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  • 1.
    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.

  • 2.
    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    KTH, Superseded Departments, Solid Mechanics.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    KTH, Superseded Departments, Solid Mechanics.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.

  • 3.
    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.

  • 4.
    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.

  • 5.
    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

  • 6.
    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.

  • 7.
    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.

  • 8. Aygül, M.
    et al.
    Al-Emrani, M.
    Barsoum, Zuheir
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Leander, John
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Investigation of distortion-induced fatigue cracked welded details using 3D crack propagation analysis2014In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 64, p. 54-66Article in journal (Refereed)
    Abstract [en]

    The behaviour of distortion-induced fatigue cracks in welded details in an existing bridge was studied analytically by performing crack propagation analysis based on linear elastic fracture mechanics. The real load history of the bridge was obtained from strain measurements. These loads were utilised to examine the crack growth rate and the residual service life of the cracked detail. Moreover, the effectiveness, accuracy and applicability of the crack propagation analysis on bridge structures were investigated by simulating a complex case of fatigue cracking using several crack propagation analyses. The results of the analyses revealed that the fatigue crack in the studied details had significantly different crack growth characteristics in different directions. In the thickness direction, for instance, the crack was seen to propagate at a certain rate, which increased with the propagated crack from the beginning and, as expected, the crack propagation rate decreased when the crack grew longer. The crack was subsequently arrested half way through the thickness of the plate. In the longitudinal direction, the crack was not, however, arrested in the same way as in the thickness direction and it continued to propagate at a reduced yet constant crack growth rate. The results also revealed that, even though distortion-induced fatigue cracking was usually caused by a mixed-mode condition (i.e. a combination of modes I, II and III), the governing propagation mode is still mode I. Furthermore, it was also observed that the contribution of modes II and III to crack propagation was very little and dependent on the location of the propagated crack front, as well as the geometrical configuration of the cross-beam.

  • 9. Baumgartner, J.
    et al.
    Yıldırım, H. C.
    Barsoum, Zuheir
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Fatigue strength assessment of TIG-dressed welded steel joints by local approaches2019In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 126, p. 72-78Article in journal (Refereed)
    Abstract [en]

    Fatigue strength assessment methods by local approaches are widely used in the literature. This paper provides a comprehensive evaluation of published data for welded steel joints improved by TIG dressing methods. Fatigue classes for the local assessment methods with the available fatigue data are recommended. The available fatigue data extracted for transverse non-load carrying welds, cruciform joints as well as butt joints. In total, 17 published test series of weld details with various yield strengths and stress ratios are presented. Fatigue strength assessment is performed by considering the weld profile geometry within Finite Element models and taking the resulting stress gradients as basis for the evaluation. In addition, the influence of the steel grade is included. The most reliable results are derived by using the critical distance approach. Fatigue classes and critical distances are recommended as a result of the evaluations.

  • 10.
    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.

  • 11.
    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.

  • 12.
    Dahlin, Peter
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Fatigue crack growth: mode I cycles with periodic mode II loading2008In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 30, no 5, p. 931-941Article in journal (Refereed)
    Abstract [en]

    The influence of periodic Mode II loading on Mode I fatigue crack growth is studied using a new type of loading device. The average rate of crack growth is found to be governed mainly by four parameters; Delta K-I and the Mode I R-ratio, the magnitude of the Mode II load and the Mode II period, M (M = number of Mode I load cycles per Mode II load). The Mode 11 load has to be large enough to create residual tangential displacements of the crack faces otherwise no effect occurs at all. Two mechanisms are found, Mode II-induced crack closure that reduces the crack propagation rate (long range) and a mechanism that increases the growth rate temporarily at every Mode I I load (short-range). For a specific Mode I load and Mode I I magnitude, it is possible to find a minimum crack growth rate for a certain Mode II period, M-opt. At high R-ratios the Mode II-induced closure mechanism disappears, but the temporary increase in growth rate at every Mode II load is still active. A semi-analytical model for the average crack growth rate during sequential Mode I and Mode II loading is presented.

  • 13.
    Dahlin, Peter
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Olsson, Mårten
    KTH, Superseded Departments, Solid Mechanics.
    Reduction of mode I fatigue crack growth rate due to occasional mode II loading2004In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 26, no 10, p. 1083-1093Article in journal (Refereed)
    Abstract [en]

    The effect of mode 11 loading on subsequent mode I crack growth is experimentally investigated on steel AISI 01. The results show that the mode I crack growth rate decreases after a single mode 11 load cycle. This effect remains also after the crack has propagated through the mode 11 plastic zone. The crack growth reduction is shown to be caused by crack closure due to tangential displacement of crack-surface irregularities. The durability of this reduction has a decisive influence on the fatigue life when the mode I R-ratio is not as high as to keep the entire load cycle above the closure level.

  • 14.
    Ericsson, Mats
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Jin, Lai-Zhe
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Fatigue of friction stir welded T-joints2005In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452Article in journal (Other academic)
  • 15.
    Ericsson, Mats
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Jin, Lai-Zhe
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Fatigue properties of friction stir overlap welds2007In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 29, no 1, p. 57-68Article in journal (Refereed)
    Abstract [en]

    Friction stir welding (FSW) is currently used for many applications involving lap or T-joints, e.g. hermetically closed boxes such as cooling elements and heat exchangers. The frequent pressure changes in these make them susceptible to fatigue. The fatigue characterization of lap joints involves a combination of shear and bending. Forces applied to the ends of lap joints result in non-axial stresses in the connection area. FSW lap joints of Al-Mg-Si alloy 6082 in the artificially aged condition T6 were studied. A pin (probe) based on the Triflute (TM) concept was used with two modifications to the pin, the pin end being either convex or concave. Tool shoulders of 15 and 18 turn respectively were utilized, producing four different weld series. Fracture was initiated in the highly stressed area where the weld cuts through the interface between the two sheets. The cracks typically propagated through the weld in the upper sheet (tool side). The broadest tool shoulder with a concave end of pin design gave the best fatigue performance. This was due to an improved flow path provided by the hollowed out end of the pin; allowing material flow around the pin which resulted in minimal hooking of the sheet interface adjacent to the weld nugget. Additionally heat energy was supplied by the increased contact area. The stress intensity factor Delta K was determined. It was found that a simplified approach, developed to estimate Delta K for overlap spot welds, could be applied to friction stir overlap joints. The corresponding crack propagation rates were in fair accordance with the experimental results.

  • 16.
    Ericsson, Mats
    et al.
    KTH, Superseded Departments, Materials Science and Engineering.
    Sandström, Rolf
    KTH, Superseded Departments, Materials Science and Engineering.
    Influence of welding speed on the fatigue of friction stir welds, and comparison with MIG and TIG2003In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 25, no 12, p. 1379-1387Article in journal (Refereed)
    Abstract [en]

    The objective of this investigation was to determine whether the fatigue strength of friction stir (FS) welds is influenced by the welding speed, and also to compare the fatigue results with results for conventional arc-welding methods: MIG-pulse and TIG. The Al-Mg-Si alloy 6082 was FS welded in the T6 and T4 temper conditions, and MIG-pulse and TIG welded in T6. The T4-welded material was subjected to a post-weld ageing treatment. According to the results, welding speed in the tested range, representing low and high commercial welding speed, has no major influence on the mechanical and fatigue properties of the FS welds. At a significantly lower welding speed, however, the fatigue performance was improved possibly due to the increased amount of heat supplied to the weld per unit length. The MIG-pulse and TIG welds showed lower static and dynamic strength than the FS welds. This is in accordance with previous comparative examinations in the literature on the fatigue strength of fusion (MIG) and FS welds. The TIG welds had better fatigue performance than the MIGpulse welds. The softening, of the alloy around the weldline has been modelled. Using a model without adjustable parameters, a fair description of the hardness profiles across the weld as a function of welding speed was obtained. The softening in front of the Friction Stir Welding tool was also estimated. At the low and high welding speeds a full and partial softening is predicted, respectively.

  • 17.
    Gamstedt, E. Kristofer
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Sjogren, B. A.
    An experimental investigation of the sequence effect in block amplitude loading of cross-ply composite laminates2002In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 24, no 04-feb, p. 437-446Article in journal (Refereed)
    Abstract [en]

    The Palmgren-Miner rule has been shown to be inexact in many cases for various composite materials. Several empirical models have been conceived to account for this discrepancy, as well as the effect of block sequence. The approach taken here is based on the underlying mechanisms. A cross-ply laminate was used as a model material. In general, composites show both initiatory and progressive mechanisms under fatigue loading. The former is active at high static stresses, whereas the latter predominates at lower stress amplitudes where they are given sufficient time to propagate, Initiatory mechanisms give rise to damage from which the progressive mechanisms can start, and conversely the progressive mechanisms continually alter the local stress state which results in further damage accumulation caused by the initiation controlled mechanisms. In a cross-ply laminate, the initiatory mechanism is the formation of transverse cracks, and the progressive mechanism is mainly delamination growth initiated from the transverse cracks. In an experimental investigation of carbon fiber/epoxy cross-ply laminates, the interaction of these mechanisms has shown why a sequence of high-low amplitude levels results in shorter lifetimes than a low-high order. Such a sequence effect seems to be a common behavior for many other composite materials, and can be mechanistically explained by a similar kind of interaction. Advantages and drawbacks of the mechanistic approach compared with empirical rules are also discussed.

  • 18.
    Karlén, Kristoffer
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    A probabilistic model for the entire HCF domain based on equivalent stress - Simulations and experiments2012In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 36, no 1, p. 9-17Article in journal (Refereed)
    Abstract [en]

    A probabilistic model for fatigue life is presented. It allows for inhomogeneous stress fields. The stressing of the whole body is summarized in an equivalent stress. The model is calibrated with experiments and can be formulated for a variety of different types of equivalent stresses (point, gradient, area and volume). Finite life experiments at constant load levels as well as tests around the infinite life fatigue limit are used. Using this new model, the PES-model (Probabilistic and based on Equivalent Stress), it is shown that the choice of equivalent stress has a small influence on the fit for finite fatigue life, i.e. the constant load level results. The fit for all tested equivalent stresses are similar over the entire failure probability domain. For the fatigue limit part, however, there is a larger difference between the choices of equivalent stress. It is found that a gradient adjusted point stress is most suited for the PES-model, and that it can be used in the entire SN-domain.

  • 19.
    Karlén, Kristoffer
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    A study of the volume effect and scatter at the fatigue limit: experiments and computations for a new specimen with separated notches2011In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 33, no 3, p. 363-371Article in journal (Refereed)
    Abstract [en]

    In this paper a new fatigue specimen is presented that has been developed in order to conduct detailed investigations of the volume effect and the scatter of fatigue data in the fatigue limit regime The specimen has two separated notches A and B with different size of the loaded volume associated with them respectively By changing the location of the applied load the fatigue failure in the experiments can be controlled to occur in either notch A or B Comparisons were made with simulations based on statistical weakest link (WL) theory In the weakest link statistics the three parameter Weibull distribution is used for estimation of the fatigue failure probability It is shown that the predictive capability of WL-models is poor for high and low failure probabilities when the two separated notches A and B are considered If only one notch (either A or B) is considered and the other one is disregarded the predictive capability of WL is drastically improved Experiments with almost equal failure WL-probability in A and 8 (28% and 27%) did not result in equal experimental failure probability WL-statistics can not predict the experimental outcome when separated notches are present.

  • 20.
    Karlén, Kristoffer
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    An investigation of a fatigue model with two competing failure mechanisms2014In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 64, p. 131-139Article in journal (Refereed)
    Abstract [en]

    A new combined fatigue model that considers a global and a local fatigue mechanism is presented. The global mechanism is assumed to describe fatigue failure from internal defects. It is modeled using weakest link theory. The local mechanism describes fatigue failure occurring in a very clean material (i.e. almost defect-free). This mechanism is described by the normal distribution, where the stress is the largest occurring stress value, the point stress, or the point stress adjusted with the stress gradient, denoted the gradient adjusted point stress. Experiments have been performed on notched specimens with different notch root radii. At high failure probabilities, the local model dominates in the combined model. At low failure probabilities, results are less clear. There, the local model is not accurate and the global model dominates. Using the gradient adjusted point stress as the local model and the volumetric weakest link integral as the global model gives the best overall fit to the probability of failure. It is noted that for design with respect to high failure probabilities (> 20%), it is enough to use only the gradient adjusted local model.

  • 21.
    Karlén, Kristoffer
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    An investigation of the location of fatigue initiation - Deterministic and probabilistic aspects2014In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 66, p. 65-77Article in journal (Refereed)
    Abstract [en]

    Fatigue limit tests where there are two competing fatigue failure locations, two notches of different size, are performed in this paper. The normal distribution is used for describing the experimental outcome, where the largest occurring stress value or the gradient adjusted stress is used as a stress measure. Another model for describing the experimental outcome is the Weakest Link (WL)-integral where the integration is performed over the specimen surface area or the specimen volume. In addition, the positions of the fatigue failure initiation sites are measured and investigated. The predictive capabilities of the different models are evaluated with respect to determination of the longitudinal failure location. A notable result is that the longitudinal location of fatigue failure differs considerably from one experiment to the other, even when they are numerically equal. Nominally the stress at the fatigue failure site is much lower than the maximal stress. The load cases where there should be an equal amount of failures in both notches according to the models used do not agree with the experimental findings. Neither the point stress model nor the gradient adjusted point stress, nor the WL-integral can explain the experimental outcome from the competing notch tests.

  • 22.
    Karlén, Kristoffer
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Ahmadi, Hamidreza
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Härkegård, Gunnar
    On the effect of random defects on the fatigue notch factor at different stress ratios2012In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 41, p. 179-187Article in journal (Refereed)
    Abstract [en]

    The effect of randomly distributed defects of different sizes on the fatigue performance of a notched member has been studied. Both the effect of the volume density of defects and the size distribution of the defects have been studied. The fatigue life and the fatigue limit distributions of a smooth or notched specimen have been estimated. Using this probabilistic approach, the fatigue notch factor as function of failure probability can be estimated. It is shown that the approach with randomly distributed defects has better predictive capabilities than classical methods. The weakest link approach can be seen as an alternative to simulation of the randomly distributed defects. The two methods have similar predictive capabilities. The stress based methods generally do not take the load ratio into account when estimating the fatigue notch factor. It is also shown that the fatigue limit estimated with defect analysis is lower than the experimental one.

  • 23.
    Khurshid, Mansoor
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Barsoum, Zuheir
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Daeuwel, Thomas
    Barsoum, Imad
    The Petroleum Institute, United Arab Emirates.
    Root fatigue strength assessment of fillet welded tube-to-plate joints subjected to multi-axial stress state using stress based local methods2017In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 101, p. 209-223Article in journal (Refereed)
    Abstract [en]

    In this study the fatigue strength of fillet welded tube-to-plate joints failing at the weld root and subjected to multi-axial stress states is investigated. The fatigue test data is collected from the literature and it is assessed together with the experimental data generated in this study. Finite element analysis is used to analyze the stress state at the weld root. The fatigue strength estimation capabilities of local stress based methods such as the Principal Stress Hypothesis (PSH), von Mises Stress Hypothesis (vMH), Modified Wohler Curve Method (MWCM), and Effective Equivalent Stress Hypothesis (EESH) are compared. The applicability of modified Gough Pollard Equation (GPE) in local stress system is also assessed. It is observed that most of the proposed local stress assessment methods can estimate the fatigue strength of fillet welds subjected to multiaxial stress states with constant principal stress direction, e.g. proportional loading. In case of load histories which produce varying principal stress directions with respect to time, e.g. non-proportional loading, better estimation capability is shown by MWCM and EESH. In most of the cases of varying principal stress direction load histories, vMH and PSH fail to estimate the fatigue strength. The fatigue strength of specimens tested with combined loading is reduced in comparison to the fatigue strength of specimens tested with only internal pressure and only bending loading. Out-of-phase loading does not affect the fatigue strength significantly for the specimens in this study. However; a decrease in fatigue strength is observed for the test data for out-of-phase loading collected from the literature.

  • 24.
    Leander, John
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Al-Emrani, Mohammad
    Chalmers University of Technology.
    Reliability-based fatigue assessment of steel bridges using LEFM: A sensitivity analysis2016In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 93, no 1, p. 82-91Article in journal (Refereed)
    Abstract [en]

    The lack of an established safety format prohibits a widespread use of linear elastic fracture mechanics (LEFM) for the fatigue assessment of steel bridges. The aim of this study is to facilitate a future development of a deterministic design approach. A probabilistic sensitivity analysis has been performed to study the influence of different modeling options on the resulting time variant reliability. The analyses have been performed by the first order reliability method (FORM) together with a model correction factor. The result shows the importance of modeling the crack shape in an adequate manor and, the importance of the material parameters. Other parameters as the load sequence and the option between a linear and a bi-linear crack growth law are less important. A calculation of the omission sensitivity factors shows that the uncertainties of the material parameters in the crack growth law have the most decisive influence and, thereafter, the uncertainty of the stress intensity factor.

  • 25.
    Leander, John
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Aygül, Mustafa
    Chalmers University of Technology.
    Norlin, Bert
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Refined fatigue assessment of joints with welded in-plane attachments by LEFM2013In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 56, p. 25-32Article in journal (Refereed)
    Abstract [en]

    The fatigue life of joints with welded in-plane gusset plates is studied. The purpose is to suggest a descriptive detail category to be used in assessment of existing structures. Linear elastic fracture mechanics is used together with fictitious initial crack depths to predict the fatigue life. By calibrating the fictitious crack depths to real fatigue tests and simulating new test results, fatigue strengths dependent on the dimensions of the joint have been derived. The results show that a decreasing length and thickness of the gusset give decreasing stress intensity and higher fatigue strength. A numerical example is presented showing the merit of the proposed strengths in fatigue life.

  • 26. Linder, Jan
    et al.
    Axelsson, Markus
    Nilsson, Henrik
    The influence of porosity on the fatigue life for sand and permanent mould cast aluminium2006In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 28, no 12, p. 1752-1758Article in journal (Refereed)
    Abstract [en]

    The automotive industry always strives to achieve light weight components to reduce fuel consumption and to meet environmental requirements. One way to obtain weight reduction is to replace steel components with components made of aluminium or other light weight materials. Aluminium has good corrosion properties and a high strength to weight ratio which makes it favourable in many applications. The increased use of aluminium castings in the automotive industry does also imply that the need for design data for aluminium, increases. Especially for castings, the influence of casting defects are always an issue. For this reason fatigue properties for as-cast sand and permanent mould specimens with different contents of porosity have been studied. Sand cast and permanent mould cast aluminium specimens of two different geometries were fatigue tested in cyclic bending at R= -1. Prior to fatigue test specimens were examined by X-ray and sorted into three quality groups depending on the porosity level. The aim of this work was to investigate the fatigue life for sand cast and permanent mould cast AlSi10Mg with different amounts of porosity. An additional aim was to predict the largest defect contained in a specified volume of a component, by using a statistical analysis of extreme values, and relate it to the fatigue life. The results showed that fatigue strength for a smooth specimen geometry decreases by up to 15% with increased porosity. For specimens with a notched geometry, no influence of porosity on the fatigue strength was found. This is believed to be due to a much smaller volume subject to high stress than for specimens with low stress concentration.

  • 27. Linder, Jan
    et al.
    Melander, Åke
    Fatigue strength of spot welded stainless sheet steels exposed to 3% NaCl solution1998In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 20, no 5, p. 383-388Article in journal (Refereed)
    Abstract [en]

    Fatigue properties of spot welded stainless sheets steels have been investigated in a 3% NaCl solution and, for comparison, also in air at ambient temperature. Corrosion fatigue tests have been conducted both for one austenitic Stainless steel type AISI304, and for one duplex (similar to 50% austenite, 50% ferrite) stainless steel SAF2304. For the duplex steel, the effect of preexposure of unloaded specimens to a 3%NaCl solution before corrosion fatigue testing was also investigated. Corrosion fatigue strength in 3% NaCl for the stainless steels investigated was found to be reduced by approximately 30-40% compared to fatigue strength in air. Preexposure of unloaded specimens prior to fatigue testing was shown not to reduce fatigue strength further.

  • 28. MackAldener, M.
    et al.
    Olsson, Mårten
    KTH, Superseded Departments, Solid Mechanics.
    Tooth Interior Fatigue Fracture - computational and material aspects2001In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 23, no 4, p. 329-340Article in journal (Refereed)
    Abstract [en]

    The hypothesis of the gear failure mode Tooth Interior Fatigue Fracture (TIFF), i.e. a fatigue crack is initiated in the interior of the tooth, is analysed by numerical simulations using FEM and the critical plane fatigue initiation criterion according to Findley. The residual stress profile in the gear is simulated and calibrated versus a neutron diffraction residual stress measurement. Fatigue properties are determined experimentally and are taken to vary with depth in the numerical analysis. In addition, the effect of shot peening is included. The analysis shows that a TIFF-crack is initiated approximately mid-height of the tooth and slightly below the case-core boundary. Furthermore, the analysis shows that TIFF is more pronounced in idlers than in single stage gears.

  • 29.
    Mahdavi Shahri, Meysam
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Osikowicz, Wojciech
    Sapa Technology.
    Critical distance method to estimate the fatigue life time of friction stirwelded profiles2012In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 37, p. 60-68Article in journal (Refereed)
    Abstract [en]

    The fatigue failure of friction stir (FS) welded aluminium panels of alloy 6005A has been analysed. Thesepanels are produced with two main joint geometries: namely half overlap and hourglass. Presence ofcrack like notches (interface between the welded work pieces) and blunt notches (corners and bendsin the base metal) have been studied with finite element method (FEM) stress analysis combined withthe critical distance method. It was shown that the failure location and fatigue limit could be predictedfor three of the four types of profiles considered. Choice of the welding procedure (clamping condition),however, can significantly influence the fatigue life and prediction accuracy. When different welding procedurewas used the weld failure was not reproduced by model and failure location was not predictedcorrectly. In this case fracture mechanics approach along with residual stress analysis has been used toanalyse the failure. It is also shown that local plastic deformation on the crack tip is induced by clampingresulting in tensile residual stresses at the crack tip. Taking stress intensity factor of the residual stressinto account, the position of the failure can be explained

  • 30.
    Mandal, Nirmal Kumar
    et al.
    Cent Queensland Univ, Ctr Railway Engn, Rockhampton, Qld 4702, Australia..
    Spiryagin, Maksym
    Cent Queensland Univ, Ctr Railway Engn, Rockhampton, Qld 4702, Australia..
    Berg, Mats
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Stichel, Sebastian
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    On the railhead material damage of insulated rail joints: Is it by ratchetting or alternating plasticity?2019In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 128, article id 105197Article in journal (Refereed)
    Abstract [en]

    Railhead becomes severely stressed due to localised contact loadings. This is more critical in the vicinity of endposts of an insulated rail joint. Popular IRJ endpost materials are fibreglass (fb), polytetrafluoroethylene (ptfe) and nylon66 (ny). A 3D finite element analysis is carried out to study stress distributions in railhead material near the endposts. The progressive damage parameter, von Mises stress and other residual stress and strain plots indicate that plastic deformation and material degradation occur at sub-surface levels of the railhead material. Although fb and ptfe are better endpost materials than ny, ny displays superior performance over the other two.

  • 31. Mansson, T.
    et al.
    Skantz, J.
    Nilsson, Fred L.
    KTH, Superseded Departments, Solid Mechanics.
    High temperature fatigue crack growth in two metals under constant and variable amplitude loading2002In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 24, no 11, p. 1159-1168Article in journal (Refereed)
    Abstract [en]

    Results are presented for fatigue crack growth at elevated temperatures during both constant amplitude and variable amplitude loading. A careful experimental investigation is presented to show that the concept of the effective stress-intensity factor range DeltaK(I)(eff) can be used to eliminate the load ratio effect on fatigue data and produce one single set of crack growth data. The fatigue crack propagation data corrected for crack closure is then used for comparison of mean crack propagation rates for variable amplitude loads with simple time histories. It is concluded that measured crack closure can not fully explain the discrepancies between measured and predicted propagation rates. A reduction factor fitted to the experimental data could be used to successfully correlate the results.

  • 32.
    Norberg, Sven
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    A fast, versatile fatigue post-processor and criteria evaluation2005In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 27, no 10-12, p. 1335-1341Article in journal (Refereed)
    Abstract [en]

    This paper is concerned with the development and use of a general High Cycle Fatigue (HCF) post-processor, FAST, Fatigue AnalysiS Tool. It is used to evaluate and develop HCF criteria. FAST can handle many kinds of criteria and evaluate them with fatigue test results from both standard test specimens and actual components. When a reliable criterion exists, FAST will be used for design purposes. The principles of FAST are outlined and an example is given where seven local multiaxial stress based criteria are evaluated, along with the commercial fatigue post-processor Femfat. The conclusion is that a correct treatment of the volume or stress gradient effect is more important than the choice of criterion.

  • 33.
    Norberg, Sven
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    The effect of loaded volume and stress gradient on the fatigue limit2007In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 29, no 12, p. 2259-2272Article in journal (Refereed)
    Abstract [en]

     In this paper an investigation of multiaxial stress based criteria and evaluation methods is presented. The criteria are used with the point, gradient and volume methods. The purpose is to determine the combination of criteria and methods that is best suited for design against the fatigue limit. The evaluation is based on elastic FE-analysis of 15 geometries for which the fatigue limit loads are known. The point method is based on the maximum values of the fatigue stress in each specimen. With the gradient method, the fatigue stress is adjusted with the relative or absolute gradient of the fatigue stress itself. With the volume method, a statistical size effect is considered, by use of a weakest link integral. Thus, the probability of fatigue depends on the fatigue stress distribution. Also, the gradient and volume methods are combined. The results show that the point and gradient methods are not good for prediction of the fatigue limit. It is recommended to use the volume method in fatigue design. It is accurate enough for prediction of the fatigue limit, straightforward to use and easy to interpret. The choice of method is much more important than the choice of criteria.

  • 34.
    Sadek, Salar
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    A Probabilistic Method for Multiaxial HCF based on highly loaded regions below the threshold depth2016In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 87, p. 91-101Article in journal (Refereed)
    Abstract [en]

    For High Cycle Fatigue (HCF) assessment of structural components, a variety of evaluation methods that take important factors into account are available. The application of probabilistic evaluation methods is not always straightforward. Investigations show that the methods have limited predictive capability and most often poor transferability of fatigue test results for more complex geometries. Here, a new probabilistic HCF model based on the concept of highly loaded region is proposed. A depth parameter is taking on an important role in the proposed model. Different local stress criteria in combination with the new model and two other evaluation methods are studied. The purpose is to determine the combination of criteria and evaluation method that gives the best transferability. The results show that the new model has superior predictive capability than other models regardless the choice of fatigue stress criteria.

  • 35.
    Sadek, Salar
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    New models for prediction of high cycle fatigue failure based on highly loaded regions2014In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 66, p. 101-110Article in journal (Refereed)
    Abstract [en]

    The stress gradient and the structural size are known to be very important factors affecting the fatigue life of metallic mechanical components. For multiaxial stress conditions, a relevant criterion must be used to account for both these effects as well as the history of a non-homogeneous stress state. The new probabilistic HCF models in this work fulfill these requirements. A stress field resulting from rotating bending is analyzed. Also, the well-known weakest link model has be compared to the new proposed HCF models based on the concept of the highly loaded regions. The new models take the highly loaded regions (volume, area) and the stress gradient effect into account. These models show significant improvement of failure probability prediction capability compared to the weakest link integral.

  • 36.
    Sadek, Salar
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    The probability of HCF - Surface and sub-surface models2016In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 92, p. 147-158Article in journal (Refereed)
    Abstract [en]

    In this paper, new probabilistic HCF evaluation models based on the surface and the sub-surface region is presented. The sub-surface model stem from the hypothesis that the highest fatigue risk depends on the loading in a region slightly below the free surface area. There, the materials microstructural barriers must be overcome by fatigue crack propagation. The size of the region with high load below the surface affects the fatigue risk. The predictive capability of the new proposed HCF model is studied along with comparisons to other state of the art HCF probabilistic models. Fatigue data originating from different geometries and materials conducted at different loading modes, rotating bending and uniaxial loading is used for the analysis. The investigation performed in this work indicate that the new proposed models increase the prediction accuracy and hence predictive capability.

  • 37.
    Sandberg, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Mansour, Rami
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Fatigue probability assessment including aleatory and epistemic uncertainty with application to gas turbine compressor blades2017In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 95, p. 132-142Article in journal (Refereed)
    Abstract [en]

    In, this work, a new method for fatigue probability assessment is introduced. The method is applied to a bladed disk in a gas turbine for computation of the high cycle fatigue probability. Both epistemic and aleatory uncertainties are modeled. The aleatory uncertainty is of two types: Variable aleatory uncertainty is modeled by use of stochastic variables that influence the problem, including both design variables and stochastic parameters. Physical aleatory uncertainty is modeled by use of a probability that remains even if all stochastic variables are replaced by deterministic values. The fatigue behavior of a material exhibits physical aleatory uncertainty. The results show that the epistemic uncertainty in the modeling of the aero-forcing gives the major contribution to uncertainty in the computed failure probability. The new method is also used to study the influence on the probability of high cycle fatigue that comes from the stochastic variables.

  • 38.
    Sandberg, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    An investigation of the prediction accuracy for volume based HCF models using scaled geometries and scaled loading2015In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 82, no 2, p. 317-324Article in journal (Refereed)
    Abstract [en]

    he prediction accuracy for the volume based Weibull- and V* high cycle fatigue models is investigated and compared to the point method. A high number of fatigue tests are performed in rotating bending for single notched cylindrical specimens manufactured in a 12% Cr-steel, a high quality structural steel. The specimens are designed with different highly stressed volume and stress gradient by scaling the geometry, but with the same stresses at the corresponding scaled points. Thus, the maximum stress in the notch is the same. Experiments are performed for three specimen sizes at several stress levels. The volume based Weibull- and V*-models, as well as the point stress method are fitted to the experimental results. Based on the results, the V*-model is favored for design purposes.

  • 39.
    Sandberg, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    On the optimal choice of experiments for determination of parameters in multiaxial HCF-criteria2014In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 61, p. 315-324Article in journal (Refereed)
    Abstract [en]

    Material parameters in multiaxial HCF-criteria must be estimated from experiments, which always are affected by scatter from several sources. A higher number of tests provides a more reliable parameter estimation, but the cost for the testing increases. In this paper, by starting from a known fatigue limit distribution for the titanium alloy Ti-6-4, test series are simulated according to different predefined test strategies. In Monte Carlo simulations, scatter in the material properties and in specimen alignment in the testing machine is taken into account. Based on the results obtained from the simulations, it is determined which one of the defined strategies that is most beneficial for HCF-criteria material parameter estimation, and also how many tests that should be performed in order to maximize the benefit of the information obtained from the fatigue testing.

  • 40.
    Shahri, Meysam Mahdavi
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Sandstrom, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
    Fatigue analysis of friction stir welded aluminium profile using critical distance2010In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 32, no 2, p. 302-309Article in journal (Refereed)
    Abstract [en]

    A friction stir (FS) welded extruded aluminium alloy 6005A, which is used for train wall sides, was fatigue tested. The friction stir weld was a lap-butt joint with a sharp notch (interface between the work pieces) next to the weld nugget. Fatigue cracks and failure appeared at notches in the profile. In most profiles, cracks also initiated at the sharp notch at the weld, but the propagation was slow and complete fracture never took place there. Finite element method (FEM) stress analysis combined with the theory of critical distance was used to estimate the fatigue limit. Results from the analysis for the fatigue limit were within 3-28% of the observations. The stress analysis correctly predicted that failure would not occur in the welds.

  • 41.
    Svärd, Henrik
    KTH, School of Engineering Sciences (SCI), Mathematics (Dept.), Optimization and Systems Theory.
    A branch and bound algorithm for evaluation of the Findley fatigue criterion2015In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 73, p. 27-38Article in journal (Refereed)
    Abstract [en]

    In this manuscript, a new algorithm for evaluation of the Findley fatigue criterion is proposed. The algorithm uses a branch and bound technique to limit the number of investigated planes in the search for the critical one. The algorithm has two major advantages over currently existing methods. Firstly, for a given tolerance on the error of the evaluation, it needs to investigate fewer planes on average, thereby reducing the execution time compared to state of the art methods. Secondly, the algorithm is guaranteed to give results within a tolerance of the global maximum, and this tolerance may be freely chosen by the analyst.

  • 42.
    Wentzel, Henrik
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Huang, Xiyue
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Experimental characterization of the bending fatigue strength of threaded fasteners2015In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 72, p. 102-108Article in journal (Refereed)
    Abstract [en]

    The fatigue strength in bending of pre-stressed steel bolts is investigated and compared to the fatigue strength in axial tension. The strength is measured in terms of maximum engineering stress amplitude, neglecting any stress concentration in the threads. The experimental results reveal that the fatigue limit is 76% higher in bending than in axial tension. A finite element model is used to compute the stress state in the threaded region for both axial tension and bending. It allows fitting a volume based weakest link model to the experimentally observed failure probabilities. Based on the good fit of the weakest link model it is argued that randomly distributed defects in the highly stressed thread root determine the fatigue strength.

  • 43. Yildirim, Halid Can
    et al.
    Marquis, Gary B.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures. Aalto University, Finland.
    Barsoum, Zuheir
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Fatigue assessment of high frequency mechanical impact (HFMI)-improved fillet welds by local approaches2013In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 52, p. 57-67Article in journal (Refereed)
    Abstract [en]

    Local fatigue assessment methods like the structural hot spot stress and effective notch stress methods as defined by the International Institute of Welding are widely used by design engineers and researchers to assess the fatigue strength of welded components. This paper provides a comprehensive evaluation of published data for welded joints which had been improved using high frequency mechanical impact (HFMI) treatment. All of the published data for HFMI-treated welds are presented in terms of nominal stress. The goal of the current paper is to establish local fatigue assessment procedures for improved fillet welds. In total, 160 published experimental results for longitudinal and cruciform welds subjected to R = 0.1 axial loading are evaluated. Local stress quantities for each joint were assessed based on the finite element analyses and reported nominal stress values. A correction procedure for yield strength that was previously verified for nominal stress-based fatigue assessment is also applied to the local stress methods studied in this paper. For both the structural hot spot stress and effective notch stress methods, sets of characteristic fatigue strength curves as functions of yield strength are proposed and verified. The structural hot spot stress method includes one set of fatigue strength curves for load-carrying welds and a second set for non-load carrying welds. The effective notch stress method includes a single set of curves for all welds. All of the design curves proposed in this study are conservative with respect to available fatigue test data.

  • 44.
    Zenkert, Dan
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Burman, Magnus
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering.
    Failure mode shifts during constant amplitude fatigue loading of GFRP/foam core sandwich beams2011In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 33, no 2, p. 217-222Article in journal (Refereed)
    Abstract [en]

    This paper presents fatigue results for sandwich beams that exhibit a transition in failure mode, from core shear failure to face laminate tensile failure, as function of load amplitude only. The basis of this are fatigue tests of foam cores in shear and tensile tests on composite laminates. These results show that the slopes of the stress-life (S-N) relation are different for the core and laminates. By using the obtained stress-life relations, a simple design scheme is given for sandwich beams which are anticipated to have a transition of failure mode for a particular load level. Two designs are manufactured and tested in fatigue under constant amplitude loading. The results clearly show the aim of investigation with transitions in failure modes giving a structural stress-life diagram a bi-linear shape. For high load and small number of cycles to failure, the beams fail by core shear fracture while for lower loads, and large number of cycles to failure the beams fail by face sheet tensile failure.

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