Fatigue Behavior And Modeling Of Short Fiber Reinforced Polymer Composites

A common application of short fiber reinforced polymer composites (SFRPCs) is in the automotive industry. A high demand of these polymer composites is due to their low cost, ease of manufacturing in complex geometries, high production rate, and a low ratio of weight to strength. In structures such as automobile, components are typically subjected to fluctuating loads and generally prone to fatigue failure. Therefore, a profound understanding of fatigue prevents premature failures of structural components. This study investigates uniaxial fatigue behavior of two short glass fiber polymer composites including 30 wt% short glass fiber polybutylene terephthalate (PBT) and 35 wt% short glass fiber polyamide-6 (PA6) under a number of load and environmental conditions. The main objectives are to evaluate the behavior of these materials under monotonic and cyclic loadings and present fatigue life prediction methodologies to reduce their development expenses and time. The considered environmental effects include those of low and elevated temperatures as well as moisture (or water absorption) effect. Fatigue behavior is also explored under the action of nonzero mean stress (or R ratio) as well as various cyclic loading frequencies. Material anisotropy and geometrical discontinuity effects (i.e. stress concentration) are also considered in this study. Microscopic failure analysis is also performed, when necessary, to identify failure mechanisms. Tensile tests were performed in various mold flow directions and with two thicknesses at a range of temperatures and strain rates. A shell-core morphology resulting from orientation distribution of fibers influenced the degree of anisotropy. Tensile strength and elastic modulus nonlinearly decreased with specimen angle and Tsai-Hill criterion was found to correlate variation of these properties with the fiber orientation. Kinetics of water absorption was studied and found to follow the Fick's law. Tensile tests were performed at room temperature with specimens in the longitudinal and transverse directions and with various degrees of water absorption. Mathematical relations were developed to represent tensile properties as a function of water content. Mathematical relationships were developed to represent the stress-strain response, as well as tensile properties in terms of strain rate and temperature. Time-temperature superposition principle was also employed to superimpose the effect of temperature and strain rate on tensile strength. A temperature dependent shift factor of Arrhenius type is suggested, which is independent of the mold flow direction. Variation of tensile toughness with fiber orientation and strain rate was evaluated and mechanisms of failure were identified based on fracture surface microscopic analysis and crack propagation paths. Fiber length, diameter, and orientation distribution mathematical models were also used along with analytical approaches to predict tensile strength and elastic modulus form tensile properties of constituent materials. Laminate analogy and modified Tsai-Hill criteria provided satisfactory predictions of elastic modulus and tensile strength, respectively. Short-term creep experiments were performed at high temperatures and mathematical equations were developed to relate temperature, stress and rupture time. Effect of frequency on fatigue life was evaluated and incremental step frequency tests were performed at different stress ratios and stress levels. Surface temperature rise was found to be material, frequency and stress level dependent. Three energy-based models were applied to the incremental step frequency data and relationships were developed for each material to estimate surface temperature rise as a function of test frequency and stress level. Cyclic deformation tests were performed at different temperatures and in different directions of mold flow. Effect of mold flow direction on fatigue behavior was significant at all temperatures and stress ratios and the Tsai-Hill criterion was used to predict off-axis fatigue strengths. Fatigue behavior of PA6 was influenced by water absorption, while PBT indicated a high resistance against water. Temperature also greatly influenced fatigue strength and a shift factor of Arrhenius type was developed to correlate fatigue data at various temperatures, independent of the mold flow direction and stress ratio. Micromechanisms of fatigue failure at different temperatures and water absorption were also investigated. Good correlations between fatigue strength and tensile strength were obtained and a method for obtaining strain-life curves from load-controlled fatigue test data is presented. A fatigue life estimation model is also presented which correlates data for different temperatures, fiber orientations, and stress ratios. Effect of mean stress on fatigue life was found to be significant at all temperatures. Several mean stress parameters were evaluated for their ability to correlate mean stress data. Notched fatigue tests of an unreinforced polymer and a short glass fiber thermoplastic composite were also conducted using plate type specimens with a central circular hole. Effect of stress concentration was found to be considerable, with or without mean stress and in both the longitudinal and transverse directions. The commonly used Neuber's rule for metallic materials, nonlinear FEA, as well as critical distance approaches were utilized for notch deformation and fatigue life analyses.