Papers by Keyword: Crack Initiation

Abstract: The very high cycle fatigue behaviour of high-strength steels is mostly affected by cracks which were initiated at non-metallic inclusions inside the fatigue specimens and hence under the surface. By separating the data points in the S-N-diagram by the chemical composition of the inclusion at which a crack is initiated the authors recognized that the lifetime depends strongly on the inclusion type. The authors further deduce that the chemical composition of an inclusion as well as its geometry influences the mechanism by which such an inclusion triggers crack initiation. Thus, titanium nitride and homogeneous calcium oxide inclusions have been observed to show fractures caused by the stress concentration in the inclusion. The broken inclusions then present sharp notches in the steel matrix and cause further damage from there. In contrast, aluminium calcium oxide inclusions decay or detach from the steel matrix during loading on account of a rather low interface stress. This detachment process results in holes in the steel matrix, which act as logical starting points of fatigue cracks. Both processes described above occur at different stress intensity factors and lead to failure before an ultimate number of cycles of 10^9. Furthermore, it was possible to determine threshold values of failure for each inclusion type by stressing run out specimens on a higher stress level. These threshold values are in accordance with those of specimens which failed during one-level stressing. The fatigue tests were performed with 100Cr6 in martensitic and bainitic condition. Tension/compression tests at a load ratio of R = -1 were conducted on an ultrasonic fatigue testing facility. Failures were only initiated at non-metallic inclusions. The fracture surfaces were analyzed by scanning electron microscopy and by energy dispersive X-ray spectroscopy.

Abstract: In this paper the TMF crack initiation behaviour of the single-crystal nickel-base superalloyMD2 is investigated and modelled. TMF tests were performed in both IP and OP for varying mechanicalstrain ranges in the [001] crystallographic direction until TMF crack initiation was obtained. Acrystal plasticity-creep model was used in conjunction with a critical-plane approach, to evaluate thenumber of cycles to TMF crack initiation. The critical-plane model was evaluated and calibrated ata stable TMF cycle, where the effect of the stress relaxation had attenuated. This calibrated criticalplanemodel is able to describe the TMF crack initiation, taking tension/compression asymmetry aswell as stress relaxation anisotropy into account, with good correlation to the real fatigue behaviour.

Abstract: Cyclic plastic straining in crystalline materials is localized to persistent slip bands (PSBs) and results in formation of persistent slip markings (PSMs) consisting of extrusions and intrusions. Intensive plastic strain in PSBs results in dislocation interactions and formation of point defects. The extended model based on point defect formation, migration and annihilation is presented describing surface relief formation in the form of extrusion-intrusion pairs. Point defect migration and resulting mass transfer is the principle source of cyclic slip irreversibility leading to crack-like defects - intrusions. Fatigue cracks start in the tip of sharp intrusions.

Abstract: The aim of this study is to discuss an effect of stress ratio and loading mode on high cycle fatigue performances of extruded magnesium alloys. Axial loading fatigue tests under three conditions of stress ratio, R, of 0, -1 and-1.5, and also rotating bending fatigue tests have been performed in laboratory air at room temperature using hourglass shaped specimens of AZ31, AZ61, AZ80 and T5-treated AZ80 alloy. From the experimental results, some materials showed a specific stepwise S-N curve on which two knees appear. The shape of S-N diagram depended on a kind of tested materials, applied stress ratio and loading mode. It was suggested from the detail observation of fracture surface that fatigue crack initiation mechanism changed from a twin-induced failure mode at high stress amplitude level to a slip-induced one at low stress amplitude level. This transition was determined with the relation between the minimum stress during a fatigue cycle and the compressive yield stress at which deformation twin occurs.

Abstract: Fatigue lifetime assessment is essential in the design of structures. Under-estimated predictions may result in unnecessary in-service inspections. Conversely, over-estimated predictions may have serious consequences on the integrity of structures. In some nuclear power plant components, the fatigue loading may be equibiaxial because of thermal fatigue. So the potential impact of multiaxial loading on the fatigue life of components is a major concern. Meanwhile, few experimental data are available on austenitic stainless steels. It is essential to improve the fatigue assessment methodologies to take into account the potential equibiaxial fatigue damage. Hence this requires obtaining experimental data on the considered material and with a strain tensor in equibiaxial tension. This paper describes an experimental program on austenitic stainless steel carried out on the new experimental fatigue device FABIME2 developed in the LISN in collaboration with EDF and AREVA. This new device allows accurate quantification of the effects of both equibiaxial strain state as well as structural parameters (such as mean stress) on the fatigue life. It also allows studying the complexity of combinations between potential detrimental effects like surface roughness, mean stress and equibiaxial loading. Different load ratios can be tested by adjusting the loading conditions. A Finite Element Modeling is performed in order to obtain a precise description of the strain state in the specimen. The results of the on-going test campaign will be presented.

Abstract: A three-dimensional crystal plasticity (CP) finite element model is developed to reproduce the grain level stress concentration and deformation of polycrystalline aluminium alloy 7075 (AA7075) during fatigue experiments. The grains contained in the model possess the same size and crystallographic orientations obtained from electron back-scatter diffraction experiments. A modified CP constitutive model, which considers the backstress evolution, is employed to describe the mechanical behaviour of AA7075 under cyclic loading. A round-notched specimen from a fatigue test is simulated using the proposed CP model. Convergence studies in terms of mesh density and plastic deformation zone size are carried out to determine the appropriate conditions for the simulation. The simulation results are compared with those obtained using the elasto-plastic model and the CP model without grain morphology. The comparison indicates that with the embedded grain morphology, the proposed model can capture very well the local response induced by the microstructure features, which is vital to the accurate fatigue life prediction of aluminium alloys.

Abstract: Orientation changes during fatigue crack initiation in ferrite and ferritepearlite steel were evaluated by electron backscatter diffraction (EBSD). Ferrite steel with different grain sizes and ferritepearlite steel with different carbon contents were prepared. EBSD measurements and fatigue tests were alternately performed using a small specimen. The tests on both ferrite and ferritepearlite steel suggest that the initial cracks were observed in the ferrite matrix. Thus, crystal rotation induced by fatigue in ferrite matrix is quantitatively evaluated by two misorientation parameters: grain reference orientation deviation, which is the misorientation between measuring points and the average orientation in each grain, and crystal misorientation at the same point before and after fatigue testing.

Abstract: Advanced High Strength (AHS) steels have been increasingly applied in the automotive industries due to their distinguished mechanical properties. Microstructures of these steels play an important role and are designed by constituent phases with distinct characteristics. AHS steels exhibit sophisticated damage mechanisms that complicate the prediction of material formability. In this work, Ductile Crack Initiation Locus (DCIL) was developed for describing failure behavior of dual phase steel sheet. A hybrid experimental and numerical analysis was used to determine the DCIL. Tensile tests of various sample geometries were experimentally carried out and crack initiation occurred during forming was identified by the Direct Current Potential Drop (DCPD) method. Then, FE simulations of the corresponding tests were performed to evaluate local stress triaxialities and equivalent plastic strains of the critical area. The damage curves for both crack initiation and localized necking were obtained. Additionally, the von Mises, Hill48 and Yld2000-2d yield criterion were defined in the calculations in order to examine effect of yield model on the resulted curves. To verify applicability of the damage curves, Nakazima test of uniaxial sample was taken into account.

Abstract: The work is focused on the study of degradation of ZrO₂ stabilized by Y₂O₃ (YSZ) thermal barrier-coating system with CoNiCrAlY bond coat applied on cast polycrystalline nickel-based superalloy Inconel 713LC. Cylindrical specimens in as-coated conditions were cyclically strained under strain control with constant total strain amplitude in symmetrical cycle at high temperature (900 °C) in air. Coating system YSZ with CoNiCrAlY bond coat were prepared by APS method on blasted surface. The microstructure of TBC was characterized with scanning electron microscopy and energy dispersion X-ray analysis. The coating thickness and hardness profile was measured. Fracture surface, surface relief and polished sections parallel to the specimen axis were examined to study damage mechanisms in coatings under cyclic loading at high temperature. It was find that initiation of the fatigue crack usually occurs on interface YSZ-CoNiCrAlY and the trajectory of the further crack propagation was documented.

Abstract: This paper proposes a criterion for crack opening in FCC single crystals based on analyses of lattice orientation and interface energy of two adjacent crystals in a crystal plasticity finite element model (CPFEM). It also demonstrates the implementation of the criterion in Abaqus/Standard to simulate crack initiation and propagation in single-edged notch single crystal aluminium samples. Elements in the FEM mesh that have crystalline structures satisfying the crack opening criterion are removed from the mesh at the end of every loading step and FEM analyses are restarted on the new mesh in the next loading step. Removed elements effectively act as voids in the material due to crack nucleation. Similarly, the coalescence of newly removed elements at the end of a loading step with the existent ones simulates crack growth in the material. Two advantages of this approach are noted. Firstly, crack nucleation and its subsequent growth in the material is simulated solely based on lattice evolution history in the material without any presumptions of crack paths or regions where cracks are likely to occur. Secondly, as the criterion for crack nucleation is evaluated based on, and thus changes with, the lattice evolution during loading, a predefined energy criterion for crack opening, which could be erroneous, is avoided. Preliminary results of void nucleation and void growth around the notch tip in Cube and Brass oriented samples using CPFEM modelling appear to agree with molecular dynamics simulations of void growth in FCC single crystals.