Life assessment in constant and variable amplitude high‐temperature fatigue of ductile cast iron and metastable austenitic steel based on in situ measurement of physical properties

Abstract

As effective methods for characterization of temperature and load dependent fatigue behavior, the present paper describes (i) application of strain increase and temperature increase tests for comprehensive quantitative characterization of the dynamic strain aging influence on fatigue properties of cast iron and (ii) analysis of electromagnetic acoustic transducer signals for early detection of fatigue damage of AISI 347 metastable austenitic stainless steel. In case of cast iron, plastic strain and stress amplitude recorded in temperature increase tests allow identification of the onset and peak of dynamic strain aging at frequencies up to 92 Hz. Isothermal strain increase tests and application of Morrow's relationships between cyclic deformation behavior and slopes of a Wöhler curve in the high cycle / low cycle fatigue‐regime allow determining the total strain Wöhler curve in excellent agreement with conventional constant amplitude experiments. In‐situ characterization of fatigue processes in AISI 347 was performed by measurement of ultrasonic signal time‐of‐flight as well as amplitude decay by electromagnetic acoustic transducers which generate ultrasonic waves directly in electrically conductive materials without the use of a couplant. Especially the time‐of‐flight signal reflects the actual specimen loading as well as cyclic hardening / softening and early fatigue damage with excellent sensitivity.