Analysis of stress concentration during tension of round pultruded composite rods

The stress state in a solid round transversely isotropic rod loaded with balanced shear stresses on a part of its lateral surface is considered. The change in gripping conditions is simulated by varying the distribution of statically equivalent shear stresses. The solution for the stress state was obtained using exact equations of linear theory of elasticity of anisotropic body. An analysis of the distribution of longitudinal and hoop stresses showed that at the grip edges a very high stress concentration is possible, which depends on the radius of the tensioned rod and the distribution of shear stresses on the gripped surface. At the grip edges, the longitudinal and hoop stresses tend to infinity. It is suggested to estimate the stresses at the singular point by averaging the stresses across the thickness of a subsurface layer; this thickness depends upon the susceptibility of the composite to the stress concentration. As a numerical example, the stress state of an intermediate-modulus carbon-fiber reinforced polymer pultruded rod is considered. The results obtained show that the decrease of tensile strength with increasing diameter of the rod is related to the nonuniformity of stress distribution near the grips, independent of any type of size dependent variation in material strength. By changing the distribution of the applied shear stresses it is possible to mitigate this effect and to increase the tensile load-carrying capacity of round pultruded rods.