Sharif Digital Repository

Adirect numerical simulation approach is used to investigate the effective non-linear viscoelastic stress response of non-gap-spanning magnetic chains suspended in a Newtonian fluid. The suspension is confined in a channel and the suspended clusters are formed under the influence of a constant external magnetic field. Large amplitude oscillatory shear (LAOS) tests are conducted to study the non-linear rheology of the system. The effect of inertia on the intensity of non-linearities is discussed for both magnetic and non-magnetic cases. By conducting magnetic sweep tests, the intensity and quality of the non-linear stress response are studied as a function of the strength of the external... 

The microstructure of cold rolled Al–6Mg alloy is investigated after two steps annealing at different coupled temperatures of 250–320 °C and 320–400 °C for various times. Dynamic strain aging behavior in terms of serrated flow and strain rate sensitivity is investigated. The effect of three microstructural features, cell structure, recovered and recrystallized microstructures, on the strain rate sensitivity is elucidated. Two steps annealing process is utilized to capture the effect of recovery and precipitation phenomena on recrystallization and dynamic strain aging behaviors. The results show that the negative strain rate sensitivity of cold rolled specimen increases to positive values in... 

In this paper, a method for modeling of elastic-plastic hardening materials under large deformations is proposed. In this model the generalized strain rate tensor is used. Such a tensor is obtained on the basis of the method which was introduced by the authors. Based on the generalized strain rate tensor, a flow rule, a Prager-type kinematic hardening equation and a kinematic decomposition is proposed and the governing equations for such materials are obtained. As an application, the governing equations for the simple shear problem are solved and some results are compared with those in the literature. Copyright © 2008 by ASME  

Fracture tests on Sn93Pb37 solder joints in a double cantilever beam (DCB) configuration were performed at two different strain rates of 10−5 and 0.03 s−1 under mode I loading conditions. In each case, the critical strain energy release rate for crack initiation, Jci, was obtained. Effects of storing specimens at a constant temperature of 75 °C and cyclic temperature varying between 32 and 75 °C were examined at these strain rates. In the strain rate of 0.03 s−1, storing samples in a constant or cyclic temperature caused the fracture energy to decrease significantly with respect to the specimens maintained in ambient temperature. The significant reduction in fracture energy by placing the... 

The hot deformation behavior of nickel-base superalloy UDIMET 720 in solution-treated conditions, simulating the forging process of the alloy, was studied using hot compression experiments. Specimens were deformed in the temperature range of 1000 °C to 1175 °C with strain rates of 10-3 to 1 s-1 and total strain of 0.8. Below 1100 °C, all specimens showed flow localization as shear band through the diagonal direction, with more severity at higher strain rates. A uniform deformation was observed when testing between 1100 °C and 1150 °C with dynamic recrystallization as the major flow softening mechanism above 1125 °C. Deformation above γ' solvus temperature was accompanied with grain boundary... 

In this work, deformation behavior as well as creep and cavitation of AA7075-T76 were studied. The as-received plate was first stabilized utilizing solution treatment followed by two-stage artificial aging at 120 and 180 °C. Then, tensile tests were carried out on the aged-alloy in the temperature range between 120 and 250 °C under strain rates of 0.0005 and 0.005 s−1. Furthermore, stress-controlled creep tests were performed at temperatures varying between 120 and 210 °C at stresses ranging from 130 to 250 MPa. Microstructural evolution was then conducted to assess the microstructural changes and growth of cavities during creep employing optical metallography and scanning electron... 

Polypropylene (PP) is a semi-crystalline polymer with a very wide range of industrial applications. This is why its mechanical properties, including yielding, have been the subject of numerous studies. The current study has examined the influence of strain rate and thermal softening on yield stress and post yielding behavior of an isotactic PP. A two-process Ree and Eyring model and the heat equation in adiabatic deformation were coupled with a physical parameter described as the temperature sensitivity of the flow stress at yielding, to simulate the effect of gradual temperature rise in adiabatic heating on yielding behavior. Along with that, tensile bars were subjected to tests at... 

Serrated flow was investigated in superalloy IN738LC, a nickel-base γ′ age-hardened alloy. In this material serrated flow appeared between 350 and 450 °C and strain rate of (8.77 × 10-5 to 8.77 × 10-3) s-1. Activation energy for this process was calculated to be 0.69-0.86 eV which is in good agreement with the values reported for similar alloys. Results show that the diffusion rate of substitutional solute atoms at this temperature range is too low to cause this effect. This suggests that the interaction of solute atoms and moving dislocation is responsible for the observed serrated flow in this alloy. © 2007 Elsevier B.V. All rights reserved  

A thermo-mechanical model is developed to predict metal behavior during hot working operations. At first, a neural network model is trained to calculate flow stress of deforming metal as a function of temperature, strain and strain rate and then by coupling the neural network model and a thermo-viscoplastic finite element model, temperature and velocity fields during hot open die forging process are predicted. To examine the model, hot nonisothermal upsetting on a low carbon steel is performed while force-displacement behavior and temperature history during hot working are recorded. A good agreement is observed between the predicted data and the measured results. © 2007 Elsevier B.V. All... 

A modified analytical model is developed for analysis of 3D elastic stress fields in short fiber composites subjected to an applied axial load. Two sets of exact displacement solutions for the matrix and fiber are derived based on the theory of elasticity. The superposition state of these solutions are then used to obtain the analytical expressions for the 3D stress field components over the entire composite system including the fiber end region, which is modeled by the use of imaginary fiber technique. The main difference with the previous works here is that the stress field is considered to be a function of both radial and axial directions. Such an assumption made it possible to calculate... 

The hot deformation behavior of A-286 superalloy has been characterized using hot compression experiments in the temperatures between 1000 and 1100 °C and strain rates varying between 0.001 and 0.1 s-1. In addition, hot workability of this alloy has been analyzed by employing flow-localization parameter. The results show that both kinds of softening mechanism, dynamic recovery and dynamic recrystallization, occur during hot working, where at 1000 °C the main mechanism is dynamic recovery and at higher temperatures and strain rate of 0.001-0.01 s-1 dynamic recrystallization takes place. Calculations demonstrates that this alloy mainly have a good workability for the utilized deformation... 

In this paper, two corotational modeling for elastic-plastic, mixed hardening materials at finite deformations are introduced. In these models, the additive decomposition of the strain rate tensor as well as the multiplicative decomposition of the deformation gradient tensor is used. For this purpose, corotational constitutive equations are derived for elastic-plastic hardening materials with the non-linear Armstrong-Frederick kinematic hardening and isotropic hardening models. As an application of the proposed constitutive modeling, the governing equations are solved numerically for the simple shear problem with different corotational rates and the stress components are plotted versus the... 

In this study, hot rolling behavior of a low alloy steel in the dual-phase region is studied. The effects of various process parameters such as initial temperature, soaking time, rolling speed and the cooling conditions are investigated. Then, mechanical testing and microstructural studies are performed and the effects of process parameters are studied and finally the optimum rolling program is determined based on the achieved results. The results show that rolling speed significantly alters the final microstructures and mechanical properties. Higher rolling speed results higher volume fraction of ferrite phase. In addition, the optimum dual-phase microstructure for this steel can be... 

In this study, the kinetics of dynamic recrystallization is predicted under hot rolling conditions employing a first-order rate equation. The proposed model considers the effects of temperature and strain rate variations on recrystallization kinetics by simultaneously modelling heat transfer and plastic deformation phenomena. To do so, a two-dimensional finite element method is coupled with the transformation equation to calculate the temperature distribution and the strain rate field as well as the kinetics of dynamic recrystallization within the metal, concurrently, The model developed was examined for a low carbon steel. A series of hot compression tests were carried out at different... 

The hot working behavior of A286 was studied using hot compression tests over temperature range of 950-1100 °C and at strain rates of 0.001-1 s -1. The flow curves of the material over the studied temperatures and strain rates were typical of dynamic recrystallization. However, some points reflected a change in the mechanism of softening with the change of strain rate. The relation between flow stress, deformation temperature and strain rate was examined via power-law, hyperbolic sine and exponential constitutive equations and the hyperbolic sine function was found more appropriate. The peak strain increased with strain rate up to 0.01 s-1 and then unexpectedly decreased at higher strain... 

The 7075 aluminium alloy is one of the most important engineering alloys utilised extensively in aircraft and transportation industries due to its high specific strength. In the present research, the flow behaviour of this alloy has been investigated using hot compression test at strain rates of 0.001, 0.01, 0.1 and 1 s-1 and temperatures of 350, 400 and 450°C. The results reveal that dynamic softening occurred in these temperatures and strain rates. The activation energy, strain rate sensitivity and two constitutive equations (hyperbolic sine law and the power law) are derived from the results. It is shown that the hyperbolic sine law has a better agreement with the experimental results  

The dynamic and static softening phenomena in Al-6Mg alloy were studied through interrupted two-stage hot compression test performed isothermally at 480°C and strain rate range of 0.001-0.1 s-1. The interruptions of 29 and 90 s were considered when the true strain reached 0.5. It was concluded that the effect of static softening on the flow stress was not highlighted by extending the interruption at a constant strain rate. Also, it was exhibited that softening rate highly enhanced with the strain rate decrement at a constant time. Moreover, the static and dynamic recrystallization was revealed as the dominant softening mechanisms at low and high strain rates, respectively  

In order to analyze the flow behavior and workability of Ni–42Cu in cast and wrought conditions, hot deformation tests were performed at temperatures and strain rates within the ranges of 900–1150 °C and 0.001–1 s−1, respectively. Tensile tests showed a “hot ductility trough” at 950 °C for both alloys. The drop in hot ductility was more considerable in the cast alloy because of the sluggish dynamic recrystallization. The hot ductility drop and grain boundary cracking, particularly in the cast alloy, were attributed to the segregation of detrimental atoms to the boundaries. It was shown that the hot ductility of the wrought alloy could be improved with increasing strain rate. It was... 

Flow stress behavior of AA5086 was determined using tensile testing at different temperatures from room temperature to 500 °C and strain rates varying between 0.002 and 1 s−1. The strain rate sensitivity parameter and occurrence of dynamic strain aging were then investigated in which an Arrhenius-type model was employed to study the serrated flow. Additionally, hot deformation behavior at temperatures higher than 320 °C was evaluated utilizing hyperbolic-sine constitutive equation. Finally, a feed forward artificial neural network model with back propagation learning algorithm was proposed to predict flow stress for all deformation conditions. The results demonstrated that the strain rate... 

In this study, the influence of temperature on serrated flow behavior, strain rate sensitivity (SRS) and effective activation volume in a Ti-based bulk metallic glass (BMG) bent at different strain rates has been investigated. Results reveal that the fundamental origin of shear deformation is related to the time-dependent structural relaxations, which occur during deformation at special temperatures and strain rates. Furthermore, the localized structural relaxation processes at intermediate temperatures cause an inelastic shuffling within cages and mitigate the activation of STZs. The main reason of negative SRS at intermediate temperatures is due to the insufficient time for structural...