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Matrix damage, in the form of cracks parallel to the fiber, is a common failure mode for composite laminates, yet little is known concerning the associated shear modulus degradation. This study proposes a novel experimental technique to study shear modulus degradation under biaxial loading. An Iosipescu coupon was modified that induced transverse tension and compression loads while applying shear loading. The axial loads were induced using unbalanced layups with coupled normal–shear response. An analytical, closed-form solution was generalized to include unbalanced layups under a shear loading boundary condition. It was observed that the shear modulus degradation depends on the normal stress acting on the matrix transverse cracks. While compressive normal stress increased the effective shear modulus, the tension normal stress reduced the shear modulus, compared to pure shear loading. [ABSTRACT FROM AUTHOR]

Abstract: Matrix damage, in the form of cracks parallel to the fiber due both to in-plane tensile loading perpendicular to the fibers and to in-plane shear loading, is a common failure mode for composite structures, yet little is known concerning their interaction. Past work has focused on experimental and analytical studies of axial and transverse stiffness reduction due to matrix cracks. By comparison, there is relatively little experimental work addressing shear modulus degradation from matrix damage. In this paper, a modified Isoipescu coupon is proposed to study the shear modulus degradation due to loading perpendicular to the fibers direction. The layup and coupon geometry were selected in a way that controls the severity of the damage and allows the measurement of shear and transverse stiffness degradation in the same coupon. The proposed method showed good agreement with results from tubular specimens and has advantages of simplified specimen fabrication using standard test fixtures. The results provided the first experimental comparison of shear modulus reduction, from transverse damage, to the predictive models. The results were compared with existing analytical and numerical models which over predicted the observed shear modulus reduction. [Copyright &y& Elsevier]