Fracture mechanics evaluation of filament wound case materials subjected to operational environments

Abstract

The fracture characteristics of several filament wound composite materials were evaluated over operational conditions typical of solid rocket motor composite cases. The fracture toughness (KQ) for Mode I crack growth was determined for three composite systems, a Kevlar-49/AS4 graphite laminate and two AS4 graphite laminates of different orientation layup. Double-edge notch tensile coupons were used to generate KQ data at several crack sizes at room temperature to determine that value of KQ which was independent of crack length. The fracture toughness and net stress were found to depend on crack length over a fairly large range. These results were in good agreement with other work reported on flat plate, autoclaved laminate systems. The effects of elevated temperatures (150° to 200°F) were compared to the ambient (77°F) results for single cycle loading and found to be within the scatter of the tests. Since potential applications of the composite systems involved multiple cycle loading, high humidity exposure, and salt water submersion, several tests were conducted to evaluate these effects, on KQ. All samples were subjected to three loading cycles to 75% of ultimate failure and then placed in other environments. High humidity (15% RH) and salt water exposure effects were evaluated in two sample sets. These sets, plus one control set, were then tested at 200°F after removal from the environment. The data show that the fracture toughness, KQ, was unaffected by the operational environmental exposure and multiple load cycles. Nomenclature a = crack length of edge notch b = half-width of tensile specimen c v = coefficient of variation D = diameter F(a/b) = geometry correction factor for finite dimension specimens Kj = Mode I stress intensity factor K lc = critical Mode I stress intensity factor or fracture toughness KQ = quasifracture toughness for Mode I loading on composite materials L = length W -sample width a = far-field tensile stress o c = critical far-field tensile stress o net = net tensile stress in crack region 6 = helical winding angle