Shear properties and a stress analysis obtained from vinyl-ester losipescu specimens

The use of losipescu specimens for the determination of the shear properties of a vinyl-ester resin was investigated. The antisymmetric four-point bend and the Adams and Walrath fixtures were studied for their suitability in loading these specimens. Photoelastic and strain-gage data in addition to published finite-element results show that the latter fixture distorts the stress field in the gage section. The antisymmetric four-point bend fixture is found to give the purest shear-stress field in the gage section and to yield the most reliable shear-modulus values. A refined photoelastic analysis shows that the shear-stress distribution between the notch roots is essentially uniform with a relative maximum or minimum at the centroid depending on the depths of the notches. Also, stress risers of up to 30 percent are observed near the notch roots. Except at the roots, finite-element predictions are presented which are in excellent agreement with photoelastic data. The failure mode of this vinyl-ester resin is tensile and the corresponding tensile stress calculated from the average shear stress in the gage section of the losipescu specimen is in excellent agreement with failure data acquired in tension.