Increasing Impact Strength of a Short Glass Fiber Compression Molded BMC by Shortening Fibers without Change in Equipment

Bird strike, volcanic rock, hailstones, micrometeoroids, or space debris can cause damage to aircraft and space vehicles, therefore their composite materials must have high impact resistance to maximize safety. In a 55% wt. CaCO(3) compression molded short glass fiber polyester GFRP-BMC (bulk molded compound), shortening the nominal 6.4 mm fiber length formulation, by 30 min extended mixing, to 0.44 mm was found to increase Charpy impact values, a(uc), without a change in the compression molding equipment. Specimens were cut from square panels in a spiral configuration in conformity with ASTM D 6110-02 for orthotropic panels, the flow direction approximately radially outward from the charge. At a median-fracture probability of P(f) = 0.50, extended mixing improved a(uc) by 29%, from 7.43 to 9.59 kJm(−2), and for each solidification texture angle, namely, 0 to 90 (random), 71, 45 and 18 deg, the a(uc) increased by 25% (6.26 to 7.86 kJm(−2)), 18% (9.36 to 11.07 kJm(−2)), 35% (7.68 to 10.37 kJm(−2)), and 20% (6.96 to 8.36 kJm(−2)), respectively. This strengthening can be explained by an increased number of thermal compressive stress sites between the glass fiber and matrix due to a difference in the coefficient of thermal expansion (CTE) during cool-down, and shrinkage, with an increased number of spaces between fibers, |S(f)| from 217 to approximately 2950 per mm(3), enhancing impact energy.