Paper Title
Numerical Modeling of Hydraulic Autofrettage Process on Inner Surface of Fluid End Blocks of Fracture Pumps based on True Material Model

Abstract
Fluid endblocks (FEBs) are the most important components in the hydraulic fracturing pumps. A potential important application of the hydraulic autofrettage process (HAP) is to strengthen the inner surfaces of the FEBs. This is achieved by creating a favorable compressive residual stress field near to the inner surfaces of the component to increase its pressure-bearing capacity and/or improve lifetime. This requires a fundamental understanding and modeling of the complex mechanics of the HAPin order to accurate predict such residual stresses. The key outstanding modeling issue is the complex material behavior, dominated by the Bauschinger effect and associated with reversed yielding. This effect has been modeled for axisymmetric cylinders but has not been incorporated into FEBs; it seems that the combination of geometrical and material complexities has deterred such analysis. In this paper, a newly developed finite element analysis (FEA)-based user programmable function (UPF), featuring true material constitutive behavior, i.e., replicating an existing Bauschinger-effect characterization (BEC), will be adopted to accurately simulate the HAPto quantitatively investigate the stress-strain evolution and residual stress fieldsnear the inner surfaces of the FEBs of the fracture pumps. This simulation will then be compared with the FEA modelingbya traditional bilinear kinematic hardening material model to clearly indicate the importance of the accuracy of the material constitutive model in determining appropriate residual stresses and strains. Keywords -Hydraulic Autofrettage Process (HAP), Fluid End Blocks (FEBs), Fracture Pumps, Computer Simulation, Bauschinger-Effect Characterization (BEC), True Material Constitutive, User Programmable Function (UPF).