Chapter 15 Fatigue Behavior of Fluid End Crossbore Using a Coupon-Based Approach Mahdi Kiani, Rayford Forest, Steven Tipton, and Michael W. Keller 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. Keywords Fluid end module • Fatigue failure • Autofrettage • Compressive residual stresses • FEA 15.1 Introduction Many high-pressure components have intersecting bore geometries, such as in reciprocating pumps that are widely used in gas and oil industries for well stimulation. These intersecting bore geometries are critical regions when designing fluid ends on positive displacement pumps, because the intersection between two bores is a stress concentrator [1–4]. This is especially true as the fluctuating pressure history in a fluid end can be extreme, with pressures alternating between 0 and 15,000 psi at frequencies of up to 5 Hz. At stress concentration points in the fluid ends, the maximum tensile stress can rise up to several times of the fluid pressure in other sections. As a result, fatigue often limits the useful life of the pump. To address these large alternating stresses, compressive residual stresses at crossbore intersections can be introduced to improve fatigue life. These residual stresses can be applied in several ways, such as autofrettage or shot peening [4–6]. The autofrettage approach is one of the most common due to its simplicity and low-cost. During the autofrettage process, the fluid end is pressurized above the design operating pressure, which causes partial yielding. This plastic deformation is concentrated at the crossbore intersection, because of high stress concentration factor in this section. When the autofrettage pressure is released the bulk material elastically relaxes, but the permanent plastic deformation at the crossbore intersection resists. As a result, the bulk elastic material compresses the plastically deformed crossbore intersection leading to imposition of compressive residual stresses exactly where they are most effective, the crossbore intersection. The mean compressive stress generated during autofrettage serves to offset the fluctuating tensile stresses during operation and increase the fatigue life of the fluid end module [4, 7]. To optimize the autofrettage pressure and enhance the compressive residual stresses, understanding the stress-strain response during autofrettage in the crossbore area of the fluid end is critical. Moreover, evaluation of the evolution of the compressive residual stresses after autofrettage is required for fatigue life estimation and design. Understanding the behavior of the imposed residual stresses is critical for evaluating the proper time interval for reapplying the surface treatment process [1, 8, 9]. M. Kiani ( ) • R. Forest • S. Tipton • M.W. Keller The University of Tulsa, 800 South Tucker Drive, Tulsa, OK 74104, USA e-mail: mahdi-kiani@utulsa.edu © The Society for Experimental Mechanics, Inc. 2016 A.M. Beese et al. (eds.), Fracture, Fatigue, Failure and Damage Evolution, Volume 8, Conference Proceedings of the Society for Experimental Mechanics Series, DOI 10.1007/978-3-319-21611-9_15 115
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