Sharif Digital Repository

In this paper, a new rate type endochronic constitutive model is introduced to describe deformations in the finite strain range. A new material dependent objective rate of Cauchy stress is suggested based on the general form of spin tensors, defining objective stress rates. The endochronic constitutive equations are extended using the concept of corotational stress rates and additive decomposition of deformation rate. The constitutive relations are specialized for thin-walled tubes under torsion and a procedure for solving the ordinary differential equations for cases of simple and pure torsion is developed. The axial effects for various materials, subjected to simple and pure torsion, are... 

The delamination phenomenon has been studied in macroscopic scale by different approaches. Here, using micro-mechanical models, it is tried to find the possible relationship between the problem and the stress fields around a fiber. Using the ideas and the results on the free surface cracking of unidirectional composite, the FEM model is developed. The model verification is done by comparing the results by the results obtained in presence of damaged incorporated by the damage theory. Here, on the free surfaces fiber/matrix debonding is applied as a composite damage and the results show a good agreement with the available results  

In current studies, equal channel angular pressing process (ECAP) and modified ECAP processes are simulated under ideal conditions to compare the deformation characteristics. The deformation behaviour is more complicated and the strain induced during the processes is highly non-uniform in the modified ECAP processes except in the equal channel multi-angular pressing (ECMAP) process with Route C. The strain homogeneity is more of a possibility with ECAP and ECMAP with Route C processes. The deformation stress state is widely distributed in nature in modified ECAP processes than in conventional ECAP. In addition, the load requirements are also higher in modified ECAP processes with that of the... 

An integrated mathematical model was developed to study the thermo-mechanical behavior of strips and work-rolls during warm rolling process of steels. A two-dimensional finite element analysis was first employed to solve for the thermo-mechanical response of the rolled strip under steady-state conditions. The calculated roll pressure and temperature fields were then used to apply proper boundary conditions for solving the governing thermo-mechanical problem for the work-roll. The obtained results indicate that in warm strip rolling of steels, the thermal and mechanical stresses developed within the work-roll are comparable; however, the more significant influence is due to heating and... 

Recent research studies on ductile fracture of metals have shown that the ductile fracture initiation is significantly affected by the stress state. In this study, the effects of the stress invariants as well as the effect of the reverse loading on ductile fracture are considered. To estimate the reduction of load carrying capacity and ductile fracture initiation, a scalar damage expression is proposed. This scalar damage is a function of the accumulated plastic strain, the first stress invariant and the Lode angle. To incorporate the effect of the reverse loading, the accumulated plastic strain is divided into the tension and compression components and each component has a different weight... 

In this study, based on the reduced form of elasticity displacement field for a long laminate, an analytical method is established to exactly obtain the interlaminar stresses near the free edges of generally laminated composite plates subjects to extension, torsion, and bending. The constant parameters being in the displacement field, which describe the global deformation of a laminate, are appropriately calculated by using the improved first-order shear deformation theory. Reddy's layerwise theory is subsequently employed for analytical and numerical examinations of the boundary layer stresses within arbitrary laminated composite plates. Various numerical results are developed for the... 

Accurate prediction of static and dynamic response of nano structures is one of the important interests of scientists in the last decade. Nano bearing as an important part of nano machines has been extensively implemented in recognizing and disassembling cancerous cells and building molecular support structures for strengthening bones. For this reason, Molecular Dynamic Method and some experimental methods are implemented in this area. As nano ball bearing is one of the most important components of nano machines, a large number of studies are concentrated to analyze it statically and dynamically. In this paper, a Fullerene is simulated by a spherical super element whose stress, deformation... 

In this study, based on the reduced from of elasticity displacement field for a long laminate, an analytical method is established to exactly obtain the interlaminar stresses near the free edges of generally laminated composite plates under the extension and bending. The constant parameters, which describe the global deformation of a laminate, are properly computed by means of the improved first-order shear deformation theory. Reddys layerwise theory is subsequently utilized for analytical and numerical examinations of the boundary layer stresses within arbitrary laminated composite plates. A variety of numerical results are obtained for the interlaminar normal and shear stresses along the... 

In aerospace applications structures weight is considered as a major performance parameter. One of the most effective manufacturing methods for weight reduction is superplastic forming. By this technique, not only the weight, but also the stress concentration, the cost, and the manufacturing time are noticeably reduced. Additionally, the components with complicated shapes could be formed in a single manufacturing step. Due to the wide demand for whiskers reinforced metal matrix composites (MMCs) in aerospace industries, many research works have been designed to extend the borders of superplastic forming to MMCs materials. In the present study, a 3-D symmetric micromechanical finite element... 

The effect of the interface stresses is studied upon the size-dependent elastic deformation of an elastic half-plane having a cylindrical inclusion with distinct elastic properties. The elastic half-plane is subjected to either a uniaxial loading at infinity or a uniform non-shear eigenstrain in the inclusion. The straight edge of the half-plane is either traction-free, or rigid-slip, or motionless, which represents three practical situations of mechanical structures. Using two-dimensional Papkovich-Neuber potentials and the theory of surface/interface elasticity, the elastic field in the elastic half-plane is obtained. Comparable with classical result, the new formulation renders the... 

A plastic temperature-dependent constitutive model is developed for 0.05 mm thickness pure aluminum diaphragms at large strains by using bulge test method. Rupture strain of the material is recorded below two percent according to tensile tests. In order to achieve the material behavior at larger strains, an own bulge test apparatus is used to extract equi-biaxial stress-strain curves at different temperatures, i.e. from room to 150 °C. Effective stress-strain behavior is then computed via a transformation scheme using the plastic work definition as well as the room temperature uniaxial stress-strain curve. It is illustrated that the Johnson-Cook constitutive model is not able to capture this... 

Single lap-shear (SLS) specimens of 63Sn37Pb solder joints were prepared with three different adherend thicknesses at three varying joint lengths. The fracture force was measured at a shear strain rate of 0.01 s-1 for different geometries. The elastic-plastic fracture mechanics (EPFM) theory was used to find the energy dissipated in each case using a finite element model (FEM), and the fracture energy was obtained by cohesive zone modeling (CZM). Both 2-D and 3-D models were used to explain the variations in fracture energy by the level of constraint on the joint. Also, the plastic zone area and stress distribution along the solder layer were calculated at the moment of fracture. A phase...