Free-edge effects in curved composite laminates: analysis, optimization and test

在许多应用情况下，曲线层合板结构的失效是由曲率引起的层间剥离应力引起的分层。此外，在层合复合材料结构中，在自由层合板边缘附近出现局部的和潜在的奇异应力场，这增加了复合材料层合板的分层临界性。到今天为止，只有纯粹的数值分析手段可以用来评估弯曲层合结构中的自由边缘应力场。因此，本研究计划致力于发展封闭形式的解析和半解析方法，以分析弯曲复合材料层合板的自由边缘效应。闭合形式的解析近似方法可以通过使用各种策略来发展，其中以分层脱层力和力矩的形式来评估应力结果是最简单和直接的方法。此外，也可以采用所谓力平衡法形式的基于层应力的方法。最后，本文拟采用基于层间位移的分析方法。所有的分析方法都将通过比较有限元模拟进行验证。此外，所有的分析方法都将通过实验结果进行验证。一旦建立了近似的闭合解析方法，曲线层合板中的自由边效应将被视为一个优化问题。通过数学规划的方法，将探索在曲线层合板结构的情况下可以发挥哪些优化潜力，以及是否可以最大限度地减少自由边缘效应。所开发的分析方法的质量和优化程序的结果将通过对弯曲层合板的试件进行测试来评估。

In many application cases, curved laminated structures fail by delaminations which are triggered by interlaminar peeling stresses that arise due to the curvature. Further, in laminated composite struc-tures localized and potentially singular stress fields arise in the vicinity of the free laminate edges which add to the delamination criticality of a composite laminate. As of today there are only purely numerical means of analysis available by which free-edge stress fields in curved laminated struc-tures can be assessed. Consequently, this research proposal is devoted to the development of closed-form analytical and semi-analytical methods for the analysis of free-edge effects in curved composite laminates. Closed-form analytical approximate methods can be developed by using various strategies where the assessment of stress resultants in the form of layerwise delamination forces and moments is the most simple and straightforward one. Furthermore, layerwise stress based approaches in the form of the so-called force-balance-method can be employed as well. Lastly, it is intended to use layerwise displacement based analysis approaches. All analysis methods will be verified by comparative finite element simulations. Furthermore, all analysis methods will be validated by means of experimental results. As soon as approximate closed-form analytical methods have been established, the free-edge effect in curved laminated will be considered as an optimization problem. By using the methods of mathematical programming it will be explored which optimization potential can be exploited in the case of curved laminated structures and if free-edge effects can be minimized. The quality of the developed analysis approaches and the results of the optimization procedures will be assessed by tests performed on specimens of curved laminates.