Experimental Analysis of multiaxial residual stress states induced by laser surface hardening

拟议的后续应用的主题是制定一项测量和评价战略，以便用中子衍射法无损地确定在约十分之一毫米至几毫米范围内的三轴残余应力深度分布。这种方法在用传统的衍射法确定局部残余应力深度梯度的情况下具有重要意义，即根据SINY方法重复X射线应力分析，结合连续的子层去除，伴随着由于材料去除而产生的大量残余应力重分布。通过这种方法，将确定错误的残余应力分布。将开发和建立SAE 4140钢激光硬化表面状态的测量和评估策略。激光硬化导致了局部复杂的残余应力分布，加工区内存在压缩残余应力，这些残余应力被加工区外相当高的拉伸残余应力所平衡，而从压缩到拉伸的转变则表现出陡峭的梯度。该方法基于直通表面扫描，为此将制定一种策略来校正由表面效应引起的虚假应变，表面效应是由于一次和二次孔限定的标称测量体积只被部分填充而产生的。这些虚假应变与由于残余应力引起的弹性晶格应变具有相同的量级，导致错误的残余应力。在本项目的第一个供资阶段，对中子衍射仪Stress-Spec@FRM II的表面效应进行了详细的实验和数值研究。作为结果，确定了使表面效应最小化的仪器设置。此外，由表面效应引起的杂散应变现在可以通过利用蒙特卡罗模拟和使用分析模型来模拟衍射实验来计算。因此，可以对测量数据进行相应的校正。然而，将该策略应用于激光表面硬化材料状态仍然是有问题的；因为我们可以证明在激光硬化马氏体加工区和铁素体基材之间的界面处发生第二表面效应。这一效应将在成功项目的继续过程中进行详细的实验和数值研究。最后，提出了一种避免和数值补偿该界面区域表面效应的策略。为此，将对中子实验模拟的分析模型进行相应的扩展。此外，将实现将该方法转变为进一步专门用于残余应力分析的中子实验(例如SALSA@ILL)。

Subject of the proposed follow-up application is the development of a measuring and evaluation strategy for the non-destructive determination of triaxial residual stress depth distributions in the range of some tenth of a millimeter to some millimeters with neutron diffraction. This method is of essential significance in the case that the conventional approach for the determination of local residual stress depths gradients with diffraction methods, i.e. repeated X-ray stress analysis according to the sin²y-method in combination with a successive sub-layer removal, is accompanied with massive residual stress redistributions due to the materials removal. By this means erroneous residual stress distributions will be determined. The measuring and evaluation strategy will be developed and established for laser hardened surface states of steel SAE 4140. Laser hardening results in a local, complex residual stress distribution with compressive residual stresses in the process zone, which are balanced by rather high tensile residual stresses outside the process zone, while the transition from compression to tension shows a steep gradient. The method is based on through surface scans, for which a strategy will be developed to correct for spurious strains that result from the surface effects, which occur due to the nominal gauge volume defined by the primary and secondary apertures, which is only partially filled. These spurious strains, which are of the same magnitude as the elastic lattice strain due to the residual stresses, result in erroneous residual stresses. Within the first funding phase of the current project the surface effects were detailed studied experimentally and numerically for the neutron diffractometer Stress-Spec@FRM II. As a results instrument settings were determined for the minimization of the surface effects. Furthermore, the spurious strains that result from the surface effect can now be calculated through simulation of the diffraction experiment by means of Monte-Carlo simulations and by using an analytical model. As a consequence, the measuring data can be corrected accordingly. However, the application of the strategy to laser surface hardened material states is still problematic; since we could demonstrate that a second surface effect occurs at the interface between the laser hardened martensitic process zone and the ferritic base material. This effect will be experimentally and numerically studied in detail within the continuation of the successful project. Finally, a strategy for avoiding and for the numerical compensation of the surface effect for this interfacial zone will be developed. For this purpose, the analytical model for simulation of the neutron experiment will be extended accordingly. Moreover, the transformation of the method to further neutron experiments dedicated for residual stress analyses (e.g. Salsa@ILL) will be achieved.