Specified criterion for delamination upon bending of a composite beam

The interlayer strength in polymer fiber composites is characterized mostly by the strength of the matrix, which is much lower than fiber strength. For this reason, the analysis of fracture occurred through delamination is extremely important for assessing the operability of composite structural elements. When designing critical structures, it is necessary to know the interlayer shear strength, for which the method of bending a short beam has been standardized. The shear stresses and the interlayer shear strength in bending theory are traditionally assumed to be independent of the length and width of the beam. However, a large number of experimental studies prove the opposite fact that the geometry of the specimen affects the value of critical stresses. The linear fracture criterion proposed by the authors allows explanation and quantitatively description of the interlayer shear strength dependence on the geometry of the specimen. The influence of the heterogeneity of interlayer shear stresses across the beam on the critical stresses is analyzed. A strict solution of the bending problem showed that taking into account the specified shear stress distribution gives an insignificant correction to the determined value of the interlayer strength, which makes it possible to use a simplest parabolic distribution in height. The results of the analysis are confirmed in three-point bending tests of short composite beams of different widths. The results of fatigue tests of short beams made of carbon fiber reinforced plastic are analyzed. The relationship between tensile fatigue curves of polymer fiber composites and the fatigue curves obtained in cyclic three-point bending test of short beams has been revealed using the proposed linear fracture criterion. The estimation of the strength scale effect on the basis of the energy criterion of delamination with and without taking into account the refined distribution of interlayer shear stresses is presented.