Abstract
The present paper deals with the in-situ observation of local fatigue damage processes by using a conventional servohydraulic testing machine and a new developed piezo-driven miniature testing system in the scanning electron microscope in combination with electron backscatter diffraction to correlate crack initiation and propagation with local microstructural features. In the case of metastable austenitic 304L steel it is shown and supported by finite element calculations that fatigue-induced strain localization causes the formation of α' martensite. Martensite nuclei lying parallel to the slip planes in the austenite grains and twin boundaries can be considered as crack-initiation sites. Fatigue crack propagation causes an increasing martensite transformation rate ahead of the crack tip leading to the occurrence of crack closure effects. As shown in an earlier study, crack closure in the case of short cracks follows a transient regime, i.e. immediately after crack initiation the crack stays open almost during the complete fatigue cycles. The development of a short crack model based on the boundary element method aims to the prediction of crack initiation and short fatigue crack propagation rates.
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