Characterization of Discontinuous Fiber Reinforced Sheet Molding Compounds Under Tension-Tension Fatigue Load

Abstract

Sheet molding compounds (SMC) have long been successfully used for nonstructural applications, as for example in the automotive industry. However, the prediction of their fatigue behavior is still proving difficult. This is due to a variety of possible material compositions and their complex microstructure resulting in a complex damage evolution. Comprehensive experimental investigations are still necessary to understand the damage evolution and its impact on the macroscopic material behavior during cyclic loading. In this work the fatigue behavior of discontinuous glass fiber reinforced sheet molding compounds (SMC) was investigated in a load-controlled tension-tension fatigue test for different loads. The material is based on an unsaturated polyester polyurethane hybrid (UPPH) resin system without fillers. The reinforcing glass fibers feature a length of 25.4 mm (1 inch). The macroscopic material behavior shows statistical scattering in the range of two times the power of ten. Due to the manufacturing process, the fiber orientation distribution and volume contents vary within one SMC plate, which influences the fatigue life. The evolution of stiffness was determined for each test. Characteristic stages of damage can be clearly distinguished by means of stiffness reduction. Digital images were captured during fatigue loading to visualize local damage without interrupting the test by triggering the camera via the load signal. The dominant failure mechanisms were matrix cracking and interface failure. These can be observed long before final failure. The results allow a good description of the fatigue behavior throughout all stress ranges and present a good basis for further investigations