Effects of dispersed microvoids on thermal expansion behavior of composite materials

In our earlier papers [T. Takei, H. Hatta, M. Taya, Mater. Sci. Eng. A131 (1991) 133; T. Takei, H. Hatta, M. Taya, Mater. Sci. Eng. A131 (1991) 145], we found that the coefficients of thermal expansion (CTEs) of particulate composites are lowered by dispersed microvoids in the matrix. In order to prove this void dispersion effect by an analytical model and to utilize this effect for wide control of composite CTE, an analytical parametric study is conducted in this paper. It is found in this study that the combination of fiber type reinforcement with high elastic modulus and low CTE and disk-shaped voids was identified to remarkably lower the composite CTE. This CTE control technique is based on the mechanism that microvoids in the matrix are compressed by compressive stress field developed around reinforcement when the composite is subjected to temperature rise. In order to confirm experimentally this void dispersion effect on CTEs, three types of composite materials with dispersed voids in their matrices were fabricated, particulate, short fiber, continuous fiber (3D) reinforced composites, and CTEs of these composites were measured. It was concluded from comparison between the predictions and experimentally observed CTEs that the CTE reduction by the void dispersion actually occurs and this effect is analytically predictable up to some limit volume fraction, which depends on a composite type.