On the impact of pre-deformation on the fatigue performance of metallic laminates

由于其结合对立材料特性的能力，现代金属多层材料的性能优于其单片材料。因此，这些金属层压板被用于各种应用，多年来一直是各种研究的重点。目前的加工进展允许在汽车工业中使用热轧金属层压板。适合批量生产的具有可调、分级性能的高性价比片状材料在该领域的众多应用中非常有前景。为了确保这些材料的可靠使用，需要对其疲劳特性有深刻的了解。由于由热轧层压板制成的部件是通过成形生产的，因此在加工过程中会受到预变形的影响，因此预变形的影响在这种情况下尤为重要。然而，目前的分析表明，对层状复合材料的疲劳性能还没有系统的研究，单调预变形对疲劳性能的影响也还没有得到解决。众所周知，影响疲劳行为的重要因素是缺陷密度和构件的残余应力状态。关于卷层复合材料残余应力的形成及其在预变形和循环荷载作用下的演变，公开文献中没有全面的数据。鉴于这种缺乏数据的情况，设计了本项目。本文将深入研究热轧两层复合材料的疲劳行为。将重点讨论复合材料各层以及边界层的作用。对于两层复合材料的预期应用领域，将研究预变形对材料微观结构发展、缺陷密度和分布、残余应力状态和疲劳行为的影响，并提出一个适当的模型来捕捉相关关系。该项目中使用的分析方法将允许评估表面和边界层对双层层压材料疲劳行为的影响。此外，研究人员还对目标预变形的影响进行了研究和建模，从而深入了解所产生的残余应力状态、缺陷密度和微观结构变化是否会导致疲劳性能的提高或降低。

Due to their ability to combine opposing material properties, modern metallic multi-layer materials are able to outperform their monolithic counterparts. As a result, these metallic laminates are used in a variety of applications and have been in focus of various research studies for many years. Current progress in processing now allows to use hot-rolled metallic laminates in the automotive industry. The cost-effective sheet materials suitable for mass production with specifically adjustable, graded properties are very promising for numerous applications in this field. In order to ensure a reliable use of such materials, a profound understanding of their fatigue properties is needed. As components made from hot-rolled laminates are produced by forming and, thus, are subjected to pre-deformations during processing, the influence of pre-deformation is of particular importance in this context. However, analysis of the current state-of-the-art reveals that no systematic investigations on the fatigue properties of layered composites are available and that the influence of monotonic pre-deformations on the fatigue performance has not been addressed so far. Well-known factors significantly influencing the fatigue behavior are defect density and the residual stress state of a component. Regarding formation of residual stresses in roll-laminated composite materials and their evolution upon pre-deformation and under cyclic loading no comprehensive data can be found in open literature. In light of this lack of data the present project was designed. The fatigue behavior of a two-layer composite produced by hot rolling will be thoroughly investigated. The role of the individual layers of the composite as well as the boundary layer will be in focus. With respect to the envisaged fields of application of the two-layer composite material, the effects of pre-deformation on the microstructure development, defect density and distribution, the residual stress state and the fatigue behavior of the material will be investigated and an adequate model allowing to capture relevant relationships will be proposed. The analysis methods used in the project will allow for the evaluation of the influence of the surface and boundary layer on the fatigue behavior of the two-layer laminate material. In addition, the influence of targeted pre-deformations is investigated and modelled, thus, providing insights into whether the resulting residual stress states, defect densities and microstructural changes lead to superior or inferior fatigue performance.