Abstract

Fiber-reinforced composite materials have been widely used for manufacturing load-bearing dynamic components in aerospace. One example is the helicopter’s main rotor yoke. Yokes need to be rigid and sufficiently flexible to survive many blade flexing and revolutions. To understand and improve the fatigue life of rotor yoke, a flexural fatigue behavior study was done on the thick glass/epoxy laminate with bolted holes through the thickness. 80 plies unidirectional and cross-ply specimens were manufactured by hand lay-up. The effect of debulking process was discussed. A suitable bagging and curing procedure was used to achieve uniform and acceptable thick glass/epoxy laminates for mechanical tests. The unidirectional and cross-ply specimens were tested on a clamped-clamped fatigue bending fixture. For each of the stacking sequences, test specimens were subjected to four deflection levels. All flexural fatigue tests were conducted under displacement control at 3Hz, with an R-ratio of 0.1 (single sided bending). Thermography was used to record the surface temperature increase, focusing on the crack initiation area. A crack was detected by four ways: sound, visual observation, load drop and thermography. Thermography was found to be an effective in-situ delamination detection method. A 20% reduction of initial load was taken as failure. The cross-ply specimens initiated delaminations more easily than unidirectional specimens. However, the cross-ply specimens had slower crack propagation rate. The unidirectional specimens did not initiate delaminations (at same thickness position) but had shear-out cracks (at different thickness positions) at displacements lower than 20mm. The fatigue lives were up to 1E6 cycles. When subjected to the displacement lower than 21mm, the cross-ply specimens had fatigue life up to 1E6 cycles. For both stacking sequences, the delamination initiated around the second bolted hole area.