Separation of surface and volume damage due to HCF/VHCF loading to identify the re-use potential of quenched and tempered steels

在新机器和设备修复后疲劳部件重新使用的背景下，首次在所申请的项目范围内对疲劳损伤可在表面和体积之间分配的程度进行定量评估。因此，将审查从已经加载和损坏的材料状态可以在多大程度上制造样品，以及可以使用哪些测量技术来表征损坏状态，从而对进一步使用潜力进行基于物理的评估。为此，用调质42CrMo4钢制成的圆柱形试件在5赫兹的HCF范围内进行疲劳，直到达到规定的预损伤，并通过电阻、温度和磁场测量等手段进行原位表征，然后进行扫描电子显微镜观察。在进行了这一表征之后，将损坏的体积从样品中移除，将其抛光，然后在各种测试系统上以5、60、200和1000赫兹的测试频率进行二次加载。因此，在Hcf范围内，可以直接与原始未损坏的样品进行比较。在VHCF范围内，可以确定疲劳强度的可能偏差。首次加载和二次加载后表面和体积中位错排列的透射电子显微镜表征允许指定潜在不同的疲劳寿命和损伤探测器的相关信号随微观组织的演变。假设在二次加载过程中，体积损伤导致疲劳强度或疲劳寿命降低，则进一步出现的问题是，这一条件是否可以恢复，即位错密度的增加是否可以通过恢复退火来消除。这一点也将通过二次加载和透射电子显微镜调查来验证。组件设计的关键是导致裂纹萌生的PSG的形成和扩展，这里的组件设计基于对指定深度的不可逆表面损伤。这些过程将用一个面向机制的模型来表示，以便能够描述未损坏和损坏前的样品之间的差异。实验方案包括不同的试件几何形状、测试频率和测试系统。因此，在申请人以前工作的基础上，将围绕频率效应和尺寸效应进行校准，并最终通过一些选定的大型试件实验进行验证。

Against the background of the re-use of fatigued components in new machines and plants after reconditioning, a quantitative evaluation of the extent to which the fatigue damage can be divided between surface and volume is to be carried out for the first time within the scope of the project applied for. Therefore, it will be examined to what extent specimens can be manufactured from already loaded and damaged material states and which measurement techniques can be used to characterize the damage state and thus for a physically based evaluation of the further utilization potential. For this purpose, cylindrical test specimens made of the quenched and tempered steel 42CrMo4 are fatigued in the HCF range at 5 Hz until a defined pre-damage is reached and characterized in-situ by means of resistometry, thermometry and magnetic field measurement, followed by scanning electron microscopy. Following this characterization, the damaged volume is removed from the specimen, which is polished and then subjected to secondary loading on various test systems at test frequencies of 5, 60, 200 and 1000 Hz. In the HCF range, a direct comparison with the original undamaged specimens can thus be derived. In the VHCF range, possible deviations in fatigue strength can be identified. The transmission electron microscopic characterization of the dislocation arrangement in the surface and volume after initial and secondary loading allows the assignment of potentially different fatigue lives and the related signals of the damage detectors with the microstructure evolution. Assuming that the volume damage leads to a reduction of the fatigue strength or fatigue life during secondary loading, the question further arises whether this condition can be reconditioned, i.e., whether an increase in dislocation density can be relieved by recovery annealing. This is to be verified by means of second loading and TEM investigations, too. Crucial for the component design, which is based here on irreversible surface damage to a depth to be specified, is the formation and propagation of PSGs leading to crack initiation. These processes are to be represented in a mechanism-oriented model to be able to describe the differences between undamaged and pre-damaged specimens. The experimental program includes different specimen geometries, test frequencies and test systems. Therefore, based on previous work of the applicants, a calibration around the frequency effect and the size effect will be performed and finally verified by some selected large-scale specimen experiments.