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On the asperity point load mechanism for rolling contact fatigue
KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
2007 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Rolling contact fatigue is a damage process that may arise in mechanical applications with repeated rolling contacts. Some examples are: gears; cams; bearings; rail/wheel contacts. The resulting damage is often visible with the naked eye as millimeter sized surface craters. The surface craters are here denoted spalls and the gear contact served as a case study.

The work focused on the asperity point load mechanism for initiation of spalls. It was found that the stresses at asperity level may be large enough to initiate surface cracking, especially if the complete stress cycle was accounted for.

The gear contact is often treated as a cylindrical contact. The thesis contains experimental and numerical results connected to rolling contact fatigue of cylindrical contacts. At the outset a stationary cylindrical contact was studied experimentally. The stationary test procedure was used instead of a rolling contact. In this way the number of contact parameters was minimized. The cylindrical contact resulted in four different contact fatigue cracks. The two cracks that appeared first initiated below the contact. The other two cracks developed at the contact surface when the number of load cycles and the contact load increased.

The influence of a surface irregularity (asperity) was studied numerically with the Finite Element Method (FEM). Firstly, the stationary contact was modelled and investigated numerically. At the cylindrical contact boundary a single axisymmetric was included. The partially loaded asperity introduced a tensile surface stress, which seen from the asperity centre was radially directed. Secondly, FE simulations were performed where a single axisymmetric asperity was over-rolled by a cylindrical contact. The simulations were performed for pure rolling and rolling with slip. For both situations, tensile forward directed stresses in front of the asperity were found. The presence of slip and a surface traction greatly increased the stresses in front of the asperity. Finally, when rolling started from rest with applied slip, the distance to steady-state rolling was determined for elastic similar cylindrical rollers.

Place, publisher, year, edition, pages
Stockholm: KTH , 2007. , 14 p.
Series
Trita-HFL. Report / Royal Institute of Technology, Solid mechanics, ISSN 1654-1472 ; 0440
Keyword [en]
Rolling contact fatigue, Spalling, Asperity contact, Point load; Micro-cracks, Traction, Applied slip
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:kth:diva-4569OAI: oai:DiVA.org:kth-4569DiVA: diva2:12898
Public defence
2007-12-17, F3, Lindstedsvägen 26, Stockholm, KTH, 10:15
Opponent
Supervisors
Note

QC 20100702

Available from: 2007-12-10 Created: 2007-12-10 Last updated: 2013-01-14Bibliographically approved
List of papers
1. Standing contact fatigue with a cylindrical indenter
Open this publication in new window or tab >>Standing contact fatigue with a cylindrical indenter
2005 (English)In: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 28, no 7, 599-613 p.Article in journal (Refereed) Published
Abstract [en]

A hardened steel cylinder was repeatedly pressed against a flat case-hardened steel specimen that was equally wide as the cylinder was long. Some contact end effects were noted as a result of limited plastic deformation. A strain gauge on the contact surface, just outside the contact and oriented perpendicular to the cylinder detected a surface strain when the cylinder was loaded. The non-zero surface strain was the result of boundary effects of the finite specimen. Four different types of contact fatigue cracks developed in and below the specimen contact surface. The cracks were named lateral, median, contact end and edge cracks. Changes in the measured surface strain values could be used to determine when the lateral and edge cracks developed. The order in which all four crack types typically developed was determined from optical crack observation at test termination, strain measurements and stress computations. Numerical computations using finite-element (FE) analyses were used to verify the surface strain behaviour due to loading and cracking, to verify contact end effects; crack locations and crack orientation by aid of the Findley multi-axial fatigue criterion.

Keyword
contact fatigue, lateral crack, spalling, sub-surface cracks
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-7769 (URN)10.1111/j.1460-2695.2005.00904.x (DOI)000229934300003 ()2-s2.0-21344454861 (Scopus ID)
Note
QC 20100702Available from: 2007-12-10 Created: 2007-12-10 Last updated: 2011-11-11Bibliographically approved
2. Influence of a single axisymmetric asperity on surface stresses during dry rolling contact
Open this publication in new window or tab >>Influence of a single axisymmetric asperity on surface stresses during dry rolling contact
2007 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 29, no 5, 909-921 p.Article in journal (Refereed) Published
Abstract [en]

The effect from contact loading of some single axisymmetric asperities as a potential mechanism for surface initiated rolling contact fatigue was investigated numerically using FEM. Computational results were compared to properties of some rolling contact fatigue craters, or spalls, in the teeth surfaces of four driving gear wheels. The gears were geometrically identical but had experienced slightly varying load conditions. The residual surface stresses of a used teeth with spalls were measured using the hole drilling technique. The combined cylinder asperity contact was first modelled with a stationary model in which an asperity was introduced at the contact rim. By varying asperity height, width, position and contact load dangerous asperity configurations were sought for. The gear contact close to the rolling circle was modelled as two rolling cylinders. A single asperity was introduced into the contact surface of one of them. Due to the presence of the asperity a three-dimensional contact model was required. The simulation included residual stresses from heat treatment and plastic deformation due to the first roll cycle. Thus, the stress results were computed from the second roll cycle. The important overall conclusion was that a single asperity may serve as a stress raiser in the contact surfaces. Furthermore, the computed values of the increased surface stresses were comparable to those that are reported in the literature to give cracks. Example of dangerous asperity dimensions were noted and changes in residual stresses from moderate plastic deformation during rolling were estimated. The asperity deformed plastically during over-roll but remained sufficiently high. The trajectory of the largest principal stress was computed, starting from the position in front of a loaded asperity with maximum tensile stress. The trajectory was compared to the spalling entry angle of a representative spall. For some asperity-cylinder configurations a convex region with large stress was found in the surface. The presence of such a convex stress region was compared to the convex shaped of the spalling tip that sometimes could be found.

Keyword
rolling contact fatigue, spalling, asperity, residual stress
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-7770 (URN)10.1016/j.ijfatigue.2006.08.002 (DOI)000245773000013 ()2-s2.0-33846186866 (Scopus ID)
Note
QC 20100702Available from: 2007-12-10 Created: 2007-12-10 Last updated: 2011-11-11Bibliographically approved
3. Surface stresses at an axisymmetric asperity in a rolling contact with traction
Open this publication in new window or tab >>Surface stresses at an axisymmetric asperity in a rolling contact with traction
2008 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 30, no 9, 1606-1622 p.Article in journal (Refereed) Published
Abstract [en]

Rolling contact between a smooth cylinder and a cylinder with an axisymmetric surface asperity was modelled numerically. The influence of tangential slip and friction was investigated through relative contact movement between the cylinders. As the asperity entered the rolling contact it acted as a point type contact force, which gave a tensile surface stress in the forward rolling direction. The tensile stress maximum was greatly influenced by slip and coefficient of friction.

Data for the simulations were captured from a gear example with surface initiated rolling contact fatigue or spalling. The cylindrical contact load and geometry corresponded to that at the roll-circle of the gear. The geometrical properties of the asperity were based on surface profiles of the gear flank. The combined isotropic and non-linear kinematic Chaboche material model was used with parameters determined from cyclic compression-tension tests on the gear material. The residual stress profile due to heat-treatment of the gear was included into the model.

Two different frictional set-ups were investigated. One contained a non-zero coefficient of friction throughout the rolling contact. This was believed to compare to dry contacts. The other set-up was supposed to model lubricated rolling with asperity break-through to metal contact. Here friction was non-zero on the asperity and zero elsewhere in the contact. With traction throughout the cylindrical contact a sufficiently long start distance had to be travelled before the asperity interaction. Thus, the transient rolling distance was determined together with the slip limit for sliding in the cylindrical contact. Numerical predictions of residual stresses and surface distress angles suggested that the asperity friction model agreed with gear conditions.

Evaluation of elastic-plastic asperity indentation suggested that the asperity deformation was approximately as severe as repeated macro-scale experiments with fatigue cracks. Since the stresses at the asperities were of the same size as those at the repeated indentations and since the Findley multi-axial fatigue criteria predicted fatigue damage, it was concluded that the stresses in front of the asperity could be sufficient to initiate rolling contact fatigue cracks in applications. The influences of some parameters on the stress maximum were also evaluated.

Keyword
asperity contact, rolling contact fatigue, spalling, transient rolling, gear contact
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-7771 (URN)10.1016/j.ijfatigue.2007.11.008 (DOI)000257001700008 ()2-s2.0-43949102372 (Scopus ID)
Note
QC 20100702. Uppdaterad från Accepted till Published i DiVA 20100702.Available from: 2007-12-10 Created: 2007-12-10 Last updated: 2012-03-19Bibliographically approved
4. Transient rolling of cylindrical contacts with constant and linearly increasing applied slip
Open this publication in new window or tab >>Transient rolling of cylindrical contacts with constant and linearly increasing applied slip
2009 (English)In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 266, no 1-2, 316-326 p.Article in journal (Refereed) Published
Abstract [en]

The transient start problem of a rolling cylindrical contact has been studied. The transient conditions were controlled by the applied relative slip. Two cases with start of rolling from stationary contact were investigated, with constant and with linearly increasing applied slip. At each instant during the transition stage, it was assumed that the traction distribution could be approximated with the Carter traction for steady-state tractive rolling. Based on this distribution, approximate expressions were derived for the transient rolling distance and transient behaviour of the tangential load. The transient period could end in gross sliding or steady-state creep with the Carter traction distribution and stick-slip regions in the contact. The expressions and the transient traction distributions were validated numerically using FEM. Simulations with constant applied slip showed that when rolling started from a tangentially unloaded and unstrained position, the steady-state traction distribution by Carter was a good approximation of the actual transient traction distribution. The solution was accurate for transient rolling lengths longer than a quarter of the contact width. The transient behaviour depended on the bulk geometry of the structures. For the relatively stiff structure with two elastic steel cylinders, small amounts of relative slip and high coefficients of friction, the transient rolling distance, L-0, could become large. In the present study, examples with L-0 approximate to 40 . a were identified. Thus, situations exist for which the transient conditions might be important. The transient distance increased with smaller slip, larger coefficient of friction, lower bulk stiffness, higher contact normal loads and for more compliant materials. The spur gear contact interaction with varying slip was considered as a case study.

Keyword
Transient rolling, Stick-slip, Sliding contact, Contact traction, Spur gear
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-7772 (URN)10.1016/j.wear.2008.07.008 (DOI)000261538800037 ()2-s2.0-55249116733 (Scopus ID)
Note
QC 20100702. Uppdaterad från Submitted till Published i DiVA 20100702.Available from: 2007-12-10 Created: 2007-12-10 Last updated: 2011-11-11Bibliographically approved

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