Fatigue Experiments and Modulus Degradation Modeling on Sheet Molding Compound
by
Tony Bor-Yih Tai
Abstract
Short fiber reinforced Sheet Molding Compound (SMC) has become one of the most important engineering materials in ground vehicle industry in recent years. Some of the advantages of using SMC include a high stiffness to weight ratio, superior strength, good corrosion resistance, and the ability to mold the material into complex shapes. In addition, the low manufacturing cost of SMC has prompted the industries to replace the traditional material such as sheet steel and metal alloys with SMC in many engineering applications, especially in the automotive industry.
Even though with the increasing applications using SMC, many mechanical properties are not well understood and need further research. One of the major concerns is the fatigue behavior of SMC. Specifically, the microstructural damages develop that lead to modulus degradations in long term applications and in turn determine the fatigue life of SMC.
The objectives of this research are to understand the fatigue behavior of the SMC composites and to develop a correlation between modulus, life, and the microstructural damages observed from fatigue experiments of SMC. There are six formulas of the SMC composites being studied, with each formula consisted of various compositions of the glass fiber, calcium carbonate filler, and resin matrix. The research consists of two major aspects. The first aspect includes conducting tensile and fatigue experiments on the SMC composites and performing image analysis on one of the SMC composites (FS4501, 45% glass fiber). The second aspect involves simulations of the modulus degradations in SMC composite, FS4501, using the Modulus Prediction Program developed in this study. This program calculates the degradation of elastic modulus as a function of volume fraction and orientation of cracks. The results have implications in estimating the residual life of composites in service upon obtaining a photograph of the surface and predicting the modulus reduction.
Since the introduction of glass-fiber reinforced plastics for engineering applications, efforts devoted to research and developments of fiber-reinforced composites have been growing steadily. In particular, two researches are relevant and important to the development of this study. In 1987, Advani [1] studied fiber orientation for processing considerations and predicting mechanical properties. He used a tensor description to characterize and predict the orientation of fibers during flow. The orientation of the fibers plays the role of a structural state variable to establish a complete path from processing to properties of short fiber composites. In 1985, Chim [2] studied microscopic material damage and macroscopic property degradation in random short-fiber SMC composite subjected to cyclic tensile loading. He used the self-consistent mechanics analysis in conjunction with the probabilistic microcrack distribution functions introduced in the composite under fatigue loading. Both of these studies were examined during this research to better understand and forward a new model for fatigue damage in SMC.
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