Experimental characterisation and numerical analysis of increasing fatigue strength by residual stresses for cross rolling parts

在2013年战略计划框架内，正在研究如何利用成形技术产生的残余应力来改善部件的性能。拟议项目的目的是有针对性地设置冷成形过程中的残余应力状态，以提高疲劳强度。该项目的重点是混合部件，其中轴和轮毂通过横向轧制工艺连接在一起。这会导致与工艺有关的圆周凹槽，这可能会对预期使用寿命产生不利影响。这一缺点可以通过在轧制过程中选择性地在缺口中诱导残余压应力来弥补。在前两个资金期，建立了稳定的工艺，并证明了该方法在提高疲劳强度方面的性能。在此背景下，开发了不同的数值模型来模拟该过程和疲劳寿命。展示了通过不同的工艺变量积极影响使用寿命的能力。在第三个也是最后一个资助期，现在重点放在实际组件上，考虑到加入伙伴(中心)在服务条件下的影响。为此，重点将放在关于在缺口和连接区设置残余应力状态的方法组件和工艺设计上。对于实际应用，正在建立一个非常灵活的轧制夹具，它还允许具有更多几何形状的复杂工艺变量，以满足所需残余应力分布的要求。此外，由于在操作条件下应用于实际部件，部件的疲劳性能被再次考虑并获得了重要性。在与失效相关的缺口和接头处，残余压应力的稳定性对使用寿命至关重要。因此，采用弹塑性模型对循环载荷作用下的残余应力变化进行了测量和模拟。裂纹扩展模型将扩展到3D情况，因此，作为连贯模拟策略的一部分，将考虑可变的残余应力来分析部件的疲劳寿命。在第三个资助期结束时，轧制部件将通过可转移的设计方法，针对其接头和缺口几何形状进行具体设计。在考虑残余应力稳定性的情况下对疲劳强度进行了分析，并将残余应力变化的结果纳入设计方法。灵活的新工艺运动学将使缺口和连接区的残余应力能够独立设置，对真实部件的长期测试将提供整体方法的证据。此外，还将提供关于固体部件中残余应力稳定性的可转移声明。

Within the framework of SPP 2013, research is being conducted into the use of residual stresses induced by forming technology to improve component properties. The aim of the proposed project is the targeted setting of residual stress states in cold forming processes to increase fatigue strength. The project is focused on a hybrid component in which the shaft and hub are joined by a transverse rolling process. This results in a process-related circumferential notch which can have an unfavourable effect on the expected service life. This disadvantage can be compensated by selectively inducing residual compressive stresses in the notch during the rolling process.In the first two funding periods, a stable process was established and the performance of the approach for improving fatigue strength was demonstrated. Within this context, different numerical models were developed to simulate the process and fatigue life. The ability to positively influence the service life through different process variants was demonstrated.In the third and final funding period, the focus is now on the real component, taking into account the influence of a joining partner (hub) under service conditions. For this purpose, the focus will be on the methodical component and process design with regard to the residual stress states to be set in the notch and the joining zone. For practical implementation, a very flexible rolling jig is being set up which also allows complex process variants with a greater variety of geometries in order to meet the requirements for the desired residual stresses profiles.Furthermore, the fatigue behaviour of the component is again considered and gains in importance due to the application to real components under operating conditions. The stability of the compressive residual stresses in the failure-relevant notch and at the joint is critical for the service life. Therefore, the change in residual stresses under cyclic load is measured and simulated by means of an elastic-plastic model. The crack propagation model will be extended to the 3D case and thus, as part of a coherent simulation strategy, the fatigue life of the components will be analysed taking into account the variable residual stresses.At the end of the third funding period, the rolled component will be specifically designable with respect to its joint connection and notch geometry by means of a transferable design methodology. The fatigue strength was analysed under consideration of the residual stress stability and the findings on the residual stress changes were included in the design methodology. The flexible new process kinematic will enable the residual stresses in the notch and the joining zone to be set independently, and long-term tests on real components will provide proof of the overall approach. In addition, transferable statements on residual stress stability in solid components will be available.