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

应用项目侧重于无止回纤维增强复合材料在反循环荷载作用下的多尺度损伤分析。该项目的目的是基于物理的纤维增强复合材料现有损伤模型的推广，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/实验与多尺度数学相结合的损伤研究方法。循环加载损伤的类型和数量由相对于纤维方向的载荷矢量的方向和符号来定义。在不太清楚的刚度和强度的拉-压-不对称现象、拉-压诱导损伤（被动损伤）的相互作用以及拉诱导分层对随后的压缩载荷的具体影响中，已经确定了在反向载荷下对损伤和退化进行现实建模的具体挑战。为此，通过细观和宏观循环试验，结合原位计算机层析成像和光弹性应力分析，分析了细丝水平（微观）和层水平（细观）的损伤现象和退化行为。从微观层面的附加数值分析，建立了基于物理的不同应力比循环加载下应力-应变变化行为的数学公式。开发的数学方法随后在<s:1>动力与静态研究所（ISD）的基于fe的疲劳损伤模型中实施，该模型已经对脉动循环载荷进行了首次验证。这样就可以克服目前已知的疲劳分析模型对恒幅脉动载荷的局限，并向谱载荷下纤维增强材料的寿命评估迈出了重要的一步。

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.