Estimation of Shear Moduli for Orthogonal 3-D Woven SiC Fiber/SiC Matrix Composite Using Torsional Test

Abstract

The present study examines the in-plane and out-of-plane shear moduli of an orthogonal 3-D woven SiC fiber/SiC matrix composite. A prismatic composite bar of rectangular cross section was subjected by a small torsional moment, and the torsional rigidity was measured by an optical method. Conventional Lekhnitskii's equations for orthotropic materials subjected by torsion were applied to estimate the shear moduli from the torsional rigidities obtained from the specimens with different cross sections. The estimated in-plane shear modulus agreed with the modulus measured by ±45° off-axis tensile testing. The effect of 0° tensile loading on the shear stiffness degradation was also investigated by the repeated torsion tests after step-wise tensile loading. As a result, it was revealed that both in-plane and out-of-plane shear moduli were reduced by more than 50% of the initial values due to 0° tensile load. Finite element analyses (FEA) based on the homogenization method were conducted in order to evaluate the shear moduli estimated by the torsion tests. By optimizing the void fraction between fiber bundles, the numerical results by FEA were in good agreement to the experimental results.