Mechanical characterisation of fatigued austenitic stainless steel under applied loads by in-situ neutron diffraction

Abstract

The fatigue behaviour of austenitic stainless steel, in which a martensitic phase is formed due to plastic deformation, is of some interest for practical reasons. We have studied two samples, subjected, respectively, to 6 × 10⁵ (30% level of fatigue) and 1.2 × 10⁶ (60%) tensile-compressive loading cycles at a frequency of 5 Hz, on the in situ stress rig of the ENGIN instrument at ISIS. The main results of this investigation are that (1) the macroscopic Young's modulus (i.e. the elastic slope. of the bulk stress—strain response) decreases with increasing fatigue, (2) the Rietveld determined elastic constants of the austenite and martensite phases for both the parallel and perpendicular directions are independent of the level of fatigue, (3) the Rietveld determined austenite elastic response is linear throughout the measured stress range (0–500MPa) whilst the martensite response is linear only up to 180–200 MPa. the same stress level at which bulk plasticity is observed, (4) the ratio of the martensite elastic constants in the parallel and perpendicular directions is almost twice that expected based purely on the value of Poisson's ratio.

Key Words:

• Stress
• Strain
• Neutron
• Diffraction
• Steel
• Austenite
• Martensite