PHYBAL-CHT - Short-time procedure for the determination of fatigue properties of steel

由于传统的疲劳试验需要高成本的Woehler试验，因此短时间、低成本的试验方法对评价金属材料的疲劳性能具有重要意义。为了表征金属材料的循环变形行为，在循环变形曲线中使用并绘制了塑性应变幅值与循环次数的关系。在本研究项目的范围内，将通过循环硬度测试来描述钢的塑性变形行为，该测试应与其循环变形行为相关联。在基于循环硬度测试的新方法PHYBAL-CHT(CHT=循环硬度测试)中，塑性压痕深度幅度是作为硬度测量循环的函数来确定的。与塑性应变幅值类似，循环硬度试验PHYBAL-CHT的测量变量将用于确定塑性压痕应变幅值。这种方法可以定量地描述循环硬化，因为它是在正火钢的循环变形曲线中测量的。在提交的研究项目中，42CrMo4(SAE 4140)在正火和三种调质状态下的典型塑性压痕应变幅度应通过循环硬度试验PHYBAL-CHT来确定。应将结果与常规疲劳试验的循环变形曲线进行比较。结果表明，PHYBAL-CHT适合于描述正火或调质钢的不同循环变形行为。与传统的疲劳性能测定方法相比，这种新的方法可以大大减少试验工作量。为了证明PHYBAL-CHT试验结果与常规疲劳试验结果的可比性，将研究外加载荷引起的显微组织变化。在逐步加载的PHYBAL-CHT试件和常规疲劳试验试件上，将通过二维显微硬度图、电子背散射衍射测量和半最大强度下全宽度的射线测量来确定塑性变形体积的变化。为了用这些方法证明局部组织的变化，循环硬度试验所得到的塑性变形体积是足够大的。为此，将建造一个测试设施来执行循环宏观硬度测量。

Short-time and cost effective test methods are of major interest for the evaluation of fatigue properties of metallic materials, because conventional fatigue tests require cost consuming Woehler-experiments. To characterize the cyclic deformation behavior of metallic materials the relation of the plastic strain amplitude and the number of cycles is used and plotted in cyclic deformation curves.In the scope of the presented research project the plastic deformation behavior of steel will be described by a cyclic hardness test which shall be correlated with its cyclic deformation behavior. In the new approach PHYBAL-CHT (CHT = Cyclic Hardness Test), which is based on a cyclic hardness test, the plastic indentation-depth amplitude is determined as a function of the hardness measuring cycles. In analogy to the plastic strain amplitude the measured variables of the cyclic hardness test PHYBAL-CHT will be used to determine a plastic indentation strain amplitude. This procedure allows to describe quantitatively the cyclic hardening, as it is measured in cyclic deformation curves of normalized steels. In the submitted research project representative plastic indentation strain amplitudes shall be determined by the cyclic hardness test PHYBAL-CHT for 42CrMo4 (SAE 4140) in the normalized as well as three quenched and tempered states. The results shall be compared to cyclic deformation curves of conventional fatigue tests. It will be proved, that PHYBAL-CHT is suitable to describe the different cyclic deformation behavior of normalized or quenched and tempered steels.With this new method-approach the experimental effort can be reduced significantly compared to methods which are used to determine fatigue properties in a conventional manner. To prove the comparability between the results of PHYBAL-CHT and conventional fatigue tests, microstructural changes due to the applied loading will be investigated. Changes in the plastic deformed volume will be determined on step-by-step loaded PHYBAL-CHT specimens as well as specimens of conventional fatigue tests by two-dimensional micro hardness mappings, electron backscatter diffraction measurements and radiographic measurements of the full widths at half maximum intensity. To prove local microstructural changes by these methods, it is necessary, that the volume which is plastically deformed by the cyclic hardness test is sufficient in size. For this purpose, a test facility will be built to perform cyclic macro hardness measurements.