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

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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.