Mechanism of Compressive Strength Reduction due to Impact damage in Composite Laminate and Material Design

本文以“复合材料层合板冲击损伤压缩强度降低机理及材料设计”为题进行了研究。本文的工作主要包括三个部分，即近似理论、断裂行为的有限元数值模拟和含多个圆形脱层的理想冲击损伤模型的实验研究，实验研究了含多个圆形脱层试件的屈曲和后屈曲行为，并指出含多个圆形脱层试件的最终失效是由于脱层沿载荷横向扩展引起的。这意味着复合材料层合板的层间韧性是CAI问题的重要组成部分，应采用断裂力学方法进行计算，并采用Rayleigh Ritz方法进行近似分析。通过对多个脱层的尺寸、数量和形状的参数研究，我们得到了由于冲击损伤引起的屈曲强度降低的各种有趣现象。但后屈曲分析被发现是必要的，以解释最终的破坏行为的损伤板。进行有限元分析需要大量的数值工作，因为不仅要进行三维分析，而且还必须考虑接触问题。结果很好地解释了试验中观察到的屈曲、后屈曲和破坏行为，用Rayleigh-Ritz方法得到的屈曲载荷与试验和有限元方法得到的结果吻合较好。为了解决冲击后压缩问题，必须结合冲击响应过程中的损伤累积过程来研究强度折减。

A research work is conducted under the title of "Mechanism of Compressive Strength Reduction due to Impact damage in Composite Laminate and Material Design". The present work is made of three parts, that is, Approximate theory, numerical simulation of fracture behavior by finite element method and experiment of the plate with multiple circular delaminations which is an ideal model of impact damage.In the Experiment, the buckling and postbuckling behaviors are studied and final failure of the specimen with multiple delaminations is caused by the delamination propagation in transverse direction to the load. It means that the inter-laminar toughness of the composite laminate is important for the CAI problem and a fracture mechanics approach should be conducted.The approximate analysis is conducted through the Rayleigh Ritz method. Through a parametric study done by this method with respect to the size, number and shape of the multiple delaminations, we obtained various interesting phenomena on the buckling strength reduction due to the impact damage. But postbuckling analysis is found to be necessary to explain the final failure behavior of the damaged plate. The finite element analysis conducted needed huge numerical efforts, because not only a three dimensional analysis is done but also a contact problem must be considered. The results explain well the buckling, post buckling and failure behaviors observed in the experiment.The buckling loads obtained through Rayleigh-Ritz agree well with those obtained through both experiment and finite element method. Considering its efficiency, the present method is useful to explain global phenomena.In order to solve the CAI (Compression After Impact) problem, the strength reduction must be studied relating the damage accumulation process during impact response.