In composite laminates the layer-wise material mismatch induces highly localized stress concentrations at the free edges. This is referred to as free-edge effect. As a consequence interlaminar crack initiation may occur in addition to the typically considered intralaminar failure modes. The stresses do not necessarily have to be caused by mechanical loads. Also temperature differences may induce significant interlaminar stresses. In the present study, delamination onset in symmetric angle-ply laminates caused by thermal stresses is investigated by means of the coupled stress and energy criterion within the framework of finite fracture mechanics on the basis of the well-known generalized plane strain state. Using this relatively young criterion, both failure load and the length of the initiated crack can be predicted. Besides the intrinsic material properties in terms of strength and fracture toughness, only the interlaminar stresses as well as the released energy during crack initiation are required for the evaluation of the coupled criterion. Since no analytical closed-form solution of the free-edge effect is known, the required field quantities are obtained by employing a finite element model. Effects as the influence of the thickness and the orientation of the individual layers on the failure temperature are studied. For that purpose, a dimensional analysis is performed allowing to scale the results of a certain boundary value problem to self-similar configurations yielding a significant reduction of the numerical effort. It is revealed that especially for thick angle-ply laminates with large fiber orientation angles and temperature differences as they may occur in aeronautical applications interlaminar crack initiation can be the predominant failure mode. For validation purposes the results obtained by the coupled criterion are compared to the widely-used cohesive zone model.

在复合层压板中，逐层材料不匹配会在自由边缘引起高度局部的应力集中。这被称为自由边缘效应。因此，除了通常考虑的层内破坏模式之外，还可能发生层间裂纹萌生。应力不一定是由机械负载引起的。此外，温差可能会引起显着的层间应力。在本研究中，基于众所周知的广义平面应变状态，在有限断裂力学框架内，通过耦合应力和能量准则研究了由热应力引起的对称角层层压板的分层开始。使用这个相对较新的准则，可以预测失效载荷和起始裂纹的长度。除了强度和断裂韧性方面的固有材料特性外，评估耦合准则只需要层间应力和裂纹萌生过程中释放的能量。由于没有自由边效应的解析封闭形式解是已知的，所需的场量是通过采用有限元模型来获得的。研究了厚度和各个层的取向对失效温度的影响。为此，执行量纲分析允许将某个边界值问题的结果缩放为自相似配置，从而显着减少数值工作。结果表明，特别是对于具有大纤维取向角和温差的厚角层合板，因为它们可能发生在航空应用中，层间裂纹萌生可能是主要的失效模式。出于验证目的，将通过耦合准则获得的结果与广泛使用的内聚区模型进行比较。