A generalized solution to the crack bridging problem of fiber metal laminates

The slow crack growth and high residual strength of fiber metal laminates (FMLs) results from bridging, the transfer of loads from cracked metal layers to intact fiber layers that “bridge” the cracks over delaminations between the laminae. Understanding and improving the structural performance of FMLs therefore depends on characterizing the bridging phenomenon. Previously, mathematical models of bridging load transfer have been developed and verified for simplified circumstances:thin laminates with identical thicknesses and material composition through the laminate, identical crack lengths and delamination sizes, and simple tension loads. These models use a compatibility requirement at the delamination boundary to derive the bridging stress. This paper presents a generalized approach to solving the bridging problem that solves simultaneously for compatibility at every delamination boundary in the laminate, allowing the bridging load distribution to be derived for a laminate of arbitrary layup (different thicknesses and materials) with arbitrary damage (independent crack lengths and delamination sizes and shapes) and subject to arbitrary loading (any combination of applied tension and bending moment). The approach is evaluated by comparing its results to bridging stress distributions derived from finite element analysis. Good agreement was obtained for varied laminate and damage configurations, but some discrepancy exists in the presence of bending.