Influence of loading frequency and role of surface micro-defects on fatigue behavior of metastable austenitic stainless steel AISI 304

Abstract High frequency fatigue testing always provokes questions about whether the testing conditions are representative for the real application conditions. Therefore, any influences coming from the testing procedure must be known and understood before relying on the fatigue data based on high frequency testing for component design and validation phases. For this reason and because metastable austenitic steels are well known for their strain rate sensitivity (Muller-Bollenhagen, 2011; Sorich, 2016), the steel AISI 304 and the role of surface micro-defects produced by laser beam cutting were analyzed regarding the influence of load frequency on the cyclic response and fatigue behavior, and the findings of the investigation are thoroughly discussed in this paper. Fatigue tests were performed at load frequencies of 100 Hz and 1000 Hz using two resonance pulsation test systems, as well as by means of a servo-hydraulic test machine at 1 Hz and 50 Hz. All fatigue experiments were performed at tensile-tensile loading condition (R = 0.1). The cyclic deformation behavior was characterized based on the evaluation of stress-strain hysteresis loops and temperature measurements. The deformation-induced phase transformation from γ-austenite to α′-martensite was globally and locally evaluated by means of magneto-inductive measurements and EBSD analysis, respectively. Furthermore, for the first time it was possible to compare test results generated by a 1000 Hz resonance pulsation test system with results from a conventional resonance test stand cycling at around 100 Hz. The analyses showed that higher amounts of α′-martensite and lower plastic strain amplitudes are observed when the cyclic experiments are carried out at lower frequency, promoting higher fatigue strengths. Nevertheless, the influence of test frequency for specimens containing surface micro-defects is dominant in the low cycle fatigue (LCF) regime while in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) range the fatigue life determining parameter is the severity of the micro-notches present along the laser cut surface.