CHALLENGE - In aerospace applications, composite structures must be designed for no-growth or slow fatigue crack growth to prevent manufacturing defects or in-service damage from reaching the safety limits. Fatigue delamination growth in composite laminates is strongly influenced by mean loads or load ratios. Description of this behavior currently relies on empirical curve fitting, which renders it difficult to predict fatigue lives of composite structures. The aim of this paper is to develop a scaling parameter that can account for the effects of fiber bridging on the growth behavior of delamination cracks under fatigue loading.
SOLUTION - Static and fatigue experiments were carried out on IM7/977-3 composite laminates under mode I and mode II. Large-scale fiber bridging was observed as a major toughening mechanism under both static and fatigue loading. To correctly account for the effect of fiber bridging, an inverse method was developed to determine the traction stresses acting in the crack wake. The new scaling parameter, accounting for the effect of bridging by cross-over fibers, is shown to unify the fatigue growth rates under different load ratios obtained in this study. FE analyses were performed using commercial software Abaqus Standard. A genetic algorithm available in modeFRONTIER is employed to search the appropriate values that will minimize the cost function.
BENEFITS - An inverse method has been developed to quantify the fiber bridging law under static and fatigue loading. Assuming that threshold for very slow delamination growth taken at 10-8 m/cycle, the mode I and II threshold value can be taken at 27.5 J/m2 and 200 J/m2 respectively, which represents 27% and 20% of the delamination onset value under static loads.