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On fatigue crack growth modelling of surface initiated rolling contact fatigue using the asperity point load mechanism
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Load transfer in applications or between machine components is generally achieved through contact. In case of recurrent high contact loads in combination with a rolling motion, i.e. with a relatively small amount of slip, the contact surface may eventually suffer from rolling contact fatigue (RCF). The damage consists then of cracks and craters or spalls, which can cause dysfunctionality of the application leading to inefficiency or increased maintenance costs. Ultimately the damage may cause total failure of the machine component. The damage process is still not fully understood due to the complexity of the problem. Different mechanisms have been suggested to explain initiation and propagation of RCF damage. The current work focused on crack growth modelling of surface initiated RCF in case hardened gear steel. The study was based on the asperity point load mechanism, which emphasizes the importance of the surface roughness in the damage process. Asperities on the contact surface act as stress raisers inducing locally high tensile surface stress when entering the contact. Improved understanding of the damage process and further validation of the asperity point load mechanism was achieved.

In Paper A, the crack path of surface initiated RCF was simulated in the symmetry plane of the damage with the trajectory of the largest principal stress in the uncracked material. The mode I fracture mechanism was found applicable as well as linear elastic fracture mechanics (LEFM). The evolvement of the asperity contact parameters during the load cycle was determined through a finite element (FE) contact model based on an equivalent contact geometry. The predicted RCF crack path agreed with experimental spall profiles both in entry details as in overall shape. An experimental series was performed in Paper B to investigate the crack closure behaviour in presence of large negative minimum loads. The experimental results suggested a crack closure limit close to zero. The choice of the equivalent mixed-mode stress intensity factor range and especially the crack closure limit had a significant effect on the predicted RCF or spalling life. The two-dimensional crack growth model was further developed in Paper C and used to investigate the influence of asperity size, friction and residual surface stress on the simulated RCF damage. The simulations agreed qualitatively with experimental observations where reduced surface roughness, improved lubrication and compressive residual surface stress increased RCF resistance. In Paper D, a three-dimensional stationary crack was studied using an FE model and a simplified RCF load. A new crack geometry was proposed allowing the investigation of the spall opening angle of the typical vshaped damage. Crack arrest through crack closure was suggested as explaining mechanism. A qualitative study indicated increased spread of the surface damage with increased friction. The results also depended on the crack inclination angle. The different studies supported the asperity point load mechanism to explain not only fatigue initiation but also fatigue crack propagation.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. , 46 p.
Series
TRITA-HFL. Report / Royal Institute of Technology, Solid Mechanics, ISSN 1654-1472 ; 0551
National Category
Applied Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-141151ISBN: 978-91-7501-999-4 (print)OAI: oai:DiVA.org:kth-141151DiVA: diva2:695158
Public defence
2014-02-20, Sal F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 20140210

Available from: 2014-02-10 Created: 2014-02-10 Last updated: 2014-02-10Bibliographically approved
List of papers
1. Rolling contact fatigue crack path prediction by the asperity point load mechanism
Open this publication in new window or tab >>Rolling contact fatigue crack path prediction by the asperity point load mechanism
2011 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 78, no 17, 2848-2869 p.Article in journal (Refereed) Published
Abstract [en]

The crack path of surface initiated rolling contact fatigue was investigated numerically based on the asperity point load mechanism. Data for the simulation was captured from a gear contact with surface initiated rolling contact fatigue. The evolvement of contact parameters was derived from an FE contact model where the gear contact had been transferred to an equivalent contact of a cylinder against a plane with an asperity. Five crack propagation criteria were evaluated with practically identical crack path predictions. It was noted that the trajectory of largest principal stress in the uncracked material could be used for the path prediction. Different load types were investigated. The simplified versions added some understanding but the full description with cylinder and asperity pressures was required for accurate results. The mode I fracture mechanism was applicable to the investigated rolling contact fatigue cracks. The simulated path agreed with the spall profile both in the entry details as in the overall shape, which suggested that the point load mechanism was valid not only for initiation but also for rolling contact fatigue crack growth.

Keyword
Rolling contact fatigue, Spalling, Asperity, Fatigue crack path, Mode I, Plane mixed-mode
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-47655 (URN)10.1016/j.engfracmech.2011.07.012 (DOI)000301734000002 ()2-s2.0-80054855001 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20111114Available from: 2011-11-11 Created: 2011-11-11 Last updated: 2017-12-08Bibliographically approved
2. A fracture mechanical life prediction method for rolling contact fatigue based on the asperity point load mechanism
Open this publication in new window or tab >>A fracture mechanical life prediction method for rolling contact fatigue based on the asperity point load mechanism
2012 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 83, 62-74 p.Article in journal (Refereed) Published
Abstract [en]

The purpose was to develop a fracture mechanics based method for determining the life of surface initiated rolling contact fatigue or spalling. The life simulations were based on the asperity point load mechanism, a mode I crack growth assumption and LEFM. The life prediction was verified against the spalling life in some gear teeth, which had been measured for the simulation data. The computational tool required an equivalent mixed-mode life parameter. Such are suggested in the literature and some of these were evaluated. Also, the work required material properties for crack growth at stress cycles with highly compressive minimum loads. An experimental series was performed for crack growth at R < 0. Negative crack closure limits K-I,K-cl were suggested by the compliance but not the crack growth rate. Simulations with small negative closure limits (K-I,K-cl = -0.1 MPa root m) predicted the spalling life in the gears. It was however noted that the life predictions depended more on K-I,K-cl than the equivalent mixed-mode life parameter.

Keyword
Rolling contact fatigue, Spalling, Asperity, Crack closure, Fatigue life, Plane mixed-mode
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-95123 (URN)10.1016/j.engfracmech.2011.12.012 (DOI)000302515000006 ()2-s2.0-84857916030 (Scopus ID)
Funder
Swedish Research Council
Note
QC 20120515Available from: 2012-05-15 Created: 2012-05-14 Last updated: 2017-12-07Bibliographically approved
3. Surface initiated rolling contact fatigue based on the asperity point load mechanism-A parameter study
Open this publication in new window or tab >>Surface initiated rolling contact fatigue based on the asperity point load mechanism-A parameter study
2012 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 294, 457-468 p.Article in journal (Refereed) Published
Abstract [en]

The influence of asperity size, friction and residual surface stress on surface initiated rolling contact fatigue damage was investigated using the asperity point load mechanism. A parametric study was performed in two steps, first with a classic one-parameter-at-a-time approach, then as a 2-level full factorial design. The effect on fatigue initiation, damage size and spalling life for both early and developed spalling damage was examined for a gear application. Simplified response surfaces were derived as an engineering design tool for improved spalling resistance. The parametric investigation suggested that among the investigated parameters, reduced asperity height and local asperity friction will have the largest effect on the crack initiation risk. The simulations agreed with the engineering experiences that reduced surface roughness improves rolling contact fatigue resistance and that improved lubrication lengthens spalling lives and decreases fatigue risk. With compressive residual surface stresses the predictions suggested reduced fatigue risk and substantially decreased depth of individual spalls. Finally, predictions of experimentally observed effects of changing asperity size, friction and residual surface stress on spalling further motivates the asperity point load mechanism as the source behind surface initiated rolling contact fatigue.

Keyword
Crack path, Fatigue life, Friction, Initiation, Rolling contact fatigue, Spalling
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-105261 (URN)10.1016/j.wear.2012.07.005 (DOI)000311003200027 ()2-s2.0-84866884023 (Scopus ID)
Funder
Swedish Research Council
Note

QC 20121120

Available from: 2012-11-20 Created: 2012-11-19 Last updated: 2017-12-07Bibliographically approved
4. Investigation of the spall opening angle of surface initiated rolling contact fatigue
Open this publication in new window or tab >>Investigation of the spall opening angle of surface initiated rolling contact fatigue
(English)Manuscript (preprint) (Other academic)
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-141147 (URN)
Note

QS 2014

Available from: 2014-02-10 Created: 2014-02-10 Last updated: 2014-02-10Bibliographically approved

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