Fretting fatigue of aluminum alloy in contact with steel in oil drill pipe connections, modeling to interpret test results

At many drilling sites for oil production the use of high strength aluminum alloy for the drill string can be preferable than steel, due to its better strength to weight ratio, lower stiffness and higher corrosion resistance. This leads to the need of aluminum to steel pipe connections, which can be critical in terms of fatigue. The design of this connection is prone to fretting fatigue similar to the shrink-fitted assembly of a shaft in a housing. Full scale tests are reported in the present paper, showing a reduction of the fatigue strength, due to fretting, by a factor of 2.7, in comparison to the aluminum alloy fatigue strength. The Theory of the Critical Distance along with the Modified Whöhler Curve Method can give good results in interpreting fretting fatigue tests, however a slip related parameter, to better characterize the fretting condition, is here introduced. Then, a two parameter (stress–slip) map is proposed, offering a powerful tool to evaluate modifications of the design, improving fretting fatigue.