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

在应用项目中，重点分析了环形纤维增强复合材料在循环荷载作用下的多尺度损伤。该项目的目的是对现有的纤维增强复合材料的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。针对纤维复合材料在不同尺度上的不均匀结构，选择了数值/实验和多尺度数学相结合的方法进行损伤研究。循环载荷引起的损伤类型和数量由载荷向量相对于纤维方向的方向和符号来定义。在实际模拟反向载荷作用下的损伤和退化过程中，由于对拉伸-压缩-强度不对称现象、拉伸和压缩引起的损伤(被动损伤)的相互作用以及拉伸引起的分层对后续压缩载荷的特殊影响还未被很好地理解，因此，借助微观和宏观循环试验以及原位计算机层析和光弹性应力分析，分析了纤维水平(微观水平)和层水平(细观水平)的退化行为。通过细观层面的数值分析，建立了不同应力比下循环荷载作用下应力应变行为变化的物理数学公式。所开发的数学方法随后在基于有限元的疲劳损伤模型ISD(Institut für Statik und Dynamik，ISD)中实现，该模型已经对脉动循环载荷进行了第一次验证，从而克服了目前已知疲劳分析模型对恒幅脉动载荷的限制，并朝着谱载荷下纤维增强材料的实际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.