Mechanical Characterization of IM7/8551-7 Carbon/Epoxy Under Biaxial Stress

This paper presents the results of a detailed characterization of the relatively new IM7/8551-7 carbon/epoxy material system under conditions of multiaxial stresses applied to both laminae and laminates. IM7 is a high elongation, high strength carbon fiber, and the 8551-7 matrix is a high toughness epoxy resin. The lamina tests provided a failure envelope for combinations of transverse tension or compression and in-plane shear, and illustrated that the matrix dominated strength properties exhibit a strong dependence on the state of stress. The transverse strains at failure were found to be higher than for previous epoxy systems examined, and the shear stress-strain curves showed significant nonlinear behavior. These features apparently contribute to the “toughness” of the resin. Laminated specimens in a quasi-isotropic configuration were tested in tension-tension and tension-compression stress states. In previous work on other fiber/resin systems it has been shown that laminate ultimate failure could be correlated by means of fiber direction strain in a critical ply, independent of matrix cracking and the details of the laminate configuration and state of stress. However, different fiber strain values must be used for tension and compression. The behavior of quasi-isotropic IM7/8551-7 laminates appears to follow the observations noted above, with the important exception that laminate tension failure strain is lower than unidirectional coupon strain. The compression values are significantly higher than seen previously with AS4/3501-4 laminates, but as usual lower than the tension values. The results raise presently unresolved issues about the effect of the resin and possibly the processing variables on the delivered laminate strength.