The aim of this study was to fabricate YAG/Al2O3 ceramic composites with different alumina nanopa... more The aim of this study was to fabricate YAG/Al2O3 ceramic composites with different alumina nanoparticles using slip casting and the atmospheric sintering process. In addition, some mechanical properties such as hardness and elastic modules of this novel ceramics were evaluated using the nanoindention technique. The results showed that the rheological behavior of the slurry was optimized to the solid loading of 55 wt% and the relative density of the green body was enhanced up to 65%. Relative density was increased after sintering at 1700 °C for 12 h to 99.5% and the pore size (150 nm) was reduced to half of that of powder particles. It should be noted that the optimum amount of alumina nanoparticles as a reinforcing agent in the matrix was less than 5%wt and the composite hardness was increased to 7.3%, as compared to the pure YAG ceramic.
The viewpoint about metaphor has completely changed from the past to the present especially after... more The viewpoint about metaphor has completely changed from the past to the present especially after the theory of metaphor by George Laykoff and Mark Johnson enters into the field of cognitive linguistics. Based on this approach, metaphors reflect the conceptual system of the human mind that is influenced by its culture and experiences in the form of language. In this regard, it should be noted that one of the problems of translating poetry and specially Hafez's poem is linguistic and cultural concepts. In this article, we examine the methods used by the two Arab translators, Ibrahim al-Shvarabi and Omar Shebli, in translating the metaphors of the Hafez Divan, and show the impact of the methods employed in the translation of metaphors on the representation of cultural identity. For this purpose, we highlighted Hafez ghazals’ metaphors (50 ghazals of Divan of Hafez based on Qazvini version) according to classic comments, then explaining the method of translating each metaphor using...
Mechanical properties of slab tracks on a foundation with nonlinear stiffness are accounted for. ... more Mechanical properties of slab tracks on a foundation with nonlinear stiffness are accounted for. At first, the cracking stages were inspected in FEM models, and it was gathered that slab tracks have one-way flexural behavior. Secondly, experimental full-scale models were made, and the accuracy of analyses was verified by comparing theFEM load-deflection curves with those of previous studies and validating the cracking and ultimate loads with those obtained from experiments. Finally, the effects of several parameters on the cracking and ultimate loads and the energy absorption of steel fiber-reinforced slab tracks were investigated by examining the real behavior of slab tracks on elastic foundations before and after cracking. Steel fibers increased the compressive and flexural strengths as well as ductility and energy absorption. The 2.5 m width was the optimal width and the fracture pattern changed at this width. Finally, based on the obtained fracture loads, design curves were plot...
The effect of axial deformation of shell particles on the dynamic instability (flutter) of cantil... more The effect of axial deformation of shell particles on the dynamic instability (flutter) of cantilevered cylindrical shells made of functionally graded materials (FGM) under an end axial follower force is addressed. To this end, at first, results for free vibration of FGM cylindrical shells were verified with previous outcomes and they were in very good agreement. Then, the effect of axial deformation of the shell, acting like a reducing linearly-distributed follower load, on the critical circumferential mode number and the flutter load of FGM shells was accounted for. Finally, the effect of axial deformation of the shell particles on the critical circumferential mode number and the flutter load of FGM shells were investigated. In this case, three homogeneous shells with different elasticity moduli and densities and two FGM materials were considered: nickel-stainless steel and stainless steel-alumina. Results include the increasing critical circumferential mode number and the increas...
The use of non-linear nite elements to assess the e ect of transverse reinforcement amount on the... more The use of non-linear nite elements to assess the e ect of transverse reinforcement amount on the shape of shear-torsion interaction curve for RC beams is addressed. At rst, FEM results for specimens under pure torsion and combined shear and torsion were validated with previous outcomes. Secondly, reference specimens were made with the minimum required transverse reinforcement at rst. Each specimen was analyzed with the nite element method and tested under similar conditions in the laboratory, and the results, including the cracking pattern, cracking and ultimate loads, and shear-torsion interaction curves, were found to be in very good agreement. Finally, the transverse reinforcement amount was increased to certain extents and similar results were determined. FEM interaction curve stood below the experimental curve. It was also deduced that, from a certain increase in the transverse reinforcement, the shear-torsion interaction curve transforms from linear to elliptical.
The effect of axial deformation of shell particles on the dynamic instability (flutter) of cantil... more The effect of axial deformation of shell particles on the dynamic instability (flutter) of cantilevered cylindrical shells made of functionally graded materials (FGM) under an end axial follower force is addressed. To this end, at first, results for free vibration of FGM cylindrical shells were verified with previous outcomes and they were in very good agreement. Then, the effect of axial deformation of the shell, acting like a reducing linearly-distributed follower load, on the critical circumferential mode number and the flutter load of FGM shells was accounted for. Finally, the effect of axial deformation of the shell particles on the critical circumferential mode number and the flutter load of FGM shells were investigated. In this case, three homogeneous shells with different elasticity moduli and densities and two FGM materials were considered: nickel-stainless steel and stainless steel-alumina. Results include the increasing critical circumferential mode number and the increas...
The use of non-linear nite elements to assess the e ect of transverse reinforcement amount on the... more The use of non-linear nite elements to assess the e ect of transverse reinforcement amount on the shape of shear-torsion interaction curve for RC beams is addressed. At rst, FEM results for specimens under pure torsion and combined shear and torsion were validated with previous outcomes. Secondly, reference specimens were made with the minimum required transverse reinforcement at rst. Each specimen was analyzed with the nite element method and tested under similar conditions in the laboratory, and the results, including the cracking pattern, cracking and ultimate loads, and shear-torsion interaction curves, were found to be in very good agreement. Finally, the transverse reinforcement amount was increased to certain extents and similar results were determined. FEM interaction curve stood below the experimental curve. It was also deduced that, from a certain increase in the transverse reinforcement, the shear-torsion interaction curve transforms from linear to elliptical.
A micromechanics-based ductile fracture initiation theory is developed and applied for high-throu... more A micromechanics-based ductile fracture initiation theory is developed and applied for high-throughput assessment of ductile failure in plane stress. A key concept is that of inhomogeneous yielding such that microscopic failure occurs in bands with the driving force being a combination of band-resolved normal and shear tractions. The new criterion is similar to the phenomenological Mohr–Coulomb model, but the sensitivity of fracture initiation to the third stress invariant constitutes an emergent outcome of the formulation. Salient features of a fracture locus in plane stress are parametrically analyzed. In particular, it is shown that a finite shear ductility cannot be rationalized based on an isotropic theory that proceeds from first principles. Thus, the isotropic formulation is supplemented with an anisotropic model accounting for void rotation and shape change to complete the prediction of a fracture locus and compare with experiments. A wide body of experimental data from the ...
Journal of the Mechanics and Physics of Solids, 2021
Abstract An isotropic multi-surface model of porous material plasticity is derived and employed t... more Abstract An isotropic multi-surface model of porous material plasticity is derived and employed to investigate the effects of the third stress invariant in ductile failure. The constitutive relation accounts for both homogeneous and inhomogeneous yielding of a material containing a random distribution of voids. Individual voids are modeled as spheroidal but the aggregate has no net texture. Ensemble averaging is invoked to operate a scale transition from the inherently anisotropic meso-scale process of single-void growth and coalescence to some macroscopic volume that contains many voids. Correspondingly, expressions for effective yield and associated evolution equations are derived from first principles, under the constraint of persistent isotropy. It is found that the well-known vertex on the hydrostatic axis either disappears for sufficiently flat voids or develops into a lower-order singularity for elongated ones. When failure is viewed as the onset of an instability, it invariably occurs after the transition to inhomogeneous yielding with the delay between the two depending strongly upon the Lode parameter. The strain to failure is found to be weakly dependent on the Lode parameter for shear-dominated loadings, but strongly dependent on it near states of so-called generalized tension or compression. Experimentally determined fracture loci for near plane stress states are discussed in light of the new findings.
Proper design and optimization in stent parameters are required for efficient use of stents in co... more Proper design and optimization in stent parameters are required for efficient use of stents in coronary arteries. Efficiency depends on features including resistance against vessel pressures, (Radial Strength) limited variation in length during expansion (Foreshortening & Recoil) and resistance of stents in periodic/cyclic pressure & tension of heart pulsation (Fatigue Life Cycle). It is customary to design a stent based upon materials endurance limit which is equivalent to 0.5 of the yield strength for stainless steel in order to accomplish physically infinite life cycle for the stent. Different atherosclerosis removal rates are reported by various stent designs. Hence, considering the significance of designs and costly classic methods of producing prototypes, using finite element analysis is beneficial due to the high efficiency and accuracy as well as lower costs. In this study a biocompatible polymeric stent (PLA) is designed by SolidWorks software and influence of designing parameters including longitudinal pattern (Cell) number, circular-circumferential pattern (Cell) number, connection of cells by N, M, or W type flex connector, radius curve, thickness, maximum and minimum width on the mechanical properties such as contact force, foreshortening and maximum stress are estimated by FEM on over 58000 models. The effective design parameters are subsequently optimized to obtain the best model for the stent. Performance of the optimized stent is then investigated in a simulated artery suffering from a 49% non-uniform plaque and after the implementation of the stent in the artery, the inner area of the cross section of the artery increased by 74 percent which demonstrates improvement in blood flow by 74 percent. Results can be useful in designing and optimizing stents for non-uniform restenosis conditions. c
An analytical micromechanics-based yield criterion is developed to describe both void growth and ... more An analytical micromechanics-based yield criterion is developed to describe both void growth and coalescence under combined tension and shear, with smooth transition between growth and coalescence, thus its name unified. The model is obtained by limit analysis over a cylindrical elementary cell embedding a coaxial cylindrical void of finite height. The velocity field employed is an extended counterpart of the discontinuous, yet kinematically admissible trial field utilized in a recent work. Plasticity in the deformable matrix is modeled using rate-independent J 2 flow theory, and the effective dissipation function is calculated by exact as well as approximate integration techniques, the latter generating a simpler flow potential. The model is aimed to predict void growth as well as coalescence by internal necking or shearing. The complete yield surface , being function of normal as well as shear stresses, exhibits curved and planar parts signifying void coalescence. The transition between the curved and planar parts is cornerless. The analytical predictions are compared to results of FEM single-step cell-model calculations of limit analysis executed on an identical geometry exposed to quasi-periodic boundary conditions.
We have carried out dilatant plasticity simulations to investigate the process of void-mediated f... more We have carried out dilatant plasticity simulations to investigate the process of void-mediated failure inside a shear band. The constitutive model accounts for possibly inhomogeneous flow within the band, void rotation and void elongation. We found that the material in the band may soften with no increase in the void volume fraction. For a given matrix hardening capacity, the rate of softening was found to depend strongly on the ratio of shear band width to in-plane void spacing. The emergent softening led to complete loss of load bearing capacity thereby providing a physical mechanism of failure in shear bands. The mechanism is consistent with essential features of shear-fractured specimens in terms of surface roughness, porosity and dimple shape.
Micromechanics-based constitutive relations for post-localization analysis are obtained, to be us... more Micromechanics-based constitutive relations for post-localization analysis are obtained, to be used in a multisurface representation of porous metal plasticity. Each yield surface involves a number of internal parameters. Hence, the constitutive relations must be closed with evolution equations for the internal parameters. The latter are essential to describing the gradual loss of load-bearing capacity under shear-dominated loading. We also briefly discuss potential void closure due to void rotation and elongation in shear and show additional details regarding the simulations reported in a recent paper (A mechanism of failure in shear bands (2018) Extreme Mechanics Letters, 23, pp. 67-71.) The method can be more broadly used in a range of ductile failure problems involving combined tension and shear loadings.
ABSTRACT The one-way (two-way) flexural strength of RCC prisms (circular slabs) reinforced with g... more ABSTRACT The one-way (two-way) flexural strength of RCC prisms (circular slabs) reinforced with glass fiber textiles is addressed. To this end, alkaline-resistant glass fiber textiles with three surface weights were used in the composite, the matrix concrete was designed with zero/nonzero slump, and the textiles were used with/without an intermediate layer provided by epoxy resin and sand mortar. Prisms were tested under a four-point loading apparatus and circular slabs were placed on simple supports under a central load. Effects of the amount and geometry of reinforcement, matrix workability, and the intermediate layer on the ultimate load and deflection were investigated. Results revealed that, with a specific reinforcement amount, there is an optimum textile tex for each case, depending on the matrix mix design and the presence of intermediate layer. Similar results were obtained in one-way and two-way bending tests.
International Journal of Structural Integrity, 2017
PurposeSteel shear walls have recently received exclusive remark. Respective of most building cod... more PurposeSteel shear walls have recently received exclusive remark. Respective of most building code requirements, design of shear wall vertical boundary elements (VBEs) and local boundary elements (LBEs) against web yielding triggers exaggerated stiffness. The extent of stiffness reduction effects in boundary elements thus calls for more exhaustive investigation. The paper aims to discuss these issues.Design/methodology/approachTo this end, FEM-based push-over curves demonstrating base shear vs roof displacement, and von Mises plastic strains were scrutinized in half-scale and full-size models. Analyses were in perfect conformity with experimental data.FindingsWith reference to the AISC requirement, up to 35 percent decrease in the VBE moments of inertia could be imparted in higher levels without the ultimate load capacity nor displacement to failure being reduced. Also considered was open shear walls with reduced or minimum-design LBEs, the latter being used in continuous or abridge...
Micromechanics-based constitutive relations are developed to model plasticity in solids with rela... more Micromechanics-based constitutive relations are developed to model plasticity in solids with relatively high levels of porosity. They are especially appropriate to model void coalescence in ductile materials. The model is obtained by limit analysis of a cylindrical cell containing a coaxial void of finite height with plastic flow confined to the ligaments, and loaded under combined tension and shear. Previously obtained analytical estimates were not upper-bound preserving when shear was present and, in addition, were assessed against numerical results obtained for different cell geometries. Here, a rigorous upper-bound model is developed and its predictions are consistently compared with finite-element based estimates of limit loads on the same cylindrical unit cell exploiting quasi-periodic boundary conditions. The numerical results are used to guide a heuristic modification of the model in order to capture the behavior for extremely flat or extremely elongated voids.
ABSTRACT In this paper, the effects of both pozzolans and (steel and poly-propylene) fibers on th... more ABSTRACT In this paper, the effects of both pozzolans and (steel and poly-propylene) fibers on the mechanical properties of roller compacted concrete are studied. Specimens for the experiments were made using a soil-based approach; thus, the Kango's vibration hammer was used for compaction. The tests in the first stage were carried out to determine the optimal moisture requirements for mix designs using cubic mm specimens. In the tests of the second stage, the mechanical behaviors of the main specimens made using the optimal moisture obtained in the previous stage were evaluated using 28, 90, and 210 day cubic specimens. The mechanical properties of RCC pavements were evaluated using a soil-based compaction method and the optimum moisture content obtained from the pertaining experiments, and by adding different percentages of Iranian pozzolans as well as different amounts of steel fibers, each one accompanied by 0.1% of poly-propylene fibers. Using pozzolans, maximum increase in compressive strength was observed to occur between 28 and 90 days of age, rupture modulus was found to decrease, but toughness indices did not change considerably. The influence of steel fibers on compressive strength was often more significant than that of PP fibers, but neither steel nor PP fibers did contribute to increase in the rupture modulus independently. Also, the toughness indices increased when steel fibers were used.
The literature lacks exhaustive study on CFDST hybrid columns circumscribed by FRP layers. Extend... more The literature lacks exhaustive study on CFDST hybrid columns circumscribed by FRP layers. Extended FEM analysis was carried out on 70 real-scale models with varying parameters including the material and number of FRP layers, concrete strength, length-to-diameter ratio (specific length), and hollow section ratio. Carbon fibers proved stronger than glass fibers, leading to higher ultimate stress associated with lower strain. Specimens with high specific lengths suffered from steel premature buckling, thus stiffened with steel plates. Specimens with various hollow section ratios were finally compared, showing an increase range of 70% between the maximum (0.75) and minimum (0.25) ratios.
The aim of this study was to fabricate YAG/Al2O3 ceramic composites with different alumina nanopa... more The aim of this study was to fabricate YAG/Al2O3 ceramic composites with different alumina nanoparticles using slip casting and the atmospheric sintering process. In addition, some mechanical properties such as hardness and elastic modules of this novel ceramics were evaluated using the nanoindention technique. The results showed that the rheological behavior of the slurry was optimized to the solid loading of 55 wt% and the relative density of the green body was enhanced up to 65%. Relative density was increased after sintering at 1700 °C for 12 h to 99.5% and the pore size (150 nm) was reduced to half of that of powder particles. It should be noted that the optimum amount of alumina nanoparticles as a reinforcing agent in the matrix was less than 5%wt and the composite hardness was increased to 7.3%, as compared to the pure YAG ceramic.
The viewpoint about metaphor has completely changed from the past to the present especially after... more The viewpoint about metaphor has completely changed from the past to the present especially after the theory of metaphor by George Laykoff and Mark Johnson enters into the field of cognitive linguistics. Based on this approach, metaphors reflect the conceptual system of the human mind that is influenced by its culture and experiences in the form of language. In this regard, it should be noted that one of the problems of translating poetry and specially Hafez's poem is linguistic and cultural concepts. In this article, we examine the methods used by the two Arab translators, Ibrahim al-Shvarabi and Omar Shebli, in translating the metaphors of the Hafez Divan, and show the impact of the methods employed in the translation of metaphors on the representation of cultural identity. For this purpose, we highlighted Hafez ghazals’ metaphors (50 ghazals of Divan of Hafez based on Qazvini version) according to classic comments, then explaining the method of translating each metaphor using...
Mechanical properties of slab tracks on a foundation with nonlinear stiffness are accounted for. ... more Mechanical properties of slab tracks on a foundation with nonlinear stiffness are accounted for. At first, the cracking stages were inspected in FEM models, and it was gathered that slab tracks have one-way flexural behavior. Secondly, experimental full-scale models were made, and the accuracy of analyses was verified by comparing theFEM load-deflection curves with those of previous studies and validating the cracking and ultimate loads with those obtained from experiments. Finally, the effects of several parameters on the cracking and ultimate loads and the energy absorption of steel fiber-reinforced slab tracks were investigated by examining the real behavior of slab tracks on elastic foundations before and after cracking. Steel fibers increased the compressive and flexural strengths as well as ductility and energy absorption. The 2.5 m width was the optimal width and the fracture pattern changed at this width. Finally, based on the obtained fracture loads, design curves were plot...
The effect of axial deformation of shell particles on the dynamic instability (flutter) of cantil... more The effect of axial deformation of shell particles on the dynamic instability (flutter) of cantilevered cylindrical shells made of functionally graded materials (FGM) under an end axial follower force is addressed. To this end, at first, results for free vibration of FGM cylindrical shells were verified with previous outcomes and they were in very good agreement. Then, the effect of axial deformation of the shell, acting like a reducing linearly-distributed follower load, on the critical circumferential mode number and the flutter load of FGM shells was accounted for. Finally, the effect of axial deformation of the shell particles on the critical circumferential mode number and the flutter load of FGM shells were investigated. In this case, three homogeneous shells with different elasticity moduli and densities and two FGM materials were considered: nickel-stainless steel and stainless steel-alumina. Results include the increasing critical circumferential mode number and the increas...
The use of non-linear nite elements to assess the e ect of transverse reinforcement amount on the... more The use of non-linear nite elements to assess the e ect of transverse reinforcement amount on the shape of shear-torsion interaction curve for RC beams is addressed. At rst, FEM results for specimens under pure torsion and combined shear and torsion were validated with previous outcomes. Secondly, reference specimens were made with the minimum required transverse reinforcement at rst. Each specimen was analyzed with the nite element method and tested under similar conditions in the laboratory, and the results, including the cracking pattern, cracking and ultimate loads, and shear-torsion interaction curves, were found to be in very good agreement. Finally, the transverse reinforcement amount was increased to certain extents and similar results were determined. FEM interaction curve stood below the experimental curve. It was also deduced that, from a certain increase in the transverse reinforcement, the shear-torsion interaction curve transforms from linear to elliptical.
The effect of axial deformation of shell particles on the dynamic instability (flutter) of cantil... more The effect of axial deformation of shell particles on the dynamic instability (flutter) of cantilevered cylindrical shells made of functionally graded materials (FGM) under an end axial follower force is addressed. To this end, at first, results for free vibration of FGM cylindrical shells were verified with previous outcomes and they were in very good agreement. Then, the effect of axial deformation of the shell, acting like a reducing linearly-distributed follower load, on the critical circumferential mode number and the flutter load of FGM shells was accounted for. Finally, the effect of axial deformation of the shell particles on the critical circumferential mode number and the flutter load of FGM shells were investigated. In this case, three homogeneous shells with different elasticity moduli and densities and two FGM materials were considered: nickel-stainless steel and stainless steel-alumina. Results include the increasing critical circumferential mode number and the increas...
The use of non-linear nite elements to assess the e ect of transverse reinforcement amount on the... more The use of non-linear nite elements to assess the e ect of transverse reinforcement amount on the shape of shear-torsion interaction curve for RC beams is addressed. At rst, FEM results for specimens under pure torsion and combined shear and torsion were validated with previous outcomes. Secondly, reference specimens were made with the minimum required transverse reinforcement at rst. Each specimen was analyzed with the nite element method and tested under similar conditions in the laboratory, and the results, including the cracking pattern, cracking and ultimate loads, and shear-torsion interaction curves, were found to be in very good agreement. Finally, the transverse reinforcement amount was increased to certain extents and similar results were determined. FEM interaction curve stood below the experimental curve. It was also deduced that, from a certain increase in the transverse reinforcement, the shear-torsion interaction curve transforms from linear to elliptical.
A micromechanics-based ductile fracture initiation theory is developed and applied for high-throu... more A micromechanics-based ductile fracture initiation theory is developed and applied for high-throughput assessment of ductile failure in plane stress. A key concept is that of inhomogeneous yielding such that microscopic failure occurs in bands with the driving force being a combination of band-resolved normal and shear tractions. The new criterion is similar to the phenomenological Mohr–Coulomb model, but the sensitivity of fracture initiation to the third stress invariant constitutes an emergent outcome of the formulation. Salient features of a fracture locus in plane stress are parametrically analyzed. In particular, it is shown that a finite shear ductility cannot be rationalized based on an isotropic theory that proceeds from first principles. Thus, the isotropic formulation is supplemented with an anisotropic model accounting for void rotation and shape change to complete the prediction of a fracture locus and compare with experiments. A wide body of experimental data from the ...
Journal of the Mechanics and Physics of Solids, 2021
Abstract An isotropic multi-surface model of porous material plasticity is derived and employed t... more Abstract An isotropic multi-surface model of porous material plasticity is derived and employed to investigate the effects of the third stress invariant in ductile failure. The constitutive relation accounts for both homogeneous and inhomogeneous yielding of a material containing a random distribution of voids. Individual voids are modeled as spheroidal but the aggregate has no net texture. Ensemble averaging is invoked to operate a scale transition from the inherently anisotropic meso-scale process of single-void growth and coalescence to some macroscopic volume that contains many voids. Correspondingly, expressions for effective yield and associated evolution equations are derived from first principles, under the constraint of persistent isotropy. It is found that the well-known vertex on the hydrostatic axis either disappears for sufficiently flat voids or develops into a lower-order singularity for elongated ones. When failure is viewed as the onset of an instability, it invariably occurs after the transition to inhomogeneous yielding with the delay between the two depending strongly upon the Lode parameter. The strain to failure is found to be weakly dependent on the Lode parameter for shear-dominated loadings, but strongly dependent on it near states of so-called generalized tension or compression. Experimentally determined fracture loci for near plane stress states are discussed in light of the new findings.
Proper design and optimization in stent parameters are required for efficient use of stents in co... more Proper design and optimization in stent parameters are required for efficient use of stents in coronary arteries. Efficiency depends on features including resistance against vessel pressures, (Radial Strength) limited variation in length during expansion (Foreshortening & Recoil) and resistance of stents in periodic/cyclic pressure & tension of heart pulsation (Fatigue Life Cycle). It is customary to design a stent based upon materials endurance limit which is equivalent to 0.5 of the yield strength for stainless steel in order to accomplish physically infinite life cycle for the stent. Different atherosclerosis removal rates are reported by various stent designs. Hence, considering the significance of designs and costly classic methods of producing prototypes, using finite element analysis is beneficial due to the high efficiency and accuracy as well as lower costs. In this study a biocompatible polymeric stent (PLA) is designed by SolidWorks software and influence of designing parameters including longitudinal pattern (Cell) number, circular-circumferential pattern (Cell) number, connection of cells by N, M, or W type flex connector, radius curve, thickness, maximum and minimum width on the mechanical properties such as contact force, foreshortening and maximum stress are estimated by FEM on over 58000 models. The effective design parameters are subsequently optimized to obtain the best model for the stent. Performance of the optimized stent is then investigated in a simulated artery suffering from a 49% non-uniform plaque and after the implementation of the stent in the artery, the inner area of the cross section of the artery increased by 74 percent which demonstrates improvement in blood flow by 74 percent. Results can be useful in designing and optimizing stents for non-uniform restenosis conditions. c
An analytical micromechanics-based yield criterion is developed to describe both void growth and ... more An analytical micromechanics-based yield criterion is developed to describe both void growth and coalescence under combined tension and shear, with smooth transition between growth and coalescence, thus its name unified. The model is obtained by limit analysis over a cylindrical elementary cell embedding a coaxial cylindrical void of finite height. The velocity field employed is an extended counterpart of the discontinuous, yet kinematically admissible trial field utilized in a recent work. Plasticity in the deformable matrix is modeled using rate-independent J 2 flow theory, and the effective dissipation function is calculated by exact as well as approximate integration techniques, the latter generating a simpler flow potential. The model is aimed to predict void growth as well as coalescence by internal necking or shearing. The complete yield surface , being function of normal as well as shear stresses, exhibits curved and planar parts signifying void coalescence. The transition between the curved and planar parts is cornerless. The analytical predictions are compared to results of FEM single-step cell-model calculations of limit analysis executed on an identical geometry exposed to quasi-periodic boundary conditions.
We have carried out dilatant plasticity simulations to investigate the process of void-mediated f... more We have carried out dilatant plasticity simulations to investigate the process of void-mediated failure inside a shear band. The constitutive model accounts for possibly inhomogeneous flow within the band, void rotation and void elongation. We found that the material in the band may soften with no increase in the void volume fraction. For a given matrix hardening capacity, the rate of softening was found to depend strongly on the ratio of shear band width to in-plane void spacing. The emergent softening led to complete loss of load bearing capacity thereby providing a physical mechanism of failure in shear bands. The mechanism is consistent with essential features of shear-fractured specimens in terms of surface roughness, porosity and dimple shape.
Micromechanics-based constitutive relations for post-localization analysis are obtained, to be us... more Micromechanics-based constitutive relations for post-localization analysis are obtained, to be used in a multisurface representation of porous metal plasticity. Each yield surface involves a number of internal parameters. Hence, the constitutive relations must be closed with evolution equations for the internal parameters. The latter are essential to describing the gradual loss of load-bearing capacity under shear-dominated loading. We also briefly discuss potential void closure due to void rotation and elongation in shear and show additional details regarding the simulations reported in a recent paper (A mechanism of failure in shear bands (2018) Extreme Mechanics Letters, 23, pp. 67-71.) The method can be more broadly used in a range of ductile failure problems involving combined tension and shear loadings.
ABSTRACT The one-way (two-way) flexural strength of RCC prisms (circular slabs) reinforced with g... more ABSTRACT The one-way (two-way) flexural strength of RCC prisms (circular slabs) reinforced with glass fiber textiles is addressed. To this end, alkaline-resistant glass fiber textiles with three surface weights were used in the composite, the matrix concrete was designed with zero/nonzero slump, and the textiles were used with/without an intermediate layer provided by epoxy resin and sand mortar. Prisms were tested under a four-point loading apparatus and circular slabs were placed on simple supports under a central load. Effects of the amount and geometry of reinforcement, matrix workability, and the intermediate layer on the ultimate load and deflection were investigated. Results revealed that, with a specific reinforcement amount, there is an optimum textile tex for each case, depending on the matrix mix design and the presence of intermediate layer. Similar results were obtained in one-way and two-way bending tests.
International Journal of Structural Integrity, 2017
PurposeSteel shear walls have recently received exclusive remark. Respective of most building cod... more PurposeSteel shear walls have recently received exclusive remark. Respective of most building code requirements, design of shear wall vertical boundary elements (VBEs) and local boundary elements (LBEs) against web yielding triggers exaggerated stiffness. The extent of stiffness reduction effects in boundary elements thus calls for more exhaustive investigation. The paper aims to discuss these issues.Design/methodology/approachTo this end, FEM-based push-over curves demonstrating base shear vs roof displacement, and von Mises plastic strains were scrutinized in half-scale and full-size models. Analyses were in perfect conformity with experimental data.FindingsWith reference to the AISC requirement, up to 35 percent decrease in the VBE moments of inertia could be imparted in higher levels without the ultimate load capacity nor displacement to failure being reduced. Also considered was open shear walls with reduced or minimum-design LBEs, the latter being used in continuous or abridge...
Micromechanics-based constitutive relations are developed to model plasticity in solids with rela... more Micromechanics-based constitutive relations are developed to model plasticity in solids with relatively high levels of porosity. They are especially appropriate to model void coalescence in ductile materials. The model is obtained by limit analysis of a cylindrical cell containing a coaxial void of finite height with plastic flow confined to the ligaments, and loaded under combined tension and shear. Previously obtained analytical estimates were not upper-bound preserving when shear was present and, in addition, were assessed against numerical results obtained for different cell geometries. Here, a rigorous upper-bound model is developed and its predictions are consistently compared with finite-element based estimates of limit loads on the same cylindrical unit cell exploiting quasi-periodic boundary conditions. The numerical results are used to guide a heuristic modification of the model in order to capture the behavior for extremely flat or extremely elongated voids.
ABSTRACT In this paper, the effects of both pozzolans and (steel and poly-propylene) fibers on th... more ABSTRACT In this paper, the effects of both pozzolans and (steel and poly-propylene) fibers on the mechanical properties of roller compacted concrete are studied. Specimens for the experiments were made using a soil-based approach; thus, the Kango's vibration hammer was used for compaction. The tests in the first stage were carried out to determine the optimal moisture requirements for mix designs using cubic mm specimens. In the tests of the second stage, the mechanical behaviors of the main specimens made using the optimal moisture obtained in the previous stage were evaluated using 28, 90, and 210 day cubic specimens. The mechanical properties of RCC pavements were evaluated using a soil-based compaction method and the optimum moisture content obtained from the pertaining experiments, and by adding different percentages of Iranian pozzolans as well as different amounts of steel fibers, each one accompanied by 0.1% of poly-propylene fibers. Using pozzolans, maximum increase in compressive strength was observed to occur between 28 and 90 days of age, rupture modulus was found to decrease, but toughness indices did not change considerably. The influence of steel fibers on compressive strength was often more significant than that of PP fibers, but neither steel nor PP fibers did contribute to increase in the rupture modulus independently. Also, the toughness indices increased when steel fibers were used.
The literature lacks exhaustive study on CFDST hybrid columns circumscribed by FRP layers. Extend... more The literature lacks exhaustive study on CFDST hybrid columns circumscribed by FRP layers. Extended FEM analysis was carried out on 70 real-scale models with varying parameters including the material and number of FRP layers, concrete strength, length-to-diameter ratio (specific length), and hollow section ratio. Carbon fibers proved stronger than glass fibers, leading to higher ultimate stress associated with lower strain. Specimens with high specific lengths suffered from steel premature buckling, thus stiffened with steel plates. Specimens with various hollow section ratios were finally compared, showing an increase range of 70% between the maximum (0.75) and minimum (0.25) ratios.
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Papers by Mohammad Torki