FEM simulations of tensile deformation and fracture analysis for CuW alloys at mesoscopic level

Considering the mechanical properties and fracture mechanism of CuW alloys determined by microstructures, finite element method (FEM) is employed to analyze the mesoscopic model using Voronoi tessellation technology. Heterogeneous plastic strain and stress distributions are obtained at mesoscopic level in order to reveal the deformation process and fracture mechanism of CuW alloys. Shear bands or microcracks induced by shear band appear at the Cu/W and W/W interfaces mostly, and then propagate along the interfaces. Therefore, mechanical strength of CuW alloys is determined by sintering neck mainly. Additionally, the microstructures of fracture surfaces are applied to verify the simulation results.     

Dynamic characteristics of the plastic deformation kinetics of polycrystals

Methods of spectral analysis of local plastic deformations in polycrystalline copper on three structural levels are used to show that the tendency to synchronization is a general dynamic characteristic of the plastic deformation kinetics, and is a measure of the self-organization, evolution, stability, and decay of the dissipative structures in deformable polycrystals. The synchronization effect is directly related to the strength and plasticity characteristics. Pis’ma Zh. Tekh. Fiz. 24, 50–53 (August 26, 1998)     

Localization of plastic deformation and fracture in aluminum polycrystals

The effect of the grain size as a basic structural parameter on plastic strain macrolocalization has been studied for polycrystalline aluminum. The mathematical form of the above dependence has been verified. The limiting cases have been defined both for small-and coarse-grain ranges. The effect of sample dimension on the macrolocalization period has been considered. __________ Translated from Problemy Prochnosti, No. 1, pp. 52–55, January–February, 2008.     

Modeling of the kinetics of plastic deformation of polycrystals using Markov chains

Discrete Markov chains are used to model the kinetics of microplastic deformations in polycrystalline copper as a multilevel hierarchical process. The quantitative contribution of each structural level to the average deformation is determined. Pis’ma Zh. Tekh. Fiz. 24, 8–11 (August 12, 1998)     

Plastic deformation of Zr-Sn polycrystals at intermediate temperatures

The yield stress and the activation volume for Zr-Sn alloys with 0.74, 2.85, 4.27 and 6.19 wt% Sn have been measured at temperatures between 400 and 750 K. The temperature dependence of the yield stress exhibits a plateau except for the alloy with the highest content of tin. The yield stress increases with increasing content of tin atoms. A non-monotonic variation of the activation volume with temperature has been observed for pure zirconium and for Zr-Sn alloys with 0.74 and 2.85 wt% Sn. The maximum value of the activation volume (at about 600 K) decreases with increasing content of tin. Dynamic strain ageing is considered to be responsible for the maximum in the temperature dependence of the activation volume. The dislocation structure has been observed. The experimental results are interpreted in terms of a simple model which considers that the flow stress is determined by thermally activated glide of dislocations through obstacles, dynamic strain ageing and a strengthening effect of tin atoms.     

On the kinetics of localized plasticity domains emergent at the pre-failure stage of deformation process

The behavior of localized plasticity domains occurring at the final stage of plastic flow process has been investigated. A series of runs was conducted on materials differing in crystal lattice type, which enabled establishment of the regular features exhibited by the flow process upon a transition to macroscopic necking and viscous failure. It is found that the most distinctive regularity is the occurrence of flow domains, which are moving in a concerted manner towards the pole of a bundle of domain trajectories plotted in the time–space co-ordinates. The deformation patterns are found to be related to the kinetics of nucleation and motion of localized plasticity fronts. The probable origin of the observed effects is considered.     

A micropolar model of plastic deformation of polycrystals at the mesolevel

Experimental and theoretical research of the last few years have shown that it is impossible in principle to describe macroscopic properties of plastic flow when only the dislocation theory is adopted. It is necessary to take into consideration one or more intermediate mesoscopic levels at which volumes of finite sizes are the objects of study. The heterogeneity of a material's internal structure and formation of substructures of essentially a greater scale than separate dislocation are to be taken into account obviously during plastic deformation in mesovolumes. Hence, an adequate model of the mesovolume behaviour should consider displacement, rotations and interaction of structural elements, which are available in the mesovolume. A medium like this requires to take account of internal non-compensated moments and only for a representative mesovolume the total (averaged) moment is equal to zero. A simple model is offered here which allows one to consider independent rotations of mesofragments of finite size without obvious calculation of couple-stress and torsion-curvature. As rotation and displacement of individual mesofragment, and its strain as well, are completely determined by its velocity field, it is necessary to define an asymmetrical part of the force-stress tensor from additional physical reasons. It is supposed in the model that at the stage of perfectly elastic deformation the stress tensor is symmetric, and its asymmetrical part appears only at the stage of plastic deformation and is proportional to the function of plastic strain accumulated in the considered local area. Test calculations carried out have shown that the model is able to simulate the behaviour of crystals with a limited number of active slip systems well. In the case of two-dimensional flow, a spin parameter determines both magnitude and direction (by the sign of the parameter) of rotation of local mesovolumes of a material.     

Investigation of softwood fracture criteria at the mesoscopic scale

In order to improve the knowledge of softwood mechanical behaviour and particularly fracture mechanisms, investigation at small scales is needed. For that purpose, new numerical tools based on the real wood morphology in the transverse plane are specifically used. A mode I fracture criterion applicable to cracks oriented in the transverse plane in softwood is then studied at the mesoscopic scale, i.e. the annual ring scale. Numerical investigation combined with digital image correlation help to obtain wood fracture parameters in the annual ring. These parameters give additional knowledge and understanding of wood fracture according to local specificities such as crack tip position in an annual ring and notch orientation in relation to natural wood orientation and loading orientation. Mesoscopic fracture criteria can help our understanding of secondary crack appearance and crack arrest phenomena. Mixed mode study and coupling between mode I and mode II of fracture will be investigated further. Mixed mode fracture criteria can then help to better understand crack bifurcation phenomenon.     

A study of craze deformation in the fatigue fracture of polymethylmethacrylate

Continuous measurements of the craze contour have been undertaken for fatigue crack propagation in polymethylmethacrylate. The craze contour was determined by an interference fringe method, using equipment specially constructed for continuous load cycling, mounted on the stage of an optical microscope. It was found that there was a very great variation in the size of the craze for a fixed loading programme, although the crack growth rate showed a good correlation with the applied stress intensity range. A general result is that the craze length in fatigue was found to be greater than that for continuous crack propagation under simple fracture. This result suggested that it would be instructive to examine an extension of the Dugdale line-zone model, where the craze stress is not assumed to be constant along the length of the craze. It is shown that this modified line-zone model can provide some understanding of the observed results for craze deformation in fatigue.     

Physics of deformation and fracture at impact loading and penetration

The structure, dislocation substructure, and mechanical properties of the targets made of four aluminum alloys after a impact loading by kinetic energy projectile have been investigated. The formula for approximation of the ballistic limit velocity by indentation technique is proposed. It has been shown that the maximum nonequiaxiality of the grain shape, increase of dislocation density, and decrease of dislocation cell size correspond to the 40–70% of plastic deformation at static compression for the investigated aluminum alloys.