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

该项目涉及聚碳酸酯（PC）薄膜的实验表征，建模和参数识别，这些薄膜通过冷成型（预拉伸）获得所谓的自增强。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，诸如强度和延展性的性质可以取决于在塑料的冷成形期间利用的期望的加载方向而受到影响。主要目的是对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能够模拟冷成形过程中应变诱导各向异性的模型.本项目的结果将扩展冷成型聚合物的基础知识，例如在拉伸和自增强薄膜的制造过程中。实验部分涉及Dammann，Caylak，Mahnken（2014）通过连续双轴加载的方式对PC制成的拉伸棒和薄膜的诱导各向异性的扩展。光学测量支持高度不均匀的应变场的测定。进一步的目标是调查当地的材料行为和可压缩的非弹性。为此目的，在初步实验中的体积应变的测定的两个独立的方法的比较显示出有前途的结果。此外，正如初步调查所表明的那样，硬化和软化的调查结果是必不可少的。而不是全球观察到的软化，局部这种效果几乎没有发生。不同的全球应变率将被表征。材料建模部分涉及Mahnken，Dammann（2014）中应变诱导各向异性的发展。因此，依赖于加载方向发展的结构张量被用于通过依赖于当前所谓的加载角的加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FE网格依赖性，并将体积流量函数纳入现有模型。在这样做时，新的信息，从实验中，初步调查表明，从文献中不知道，被认为是。除了均匀化的应力-应变曲线，也用于识别的正则化参数的逆有限元方法的非均匀位移数据。利用所获得的材料参数对材料模型进行了模拟，如薄膜的冷拉伸。

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.