Electrical resistance-based fatigue damage assessment of steels

Electrical resistance measurements are a common method for the characterization of various microstructural properties and damage mechanisms, e.g., dislocation density, void volume fraction and microcracks. Additionally, influence affecting the electrical resistance, such as specimen geometry or temperature, must be considered. Therefore, ex-situ measurement techniques are frequently employed during fatigue due to their simpler measurement. However, ex-situ investigations have the potential to result in unintended influences due to disruptions, and only discrete states are analyzed limiting the characterization. Consequently, in-situ measurements were performed in this study to investigate damage mechanisms and evolution during fatigue loading. To quantify and compensate for the influence of geometry, temperature, and martensite volume fraction change during fatigue tests, a complex experimental setup was developed for in-situ electrical resistance measurements. A new developed combination of measurement systems enables the direct transfer of measured strain to electrical resistance. The method was tested on high-temperature vacuum brazed joints with a metastable austenite as base material and Ni-based filler metal. Finally, the change of the microstructure was evaluated through scanning electron microscopy analyses at different load cycles.