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

纤维增强复合材料的力学性能在过去的几十年里得到了国内外研究者的广泛关注。然而，在焦点是连续纤维增强层压板通常与热固性基体。一方面，这类材料的技术和经济动机非常高。另一方面，这些材料可以相对容易地进行几何描述。考虑到复合材料在工业应用中的份额，短纤维增强热塑性塑料是应用最广泛的复合材料。短纤维增强热塑性塑料-通常通过注塑成型加工-正在获得越来越多的高性能应用。因此，积累的需求存在调查其力学行为。应力-循环曲线（S-N曲线或Wöhler曲线）的确定，特别是高周疲劳（HCF）强度的确定，需要大量的实验工作。然而，HCF强度通常用于设计和尺寸确定。因此，本文所要求的研究项目的目的是开发一种方法，该方法允许短纤维增强热塑性塑料（SFRT）的HCF强度的时间有效的实验测定。纤维增强聚合物的S-N曲线不能用高周疲劳的水平曲线来近似，因为不存在金属材料那样的疲劳极限。因此，中止标准定义在100万至1000万次循环范围内，并且在S-N图中标识了未破裂的每个受试样本。由于SFRT的S-N曲线在HCF范围内的平坦梯度，这是合理的简化。因此，下面将属于1000万次循环的应力定义为HCF强度。将该应力定义为金属材料的疲劳极限，结合试验和细观力学有限元模拟，分析了SFRT在准静态拉伸和疲劳载荷作用下的损伤机理。基于声发射测试，开发了一种方法，使一个高效的时间确定的HCF强度。此外，决定性的材料性能，导致失败的SFRT拉-拉疲劳载荷下，被确定。在项目成功完成后，开发了一种方法，通过单调拉伸试验确定HCF强度。

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.