Sharif Digital Repository

In order to predict the flow stress curves up to peak under the hot working conditions two crucially modified models are presented. The modification is done by considering the effects of both temperature and strain rate. Besides, the ability of prediction at very low strains is added to them. Furthermore a new mathematical model has been developed to estimate the stress-strain curves beyond the peak via a secant hyperbolic function. The predicted results are found to be in accord with empirical values  

A constitutive model is developed for a cemented gravelly sand. The model is based on the separation of the cemented soil to the uncemented part and cemented bonds and combining the mechanical behaviour of each part using consistency and energy equilibrium equations. The uncemented part was modelled using the Pastor et al. (1985) model for sands. A new model was developed and proposed for the cemented bonds. Combination of these parts resulted in the modelling of cemented soil behaviour with a very good consistency both in drained and undrained conditions. The pore pressure in undrained conditions and the volumetric strains in the drained state were also modelled successfully using this... 

In this paper, a unified explicit tensorial relation is sought between two stress tensors conjugate to arbitrary and general Hill strains. The approach used for deriving the tensorial relation is based on the eigenprojection method. The result is, indeed, a generalization of the relations that were derived by Farahani and Naghadabadi [1] in 2003 from a component to intrinsic form. The result is unified in the sense that it is valid for all cases of distinct and coalescent principal stretches. Also, in the case of three dimensional Euclidean inner product space, using the derived unified relation, some expressions for the conjugate stress tensors are presented. © Sharif University of... 

Most engineering components are subjected to multiaxial rather than uniaxial cyclic loading, which causes multiaxial fatigue. The pre-requisite to predict the fatigue life of such components is to determine the multiaxial stressstrain relationship. In this paper the multiaxial cyclic stress-strain model under proportional loading is derived using the modified power law stress-strain relationship. The equivalent strain amplitude consisted of the normal strain excursion and maximum shear strain amplitude is used in the proportional model to include the additional hardening effect due to nonproportional loading. Therefore a new multiaxial cyclic stress-strain relationship is devised for out of... 

The effect of strain-hardening exponent was studied on the occurrence of central burst defects in the aluminum wiredrawing process. The study showed that in addition to geometrical factors, strain hardening is also a significant parameter related to central burst defects. The possibility of formation of central burst increases with decrease in strain hardening exponent. The study also revealed that during multipass drawing, metals show different behavior with respect to the central burst due to a new yield stress in each pass, especially during cold drawing  

The size of the rock specimen affects the stress concentrates in the vicinity of the top/bottom of the rock specimen during the evaluation of the geomechanical parameters in the laboratory, which causes unreliable results. However, the appropriate size for geomechanical evaluation is not well understood yet because of limitations in the sampling and analysis. In this study, a series of numerical simulations using a finite element package was conducted to investigate the effect of sample aspect ratio, fluid saturation, and porosity, on the mechanical behavior of the rock under elastic and poroelastic conditions. In addition, two concepts, stress/strain homogeneity index (SHI) and... 

The linear unfolding inverse finite element method (IFEM) has been modified and enhanced by implementing large deformation relations. The method is helpful to predict forming severity of the part that should be deep drawn as well as its blank shape and strain distribution in preliminary design stage. The approach deals with minimization of potential energy and large deformation relations with membrane elements. To reduce the computation time, the part is unfolded properly on the flat sheet and treated as 2D problem. Moreover, the nonlinear stress-strain relationship of plastic material properties has been considered to increase the accuracy of the results. An experiment set up has been... 

In this paper, the predictions of different beam theories for the behavior of a shape memory polymer (SMP) beam in different steps of a thermomechanical cycle are compared. Employing the equilibrium equations, the governing equations of the deflection of a SMP beam in the different steps of a thermomechanical cycle, for higher order beam theories (Timoshenko Beam Theory and von-Kármán Beam Theory), are developed. For the Timoshenko Beam Theory, a closed form analytical solution for various steps of the thermomechanical cycle is presented. The nonlinear governing equations in von-Kármán Beam theory are numerically solved. Results reveal that in the various beam length to beam thickness... 

A thermodynamically-consistent phase field approach for crack propagation which includes the following novel features is presented. (1) Scale dependency was included by relating the length scale to the number of cohesive interatomic planes at the crack tip. Because of this, the developed theory is applicable from the atomistic to the macroscopic scales. (2) The surface stresses (tension) are introduced by employing some geometrical nonlinearities even in small strain theory. They produce multiple contributions to the Ginzburg-Landau equation for crack propagation. (3) Crack propagation in the region with compressive closing stresses is eliminated by employing a stress-state-dependent kinetic... 

In this paper, the mechanical properties of graphene nanosheets are evaluated based on the nonlinear modified Morse model. The interatomic interactions including stretching and bending of the covalent bonds between carbon atoms, are replaced by nonlinear extensional and torsional spring-like elements. The finite element method is implemented to analyze the model under different loading conditions and linear characteristics of the graphene structure including the Young's modulus, surface modulus, shear modulus and Poisson's ratio are evaluated for various geometries and chirality where these properties are shown to be size and aspect ratio dependent. It is also found that when the dimensions... 

This paper presents the results of experimental and analytical study on a new technique of seismic retrofitting of concrete columns. In this technique, standard strapping devices, which are used in the packaging industry, are used to post-tension metal strips around concrete columns. The two ends of the strip are tightened in sealed clamps to maintain the pre-stress force in the strip. Experimental program included axial compressive tests on 30 cylindrical as well as 30 prismatic small-scale columns which were actively confined by pre-stressed metal strips. Longitudinal and lateral strain of concrete and strain in the strips were monitored. Test Results showed significant increase in... 

Concrete is widely used for many practices in Civil Engineering. Therefore, an understanding of its behavior helps engineers and researchers to perform more accurate and cost-effective analyses and designs. In this respect, several models have been proposed to predict the behaviors of concrete most of which are satisfactorily accurate. However, by increasing the accuracy of the models, their computational cost increases, too. In this study, a model with the least computational cost is proposed to predict the behaviors of various concretes and confinement conditions. This model does not require any experimental tests to determine its parameters. It was proved to be able to predict 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... 

In this work, the human lung alveoli are idealized by a three dimensional honeycomb like geometry and a fluid-structure analysis is performed to study the normal breathing mechanics. In contrast to previous works in which the inlet flow rate is predefined, in this model, we have applied a negative pressure on the outside surface of the alveolus which causes air to flow in and out of the alveolus. The integration of the experimental curve of breathing flow rate was used to approximate the shape of the external applied pressure. Our Fluid-Structure Interaction (FSI) model has an advantage over other literature since it addresses both the fluid dynamics and solid mechanics, simultaneously. The... 

This paper investigates the behaviour of C-shaped and L-shaped angle shear connectors embedded in solid concrete slabs. An effective finite element model is proposed to simulate the push out tests of these shear connectors that encompass nonlinear material behaviour, large displacement and damage plasticity. The finite element models are validated against test results. Parametric studies using this nonlinear model are performed to investigate the variations in concrete strength and connector dimensions. The finite element analyses also confirm the test results that increasing the length of shear connector increases their shear strength proportionately. It is observed that the maximum stress... 

In this paper, a hierarchical RVE-based continuum-atomistic multi-scale procedure is developed to model the nonlinear behavior of nano-materials. The atomistic RVE is accomplished in consonance with the underlying atomistic structure, and the inter-scale consistency principals, i.e. kinematic and energetic consistency principals, are exploited. To ensure the kinematic compatibility between the fine- and coarse-scales, the implementation of periodic boundary conditions is elucidated for the fully atomistic method. The material properties of coarse-scale are modeled with the nonlinear finite element method, in which the stress tensor and tangent modulus are computed using the Hill-Mandel... 

In this paper, Mindlin's second strain gradient theory is formulated and presented in an arbitrary orthogonal curvilinear coordinate system. Equilibrium equations, generalized stress-strain constitutive relations, components of the strain tensor and their first and second gradients, and the expressions for three different types of traction boundary conditions are derived in any orthogonal curvilinear coordinate system. Subsequently, for demonstration, Mindlin's second strain gradient theory is represented in the spherical coordinate system as a highly-practical coordinate system in nanomechanics. Second strain gradient elasticity have been developed mainly for its ability to capture the...