Improvement of the mapping accuracy in material modelling with the integration of the yield locus at plane strain

在许多行业，通过轻量化建设的概念来减轻产品的整体重量是一种趋势。例如，在汽车行业，减少二氧化碳排放的法律要求正在进一步加速这一发展。要实现这一点，一个核心方面是资源节约型生产。由薄板厚度较大的传统材料制成的部件被厚度较小的高强度材料或比密度较低的材料取代，从而减轻了重量。在这种情况下的挑战是这些创新材料的有限成形性。尤其是材料在平面应变下的行为是决定性的，因为那里的成形性最低，即使在低变形时也会发生严重的板材减薄。结果表明，汽车工程中拉深过程中80%以上的失效是由平面应变或近平面应变引起的。因此，有必要对平面应变状态下的材料性能进行研究。具有变形硬化机制的材料，如DC级的深冲钢，表现出取决于变形程度和加载路径的硬化行为。对于这些材料，在数学屈服轨迹模型中以足够的精度映射与变形相关的材料属性是至关重要的。商业有限元软件中提供的传统材料模型不提供这种自由度。因此，本研究项目的主要目的是对材料在平面应变状态下的硬化进行基础性研究，并研究变形硬化对初始模型屈服轨迹精度的影响。通过采样过程，开发了能够进行优化参数识别的指示器。根据材料在不同应变和不同加载路径下的特性，识别出屈服轨迹。通过比较模型的屈服应力和实验的屈服应力，可以计算出一个误差值，从而可以说明模型的质量。此外，还开发了一个分析指标，该指标定义了基于成形过程的变形程度，在该程度上，屈服轨迹模型的参数识别将导致最大的映射精度。其结果是增加了对材料在平面应变下的行为的了解，并由此获得了优化材料模型的资格，所需的材料试验次数尽可能少，从而获得了有效的材料表征。

In many industries, there is a trend to reduce the overall weight of the product by the concept of lightweight construction. In the automotive industry, for example, this development is being further accelerated by legal requirements to reduce CO2 emissions. To achieve this, a central aspect is resource efficient production. Components made of conventional materials with a high sheet thickness are substituted by high strength materials with lower thickness or materials with a lower specific density and thus a reduced weight. The challenge in this context is the limited formability of these innovative materials. Especially the material behavior at plane strain is of decisive importance, since the lowest formability is present there and severe sheet thinning occurs even at a low deformation. As a result, more than 80% of the failures in deep drawing processes in automotive engineering are due to a state of plane strain or near plane strain. Therefore, it is necessary to investigate the material properties in the state of plane strain. Materials with a distortional hardening mechanism, such as deep-drawing steel of the DC grade, exhibit a hardening behavior that depends on the degree of deformation and the load path. For these materials, it is crucial to map the relevant deformation-dependent material properties with sufficient precision in the mathematical yield locus model. Conventional material models, which are available in commercial FE software, do not offer this degree of freedom. The primary goal of this research project is therefore the fundamental investigation of material hardening in the plane strain state and the influence of distortional hardening on the accuracy of the initial modelled yield locus. By means of sample processes, indicators are developed that enable an optimized parameter identification. Based on the material characterization at different strains and different load paths, yield loci are identified. By comparing the modelled and experimental yield stress, an error value can be calculated which allows a statement about the model quality. Furthermore, an analytical indicator is developed, which defines a degree of deformation based on the forming process, at which the parameter identification of the yield locus model leads to a maximum of mapping accuracy. The result is an increased knowledge about the material behavior at plane strain and, derived from this, the qualification of an optimized material modelling, with the lowest possible number of required material tests and thus an efficient material characterization.