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Life prediction and mechanisms for the initiation and growth of short cracks under fretting fatigue loading
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
2006 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Fretting fatigue is a damage process that may arise in engineering applications where small cyclic relative displacements develop inside contacts leading to detrimental effects on the material fatigue properties. Fretting is located in regions not easily accessible, which makes it a dangerous phenomenon. It is therefore important to be able to make reliable predictions of the fretting fatigue lives.

The work presented in this thesis has its focus on different aspects related to fretting fatigue in the titanium alloy Ti-17. A fretting experiment was developed which allowed for separate control of the three main fretting loads. Initially, the evolution of the coefficient of friction inside the slip region was investigated experimentally and analytically. Subsequently, 28 fretting tests were performed in which large fatigue cracks developed.

The fretting tests were firstly evaluated with respect to fatigue crack initiation through five multiaxial fatigue criteria. The criteria predicted a too high fretting fatigue limit. A possible clue to the discrepancy was found in the fretting induced surface roughness with the asperity-pit interactions.

The fatigue growth of the large fretting cracks was numerically modelled through a parametric crack growth procedure. The predicted lives were compared to the experimental outcome. The numerical simulations showed that linear elastic fracture mechanics was an appropriate tool for the prediction of fretting fatigue propagation lives in the long crack regime.

Fatigue cracks spend most of their propagation life in the small crack regime. The possibility of modelling the small crack behaviour is therefore very important from the engineering point of view. The fatigue growth of through thickness short cracks was studied experimentally and numerically in the four-point bend configuration. It was found that linear elastic fracture mechanics and closure-free material growth data furnished conservative estimates for cracks longer than 50 μm.

One method to improve fretting fatigue life is to shot peen the contact surfaces. Experimental results on fretting life with or without shot peening were simulated. The fatigue life enhancement in shot peened specimens could be explained by slower crack growth in the surface material layer with residual compressive stresses.

Place, publisher, year, edition, pages
Stockholm: KTH , 2006. , p. 29
Series
Trita-HFL. Report / Royal Institute of Technology, Solid mechanics, ISSN 1654-1472 ; 0423
Keywords [en]
Fretting fatigue; Fretting experiment; Friction evolution; Fatigue crack initiation; Fatigue crack growth; Short crack; Non-destructive testing; Acoustic emission; Potential drop; Shot peening; Stress relaxation; Crack closure; Finite element method; Titanium alloy.
National Category
Mechanical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-4185OAI: oai:DiVA.org:kth-4185DiVA, id: diva2:11130
Public defence
2006-12-06, F3, Lindstedtsvägen 28, KTH - Stockholm, 10:00
Opponent
Supervisors
Note

QC 20100827

Available from: 2006-11-23 Created: 2006-11-23 Last updated: 2013-01-14Bibliographically approved
List of papers
1. A study on fretting friction evolution and fretting fatigue crack initiation for a spherical contact
Open this publication in new window or tab >>A study on fretting friction evolution and fretting fatigue crack initiation for a spherical contact
2004 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 26, no 10, p. 1037-1052Article in journal (Refereed) Published
Abstract [en]

A new design for fretting experiments is presented. The normal and tangential contact loads as well as the specimen bulk stress are separately controlled. The separate control of load systems enables more accurate simulations of the fretting situations in component interfaces. Also, the influence of the salient parameters can be investigated individually. The initial test series comprised a spherical indenter and constant normal load and bulk stress. The evolution of the slip zone coefficient of friction at a spherical fretting contact was evaluated in four different ways. For two of these methods new equations were derived. The importance of a correct coefficient of friction and the advantages of each evaluation method are discussed. The experimental results were evaluated with respect to fretting fatigue crack initiation. Five multi-axial fatigue criteria were evaluated and ranked with respect to their ability to predict fretting fatigue initiation properties. The endurance limits of all criteria were too high as compared to the experimental fretting fatigue endurance level. A qualitative explanation for the discrepancy was found in the surface profile of the slip zone.

Keywords
fretting fatigue; fretting experiment; friction evolution; multiaxial fatigue; fatigue initiation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-6421 (URN)10.1016/j.ijfatigue.2004.03.010 (DOI)000223223300002 ()2-s2.0-3042815179 (Scopus ID)
Note
QC 20100827 QC 20110922Available from: 2006-11-23 Created: 2006-11-23 Last updated: 2017-12-14Bibliographically approved
2. Fretting fatigue crack growth for a spherical indenter with constant and cyclic bulk load
Open this publication in new window or tab >>Fretting fatigue crack growth for a spherical indenter with constant and cyclic bulk load
2005 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 72, no 11, p. 1664-1690Article in journal (Refereed) Published
Abstract [en]

Fatigue growth of edge cracks subjected to non-proportional fretting loads was investigated experimentally and numerically. The cracks were produced during fretting experiments with a spherical contact between two alpha + beta titanium alloys. Constant normal load was combined with cyclic tangential load and constant or cyclic bulk load. Crack propagation was detected during the experiments by strain gauges on the specimen surface and acoustic emission measurements. A parametric crack growth description procedure was used to model fatigue growth of the three-dimensional fretting cracks that were loaded with multiaxial and non-proportional stresses from the fretting contact. The predicted crack growth lives and crack shapes agreed with the experimental results. A crack path prediction based on the maximum principal value of the stress range tensor Delta sigma(ij) was evaluated.

Keywords
fretting fatigue; fretting experiment; titanium alloys; fatigue crack growth; crack path
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-6422 (URN)10.1016/j.engfracmech.2005.01.004 (DOI)000228682300002 ()2-s2.0-15544367252 (Scopus ID)
Note
QC 20100827Available from: 2006-11-23 Created: 2006-11-23 Last updated: 2017-12-14Bibliographically approved
3. Fatigue growth of short cracks in Ti-17: experiments and simulations
Open this publication in new window or tab >>Fatigue growth of short cracks in Ti-17: experiments and simulations
2007 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 74, no 15, p. 2293-2310Article in journal (Refereed) Published
Abstract [en]

The fatigue behaviour of through thickness short cracks was investigated in Ti-17. Experiments were performed on a symmetric four-point bend set-up. An initial through thickness crack was produced by cyclic compressive load on a sharp notch. The notch and part of the crack were removed leaving an approximately 50 mu m short crack. The short crack was subjected to fatigue loading in tension. The experiments were conducted in load control with constant force amplitude and mean values. Fatigue growth of the short cracks was monitored with direct current potential drop measurements. Fatigue growth continued at constant R-ratio into the long crack regime. It was found that linear elastic fracture mechanics (LEFM) was applicable if closure-free long crack growth data from constant K-1max test were used. Then, the standard Paris' relation provided an upper bound for the growth rates of both short and long crack.The short crack experiments were numerically reproduced in two ways by finite element computations. The first analysis type comprised all three phases of the experimental procedure: precracking, notch removal and fatigue growth. The second analysis type only reproduced the growth of short cracks during fatigue loading in tension. In both cases the material model was elastic-plastic with combined isotropic and kinematic hardening. The agreement between crack tip opening displacement range, cyclic J-integral and cyclic plastic zone at the crack tip with Delta K-1 verified that LEFM could be extended to the present short cracks in Ti-17. Also, the crack size limits described in the literature for LEFM with regards to plastic zone size hold for the present short cracks and cyclic softening material.

Keywords
short crack, fatigue crack growth, potential drop method, titanium alloy, crack closure
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-6423 (URN)10.1016/j.engfracmech.2006.11.016 (DOI)000247823000001 ()2-s2.0-34249659123 (Scopus ID)
Note
QC 20100827Available from: 2006-11-23 Created: 2006-11-23 Last updated: 2017-12-14Bibliographically approved
4. Influence of residual stresses from shot peening on fretting fatigue crack growth
Open this publication in new window or tab >>Influence of residual stresses from shot peening on fretting fatigue crack growth
2007 (English)In: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 30, no 10, p. 947-963Article in journal (Refereed) Published
Abstract [en]

One method to improve fretting fatigue life is to shot peen the contact surfaces. Experimental fretting life results from specimens in a Titanium alloy with and without shot peened surfaces were evaluated numerically. The residual stresses were measured at different depths below the fretting scar and compared to the corresponding residual stress profile of an unfretted surface. Thus, the amount of stress relaxation during fretting tests was estimated. Elastic-plastic finite element computations showed that stress relaxation was locally more significant than that captured in the measurements. Three different numerical fatigue crack growth models were compared. The best agreement between experimental and numerical fatigue lives for both peened and unpeened specimens was achieved with a parametric fatigue growth procedure that took into consideration the growth behaviour along the whole front of a semi-elliptical surface crack. Furthermore, the improved fretting fatigue life from shot peening was explained by slower crack growth rates in the shallow surface layer with compressive residual stresses from shot peening. The successful life analyses hinged on three important issues: an accurate residual stress profile, a sufficiently small start crack and a valid crack growth model.

Keywords
fatigue crack growth, finite element method, fretting fatigue, peening, residual stress relaxation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-6424 (URN)10.1111/j.1460-2695.2007.01165.x (DOI)000249428500005 ()2-s2.0-34548702218 (Scopus ID)
Note
QC 20100827Available from: 2006-11-23 Created: 2006-11-23 Last updated: 2017-12-14Bibliographically approved

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