Bearing Failure of Bolted Composite Joints. Part I: Experimental Characterization

Abstract

An investigation was performed to study the bearing failure of mechanically fastened fiber-reinforced laminated composite joints. Only double-lap metal/composite/metal bolted joints were investigated. The results of this study will be presented in two parts: experimental characterization and analytical prediction. This paper summarizes the experimental work. The major focus of the experiments was to characterize the bearing failure mechanism and mechanics, and to evaluate the effect of clamping pressure on the bearing response and bearing strength of bolted joints. The bearing damage was characterized either as pure bearing, which had no lateral supports, or as bolt bearing, which contained lateral supports with various degrees of clamping pressure. A specially designed semi-circular notched specimen was proposed to characterize pure bearing damage. For bolt bearing, a load cell was designed and manufactured which was mounted on the fastener to monitor the bolt clamping pressure as a function of the applied load. T800/3900-2 graphite/epoxy prepregs were selected to fabricate the specimens. All the specimens were x-rayed and sliced at different load levels to examine internal damage. Based on the experiments, it could be concluded that lateral support is crucial for bolted laminated composite joints. Bearing damage can be catastrophic if there is no lateral support. Shear cracks induced by accumulated compression failure appeared to be the primary failure mode of the bearing damage. Lateral supports could suppress the shear crack propagation and change failure from a catastrophic to a progressive failure mode. Clamping pressure could increase bearing strength.