Entwicklung robuster Interface-Elemente für die Simulation von Delaminations-Prozessen in CFK-Laminaten

分层是复合材料中一种常见的失效模式。由于刚度的变化和厚度方向上缺乏配筋，在低荷载作用下已经发生分层，并可能导致结构整体倒塌。例如，界面元素用于模拟层之间的薄树脂层，为充分模拟裂纹的萌生和扩展提供了机会。通常，线性和指数黏聚律用于模拟软化行为。内聚区模型的缺点是需要正确地再现裂纹尖端前的应力分布。这意味着由于分层过程区的高应力梯度和一个元件中相关的部分键合，需要非常精细的网格。因此，沿分层部分的单元尺寸由界面单元定义，而不是通常由相邻的壳单元定义。本研究的目的是利用不同的数值方法来提高不连续界面单元的鲁棒性。在应用研究中，对过去的工作成果进行了总结，并对今后的工作进行了展望。在迄今为止所做的研究中，界面单元已由Hu-Washizu的多混合配方改进，具有双线性和特殊开发的应力和应变形状函数，单元尺寸可缩小50%。研究了高阶积分和自适应积分方案来描述单单元的总应力分布。最后，将双线性单元的运动学增强为二次和偶然性形状函数，以更好地逼近分层过程区域内的位移。在随后的研究中，将开发Hellinger-Reissner （HR）的另一个混合if公式，并将其实现到元素算法中。混合hr -公式为应力提供了自由的形状函数，可以很好地逼近裂纹前沿的应力分布，提高了鲁棒性。在增强运动学领域的进一步研究应软化刚度，避免载荷-位移曲线的振荡。除各节点当前节点位移自由度外，还将在界面公式中实现加性旋转自由度和加性虚元自由度。然后，将额外的自由度集成到开发的混合界面中。最后，着眼于鲁棒性的提高，将内聚规律放大。在当前工作状态下，指数内聚律是基于相对位移的。接触公式和混合模式问题，包括依赖于层间断裂韧性的内聚规律，将与DLR和亚琛工业大学合作进行审查。

Delamination is a commonly observed failure mode in composite materials. Due to stiffness changes and the lack of reinforcement in thickness direction, delamination already occurs by low loads and can yield total structural collapse. For instance, interface elements are used to simulate thin resin layers between lamina, providing the opportunity for an adequate simulation of crack initiation and propagation. Typically, linear and exponential cohesive laws are used to simulate the softening behavior. The drawback of the cohesive zone model is the requirement of a correct reproduction of the stress distribution ahead of the crack tip. This means that very fine meshes, due to high stress gradients in the delamination process zone and the associated partial bonding in one element, are needed. Thus, the element size along the delamination part is defined by the interface elements and not as usual by the adjacent shell elements. The purpose of this research is to improve the robustness of discontinuous interface elements with different numerical techniques. In the application for research the results of the past work is summarized and later on providing prospective works are presented. In the research done so far, the interface element has been improved by a multi hybrid formulation from Hu-Washizu with bilinear and special developed shape functions for stresses and strains whereby the element size could be reduced by 50 percent. High order and adapted integrations schemes were investigated to describe the total stress distribution in one element. Finally the bilinear element kinematic has been enhanced to quadratic and serendipity shape functions to give a better approximation of the displacements within the delamination process zone. In the subsequent research an additional mixed IF-Formulation by Hellinger-Reissner (HR) will be developed and implemented into the element algorithm. Providing free shape functions for stresses, the mixed HR-Formulation offers a good approximation of the stress distribution ahead of the crack front and can advance the robustness. Further investigations in the field of enhanced kinematics shall soften the stiffness and avoid oscillation in the load-displacement curve. Beside the current nodal displacement degrees of freedom at each node, additive rotational degrees of freedom and additive virtual element degrees of freedom are to be implemented in the interface formulation. Afterwards, the additional degrees of freedom will be integrated in the developed hybrid interface. Finally, focusing on the improvement of robustness the cohesive law will be amplified. In the current state of work the exponential cohesive law is based on relative displacements. The contact formulation and the mixed mode problem, including a cohesive law depending on the interlaminar fracture toughness, are to be reviewed in collaboration with the DLR and the RWTH Aachen.