On the optimal choice of fibre orientation angle in off-axis tensile test using oblique end-tabs: Theoretical and experimental studies

Shear strength evaluation in composite materials still remains a not fully solved problem. One of most promising proposals to characterise a material in shear by means of a simple tensile test is the off-axis tension test with oblique end-tabs whose inclination coincides with that of the longitudinal isodisplacement lines. This end-tab configuration presents an essential difficulty since the tab angle depends on the material properties including the shear modulus G12, which is one of the values to be obtained from off-axis tension tests. In the present work, the study of the most suitable fibre orientation for the performance of off-axis tests under an oblique tabs configuration has been addressed. An analytical study on the dependence of the tab angle ϕ on both the fibre orientation θ and the material properties E11, E22, ν12 and G12 has been carried out, the quotient G12/E11 being identified as the parameter that controls the evolution of the tab angle. The influence of deviations in the tab angle on the stress state near the corners of specimen has been analysed by means of a novel study of the nominal singular stress state at these points. The results of this study show that negative deviations in the angle of the tabs contribute to decrease the severity of the stress state at the corner, whereas positive deviations contribute to increase the order of the stress singularity. Additionally, the range of fibre orientations in which the evaluation of shear strength S is feasible has been determined, i.e., the effect of σ12 in the failure is clearly dominant versus the effect of the rest of the stress-state components. In the experimental part of this work, four different fibre orientation angles (5°, 10°, 15° and 20°) have been considered to perform the off-axis tension test under oblique end-tabs configuration. The experimental results of these tests show that only for 10° fibre orientation angle the majority of specimens fail at the central zone under a uniform stress state. Finally, for a fibre orientation angle θ = 10°, specimens with induced positive deviations in tab angle of +3° and +7° have been tested, it being observed that all the failures appears at the end of specimens. On the basis of both theoretic and experimental results, the optimal fibre orientation angle to use in the tests is θ = 10°. The tab angle must be evaluated on the basis of an estimation of the material properties (quotient G12/E11), it being recommendable, in case of ignoring the value of G12, to use a tab angle lesser than the theoretical value to avoid positive deviations.