Evaluation and modelling of the fatigue damage behaviour of polymer composites at reversed cyclic loading

本课题主要研究循环载荷作用下无限长纤维增强复合材料的多尺度损伤问题。该项目的目的是对现有的纤维增强复合材料损伤模型进行物理概括，作为复合材料结构疲劳寿命预测的重要组成部分。由于纤维复合材料在不同尺度上的结构不均匀性，选择了数值/实验和多尺度数学相结合的方法来研究损伤。由于循环载荷引起的损伤的类型和数量由载荷矢量相对于纤维方向的方向和符号定义。已经确定了在反向载荷下的损伤和退化的真实建模的具体挑战，这些挑战包括：刚度和强度的拉-压不对称现象，拉和压引起的损伤的相互作用，（被动损伤）和拉伸引起的脱层对随后的压缩载荷的具体影响。借助微观和宏观循环试验，结合原位计算机断层扫描和光弹性应力分析，分析了纤维水平（微观水平）和层水平（介观水平）的降解行为。从微观层次上的附加数值分析，物理为基础的数学公式的变化应力-应变行为下的循环载荷与不同的应力比制定。所开发的数学方法随后在统计与动力研究所（ISD）的基于有限元的损伤模型中实现，因此，可以克服目前已知的疲劳分析模型对等幅脉动载荷的局限性，并朝着谱下纤维增强材料的实际寿命评估迈出重要的一步装载已完成。

The project in application focusses the analysis of multi-scale damage of endless fibre reinforced composites under reversed cyclic loading. The aim of the project is the physically based generalisation of existing damage models for fibre reinforced composites as an essential part of the fatigue life prediction of composites structures. Due to the inhomogeneous structure of fibre composites on different scales a combined numerical/experimental and multiscale mathematical approach for the investigation of damage has been chosen. The type and quantity of damage due to cyclic loading is defined by the orientation and sign of the load vector in relation to the fibre direction. Specific challenges for realistic modelling of damage and degradation under reversed loading have been identified in the not well understood phenomena of tension-compression-asymmetry in stiffness and strength, the interaction of tension and compression induced damage (passive damage) and the specific influence of tension induced delaminations to subsequent compression loads.For that reason the damage phenomena, the degradation behaviour on filament level (micro level) and on layer level (meso level) are analysed with the help of microscopic and macroscopic cyclic tests accompanied by in-situ computer tomography and photoelastic stress analysis. From additional numerical analyses on micro level, physically based mathematical formulations for the varying stress-strain behaviour under cyclic loading with different stress ratios are formulated. The mathematical approaches developed are subsequently implemented in the FE-based Fatigue-Damage-Model of the Institut für Statik und Dynamik (ISD), for which first validations for pulsating cyclic loads have already been carried out.Thus the present limitation of the known fatigue analysis models to constant amplitude pulsating loads may be overcome and a significant step towards a realistic life time assessment of fibre reinforced materials under spectrum loading is taken.