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  • 1.
    Alfredsson, Bo
    University of Waterloo, Department of Management Sciences.
    2-Machine and 3-Machine Flow-Shop Scheduling Problemns with Equal Sized Transfer Batches1992In: International Journal of Production Research, ISSN 0020-7543, E-ISSN 1366-588X, Vol. 30, no 7, p. 1551-1574Article in journal (Refereed)
  • 2.
    Alfredsson, Bo
    KTH, Superseded Departments, Solid Mechanics.
    A study on contact fatigue mechanisms2000Doctoral thesis, comprehensive summary (Other scientific)
    Abstract [en]

    Surfaces subjected to rolling and sliding contacts maysuffer from contact fatigue. This thesisdeals with solidmechanic aspects of contact fatigue including the descriptionand verificationof explaining mechanisms. The new mechanism forsurface initiated contact fatigue is basedon tensile surfacestresses from local asperity contacts. It is also realised thatsub-surfaceinitiated contact fatigue is the result of tensileresidual stresses that emanate from plasticdeformation belowthe surface. These mechanisms clearly show that contact fatiguecracksfollow the same rules as ordinary fatigue cracks inhardened steel.

    The thesis contains four papers that treat a new testprocedure named Standing ContactFatigue (SCF). The results ofthe test procedure have played an important role inthedevelopment and verification of the mechanisms for surfaceand sub-surface contact fatigue.

    The first part of the research work was experimental. Inthis part the SCF test properties wasdecided, crack resultsconfirmed and crack detection methods developed. Herecomparativestudies were performed using some differentmaterials and mechanical properties. It wasverified that SCFcould detect differences in contact fatigue resistance.

    Next a finite element model of the SCF test was evaluatedthrough the general-purposeprogram MARC. The model includedgraded material properties that originate from heattreatment.The residual surface deformation and surface compliance wereverified againstexperimental results. Crack initiation wasinvestigated in two ways. Firstly, the principalstresses atcritical locations were computed and plotted in a Haighdiagram. The diagramshowed that the cracks initiate in adirection perpendicular to the principal stress with thelargeststress range provided that the principal stress is tensilesometime during the load cycle.

    Secondly, some high cycle multiaxial fatigue criteria,including the Haigh principal stresscriterion, was evaluatedagainst the SCF crack initiation results. The surface cracklocation waspredicted by including statistical effects using aweakest-link criterion and a three-parameterWeibulldistribution.

    The SCF crack propagation was investigated by numericalevaluation ofJ1 andJ2integrals. The crack initiation and propagationphases were separated with a threshold criterionand a directioncriterion. It was found that during crack propagation bothsurface andsub-surface contact fatigue cracks follow thedirection with minimum mode II loading.

    Key words: contact fatigue mechanism; spall; spalling;surface crack; sub-surface crack;elasto-plastic indentation;contact compliance measurement; mixed-mode fatigue;fatiguecrack growth;J-integral; multiaxial fatigue; weakest-link.

  • 3.
    Alfredsson, Bo
    KTH, Superseded Departments, Solid Mechanics.
    An experimental and numerical study on contact fatigue initiation1998Licentiate thesis, comprehensive summary (Other scientific)
  • 4.
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Fretting fatigue of a shrink-fit pin subjected to rotating bending: Experiments and simulations2009In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 31, no 10, p. 1559-1570Article in journal (Refereed)
    Abstract [en]

    Fretting fatigue initiation was studied for a shrink-fit pin at rotating bending. Eight assemblies with four different grips were manufactured from soft normalized steel and tested at loads well below bending endurance. All pins displayed rust-red fretting oxides deep into the contact and black oxidised fretting scars with fretting fatigue cracks at the rim. The slip evolution was simulated in a three-dimensional FE model including assembly, bending and sufficiently many rotations to reach a steady-state. The extension of cyclic slip agreed with the black oxidised scar. Deeper into the contact a monotonic slip developed to the positions with rust-red oxides. Asymmetric slip and traction on the interface sides together with a slight twist of the pin in the hub and the slip development process, illustrated that a three-dimensional analysis was required for the interface. Both the stress amplitude and the Findley multi-axial criterion predicted fretting fatigue of the pin although the rotating bend stress was well below the endurance limit.

  • 5.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Arregui, I. Linares
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Lai, J.
    Low temperature creep in a high strength roller bearing steel2016In: Mechanics of materials (Print), ISSN 0167-6636, E-ISSN 1872-7743, Vol. 100, p. 109-125Article in journal (Refereed)
    Abstract [en]

    Noticeable low temperature creep was established for a bainitic and a martensitic microstructure of the 100CrMnMo8 high strength roller bearing steel. The response revealed primary creep that differed between the microstructures, following a power law for martensite and the logarithmic description for bainite. The detected creep was pressure sensitive, higher in tension than in compression for the same stress level, following the strength differential effect (SDE) at material yielding. Two models were proposed where the stress variable for the pressure effect was based on the Drucker-Prager yield function and deviatoric creep strains were derived from a non-associated von Mises potential. Model parameters were determined from experimental series on the respective microstructure. When the models were evaluated against the experiments the accuracy was of the same order as the effects of different heat treatment batches and different load application rates. The importance of different material parameters in the descriptions was discussed.

  • 6.
    Alfredsson, Bo
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Cadario, Alessandro
    KTH, Superseded Departments, Solid Mechanics.
    A study on fretting friction evolution and fretting fatigue crack initiation for a spherical contact2004In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 26, no 10, p. 1037-1052Article in journal (Refereed)
    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.

  • 7.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Dahlberg, Johan
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    The role of a single surface asperity in rolling contact fatigue2008In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 264, no 9-10, p. 757-762Article in journal (Refereed)
    Abstract [en]

    The effect of contact loading of single surface irregularities, i.e. asperities, as an underlying mechanism for surface initiated rolling contact fatigue was investigated numerically using FEM. Spalls in the teeth flanks of driving gear wheels were investigated for typical spalling crack initiation properties. The spalling entry angle was documented and some spalls with a convex entry tip were found. The residual surface stresses of the used teeth, with spalls, were measured with the hole drilling technique. 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 material description included J(2)-plasticity with isotropic and linear hardening. The simulation included residual stresses from material heat treatment. The first roll cycle introduced plastic deformation which altered the residual stresses. Thus, the stress results were captured during a second roll cycle. The most important result was that asperities will serve as local stress raisers in the contact surfaces. The computed stress cycle at the asperity was compared to stress cycles that gave ring/cone cracks at point loaded experiments. The principal stress trajectory into the material was compared to the cross-section profiles of the spalling entry and ring/cone crack. The surface stress profile at the asperity was compared to the convex surface profiles of the spalling tip and ring/cone crack. The asperity deformation and change in residual surface stresses from moderate plastic deformation during rolling were estimated.

  • 8.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Linares Arregui, Irene
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Hazar, Selcuk
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Numerical analysis of plasticity effects on fatigue growth of a short crack in a bainitic high strength bearing steel2016In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 92, p. 36-51Article in journal (Refereed)
    Abstract [en]

    Plasticity effects on fatigue growth were simulated for a physically short crack. The material description comprised the Drucker-Prager yield surface, non-associated flow rule and non-linear combined hardening. The simulated development of the growth limiting parameter agreed with the experimental crack behaviour with early rapid propagation followed by a transition to slow R-controlled growth. The crack was open to the tip without any crack face closure throughout all load cycles. Instead compressive residual stresses developed at the unloaded tip which supplied an explanation to the slow rate of the propagated short crack in this bainitic high strength bearing steel. The material's strength differential effect was the key difference explaining why compressive residual stresses instead of crack face closure was responsible for the short crack effect in this material.

  • 9.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Nordin, E.
    An Elastic-Plastic Model for Single Shot-Peening Impacts2013In: Tribology letters, ISSN 1023-8883, E-ISSN 1573-2711, Vol. 52, no 2, p. 231-251Article in journal (Refereed)
    Abstract [en]

    A model was developed for impacts of elastic perfectly plastic spherical particles with impact velocities up to 250 m/s. The model is based on the two master curves, for normalized pressure and projected contact area c (2), which both are functions of the representative strain I > at maximum impact. The model and its parameters were fitted to finite element results for elastic perfectly plastic and strain rate-independent materials. It was applied to a wide range of materials with different ratio between yield stress and elastic properties, different ball sizes and impact velocities. The impact model predicted the results from finite element method for contact radius, maximum impact depth in both target and ball as well as remaining impact depth in target and ball. The remaining impact depth was determined from elastic spring back with Hertzian and quadratic pressure at maximum impact. The rebound velocity was also estimated by following the load-deformation path during spring back. If the strain rate-compensated yield stress was used for the master curve parameters, then the model predicted the impact results also for the investigated strain rate-dependent materials.

  • 10.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Olsson, Erik
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Multi-axial fatigue initiation at inclusions and subsequent crack growth in a bainitic high strength roller bearing steel at uniaxial experiments2012In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 41, p. 130-139Article in journal (Refereed)
    Abstract [en]

    The behaviour of inclusion initiated fatigue was studied for a high strength bearing steel with a bainite micro-structure. The analysis included experiments and numerical simulations. It was realized that the stress-state was multi-axial in the matrix material that met the inclusion also for a uniaxial far field stress. Fatigue initiation risk at the interface between the inclusion and matrix material was therefore predicted with the Findley multi-axial critical plane criterion. The fatigue parameters were determined from independent experiments on smooth specimens with tensile surface stress gradients. Crack growth from the inclusion to final rupture was modelled as a penny shaped crack with closure compensated effective material parameters. The growth simulations suggested that the majority of the fatigue life was consumed as fatigue crack initiation at the non-metallic inclusion.

  • 11.
    Alfredsson, Bo
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Olsson, Mårten
    KTH, Superseded Departments, Solid Mechanics.
    Applying multiaxial fatigue criteria to standing contact fatigue2001In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 23, no 6, p. 533-548Article in journal (Refereed)
    Abstract [en]

    The ability of some multiaxial fatigue criteria to predict initiation of standing contact fatigue cracks is investigated. In the standing contact fatigue test an indenter subjects a case-hardened rest specimen to a stationary but pulsating contact load. The initiation of two axi-symmetric crack types appearing in the test specimen is investigated. The surface initiated ring/cone cracks circumscribe the contact area, whereas the lateral cracks are horizontal sub-surface cracks. The effect of the stress-state history at possible crack Locations is evaluated through the Sines, Haigh principal stress, Findley, Me Diarmid and Dang Van multiaxial fatigue criteria. The material fatigue parameters of each criterion are determined from independent bending and torsion fatigue testing. Finally, the mean and spread in radial position of the ring/cone crack are evaluated by considering the statistical effects of a weakest link assumption using a three parameter Weibull distribution. The investigation shows that it is hard to distinguish a single criterion that well describes all aspects of the experimental results. For the current contact situation with highly compressive mean stresses in combination with tensile maximum values the Findley criterion shows the best overall performance followed by the Haigh principal stress criterion

  • 12.
    Alfredsson, Bo
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Olsson, Mårten
    KTH, Superseded Departments, Solid Mechanics.
    Inclined standing contact fatigue2003In: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 26, no 7, p. 589-602Article in journal (Refereed)
    Abstract [en]

    An experimental method is presented, in which a sphere is repeatedly pressed against a surface with an inclined contact load. The method is a development of the normally loaded standing contact fatigue test. Experiments are performed for three inclination angles below the angle of friction and the results are compared to those of the normally loaded standing contact fatigue test. The influence of tangential load on endurance limit load, number of cycles to crack initiation, contact mark appearance and crack behaviour in the surface as well as in cut views are evaluated. The surface crack behaviour outside the contact mark is analysed based on the cyclic contact stresses in the test specimen. The trajectories of the largest principal stresses are followed in both the surface view and in the cut view on the symmetry plane. These stress trajectories are compared to the experimental crack results. The connection between the inclined standing contact fatigue cracks and surface distress micro-cracks is also discussed.

  • 13.
    Alfredsson, Bo
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Olsson, Mårten
    KTH, Superseded Departments, Solid Mechanics.
    Initiation and Growth of Standing Contact Fatigue Cracks2000In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 65, no 1, p. 89-106Article in journal (Refereed)
    Abstract [en]

    In the standing contact fatigue test an indenter subjects a case hardened test specimen to a stationary but pulsating contact load. Two crack types appearing in the test specimen are investigated. The ring/cone cracks are surface cracks that circumvent the contact area, whereas the lateral cracks are horizontal sub-surface cracks. The initial crack lengths are determined for both crack types. Actual crack paths from experiments are evaluated numerically. For each crack tip position, stress intensity factors are determined from J(1) and J(2) integrals. The stress intensity ranges are compared to criteria for fatigue crack propagation rate and direction. As the cracks propagate, they orient in the direction with mode II loading close to zero.

  • 14.
    Alfredsson, Bo
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Olsson, Mårten
    KTH, Superseded Departments, Solid Mechanics.
    Standing Contact Fatigue1999In: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 22, no 3, p. 225-237Article in journal (Refereed)
    Abstract [en]

    A novel experimental method for testing the resistance of a material to contact fatigue, called standing contact fatigue (SCF), is presented. It comprises a spherical indenter, repeatedly pressed onto a plane specimen in pure normal contact without lubrication, friction or wear. The SCF method is here applied to three case-hardened steels, and results in ring/cone cracks initiated at the surface. The connection between SCF and spalling is discussed.

    The experimental results are presented in the form of P-N curves, where P is the normal contact load and N the number of cycles required for fatigue crack initiation. The experimental results are supported by numerical simulations of the tests. The elasto-plastic properties of case-hardened materials are graded, i.e. functions of the depth from the carburized surface. The gradation is estimated from independent experiments and is included in the analysis.

  • 15.
    Alfredsson, Bo
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Olsson, Mårten
    KTH, Superseded Departments, Solid Mechanics.
    Standing Contact Fatigue Testing of a Ductile Material: Surface and Sub-Surface Cracks2000In: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 23, no 3, p. 229-240Article in journal (Refereed)
    Abstract [en]

    During standing contact fatigue testing of case hardened steel plates, four different fatigue crack types are found: ring/cone; lateral; radial; and median cracks. Fatigue results are presented as load versus cycle number, with endurance limits and initiation laws for the ring/cone and lateral cracks. The behaviour of the radial surface strain outside the contact is altered by the presence of cracks. In particular this makes in situ crack detection possible for the lateral crack.

    The ductility of the tested material is found to be important for fatigue crack initiation. Numerical elastoplastic computations are used to derive the stress cycles responsible for each crack type. Stress cycles at different locations and in different directions are compared in order to explain why a particular crack type initiates. It is noted that cracks are produced normal to principal stresses of sufficient range, which are tensile sometime during the load cycle. Implications for spalling are discussed.

  • 16.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). STandUP Wind.
    Segalini, Antonio
    KTH, School of Engineering Sciences (SCI), Mechanics, Fluid Physics. STandUP Wind.
    Wind farms in complex terrains: an introduction2017In: Philosophical Transactions. Series A: Mathematical, physical, and engineering science, ISSN 1364-503X, E-ISSN 1471-2962, Vol. 375, no 2091Article in journal (Refereed)
    Abstract [en]

    Wind energy is one of the fastest growing sources of sustainable energy production. As more wind turbines are coming into operation, the best locations are already becoming occupied by turbines, and wind-farm developers have to look for new and still available areas-locations that may not be ideal such as complex terrain landscapes. In these locations, turbulence and wind shear are higher, and in general wind conditions are harder to predict. Also, the modelling of the wakes behind the turbines is more complicated, which makes energy-yield estimates more uncertain than under ideal conditions. This theme issue includes 10 research papers devoted to various fluid-mechanics aspects of using wind energy in complex terrains and illustrates recent progress and future developments in this important field. This article is part of the themed issue 'Wind energy in complex terrains'.

  • 17.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Wåtz, Veronica
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Erik
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Fatigue crack initiation and growth at holes in a high strength bainitic roller bearing steel when loaded with non-proportional shear and compressive cycles2011In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 33, no 9, p. 1244-1256Article in journal (Refereed)
    Abstract [en]

    Fatigue initiation from an artificial defect was investigated for a bainitic high strength roller bearing steel. Thin walled pipe specimens with small holes were subjected to multi-axial and non-proportional load cycles. The experimental fatigue crack positions around the hole were predicted with the Findley critical plane criterion. The criterion also ranked the severeness of three load sequences with respect to fatigue risk. Crack growth simulations and crack life measurements with strain gauges confirmed the ranking between the load sequences. Three uni-axial fatigue series with stress gradients were used to determine surface endurance data for the Findley criterion.

  • 18.
    Alfredsson, Bo
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Öberg, Martin
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Lai, J.
    Propagation of physically short cracks in a bainitic high strength bearing steel due to fatigue load2016In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 90, p. 166-180Article in journal (Refereed)
    Abstract [en]

    Physically short cracks in a bainitic high strength bearing steel were fatigue loaded. The rapid propagation rate of early open short cracks agreed with that of long closure free cracks. After some rapid growth, the short cracks entered a transition period to the rate of growth limited long cracks. Potential drop showed that the short cracks were open to the tip throughout the growth sequence, which excluded crack face closure in the wake as the growth limiting mechanism in this material. Instead the short crack effect was related to residual stresses and other mechanisms at the crack tip. Crack manufacturing procedures were determined for straight long and short start cracks in the present material. LEFM with effective material parameters and limit compensation predicted the short crack lives.

  • 19.
    Cadario, Alessandro
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Fatigue growth of short cracks in Ti-17: experiments and simulations2007In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 74, no 15, p. 2293-2310Article in journal (Refereed)
    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.

  • 20.
    Cadario, Alessandro
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Fretting fatigue crack growth for a spherical indenter with constant and cyclic bulk load2005In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 72, no 11, p. 1664-1690Article in journal (Refereed)
    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.

  • 21.
    Cadario, Alessandro
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Fretting fatigue experiments and analyses with a spherical contact in combination with constant bulk stress2006In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 39, no 10, p. 1248-1254Article in journal (Refereed)
    Abstract [en]

    A fretting experiment with separate control of bulk stress, normal and tangential contact loads is presented. For the initial test series, constant normal and bulk loads were combined with a cyclic tangential load. Both the spherical indenter and the plane specimen were manufactured from alpha + beta titanium alloys. Strain gauges and acoustic emission measurements were employed for the determination of the time to crack initiation and the propagation life. The fretting cracks always initiated inside the slip zone at positions that were spread over the whole slip zone. Crack nucleation was investigated with five multiaxial fatigue criteria. It was concluded that macroscale stresses from the contact and bulk load alone could not explain all aspects of crack initiation. A possible explanation was found in the fretting-induced roughness. The growth of the fretting crack was simulated numerically by a parametrical description. It was noted that a cyclic bulk load was required to drive the crack growth through the specimen to final failure.

  • 22.
    Cadario, Alessandro
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Influence of residual stresses from shot peening on fretting fatigue crack growth2007In: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 30, no 10, p. 947-963Article in journal (Refereed)
    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.

  • 23.
    Dahlberg, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Influence of a single axisymmetric asperity on surface stresses during dry rolling contact2007In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 29, no 5, p. 909-921Article in journal (Refereed)
    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.

  • 24.
    Dahlberg, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Standing contact fatigue with a cylindrical indenter2005In: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 28, no 7, p. 599-613Article in journal (Refereed)
    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.

  • 25.
    Dahlberg, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Surface stresses at an axisymmetric asperity in a rolling contact with traction2008In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 30, no 9, p. 1606-1622Article in journal (Refereed)
    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.

  • 26.
    Dahlberg, Johan
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Transient rolling of cylindrical contacts with constant and linearly increasing applied slip2009In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 266, no 1-2, p. 316-326Article in journal (Refereed)
    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.

  • 27.
    Everitt, Carl-Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Contact fatigue initiation and tensile surface stresses at a point asperity which passes an elastohydrodynamic contact2018In: Tribology International, ISSN 0301-679X, E-ISSN 1879-2464, Vol. 123, p. 234-255Article in journal (Other academic)
    Abstract [en]

    Contact mechanics and tribology was combined with fundamental fatigue and fracture mechanics to form a new mechanism for surface initiated rolling contact fatigue. Following, fatigue was investigated numerically for single asperities and craters in lubricated rolling contact surfaces. The hypothesis suggests that asperity point contacts can create sufficiently large tensile stresses for fatigue. The investigated case corresponded to a heavily loaded truck gear with ground surfaces. Reynolds equation resolved the elastohydrodynamic effect of the asperity in the transient three dimensional contacts. The Findley critical plane criterion was used for multiaxial and non-proportional fatigue evaluation. The simulations confirmed the new mechanism for rolling contact fatigue and showed how asperities can create contact fatigue in the lubricated contacts even without slip.

  • 28.
    Everitt, Carl-Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    The asperity point load mechanism for rolling contact fatigue considering slip and thermal elastohydrodynamic lubricationManuscript (preprint) (Other academic)
    Abstract [en]

    Rolling contact fatigue was investigated numerically for a single asperity and crater passing through lubricated rolling contacts with slip. The purposes was to explain why rolling contact fatigue pits develop in the forward rolling direction when slip is negative and to further verify the asperity point load mechanism for pitting. The simulations included thermal effects from friction and compression of the lubricant. It was concluded that heating of the lubricant decreased the viscosity and the contact shear stress profile in the second half of the contact, which provided an asymmetric stress profile and explained the pitting damage behaviour in relation to slip direction. 

  • 29. Gutkin, R.
    et al.
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Growth of fretting fatigue cracks in a shrink-fitted joint subjected to rotating bending2008In: Engineering Failure Analysis, ISSN 1350-6307, E-ISSN 1873-1961, Vol. 15, no 5, p. 582-596Article in journal (Refereed)
    Abstract [en]

    Crack growth in a shrink-fit assembly subjected to rotating bending was studied. Two finite element (FE) models and a numerical routine were implemented to evaluate the contribution of the fretting load and to compute the crack growth. Numerical results for the crack propagation were compared to each other and to experimental data from the literature. Good agreements were found for the stress intensity factor (SIF) computations and for the crack propagation life. A significant reduction in the fatigue crack propagation life, up to 50%, was found as compared to an equivalent pure bending case.

  • 30.
    Hannes, Dave
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    A fracture mechanical life prediction method for rolling contact fatigue based on the asperity point load mechanism2012In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 83, p. 62-74Article in journal (Refereed)
    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.

  • 31.
    Hannes, Dave
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    A parametric investigation of surface initiated rolling contact fatigue using the asperity point load mechanism2014In: Advances in Fracture and Damage Mechanics XII: selected, peer reviewed papers from the 12th International Conference on Fracture and Damage Mechanics (FDM 2013), September 17-19, 2013, Sardinia, Italy, Trans Tech Publications Inc., 2014, p. 45-48Conference paper (Refereed)
    Abstract [en]

    Rolling contact fatigue (RCF) will eventually become an issue for machine elements that are repeatedly over-rolled with high contact loads and small relative sliding motion. The damage consists of cracks and craters in the contact surfaces. Asperities on the contact surfaces act as local stress raisers and provide tensile surface stresses which can explain both initiation and propagation of surface initiated RCF damage. A parametric study was performed to investigate the contribution of surface roughness, friction and a residual surface stress to the RCF damage process. The effects on initiation, crack path and fatigue life at both early and developed damage were examined for a gear application. Both a one-parameter-at-a-time approach and a 2-level full factorial design were carried out. Surface roughness and local friction properties were found to control crack initiation, whereas the simulated crack path was primarily affected by the residual surface stress, especially for developed damage. Reduced surface roughness, improved lubrication and a compressive residual surface stress all contributed to increase the simulated fatigue life. The asperity point load model could predict effects on RCF that are observed with experiments. The results further support the asperity point load mechanism as the source behind surface initiated RCF.

  • 32.
    Hannes, Dave
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    A fracture mechanical life prediction method for rolling contact fatigue based on the asperity point load mechanismReport (Other academic)
  • 33.
    Hannes, Dave
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Investigation of the spall opening angle of surface initiated rolling contact fatigueManuscript (preprint) (Other academic)
  • 34.
    Hannes, Dave
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Modelling of surface initiated rolling contact fatigue damage2013In: Fatigue Design 2013, International Conference Proceedings, Elsevier, 2013, p. 766-774Conference paper (Refereed)
    Abstract [en]

    A parametric study was performed to investigate the contribution of surface roughness, friction and a constant residual surface stress to the rolling contact fatigue damage process. The effects on initiation, crack path and fatigue life were examined for a gear application. The asperity point load model could predict effects on rolling contact fatigue that are observed with experiments. The study of a ring/cone crack subjected to a rolling contact fatigue load allowed to explain the v-shaped cracks typical for surface initiated spalling. The results further support the asperity point load mechanism as a source behind surface initiated rolling contact fatigue.

  • 35.
    Hannes, Dave
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Numerical investigation of the spall opening angle of surface initiated rolling contact fatigue2014In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 131, p. 538-556Article in journal (Refereed)
    Abstract [en]

    The spall opening angle was studied for surface initiated rolling contact fatigue with the purpose to allow assessment of the volume of detached material. The influence of friction and the crack inclination angle on the damage spread in the contact surface was investigated with the asperity point load mechanism and a simplified three-dimensional rolling contact fatigue load. Crack arrest due to crack closure was proposed as explaining mechanism for the spall opening angle of the typical v-shaped or arrowhead crack configurations. A new three-dimensional crack geometry was presented allowing the study of the spalling surface morphology in a gear application. Stress intensity factors along the crack front were computed using the eXtended Finite Element Method (XFEM) implemented in Abaqus (6.12). Both low crack inclination angles and increased friction resulted in larger spall opening angles. For cracks with small inclination angles the effects of increased friction on the spall opening angle appeared however very little. The findings increase understanding of the surface morphology and the damage process and further motivate the asperity point load mechanism as an important source for surface initiated RCF damage.

  • 36.
    Hannes, Dave
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Rolling contact fatigue crack growth prediction by the asperity point load mechanism2012In: Advances In Fracture And Damage Mechanics X / [ed] Tonkovic, Z; Aliabadi, MH, Trans Tech Publications Inc., 2012, Vol. 488-489, p. 101-104Conference paper (Refereed)
    Abstract [en]

    The crack path and growth life 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. 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. 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. Different equivalent stress intensity factor ranges were used to estimate the fatigue life, which agreed with the life of the investigated gear wheels.

  • 37.
    Hannes, Dave
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Rolling contact fatigue crack path prediction by the asperity point load mechanismReport (Other academic)
  • 38.
    Hannes, Dave
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Rolling contact fatigue crack path prediction by the asperity point load mechanism2011In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 78, no 17, p. 2848-2869Article in journal (Refereed)
    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.

  • 39.
    Hannes, Dave
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Surface initiated rolling contact fatigue based on the asperity point load mechanism-A parameter study2012In: Wear, ISSN 0043-1648, E-ISSN 1873-2577, Vol. 294, p. 457-468Article in journal (Refereed)
    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.

  • 40.
    Hazar, Selcuk
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Lai, J.
    Mechanical modeling of coupled plasticity and phase transformation effects in a martensitic high strength bearing steel2018In: Mechanics of materials (Print), ISSN 0167-6636, E-ISSN 1872-7743, Vol. 117, p. 41-57Article in journal (Refereed)
    Abstract [en]

    The stress and strain induced solid to solid phase transformation of retained austenite in a martensitic high strength bearing steel has been studied. Monotonic tension experiments that were carried out at different temperatures using this high strength steel showed that not only the strain induced but also the stress induced phase change plays a crucial role in the phase transformation of retained austenite to martensite. In the material model, plastic deformation was defined using the Drucker Prager yield surface through a nonassociated flow rule accompanied by nonlinear kinematic and isotropic hardening. The hardening was coupled with stress and strain induced phase transformations. A nonlinear elastic effect based on elastic dilation was included in the constitutive model by extending the bulk modulus with a second order term. For the finite element analysis, the material model was written as a user defined material subroutine (UMAT). The numerical simulations were done using ABAQUS and compared to monotonic tension, compression and cyclic experiments. The results showed that the strength differential effect and the volumetric change under loading are closely related to the transformation of retained austenite to martensite. At low temperatures the effect of stress induced phase transformation on yield strength was noticeable. It was concluded that at certain temperatures both strain and stress induced phase transformations significantly affect mechanical behavior of the high strength steel.

  • 41.
    Linares Arregui, Irene
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Alfredsson, B.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Elastic-plastic characterization of a high strength bainitic roller bearing steel-experiments and modelling2010In: International Journal of Mechanical Sciences, ISSN 0020-7403, E-ISSN 1879-2162, Vol. 52, no 10, p. 1254-1268Article in journal (Refereed)
    Abstract [en]

    Monotonic and cyclic deformations were studied for a high strength bainitic roller bearing steel. The temperature of 75 °C corresponded to normal roller bearing conditions. The materials showed hydrostatic influence on yielding, but no or marginal influence of plastic deformation on density change. Therefore, a linear elastic constitutive model with pressure dependent yielding, non-associated flow rule, combined non-linear kinematic and isotropic hardening was necessary to characterize the cyclic behaviour. A stepwise process is detailed for determining the material parameters of the pressure dependent model, where particular attention was placed on the hardening parameters. One set of parameters was sufficient to describe all tested load ranges including compressive ratchetting. Some comparative tests were performed at room temperature, 150 °C and on martensitic specimens at 75 °C. The temperature influence was limited to the isotropic hardening parameters.

  • 42.
    Linares Arregui, Irene
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Non-linear elastic characterisation of a high strength bainitic roller bearing steel2013In: International Journal of Mechanical Sciences, ISSN 0020-7403, E-ISSN 1879-2162, Vol. 68, p. 1-15Article in journal (Refereed)
    Abstract [en]

    A small but not negligible non-linear elastic behaviour was detected when investigating cyclic uniaxial push-pull experiments on a high strength bainitic steel. Cyclic torsion experiments led to the conclusion that the shear modulus was relatively constant. A non-linear elastic model was implemented where the bulk modulus was extended with a second order term related to the elastic dilatation and where the shear modulus was constant. The material presented a strength differential effect (SDE), with larger yield stress in compression than in tension. Consequently, the non-linear elastic model was combined with a plasticity model that incorporated a Drucker-Prager yield surface, non-associated flow rule and combined non-linear hardening. Expressions that include non-linear elasticity were derived for the elastic-plastic hardening and the compliance tensors. The extended material model predicted the elastic-plastic results from cyclic push-pull experiments. Also, a phenomenological analysis of the cyclic elastic response showed isotropic damage in the elastic moduli. The steady-state damage increased linearly with the cyclic plastic strain range.

  • 43.
    Linares Arregui, Irene
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Non-linear elastic characterization of a high strength bainitic rollerbearing steel2010Report (Other academic)
  • 44.
    Linares Arregui, Irene
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Numerical analysis on plasticity induced crack closure of a physically short fatigue crack in a high strength roller bearing steel2015Report (Other academic)
  • 45.
    Linares Arregui, Irene
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Lai, Junbiao
    Low temperature creep in a high strength roller bearing steel2015Report (Other academic)
  • 46.
    Ljustell, Pär
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Variable amplitude fatigue crack growth at monotonic large scale yielding experiments on stainless steel 316L2013In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 109, p. 310-325Article in journal (Refereed)
    Abstract [en]

    Fatigue crack growth in stainless steel 316L was investigated for two different variable amplitude block loads in the monotonic large scale yielding load range. The crack tip opening displacement, δ, was controlled in situ based on numerical computations on the coupling between the applied load, the crack length and δ. The average growth rates over the block sequences were determined from potential drop and visually measured crack lengths. The experimental growth rates were also predicted, based on small scale yielding fatigue crack growth data from closure free experiments, using a straight forward average procedure. The possibility to use small scale yielding material data was explained the by the large isotropic material hardening and closure free crack growth.

  • 47. Nilsson, F.
    et al.
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Engineering aspects on fatigue crack growth under complex loading2005In: 11th International Conference on Fracture 2005: ICF11, 2005, p. 2763-2767Conference paper (Refereed)
    Abstract [en]

    Fatigue crack growth experiments were performed for different types of complex loading involving variable amplitude loading, mixed mode loading and contact loading. The growth processes were analysed by combinations of engineering and numerical methods.

  • 48.
    Nordin, Erland
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). Scania CV AB, Sweden.
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Experimental Investigation of Shot Peening on Case Hardened SS2506 Gear Steel2017In: Experimental techniques (Westport, Conn.), ISSN 0732-8818, E-ISSN 1747-1567, Vol. 41, no 4, p. 433-451Article in journal (Refereed)
    Abstract [en]

    Shot peening is a manufacturing process commonly used to increase fatigue life in components for the automotive and aircraft industry. In this paper the effect of shot peening is described for a case hardened gear steel. For gears there are three main factors from shot peening that influence fatigue life: residual stresses, microstructure and surface roughness. The paper describes an experimental series where these parameters were measured for common industrial shot peening settings. The aim was to show how several different measurement techniques and results correspond to each other for certain shot peening parameters. The aim was also to gather experimental results that can be used for verification of shot peening simulations. To simplify measurements and decrease variation, flat steel plates were used as targets. Residual stress, full width at half maximum (FWHM), retained austenite, surface roughness/topology, hardness and Barkhausen noise were measured and related to microstructural changes. The mean indentation diameter was measured for individual shots at low coverage of each intensity which was used to determine the average velocity of the media. The mean diameter and hardness of the shot peening media was also determined.

  • 49.
    Nordin, Erland
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). Scania CV AB, Sweden.
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Measuring shot peening media velocity by indent size comparison2016In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 235, p. 143-148Article in journal (Refereed)
    Abstract [en]

    Shot peening is a manufacturing method that makes indentations on a components surface by impacting it with small steel balls (media). In a production environment the shot peening process is usually controlled by defining media size, coverage and Almen intensity. For simulating shot peening, the media velocity is needed and therefore it must be measured or correlated to the Almen intensity. This paper details a method to record indentations on a test plate and then compare them to single shot indentations with measured velocities. For small media sizes the results are in reasonable agreement with other published results but at larger sizes there is a larger spread in published results. It is therefore recommended that the media velocity is either measured directly or that the indents are analysed as presented in this paper.

  • 50.
    Sedlak, Michal
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Alfredsson, Bo
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Efsing, Pål
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.). Ringhals AB, SE-43285 Varobacka, Sweden..
    A cohesive element with degradation controlled shape of the traction separation curve for simulating stress corrosion and irradiation cracking2018In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 193, p. 172-196Article in journal (Refereed)
    Abstract [en]

    A cohesive element with extended environmental degradation capability was developed and implemented into an Abaqus user element. The element uses a virgin and a fully degraded Traction Separation Law (TLS) as input. The use of the potential based PPR model enables flexibility in the softening shapes for both TSL. When the element is degraded, the TSL gradually goes from the shape of the virgin material to the fully degraded TSL shape. This transition was made with a new parameter. that can govern a more ductile or brittle crack growth behaviour at degradation. The effect on the plastic zone due to changing the softening shape is shown, where the convex shaped softening TSL gives higher plastic dissipation and larger plastic zones than the concave and more brittle TSL. The new degradation method was evaluated against a Hydrogen Embrittlement (HE) experiment showing improved agreement with the experiment compared to the literature. The effect of different susceptibility zones at the crack tip was also investigated, showing that a uniform degradation throughout the susceptible zone is more influenced by the. parameter than a triangular susceptible zone.

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