• Juoksukangas, Janne

Abstract

Fretting is the micrometer-level relative movement between contacting surfaces that can lead to fretting fatigue and fretting wear in practical connections in machine components. Fretting has a clear tendency to nucleate cracks that can continue to grow as a result of the cyclic loads in a component, leading to premature failure. The lack of fundamental knowledge of the fretting mechanism and a universal model poses challenges for the design of modern machine components having contacts under high loading.<br/><br/>This thesis comprises seven publications. Its purpose is to study the effect of various design parameters on the fretting fatigue behavior in practical connections and also to apply the Digital Image Correlation method to fretting contact in order to measure displacement fields. Both experimental and modelling methods were employed to study complete and bolted contacts. The material used was self-mated quenched and tempered steel. A complete contact fretting test device was developed that had a large contact area and transverse loading resembling practical connections. Numerous fatigue tests were carried out. The Finite Element Method was used to analyze the contacts in greater detail.<br/><br/>Fretting significantly decreased the fatigue life in complete and bolted contacts. Increasing contact normal load decreased life in both types of contact while rounding of the sharp contact edge did not extend fatigue life. Fatigue life decreased when the amplitude of cyclic loading of the test specimen was increased. In complete contact tests, the cracking point was at the contact edge whereas in the bolted joints the area of fretting damage and cracking point was away from the geometric stress concentration (bolt hole) and corresponded to the distribution of frictional energy dissipation. Shot peening and nitriding were particularly effective in increasing fatigue life in sharp ended contacts. Cyclic relative displacement fields close to the contact interface were measured using the Digital Image Correlation method so that bulk compliances were minimized. These displacement measurements were successfully compared with numerical results. The modelling results agreed with the experiments in terms of cracking prediction and contact quantities.

Topics

• impedance spectroscopy
• surface
• experiment
• crack
• fatigue
• quenched and tempered steel