Reference Papers
A brief overview of the base models employed in FEPX, as well as the general Neper/FEPX workflow are documented in this paper (click the image to access the article):

R Quey and M Kasemer, The Neper/FEPX project: free / open-source polycrystal generation, deformation simulation, and post-processing, IOP Conference Series Materials Science and Engineering, vol. 1249, pp. 012021, 2022.
The methods implemented in FEPX are documented in this theory paper:

PR Dawson and DE Boyce, FEpX – Finite Element Polycrystals: Theory, finite element formulation, numerical implementation and illustrative examples, arXiv:1504.03296 [cond-mat.mtrl-sci], 2015.