Composite specimen bearing failure reduction in iosipescu shear tests

ASTM losipescu sheart test fixtures often crush the loaded edges of high shear strength specimens before shear failure occurs between the notches. To alleviate this problem, two new shear test fixtures were designed and built. The first uses a pivoting load surface to track the specimen shape as it changes under load. The second uses a rounded load surface optimized to produce a uniform bearing pressure along the loaded edge of a quasi-isotropic sheet-molding compound (SMC) specimen at shear failure. Finite element analyses and strain gage data are presented that describe the behavior of the baseline ASTM, pivoting load surface and rounded load surface fixtures. A substantial reduction in bearing stress and edge crushing was obtained with the rounded load surface fixture. Additionally, the modified surface optimized to test SMC specimens showed no tendency to change the shear strength measured in other composite specimens.