Experiments, modeling and parameter identification with inhomogeneous strain states for plastics with strain inducedanisotropy

该项目涉及通过冷成形(预拉伸)获得所谓的自增强的聚碳酸酯(PC)薄膜的实验表征、建模和参数识别。以这种方式，初始各向同性材料经历应变诱导的各向异性。因此，强度和延展性等性能可能会受到所需加载方向的影响，该加载方向在塑料冷成形过程中使用。主要目的是对板材的三维力学性能进行全面的实验表征，并在此基础上改进自己开发的模拟冷弯成形过程中应变诱发各向异性的模型。这个项目的结果将扩展关于冷成形聚合物的基本知识，例如在制造拉伸和自增强薄膜的过程中。实验部分涉及通过顺序双轴加载的方法扩展Dammann，Caylak，Mahnken(2014)关于PC制成的拉伸棒和薄膜的诱导各向异性的自身发展。光学测量支持高度不均匀应变场的确定。进一步的目标是研究局部材料的行为和可压缩的非弹性。为此，在初步实验中对两种独立的体积应变测定方法进行了比较，结果令人满意。此外，正如初步调查表明的那样，关于硬化和软化的发现是必不可少的。与全球观察到的软化相反，局部几乎不会出现这种效应。不同的全球应变率将被描述出来。材料模型部分涉及到自己在Mahnken，Dammann(2014)中关于应变诱导各向异性的发展。因此，用一个与加载方向相关的结构张量来表示各向异性，用与当前所谓的加载角度相关的权函数来表示各向异性。此外，该项目旨在通过梯度引入正则化，以避免对有限元网格的依赖，并在现有模型中加入体积流函数。在这样做的过程中，考虑了来自实验的新信息，这些信息是通过初步调查表明的，而不是从文献中得知的。最后，打算进行参数识别。除了均匀的应力-应变曲线外，还利用非均匀位移数据通过逆有限元方法识别正则化参数。利用所得到的材料模型的材料参数，对薄膜的冷拉伸等冷成形过程进行了模拟。

The project deals with the experimental characterization, modeling and parameter identification of polycarbonate (PC) films which receive a so-called self-reinforcement by cold-forming (pre-stretching). In this way, the initially isotropic material experiences a strain-induced anisotropy. Thus, properties such as strength and ductility can be influenced depending on the desired loading direction, which is utilized during cold-forming of plastics. The main aim is a comprehensive experimental characterization of the three-dimensional mechanical properties and on this basis to improve an own developed model which cansimulate strain-induced anisotropy during cold-forming. The basic knowledge on cold-forming polymers, for example during manufacturing of stretched and thus self-reinforced thin films, will be extended by results of this project.The experimental part deals with the extension of own developments for induced anisotropy in Dammann, Caylak, Mahnken (2014) for tensile bars and films made of PC by means of sequential biaxial loading. Optical measurements support the determination of highly inhomogeneous strainfields. Further objectives are to investigate the local material behavior and the compressible inelasticity. For this purpose, the comparison of two independent methods for the determination of the volume-strain in preliminary experiments shows promising results. Additionally, findings on hardening and softening are essential, as preliminary investigations indicate. Instead of the globally observed softening, locally this effect hardly occurs. Differentglobal strain rates will be characterized. The material modeling part is concerned with the extension of own developments on strain induced anisotropy in Mahnken, Dammann (2014). Therefore, a structure tensor which develops in dependence onthe loading direction is used to represent the anisotropy by means of weighting functions in dependence of the current so-called loading angle. Furthermore, the project aims to introduce a regularization by gradients to avoid FE-mesh dependencies and to include a volumetric flow function into the existing model. In doing so, new information from the experiments, indicated by preliminary investigations and not known from the literature, is considered.Finally, parameter identification is intended. In addition to homogenized stress-strain curves also the inhomogeneous displacement data are used for the identification of the regularization parameter by means of an inverse finite element method. With the obtained material parameters for the material model cold-forming processes, such as the cold stretching of films, are simulated.