Fatigue Behavior of Fluid End Crossbore Using a Coupon-Based Approach

Abstract

Fracture or mud pumps are known as the heart of the drilling and hydraulic fracturing. Crossbore geometries are central to the design of fluid end module in these positive displacement reciprocating pumps. Intersection between bores emerges as a stress concentrator and because the fluctuating pressure history is extreme, fatigue limits the useful life of the pump. Approaches such as autofrettage are typically used to extend fatigue lives through the imposition of compressive residual stresses at crossbore intersections. Direct investigation of the impact of residual stresses in working pumps is not typically possible. In order to improve understanding of the impact of residual stresses on fatigue life and to optimize the fatigue-strength improvement provided to fluid ends, unique sample geometry was designed to simulate the stresses in the crossbore. These samples are tested on laboratory-based servohydraulic fatigue frames and eliminate the need for complicated in-situ stress analysis on the fluid ends. Using notch strain analysis and modified Smith–Watson–Topper approach a life prediction algorithm was also developed to calculate the fatigue life of the coupon. To optimize the autofrettage load and cyclic loading simulation, elastoplastic FEA was accomplished utilizing a combined nonlinear isotropic/kinematic hardening material model for 4300-series alloy steel.