Experimental and Analytical Study of Mode II Interlaminar Failure of Bolted and Bonded Composite Structures

Abstract

The capability of a fastener to arrest and stabilize pure Mode II interlaminar failure in composite structures has been investigated both experimentally and analytically. A novel pure Mode II three-beam axially loaded specimen was designed specifically for the study. Test specimens were manufactured with [(0/45/90/-45)3]S quasi-isotropic carbon fiber composite laminates for each beam. Teflon inserts were installed as initial cracks, and several variations of plies and bonding techniques were used at the crack interface. Testing showed that crack initiation loads depended on the plies at the crack interface and the bonding technique used. The fracture toughness of the bond in several configurations resulted in the crack initiation loads being higher that the load at which filled-hole tension failure of the laminates occurred; in such cases the full arrestment capability of the fasteners was not able to be captured, but post failure C-scans showed that the presence of a fastener did provide more crack arrestment than without a fastener. However, the testing of other specimens with lower fracture toughness of crack interface bonds displayed the capability of the fastener to arrest and stabilize a crack before filled-hole tension failure. An analytical model and solution were developed to simulate crack growth in the three-beam specimen. The analytical solution was verified using FEM, and showed very good agreement of the results. The analytical model was used to determine the critical Mode II strain energy release rate, GIIC, of each specimen configuration. Also, the analytical model was used to perform a series of parametric analysis of the three-beam specimen to study the effect that changing specimen parameters, such as laminate layup and thickness, had on the crack propagation behavior of the specimen. It was advised that the analytical model be used to aid in the design of alternate specimen configurations for further study.