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  • 1. Alam, M M
    et al.
    Barsoum, Zuheir
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Jonsen, P
    Kaplan, A F H
    Haggblad, H A
    Influence of defects on fatigue crack propagation in laser hybrid welded eccentric fillet joint2011In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 78, no 10, p. 2246-2258Article in journal (Refereed)
    Abstract [en]

    Fatigue cracking of laser hybrid welded eccentric fillet joints has been studied for stainless steel. Two-dimensional linear elastic fracture mechanics analysis was carried out for this joint geometry for four point bending load. The numerical simulations explain for the experimental observations why the crack propagates from the lower weld toe and why the crack gradually bends towards the root. Lack of fusion turned out to be uncritical for the initiation of cracks due to its compressive stress conditions. The linear elastic fracture mechanics analysis has demonstrated in good qualitative agreement with fatigue test results that lack of fusion slightly (<10%) reduces the fatigue life by accelerating the crack propagation. For the geometrical conditions studied here improved understanding of the crack propagation was obtained and in turn illustrated. The elaborated design curves turned out to be above the standard recommendations.

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    KTH, Superseded Departments, Solid Mechanics.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    KTH, Superseded Departments, Solid Mechanics.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

  • 3.
    Birgisson, Björn
    et al.
    Department of Civil and Coastal Engineering, University of Florida, United States.
    Montepara, A.
    Romeo, E.
    Roncella, R.
    Napier, J.
    Tebaldi, G.
    Determination and prediction of crack patterns in hot mix asphalt (HMA) mixtures2008In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 75, no 3-4, p. 664-673Article in journal (Refereed)
    Abstract [en]

    This paper presents a comparison between predicted and measured crack patterns developing in hot mix asphalt (HMA) mixtures during common fracture tests. A digital image correlation (DIC) System was applied to obtain displacement/strain fields and for detecting crack patterns. The resulting cracking behavior was predicted using a displacement discontinuity boundary element method to explicitly model the microstructure of HMA. The predicted fracture initiation and crack propagation patterns are consistent with observed cracking behavior. The results imply that fracture in mixtures can be modeled effectively using a micro-mechanical approach and that crack propagation patterns can be captured using the DIC System.

  • 4.
    Boåsen, Magnus
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Stee, Mateusz
    Swedish Def Res Agcy FOI, SE-16490 Stockholm, Sweden..
    Efsing, Pål
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Faleskog, Jonas
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    A generalized probabilistic model for cleavage fracture with a length scale - Influence of stress state and application to surface cracked experiments2019In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 214, p. 590-608Article in journal (Refereed)
    Abstract [en]

    A probabilistic model for the cumulative probability of failure by cleavage fracture with a material related length scale is further developed in this study. A new generalized effective stress measure is proposed, based on a normal stress decomposition of the stress tensor, capable of describing a state of normal stress in the range from the mean stress to the maximum principal stress. The effective stress measure associated with a material point is evaluated from the stress tensor averaged over the material related length scale. The model is shown to be well capable to predict both the fracture toughness at loss of both in-plane and out-of-plane constraint by model application to two different datasets from the open literature. The model is also shown to be well capable of predicting the probability of failure of discriminating experiments on specimens containing semi-elliptic surface cracks. A comparison where the master curve methodology is used to predict the probability of failure of the experiments is also included.

  • 5.
    Bremberg, Daniel
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Dhondt, Guido
    Automatic crack-insertion for arbitrary crack growth2008In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 75, no 3-4, p. 404-416Article in journal (Refereed)
    Abstract [en]

    A crack-insertion tool for automatic non-planar crack propagation calculations is under development. The final volume mesh of the cracked structure is achieved by a new approach. The cracked structure boundary is modelled as a discretized skin. A tube-like domain enclosing the crack-front is traced and filled with hexahedral elements while the remaining cracked structure is meshed with tetrahedral elements. The two separate meshes are finally connected by linear MPC equations. Results show that the method works very well for curved crack shapes in complex structures.

  • 6.
    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.

  • 7.
    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.

  • 8.
    Dahlin, Peter
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Olsson, Mårten
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Mode I fatigue crack growth reduction mechanisms after a single mode II load cycle2006In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 73, no 13, p. 1833-1848Article in journal (Refereed)
    Abstract [en]

    A single Mode 11 load cycle, large enough to create residual displacements, decreases the subsequent Mode I crack growth rate. The distance for Mode I crack growth rate to fully recover, i.e., revert to the same da/dN as before Mode 11 load, is much longer than Mode 11 plastic zone size. The higher Mode 11 load, the larger is the reduction in growth rate and the longer the recovery distance. Higher Mode I R-ratio means smaller reduction in growth rate. Above a certain R-ratio, no reduction occurs at all. In the present study it is found that the reduction in growth rate is solely caused by crack closure due to tangential displacement of crack-surface irregularities that induce a surface mismatch between the upper and lower crack faces. The mechanism is called Mode II-induced crack closure. A model based on both analytical and experimental results is developed in order to estimate the degree of Mode II-induced crack closure after a Mode 11 load.

  • 9.
    Elmukashfi, Elsiddig
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Kroon, Martin
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Numerical analysis of dynamic crack propagation in biaxially strained rubber sheets2014In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 124, p. 1-17Article in journal (Refereed)
    Abstract [en]

    This paper proposes a computational framework for dynamic crack propagation in rubber in which a nonlinear finite element analysis using cohesive zone modeling approach is used. A suddenly initiated crack at the center of biaxially stretched sheet problem is studied under plane stress conditions. A transient dynamic analysis using implicit time integration scheme is performed. In the constitutive modeling, the continuum is characterized by finite-viscoelasticity theory and coupled with the fracture processes using a cohesive zone model. This computational framework was introduced previously by the present authors (Elmukashfi and Kroon, 2012). In the current work, the use of a rate-dependent cohesive model is examined in addition to investigation of generalized biaxial loading cases. A Kelvin-Voigt element is used to describe the rate-dependent cohesive model wherein the spring is described by a bilinear law and dashpot with a constant viscosity is adopted. An explicit integration is used to incorporate the rate-dependent cohesive model in the finite element environment. A parametric study over the cohesive viscosity is performed and the steady crack propagation velocity is evaluated and compared with experimental data. It appears that the viscosity varies with the crack speed. Further, the total work of fracture is estimated using rate-independent cohesive law such that the strength of the cohesive zone is assumed to be constant and the separation work per unit area is determined form the experimental data. The results show that fracture-related processes, i.e. creation of new surfaces, cavitation and crystallization; contribute to the total work of fracture in a contradictory manner.

  • 10.
    Faleskog, Jonas
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Kroon, Martin
    KTH, Superseded Departments, Solid Mechanics.
    Öberg, Hans
    KTH, Superseded Departments, Solid Mechanics.
    A probabilistic model for cleavage fracture with a length scale - parameter estimation and predictions of stationary crack experiments2004In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 71, no 1, p. 57-79Article in journal (Refereed)
    Abstract [en]

    This study presents a large experimental investigation in the transition temperature region on a modified A508 steel. Tests were carried out on single-edge-notch-bend specimens with three different crack depth over specimen width ratios to capture the strong constraint effect on fracture toughness. Three test temperatures were considered, covering a range of 85 degreesC. All specimens failed by cleavage fracture prior to ductile tearing. A recently proposed probabilistic model for the cumulative failure by cleavage was applied to the comprehensive sets of experimental data. This modified weakest link model incorporates a length scale, which together with a threshold stress reduce the scatter in predicted toughness distributions as well as introduces a fracture toughness threshold value. Model parameters were estimated by a robust procedure, which is crucial in applications of probabilistic models to real structures. The conformity between predicted and experimental toughness distributions, respectively, were notable at all the test temperatures.

  • 11.
    Grenestedt, Joakim L
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Kuttenkeuler, Jakob
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    On cracks emanating from wedges in expanded PVC foam1996In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 54, no 4, p. 445-456Article in journal (Refereed)
    Abstract [en]

    An experimental and analytical study was made on the effect of stress singularities on the strength of expanded PVC foam materials of different densities. Experiments were performed on specimens with different wedge geometries ranging from sharp cracks, with the ordinary inverted square root stress singularity, to shallow re-entrant corners with weak singularities. A brittle fracture criterion based on a generalised stress intensity factor, called Q, at the wedge tip was fit to experimental data. The critical stress intensity factor, Qcr, for crack initiation depends on the wedge geometry. This dependence was estimated from simple point-stress criteria and a criterion due to Seweryn [Brittle fracture criterion for structures with sharp notches. Engng Fracture Mech. 47, 673-681 (1994)], and good agreement with experimental data was obtained. When the point-stress criterion was applied to Mode II sharp cracks, poor agreement with published data was found. A critical study of the Mode II crack specimen was therefore initiated, leading to the conclusion that the commonly used specimen gives erroneous values of KIIc and the reason seems to be due to crack surface friction. A new Mode II crack specimen which eliminates crack surface friction was proposed and tested, and good agreement with the point-stress criterion was obtained. A criterion for homogeneous materials proved to be adequate also for the porous PVC foams.

  • 12.
    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.

  • 13.
    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.

  • 14.
    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.

  • 15.
    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, Junbiao
    SKF Engn & Res Ctr, POB 2350, NL-3430 DT Nieuwegein, Netherlands..
    Martensite transformation in the fatigue fracture surface of a high strength bearing steel2019In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 220, article id UNSP 106650Article in journal (Refereed)
    Abstract [en]

    Phase transformation of retained austenite (RA) to martensite was studied considering both stress and strain induced transformations during fatigue crack propagation. X-ray diffraction measurements, performed on the fatigue crack surface obtained through push-pull experiments, showed that almost all RA in the fatigue surface transformed to martensite. A material model that can simulate the phase change including nonlinear isotropic and kinematic hardening behaviors and nonlinear elasticity was used to simulate the fatigue crack growth. Numerical results showed that only a very small amount of RA remained in the crack surface, which agreed with the X-ray diffraction measurements. The effect of phase transformation on crack closure was studied and it was observed in the FE simulations that the crack faces close at an earlier stage when phase change has been taken into account than when it is absent.

  • 16.
    Hoffelner, W.
    et al.
    Brown Boveri Research Centre.
    Gudmundson, Peter
    Brown Boveri Research Centre.
    A fracture mechanics analysis of ultrasonic fatigue1982In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 16, no 3, p. 365-371Article in journal (Refereed)
    Abstract [en]

    The method of ultrasonic fatigue finds increasing interest in materials science. Especially, fatigue crack growth rates near the threshold stress intensity range, Δ”K0, can be determined with this method in reasonable times providing no frequency and corrosion effects exist. But for an accurate application of this technique it is necessary to improve the testing systems and also the determination of the dynamic cyclic stress intensity range, Δ”K. In this paper, fatigue crack growth experiments at ultrasonic frequencies with different mean stresses and also the calculation of the dynamic stress intensity range with finite elements are treated. On this basis fatigue crack growth curves at room temperature of the alloys Hastelloy X and IN 800 were measured and compared with results obtained at low frequencies. No significant influence of frequency could be found in these materials.

  • 17.
    Kroon, Martin
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    An asymptotic analysis of dynamic crack growth in rubber2011In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 78, no 17, p. 3111-3122Article in journal (Refereed)
    Abstract [en]

    Asymptotic analyses of the mechanical fields in front of stationary and propagating cracks are important for several reasons. For example, they facilitate the understanding of the mechanical and physical state in front of crack tips, and they enable prediction of crack growth. Furthermore, efficient modelling of arbitrary crack growth by use of XFEM (extended finite element method) requires accurate knowledge of the asymptotic crack tip fields. The present study focuses on the asymptotic fields in front of a crack that propagates dynamically in rubber. Static analyses of this type of problem have been made in previous studies. In order to be able to compare the present results with these earlier studies, the constitutive model from Knowles and Sternberg (J. Elast. 3: 67-107, 1973) was adopted. It is assumed that viscoelastic stresses become negligible compared with the singular elastic stresses close to the crack tip. The present analysis shows that in materials with a significant hardening, the inertia term in the equations of motion becomes negligible in the asymptotic analysis. However, for a neoHookean type of model, inertia comes into play and causes a maximum theoretical crack speed that equals the shear wave speed.

  • 18.
    Kroon, Martin
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Biomechanics.
    Faleskog, Jonas
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Öberg, Hans
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    A probabilistic model for cleavage fracture with a length scale - Parameter estimation and predictions of growing crack experiments2008In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 75, no 8, p. 2398-2417Article in journal (Refereed)
    Abstract [en]

    A probabilistic model for the cumulative probability of failure by cleavage fracture was applied to experimental results where cleavage fracture was preceded by ductile crack growth. The model, introduced by Kroon and Faleskog [Kroon M, Faleskog J. A probabilistic model for cleavage fracture with a length scale - influence of material parameters and constraint. Int J Fract 2002; 118:99-118], includes a non-local stress with an associated material related length scale, and it also includes a strain measure to account for the number of nucleated cleavage initiation sites. The experiments were performed on single edge cracked bend test specimens with three different crack lengths at the temperature 85 degrees C, which is in the upper transition region for the steel in question. The ductile rupture process is modelled using tile cell model for nonlinear fracture mechanics. The original cleavage fracture model had to be modified in order to account for the substantial number of cleavage initiators being consumed by the ductile process. With this modification, the model was able to accurately capture the experimental failure probability distribution.

  • 19.
    Ljustell, Pär
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Fatigue crack growth experiments on specimens subjected to monotonic large scale yielding2013In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 110, p. 138-165Article in journal (Refereed)
    Abstract [en]

    Fatigue crack growth behavior was investigated for through thickness long cracks in two different geometries at monotonic large scale yielding in a stainless steel 316L. Finite element computations on the coupling between the applied load, the crack length and the crack tip opening displacement, delta, were conducted. These coupling equations were used to control Delta delta in situ in the experiments based on the potential drop measured crack length. The Delta delta was able to characterize and correlate the fatigue crack growth load state for the present geometries and loads. Also the stress-intensity factor range, Delta K-I, could predict the growth rates due to large isotropic hardening at cyclic conditions and absence of crack closure.

  • 20.
    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.

  • 21.
    Ljustell, Pär
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Nilsson, Fred L.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Variable amplitude crack growth in notched specimens2005In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 72, no 18, p. 2703-2720Article in journal (Refereed)
    Abstract [en]

    Predictions of variable amplitude fatigue crack growth in notched tensile specimens at an elevated temperature and relative high loads were conducted. The predictions were based on constant amplitude fatigue data and mean crack growth rates were calculated. Crack closure levels were calculated by comparison of crack closure free experimental data obtained with the so called constant K1(max)-method and nominal data. Also, numerical simulations of the closure level throughout the specimens were performed. Although experimental data showed rather large a scatter it was concluded that variable amplitude crack growth rates can be predicted with reasonable agreement using constant amplitude fatigue data. Also, crack growth rates cannot be predicted by LEFM in the inelastic notch affected zone, even if absence of crack closure is assumed.

  • 22. MackAldener, M.
    et al.
    Olsson, Mårten
    KTH, Superseded Departments, Solid Mechanics.
    Analysis of crack propagation during tooth interior fatigue fracture2002In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 69, no 18, p. 2147-2162Article in journal (Refereed)
    Abstract [en]

    Tooth interior fatigue fracture is a failure mode that is initiated as a fatigue crack in the interior of the tooth of a gear. TIFF cracks have been observed in case hardened idler gears. A fracture mechanical analysis of a TIFF crack is performed utilising FEA. A 3D TIFF crack is modelled at a position in the tooth that corresponds with an observed crack surface. The different material properties in the case and the core, determined by mechanical testing, are considered, as well as the residual state of stress due to case hardening. Various crack lengths are analysed to estimate crack propagation both into the core and into the case. The following results have been found: A TIFF crack initiated slightly under the case layer will propagate into the case layer where it stops. . The main crack propagation will take place in the core. . The crack propagation is only a small portion of the total life (order of 10(5) cycles). . After reaching the case layer the TIFF crack eventually deflects toward the tooth root and the upper part of the tooth falls off. The crack deflection is due to redistribution of contact loading. Several gear teeth pairs are simultaneously in contact and the cracked tooth is loaded less than the uncracked during this stage of life.

  • 23. Makela, Petri
    et al.
    Östlund, Sören
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    Cohesive crack modelling of thin sheet material exhibiting anisotropy, plasticity and large-scale damage evolution2012In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 79, p. 50-60Article in journal (Refereed)
    Abstract [en]

    The crack tip region in notched structures generally exhibit damage evolution before ultimate failure occurs. In some materials, the damaged regions may reach considerable sizes prior to structural collapse. In this work, a cohesive crack model suitable for static fracture mechanics analysis of thin sheet materials exhibiting anisotropy, plasticity, and large-scale damage evolution was developed. The material parameters of the model were calibrated solely by tensile testing of unnotched test specimens. The predictive capability of the model was verified by comparisons with experiments on notched test specimens with different crack sizes. The predictions of failure were shown to be in excellent agreement with the experiments.

  • 24.
    Månsson, Tomas
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Öberg, Hans
    KTH, Superseded Departments, Solid Mechanics.
    Nilsson, Fred L.
    KTH, Superseded Departments, Solid Mechanics.
    Closure effects on fatigue crack growth rates at constant and variable amplitude loading2004In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 71, no 10-sep, p. 1273-1288Article in journal (Refereed)
    Abstract [en]

    Accurate measurements of fatigue propagation rates after single and during repetitive overloads were performed. It was shown that plastic deformations in the crack tip region and in the crack wake cause crack closure that is of major importance for crack growth rates. The consequences of this for methods to determine certain material properties are discussed. An improved method to measure closure free propagation rate data was utilised. The possibilities and shortcomings of compliance methods to determine crack closure levels are pointed out. The relevance of using DeltaK(I,eff) = K-I,K-max - K-I,K-lim, where K-I,K-lim possibly could be interpreted as the crack closure level was investigated and is discussed. A method to predict average crack propagation rates at block loadings consisting of well-defined load cycles of different amplitudes and R-factors was suggested. Three-dimensional effects after overloads were observed. These observations confirmed that the crack closure behaviour also depends on geometry of the specimen.

  • 25.
    Nilsson, F
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    The 15th European Conference of Fe racture2006In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 73, no 2, p. 159-159Article in journal (Other academic)
  • 26.
    Nilsson, Fred L.
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Hansson, T.
    Mansson, T.
    Growth of surface cracks under constant and variable amplitude loading2004In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 71, no 12, p. 1725-1735Article in journal (Refereed)
    Abstract [en]

    Fatigue crack growth experiments were performed on surface cracked tensile specimens of Inconel 718 at 400 degreesC. The loading was carried out at constant as well as at variable amplitude. The experimental results for the mean growth rate were compared with predictions based on data obtained from testing of compact tension specimens. Both nominal data as well as data corrected from measured crack closure were used in the predictions. The corrected data provided much better predictions than the nominal ones indicating that the level of crack closure during the testing of the surface cracked specimens was much lower than in compact tension specimens.

  • 27.
    Onifade, Ibrahim
    et al.
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Building Materials.
    Birgisson, Björn
    Balieu, Romain
    KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
    Energy-Based Damage and Fracture Framework for Viscoelastic Asphalt Concrete2015In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 145, p. 67-85Article in journal (Refereed)
    Abstract [en]

    A framework based on the continuum damage mechanics and thermodynamics of irreversible processes using internal state variables is used to characterize the distributed damage in viscoelastic asphalt materials in the form of micro-crack initiation and accumulation. At low temperatures and high deformation rates, micro-cracking is considered as the source of nonlinearity and thus the cause of deviation from linear viscoelastic response. Using a non-associated damage evolution law, the proposed model shows the ability to describe the temperature-dependent processes of micro-crack initiation, evolution and macro-crack formation with good comparison to the material response in the Superpave indirect tensile (IDT) strength test.

  • 28.
    Pilhagen, Johan
    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.
    Loss of constraint during fracture toughness testing of duplex stainless steels2013In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 99, p. 239-250Article in journal (Refereed)
    Abstract [en]

    Delamination of the fracture surfaces, so called splits, is an important phenomenon that occurs at sub-zero temperature for hot-rolled duplex stainless steels during impact and fracture toughness testing. To evaluate how the splits influence the fracture toughness, sub-zero temperature fracture toughness testing of 50, 30 and 10 mm thick plates of hot rolled 2205 duplex stainless steel was performed. The results show that the splits cause loss of constraint along the crack front. This can be observed as local difference in crack growth in the specimen. The initiation fracture toughness is not influenced by the specimen thickness. Furthermore, due to the delamination the material exhibits a stable fracture process despite the presence of cleavage fracture. This is interfering with the master curve method so for evaluating the fracture toughness at sub-zero temperatures an assessment of the fracture resistance curve is instead suggested. For assessing the brittle crack behaviour at sub-zero temperatures it is proposed to use the split initiation as a "failure" criteria. The splits are also the cause of the pop-in behaviour observed for the duplex stainless steels. The susceptibility for pop-in is influenced by the microstructure.

  • 29.
    Ribeiro-Ayeh, Steven
    et al.
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Hallström, Stefan
    KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
    Strength prediction of beams with bi-material butt-joints2003In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 70, no 12, p. 1491-1507Article in journal (Refereed)
    Abstract [en]

    Failure of bi-material interfaces is studied with the aim to quantify the influence of the induced stress concentrations on the strength of the interfaces. The suggested approach is applied to a specimen configuration where two different materials are butt-joined to form a two-material beam. Strength predictions for different interface bias are made and verified in experiments where a polymer foam material is joined with members of either aluminium or Plexiglas.The predictions are made using a simple point-stress criterion in combination with highly accurate finite element calculations. The point-stress criterion was known from earlier work to give accurate predictions of failure at cracks and notches but had to be slightly modified to become applicable for the studied configurations. Both the strength and the cross-over from local, joint-induced failure to global failure were predicted with reasonable accuracy.

  • 30.
    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.

  • 31.
    Sedlak, Michal
    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.).
    Efsing, Pål
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.), Solid Mechanics (Div.).
    A coupled diffusion and cohesive zone model for intergranular stress corrosion cracking in 316L stainless steel exposed to cold work in primary water conditions2019In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 217, article id UNSP 106543Article in journal (Refereed)
    Abstract [en]

    A multi-physics model was developed to simulate intergranular stress corrosion cracking (IGSCC) in austenitic stainless steel. The model is implicit, coupled with a segregated solution scheme including a diffusion equation based on Fick's second law and a cohesive zone description for the fracture mechanics part. The degradation is modelled with an anodic slip-dissolution equation that uses the effective cohesive traction and concentration as the main parameters. The diffusivity in Fick's second law creates a moving boundary. The cohesive zone is modelled using the PPR model with extended degradation properties using the degradation parameter chi. The model was evaluated against experiments on the effects of cold work on IGSCC. The model showed good agreements for both shifting amount of cold work, illustrated by only changing the yield stress in the bulk material and for shifting the stress intensity factor. The model versatility was also shown by simulating IGSCC in Alloy 600, also with good agreements. The change in the bulk material made crack propagation more disadvantageous for the lower yield stress where the crack blunts, creates more plastic strain and lowers the cohesive traction. The model predicts that cold work of the bulk material creates a faster crack growth velocity due to lower amount of plastic deformation in the bulk and higher cohesive traction. The higher crack growth rate is a coupled effect of both fracture and oxidation properties.

  • 32.
    Sieurin, Henrik
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Sandström, Rolf
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Fracture toughness of a welded duplex stainless steel2005In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 73, p. 377-390Article in journal (Refereed)
    Abstract [en]

    The present work includes fracture toughness testing on 30 and 50 mm thick duplex stainless steel 2205 (22% Cr, 5.5% Ni, 3% Mo, 0.15% N). Base metal and submerged arc weldments (SAW) at subzero temperatures have been tested using full size three point bending. The evaluation of the results has been carried out using J-integral calculations and correlations to Charpy impact values. It is shown that the temperature dependence of the fracture toughness can be described by a transition temperature curve, the master curve. The reference temperature was evaluated to -143 and -101 degrees C for base and weld material respectively.

  • 33.
    Stec, Mateusz
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Faleskog, Jonas
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    A micromechanics based probabilistic model for cleavage fracture: Application to SE(B) experiments2008In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315Article in journal (Other academic)
  • 34.
    Stec, Mateusz
    et al.
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Faleskog, Jonas
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Öberg, Hans
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Validation of two probabilistic cleavage fracture models for half elliptic surface crack2008In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315Article in journal (Other academic)
  • 35. Wu, L.
    et al.
    Tjahjanto, Denny
    KTH, School of Engineering Sciences (SCI), Solid Mechanics (Dept.).
    Becker, G.
    Makradi, A.
    Jérusalem, A.
    Noels, L.
    A micro-meso-model of intra-laminar fracture in fiber-reinforced composites based on a discontinuous Galerkin/cohesive zone method2013In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 104, p. 162-183Article in journal (Refereed)
    Abstract [en]

    The recently developed hybrid discontinuous Galerkin/extrinsic cohesive law framework is extended to the study of intra-laminar fracture of composite materials. Toward this end, micro-volumes of different sizes are studied. The method captures the debonding process, which is herein proposed to be assimilated to a damaging process, before the strain softening onset, and the density of dissipated energy resulting from the damage (debonding) remains the same for the different studied cell sizes. Finally, during the strain softening phase a micro-crack initiates and propagates in agreement with experimental observations. We thus extract a resulting mesoscale cohesive law, which is independent on the cell sizes, using literature methods.

  • 36.
    Zang, Weilin
    et al.
    KTH, Superseded Departments, Solid Mechanics.
    Gudmundson, Peter
    KTH, Superseded Departments, Solid Mechanics.
    An Integral-Equation Method For Piece-Wise Smooth Cracks In An Elastic Half-Plane.1989In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 32, no 6, p. 889-897Article in journal (Refereed)
    Abstract [en]

    A boundary integral equation method for piece-wise smooth cracks in an elastic half-plane is presented. Both internal and surface cracks have been considered. The numerical formulation is based on an element description of the unknown dislocation densities along the crack line. Crack line kinks are treated by an introduction of double nodes at each kink. Numerical test examples are presented and compared, either to existing solutions, or to own finite element calculations. Excellent agreements were obtained for all cases.

1 - 36 of 36
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