Session: 16-01-01: Poster Session: NSF-Funded Research (Grad & Undergrad)

Paper Number: 99405

99405 - In Situ Nonlinear Ultrasonic Wave Measurements to Correlate β to Tension and Fatigue Damage of Stainless Steel 316l: The Effect of Grain Orientations and Slip Irreversibility. 

Dislocations are the early stage damage indicator of plastic deformation, the accumulation of which eventually leads to cracks and failures in structures. Nonlinear ultrasound (NLU) as a new technique for nondestructive evaluation(NDE) has been studied to such early damage accumulation, which is not possible with a conventional ultrasonic NDE technique. The NLU parameter (β) is sensitive to dislocation parameters such as dislocation substructures (e.g. vein structures, channels, and persistent slip bands), microstructural features (e.g. precipitates, grain size), and internal stress; thus, prior works have shown that β continuously changes during plastic deformation and there is a correlation between β and fatigue life. This research utilizes an in situ experimental setup for nonlinear longitudinal and Rayleigh wave measurements to correlate β to plastic/fatigue damage of stainless steel 316L. The overall goal of our research is to understand the relationship between β and microstructural features such as grain orientations and slip irreversibility.  

The first part of the research is to study the orientation effect on β. The variation in β is caused by the interaction of the ultrasonic waves with the dislocations, and when dislocations are present in the crystal two orientation-dependent factors affect β: one between the applied stress and the resolved shear stress, the other between the shear strain in the slip direction and measured strain. To correlate β with the applied stress, first, β was measured in situ as the monotonic tensile load increased using in situ nonlinear longitudinal/Rayleigh wave techniques. Specifically, multiple paths of incremental tensile load were applied to a stainless steel 316L sample under stress-controlled loading. During each path, ultrasonic in situ measurements were made at several intervals as the stress increased up to and above the yield stress. After the in situ tests, we set multiple points on the deformed sample and cut the sample into a cube at each point considering each cube has different a degree of deformation. The same β measurement was performed on the cubes for two different orientations to confirm the effect of orientations and Electron Backscatter Diffraction (EBSD) was used to characterize grain orientations and Schmid factors. By comparing Schmid factors and β measured for two different orientations, the effect of orientations on β was experimentally determined. 

Second, to track the effect of irreversible slip on β, Rayleigh wave measurements were conducted in situ with respect to the load frame at zero stress after the tension and compression fatigue cycles. During reverse loading, dislocation activities during the forward loading are not fully reversed during the reverse loading. The accumulation of irreversible slip is one of the mechanisms leading to crack initiation. The measured β shows a rapid decrease during hardening followed by a moderate decrease during softening. The parameter Δβ, the difference between β measured after tension and compression, is dependent on strain amplitude. The dependence of Δβ on strain amplitude is related to fatigue life through a power law relationship, similar to slip irreversibility. These results suggest that in situ nonlinear ultrasound could be used to nondestructively evaluate and predict fatigue life. 

The in situ Rayleigh/longitudinal wave measurement procedures developed in this work enabled us to better understand the relationship between β and microstructural features such as grain orientations and slip irreversibility. Further, this research introduced an experimentally-derived relationship between cumulative plastic shear strain and NLU wave propagation and advanced our insights into NLU wave responses that evolve during tensile deformation/fatigue. This work will fill a gap in the current understanding of how nonlinear wave propagation is related to dislocation-based damage at the mesoscale and ultimately enable NDE NLU techniques to characterize very early-stage damage.

Presenting Author: Hyelim Do University of Illinois, Urbana-Champaign

Presenting Author Biography: Ph.D. Student<br/>Wave Propagation and Metamaterials Laboratory (WPM Lab)<br/>Department of Mechanical Science &amp; Engineering<br/>University of Illinois at Urbana-Champaign<br/><br/>B.S. in Energy Resources Engineering, Seoul National University, Korea<br/>M.S. in Computational Science and Technology, Seoul National University, Korea

Authors:

Hyelim Do University of Illinois, Urbana-Champaign
Changgong Kim University of Illinois, Urbana-Champaign
Kathryn Matlack University of Illinois, Urbana-Champaign