Fatigue Crack Growth in ARALL: A Hybrid Aluminum Aramid Composite Material. Crack Growth Mechanisms and Quantitative Predictions of the Crack Growth Rates

摘要

The fatigue crack growth behavior of small and large cracks in ARALL was investigated under constant amplitude and flight simulation loading. Static properties, delamination in the fiber-adhesive layer under cyclic loads, and shear deformations in the fiber-adhesive layer under static and cyclic loads were tested to provide material data for a model for the prediction of fatigue crack growth in ARALL. The thickness of the aluminum sheet layers and the fiber-adhesive layers; different types of adhesives and fibers; and residual stress systems introduced by prestraining or prestressing were examined. The excellent crack growth resistance of ARALL under constant-amplitude and variable-amplitude loading is confirmed. The resistance is considerably enhanced by a favorable residual stress system, i.e., compressive stress in the aluminum alloy layers and tensile stress in the aramid fibers. Thinner individual layers also lead to better properties. The favorable residual stress system is very effective under constant-amplitude loading at low R-ratios. At high R-ratios the effect is less pronounced. Truncation of the TWIST load spectrum has a significant effect on fatigue crack growth in ARALL. A high truncation level considerably reduces crack growth rate. Initiation of small cracks in side notched specimens occurs relatively early in the fatigue life. The delamination growth rate observed in delamination tests, and crack opening displacements due to adhesive shear deformation, are directly dependent on the load transfer from the fibers to the aluminum alloy layers.