Sharif Digital Repository

In spite of the fact that the experimental results indicate the significant effect of strain rate on forming limits of sheets, this effect is neglected in all theoretical methods of prediction of Forming Limit Diagrams (FLDs). The purpose of this paper is to modify the most renowned theoretical method of determination of FLDs (e.g., M-K model) so as to enable it to take into account the effect of strain rate. To achieve this aim, the traditional assumption of preexistence of an initial geometrical inhomogeneity in the sheet has been replaced with the assumption of a preexisting "material" inhomogeneity. It has been shown that using this assumption, the strain rate would not be omitted from... 

In this paper, effects of strain rate and mean strain on the cyclic behavior and the lifetime of aluminum-silicon alloys are investigated under thermo-mechanical and isothermal fatigue loadings. To achieve these goals, low cycle fatigue tests are accomplished at evaluated temperatures under various strain rates (by changing the loading frequency) and different strain ratios (minimum to maximum strain). Thermo-mechanical fatigue experiments are performed in an out-of-phase condition where the temperature varies between 50 and 250 °C. Various heating/cooling rates are taken into account to assess the strain rate effect and different starting temperatures are considered to study the mean strain... 

In the present work, the process of equal channel angular pressing of dispersion strengthened Cu-1̇1 wt-%Al2O3 containing nanometric alumina particles was investigated by means of mathematical modelling and experimental testing. Through the modelling, deformation parameters such as hydrostatic pressure distribution and strain field were determined, and the effect of deformation path on these parameters was estimated. Equal channel angular pressing as well as mechanical and microstructural evaluations were also conducted to assess mechanical properties, grain structure and void volume fraction after deformation in different routes. The results indicate that distributions of plastic strain and... 

In this paper the effect of strain and impurity on the quantum conductance of semiconducting carbon nanotubes (CNTs) have been studied by ab-initio calculations. The effect of strain and impurity on the CNT conducting behavior and physical characteristics, like density of states (DOS), band structure, and atomic local density of state (LDOS), is considered and discussed separately and simultaneously. Our results show that the quantum conductance of semiconductor CNTs is increased by compression strain, elongation strain, and replacing nitrogen and boron doping in its structure. The amount of increasing in the conductance depends on the type of strain and impurity. Conductance of CNT can be... 

Since the constitutive information is one of the most important aspects of material deformation analysis, here a new constitutive model is proposed that can investigate the behavior of material during intense deformation better than existent models. The model that is completely based on physical mechanisms can predict all stages of flow stress evolution and also can elucidate the effects of strain and strain rate on flow stress evolution of material during intense plastic deformation. Here as an application, implementation of the constitutive model in finite element method (FEM) is used to compare two methods of sever plastic deformation (SPD) processes of copper sheet; repetitive... 

In this study, effect of rolling speed on strain aging phenomena in warm rolling of a carbon steel has been investigated. For this purpose, by using a mathematical model and predicting temperature and strain rate fields, the possibility of occurrence of dynamic strain aging during the warm rolling was first evaluated. In the next stage, warm-rolled samples were aged up to 11 months at room temperature for studying the kinetics of static strain aging, while mechanical tests as well as microstructural evolutions have been performed to determine the effect of strain aging on material behavior. The results indicate that dynamic strain aging may not occur for the employed rolling program;... 

The interfacial behavior of composites is often simulated using a cohesive zone model (CZM). In this approach, a traction-separation (T-S) relation between the matrix and reinforcement particles, which is often obtained from experimental results, is employed. However, since the determination of this relation from experimental results is difficult, the molecular dynamics (MD) simulation may be used as a virtual environment to obtain this relation. In this study, MD simulations under the normal and shear loadings are used to obtain the interface behavior of Al/Al2O3 composite material and to derive the T-S relation. For better agreement with Al/Al2O3 interfacial behavior, the exponential form... 

As the reinforcing bars used in concrete structures located in earthquake-prone areas experience strain rates higher than normal quasi-static ones, it is necessary to have a comprehensive understanding of the behaviour of such materials under these rates of loading. In this work, in order to study the behaviour of grade A-III reinforcing-bar steel (based on the GOST standard, a set of technical standards maintained by the Euro-Asian Council for Standardization, Metrology and Certification), a number of monotonic tests on its tensile and compressive strength on (short and long) specimens at various strain rates, 0·002, 0·01, 0·02 and 0·04 s-1, experienced during earthquakes, were carried out.... 

In this study, the effect of strain ageing on mechanical properties of warm-rolled steel is investigated. Warm-rolling experiments together with mechanical testing are employed to investigate the effect of processing parameters on the ageing kinetics during and after warm rolling of a low-carbon steel. To do so, the occurrence of dynamic strain ageing is first determined by means of a two-dimensional thermo-mechanical model and then the effect of this phenomenon on the subsequent static strain ageing after warm rolling is studied employing mechanical testing. The results show that the occurrence of dynamic strain ageing during warm rolling may effectively alter the progress of the subsequent... 

The effect of strain rate on the ductility of a La-based bulk metallic glass (BMG) over a wide temperature range, and the correlation between ductility and relaxation processes within this alloy are investigated in the present work. The three point bending test and dynamic mechanical analysis are employed to study these phenomena. It is found that the activation energies of the nearly constant loss (NCL) relaxation and intermediate temperature brittleness are almost identical. This observation reveals that the NCL relaxation, as a locally confined or caged dynamics, contributes as the main source of intermediate temperature brittleness in La-based BMGs. © 2018 Elsevier B.V  

In this study, multi-directional forging (MDF) as a severe plastic deformation method, annealing, and compression test at different strain rates are applied on pure aluminum. One of the main observations is when the number of MDF passes is increased, the hardness is risen. The most noticeable reduction in hardness occurs when the MDFed samples are annealed at 350 and 450 °C. Moreover, increasing the number of MDF passes and the strain rate of the compression test cause the increase in the yield stress of the samples. If the MDFed samples are non-annealed or if they are annealed at a maximum temperature of 250 °C, the effect of an increase in the number of passes on the yield stress is more... 

Molecular structural mechanics is implemented to investigate the vibrational characteristics of defect-free single-layered graphene sheets (SLGSs), which have potential applications as strain sensors. The effect of strain on the fundamental frequencies of the defect-free zigzag and armchair models with clamped-clamped boundary condition is studied. The atomistic modeling results reveal while sensitivities of the strain sensors are not influenced significantly by chirality, they can be slightly increased by decreasing aspect ratios of the sheets. It is further shown that the SLGSs-based strain sensors are more sensitive to the applied stretch than the SWCNTs versions. © 2008 Elsevier Ltd. All... 

Central burst defect, also called chevron crack, is an internal defect which can be encountered during cold drawing processes. It causes serious problems on the quality control of products, since it is impossible to detect them by a simple surface inspection of the work piece. Therefore, its prediction is of great importance in wire drawing processes. The present investigation is concerned with the effect of strain hardening exponent on the prediction of central burst defect in aluminum wires. The major conclusion of this study is that, for a range of combination of drawing conditions, geometrical parameters and strain-hardening exponent, central burst can be predicted in wire drawing....