Systematic identification of damage mechanisms of short fibre reinforced thermoplastics under fatigue loading and development of a method for time efficient determination of the high cycle fatigue strength

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The mechanical behavior of fiber reinforced composite materials gained a lot of attention in national and international research activities during the last decades. However, in the focus were continuously fiber reinforced laminates usually with thermoset matrix. On the one hand, the technical and economical motivation for this class of materials is very high. On the other hand, these materials can be geometrically described relatively easily. Considering the share of composite materials in industrial applications, short fiber reinforced thermoplastics are the most widely used composites. Short fiber reinforced thermoplastics - usually processed by injection molding - are capturing more and more high performance applications. Therefore, accumulated needs exist for investigating their mechanical behavior. This is especially true for the fatigue behavior.The determination of stress-cycle curves (S-N curves or Wöhler curves) with emphasis on the high-cycle fatigue (HCF) strength requires high experimental effort. Nevertheless, the HCF-strength is commonly used for design and dimensioning. Therefore, the aim of the herein requested research project is to develop a method that allows the time efficient experimental determination of the HCF-strength for short fiber reinforced thermoplastics (SFRT). The S-N curve for fiber reinforced polymers cannot be approximated by a horizontal curve for high cycle fatigue, because no fatigue limit like for metallic materials exists. Therefore an abort criterion is defined in the range of 1 million and 10 million cycles and each tested specimen without rupture is identified in the S-N diagram. Due to the flat gradient of the S-N curve of SFRT within the HCF range this is a legitimate simplification. Therefore, the stress belonging to 10 million cycles is defined as HCF-strength in the following. For metallic materials this stress is defined as fatigue limit.The damage mechanisms of SFRT under quasi-static tension and fatigue loading are analyzed by combining experiments with micro-mechanical finite element modeling. Based on acoustic emission tests, an approach is developed that enables a time-efficient determination of the HCF strength. Additionally, the decisive material properties, which lead to failure of SFRT under tension-tension fatigue loading, are identified. After successful completion of the project, a method is developed which enables determination of the HCF-strength by a monotonic tensile test.