Integrated neutron and surface-based characterization of stress/strain fields and microstructural evolution during fatigue crack growth in SUS304 stainless steel

Su, Y. H.  ; Shinohara, Takenao   ; Parker, J. D.*; Oikawa, Kenichi   ; Kai, Tetsuya   ; Gong, W.   ; Ito, Tatsuya   ; Harjo, S.   ; Aizawa, Kazuya  ; Kiyanagi, Yoshiaki*; Tomota, Yo*

Local variations in residual stress/strain and microstructure during fatigue crack growth in compact tension specimens of SUS304 austenitic stainless steel were investigated using complementary methods: pulsed neutron Bragg-edge imaging (NBEI), neutron diffraction (ND), digital image correlation (DIC) and electron backscatter diffraction (EBSD). Surface strain fields were evaluated using DIC and EBSD. NBEI provided two-dimensional averaged microstructure information, whereas ND yielded detailed three-dimensional distributions of residual lattice (elastic) strain and stress. This study particularly focused on how NBEI is helpful in understanding overall damage characteristics in the vicinity of a crack tip two-dimensionally. Distribution of microstructural parameters associated with crack growth, such as lattice constant, crystallite size, and texture, across the entire specimen was considered using Bragg-edge spectral analysis. The plastic zone in front of the crack tip, characterized by reduced crystallite size, was observed during crack propagation. After final fracture, two distinct zones with reduced crystallite size were identified: one caused by localized plastic deformation near the crack tip, and the other by bending at the back end of the specimen. This paper introduces the four experimental techniques and describes their respective features. Each method has its own advantages and limitations; However, by integrating their results, a more comprehensive understanding of the overall stress/strain field can be achieved.