Development and analysis of a material model based on the reptation theory for the description of the strain behaviour of PET at high strain rates

建立了两段拉伸吹塑工艺，作为一种生产具有优异力学性能和光学性能的高质量塑料中空体的方法。在此过程中，材料的拉伸导致分子链的强定向，并形成片层状结构。这些结构导致材料强度的急剧增加。对材料行为的预测有助于解释过程，并有助于提高生产空心体或薄膜的材料效率。为了预测材料的性能，使用了不同的材料模型。材料模型目前通过弹簧-阻尼器方法描述，并通过应力-应变曲线进行校准。这些模型可以很好地预测校准范围内的变形，但在此范围外精度较低。本研究项目的目的是开发和分析一个材料模型，该模型描述了基于重复理论的聚乙二甲酸乙二醇酯（PET）的应力应变行为。为此，利用确定的非晶(A)和半晶(C) PET的材料数据，实现并校准了描述聚合物熔体中产生应力的重复理论方程集。项目的第一步是材料模型的开发。为此，描述塑料熔体的重复理论方程体系适合于描述PET的行为。针对不同的PET类型，对模型进行了不同条件下的性能测试。将计算得到的不同温度下的应力应变曲线与实验结果进行了对比，验证了模型的正确性。并将该模型应用于薄膜膨胀流变仪（MIR）和拉伸吹塑过程的仿真，通过与实验的比较，分析了模型的准确性。MIR的实际测量和模拟测量之间的比较允许在变形过程中的小时间步长的应变行为分析。因此，可以识别模型的准确性，但也可以识别其弱点。最后，将材料模型嵌入到两阶段拉伸吹塑过程的仿真中。通过实验验证，并与现有的PET超弹性材料模型进行了比较。预成形体的复杂几何形状和材料内部温度分布的不均匀性对模型的精度提出了很高的要求。通过对PET材料性能的了解和对模型质量的分析，解决了材料和能源效率问题。

The two-stage stretch blow moulding process is established as a method for producing high-quality plastic hollow bodies with excellent mechanical and optical properties. The stretching of the material during the process leads to a strong orientation of the molecular chains and to a formation of lamella like structures. These structures lead to a sharp increase in the strength of the material. The prediction of the material behaviour helps in the interpretation of the process and in the increase of the material efficiency in the production of hollow bodies or films. For the prediction of the material behaviour, different material models are used. Material models are currently described through spring-damper approaches and are calibrated through stress-strain curves. These models can predict the deformation within the calibrated range well, but are less accurate outside this range. The aim of this research project is the development and analysis of a material model, which depicts the stress strain behaviour of Polyethylenterephthalat (PET) based on the reptation theory. For this purpose, the equation set of the reptation theory for the description of the resulting stress in polymer melts is implemented and calibrated with the material data for amorphous (A) and semi-crystalline (C) PET determined. The first step of the project is the development of the material model. For this purpose, the equation system of the reptation theory for the description of plastic melts is fit to describe the behaviour of PET. The performance of the model is examined under different conditions for the specified PET types. The validation of the model is carried out by comparing the calculated stress strain curves for different temperatures with experiments. Furthermore, the model is applied in simulations of membrane-inflation rheometer (MIR) and the stretch blow moulding process to analyse the models accuracy by the comparison with experiments. A comparison between practical and simulated measurement of the MIR allows the analysis of the strain behaviour in small time steps during the deformation process. Therewith, the accuracy, but also weaknesses, of the model can be identified. Finally, the material model is embedded in a simulation of the two-stage stretch blow moulding process. The validation is realised through experiments and compared to an existing hyper elastic material model for PET. The complex geometry of the preforms and the inhomogeneous temperature distribution in the material make high demands on the accuracy of the model. With the acquired knowledge of the material behaviour of PET and the analysed quality of the model, the material and energy efficiency is addressed.