Investigation of Delamination Initiation and Growth Behavior at Fastener Locations in Primary Composite Structures

Abstract

Current maintenance practice dictates that aircraft are routinely inspected after performing a fixed number of flight hours to mitigate risks from damage that grows during operation. This research aims to support maintenance operations and fleet management, and also to help improve aircraft design practice, by investigating delamination initiation and propagation in the vicinity of fastener holes within fiberreinforced composite materials. Static and fatigue bearing tests were performed on modified countersunk double lap shear (DLS) and single lap shear (SLS) test configurations. DLS static test results presented damage onset that occurred at a lower bearing stress level but higher ultimate stress compared to the SLS static test. The fatigue bearing test data showed that when the bolted hole elongates, stiffness decreases and damage area growth becomes detectable through C-scan. From microscopy observations, matrix cracking and compression-induced fiber kinking were found to occur in the straight shank region of the hole. Complex damage morphology forms in this region, emanating from the loaded bearing face, creating large wedge-shaped regions that act to drive delamination propagation with additional loading cycles. Furthermore, these results indicate that the straight shank region of the countersink-head bolt hole carries most of the bearing loads. For the SLS test configurations with fullytorqued bolts, damage beyond the countersink conical region was not detected. Thus, bolt pre-load adds out-of-plane constraints that act to prevent damage from growing during fatigue tests.