A micromechanical damage model for carbon fiber composites at reduced temperatures

Fiber-reinforced composites are seeing increased use in civil infrastructure applications where they may be exposed to moisture, sub-ambient temperatures (below -40 degrees C in northern regions), and reduced-temperature (freeze thaw) thermal cycling. These environments may induce internal damage in the composite due to residual stresses caused by mismatches in coefficients of thermal expansion between the constituents. In this study, a micromechanical damage model is presented for the prediction of matrix crack development in a unidirectional carbon/epoxy composite resulting from exposure to sub-ambient temperatures. Two thermal loadings are considered: cool-down from the cure temperature of the composite (121 degrees C) to 25 degrees C (Delta T = -96 degrees C) and to -50 degrees C (Delta T = -171 degrees C). A finite element model is utilized to determine the internal stresses due to differences in the CTE of the constituents, and a shear-lag model is developed to predict subsequent crack spacing in the matrix. The influence of fiber spacing (or fiber volume fraction) is addressed. Model predictions indicate that, at 25 degrees C, the internal stresses are not large enough to cause matrix cracking in this composite. However, at -50 degrees C, the longitudinal tensile stress in the matrix exceeds the strength of the epoxy at most typical fiber volume fractions, which will result in matrix cracking. The shear-lag model was used to predict the subsequent crack spacing, and demonstrated a strong dependence on service temperature and fiber spacing. The model predictions provide good qualitative agreement with experimental observations, and indicate that the development of damage in a composite should be considered in the design of composite structures for reduced temperature environments.