Fatigue analysis of GFRE composites and aircraft flat panel under various stress ratios and notch designs

Owing to concerns with serious crack growth behaviour and fracture surfaces of GFRE composite and FLE aircraft wing flat panels under fatigue loading after the pre-rivet pinhole process in recent years, researchers have been driven to investigate the behaviour of GFRE after pre-riveting with more than one hole, and the design of holes under variable stress ratios. The existence of additional force during the manufacturing process causes premature failure of the rivet hole, which will eventually lead to the accumulation of failures in stages. The stress induced during manufacturing causes premature failure of rivets, which in turn may lead to assembly failures, with severe consequences. Rivet holes should be designed to avoid loading the rivet in tension. The specimens of a GFRE composite and an FLE aircraft wing flat panel are prepared according to ASTM D5766. Two types of cross-ply laminate orientations of GFRE are fabricated with cross-centreline angle-plies of +/-45 [0/90[+45]0/90]6 and all 0/90 degree ply, [0/90]7 using a vacuum infusion process. Tensile tests on each pinhole design are carried out to validate the maximum load before failure. Fatigue tests were carried out to validate the fatigue performance in the range from 30% to 90% of maximum load with stress ratios of R = 0, 0.5 and -1. The result of tensile test showed the design of pinholes influence the tensile strength of the composite material. The specimen D1 which is one pinhole, had the highest tensile strength and their fatigue life also longer than the other specimens. In addition the percentage of load and R ratio had influenced the fatigue life of the GFRE laminated composite specimens, which exhibited the shortest fatigue life with increase of the percentage of load and R ratio. Lastly, the SEM micrographs showed that the specimens experienced primary cracks, secondary cracks, fibre breakage, fibre pull-in/out, matrix damage and matrix cracking. Therefore the results obtained from this work can be used to predict the behaviour of GFRE under the fatigue strength of various pinhole designs and different manufacturing process factors.