Abstract
This paper aims to discuss the behavior, focused on data distribution of fatigue life for different sucker rod grades in pure fatigue and corrosion fatigue mechanisms. Analysis of the impact from several variables and the accuracy to estimate a fatigue life.
Fatigue - and more frequently - corrosion-fatigue is a major cause of premature failures in the oil and gas industry. Various technical conditions affect fatigue life and fatigue crack growth such as the material surface condition, residual stress, bulk material properties, stress conditions (multi axial, amplitude constant or variable) and environmental influence. All these conditions lead to a stochastic (not deterministic) behavior of the component. I.e., the fatigue lives of similar components under the same conditions (loads, environment) can be significantly different.
Scatter in fatigue is an inherent characteristic of materials. The performance of a sucker rod under fatigue and corrosion fatigue conditions depends on many factors, including the environment characteristics, the material properties, and the applied stresses, giving as result a stochastic behavior.
Literature available to date on this matter suggest that statistical aspects of fatigue of material are well recognized. The sources of this scatter can be essentially different for the crack initiation period and crack growth period.
Tenaris performed extensive and abundant corrosion fatigue tests at the same stress conditions to obtain the distribution function of the fatigue life. Tests were performed in fatigue tensile testing machines equipped with enclosed chambers covering the whole surface of full-scale rod samples (1" diameter × 2 ft length).
Results of corrosion fatigue testing for particular sets of samples were analyzed under Weibull density function. Even at Lab conditions with all the testing parameters as much controlled as possible, there is a dispersion of results.
The fatigue behavior of a material, mainly in its early stages, i.e. crack initiation and growth, is governed by stochastic factors giving a particular probability distribution of failure occurrence under this phenomenon. Understanding these features provides a good tool to analyze the performance of sucker rod strings and design solutions to improve the fatigue life of components.
