Development of a Numerical Methodology Able to Simulate the Unstable Mixed Mode Delamination Growth in Stiffened Composite Panels Under Cyclic Loading Conditions

Aeronautical structures, often, experience impacts with foreign objects in service and during maintenance operations. Foreign Objects Impacts (FOI) can lead to critical damages and can compromise the overall performances of structural components. Indeed, among the others, composite components can exhibit various interacting post impacts damage mechanisms, including fiber breakage, matrix fracture, and interlaminar damages, such as delamination between different layers of the laminates. Delamination represents the most critical failure mechanisms, as it is, often, undetectable by visual inspections and it may, unstably and silently, develop within the component. This Phenomenon may be amplified under cyclic loading conditions, as the residual strength and stiffness can decrease rapidly after a certain number of cycles, potentially leading to structural collapse [1]. Unstable propagation of delaminations is particularly critical, since it, actually, can take place without the need of increasing the load acting on the structure. This phenomenon, which is very dangerous for the structural integrity of components, can be very challenging to be predicted, under fatigue loading conditions, by the standard geometrically non-linear Finite Elements Methodologies (FEM) which use a sequence of simulations under force control to mimic the fatigue behaviour of composite materials. Actually, FEM simulations under controlled force levels lead to convergence issues when predicting the highly dynamic behaviour of the unstable growth of delaminations under fatigue loading conditions. The research activity, presented in this paper, is aimed to develop an alternative efficient methodology able to mimic the unstable delamination propagation under cyclic loading conditions in composite structures by non-linear static analyses. This new methodology has been demonstrated to be able to correctly consider the fast variation of delamination size associated to decrease in loading during the unstable growth phenomenon under cyclic loading conditions. To achieve this objective, the Paris Law [2, 3] approach has been implemented in the ANSYS FEM code together with an enhanced Virtual Crack Closure Technique (VCCT) based method.