Off-Axis Tensile Coupon Testing

The off-axis tension test is frequently used to characterize the strength behavior of fibrous composite materials. This test is, ideally, uniform tension applied along a single axis. Because this axis does not coincide with one of the principal axes of the material, i.e., since the fibers are neither parallel nor perpen-dicular to this loading axis, the state of stress or strain referred to the principal axes of the material includes biaxial tension and shear. Such combined stress information is needed to assess the interpolation formulas or failure theories that are based upon 0-degree and 90-degree test specimens. Recently Pagano and Halpin [1] have pointed out that the shear coupling compliance S16 causes a non-uniform state of stress and strain in such tensile specimens when the ends are clamped, and furthermore this non-uniformity can-not be eliminated by simply allowing the ends of the specimen to rotate. An analytical solution was presented in [1] which indicated the aforementioned effect. This solution was obtained by integrating the equations for the plane stress condition from the linear theory of elasticity, but in order to solve the problem only the centerline at the specimen ends was rigidly clamped. Rizzo [2] has presented numerical solutions utilizing the finite element method to indicate the effects of full clamping and free rigid rotation of the ends of the specimens. These results are consistent with the closed form solutions of reference [1] ] and also with the experimental data presented by Wu and Thomas [3]. In the present note, further numerical results are presented which indicate the accuracy of the closed form solution presented in [1], and these results are further used to design a reasonable off-axis tensile coupon. ANALYSIS The analysis presented in reference [1] is considered valid except that the boundary conditions imposed on the displacements u and v to obtain the solution, namely, J. COMPOSITE MATERIALS, Vol. 3 (July 1969)