In-situ monitoring of dislocation proliferation during plastic deformation of 304L steel using ultrasound

The dislocation density as a function of applied stress is monitored in-situ, continuously, for 304L steel in a standard tension test. This is done by measuring the speed of acoustic waves, both longitudinal (L) and transverse (T). In a first experiment, both wave velocities, strain, and sample thickness are measured simultaneously and independently for a single elongation of the sample. Measurements are in agreement with theoretical predictions relating acoustic wave speed and dislocation density. The resulting data for dislocation density as a function of stress in the plastic region obeys the Taylor rule, and the simultaneous measurements of acoustic velocity and applied stress allows for the determination of the two parameters appearing in said rule. X-ray diffraction (XRD) measurements are performed using the Rietveld refinement, and a Croussard-Jaoul (CJ) analysis for the stress-strain curve is carried out. Both indicate the presence of an increasing number of twins at high strains. In a second experiment three consecutive loading-unloading cycles are performed for the same steel sample. In this case the shear wave velocity, applied force and sample strain are measured. Additionally, the stress, the strain and the sample thickness are obtained through a finite element modeling, calibrated with experimental data. The plastic behavior in the first and third cycles is similar to that of the first experiment: There is an increase in dislocation density, as determined through the change in the speed of shear waves, and said increase obeys the Taylor rule. The extent to which these results constrain the values of the parameters appearing in the Taylor rule for the third cycle is discussed. The material behavior during the second cycle is markedly different, with an increase, rather than decrease, in the speed of shear waves. Using classical theory of elastic wave behavior in polycrystals, it is found that this change is consistent with a decrease in grain size due to a proliferation of twins, as foreshadowed by the XRD measurements and C-J analysis. Our work adds ultrasound as a tool to those available for in-situ testing of steels, and paves the way for the development of devices for the evaluation of pieces in service.

Automated summary

This study monitors the dislocation density of 304L steel in a tension test by measuring acoustic wave velocities. The results show that the measured velocities are consistent with theoretical predictions. XRD measurements and C-J analysis also indicate the presence of twins at high strains. Additionally, the study investigates the differences in plastic behavior during different loading and unloading cycles.