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.

Three-dimensional size effects in composite pin joints

“…8 that the bearing strengths in each group increase as the thickness scale increases. A similar result based on the thickness study has also been reported by Wang et al 23 The interpretation of the change in bearing strength due to the thickness scaling was more complicated than that due to in-plane scaling. In addition to the negative influence based on the Weibull theory for specimens subjected to pin loading (also true for specimens subjected to compressive loading reported in the section "Effect due to Compressive Strength"), there was apparently a positive contribution due to the thickness scaling.…”

On the prediction of bolted single-lap composite joints

“…From these failure modes only bearing damage produces a progressive failure, thus composite bolted joints are designed to fail under this mode. Bearing failure occurs in the material immediately adjacent to the contacting bolt surface due to primar ily compressive stresses and leading to a non linear behaviour of the composite plates [7].…”

Pin-bearing strengths for bolted connections in fibre-reinforced polymer structures

“…The load arrangement has previously been used by Wang et al, (1996) to characterise bearing strengths of laminated FRPs for the aerospace industry. The Warwick University test arrangement is shown in Figure 5.…”