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

在修复后在新机器和设备中重新使用疲劳部件的背景下，首次在申请项目范围内对表面和体积之间的疲劳损伤程度进行定量评估。因此，它将被检查到什么程度的样本可以从已经加载和损坏的材料状态和测量技术可以用来表征的损坏状态，从而为进一步利用潜力的物理为基础的评估制造。为此，将由淬火和回火钢42CrMo4制成的圆柱形试样在HCF范围内以5 Hz进行疲劳试验，直到达到规定的预损伤，并通过电阻测量、温度测量和磁场测量原位表征，然后通过扫描电子显微镜进行表征。在该表征之后，从样本中移除受损体积，将其抛光，然后在5、60、200和1000 Hz的测试频率下在各种测试系统上进行二次加载。在HCF范围内，可以直接与原始未损坏的样本进行比较。在VHCF范围内，可以识别疲劳强度的可能偏差。初始和二次加载后的表面和体积中的位错排列的透射电子显微镜表征允许分配潜在的不同的疲劳寿命和损伤检测器的相关信号与微观结构的演变。假设在二次加载期间体积损伤导致疲劳强度或疲劳寿命的降低，则进一步产生的问题是，这种情况是否可以修复，即，位错密度的增加是否可以通过恢复退火来缓解。这也将通过第二次加载和TEM研究来验证。部件设计的关键是导致裂纹萌生的PSG的形成和扩展，此处部件设计基于规定深度的不可逆表面损伤。这些过程将在一个机制为导向的模型中表示，能够描述未损坏和损坏前的标本之间的差异。实验程序包括不同的试样几何形状，测试频率和测试系统。因此，基于申请人先前的工作，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规模样本实验进行验证。

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.