Failure analysis of bolted and pinned composite joints under temperature effects

In the first part of the study, experimental failure analysis has been carried out to determine the effects of high temperatures and tightening torques on the failure load and failure behavior of single lap double serial fastener glass fiber / epoxy composite joints. 40, 50, 60, 70, and 80 degrees centigrade temperatures were exposed to the specimens during tensile tests. It was seen that, the load-carrying capacity of the joint is decreased gradually by increasing temperature level. In proportion to the room temperature, the maximum decrease of failure loads occurs at 70 and 80 degrees centigrade with the rate of nearly 55 and 70 percent respectively, because of the heat damage to the resin matrix. In the second part of the study, experimental investigations were conducted on failure responses of single lap double serial fastener joints in glass fiber / epoxy composite laminates when subjected to low temperature environment. The results of experiments, implemented at five different low temperature levels ranging from 0 to -40 degrees centigrade, were evaluated in comparison with room temperature tests. Joints exhibited relatively higher load-carrying capacities with increased stiffness by decreasing temperature. Both in the first and second part of experiments, bolts were fastened under M= 6 Nm and M= 0 Nm (finger tightened) torques in order to examine tightening torque effects at each temperature condition. As expected, a greater amount of bearing load could be carried by the joints with pre-tightened fasteners. The results proved that, tightening torque is still effective at elevated and low temperature conditions, Furthermore, any reduction in temperature at subzero degrees centigrade is observed to lift the effectiveness of tightening torque on the joint strength. In addition, bearing mode, the most desirable failure type in mechanically fastened joints was monitored as the main failure mode, regardless of the temperature exposed.Keywords: glass fibers, composite joints, high temperature, low temperature