Damage Progression in Open-hole Tension Laminates by the SIFT-EFM Approach

Predicting and modeling progressive damage in fiber-reinforced composite structures up to and including final failure is a considerable challenge because damage in composite materials is extremely complex, involving multiple modes, such as delamination, transverse microcracking, fiber breakage, fiber pullout, etc. Indeed, damage in composites should be studied at different length scales, ranging from the micromechanical to the macromechanical specimen and structural scales. The challenge, however, is in finding theories and methodologies that will faithfully reflect the structural effects of damage progression without involving an inordinate amount of detail (and effort) in the model, so that designers and engineers will have practical tools. In this article, a novel finite element-based method for modeling progressive damage in fiber-reinforced composites is presented. The element-failure method (EFM) is based on the simple idea that the nodal forces of an element of a damaged composite material can be modified to reflect the general state of damage and loading. This has an advantage over the usual material property degradation approaches in that because the stiffness matrix of the element is not changed, computational convergence is guaranteed, resulting in a robust modeling method. When employed with a suitable micromechanics-based failure criterion, it may evolve into an engineering tool for mapping damage initiation and propagation in composite structures. Here, we have utilized the micromechanical information contained in a new strain invariant failure theory (SIFT) to guide the nodal force modification scheme to model progressive damage. As an application of the SIFT-EFM approach, we present a rational nodal force modification scheme for the modeling of progressive damage in quasi-isotropic composite laminates with open holes, subjected to remote tensile loads. The proposed nodal force modification scheme assumes loss of load-bearing capability in the direction transverse to the fibers for the case of local transverse microcracking, and assumes total loss of load-bearing capability when both transverse microcracking and fiber rupture occur. The study investigates the effect of stacking or layup sequence and shows that it is important to refine the model in the through-thickness dimension and includes nodal force modification for the out-of-plane component. It reinforces the view that damage propagation in composites is a complex three-dimensional event. When compared with experimental data, the predicted damage maps and final failure loads show correct trends and reasonable agreement.