Key Engineering Materials Vols. 353-358

Abstract: This work has investigated the effect of spherical dent on rolling contact fatigue (RCF). A 3-D finite element simulation model of bearing rolling contact incorporating critical plane approach has been developed to study the fatigue failure location. It was found that the fatigue failure locations were significantly influenced by the dent. The calculation results are in good agreement with the experimental results and comparable with the results from the published literatures in which 2-D models were generally used.

Abstract: Functionally graded materials (FGMs) with continuous varying properties have absorbed great attention for the purpose of eliminating the mismatch of material properties which may result in cracking. In this paper, three-dimensional finite element method (3D FEM) based on nonhomogeneous elements is used to study the fracture behaviors of a 3D FGM plate. Since real material properties at Gaussian integration points are adopted during forming the element stiffness matrix, the nonhomogeneous material properties can be applied in each element. Moreover, 20-node singular elements are used around the crack front to deal with the singularity of stress fields at the crack front. By this way, the stress intensity factors (SIFs) can be calculated with high efficiency and accuracy. Therefore, compared with the general FEM using homogeneouos elements, the calculating efficiency and accuracy can be increased. Finally, parameter analysis is conducted. It is found that the material nonhomogeneity constant and the crack parameter have significant influences on the SIFs.

Abstract: The present paper describes the FEM code the present authors have developed based on the theory of the polycrystal plasticity with dislocation distributions taken into account and the simulations of tensile deformation behavior in FCC polycrystalline materials having bimodal structures by using the developed FEM code. In order to simulate the deformation behavior of materials having bimodal structures, it is necessary for the code to simulate the mesoscopic deformation behavior with the size effect of the initial yield strength, or the 0.2% proof strength. The present study has attempted to simulate the size effect of 0.2% proof strength by modifying the Bailey-Hirsch relation. By using the modified relation, the size effect of the initial plastic yield is successfully reproduced by FE polycrystal plasticity analysis. The results also showed that the 0.2% yield strength is decreased as the volume fraction of coarse grains is increased in the bimodal structure. As the ratio of the average diameter of fine grains to that of coarse grains is increased, the yield strength of the bimodal structure is decreased. The yield strength and work hardening rate of the bimodal structure, however, is not so much decreased as that of fine grain models. It was also revealed that the reason why materials having bimodal structures show higher ductility is that coarse grains yield in earlier stage of deformation and lower the maximum stress in the materials.

Abstract: The cyclic crack tip opening displacement is well related to fatigue crack opening behavior. In this paper, we investigate the effect of the maximum stress intensity factor, Kmax, when predicting fatigue crack opening behavior using the cyclic crack tip opening displacement obtained from FEA. The commercial finite element code, ANSYS, for fatigue crack closure analysis in this study is used. We derive the prediction formula of crack opening behavior when using the cyclic crack tip displacement obtained from the FEA. The numerical prediction shows the good results regardless of stress ratios. It is confirmed that the crack opening behavior depends upon the maximum stress intensity factor Kmax.

Abstract: For the purpose of studying material cluster design and shape design of a certain arc-shaped thermal-protection component rationally, the ablation behavior and thermal stress distribution are studied by using the method of finite element numerical simulation. The study includes ablation tests, numerical simulation of temperature field, calculation of ablation thickness and numerical simulation of unsteady thermal stress field of the component. The simulation results are consistent with the results of ablation tests, which shows that the shape design of the arc-shaped thermal-protection component is rational and the dangerous periods of the component ablation are the time of initial heating and initial ablation boundary retreat.

Abstract: Rock is a kind of complex and high-disordered geological material, its damage and fracture process usually shows obvious criticality. In this paper, percolation theory is applied to analyze and describe this critical property. First, we discuss the critical fracture probability of rock through percolation and renormalization analysis, and present the equivalence between fracture probability and damage variable. Based on scaling law and the relationship between critical exponents, a critical fractal dimension is obtained. Furthermore, according to the analysis of relationship between damage and fractal dimension, we suggest a damage-fractal formula, ω=ω0+ (D-D0)/Dc. This formula can not only be used to describe the damage evolution through the variation of fractal dimension, but also to define initial damage in rock. Finally, the theoretical conclusions are validated by a series of model experiments, and the experimental results agree with that of theoretical.

Abstract: Explosive lining is a new method to construct underground space in soil. By making the most of compressibility of soil and thixotropy of concrete under explosive loading, this method offers an efficient path to form a cavity and its concrete support layer synchronously. In order to investigate the forming effect, a series of contrastive laboratory tests, including explosive lining method and conventional explosive compaction method, were performed under same soil and explosive conditions. Results show that measured dynamic stress and displacement by explosive lining method are higher than that of by conventional explosive compaction method under same equivalent radius, and the range of compact region in soil is larger too. Similarly, the physical and mechanical performance indexes of soil, such as water content and cohesion are superior to that of by conventional explosive compaction. It is approved that an even thickness concrete support layer can be formed in one-shot forming process by explosive lining, and there is no evident cranny region in the soil around the cavity.

Abstract: Modes I and II stress intensity factors are analyzed by means of a variational boundary integral method (VBIM) for slant surface-breaking cracks in a half-plane with surface steps subject to contact loadings. This method represents the crack as a continuous distribution of dislocation loops. The crack opening displacements, which are related to the geometry of loops and their Burgers vectors, can be determined by minimizing the elastic potential energy, obtained from the known expressions of the interaction energy of a pair of dislocation loops, of the solid. In contrast to other methods, this approach finally reduces to a symmetric system of equations with milder singularities of the type 1/R, which facilitate the numerical treatments. By modeling the surface boundary of the half-plane as half part of an infinite crack breaking through an infinite solid, this paper demonstrates that the VBIM can be well extended to solve the fracture problems of inclined surface-breaking cracks in a half-plane with curve or step notches subject to combined contact loadings, and presents results of stress intensity factors for a variety of loadings, cracks and step surface configurations. Numerical results of test examples are in good agreement with the existing results in the literature.

Abstract: Effects of joint width (JW) on the macroscopic stress-strain curve, the failure process and mode of jointed rock specimen (JRS) in plane strain compression are modeled by use of FLAC. The failure criterion of intact rock outside the inclined joint is a composite Mohr-Coulomb criterion with tension cut-off and the linear strain-softening post-peak constitutive relation is adopted. The joint is treated as quadrate elements of ideal plastic material beyond the peak strength. A written FISH function is used to automatically find elements in the joint. Numerical results show that the peak strength of JRS depends on JW and is lower than that of intact rock specimen without joint. For JRS, the shear strains are concentrated into the joint or the new generated shear bands (NGSBs); the peak strength decreases with an increase of JW. At lower or higher joint inclination angle (JIA), the failure mode and pattern of NGSBs are not related to JW. The post-peak response becomes ductile at wider JW and higher JIA. The post-peak slope of stress-strain curve at lower JIA is not dependent on JW since the width and inclination angle of NGSBs are not affected by JW.

Abstract: The failure process of heterogeneous rock specimen with initially random material imperfections in uniaxial plane strain compression and the macroscopically mechanical response are numerically modeled by using FLAC (Fast Lagrangian Analysis of Continua). A FISH function is generated to prescribe the initial imperfections within the heterogeneous specimen by using Matlab. The imperfection is weaker than the intact rock. Beyond the failure of the imperfection, it undergoes ideal plastic behavior, while intact rock exhibits linear strain-softening behavior and then ideal plastic behavior once failure occurs. The specimen with smooth ends is loaded at a constant strain rate and is divided into 3200 elements. The maximum numbers of the initial imperfections in five schemes are 100, 300, 500, 700 and 900. The effects of the number of the imperfections on the fracture process, the final fracture pattern and the complete stress-strain curve are investigated. Prior to the peak stress, some imperfections extend in the axial direction and then a part of them coalesce to form inclined shear bands. Beyond the peak stress, shear bands progressively intersect the specimen; in the process the number of the yielded elements approximately remains a constant. With an increase of the number of the initial imperfections, the spacing of shear fractures decreases, the peak stress and corresponding axial strain decrease; the post-peak branch of stress-strain curve becomes steeper; much more elements fail in tension; the number of the yielded elements in tension in the vicinity of the two lateral edges of the specimen remarkably increases.