Test Specimens for Bearing and By-Pass Stress Interaction in Carbon Fibre Reinforced Plastic Laminates,

Abstract : Compact test specimens for measuring the strength interaction behaviour of bolted joints subject to combined bearing and by-pass stresses have been studied. Multi-bolt specimens which have been successfully used to study these effects in aluminum alloy were found to be unsatisfactory because of the uncertainty in load transfer, and a new specimen based on parallel plates was developed. Bearing load at the holes is achieved through load transfer from the central CFRP coupon to the parallel plates. The maximum ratio of bearing to bypass loads is limited by the initial fit of the bolt and by subsequent bolt/hole deformation under load. However, the specimens recommended are capable of applying a wide range of bearing/bypass load ratios. Sample strength interaction envelopes were produced for a Hercules IM6 fiber and Ciba-Geigy Fibredux 6376 resin laminate of typical wing skin lay-up, 5.5 mm thick. Both tension and compression quadrants were studied, with two hole sizes, in both double shear and single shear. The interaction behaviour was similar in both tension and compression for 6.35 mm holes in double shear, but in the case of 9.5 mm bolts there was less interaction in compression than in tension. Countersunk fasteners in tension appeared to suffer little reduction in net strength due to bearing stresses. (SDW)