Key Engineering Materials Vols. 592-593

Abstract: We present a numerical study on virtual concrete beams with shallow midspan notches loaded in three-point bending. We document how spatial randomness of material parameters allows a crack to initiate outside the shallow notch. The interacting effects of notch depth and fluctuation rate of material parameters are shown. We use a discrete meso-level model to simulate material behavior. An approach that introduces material spatial randomness into the discrete mechanical model for concrete fracture through an autocorrelated random field is briefly presented.

Abstract: Lifetime prediction, Isothermal fatigue, damage, cracking, aluminium alloys. Abstract. In this work, a tool for lifetime prediction of aluminium alloys parts employed under isothermal fatigue was developed. This tool is a computer code implemented in a "Visual-Fortran" environment and based on a coupled model of the elastoplastic damage accumulation presented by "Lemaitre and Chaboche" [. This model allows us to describe the damage evolution depending on the number of cycles until failure of the representative volume element (RVE). This failure results in the creation of a macro-crack, its propagation over loading cycles to reach a critical size which will cause the part breakdown. The crack growth phase cannot be neglected because it may contain a considerable number of cycles, this is why we have included in the developed model, this phase of crack growth after the initiation phase; it contains mainly the numerical integration of the "Paris" law.

Abstract: The mechanical behaviour of structural components subjected to multiaxial fatigue loading is very important in modern design. Several approaches have been introduced in recent decades to analyse this problem. The so-called critical plane approach, based on the stresses acting on the plane where the crack nucleation is expected to occur, is widely used. This criterion can give us a fatigue damage measurement, which can be used to evaluate fatigue life. On the other hand, fatigue life under general multiaxial stress histories can also be assessed by applying the damage accumulation method. In such a method, a scalar damage parameter is quantified through the damage increments which develop during the fatigue process up to the critical damage value corresponding to the final failure of the structures. The damage increment approach to fatigue has recently been discussed and connected to the classical crack propagation approach. In the present paper, the interpretation of the critical plane approach based on the continuum damage mechanics concepts is examined. In particular, the physical meaning of the critical plane approach is shown, that is, such an approach can be interpreted as a damage method which takes into account the scalar damage parameter evaluated along preferential directions. Finally, the fatigue behaviour of a metallic material under multiaxial cyclic load histories is analysed through the two above approaches.

Abstract: The fatigue life of wire-bonds was measured with use of a 20 kHz ultrasonic testing system. Thereafter, the experiments were interpreted by Finite Element Analysis. First, simulations were performed according to classical continuum mechanics. They predicted stress concentrations at the location where crack initiation was found experimentally. However, the stress concentrations showed the characteristics of stress singularities even for loading cases corresponding to the regime of very high cycle fatigue, i.e. fairly moderate loads. In subsequent simulations, the singularities of stress concentrations could be removed on the basis of a newly developed version of strain gradient elasticity, resulting in predictions that agreed well with experimental results.

Abstract: Surface cracks arising during rolling sliding contact of a wheel and a rail are investigated. A two-dimensional crack model is proposed which calculates the crack driving force using the configurational force concept. The numerical applicability of the configurational force concept for surface shear cracks under cyclic contact loading is discussed and compared to the J-integral concept. A single inclined crack in a rail loaded by an accelerated wheel is investigated. The material of the rail is described by a cyclic plastic kinematic hardening model. The evolution of the crack driving force during several cycles is investigated.

Abstract: The paper deals with the influence of order of cycles in the loading block on the fatigue crack growth rate in railway axle. The railway axle can include some cracks from manufacturing process or initiated fatigue cracks from previous operation. It is advantageous to know how the crack will behave during further service of the train to ensure its safe operation. The most common approaches describing the fatigue crack growth do not take into account the effects of overload cycles, which enlarge the plastic zone ahead of the crack tip. The enlarged plastic zone generates residual compressive stresses, which cause a retardation of the fatigue crack growth. Finite element numerical calculations were used together with the generalized Willenborg model to determine influence of overload cycles on the increment of fatigue crack growing in railway axles. Real geometry of the axle, the crack front shape and typical loading spectrum were taken into account.

Abstract: This paper presents the calibration of three universal ductile fracture criteria with the stress triaxiality and the normalized third invariant of deviatoric stress dependence. Xue-Wierzbicki, Bai-Wierzbicki and Extended Mohr-Coulomb criteria are calibrated using butterfly specimen and newly designed notched tube specimen for an austenitic stainless steel. Different stress states necessary for successful calibration are generated by suitable combination of tension/compression-shear or tension/compression-torsion loading of both specimens, respectively. Suitability of the specimens for ductile fracture criteria calibration is evaluated comparing the range of reached stress states, the homogeneity of stress in process zone and appropriateness for the computational simulation of tests. Possible combination of above mentioned results with tensile testing of standard and notched cylindrical specimens is discussed, too.

Abstract: A multi-parameter fracture mechanics concept based on the Williams power series is applied on novel cracked specimen geometries utilizing combined boundary conditions of the wedge splitting and the three-point bending test. Crack tip stress fields for various configurations (causing different constraint conditions at the crack tip and thus also different fracture process zone extents) are numerically investigated and subsequently analytically reconstructed using developed procedure. An importance of using higher order terms of the Williams series is demonstrated.

Abstract: Different damage mechanics models have been proposed by researchers to calibrate the failure behavior of materials. Continuum damage mechanics (CDM) models are one of the main categories of damage models that can be exploited in numerical simulations. In this paper Lemaitres damage model, has been applied to finite element models of flat specimens. These models allow assessing the geometry transferability of the previously calibrated CDM model investigating in different geometry and loading conditions. Four different types of plane stress specimens have been designed to get different stress triaxialities which cover shear dominant and high triaxiality failure. Experimental tests were also done and the obtained data were critically compared with the results from numerical models. The tested material is Ti-6Al-4V titanium alloy which is a widely used material in aerospace industry because of its high strength and low density.

Abstract: The localization of plastic deformation was examined for polycrystalline aluminum samples having grain sizes in the range from 8·10^-3to 10 mm. It is found that the length of localized deformation autowaves is determined by the grain size of material. The localized plastic flow patterns emergent in the polycrystalline aluminum samples are found to be connected to the Hall-Petch relation. Two types of flow stress dependencies of grain size are distinguished.