Critical examination of the Iosipescu shear test as applied to 0 degrees unidirectional composite materials

Several issues regarding the application of the shear and biaxial Iosipescu tests for the shear strength characterization of unidirectional composite materials are addressed in this article. First, the nonlinear effects of sp ecimen sliding and geometric nonlinearity on the mechanical response of 0 d egrees standard unidirectional graphite/polyimide Iosipescu specimens with different loading conditions and loading block geometries have been investi gated. Second, an attempt has been made to improve the Iosipescu shear test to eliminate normal compressive stresses in the specimen gauge section and at the same time prevent axial splitting. Finally, several Iosipescu shear and biaxial experiments have been performed to select proper specimen geom etry and loading conditions for the shear strength measurements of unidirec tional composites. The nonlinear effects are examined with respect to various coefficients of friction, displacements, loading angles, and fixtures (biaxial with short a nd modified biaxial with long loading blocks) using nonlinear finite-elemen t techniques. It is shown that the effect of nonlinearity is small on the s tresses at the center of the standard Iosipescu specimen, but significant f or the stresses near the notch root up to 2 mm applied displacements. In so me cases, significant differences in the stresses calculated for different coefficients of friction have been observed. All of these results are somew hat consistent for both fixtures, but with the stress components a,, a,, an d a,, significantly lower in the standard Iosipescu specimens tested in the fixture with the long blocks. Numerical load/displacement diagrams show th at specimen sliding and geometric nonlinearity have a negligible effect on reaction forces in the biaxial fixture, and a significant effect on the rea ction forces in the modified biaxial fixture. Since the various combination s of the loading conditions evaluated in this study do not eliminate transv erse compressive stresses in the gauge section of the standard Iosipescu sp ecimens, a major improvement to the Iosipescu shear test has been proposed. Using an optimized specimen geometry subjected to biaxial shear/tension lo ading conditions, a state of almost uniform pure shear stress can be genera ted in 0 degrees unidirectional composite Iosipescu specimens without the p ossibility of axial splitting along the fibers at the roots of the notches. However, it is shown in the experimental part of this study that for the o ptimized Iosipescu specimen, crushing ar the inner loading blocks can signi ficantly affect the shear intralaminar failure process. Only by reducing th e cross-sectional area of the optimized Iosipescu specimen can the effect o f crushing on the failure process be reduced without, however, high-quality shear stress fields present in the gauge section at failure.