Quantifying microscale drivers for fatigue failure via coupled synchrotron X-ray characterization and simulations, S. Gustafson (a), W. Ludwig (b,c), P. Shade (d), D. Naragani (a), D. Pagan (e), P. Cook (c), C. Yildirim (c), C. Detlefs (c) and
M.D. Sangid (a), Nat. Commun. 11, 3189 (2020); https://doi.org/10.1038/s41467- 020-16894-2. (a) Purdue University, West Lafayette (USA) (b) University Lyon I, Villeurbanne (France) (c) ESRF
(d) Air Force Research Laboratory, Dayton (USA) (e) Cornell High Energy Synchrotron Source, Ithaca (USA)
PRINCIPAL PUBLICATION AND AUTHORS
was simulated twice, once without the twin boundary instantiated (Figures 111a,d) and once with (Figures 111b,e), to directly visualise the effect of the twin boundary on the local micromechanics. To compare the experimental DFXM to the simulated CP-FFT model, the elastic strains extracted from the simulation were projected normal to the planes of interest, allowing for a direct comparison (Figures 111a-c), while an equivalent von Mises plastic strain was extracted from the model to qualitatively compare to misorientation (Figures 111d-f). The elastic strains from DFXM exhibit steep gradients in the region in direct proximity to the twin boundary (circled in Figure 111c). The model confirms the experimentally observed strains when the twin boundary is instantiated (circled in Figure 111b), whereas the strain gradient is not present when the twin is absent (Figure 111a). This indicates that the large strain gradient is due entirely to the microstructural interactions created by the twin boundary. Additionally, the misorientation map acquired from DFXM
showed sharp gradients in direct proximity to the twin boundary (circled in Figure 111f). Similarly, the CP-FFT model displayed a gradient feature in the equivalent von Mises plastic strain, but only when the twin boundary was instantiated (circled in Figure 111e).
The experimental data demonstrate that the subsurface region of the parent grain, in direct proximity to the twin boundary, exhibits sharp gradients in both stress and strain. The abnormally high magnitude of these gradients within the bulk material has not been previously reported, as such a measurement is only possible through the combination of X-ray diffraction techniques employed in this study. The experimental characterisation and the CP- FFT model confirm that the twin boundary is the direct cause of these large gradients. These results provide insight into the development of the micromechanical fields prior to fatigue crack initiation and into the metrics, such as strain gradients, which could help define a fatigue failure criterion.
Fig. 111: a,b) CP-FFT model of elastic strain instantiated without and with the twin boundary. c) DFXM scan of elastic strain. d,e) CP-FFT model of equivalent plastic strain instantiated without and with the twin boundary. f) DFXM scan of misorientation.