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Use of µmagnetic test methods for the evaluation of the damage evolution in metallic materials subjected to cyclic loading

 

In order to achieve the goal of a (residual) fatigue life calculation for metallic materials or structures, it is necessary that comprehensive information is determined and made available. This information describes the microstructure and the associated material mechanisms, whereby their non-destructive determination is an essential aspect, especially for components in service. In addition to external and internal loads and strains, it is the chemical composition and the material’s condition in particular that define and ultimately limit the residual service life. Using conventional analytical methods, these influencing factors can usually only be measured destructively.

In principle, magnetic measuring methods are able to detect microstructural differences in metallic materials, which has been proven by numerous investigations in the past. However, so far this evaluation could only be carried out qualitatively, which inevitably requires further systematic investigations as calibration for a quantitative characterization. This is where the planned project comes in and is intended to make a further contribution to the improvement of magnetic measuring methods within the framework of (residual) fatigue life calculation as well as estimation methods for metallic materials.

Within the investigations on the unalloyed and low-alloyed carbon steels C22R (SAE 1023) and 20MnCrS5 (SAE 5120), the magnetic property-stress-microstructure relationship is to be developed and made usable for the evaluation by means of magnetic measuring procedures. The two materials intended for the investigations have comparable chemical compositions which differ significantly only in manganese and chromium content. In addition, the influence of the condition of the material as a result of heat treatments as well as the load due to superimposed mean stresses will be investigated.

These parameters to be developed will be integrated into the StressLife fatigue life calculation method and finally validated on the above-mentioned investigated materials. In addition to the characterization of fatigue behavior by means of stress-strain hysteresis measurement as well as thermometry and resistometry, the Barkhausen noise in particular will be used as a µmagnetic method and parameters describing the microstructure will be derived by means of a signal analysis.

Prior investigations show that methods based on µmagnetics can in principle be used to characterize the fatigue behavior. For further quantification of the results in the context of a fatigue life calculation, however, systematic investigations of the measuring phenomena and the material mechanisms behind them are necessary.

 

Keywords: Fatigue life calculation, non-destructive testing methods, µmagnetics, fatigue