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Showing 24 results for Plasticity

S.a. Sadrnejad,
Volume 4, Issue 2 (6-2006)

An important concern in rock mechanics is non-homogeneity as joints or fault. Adopting the joints as fractures, fractures are well known for their effects on the mechanical and transport properties of rock. It has been postulated that through fractured/jointed rock, mainly, the polygons turned to the shear vector (ti) are involved in the mobilization of shear resistance. Consequently, in order to locate the contact areas implicated into the shear-test it was firstly necessary to fix the shear direction. Moreover, since laboratory observations clearly show that only the steepest polygon surfaces touch the other sample, the identification of the potential sliding areas only requires the determination of the polygons which are faced to the shear direction and which, among them, are steep enough to be involved. The methodology to be discussed here is modeling of slip on the local and global levels due to the distribution of deformation procedure of the rock joint. Upon the presented methodology, more attention has been given to slip initiation and propagation through rock joint. In particular, softening in non-linear behaviour of joint in going from the peak to residual strengths imparts a behaviour often associated with progressive failure. A multi-plane based model is developed and used to compute plastic strain distribution and failure mechanism of rock joints. Validity of the presented model was examined by comparing numerical and test results showing the behavior of both homogeneous and jointed rock samples under general stress conditions.
S.a. Sadrnejad, M. Labibzadeh,
Volume 4, Issue 4 (12-2006)

Analysis and prediction of structural response to static or dynamic loading requires prediction of concrete response tovariable load histories. The constitutive equations for the mechanical behavior of concrete capable of seeing damage effects or crack growth procedure under loading/unloading/reloading was developed upon micro-plane framework. The proposed damage formulation has been built on the basis of five fundamental types of stress/strain combinations, which essentially may occur on any of micro-planes. Model verification under different loading/unloading/reloading stress/strain paths has been examined. The proposed model is capable of presenting pre-failure history of stress/strain progress on different predefined sampling planes through material. Many of mechanical behavior aspects happen during plasticity such as induced anisotropy, rotation of principal stress/strain axes, localization of stress/strain, and even failure mechanism are predicted upon a simple rational way and can be presented.
S. Mohammadi, A. Bebamzadeh,
Volume 4, Issue 4 (12-2006)

Explosion has always been regarded as one of the most complicated engineering problems. As a result, many engineers have preferred rather simplified empirical approaches in comparison to extremely complex deterministic analyses. In this paper, however, a numerical simulation based on the combined finite/discrete element methodology is presented for analyzing the dynamic behavior of fracturing rock masses in blasting. A finite element discretization of discrete elements allows for complex shapes of fully deformable discrete elements with geometric and material nonlinearities to be considered. Only a Rankine strain softening plasticity model is employed, which is suitable for rock and other brittle materials. Creation of new lines/edges/bodies from fracturing and fragmentation of original objects is systematically considered in the proposed gas-solid interaction flow model. An equation of state is adopted to inexpensively calculate the pressure of the detonation gas in closed form. The model employed for the flow of detonation gas has resulted in a logical algorithmic procedure for the evaluation of spatial distribution of the pressure of detonation gas, work done by the expanding gas and the total mass of the detonation gas as functions of time indicating the ability of model to respond to changes in both the mass of explosive charge and the size of the solid block undergoing fracture. Rock blasting and demolition problems are amongst the engineering applications that are expected to benefit directly from the present development. The results of this study may also be used to provide some numerical based reliable solutions for the complex analysis of structures subjected to explosive loadings.
A.a. Khosroshahi, S.a. Sadrnejad,
Volume 5, Issue 1 (3-2007)

A framework for development of constitutive models including damage progress, based on semi-micromechanical aspects of plasticity is proposed for concrete. The model uses sub-loading surface with multilaminate framework to provide kinematics and isotropic hardening/softening in the ascending/descending branches of loading and can be able to keep stress/stain paths histories for each plane separately. State of stresses on planes is divided to four basic stress patterns i.e. pure compression, increasing compression-and shear, decreasing compression-shear and tension-shear and used in derivation of plasticity equations. Under this kind of categorized form the model is capable of predicting behavior of concrete under any stress/strain path such as uniaxial, biaxial and triaxial in the monotonic and cyclic loading, Also this model is capable of predicting the effects of principal stress/strain axes rotations and consequent plastic flow and has the potential to simulate the behavior of material with anisotropy, fabric pattern, slip/weak planes and crack opening/closing. The material parameters of model are calibrated by optimum fitting of the basic test data available in the literature. The model results under both monotonic and cyclic loading have been compared with experimental results to show capability of model.
Mahmood R. Abdi, Ali Parsapajouh, Mohammad A. Arjomand,
Volume 6, Issue 4 (12-2008)

Clay soils and their related abnormal behavior such as excessive shrinkage, swelling, consolidation settlement and cracking on drying has been the subject of many investigations. Previous studies mainly evaluated the effects of additives such as lime, cement and sand on these characteristics. Initial results indicated that the soil characteristics were improved. However, reportedly in many cases, these additives resulted in a decrease in plasticity and increase in hydraulic conductivity. As a result, there has been a growing interest in soil/fiber reinforcement. The present investigation has focused on the impact of short random fiber inclusion on consolidation settlement, swelling, hydraulic conductivity, shrinkage limit and the development of desiccation cracks in compacted clays. To examine the possible improvements in the soil characteristics, samples consisting of 75% kaolinite and 25% montmorillonite were reinforced with 1, 2, 4 and 8 percent fibers as dry weight of soil with 5, 10 and 15mm lengths. Results indicated that consolidation settlements and swelling of fiber reinforced samples reduced substantially whereas hydraulic conductivities increased slightly by increasing fiber content and length. Shrinkage limits also showed an increase with increasing fiber content and length. This meant that samples experienced much less volumetric changes due to desiccation, and the extent of crack formation was significantly reduced.
S.a. Naeini, R. Ziaie_moayed,
Volume 7, Issue 2 (6-2009)

In recent years, soil reinforcement is considered of great importance in many different civil projects. One of the most significant applications of soil reinforcement is in road construction. Sub grade soil and its properties are very important in the design of road pavement structure. Its main function is to give adequate support to the pavement from beneath. Therefore, it should have a sufficient load carrying capacity. The use of geosynthetics in road and airfield construction has shown the potential to increase the soil bearing capacity. One category of geosynthetics to particular, geogrid, has gained increasing acceptance in road construction. A geogrid is a geosynthetic material consisting of connected parallel sets of tensile ribs with apertures of sufficient size to allow strike-through of surrounding soil, stone, or other geotechnical material. Geogrid reinforcement of sub grade soil is achieved through the increase of frictional interaction between the soil and the reinforcement. Geogrid have been successfully used to provide a construction platform over subgrades. In this application, the geogrid improves the ability to obtain compaction in overlying aggregates, while reducing the amount of material required be removing and replacing. Relative agreement exists that substantial benefits can be achieved from the inclusion of geogrids within the pavement systems however, the quantity of the improvement is in relative disagreement. This paper presents the effects of plasticity index and also reinforcing of soft clay on CBR values. Three samples of clay with different plasticity index (PI) values are selected and tested without reinforcement. Then by placing one and two layer of geogrid at certain depth within sample height, the effects of reinforcement and PI on CBR values are investigated in both soaked and unsoaked conditions. The results shows that as the PI increase the CBR value decreases and reinforcing clay with geogrid will increase the CBR value.
S. A. Sadrnejad, S. A. Ghoreishian Amir,
Volume 8, Issue 2 (6-2010)

A semi-micromechanical multilaminate model is introduced here to predict the mechanical behavior of soils.

This model is like a bridge between micro and macro scale upon the satisfaction of minimum potential energy level

during any applied stress/strain increments. The concept of this model is based on a certain number of sampling planes

which constitute the elastic-plastic behavior of the soil. The soil behavior presents as the summation of behavior on

these planes. A simple unconventional constitutive equations are used in each of the planes to describe the behavior

of these planes separately. An unconventional plasticity can predict the soil behavior as a smooth curve with

considering plastic deformation due to change of stress state inside the yield surface. The model is capable of

predicting softening behavior of the soil in a reasonable manner due to using unconventional plasticity. The influences

of induced anisotropy are included in a rational way without any additional hypotheses owing to in-nature properties

of the multilaminate framework. Results of this model are compared with test data and reasonable agreement is found.

O. Omidi, V. Lotfi,
Volume 8, Issue 3 (9-2010)

 Neither damage mechanics model nor elastoplastic constitutive law can solely describe the behavior of concrete satisfactorily. In fact, they both fail to represent proper unloading slopes during cyclic loading. To overcome the disadvantages of pure plastic models and pure damage approaches, the combined effects need to be considered. In this regard, various classes of plastic-damage models have been recently proposed. Here, the theoretical basics of the plastic-damage model originally proposed by Lubliner et al. and later on modified by Lee and Fenves is initially presented and its numerical aspects in three-dimensional space are subsequently emphasized. It should be mentioned that a part of the implementation in 3-D space needs to be reformulated due to employing a hyperbolic potential function to treat the singularity of the original linear form of plastic flow proposed by Lee and Fenves. The consistent algorithmic tangent stiffness, which is utilized to accelerate the convergence rate in solving the nonlinear global equations, is also derived. The validation and evaluation of the model to capture the desired behavior under monotonic and cyclic loadings are shown with several simple one-element tests. These basic simulations confirm the robustness, accuracy, and efficiency of the algorithm at the local and global levels. At the end, a four-point bending test is examined to demonstrate the capabilities of the model in real 3-D applications.

Mahmoud Reza Abdi,
Volume 9, Issue 2 (6-2011)

The use of various slags as by-products of steel industry is well established in civil engineering applications. However, the use

of BOS slag in the area of soil stabilization has not been fully researched and developed despite having similar chemical

composition and mineralogy to that of Portland cement. This paper reports on efforts to extend the use of BOS slag to soil

stabilization by determining possible beneficial effects it may have on compressive strength and durability. Results of laboratory

tests conducted on kaolinite samples stabilized with lime and treated with various percentages of BOS slag are presented. Tests

determined strength development of compacted cylinders, moist cured in a humid environment at 35° C and durability by freezing

and thawing method. Results showed that additions of BOS slag to kaolinite samples singularly or in combination with lime

increased unconfined compressive strength and durability. These characteristics were significantly enhanced by the concurrent

use of lime and BOS slag for stabilization of kaolinite.

Seyed Amirodin Sadrnejad, Hamid Karimpour,
Volume 9, Issue 2 (6-2011)

The present paper is devoted to a new critical state based plasticity model able to predict drained and undrained behaviour of

granular material. It incorporates a bounding surface plasticity model describing in multilaminate framework to capitalize on

advantages of this mathematical framework. Most of the models developed using stress/strain invariants are not capable of

identifying the parameters depending on directional effects such as principal stress rotation and fabric this is mainly because

stress/strain invariants are scalar quantities. The principal features of this model can be postulated as considering both inherent

and induced anisotropy, principal stress rotation. Since the local instability of saturated sand within post-liquefaction is highly

dependent on the residual inherent/induced anisotropy, bedding plane effects and also the stress/strain path the new mode is

competent to be employed in this regard. The constitutive equations of the model are derived within the context of non-linear

elastic behaviour for the whole medium and plastic sliding of interfaces of predefined planes. As follows, the constitutive

equations are described in detail and then the experimental results and sensitive analysis of key material constants are shown

which all imply the power of the model in predicting of soil behaviour under any condition in soil structures.

Reza Abbasnia, Neda Mirzadeh, Kamyar Kildashti,
Volume 9, Issue 3 (9-2011)

In recent years, different damage indexes have been introduced in engineering literature. The most prominent one among other

counterparts is the 1985 Park and Ang's damage index (DIPA), which demonstrates well calibration against experimental

results. Hence, it has traditionally had broad application in the field of structural engineering. Commonly, in DIPA relevant

parameters are assessed based on plastic-hinge approach, which is not well suited to consider the coupled response between

stress resultants (axial force and flexural moment) especially in grossly nonlinear domain. The reason is that named approach

is utilized constant shape plastic moment-curvature curve, which is not capable of varying the shape throughout loading history.

Another drawback of plastic-hinge method is the difficulty of representing precisely partial yielding of the cross-section. To

remedy the situation, the fiber discretization technique is used in this paper. Based on the fiber discretization strategy, not only

have the stiffness and strength degradation been characterized more accurately, but also the distribution of plasticity along the

plastic zone has been considered. Besides, the multi-directional effect of axial force and flexural moment is considered to assess

DI parameters. Additionally, this strategy directly incorporates the effect of transverse confinement into cross sectional

constitutive behaviour.

A. Hassanipour, A. Shafiee, M.k. Jafari,
Volume 9, Issue 4 (12-2011)

Shear modulus and damping ratio are important input parameters in dynamic analysis. A series of resonant column tests was

carried out on pure clays and sand-clay mixtures prepared at different densities to investigate the effects of aggregate content,

confining stress, void ratio and clay plasticity on the maximum shear modulus and minimum damping ratio. Test results revealed

an increase in the maximum shear modulus of the mixture with the increase in sand content up to 60%, followed by a decrease

beyond this value. It was also found that the maximum shear modulus increases with confining stress, and decreases with void

ratio. In addition, minimum damping ratio increases with sand content and clay plasticity and decreases with confining stress.

Finally, on the basis of the test results, a mathematical model was developed for the maximum shear modulus.

M. Jahanandish, M. Veiskarami, A. Ghahramani,
Volume 9, Issue 4 (12-2011)

Foundations behavior is affected by soil behavior which can vary from dilative to contractive depending on the stress level,

particularly in dense frictional soils. The Zero Extension Lines (ZEL) method has been generally developed to predict the

foundations behavior. Knowledge of soil behavior enables the ZEL method to predict the general and local shear failure modes.

In this paper, a relatively simple work hardening/softening soil constitutive model is developed to represent dense frictional soils

behavior under different stress levels. This model is based on the accumulation of the plastic work during a simple direct shear

test and its relationship to stress ratio to establish the hardening law. Verifications have been made for the developed soil model.

The model is then implemented into the ZEL method to theoretically investigate the bearing capacity and load-displacement

behavior of foundations over dense frictional soils. Utilization of this model enables the ZEL method to capture different modes

of failure depending on the foundation size. A numerical study on foundations behavior was performed showing the ability of the

presented approach in capturing both failure modes.

A. R. Rahai, S. Fallah Nafari,
Volume 11, Issue 4 (12-2013)

The seismic behavior of frame bridges is generally evaluated using nonlinear static analysis with different plasticity models hence this paper tends to focus on the effectiveness of the two most common nonlinear modeling approaches comprising of concentrated and distributed plasticity models. A three-span prestressed concrete frame bridge in Tehran, Iran, including a pair of independent parallel bridge structures was selected as the model of the study. The parallel bridges were composed of identical decks with the total length of 215 meters supported on different regular and irregular substructures with non-prismatic piers. To calibrate the analytical modeling, a large-scale experimental and analytical seismic study on a two-span reinforced concrete bridge system carried out at the University of Nevada Reno was used. The comparison of the results shows the accuracy of analytical studies. In addition, close correlation between results obtained from two nonlinear modeling methods depicts that the lumped plasticity approach can be decisively considered as the useful tool for the nonlinear modeling of non-prismatic bridge piers with hollow sections due to its simple modeling assumption and less computational time.
A. R. Habibi, Keyvan Asadi,
Volume 12, Issue 1 (3-2014)

Setback in elevation of a structure is a special irregularity with considerable effect on its seismic performance. This paper addresses multistory Reinforced Concrete (RC) frame buildings, regular and irregular in elevation. Several multistory Reinforced Concrete Moment Resisting Frames (RCMRFs) with different types of setbacks, as well as the regular frames in elevation, are designed according to the provisions of the Iranian national building code and Iranian seismic code for the high ductility class. Inelastic dynamic time-history analysis is performed on all frames subjected to ten input motions. The assessment of the seismic performance is done based on both global and local criteria. Results show that when setback occurs in elevation, the requirements of the life safety level are not satisfied. It is also shown that the elements near the setback experience the maximum damage. Therefore it is necessary to strengthen these elements by appropriate method to satisfy the life safety level of the frames.
M. Afzalirad, M. Kamalian, M. K. Jafari, A. Sohrabi-Bidar,
Volume 12, Issue 1 (1-2014)

In this paper, an advanced formulation of time-domain, two-dimensional Boundary Element Method (BEM) with material damping is presented. Full space two-dimensional visco-elastodynamic time-convoluted kernels are proposed in order to incorporate proportional damping. This approach is applied to carry out site response analysis of viscoelastic topographic structures subjected to SV and P incident waves. Seismic responses of horizontally layered site, semi-circular canyons, slope topography and ridge sections subjected to these incident waves are analyzed in order to demonstrate the accuracy of the kernels and the applicability of the presented viscoelastic boundary element algorithm. The results show an excellent agreement with recent published results obtained in frequency domain. Also, the effects of different material damping ratios on site response are investigated.
J. Nazari Afshar, M. Ghazavi,
Volume 12, Issue 1 (1-2014)

The Stone-column is a useful method for increasing the bearing capacity and reducing settlement of foundation soil. The prediction of accurate ultimate bearing capacity of stone columns is very important in soil improvement techniques. Bulging failure mechanism usually controls the failure mechanism. In this paper, an imaginary retaining wall is used such that it stretches vertically from the stone column edge. A simple analytical method is introduced for estimation of the ultimate bearing capacity of the stone column using Coulomb lateral earth pressure theory. Presented method needs conventional Mohr-coloumb shear strength parameters of the stone column material and the native soil for estimation the ultimate bearing capacity of stone column. The validity of the developed method has been verified using finite element method and test data. Parametric studies have been carried out and effects of contributing parameters such as stone column diameter, column spacing, and the internal friction angle of the stone column material on the ultimate bearing capacity have been investigated.
O. Farzaneh, F. Askari, J. Fatemi,
Volume 12, Issue 4 (12-2014)

AWT IMAGEPresented is a method of two-dimensional analysis of the active earth pressure due to simultaneous effect of both soil weight and surcharge of strip foundation. The study’s aim is to provide a rigorous solution to the problem in the framework of upper-bound theorem of limit analysis method in order to produce some design charts for calculating the lateral active earth pressure of backfill when loaded by a strip foundation. A kinematically admissible collapse mechanism consisting of several rigid blocks with translational movement is considered in which energy dissipation takes place along planar velocity discontinuities. Comparing the lateral earth forces given by the present analysis with those of other researchers, it is shown that the results of present analysis are higher (better) than other researchers’ results. It was found that with the increase in AWT IMAGE, the proportion of the strip load (q) which is transmitted to the wall decreases. Moreover, Increasing the friction between soil and wall ( AWT IMAGE) will result in the increase of effective distance ( AWT IMAGE). Finally, these results are presented in the form of dimensionless design charts relating the mechanical characteristics of the soil, strip load conditions and active earth pressure.

Mr. Mehdi Goorani, Dr. Amir Hamidi,
Volume 13, Issue 2 (6-2015)

This paper presents a model for prediction of the mechanical behavior of sand-gravel mixtures using generalized plasticity and critical state concepts. Proposed model is based on the difference between critical state lines of sand and sand-gravel mixture in e-Lnp' plane. A generalized plasticity model is considered as the base model for sandy soil. Its state parameter, dilation rate and hardening function are modified to involve the effects of gravel particles on the behavior of mixture. Gravel content is considered as a physical parameter for determination of four new added parameters of the model. Verification of the proposed model performed considering four sets of experiments conducted by different researchers on poorly graded sand-gravel mixtures. According to the results, proposed model provides satisfactory qualitative and quantitative predictions of the behavior of sand-gravel mixture. Stress- strain behavior besides volumetric strains in drained condition and induced pore pressure during undrained loading are satisfactory predicted which indicates the possibility of its application in boundary value problems of geotechnical engineering.
Guray Arslan, Muzaffer Borekci, Muzaffer Balci, Melih Hacisalihoglu,
Volume 14, Issue 3 (4-2016)

The contribution of concrete to inelastic deformation capacity and shear strength of reinforced concrete (RC) columns failing in shear has been investigated extensively by various researchers. Although RC members are designed to have shear strengths much greater than their flexural strengths to ensure flexural failure according to the current codes, shear degradation of RC columns failing in flexure has not been studied widely. The aim of this study is to investigate the shear degradation of RC columns using finite element analyses (FEA). The results of FEA are compared with the results of experimental studies selected from literature, and it is observed that the lateral load-deflection curves of analysed columns are compatible with the experimental results. Twenty-six RC columns were analysed under monotonically increasing loads to determine the concrete contribution to shear strength. The results of analyses indicate that increasing the ratio of shear to flexural strength reduces the concrete contribution to shear strength of the columns.

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