Shear characterization of unidirectional composites with the off-axis tension test

The influence of end constraints on accurate determination of the intralaminar shear modulusG ₁₂ from an off-axis tension test is examined both analytically and experimentally. The Pagano-Halpin model is employed to illustrate that, when the effect of end constraints is properly considered, the exact expression forG ₁₂ is obtained. When the effect of end constraints is neglected, expressions for the apparent shear modulusG ₁₂ ^* and apparent Young's modulusE _xx ^* are obtained. Numerical comparison for various off-axis configurations and aspect ratios is carried out using typical material properties for graphite/polyimide unidirectional composites. It is demonstrated that the end-constraint effect influences accurate determination ofG ₁₂ more adversely than it affects the laminate Young's modulusE _xx in the low off-axis range. Experimental results obtained from off-axis tests on unidirectional Gr/Pi specimens confirm the above. Based on the presented analytical and experimental evidence, the 45-deg off-axis coupon is proposed for the determination of the intralaminar shear modulusG ₁₂.