Mechanisms for Kink Band Evolution in Polymer Matrix Composites: A Digital Image Correlation and Finite Element Study

Abstract

Polymer matrix composites (PMCs) are attractive structural materials due to their high stiffness and low weight to strength ratio. However, unidirectional PMCs have low shear strength and failure can occur along kink bands that develop on compression due to plastic microbuckling that carry strains large enough to induce nonlinear matrix deformation. The study of kink band nucleation and evolution in unidirectional composites (UDCs) is an active area of research. In the last five decades, a large body of research has been done to understand kink band mechanisms using theory and experiments. However, a large fraction of the existing work is for uniaxial compression. The effects of stress gradients, such as those present during bending, have not been as well explored, and these effects are bound to make difference in terms of kink band nucleation and growth. Furthermore, reports on experimental measurements of strain fields leading to and developing inside these bands in the presence of stress gradients are also scarce. This need to be addressed to gain a full understanding of their behavior when UDCs are used under bending and other spatially complex stress states, particularly given that the compressive strength of these composites is a function of stress-gradient. Therefore, the primary focus of this work is to understand mechanisms for kink band evolution under an influence of stress-gradients induced during bending. Digital image correlation (DIC) is used to measure strains inside and around the kink bands during 3-point bending of samples with 0°/90° stacking made of Dyneema HB80, a trademark of DSM. Measurements indicate bands nucleate at the compression side and propagate into the sample carrying a mixture of large shear and normal strains, while also decreasing its bending stiffness. Failure was produced by a combination of plastic microbuckling and axial splitting. The microstructure of the kink bands was studied and used in a microstructurally explicit finite element model (FEM). It has been used to analyze stresses and strains at ply level in the samples during kink band evolution, using cohesive zone elements to represent the interfaces between plies. Cohesive element properties were deduced by a combination of delamination, fracture and three-point bending tests used to calibrate the FEMs. Modeling results show that progressive buckling of plies leads to kink band nucleation and propagation and that the band morphology is sensitive to the shear and opening properties of the interfaces between the plies.