High-temperature strength and storage modulus in unidirectional hybrid composites

The mechanical properties of pultruded rods of unidirectional hybrid glass/carbon-epoxy composites were evaluated with respect to retention of high-temperature properties up to 250 °C. Two prototype matrices were produced using developmental epoxy formulations with glass transition temperatures (T_g) of 176 and 202 °C, respectively. The two prototype composites showed diminished tensile strength at high temperatures, but with different temperature thresholds. The high-T_g composite retained full strength to 170 °C, while the lower-T_g composite retained full strength to 80 °C. Matrix softening and loss of fiber-matrix adhesion was a major factor affecting the strength reduction observed at high temperatures. The storage modulus, measured by dynamic mechanical analysis (DMA), showed temperature dependence nearly identical to the tensile strength for both composites. The correlation between storage modulus and tensile strength was analyzed in terms of the effect of temperature on the shear modulus of the matrices. The storage modulus behavior is presented as a basis for projecting the strength-temperature dependence of unidirectional composites.