Strain and microstructure distributions around a fatigue crack tip by neutron Bragg-edge imaging and diffraction

Su, Y. H.  ; Parker, J. D.*; 篠原 武尚   ; 及川 健一   ; 甲斐 哲也   ; Gong, W.   ; Harjo, S.   ; 川崎 卓郎   ; 相澤 一也  ; 鬼柳 善明*

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

Fatigue damage is known as one of the major concerns in maintaining the integrity of large-scale engineering components. The nonlinear strain/stress distribution around a fatigue crack tip has a great impact to understand the fatigue and crack growth behavior of engineering component. We consider that it is possible to reveal local variations in strain and microstructure around a fatigue crack tip throughout the thickness of the material by using the non-destructive Bragg-edge imaging and diffraction probes. To investigate the influence of austenitic and ferritic phases on the changes in the strain and microstructure of steels during cyclic fatigue, we selected a typical single austenite stainless steel SUS304 and a duplex stainless steel SUS329J4L. The fatigue tests on the compact-tension samples were conducted with a servo-hydraulic fatigue testing machine. The pulsed Bragg-edge imaging experiment on the fatigued samples was performed at BL22 RADEN in MLF/J-PARC. The time-of-flight neutron diffraction strain mapping was performed at BL19 TAKUMI in MLF/J- PARC. Two-dimensional maps of inhomogeneous residual strain and microstructure for the samples, which had been under different fatigue loading conditions, were determined by Bragg-edge spectral analysis. The obtained Bragg-edge imaging results were quantitatively compared with those determined by diffraction. Surface strains measured by digital image correlation during the fatigue tests will also be presented.