Modelling and Experimental Analysis of Fretting Fatigue in Complete and Bolted Contacts

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.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.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

An insight into the effect of rough surfaces and contact orientation on the fretting characteristics of quenched and tempered steel

The effect of a rough, textured surface and contact orientations on the fretting behavior of self-mated martensitic 34CrNiMo6 +QT steel was examined via a large annular flat-on-flat contact. A friction study accompanied by microscopy analyses was performed to provide a clear insight into fretting characteristics. In gross sliding, the rough surface revealed a lower delayed friction peak and the same steady-state friction as the fine-ground smooth one. In partial slip, the stable friction threshold was around 0.5 for both surfaces. The oxidation-abrasion and its combination with adhesion were observed as wear mechanisms in partial slip and gross sliding, respectively.Peer reviewe

A review of non-destructive testing techniques for the in-situ investigation of fretting fatigue cracks

© 2020 The Authors Fretting fatigue can significantly reduce the life of components, leading to unexpected in-service failures. This phenomenon has been studied for over a century, with significant progress being made during the past decade. There are various methods that have been used to study fretting fatigue cracks in order to gain a greater understanding of the effects of fretting fatigue. Destructive methods are traditionally used to observe fretting fatigue cracks. Although useful in determining crack location, crack length, crack propagation modes, crack path and shape, it is not efficient or reliable for time based measurements. Non-destructive testing has developed in recent years and now in-situ monitoring can be used during testing in order to increase the understanding of fretting fatigue. This paper presents a review of non-destructive testing techniques used in-situ during fretting fatigue testing, which are compared in order to conclude the suitability of each technique. Recent developments in non-destructive techniques that could be also applied for fretting fatigue tests are also discussed, as well as recommendations for future research made

Palkkakirjanpitäjän ammatillinen kehittyminen

Tämä opinnäytetyö on toteutettu päiväkirjamuotoisena. Opinnäytetyössä kuvataan palkkakirjanpitäjän työtä Helsingin kaupungin palkkahallinnossa kymmenen viikon ajalta 7.12.2020- 19.2.2021. Seurantaviikkojen sisältö koostuu päiväkirjamerkinnöistä sekä viikoittaisista analyyseista, joissa perehdytään valittuihin aiheisiin syvällisemmin. Palkkakirjanpitäjä työskentelee Helsingin kaupungin talouspalveluissa palkkahallinnossa palkkakirjanpitäjänä. Helsingin kaupunki on suomen suurin työnantaja ja palkkahallinnossa maksetaan noin 38 000 palkansaajan palkat. Palkkakirjanpitäjä on työskennellyt tehtävässään kahdeksan kuukauden ajan ennen opinnäytetyön aloittamista. Työtehtävät koostuvat palkkoihin liittyvien tasetilien ja vuosiansioiden täsmäytyksestä sekä tilinpäätökseen liittyvistä tehtävistä. Viikoittaisten analyysien tavoitteena on seurata omaa oppimista ja perehtyä valittuihin aiheisiin tarkemmin. Lopputuloksena on opinnäytetyön lopussa olevat pohdinnat ja päätelmät. Pohdinnoissa ja päätelmissä on kuvattu seurantaviikkojen aikana ilmenneet haasteet, oivallukset ja opitut asiat sekä oma kehittyminen

Modelling and Experimental Analysis of Fretting Fatigue in Complete and Bolted Contacts

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.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.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

Non-idealities in fretting contacts

There is direct link between non-idealities in fretting wear, friction and fretting fatigue, especially in the case of adhesion spots. Friction is not a constant and varies as a function of load cycles and non-Coulomb friction may occur. Fretting wear may lead to material transfer, resulting in tangential fretting scar interactions, and in the long run, wear debris is entrapped and cumulated in the interface. Fatigue failure can occur at low nominal stress amplitudes due to non-Coulomb effects. Novel tools are required in component design to fully capture these non-ideal phenomena.publishedVersionPeer reviewe

Prediction of contact condition and surface damage by simulating variable friction coefficient and wear

A simulation method to predict the reliability of clamped metal contacts under cyclic loading is presented. The main idea is to predict the development of contact condition of a joint by simulating a spatially variable coefficient of friction (COF) and wear. Frictional energy dissipation drives the COF evolution rule, and classic Archard's equation is employed as the evolution rule for wear depth. As both the COF and wear evolution are considered, the presented approach is capable of predicting changes in the contact condition over time. The approach is based on the Finite Element Method (FEM) and is generally applicable to industrial cases. The method is implemented as a subroutine to a FEM solver Abaqus to define a contact formulation in both normal and tangential directions. The subroutine allows full coupling between normal and tangential contact variables, which makes the approach robust also in complex industrial applications. As the effect of wear is described in the contact pressure calculation, there is no need for mesh modification. The presented approach was validated by simulating cylinder-on-plane configuration. The presented method provides similar results obtained with a simulation where geometry is updated due to wear. The results of the case study were qualitatively verified against a bolted joint type fretting experiment. The area of slip after stabilized COF distribution corresponds well with the experimental fretting scars. However, Archard's wear law seems to be limited, at least in partial slip cases, as it overestimates the amount of wear without considering entrapment of wear debris in the contact. A case study of medium speed combustion engine component is presented to show how the simulation method can be used in engine development to ensure reliable contact interfaces.acceptedVersionPeer reviewe