Papers by Keyword: Plastic Deformation

Abstract: The rheological properties of cement-based materials can be used to guide its construction. In this paper, rheological characteristics of fresh cement mortar (FCM) were investigated. The rheological model and its parameters of the FCM under different hydration time, different water to cement ratio (w/c) and sand to cement ratio (s/c) were obtained by the rheological testing and model analyzer. The result indicated that rheological model of the FCM is transformed from Burgers model to Bingham model with the extension of hydration time. And the model transformed from Burgers model to five component model with the decrease of w/c and the increase of s/c. However, the superplasticizer has little effect on the rheological model of cement mortar. The plastic deformation of the fresh cement mortar increased gradually with the prolongation of the hydration time, and the plastic yield value increased gradually from 50Pa to 250Pa with the increase of s/c. The change rate of plasticity of mortar is reduced from 422Pa/h to 107Pa/h with the increase of superplasticizer’s dosage, which shows the addition of superplasticizer retards the hydration rate.

Abstract: The object of the research is creep deformation proceeding in the conditions of electrostatic field effect. The purpose of the research is to develop the mathematical model of creep under the electrostatic field effect from the positions of representations about the wave nature of plastic deformation process. The theoretical studies of electrostatic field effect being characterized by small (up to ± 1V) potentials on the basis of mass, momentum and energy conservation in two-dimensional formulation were carried out in the process of research. The material being deformed was represented as two phase heterogeneous medium. The first component is excited and being responsible for structure transformation, the second one is unexcited and disconnected with them. For each of the components the laws of mass and momentum conservation were written. For electric fields the Maxwell equations were written. For the first time the two phase filtration model of creep was developed as a result of the research. The model takes into account the inhomogeneity of plastic deformation under electrostatic field effect. The dispersion relation for the waves of plasticity is obtained.

Abstract: During the moving stage of the projectile, the impact load produced by the detonation of the explosive powder acts on the ribbon, causing the plastic band deformation to occur rapidly and the surface temperature rapidly increases. In this paper, the evolution mechanism of the plastic deformation of brass band is studied, and the recrystallization process of the surface metal is still at the meso-scale scale. The recrystallization and grain growth stage sexual characteristics.

Abstract: Molecular dynamics simulations were carried out to investigate the change in the crystal orientation of polycrystalline materials placed under an external load. Two models were prepared, both comprising four grains but with different grain arrangements. Each grain had a face-centered cubic structure with (001) face on the x-y plane, whereas each grain had a different rotation of orientation around the z-axis. A tensile load was applied by extending the edge length in the y direction while the other directions were kept stress-free. As a result, a significant change in the microstructure was observed, with changes in both crystal orientation and shape along with the formation of subgrains. The structure and direction of the grain boundary against the external load were also found to affect the change in the microstructure.

Abstract: A new method for non-destructive evaluation of the mechanical properties of structural materials has been developed. This is based on measurements of the ultrasound propagation velocity in deforming materials. Preliminarily investigations were carried out in order to relate the ultrasound propagation velocity to the mechanical characteristics of the deforming material. A detailed description of suitable devices intended for ultrasound propagation velocity measurement with high accuracy is presented. Using Zr-based alloys as an example, it is shown that the method can be used for the monitoring of zirconium billets from which nuclear reactor fuel cladding is fabricated by cold rolling.

Abstract: Aluminium alloys are widely used in engineering fields because of its characteristics. Components used in the fields of automobile, aerospace such as gears, shafts, turbines. In the mechanical field materials can be joined using many processes like riveting, bolted joints or by using permanent joining techniques like welding, brazing or soldering. The welded joints are often used for structural applications due to the reason that they provide nearly the same strength of the parent material. But researches show that even though they provide good strength in tension, they fail much faster when they exposed to fatigue loading. This may be due to various reasons like improper welding sequence, improper preparation or improper welding parameters of the joint materials. The fatigue life may also get reduced due to stress concentration developed during the welding process. Fatigue failures often results in catastrophic failures. Many designs have been reported, have been occurred due to fatigue failures which resulted in heavy loss of human lifespan and goods. Materials with even higher tensile strength are found to fail at less number of cycles after welding.

Abstract: Plasticity of oxide fuel based on uranium dioxide is one of the less-covered topics in nuclear materials science. A useful tool to study the deformation mechanisms in this material at elevated temperatures is hot pressing. In this work, UO$_{2.06}$ powder prepared via~ADU route was uniaxially compacted at temperatures within the~$250$--$600$~{\textdegree}C range under a compressive axial stress from~$95$ to~$220$~MPa applied for a time interval between~$10$ and~$60$~min. Examinations performed with scanning electron microscopy~(SEM) and density measurements revealed a temperature effect on densification when increasing the compaction temperature from~$400$ to~$600$~\textdegree{C} with other conditions being equal. By varying the loading duration for hot pressing at~$600$~{\textdegree}C under constant compressive stress~$95$~MPa, different stages of compactions were analyzed. In addition,~XRD measurements revealed a texture developing in the material during compaction and a possible increase in dislocation density. It is argumented that dislocation-mediated plasticity contributes to densification at elevated temperatures.

Abstract: Positron lifetime calculation has been performed on a computer-generated nanocrystalline copper with a mean grain size of 9.1 nm during its deformation. For the undeformed and deformed nanocrystalline copper, calculated positron lifetimes are around 157 ps which come from the positron annihilation in the free volume in grain boundaries. Due to the grain-boundary deformation mechanism, no vacancies or vacancy clusters will be induced in grains during the plastic deformation of the nanocrystalline copper, which is different to the deformation of the conventional polycrystal. From this point of view, in-situ positron annihilation measurements can provide important experimental information on the deformation mechanism of nanocrystalline metals.

Abstract: Dislocations would be induced after plastic deformation, which might change the mechanical properties of solids. FeCrNi austenitic model alloy and its Mo-diluted alloy were cold rolled with different degree of thickness reduction. Positrons are sensitive to point defects, which are easily trapped and annihilated around the trapping sites. The mean positron lifetimes have been used to estimate the average dislocation concentration in solids. Meanwhile, the trapping efficiency μ was calculated from the lifetime results. The trapping efficiency value is estimated about 3.31×10^-7 cm³s^-1 for FeCrNi alloy and 3.31×10^-7 cm³s^-1 for Mo-diluted alloy, respectively. The increment of the hardness value during plastic deformation is related to the increase of the dislocation density and dislocation pile up in solids.

Abstract: Due to the great effect of defects on the properties of the material including strength, ductility, resistivity and opacity, there are many techniques that are used in defect detecting. Positron annihilation spectroscopy (PAS), Vickers hardness, and X-ray diffraction were used to study the influence of plastic deformation on the properties of 8006 Al-alloy in this work. An increase in the positron lifetime and Vickers hardness with a bit Broadening of XRD peaks was observed with increasing the degree of deformation reflecting a large dislocation density produced by plastic deformation.