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Abstract: The investigation of fatigue strength of lotus-type structure with nodular cast iron as a base material using computational model is analysed in present study. The irregular pores distribution in transversal and longitudinal direction, regarding the external loading, is considered in the computational models. The complete fatigue process of analyzed porous structure is then divided into the crack initiation (Ni) and crack propagation (Np) period where the total fatigue life (N) is defined as: N = Ni + Np. The crack initiation period is determined using strain life approach where elastic-plastic numerical analysis is performed to obtain the total strain amplitude in the critical stress fields around the pores. The simplified universal slope method is then used to determine the number of stress cycles, Ni, required for formation of initial cracks. The number of stress cycles, Np, required for crack propagation from initial to the critical crack length is also numerically determined using finite element (FE) models, in the frame of Abaqus computation FEM code. The maximum tensile stress (MTS) criterion is considered when analyzing the crack path inside the porous structure. The performed computational analyses show that stress concentrations around individual pores are higher when external loading is acting in transversal direction in respect to the pore distribution. Therefore, further computational analyses regarding crack initiation and crack propagation period have been done only for pores distribution in transversal direction.
Keywords: lotus-type porous structures, fatigue crack initiation, fatigue crack propagation, numerical analysis
Published in DKUM: 02.08.2017; Views: 1467; Downloads: 424
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Abstract: The distribution and amount of residual stresses can significantly contrubute to the fatigue fracture behaviour of welded joints in structures. Interior residual stresses, which interact with the plane strain state, are more dangerous than residual stresses at the surface of the welded joint. If the surface of the welded joint is mechanically treated (e.g. shapering), then significant differences between the stress stated at the surface and within the volume occur. Hence, different non-destructive methods (e.g. gamma radiation, neutron diffraction) have been developed to determine strains caused by residual stresses in the volume of polycrystalline materials. However, the measurement of strains caused by residual stresses becomes difficult for large samples. In this paper the measurement procedure on a sample taken from the weld joint is presented and the effecet of residual stresses on fatigue crack propagation is assessed for low and high cycle loading fatigue.
Keywords: zaostale napetosti, zvarni spoj, neutronski lom žarkov, utrujenostno širjenje razpoke, neutron diffraction method, residual stresses, fatigue crack propagation, weld joint
Published in DKUM: 10.07.2015; Views: 1308; Downloads: 36
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Abstract: The presented paper describes the experimental determination of fatigue crack growth parameters for high strength low-alloy steel 42CrMo4. The experiments were performed according to the ASTM E647 standard and the parameters of the Paris equation (C and m) were determined. Test specimens were subjected to impulsive cyclic tensile loading on a testing machine. During that the fatigue crack propagation was monitored as a function of elapsed fatigue cycles. Taking these experimental results into account the fatigue crack growth rates at different stress-intensity factor ranges was determined. To consider the variable material properties of the slewing bearing ring trough depth, specimens of different hardnesses were tested. Beside the experimental result a 2D contact fatigue crack propagation model is also presented. This model can be used for the simulation of surface initiated contact fatigue crack propagation on the roller slewing bearing's raceway. As a computational example the results of a finite element method contact fatigue crack propagation simulation are presented.
Keywords: crack propagation, slewing bearings, numerical modelling, experiments
Published in DKUM: 07.06.2012; Views: 2090; Downloads: 97
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Abstract: A two-dimensional computational model for simulation of contact fatigue of gear teeth flanks is presented. In the model, it is assumed that the initial crack of length 0.015 mm is initiated at the surface due to previous mechanical or heat treatment of the material as well as a consequence of the running in process. The discretized model with the initial crack is then subjected to normal contact pressure, which takes into account the elasto-hydro-dynamic (EHD) lubrication conditions, and tangential loading due to friction between contacting surfaces. The model also considers the moving contact of gear flanks, fluid trapped in the crack and residual stresses due to heat treatment of the material on crack propagation. The virtual crack extension (VCE) method, implemented in the finite element method, is then usedfor simulating the fatigue crack growth from the initial crack up to the formation of the surface pit. The computational results show that the initial surface crack of length 15 m and the considered boundary conditions lead to the appearance of very small surface pits, which can be termed as micro-pitting on gear teeth flanks. The numerical results correspond well withavailable experimental data.
Keywords: machine elements, gearing, fracture mechanics, numerical analysis, micro pitting, pitting simulation, matematical model, fatigue, contact mechanics, crack propagation
Published in DKUM: 01.06.2012; Views: 2660; Downloads: 0

Abstract: This paper describes the problem of determining crack initiation location and its influence on crack propagation in a gear tooth's root. Three different load positions on the gear tooth's flank were considered for this investigation of crack initiation and propagation. A special test device was used for the single tooth test. It can be concluded from the measurements thata crack can be initiated at very different locations in a tooth's root and then propagate along its own paths. A numerical investigation into a crack initiation's position and its influences on its propagation were carried out within the framework of linear fracture mechanics. The influence of a tooth's load position, the geometry of the tooth's root, and the influence of nonparallel load distribution on the tooth's flank were considered when investigating the crack initiation's position. Results show that linear fracture mechanics can be used for determining crack propagation, if better initial conditions for crack initiation are considered.
Keywords: fracture mechanics, crack initiation, experimental crack propagation, numerical crack propagation, crack location, gearbox, tooth root, load distribution, linear elastic fracture mechanics
Published in DKUM: 01.06.2012; Views: 1825; Downloads: 106
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