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[en] Damage evolution in quasi-brittle rocks is a complicated procedure in which both the post-peak strain-softening behavior and the immobile residual strength play key roles. The post-peak behavior observed from existing experiments is critical for the determination of the complete constitutive relation of quasi-brittle rocks. Based on the test observation on the post-peak immobile residual strength of quasi-brittle rock specimens, a proper damage tolerance is defined in this study. Following this theme, a damage tolerance principle is presented by adopting an alternative methodology, which incorporates a loading function to represent the reaction to the stress state and a condition of damage tolerance based on the Mohr–Coulomb criterion without cohesive strength. Then, an iterative solution is proposed to identify the parameters of the proposed model. The identified constitutive law is employed to determine the load–deformation behavior of quasi-brittle rocks including sandstone and quartzite, and the calculated results are compared with the test results under different confinement stress levels. Results show that the developed constitutive law for quasi-brittle rocks with damage tolerance principle can be used to predict the post-peak behavior including the strain-softening behavior and the immobile residual strength with acceptable accuracy. Finally, a parametric study is conducted to essentially investigate the effect of parameters of the proposed model on the observed characteristics of quasi-brittle rocks.
[en] The traditional BCS superconductors AOs_2O_6 (A=K, Rb, and Cs) were investigated to find the relationship between their structures and superconducting transition temperatures. The T_c decreases with increasing the unit cell parameter of AOs_2O_6. This is in contrast to the case of conventional BCS superconductivity in a single bond model, where T_c may increase with increasing the the unit cell parameter since the DOS at Fermi level increases as the unit cell parameter increases. Instead, the T_c of a β-pyrochlore oxide is proportional to the lattice softness of the compound
[en] The effects of nitrogen on the nanomechanical property and localized plasticity of nitrogen alloyed 316 series austenitic steels were investigated by using nanoindentation. Slip steps around indentations have been imaged and have indicated that nitrogen increases slip planarity greatly. The load versus displacement curves show that the addition of nitrogen increases the first excursion load and the probability of occurrence of the second excursion, which is attributed to slip planarity caused by nitrogen and the local work softening on the operative slip plane. Additionally, nitrogen increases hardness and elastic modulus, while it promotes nanoindentation creep. Planar slip caused by nitrogen inhabits the formation of strain-induced α′ martensite and then promotes the creep deformation. (paper)
[en] The effect of rapid low-temperature heat treatment on properties of heavily deformed Steel 80 has been investigated. The establishment has been made of the dependences of ultimate and yield strengths, and relative elongation on heating temperature and rate and time of isothermal holding at the heating temperature. (authors).
[en] The shear-transformation-zone (STZ) theory of plastic deformation predicts that sufficiently soft, noncrystalline solids are linearly unstable against forming periodic arrays of microstructural shear bands. A limited nonlinear analysis indicates that this instability may be the mechanism responsible for strain softening in both constant-stress and constant-strain-rate experiments. The analysis presented here pertains only to one-dimensional banding patterns in two-dimensional systems, and only to very low temperatures. It uses the rudimentary form of the STZ theory in which there is only a single kind of zone rather than a distribution of them with a range of transformation rates. Nevertheless, the results are in qualitative agreement with essential features of the available experimental data. The nonlinear theory also implies that harder materials, which do not undergo a microstructural instability, may form isolated shear bands in weak regions or, perhaps, at points of concentrated stress
[en] The Fictitious Crack Method (FCM) is applied to determine the load-deflection diagrams of notched plain concrete bean's under 3-point bending using different forms of strain-softening in the form of stress-deformation relationship. The results indicate that there is a need to determine a more realistic relationship. (author)
[en] This paper presents the results of the three-dimensional (3D) discrete element method (DEM) simulations of undrained axisymmetric/triaxial tests on loose assemblies of polydisperse spheres with and without preshearing history using a periodic cell. Undrained tests are modelled by deforming the samples under constant volume conditions. The simulations show that the preshearing process will not induce initial structural anisotropy, and that the presheared and unpresheared samples follow the same initial stress path along a unique limiting boundary in the q–p space, as observed in the published experimental literature, which was not crossed over by any of the stress paths of the presheared samples. It is also shown that the presheared samples are denser compared with the original unpresheared one, and therefore exhibit higher resistance to (temporary) liquefaction. At the grain scale, such higher resistance is found to be attributed to the evolution of a redundancy factor, a microscopic definition of liquefaction (temporary liquefaction). The Lade instability (peak deviator stress) is found to correspond to a unique mechanical coordination number of 4.5, independent of preshearing history. It is also found that the onset of liquefaction (temporary liquefaction) in terms of the redundancy factor lags behind the onset of macroscopic strain softening in terms of the Lade instability for the presheared and unpresheared samples under undrained conditions. (paper)
[en] A deterministic model with material strain-softening has been employed to predict the failure process of quasi-brittle materials subjected to different strain gradients. The failures of beams in pure bending and L-shape specimens in tension are simulated using this model, and the influence of material damage rate on the failure process is studied. The effect of statistical variations in the material properties on the fracture statistics of the components is then considered. The model correctly predicts the changes in the magnitude and distribution of the failure loads of specimens with different strain gradients. The results can help explain the influence of strain gradient on the fracture statistics of quasi-brittle materials.
[en] A model reproducing strain softening behavior in ceramic materials is proposed. This model is base on a critical treatment of previous mechanical experimental results, mainly on uranium dioxide. The main hypothesis is that the strain softening phenomenon is related to an aging process, where some point defects move towards the dislocations and modify their velocity. This is different from most of models used up to now, as they were based on the hypothesis that only the initial lack of dislocations was responsible of the strain softening behavior. A model is first developed in a simple 1D framework. Evolution of the mechanical behavior with strain rate and temperature is well reproduced by this model. Then, the 1D model is extended to a 3D mechanical model, and mechanical compressive tests on UO_2 pellets are simulated. The 3D model well reproduces the observed asymmetrical shape of the compressed pellet if one considers that the material is not initially perfectly homogeneous, which highlights the importance of accounting for spatial heterogeneity of materials in models. (authors)
[en] The cyclic softening behavior in low cycle fatigue of disk-forged Alloy718 by direct aged process was investigated at high temperatures of 500 .deg. C and 650 .deg. C. It resulted that the softening phenomena during fatigue cycles should be related to precipitates and dislocation behavior in the microstructure of DA Alloy718. The slip bands by shearing matrix were composed of the main deformation mode in both of the an forged and the cyclic tested. The dislocations in the slip bands might get distangled in progressing the repeated cyclic motion, resulting the diffused configuration of the slip band lines. The precipitates of γmoreover, have made an appearance of shearing by the dissolved dislocations