Estimating Stress Concentration near Grips by the Tension of Polymer Composite Samples

Abstract

This work aims at the problem of justification of the tensile strength test of flat glass- and carbon-fiber-reinforced plastic samples. Any technique of sample holding causes a concentration of stress and influences negatively on the strength; quantitative estimates of this effect are presented in this work. Based on a simplified equation of shear analysis, the stress distributions in the samples were calculated by different ways of simulation of the diagram of shear stresses applied from the grip side. The advantage of the analytical solution consists in the fact that a clear description of the influence on strength of the following factors is possible, namely, the lengths of the grips and the working part and, particularly, the sample thickness. A similar scale effect is often explained in terms of statistical strength theory, but the analysis shows that the decrease in strength observed with an increase in the sample thickness can be explained by the effect of nonuniformity of the stress field near the grips. The appearance of this edge effect like a Saint-Venant effect is especially pronounced in polymer composites with a low ratio (as compared with metals) of the interlaminar shear modulus to the longitudinal Young’s modulus.