3D-Analysis of surface damage in metallic materials under fatigue loading

这项跨学科的研究项目是对金属材料在疲劳载荷作用下的表面损伤积累进行现场分析。在这里，研究的重点是计量学和材料科学，目的是用高分辨率白光干涉仪研究承受疲劳载荷的技术钢材。在第一个项目阶段开发的用于调查表面形貌广泛变化和振动补偿的计量学方法都应适用于高分辨率林尼克白光干涉仪。该干涉仪将直接在伺服液压试验机上分析滑移带的局部损伤过程以及晶界的塑性变形。被测双相钢的细小晶粒度对测量系统提出了很高的要求。由于测量的复杂性和加载周期之间有针对性的短期中断，因此需要一个快速、高度并行化、计算时间优化和自动化的测量和评估过程。此外，从计量学的角度，将研究白光干涉仪在近距离机器应用中的应用。从获得的测量结果来看，表面形貌的时间发展将与试件加载有关。此外，还将定义表征损害程度的参数。这些损伤参数的时间发展可以在现场装置中使用干涉表面分析可靠地确定，因为不存在因试件的安装和拆卸而引起的干扰，并且因为可以容易地分析相对较大的区域。因此，将有一个一致的数据库，从中可以得出统计相关性。双相钢中的疲劳损伤既以孤立滑移线的形式出现，也以塑性变形的晶粒区(约占晶界面积的10%到100%)的形式出现。到目前为止，这种损伤过程在文献中还很少引起关注，但似乎是体心立方铁素体相的特征。它通常在碳含量较低的相中起作用，似乎对裂纹的萌生以及整个损伤积累过程都有很大贡献。高分辨率扫描电子显微镜的进一步研究将阐明表面起伏的变化与微塑性变形之间的关系

This interdisciplinary project deals with the in-situ analysis of surface damage accumulation under fatigue loading in metallic materials. Here, the focus of the research is on metrological aspects as well as on materials science with the aim of investigating technical steels subjected to fatigue loading with a high-resolution white-light interferometer. Both, the metrological approaches for the investigation of extensive changes in surface topography and the vibration compensation - developed during the first project phase - should be adapted to a high-resolution Linnik white-light interferometer. Local damage processes in terms of slip bands as well as plastic deformation of grain areas will be analyzed with this interferome-ter set-up directly at the servo hydraulic testing machine. The small grain size of the investigated dual phase steel place high demands on the measurement system. Due to the complexity of measurements and the targeted short interrupts between the load cycles, a fast, highly parallelized, computing time optimized and automated measurement and evaluation process will be aspired. Furthermore, from the metrological perspective the use of white-light interferometers in close-to-machine applications will be inves-tigated. From the gained measurement results, the chronological development of the surface topography will be related to specimen loading. Moreover, parameters that characterize the extent of the damage will be defined. The time development of these damage parameters can be reliably determined using interferometric surface analysis in an in-situ set-up, because there are no disturbances caused by mounting and dismounting of specimens, and because a comparatively large area can be easily analyzed. Consequent-ly, a consistent database will be available from which statistical correlations can be derived. Fatigue damage in a dual phase steel occurs both in terms of isolated slip lines and as plastically deformed grain regions (ranging from about 10% to 100% of the grain area). This damage process has gained little interest in the literature up to now, but seems to be characteristic for the body-centered cubic ferrite phase. It typically plays a role in phases with low carbon content and seems to contribute significantly to crack initiation and hence to the damage accumulation process as a whole. Additional investigations with a high resolution scanning electron microscope will elucidate the relation between change in the surface relief and micro-plastic deformation