Size Effects and Thickness Constraints in Composite Joints

Mechanical fasteners are commonly used in assembling composite structures. This study investigated the strengths of double-lap-single-pin joints made of glass/epoxy composite laminates. Three-dimensional size effects were of primary interest. Experimental results revealed that the joint strength decreased as the joint size increased. Weibull's theory seemed to be able to describe the strength reduction up to some extent. As the joint size increased, the damage modes of composite joints also shifted from initial bearing damage followed by final net-section failure to directly catastrophic net-section failure. It was also found from experiments that the constraints in the thickness direction played a key role in damage process. Three factors defined the constraints in the thickness direction: composite thickness, bonding strength through laminate thickness and clamping force from bolting. Composite joints with low thickness constraints tended to have fiber buckling and delamination that resulted in initial bearing damage before final net-section failure. Since they showed larger displacement to failure and higher energy absorption during damage processes, composite joints having initial bearing damage followed by final net-section failure were more ductile than those experienced directly catastrophic net-section failure.