Identifying failure mechanisms of composite structures under compressive load

Abstract

Various failure mechanisms involving both local and global deformation mechanisms of layered structures, consisting of differently oriented orthotropic laminae, are investigated with a large deformation finite element analysis. Under large compressive loads, fiber-reinforced composite structures can fail by buckling, kink band formation and/or delamination growth. Our aim is to identify the dominant mode which leads to the structural failure under a given boundary condition and geometrical shape. It is assumed that these structures contain initial interlaminar flaws represented by embedded delaminations. Such flaws can play a significant role in defining overall structural integrity and deformation mechanisms. Our study considers two structures with distinguished shapes, both consisting of four layer laminae. The first structure is a flat panel under compressive load and the second structure is a cylindrical shell subjected to external pressure. In both cases, the energy release rate and mixed-mode stress intensity factors are computed to quantify the crack driving force and used to measure likelihood of delamination growth. In the flat panel model, two buckling modes, a global and a local, are observed under quasi-static loading conditions. Depending upon the delamination length and laminate stacking orientations, the interaction of these two modes can produce an unstable post-buckling behavior. It is found that the energy release rate exceeds experimentally estimated fracture toughness values only after buckling occurs. In the cylindrical shell study, lower critical buckling loads are observed for models with longer interlaminar delamination as in the flat panel model. However, unlike the flat panel case, the energy release rate surpasses the critical toughness well before the applied pressure reaches the buckling load of the flawed cylindrical shell. This behavior implies that a shell containing an embedded defect along an interface can fail by delamination growth and therefore has a failure load lower than its critical buckling load. Also, for thicker cylindrical shells, the compressive internal stress exceeds the predicted stress required for kink band formation prior to structural buckling. The competition among these failure modes, buckling, delamination growth and compressive kinking in fiber-reinforced composite structures, is studied and a design diagram is introduced here. This diagram predicts a likely dominant failure mode for given structural dimensions and material properties. In addition, unstable delamination growth along the lamina interface under dynamic loading conditions is simulated using an iterative computational procedure. The results predict that the crack tip velocity can reach a fraction of the material wave speed leading to total structural failure in a very short time. For the cylindrical shell, the inertial effect on the pressure-deformation relation is investigated by comparing it with the static results.