room temperature to 1000°C under vacuum. Performing interfacial measurements at elevated temperatures can be used to optimize interfacial performance at service temperatures and to better evaluate the effects of residual stresses and matrix ductility on fiber debonding and sliding.