Experimental investigation of the combined effects of corrosion and mean stress on fatigue strength of low carbon steel

Student thesis: Doctoral Thesis

Abstract

This thesis presents an experimental investigation of the combined effects of corrosion and mean stress on the fatigue strength of a low carbon steel.Corrosion fatigue is a large problem in many engineering fields, which also affects the mining and oil and gas industries, hence the interest from Weir Group. This phenomenon can significantly reduce the fatigue strength of materials by damaging surface structures and accelerating fatigue crack growth. Positive displacement GEHO diaphragm pumps are widely used in mining industries because of the advantage of separating the power end components from the pumped fluid. However, the Fluid End components are operated in harsh environments and are subjected to fatigue and mean stress effects. Understanding the fatigue behaviour of Fluid End components material exposed to freshwater environments and the effects of positive mean stress on the corrosion fatigue strength is of paramount importance to ensure a fatigue life of 10 9 cycles, equals to the design life of GEHO pumps.The scope of this research work is to evaluate the effect of freshwater environments on the fatigue behaviour of low carbon steel S355J2, the material used to manufacture Fluid End components, and to develop a model to evaluate the effect of mean stresses on the corrosion fatigue strength at elevated fatigue cycles to be applied in the design of GEHO pumps.This is achieved by developing and performing an experimental programme consisting of corrosion tests on unloaded specimens, uniaxial fatigue tests in air and in the freshwater corrosive environment at different stress ratio conditions. Stress-based experimental results show a decrease in fatigue strength due to the corrosive environment and a continuous decrease of the corrosion fatigue strength with increasing fatigue life. The mean stress effects on corrosion fatigue lives are evaluated by the construction of Haigh diagrams based on experimental results. The sensitivity of S355J2 to mean stress in the corrosive environment is higher at high stress ratios, compared to trends in air. A modified FKM approach is proposed, based on the definition of mean stress sensitivity factor parameters, to predict the allowable stress up to 10 9 fatigue cycles in the freshwater environment.Industry scale specimens were designed to reproduce critical conditions of Fluid End components of GEHO pumps. Modifications of a test rig available in Weir Minerals Venlo was implemented and two industry scale tests were completed under cyclic pressure loading at different nominal load ratio conditions and results were in good agreement with the proposed predictive model.
Date of Award13 Jul 2020
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SupervisorDonald MacKenzie (Supervisor) & Yevgen Gorash (Supervisor)

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