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

由于传统的疲劳试验需要耗费大量成本的维勒试验，因此对金属材料的疲劳性能进行评估的主要兴趣是短时间和经济有效的试验方法。为了描述金属材料的循环变形行为，采用了塑性应变幅值与循环次数的关系，并绘制了循环变形曲线。在本研究项目的范围内，钢的塑性变形行为将通过循环硬度试验来描述，该试验应与钢的循环变形行为相关联。基于循环硬度试验的新方法PHYBAL-CHT （CHT =循环硬度试验）确定了塑性压痕深度振幅作为硬度测量周期的函数。与塑性应变幅类似，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。这个程序允许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，应通过42CrMo4 （SAE 4140）在正火状态和三种调质状态下的循环硬度试验PHYBAL-CHT来确定具有代表性的塑性压痕应变幅值。结果应与常规疲劳试验的循环变形曲线进行比较。结果表明，PHYBAL-CHT适用于描述正火钢和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，采用这种新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之间的可比性，将对施加载荷引起的微观组织变化进行研究。通过二维显微硬度映射、电子后向散射衍射测量和半最大强度下全宽度的射线照相测量，逐步加载PHYBAL-CHT试样以及常规疲劳试验试样的塑性变形体积变化将被确定。为了用这些方法证明局部的显微组织变化，循环硬度试验塑性变形的体积必须足够大。为此，将建立一个测试设备来进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.