Investigation of the cyclic behavior of carbon steels at different control modes and strain rates

本研究项目旨在拓展亚共析碳钢在不同控制类型和应变速率下的循环行为的研究现状。两个同样重要的子目标正在被追求：一方面，描述应变率和控制类型影响的定量模型得到改进，以便在超声疲劳测试设备上获得的结果可以用来推断低或中频（小于1 kHz）下力控制材料的行为。另一方面，改进了基于机理的对VHCF行为的理解。为此，在常规测试机制和超声测试机制之间的差异是跨尺度表征。本项目将对42CrMo4和50CrMo4两个热处理批次分别进行常规试验机位移和应力控制试验和超声疲劳试验机位移控制试验。基于得到的数据集，对先前研究中开发的用于转换力和位移控制试验结果的材料科学方法进行了评估。如果成功，该算法的验证将允许从超声疲劳测试设备的结果推断出应力控制材料在高频下的行为。此外，应变率对疲劳行为的影响可以直接从传统试验机和超声疲劳试验设备的位移控制试验结果中推断出来。与目前的研究状态相比，这是有利的，因为消除了干扰变量“控制类型”，从而大大降低了陈述的不确定性。在振动测试的同时，还进行了广泛的材料分析研究，如（FIB-） SEM， XPS，纳米压痕和EBSD研究。这些研究将进行调整，以帮助阐明超声和常规疲劳试验机制的差异。对机制差异的更好理解将带来对一般机制的更好理解。重点研究了以下问题：（为什么）超声检测中的微观结构演变与常规检测中的微观结构演变有什么不同？（为什么）在不同频率下进行的实验在氧气和氮气的引入以及碳的扩散方面会有所不同？应变率（单次疲劳试验中由于正弦信号而变）如何影响材料的循环性能？如果有的话，寿命和微观结构发展的差异是由什么机制造成的？

The aim of the research project is to extend the state of research on the cyclic behavior of hypoeutectoid carbon steels under different control types and strain rates. Two equally important sub-objectives are being pursued: On the one hand, quantitative models describing the influences of strain rate and control type are improved so that results obtained on ultrasonic fatigue testing equipment can be used to infer force-controlled material behavior at low or moderate frequencies (less than 1 kHz). On the other hand, the mechanism-based understanding of VHCF behavior is improved. For this purpose, the differences between the mechanisms in conventional testing and the mechanisms in ultrasonic testing are characterized across scales. In this project, displacement- and stress-controlled tests on conventional testing machines and displacement-controlled tests on ultrasonic fatigue testing machines will be performed on two heat treatment batches each of 42CrMo4 and 50CrMo4. Based on the resulting data set, the materials scientific method for converting the results of force- and displacement-controlled tests that has been developed in preceding research is evaluated. If successful, validation of the algorithm will allow inference from ultrasonic fatigue testing equipment results to stress-controlled material behavior at high frequencies. Further, the influence of strain rate on fatigue behavior is inferred directly from the results of displacement-controlled tests on conventional testing machines as well as ultrasonic fatigue testing equipment. Compared to the current state of research, this is advantageous because the disturbance variable ‘control type’ is eliminated and thus the uncertainty of the statement is significantly reduced. In parallel to the vibration tests, extensive materials analytical investigations such as (FIB-) SEM, XPS, nanoindentation and EBSD investigations are carried out. These investigations will be adjusted to help elucidate the differences in mechanisms in the ultrasonic and conventional fatigue tests. Improved understanding of the differences in mechanisms will bring improved understanding of mechanisms in general. Emphasis is placed on the following questions: (Why) is the evolution of microstructure in ultrasonic testing different from the evolution of microstructure in conventional testing? (Why) do experiments performed at different frequencies differ with respect to the introduction of oxygen and nitrogen and with respect to the diffusion of carbon? How does the strain rate (variable in single fatigue tests due to the sinusoidal signal) affect the cyclic material behavior? To what mechanisms, if any, are differences in the lifetime and in the development of the microstructure attributed?