What is the discovery?
Structural alloys can exhibit strain-rate-dependent response. This means that the material response to strain depends on how rapidly the strain is increased. High-strain-rate regimes can occur during impact or specific manufacturing processes, and these regimes show significant differences in alloy strength and hardening compared to slower “quasi-static” loading. In a new publication, a team of FAST beamline users from Purdue University, Penn State University, and Sandia National Laboratories uses High Energy X-ray Diffraction Microscopy (HEDM) to explore the microscopic differences between the various loading regimes, matching grain scale evolution to observed differences in macroscopic performance. They compare HEDM data from two samples of austenitic stainless steel, one loaded quasi-statically (using the RAMS2 load frame at CHEXS), the other at a high strain rate (using a modified Kolsky bar impact test at Sandia). They discovered that the high strain rate sample exhibited a larger spread of grain scale residual stresses than the quasi-static sample after ~2% plastic deformation. Additionally, the grains within the high strain rate sample were found to rotate less compared to the grains within the quasi-static sample. This offers new, quantitative, microscopic insight into a long-standing materials engineering problem.
Why is this important?
High strain rate deformation is common-place in material processing, impact loading, and ballistics. Alloy response to these loading conditions dictates future performance. The new paper demonstrates that individual grains within the microstructure of rapidly-strained steels have the potential to have larger magnitudes of residual stress. This residual stress can impact the fatigue performance of structural alloys - in particular, a tensile residual stress can lead to shortened fatigue lives. This work, lead by then-Purdue-graduate-student and now-CHESS-postdoc Sven Gustafson, exposes the potential for larger grain scale residual stresses after high strain rate loading as compared to a quasi-statically loaded sample. This directly bears on engineering choices matching manufacturing methods to component performance.
Why did this research need CHEXS?
The FAST beamline is dedicated to understanding materials processing and performance at the microscale using high-energy X-rays. In this capacity, FAST is the premier X-ray facility enabling both Near-Field HEDM and Far-Field HEDM techniques which were used in this study. The unique RAMS2 load frame and integrated diffraction system was critical in allowing controlled loading of the quasi-static sample and HEDM scanning of the high strain rate sample. While the high strain rate sample was studied ex-situ in this work, future upgrades to the FAST beamline and development of the 9x1 “timing mode” in CESR will allow in-situ high strain rate materials testing in the immediate future.
How was the work funded?
Sandia National Laboratories (contract number 1701331)
National Science Foundation, CMMI 16-51956
The Center for High Energy X-ray Sciences (CHEXS), NSF MPS/BIO/ENG (DMR-1829070)
Sandia National Laboratories is funded by US Dept of Energy, NNSA (DE-NA0003525)
Reference
Grain scale residual stress response after quasi-static and high strain rate loading in SS316L
Sven E Gustafson, Darren C Pagan, Brett Sanborn, and Michael D Sangid
Materials Characterization 197, 112692 (2023); https://doi.org/10.1016/j.matchar.2023.112692