Comparative Analysis of Plasticity-based GND Density Estimation Methods in Crystal Plasticity Finite Element Models

TL;DR

This paper compares different methods for estimating geometrically necessary dislocations in crystal plasticity models, highlighting their differences and proposing improvements for better accuracy in polycrystal simulations.

Contribution

It introduces a comparative analysis of GND estimation methods based on Nye tensor projections and slip gradients, and suggests improvements to enhance accuracy in polycrystal modeling.

Findings

Projection methods underestimate GND densities compared to slip gradient methods.

Using only active dislocation systems in projection techniques improves accuracy.

All methods align with analytical GND trends in simple deformation scenarios.

Abstract

In crystal plasticity finite element (CPFE) simulations, accurately quantifying geometrically necessary dislocations (GNDs) is critical for capturing strain gradients in polycrystals. We compare different methods for quantifying GNDs, all of which originate from the Nye tensor, which is computed as the curl of the plastic deformation gradient. The projection technique directly decomposes the Nye tensor onto individual screw and edge dislocation components to compute GNDs. This approach requires converting a nine-component Nye tensor into densities for a larger number of dislocation systems, a fundamentally underdetermined (non-unique) process, which is resolved using $L2$ minimization. In contrast, when employing CPFE analysis, one could directly compute dislocation densities on each slip system using shear gradients. Projection and slip gradient methods are compared with respect to their prediction of GNDs with changing grain size, strain, and grain neighborhoods, including multigrain junctions. Although these techniques match analytical GND densities for single slip, single crystal deformation, and are consistent with anticipated overall GND trends, we find that the GND densities from projection techniques are significantly lower than those predicted from CPFE-based slip gradients in polycrystals. A suggested improvement of only using the active dislocation systems in the projection technique almost entirely resolved this mismatch.