Strength and Failure Characteristics of SMC-R Composites under Biaxial Loads

Abstract

Sheet molding compound composites containing randomly oriented short fibers (SMC-R) are among the most commonly used composites in the automotive and many non-aerospace applications. They are also finding a few niche applications in the aerospace industry. Many studies have reported on static and fatigue properties of SMC-R composites under uniaxial loading conditions. However, there are many applications in which they may be subjected to biaxial loads and their biaxial properties have not been reported in the literature. This study considers the strength and failure characteristics of SMC-R under biaxial loading conditions that were generated using various combinations of normal and shear stresses ranging from uniaxial tension to shear. Glass fiber and Carbon fiber SMC-R were the materials used for quasi-static tests and carbon fiber SMC-R was used for fatigue tests. In addition to determining the strength properties, this study also includes damage development process and failure prediction under biaxial loading. Finite Element Analysis was used to understand and verify the stress distribution in the specimen. It was observed that the presence of shear stress decreases the tensile stress at failure for both glass and carbon fiber SMC-R. Depending on the stress biaxiality ratio, macroscopic damage development in both materials was initiated at 95 to 99% of the peak load. For both materials, a knee load was observed above which the material behaved non-linearly. Finite element analysis confirmed the damage development location in quasi-static tests. The biaxial failure load prediction seems to follow Hill’s anisotropic yield criterion. The carbon fiber SMC-R exhibited a high degree of scatter in both quasi-static strength and fatigue life. Weibull analysis was performed to determine its strength characteristics of this material.