Viscoplastic Research Articles

Texture comparison between room temperature rolled and cryogenically rolled pure copper

Nowadays, due to their potential for superior mechanical properties, a considerable interest in bulk ultrafine grained metals exists. One of the possible formation methods for this ultrafine grained material is cryogenic rolling. In this work, the influence of cryogenic rolling on the texture and the microstructure of pure copper is investigated by electron backscatter diffraction (EBSD), both in the deformed and the annealed state. This is done by comparing cryogenically rolled copper with room temperature rolled copper, rolled to the same thickness reductions. A texture difference between the room temperature rolled and cryogenically rolled copper is seen in the deformed state, although the largest texture difference is observed after annealing. These texture differences are mainly attributed to the presence of shear bands in the microstructure of the cryogenically rolled copper. In order to obtain a better understanding of the influence of shear bands on the texture evolution, the grain orientations inside the shear bands are analyzed both experimentally and numerically by applying the visco plastic self-consistent (VPSC) model. A number of shear band specific orientations, which are not observed in the conventional rolling texture of fcc materials, could be identified both in the experimental observations and in the simulations.

A Constitutive Model for the Creep Behavior of Offwhite Marbles

This paper reports an improved constitutive model for the shear creep behavior of offwhite marbles which are selected from slope and underground cavern and contain green schist’s weak structural planes. The shear creep behavior of the samples is characterized using the rheological tests. Based on the experimental measurements on mechanical properties under different normal stress conditions, an improved model is proposed to analyze the experimental results. It is demonstrated from a further discussion that such model can reflect the non-linear creep characteristics of structural planes, and especially, it is suitable for description of the viscoelastic and viscoplastic deformation behavior of structural planes.

Validation of metal plasticity and fracture models through numerical simulation of high velocity perforation

Perforation of circular 2024-T3(51) aluminum alloy plates of various normalized thicknesses by a rigid spherical projectile is considered as a benchmark problem to validate material plasticity and fracture models. A new set of experiments with circular 2024-T3(51) aluminum alloy plates and AISI 52100 alloy steel spheres was conducted in order to augment available in literature experimental data and to form more precise reference solution for the benchmark problem. Numerical simulations of a plate perforation using Johnson–Cook plasticity model with two sets of parameters, Johnson–Cook fracture criterion with one set of fitting parameters and pressure and Lode dependent ductile fracture criterion with three sets of material constants are performed. Problem formulation, types of plasticity and fracture models and their parameters are estimated independently in the context of their effect on the accuracy of numerical solution. Ballistic limit velocity and residual velocity tests are employed for validation procedure. With one of the considered formulation-models combination quantitative discrepancy of less than 10% and good and physically sound qualitative agreement with the experiment were obtained.

Numerical and experimental study on influence of tool plunging force and shoulder size on thermal history of friction stir welding

Friction stir welding (FSW) process is a solid stated joining process. Important process parameters include the tool rpm, travel speed, tool dimensions and the downward force on the tool. Heating is caused by rubbing of the tool faces against the workpiece and by visco–plastic dissipation of mechanical energy. There is a unique combination of shear and normal forces in FSW. In this present study gaps of FSW were determined and two very much important process parameters effect on FSW (i.e., tool plunging force and shoulder size) was presented. This study investigates the effect of tool plunging force and tool shoulder size on heat generation of FSW by using 3–D finite element (FE) transient thermal analysis. It was noticed that both plunging force and shoulder diameter have considerable effect on FSW process. Suitable correlations have been made between shoulder diameter vs. peak temperature and tool plunging force vs. peak temperature. Also generalised optimum parameters were established by considering the above said parameters. The present developed FE model results were well matched with experimental results. [Received 29 September 2013; Revised 31 July 2014; Accepted 3 October 2014]

A viscoelastic–viscoplastic model of titanium structures subjected to thermo-chemo-mechanical environment

This manuscript provides a new viscoelastic–viscoplastic deformation model for oxygen-assisted embrittlement and degradation of titanium structures subjected to combined loading and environments. The viscoelastic–viscoplastic formulation accounts for the creep/relaxation behavior at elevated temperatures within the coupled processes of oxygen transport and deformation. The viscoelastic behavior is modeled based on the Boltzmann integral and Prony series approximation of the time-dependent moduli. The concentration-dependent viscoplastic deformation response is idealized using a generalized Johnson–Cook plasticity model. The proposed deformation model is fully coupled with a transport model to describe the ingress of oxygen into the structure. The coupled transport-deformation model is implemented using a three-field formulation, in which concentration, displacement and pressure are evaluated as independent unknowns. The computational model was validated against experiments conducted in combined environments on the high temperature alloy Ti-6242S. A detailed investigation of the coupling mechanisms between the oxygen ingress induced embrittlement, relaxation and the inelastic deformation is provided, including partial validation of the interaction mechanisms based on experimental data.

Numerical investigation of effects of cutting conditions and cooling schemes on tool performance in up milling of Ti-6AL-4V alloy

Ti-6Al-4V is widely used in industry because of its high strength-to-weight ratio at elevated temperatures, its excellent resistance to fracture and corrosion, and biological properties. However, Ti-6Al-4V is classified as hard-to-cut material because of its high chemical reactivity with most tool materials and its low thermal conductivity that causes high temperature on the tool face. Consequently, prediction of the tool temperature distribution has great significance in predicting tool wear pattern. In this research, finite element method (FEM) is employed to conduct numerical investigation of the effects of cutting conditions (cutting speed, feed rate, and axial depth of cut) in corner up milling on temperature of the tool rake face. The tool material used is general carbide, and the behavior of the workpiece Ti-6Al-4V is described by using Johnson-Cook plastic model. Because of the computational expense, a separate heat transfer model is built to analyze the heat transfer process after the tooth disengages the workpiece and before it engages the workpiece again to predict change of temperature distribution during this cooling process while subjected to different cooling schemes. This research provides helpful guidance for selecting optimal cutting conditions and tool cooling strategies in up milling Ti-6Al-4V alloy.

The influence of comprehensive psychological behavior training on psychological stress and health of new recruits in training periods and its interrelation

Objective To provide references for the plan-making of mental intervention on new recruits through the exploration of the comprehensive psychological behavior training influences on psychological stress and health and relevant factors. Methods 436 new recruits in training period were divided into comprehensive psychological behavior training group (intervention group) and non intervention group (control group). By randomly selected 2 (intervention group, control group) × 2 (Before training and after training) matching design, the questionnaire survey was carried out using stress-reaction questionnaire (SRQ), symptom checklist 90(SCL-90), eysenck personality questionnaire(EPQ), trait coping style questionaire(TCSQ) at the 15th day and 75th day. t-test between group and matched t-test within group and multiple stepwise regression analysis were carried out. Results (1) The SCL-90 total point of new recruits in intervention group (125.26±36.52)and control group(124.92±34.79) before the training and the difference of SRQ and TCSQ factor scores had no statistical significance(P>0.05). The difference after the comprehensive psychological behavior training had no statistical significance (P>0.05) as the total points of SCL-90 of intervention group(125.63±36.74) and other factors' score were compared with before.Except for the somatization and force, the total points of SCL-90 (131.21±39.98)and other factor scores of the control group were all higher than what they were in the early period of the training significantly (P 0.05). (3)The regression analysis showed that factors significantly affected the total score of SCL-90 respectively was: SRQ total score, negative coping method, stability, introversion-extroversion and psychoticism (P<0.01). Five variables' coefficient of determination R2=0.665, F=247.82, P<0.01. Conclusion The comprehensive psychological behavior training could reduce the over psychological stress of new recruits, promote their positive coping attitude and protecting their psychological health. Coping method and personality characteristics are important mediating variable between new recruits' psychological stress and mental health. Key words: Recruits; Psychological stress; Mental health; Psychological intervention

A three-field (displacement–pressure–concentration) formulation for coupled transport–deformation problems

This paper provides a three-field finite element formulation for the evaluation of coupled transport–deformation problems. A stabilized advection–diffusion–reaction model is employed to idealize the mass transport of an aggressive environmental agent within a solid medium, whereas the deformation response of the medium is formulated using the mixed finite element approach with pressure and displacement as unknown variables. The proposed model accurately captures the pressure and pressure gradient fields that characterize the reaction and advection terms of the mass transport model. The concentration-dependent viscoplastic deformation response is idealized using a generalized Johnson–Cook plasticity model. The accuracy characteristics of the proposed three-field formulation are assessed by numerical simulations, which indicate the significance of accurate estimation of pressure at high stress gradient zones for correct characterization of mass transport.

THE TEXTURE EVOLUTION AT THE CENTER OF PEARLITIC STEEL WIRE DURING DRAWING AND ITS INFLUENCE ON THE MECHANICAL PROPERTIES

The cold drawing pearlitic steel wires are widely used in industry such as the automobile tire and ropes. And it possesses an ultra high strength, almost the highest in all the steel products. So many investigations are focused on the hardening mechanisms of pearlitic steel wire during cold drawing, including the microstructure fining, texture evolution and cementite dissolution. In this study, the electron backscatter diffraction (EBSD) and the visco-plastic selfconsistent (VPSC) model are used to investigate the texture evolution law at the center of wire during the drawing, as well as its influences on the mechanical behaviors. The EBSD results show that the asreceived wires after dry drawing and quenching have a little fiber texture along the drawing direction. And with wet drawing strain increasing, the intense of fiber texture increases apparently. The calculations using *贝卡尔特(亚洲)研发中心合作项目,国家自然科学基金项目51034009和广东省中国科学院全面战略合作项目2012B091100251资助 收到初稿日期: 2013-12-09,收到修改稿日期: 2014-02-27 作者简介:赵天章,男, 1987年生,博士生 DOI: 10.3724/SP.J.1037.2013.00799 第667-673页 pp.667-673

Self-Consistent Modelling of TWIP Steel during Uniaxial Tensile Loading

The Visco-Plastic (VPSC) and Elasto-Plastic (EPSC) Self-Consistent models were applied to simulate the macroscopic stress-strain response and the evolution of crystallographic texture during the tensile loading of a fully annealed Fe-24Mn-3Al-2Si-1Ni-0.06C TWIP steel. Bulk texture measurements acquired by X-ray diffraction were used to validate the modelling results. Various modelling constructs were employed to assess the contribution of twinning and latent hardening to the texture evolution. The simulations revealed the dominant role of perfect slip and the limited effect of twinning on texture development as well as a possible role for latent hardening. Comparison between the texture predictions of the VPSC and EPSC models underscored the correlation between the latent hardening effects and the employed grain interaction scheme.

Experimental and numerical testing of an innovative steam generator tube plugging procedure

This paper presents the development of a finite element model used to numerically investigate the behaviour of an innovative defective tube isolation device. The high-speed plastic deformation by electro-hydraulic shock was proposed for plug installing in a steam generator tube simulator. The objective of the study was to evaluate the influence of the geometrical, material and hydraulic shock parameters upon the performance of the plugging procedure using an advanced 3-dimensional simulation model created in LS-DYNA general code. The computational model employs the Johnson–Cook plasticity model for SS321 to study the high strain rate, elastic–plastic dynamics of the process. The results of the simulations compared against experimental data have shown the great influence of the model parameters uncertainties on the success of the plug installation.

Finite Element Modeling of Three-Dimensional Milling Process of Ti–6Al–4V

Titanium and its alloys are widely used in aerospace and medical implants due to their excellent mechanical and chemical properties. Titanium is light, strong, corrosion resistant, and biocompatible; however, it has poor machinability. The low thermal conductivity and high strength result in large cutting forces and elevated temperatures during machining, as well as a reduced tool life and a poor machined surface. The current work is based on a 3D finite element method (FEM) modeling of dry milling processes on a Ti–6Al–4V workpiece. The FEM solver Abaqus Explicit® was used with the Johnson–Cook plasticity model to describe the deformation of the workpiece. Experimental work was carried out to validate the simulated cutting forces and chip morphology; the error in the predicted cutting forces in feed and normal to feed direction was in the range of 1–34%, which was mostly within the experimental variation. The measured chip morphology showed good agreement with the simulated data in terms of the chip size and shape. A parametric study of the effect of the machining parameters on the cutting forces was carried out, where the cutting speed, depth of cut, and feed rate were varied, and the radial, tangential, and axial cutting forces were measured. An increase in the feed rate from 50 µm/tooth/rev to 200 µm/tooth/rev resulted in an increase in the tangential and axial forces of 143% and 239%, respectively. An increase in the depth of cut from 50 µm to 200 µm resulted in an increase in the tangential and axial forces of 237% and 100%, respectively. We found that increasing the feed rate was preferable to increasing the depth of cut, as it led to a smaller tangential force.

Ballistic Performance of Multi-layered Fabric Composite Plates Impacted by Different 7.62mm Calibre Projectiles

Abstract At present, the use of refined numerical simulation is gaining more and more importance, especially in extreme load cases where large experimental test programmes are not feasible. A validated numerical methodology can be exploited to investigate critical behaviour as a “virtual test”. According to this premise, a numerical investigation is presented in this work to study the ballistic resistance of Kevlar29-Epoxy fabric plates subjected to impacts of small calibre projectiles, armour piercing (AP), ball type, and a blunt shaped projectile (BSP), all with a 7.62 mm diameter. The numerical models were developed using the explicit finite element code LS - DYNA. The composite plate is 5 mm thick, made up of 12 layers. The fabric is impregnated in Epoxy matrix, to guarantee both structural and ballistic resistance, and is a 2D plain-weave. The mechanical properties of the projectiles’ deformable materials were modelled by means of the Johnson-Cook plasticity model, which also includes the failure criterion. The composite plate is modelled using a mixed Macro-homogenous / Meso-heterogeneous approach. In particular, the area around the impact adopts the Meso-heterogeneous modelling, in which the woven yarns and the matrix are separate parts able to interact, to increase the efficiency of the numerical methodology, allowing the modelling of the friction between the yarns, the delamination and the fibre-matrix debonding. This method needs to assign separately the mechanical properties to the fabric and the matrix as well as the damage criteria for the yarns, the matrix and the delamination. In the Macro-homogenous approach, which models the remainder of the composite plate, the yarns and the matrix are considered as a unique homogenous mean, to which equivalent orthotropic mechanical properties of the impregnated fabric are assigned. The numerical investigation mainly focuses on the residual velocity of the impacting projectiles, identifying the ballistic limit of each couple projectile-target.

The Use of Particle/Substrate Material Models in Simulation of Cold-Gas Dynamic-Spray Process

Cold spray is a coating deposition method in which the solid particles are accelerated to the substrate using a low temperature supersonic gas flow. Many numerical studies have been carried out in the literature in order to study this process in more depth. Despite the inability of Johnson-Cook plasticity model in prediction of material behavior at high strain rates, it is the model that has been frequently used in simulation of cold spray. Therefore, this research was devoted to compare the performance of different material models in the simulation of cold spray process. Six different material models, appropriate for high strain-rate plasticity, were employed in finite element simulation of cold spray process for copper. The results showed that the material model had a considerable effect on the predicted deformed shapes.

Configurational Forces in Crystal Plasticity

AbstractThe surface morphology of micro machined surfaces depends on the heterogeneous microstructure. A crystal plasticity model is used to describe the plastic deformation in cp‐titanium with its hcp crystal structure. Therefore the basal and prismatic slip systems are taken into account. Furthermore, self and latent hardening are considered. The rate dependency is motivated by a visco plastic evolution law. The cutting process of cp‐titanium is modeled within the concept of configurational forces for a standard dissipative media. This framework is implemented into the finite element method. An example illustrates the effects of the microstructure on plastic deformation and configurational forces. (© 2013 Wiley‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Numerical Simulation of Hot Isostatic Pressing Process for the Manufacture of Parts Used in Biomechanics

This study is devoted to numerical modeling and simulation of the process of forming of metal powders by hot isostatic compaction (HIC) for the production of parts in biomechanics. A continuous model based on a visco plastic law is used to describe the process of densification HIP (hot isostatic pressing). The model was implemented in a finite element code developed under the platform FORTRAN POWER STATION. For numerical simulation, we considered a cylindrical die subjected to isostatic pressure and a temperature field. We study variations of different parameters: relative density and radial shrinkage. To validate the numerical model, the results are compared with experimental data.

Experimental characterization and computational simulations of the low-velocity impact behaviour of polypropylene

Fil: Torres, Juan Pablo. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Mar del Plata. Instituto de Investigacion en Ciencia y Tecnologia de Materiales (i); Argentina; Universidad Nacional de Mar del Plata. Facultad de Ingenieria; Argentina;

Comparison of the Operating Life of Tank Containers, Tank Vehicles and Rail Tank Cars for the Carriage of Dangerous Goods in Practice, Analysis of Causes of Damage

More than 400 Mt of dangerous materials are transported in Germany every year, of which 150 Mt are by road. Tank containers, tank vehicles and rail tank cars are used for the carriage of dangerous goods in large quantities. Data on the operating life of tanks are only available, in practice, to a minor degree. They are only partly published, mainly after accidents.The BAM-List - Requirements for Tanks for the Carriage of Dangerous Goods, which has compatibility evaluations of metallic and polymeric materials, has been the basis for substance-related prototype approvals for tank containers and portable tanks designed for the carriage of dangerous goods by the BAM since publication of the first edition in 1989. These data are also used for the approval of rail tank cars and road tank cars and are used as a source of knowledge not only in Germany but also worldwide. Tank leakages caused by tanks or rail tank cars being made of tank and sealing materials which are not resistant to the fill goods are avoided by using the material resistance data of the BAM.Corrosion damage is among the main causes of damage. Uniform and non-uniform area corrosion without mechanical stress in aqueous substances is one of the most frequent types of corrosion during the transport of chemicals in tanks. Much damage by pitting corrosion occurs during the transport of substances containing chlorides or substances which separate chloride ions in the presence of moisture. Operational stresses are caused by the effects of both the road and the dangerous goods being transported. Mechanical damage often results from long-term overstressing and occurs after longer operating times.Operational failures cause damage which may appear during the service of tank containers, road tank cars or rail tank cars. The damage results from the inattention of employees when opening and closing the valves. Traffic accidents also cause damage to tanks and frames.Many cases of damage are due to weld area cracks resulting from four basic errors in construction, material, manufacturing and operation. Manufacturing errors can be undetected and the starting point of cracks which only grow under service conditions.Manufacturing errors result from variations in measurement and design, fittings, state of surface (hardness, abrasiveness) and mechanical surface damage (scratches, cracks). Incorrectly welded joints and errors during mechanical deformation are typical mistakes when installing the component parts. Variations in the wall thickness or combinations of different materials lead to manufacturing errors too.A lot of tanks are not used for the transport of dangerous goods before the end of their service life as they do not fulfil the revised technical safety requirements in the Dangerous Goods Regulations.Predictions in the BAM-List based on literature data and corrosion test results are reflected in the service (operating) life. Rail tank cars made of carbon steel, for example, which are mainly used for the transport of petroleum products, can achieve a service life of 40 – 50 y.Rail tank cars produced of austenitic CrNi- or CrNiMo-steel can reach an operating life of at least 30 y, whereby the corrosiveness of the transport substances plays an important role.

Constitutive modeling of fatigue damage response of asphalt concrete materials with consideration of micro-damage healing

A continuum mechanics-based viscodamage (VD) constitutive relationship is proposed to model fatigue damage of asphalt concrete. The form for the evolution of the viscodamage function is postulated based on the damage density which was determined from uniaxial constant strain rate tests that were performed at different strain rates. The proposed viscodamage model is coupled with Schapery’s nonlinear viscoelastic (VE), Perzyna’s viscoplastic (VP), and micro-damage healing (H) models to simulate the nonlinear mechanical response of asphalt concrete during fatigue. Numerical algorithms are implemented in the finite element code Abaqus via the user material subroutine UMAT. The proposed model is validated against extensive experimental data including constant strain rate, cyclic displacement controlled, and cyclic stress controlled tests over a range of temperatures, strain rates, loading frequencies, and stress/strain levels/amplitudes. The model predictions show that the VE–VP–VD–H model is capable of predicting the fatigue damage response of asphalt concrete subjected to different loading conditions. The results demonstrate that micro-damage healing occurs not only during the rest period, but also during the cyclic strain controlled tests even in the absence of the resting time.

Validation of Johnson-Cook plasticity and damage model using impact experiment

The validity of the Johnson–Cook constitutive relation and failure criterion at high strain rates, up to ∼106 s−1, was assessed by predicting the dynamic response of Ti–6Al–4V under high-speed ball impact at various velocities and angles. White-light scanning was performed to characterize impact craters formed on target surfaces. The measured crater was compared with that predicted by the corresponding finite element model developed using the finite element code Abaqus/Explicit. The target material behavior was modeled by the Johnson–Cook material model that induced both plastic deformation and damage mechanism. Good agreement was obtained between the experimental measurements and numerical predictions for all testing conditions.