Boundary element analysis of fibre bridging in perforated fibre metal laminates

Abstract

Fibre Metal Laminates (FML) are hybrid structural materials made from interlacing thin metallic and composite layers with resin adhesives. Today, FMLs are widely used in fuselage compartments and other parts of the aircraft in the aerospace industry. The newest type of FML, GLARE, possesses excellent strength and damage tolerance properties. However, more studies are required to better understand GLARE and its crack propagation resistance properties.

Our literature review will begin with a review of the different toughening mechanisms for composite materials. We will examine intrinsic and extrinsic toughening mechanisms and their applications in crack propagation resistance for composite materials. We will also delve into the three types of fracture modes and investigate the fibre bridging process in composites. The varied factors affecting the stresses and elongations of the bridging fibres during the fibre bridging process will be examined. Lastly, we will extensively review linear elastic fracture mechanics, including Griffith’s Theory and Irwin’s Modifications. The study will involve calculating the bridging stresses and the crack-tip stress intensity factor values.

Our study will use the Boundary Element Method to analyse different parameters that could affect the fibre bridging process. Namely, the factors investigated were different crack configurations, crack size ratios, fibre bridging extent, power-law indices, and the shapes of the delamination zone. The metrics used for evaluation were the bridging stresses and normalised stress intensity factor.

The computational results from the boundary element analysis were iterated to determine the stable final value for comparison. The results showed that both the fibre bridging stresses and normalised crack tip stress intensity factors increased as the crack size ratio increased. Inversely, the results for both the fibre bridging stresses and normalised crack tip stress intensity factor continue to increase as the extent of the fibre bridging in the laminate decreases. The bridging stresses tend to be lowest in the double-edge cracks, followed by the circular cracks, while the edge crack configurations have the highest bridging stress values.

Following the computational results collected, we were able to plot the relationship between different parameters and crack propagation for different extents of fibre bridging in GLARE laminates. These results gave us a better understanding of how each parameter contributes to the crack propagation process through fibre bridging.