History-Dependent Thermomechanical Properties of Graphite/Aluminum Unidirectional Composites

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A high-modulus graphite(VS0054)/aluminum(6061) unidirectional composite was tested to characterize its thermomechanical properties as influenced by the thermomechanical history. The experiments included (1) tension tests at four different temperatures under monotonically increasing load followed by unloading; (2) measurement of the cyclic thermal expansion in the longitudinal and transverse directions, conducted on annealed and T-6 heat-treated composite specimens; and (3) tests on the thermal expansion behavior of specimens simultaneously held at a constant transverse tensile stress during the thermal cycling. The tension test results show that the yield strength of the composite is greatly influenced by the residual stress, which may vary with the thermomechanical loading history. During thermal cycling the aluminum matrix of the annealed specimen undergoes elastic and plastic deformation owing to the thermal mismatch between the matrix and the fiber. This results in a characteristic cyclic nonlinear response during thermal cycling, as observed by the cyclic thermal expansion measurement. In order to interpret these results, an engineering material model (MET*MAT) was developed to characterize the salient features of this deformation. The model considers the aluminum as an elastic-perfectly plastic material with temperature-dependent yield and thermal expansion properties. The model permitted calculation of the internal (residual) stresses during the thermal and mechanical loading. The observed behavior was correlated to the predictions made by the model within an acceptable accuracy.