Fatigue behavior of thermosetting‐polyester‐matrix sheet‐molding compounds

Abstract Two sheet molding compounds, one containing 65 weight percent E‐glass fiber and no filler (SMC‐65) and the other containing 50 weight percent glass and 15 weight percent CaCO₃ filler were subjected to sinusoidal, load‐control, tension‐tension fatigue. The macroscopic behavior showed some notch sensitivity for both materials, but particularly for SMC‐65. The extent of failure damage over the specimen length decreased with decreasing load amplitude. Different failure mechanisms appear to operate under high‐ and low‐cycle fatigue conditions. Microscopic surface damage studies at stresses near the endurance limit showed several stages in the fatigue failure. For SMC‐65, these stages are, sequentially, matrix crack initiation and growth, matrix pulverization, fiber fracture, and fiber pullout. A similar, but less distinct pattern was observed for SMC‐50. Acoustic emission was also followed during fatigue cycling near the endurance limit. Distinct amplitude distribution peaks for each failure process were seen. It is suggested that matrix fracture, permanent deformation and Euler buckling are all important in high cycle fatigue failure.