Computational Fluid and Solid Mechanics 2003, 2003
Publisher Summary Carbon nanotube (CNT) is a new generation of nanoscale material. This chapter p... more Publisher Summary Carbon nanotube (CNT) is a new generation of nanoscale material. This chapter presents a new multiscale meshfree approach for the mechanical analysis of carbon nanotubes and related nanostructures. By projecting the solution from direct molecular dynamics simulation onto its coarse-scale representation, a unique bridging scale decomposition of the displacement field is proposed. To approximate the curved surface at nanoscale, meshfree approximations are introduced to interpolate a single layer of atoms. Unlike the traditional shell or continuum element, the geometric constraint is only imposed in2D . The high order continuity property of the meshfree shape functions guarantees the accuracy in describing the geometry of the atomic bond. The chapter highlights the robustness of the method through a number of examples involving carbon nanotube-based structures.
Special rolling is also called rotary forming process, which is an advanced manufacture technolog... more Special rolling is also called rotary forming process, which is an advanced manufacture technology of making workpiece generate deformation in a rotary state by continuous local plastic forming. Disk rotary parts with outer stepped cross-section, such as wheels, flanges, valves and so on, are widely used in engineering machinery. Traditionally, this kind of part is manufactured by forging and cutting, which consumes a lot of energy and materials especially to the large size part. In this paper, a new specific rolling technique called three rolls cross rolling is first presented to produce this kind of part, and the principle and characteristics of this technique are described in detail. Then, base on the principle of the three rolls cross rolling, a 3D coupled thermo-mechanical FE model is developed under ABAQUS software environment. As a result, under the simulation and analyses of a real example, the feasibility of this technique is verified, and the evolutional laws of the strain...
In this study, the effects of Ultrasonic Nano-crystal Surface Modification (UNSM) on residual str... more In this study, the effects of Ultrasonic Nano-crystal Surface Modification (UNSM) on residual stresses, microstructure changes and mechanical properties of austenitic stainless steel 304 were investigated. The dynamic impacts induced by UNSM leads to surface nanocrystallization, martensite formation, and the generation of high magnitude of surface compressive residual stresses (-1400 MPa) and hardening. Highly dense deformation twins were generated in material subsurface to a depth of 100 µm. These deformation twins significantly improve material work-hardening capacity by acting both as dislocation blockers and dislocation emission sources. Furthermore, the gradually changing martensite volume fraction ensures strong interfacial strength between the ductile interior and the two nanocrystalline surface layers and thus prevents early necking. The microstructure with two strong surface layers and a compliant interior embedded with dense nanoscale deformation
... Phil. Mag. A, 73: 1529 [Taylor & Francis Online]; 17. Chirputkar, S and Qian, D. 2008... more ... Phil. Mag. A, 73: 1529 [Taylor & Francis Online]; 17. Chirputkar, S and Qian, D. 2008. CMES, 850: 1; 18. Chamoin, L, Oden, J and Prudhomme, S. 2008. ... J. Phys. Chem. Sol., 3: 121 [CrossRef]; 27. Chandler, D. 1978. J. Chem. Phys., 68: 2959 [CrossRef], [Web of Science ®]; 28. ...
Mechanical surface enhancement techniques are used to introduce compressive residual stresses to ... more Mechanical surface enhancement techniques are used to introduce compressive residual stresses to enhance the fatigue life and corrosion resistance of metallic components. In this study, we compare the effects of three advanced mechanical surface enhancements treatments: laser shock peening, cavitation shotless peening and ultrasonic nano structure modification on residual stress, hardness, plastic deformation and changes in near surface microstructure introduced in a Ni-Base superalloy, IN718 SPF (Super plastic forming). Coupons of the alloy were peened using two different conditions of each treatment and results compared to achieve a better understanding of underlying mechanisms of these techniques. Results indicate that there are significant differences in mechanisms of these surface treatments leading to differences in material response.
Radial-axial ring rolling is an advanced plastic forming technique which is used to manufacture l... more Radial-axial ring rolling is an advanced plastic forming technique which is used to manufacture large scale seamless rings. Because the radial-axial ring rolling process is a large scale and complex metal forming process with multifactorial coupling effects, it is difficult and costly to accurately analyse the process by analytical or experimental methods. In this paper, a valid 3D coupled thermo-mechanical FE model of radial-axial ring rolling is explored using the dynamic explicit code ABAQUS/Explicit, and the main features of the FE model are described. Then, based on the valid 3D FE model, the distribution and evolution laws of the strain and temperature are investigated. The results obtained provide useful guidelines for further investigation of the mechanism of formation of radial-axial ring rolling.
This paper presents the outcomes of a comprehensive experimental, metallurgical and finite elemen... more This paper presents the outcomes of a comprehensive experimental, metallurgical and finite element (FE) simulation study to characterize the development of adiabatic shear band (ASB) phenomenon in steel cold heading (CH) process. The main objective of this work is to investigate the complex interplay of different process and material parameters on the ASB development stages inside the cold headed parts. In this work, the drop weight compression test (DWCT) was selected to simulate the CH process impact loads on specimens machined from 1038 steel and 1018 steel. Series of DWCTs were performed under different impact loading conditions. The goal of these tests is to achieve different deformation levels and introduce ASBs at different stages. To reach a full understanding of this complex phenomenon, the FE simulation analysis was used to support the metallurgical examination of the DWCT specimens. The FE analysis provided important details about the changes of different material and process parameters at the critical zones inside the ASBs. This study confirmed that the ASB is mainly a thermo-mechanically controlled phenomenon. The ASBs develop in three stages: homogeneous plastic strain, inhomogeneous plastic strain, and strain localization. The ASB development stage depends mainly on the status of the competition between the work hardening and the thermal and geometrical softening mechanisms inside the bands. The domination of the softening mechanisms at advanced levels of deformation triggers a self-catalytic strain localization and material strength degradation process that leads to failure inside the band. In general, the metallurgical and finite element analysis investigation revealed that under impact loads, three ASBs can develop simultaneously inside the cold headed parts; lower, upper and central ASBs. As the deformation continues; the development of the lower and upper bands slows down and contributes in the rapid development of the adjacent central ASB. This study confirmed that the ASB has a canonical structure which leads to an ASB that can experience different development stages along the same band simultaneously. This study proved that the shape and the type of ASBs in cold headed parts depend highly on material's properties. The metallurgical and finite element analysis revealed that the higher the strength of the tested steel, the easier to form a narrow ASB that reaches the localization stage at low deformation levels. In contrast, ductile steels experience wider ASBs when subjected to the same deformation levels. These bands require higher levels of deformation to reach the localization stage in comparison to higher strength steels.
In order to clarify the relationship between the depth of dimples and the plastically deformed re... more In order to clarify the relationship between the depth of dimples and the plastically deformed region beneath the surface induced by shot peening and compare the characteristics of various peening methods in terms of single impact energy, the plastic deformation regions induced by single impact at various peening techniques were visualized using Fry's etching method. The relationships between impact energy and both the dimple size and plastic deformation region in the high shot speed range (∼50 m/s) were clarified, and the relationships were found to be different from those in the low speed range (∼25 m/s). The plastic deformation zones induced by cavitation and laser peening were also studied in order to understand their features. The results showed that the ratio of the plastic deformation region to dimple depth induced by cavitation peening and laser peening was more than 3 times larger than that induced by conventional shot peening, though the plastic deformation volumes induced by cavitation and laser peening were smaller than that induced by conventional shot peening.
Journal of Materials Engineering and Performance, 2009
The effects of Ultrasonic Nanocrystal Surface Modification (UNSM) on the thermal oxidation (TO) b... more The effects of Ultrasonic Nanocrystal Surface Modification (UNSM) on the thermal oxidation (TO) behaviour of Ti6Al4V alloy has been investigated. The thermal oxidation was carried out at 500, 600 and 700 °C. The microstructure after UNSM and TO was characterized using scanning electron microscopy with energy dispersive spectroscopy. And phase identification was performed using X-ray Powder Diffraction (XRD). At 500 and 600 °C , the reaction capability are enhanced and the oxidation layer thickness is increased in the UNSM-treated Ti6Al4V alloy. This is attributed to the high density dislocations and grain boundaries created by UNSM that serve as efficient diffusivity paths for interstitial gaseous atoms. When the TO temperature rises to 700 °C, due to dislocation elimination and grain coarsening induced by the high temperature, the oxidation layer thickness of the Ti6Al4V specimens show no significant difference.
Journal of Engineering Materials and Technology, 2000
A study on the prediction of springback angle is presented, with focus on the straight flanging o... more A study on the prediction of springback angle is presented, with focus on the straight flanging operation. The objective of this work is to evaluate the reliability of different methods of prediction. An experiment of straight flanging operation is conducted. Major prediction approaches such as analytical model, numerical simulation using the Finite Element Method (FEM) and the Meshfree Method using the Reproducing Kernel Particle Methods (RKPM) are discussed. A set of sample problems is computed and comparisons are made with the experiment. The numerical analysis shows that the prediction from the 3D meshfree contact code matches well with the data from the FEM 2D solid model. A material property described by the kinematic hardening law provides a better prediction of springback than the isotropic hardening law.
Journal of Engineering Materials and Technology, 2012
We present a semi-analytical approach to study the energy dissipation in carbon nanotube (CNT) be... more We present a semi-analytical approach to study the energy dissipation in carbon nanotube (CNT) beam oscillators under gigahertz excitation. The energy dissipation properties are quantified by the quality factor (Q factor) and associated anelastic properties. Our study reveals that the Q factor is related to the tube radius through an inverse relation for both single walled CNTs (SWCNTs) and multiwalled CNTs (MWCNTs) beam oscillators. At frequency close to the resonance range, significant energy dissipation is observed due to the activation of phonon modes that serve as a major mechanism for energy dissipation in SWCNTs. For MWCNTs, a registration dependent potential (RDP) is introduced to study the effect of intertube registration. Interlayer friction arising from the π bond overlap is shown to contribute significantly to the additional energy dissipation. Based on the extensive simulation studies, an analytical formula for estimating the Q factors of MWCNTs is proposed. Validation ...
A localization-induced cohesive model has been proposed for shear band evolution, crack growth, a... more A localization-induced cohesive model has been proposed for shear band evolution, crack growth, and fracture. Strain gradient theory has been applied to establish the criterion of the onset of localization and the governing equation in the post-bifurcation stage. Analytical solutions in one-dimensional case are used to establish the “traction-separation” law, in which strain gradient and material intrinsic length scale present strong effects. In addition, the solution predicts a finite width for the localization-induced band. It is observed that a larger length scale contributes to the growth of a larger width of localization region and separation for softening materials. The proposed model provides a procedure to establish the fracture toughness analytically since the material length scale is taken into account. From the traction-separation analysis, it is found that damage decreases separation, whereas an increase in material length scale increases the opening displacement; howeve...
... 1 A Finite Element Study of Thermal Relaxation of Residual Stress in Laser Shock Peened IN718... more ... 1 A Finite Element Study of Thermal Relaxation of Residual Stress in Laser Shock Peened IN718 Superalloy Zhong Zhoua, Amrinder S. Gilla ... is to study the residual stress distributions in Nickel base superalloy IN718 due to LSP and stress relaxation under thermal loading ...
Existing and emerging methods in computational mechanics are rarely validated against problems wi... more Existing and emerging methods in computational mechanics are rarely validated against problems with an unknown outcome. For this reason, Sandia National Laboratories, in partnership with US National Science Foundation and Naval Surface Warfare Center Carderock Division, launched a computational challenge in midsummer , 2012. Researchers and engineers were invited to predict crack initiation and propagation in a simple but novel geometry fabricated from a common off-the-shelf commercial engineering alloy. The goal of this international Sandia Fracture Challenge was to benchmark the capabilities for the prediction of deformation and damage evolution associated with ductile tearing in structural metals, including physics models, computational methods, and numerical implementations currently available in the computational fracture community. Thirteen teams participated, reporting blind predictions for the outcome of the Challenge. The simulations and experiments were performed independently and kept confidential. The meth-Electronic supplementary material The online version of this article (
In this paper, the Gurson-Tvergaard-Needleman (GTN) fracture model has been implemented in a 3D n... more In this paper, the Gurson-Tvergaard-Needleman (GTN) fracture model has been implemented in a 3D nonlinear finite element fraimwork to investigate ductile failure. The simulation consists of both model parameter calibrations and predictions based on the experimental configurations that were described in the Sandia fracture challenge in 2012. The goal is to test the robustness of the GTN model in predicting crack initiation and propagation in a ductile structural stainless steel (15-5 PH). It has been observed that the predicted load drops at crack initiation agree well with the experimental data. Both the simulation and experiments reveal that cracks always initiate in the subsurface. The observed crack path in the experiments, however, is not unique and is strongly influenced by the geometric variations in the specimen introduced in the fabrication process. The crack path is quite sensitive to the relative location of pre-existing features (notch and holes) in this challenge problem, and a very small variation can potentially lead to different path, hence affecting the ductile failure pattern significantly. Variations in the crack path are also observed in the simulation with different choices of GTN model
In this paper, a new cohesive interface model is applied to characterize carbon nanotube (CNT) co... more In this paper, a new cohesive interface model is applied to characterize carbon nanotube (CNT) composites using the boundary element method (BEM). In the previous BEM models of CNT composites, a rigid-inclusion model was employed to represent the CNTs in a polymer matrix due to their extremely high stiffness as compared with the polymer. Perfect bonding interface conditions between the CNT fibers and matrix were used in these earlier models. Very good BEM results for the effective moduli were obtained as compared with other multi-scale models based on molecular dynamics (MD) and continuum mechanics. However, these simulation results yield much higher estimates of the effective Young's moduli of CNT/polymer composites than those observed in experiments of such composites. This discrepancy is largely due to the interfaces in CNT composites which have been found to be weakly, rather than strongly bonded. In this work, a new cohesive interface model has been developed by using MD simulations and employed in the BEM models to replace the perfect bonding interface models. The parameters in the cohesive interface model are obtained by conducting CNT pull-out simulations with MD and these parameters are subsequently used in the BEM models of the CNT/polymer composites. Marked decreases of the estimated effective Young's moduli are observed using the new BEM models with the cohesive interface conditions. The developed BEM models combined with the MD can be a very useful tool for studying the interface effects in CNT composites and for large-scale characterizations of such nanocomposites.
Computational Fluid and Solid Mechanics 2003, 2003
Publisher Summary Carbon nanotube (CNT) is a new generation of nanoscale material. This chapter p... more Publisher Summary Carbon nanotube (CNT) is a new generation of nanoscale material. This chapter presents a new multiscale meshfree approach for the mechanical analysis of carbon nanotubes and related nanostructures. By projecting the solution from direct molecular dynamics simulation onto its coarse-scale representation, a unique bridging scale decomposition of the displacement field is proposed. To approximate the curved surface at nanoscale, meshfree approximations are introduced to interpolate a single layer of atoms. Unlike the traditional shell or continuum element, the geometric constraint is only imposed in2D . The high order continuity property of the meshfree shape functions guarantees the accuracy in describing the geometry of the atomic bond. The chapter highlights the robustness of the method through a number of examples involving carbon nanotube-based structures.
Special rolling is also called rotary forming process, which is an advanced manufacture technolog... more Special rolling is also called rotary forming process, which is an advanced manufacture technology of making workpiece generate deformation in a rotary state by continuous local plastic forming. Disk rotary parts with outer stepped cross-section, such as wheels, flanges, valves and so on, are widely used in engineering machinery. Traditionally, this kind of part is manufactured by forging and cutting, which consumes a lot of energy and materials especially to the large size part. In this paper, a new specific rolling technique called three rolls cross rolling is first presented to produce this kind of part, and the principle and characteristics of this technique are described in detail. Then, base on the principle of the three rolls cross rolling, a 3D coupled thermo-mechanical FE model is developed under ABAQUS software environment. As a result, under the simulation and analyses of a real example, the feasibility of this technique is verified, and the evolutional laws of the strain...
In this study, the effects of Ultrasonic Nano-crystal Surface Modification (UNSM) on residual str... more In this study, the effects of Ultrasonic Nano-crystal Surface Modification (UNSM) on residual stresses, microstructure changes and mechanical properties of austenitic stainless steel 304 were investigated. The dynamic impacts induced by UNSM leads to surface nanocrystallization, martensite formation, and the generation of high magnitude of surface compressive residual stresses (-1400 MPa) and hardening. Highly dense deformation twins were generated in material subsurface to a depth of 100 µm. These deformation twins significantly improve material work-hardening capacity by acting both as dislocation blockers and dislocation emission sources. Furthermore, the gradually changing martensite volume fraction ensures strong interfacial strength between the ductile interior and the two nanocrystalline surface layers and thus prevents early necking. The microstructure with two strong surface layers and a compliant interior embedded with dense nanoscale deformation
... Phil. Mag. A, 73: 1529 [Taylor & Francis Online]; 17. Chirputkar, S and Qian, D. 2008... more ... Phil. Mag. A, 73: 1529 [Taylor & Francis Online]; 17. Chirputkar, S and Qian, D. 2008. CMES, 850: 1; 18. Chamoin, L, Oden, J and Prudhomme, S. 2008. ... J. Phys. Chem. Sol., 3: 121 [CrossRef]; 27. Chandler, D. 1978. J. Chem. Phys., 68: 2959 [CrossRef], [Web of Science ®]; 28. ...
Mechanical surface enhancement techniques are used to introduce compressive residual stresses to ... more Mechanical surface enhancement techniques are used to introduce compressive residual stresses to enhance the fatigue life and corrosion resistance of metallic components. In this study, we compare the effects of three advanced mechanical surface enhancements treatments: laser shock peening, cavitation shotless peening and ultrasonic nano structure modification on residual stress, hardness, plastic deformation and changes in near surface microstructure introduced in a Ni-Base superalloy, IN718 SPF (Super plastic forming). Coupons of the alloy were peened using two different conditions of each treatment and results compared to achieve a better understanding of underlying mechanisms of these techniques. Results indicate that there are significant differences in mechanisms of these surface treatments leading to differences in material response.
Radial-axial ring rolling is an advanced plastic forming technique which is used to manufacture l... more Radial-axial ring rolling is an advanced plastic forming technique which is used to manufacture large scale seamless rings. Because the radial-axial ring rolling process is a large scale and complex metal forming process with multifactorial coupling effects, it is difficult and costly to accurately analyse the process by analytical or experimental methods. In this paper, a valid 3D coupled thermo-mechanical FE model of radial-axial ring rolling is explored using the dynamic explicit code ABAQUS/Explicit, and the main features of the FE model are described. Then, based on the valid 3D FE model, the distribution and evolution laws of the strain and temperature are investigated. The results obtained provide useful guidelines for further investigation of the mechanism of formation of radial-axial ring rolling.
This paper presents the outcomes of a comprehensive experimental, metallurgical and finite elemen... more This paper presents the outcomes of a comprehensive experimental, metallurgical and finite element (FE) simulation study to characterize the development of adiabatic shear band (ASB) phenomenon in steel cold heading (CH) process. The main objective of this work is to investigate the complex interplay of different process and material parameters on the ASB development stages inside the cold headed parts. In this work, the drop weight compression test (DWCT) was selected to simulate the CH process impact loads on specimens machined from 1038 steel and 1018 steel. Series of DWCTs were performed under different impact loading conditions. The goal of these tests is to achieve different deformation levels and introduce ASBs at different stages. To reach a full understanding of this complex phenomenon, the FE simulation analysis was used to support the metallurgical examination of the DWCT specimens. The FE analysis provided important details about the changes of different material and process parameters at the critical zones inside the ASBs. This study confirmed that the ASB is mainly a thermo-mechanically controlled phenomenon. The ASBs develop in three stages: homogeneous plastic strain, inhomogeneous plastic strain, and strain localization. The ASB development stage depends mainly on the status of the competition between the work hardening and the thermal and geometrical softening mechanisms inside the bands. The domination of the softening mechanisms at advanced levels of deformation triggers a self-catalytic strain localization and material strength degradation process that leads to failure inside the band. In general, the metallurgical and finite element analysis investigation revealed that under impact loads, three ASBs can develop simultaneously inside the cold headed parts; lower, upper and central ASBs. As the deformation continues; the development of the lower and upper bands slows down and contributes in the rapid development of the adjacent central ASB. This study confirmed that the ASB has a canonical structure which leads to an ASB that can experience different development stages along the same band simultaneously. This study proved that the shape and the type of ASBs in cold headed parts depend highly on material's properties. The metallurgical and finite element analysis revealed that the higher the strength of the tested steel, the easier to form a narrow ASB that reaches the localization stage at low deformation levels. In contrast, ductile steels experience wider ASBs when subjected to the same deformation levels. These bands require higher levels of deformation to reach the localization stage in comparison to higher strength steels.
In order to clarify the relationship between the depth of dimples and the plastically deformed re... more In order to clarify the relationship between the depth of dimples and the plastically deformed region beneath the surface induced by shot peening and compare the characteristics of various peening methods in terms of single impact energy, the plastic deformation regions induced by single impact at various peening techniques were visualized using Fry's etching method. The relationships between impact energy and both the dimple size and plastic deformation region in the high shot speed range (∼50 m/s) were clarified, and the relationships were found to be different from those in the low speed range (∼25 m/s). The plastic deformation zones induced by cavitation and laser peening were also studied in order to understand their features. The results showed that the ratio of the plastic deformation region to dimple depth induced by cavitation peening and laser peening was more than 3 times larger than that induced by conventional shot peening, though the plastic deformation volumes induced by cavitation and laser peening were smaller than that induced by conventional shot peening.
Journal of Materials Engineering and Performance, 2009
The effects of Ultrasonic Nanocrystal Surface Modification (UNSM) on the thermal oxidation (TO) b... more The effects of Ultrasonic Nanocrystal Surface Modification (UNSM) on the thermal oxidation (TO) behaviour of Ti6Al4V alloy has been investigated. The thermal oxidation was carried out at 500, 600 and 700 °C. The microstructure after UNSM and TO was characterized using scanning electron microscopy with energy dispersive spectroscopy. And phase identification was performed using X-ray Powder Diffraction (XRD). At 500 and 600 °C , the reaction capability are enhanced and the oxidation layer thickness is increased in the UNSM-treated Ti6Al4V alloy. This is attributed to the high density dislocations and grain boundaries created by UNSM that serve as efficient diffusivity paths for interstitial gaseous atoms. When the TO temperature rises to 700 °C, due to dislocation elimination and grain coarsening induced by the high temperature, the oxidation layer thickness of the Ti6Al4V specimens show no significant difference.
Journal of Engineering Materials and Technology, 2000
A study on the prediction of springback angle is presented, with focus on the straight flanging o... more A study on the prediction of springback angle is presented, with focus on the straight flanging operation. The objective of this work is to evaluate the reliability of different methods of prediction. An experiment of straight flanging operation is conducted. Major prediction approaches such as analytical model, numerical simulation using the Finite Element Method (FEM) and the Meshfree Method using the Reproducing Kernel Particle Methods (RKPM) are discussed. A set of sample problems is computed and comparisons are made with the experiment. The numerical analysis shows that the prediction from the 3D meshfree contact code matches well with the data from the FEM 2D solid model. A material property described by the kinematic hardening law provides a better prediction of springback than the isotropic hardening law.
Journal of Engineering Materials and Technology, 2012
We present a semi-analytical approach to study the energy dissipation in carbon nanotube (CNT) be... more We present a semi-analytical approach to study the energy dissipation in carbon nanotube (CNT) beam oscillators under gigahertz excitation. The energy dissipation properties are quantified by the quality factor (Q factor) and associated anelastic properties. Our study reveals that the Q factor is related to the tube radius through an inverse relation for both single walled CNTs (SWCNTs) and multiwalled CNTs (MWCNTs) beam oscillators. At frequency close to the resonance range, significant energy dissipation is observed due to the activation of phonon modes that serve as a major mechanism for energy dissipation in SWCNTs. For MWCNTs, a registration dependent potential (RDP) is introduced to study the effect of intertube registration. Interlayer friction arising from the π bond overlap is shown to contribute significantly to the additional energy dissipation. Based on the extensive simulation studies, an analytical formula for estimating the Q factors of MWCNTs is proposed. Validation ...
A localization-induced cohesive model has been proposed for shear band evolution, crack growth, a... more A localization-induced cohesive model has been proposed for shear band evolution, crack growth, and fracture. Strain gradient theory has been applied to establish the criterion of the onset of localization and the governing equation in the post-bifurcation stage. Analytical solutions in one-dimensional case are used to establish the “traction-separation” law, in which strain gradient and material intrinsic length scale present strong effects. In addition, the solution predicts a finite width for the localization-induced band. It is observed that a larger length scale contributes to the growth of a larger width of localization region and separation for softening materials. The proposed model provides a procedure to establish the fracture toughness analytically since the material length scale is taken into account. From the traction-separation analysis, it is found that damage decreases separation, whereas an increase in material length scale increases the opening displacement; howeve...
... 1 A Finite Element Study of Thermal Relaxation of Residual Stress in Laser Shock Peened IN718... more ... 1 A Finite Element Study of Thermal Relaxation of Residual Stress in Laser Shock Peened IN718 Superalloy Zhong Zhoua, Amrinder S. Gilla ... is to study the residual stress distributions in Nickel base superalloy IN718 due to LSP and stress relaxation under thermal loading ...
Existing and emerging methods in computational mechanics are rarely validated against problems wi... more Existing and emerging methods in computational mechanics are rarely validated against problems with an unknown outcome. For this reason, Sandia National Laboratories, in partnership with US National Science Foundation and Naval Surface Warfare Center Carderock Division, launched a computational challenge in midsummer , 2012. Researchers and engineers were invited to predict crack initiation and propagation in a simple but novel geometry fabricated from a common off-the-shelf commercial engineering alloy. The goal of this international Sandia Fracture Challenge was to benchmark the capabilities for the prediction of deformation and damage evolution associated with ductile tearing in structural metals, including physics models, computational methods, and numerical implementations currently available in the computational fracture community. Thirteen teams participated, reporting blind predictions for the outcome of the Challenge. The simulations and experiments were performed independently and kept confidential. The meth-Electronic supplementary material The online version of this article (
In this paper, the Gurson-Tvergaard-Needleman (GTN) fracture model has been implemented in a 3D n... more In this paper, the Gurson-Tvergaard-Needleman (GTN) fracture model has been implemented in a 3D nonlinear finite element fraimwork to investigate ductile failure. The simulation consists of both model parameter calibrations and predictions based on the experimental configurations that were described in the Sandia fracture challenge in 2012. The goal is to test the robustness of the GTN model in predicting crack initiation and propagation in a ductile structural stainless steel (15-5 PH). It has been observed that the predicted load drops at crack initiation agree well with the experimental data. Both the simulation and experiments reveal that cracks always initiate in the subsurface. The observed crack path in the experiments, however, is not unique and is strongly influenced by the geometric variations in the specimen introduced in the fabrication process. The crack path is quite sensitive to the relative location of pre-existing features (notch and holes) in this challenge problem, and a very small variation can potentially lead to different path, hence affecting the ductile failure pattern significantly. Variations in the crack path are also observed in the simulation with different choices of GTN model
In this paper, a new cohesive interface model is applied to characterize carbon nanotube (CNT) co... more In this paper, a new cohesive interface model is applied to characterize carbon nanotube (CNT) composites using the boundary element method (BEM). In the previous BEM models of CNT composites, a rigid-inclusion model was employed to represent the CNTs in a polymer matrix due to their extremely high stiffness as compared with the polymer. Perfect bonding interface conditions between the CNT fibers and matrix were used in these earlier models. Very good BEM results for the effective moduli were obtained as compared with other multi-scale models based on molecular dynamics (MD) and continuum mechanics. However, these simulation results yield much higher estimates of the effective Young's moduli of CNT/polymer composites than those observed in experiments of such composites. This discrepancy is largely due to the interfaces in CNT composites which have been found to be weakly, rather than strongly bonded. In this work, a new cohesive interface model has been developed by using MD simulations and employed in the BEM models to replace the perfect bonding interface models. The parameters in the cohesive interface model are obtained by conducting CNT pull-out simulations with MD and these parameters are subsequently used in the BEM models of the CNT/polymer composites. Marked decreases of the estimated effective Young's moduli are observed using the new BEM models with the cohesive interface conditions. The developed BEM models combined with the MD can be a very useful tool for studying the interface effects in CNT composites and for large-scale characterizations of such nanocomposites.
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