STRESS SINGULARITY DUE TO KINK BAND WEAKENING A UNIDIRECTIONAL COMPOSITE UNDER COMPRESSION

A Williams type of eigenfunction expansion approach is used to compute the local stress singularity representing a measure of the degree of inherent flaw sensitivity of unidirectional fiber reinforced composite s subjected to compression. Previous experimental studies have qualita tively linked the formation of kink bands to the presence of fabricati on defects, such as fiber misalignment. These local singular stress re gions serve as the primary trigger mechanism for kink band propagation in 0 degrees-plies. The present analysis also explains the previous t est results relating to propagation of failure from a notch in a unidi rectional composite under compression. Furthermore, the present invest igation is the first to quantify, in the context of LEFM (linear elast ic fracture mechanics), the sensitivity of these composites to inheren t local flaws, such as fiber misalignments, and also shows the inadequ acy of the conventional elastic micro-buckling type of analysis to ful ly explain the experimental results. Although because of the assumed i sotropy of the fiber and matrix materials, the present study is primar ily suited to glass fiber reinforced composites, the conclusions drawn here are general enough to apply to carbon fiber reinforced composite s as well. Numerical results presented include the effects of fiber in cluded wedge angle, and the ratios of fiber-matrix shear moduli and Po isson's ratios on the strengths of the mode I and mode II singularitie s. Of special practical interest is the present LEFM type of analysis applied to quantitatively investigate the inherent flaw sensitivity of two E-glass/epoxy composites experimentally investigated earlier. Com pression fracture type of failure of these composites can be fully exp lained and quantified by the present two-dimensional LEFM-based method , which is beyond the scope of one-dimensional micro-buckling approach .