In-Plane Shear Fatigue Evaluation of SMC

Numerous studies, experimental and analytical, exist in the literature which describe in-plane shear test techniques including picture frame, two rail, three rail, cross sandwich beam, Iosipescu, and torsion tube. All of the available testing techniques, except for the torsion tube, have limitations. They quite often suffer from several of the following deficiencies: small sample size, edge influencing concentrations and not being suitable for some fiber orientations. Experimental results for the in-plane shear fatigue of automotive sheet molding compound (SMC) material are presented in this paper based on torsion tube testing. Two identical halves of a tube were compression molded and subsequently bonded together to form the test specimen. The geometry of the specimen was chosen to insure that the failure occurred away from both the bonded joints and the ends of the specimen. One variable, considered in this paper, was that of molding the test specimen in a vacuum, which proved to have a significant beneficial effort on the measured shear response. Fatigue data is presented for three ratios of minimum shear stress divided by the maximum shear stress (R). When the resulting data are plotted on a modified Goodman diagram, interpolation is possible for the shear fatigue strength for all other R values.