Deformation Processes In Materials Research Articles

Identification of Upper and Lower Level Yield Strength in Materials.

This work evaluates the possibility of identifying mechanical parameters, especially upper and lower yield points, by the analytical processing of specific elements of the topography of surfaces generated with abrasive waterjet technology. We developed a new system of equations, which are connected with each other in such a way that the result of a calculation is a comprehensive mathematical–physical model, which describes numerically as well as graphically the deformation process of material cutting using an abrasive waterjet. The results of our model have been successfully checked against those obtained by means of a tensile test. The main prospect for future applications of the method presented in this article concerns the identification of mechanical parameters associated with the prediction of material behavior. The findings of this study can contribute to a more detailed understanding of the relationships: material properties—tool properties—deformation properties.

Numerical prediction of the ductile damage in single point incremental forming process

The incremental sheet forming (ISF) is a flexible forming technology in the sheet metal engineering. This process reaches a higher formability of manufactured sheets than conventional sheet metal stamping process. A deep insight into parameters influencing this process is crucial for more understanding of the process. This paper presents a numerical investigation of damage mechanism during the Single Point Incremental sheet metal Forming (SPIF) of a part with conical shape. An elasto-plastic constitutive model with quadratic yield criteria of Hill’48 and mixed isotropic/kinematic hardening behavior is adopted to simulate ISF operation. A user material subroutine (VUMAT) is employed to implement the material behavior of the sheet metal. The efficiency of the finite element model to predict the material deformation process is investigated by comparing numerical and experimental results. A comparison between two hardening models in terms of deformed shape after springback and thickness evolution is presented. Damage evolution is investigated by comparing the results of two hardening models. The final part is devoted to study the influence of some process parameters on the damage evolution and forming effort during the SPIF of studied parts.

Elastic crack propagation model for crystalline solids using a self-consistent coupled atomistic–continuum framework

Deformation and failure processes of crystalline materials are governed by complex phenomena at multiple scales. It is necessary to couple these scales for physics-based modeling of these phenomena, while overcoming limitations of modeling at individual scales. To address this issue, this paper develops self-consistent elastic constitutive and crack propagation relations of crystalline materials containing atomic scale cracks, from observations made in a concurrent multi-scale simulation system coupling atomistic and continuum domain models. The concurrent multi-scale model incorporates a finite temperature atomistic region containing the crack, a continuum region represented by a self-consistent crystal elasticity constitutive model, and a handshaking interphase region. Atomistic modeling is done by the molecular dynamics code LAMMPS, while continuum modeling is conducted by the finite element method. For single crystal nickel a nonlinear and nonlocal crystal elasticity constitutive relation is derived, consistent with the atomic potential function. An efficient, staggered solution scheme with parallel implementation is designed for the coupled problem. The atomistic–continuum coupling is achieved by enforcing geometric compatibility and force equilibrium in the interphase region. Quantitative analyses of the crack propagation process focuses on size dependence, strain energy release rate, crack propagation rate and degradation of the local stiffness. The self-consistent constitutive and crack propagation relations, derived from the concurrent model simulation results are validated by comparing results from the concurrent and full FE models. Excellent accuracy and enhanced efficiency are observed in comparison with pure MD and concurrent model results.

Development of Criteria for Reliability of the Prediction of the Deformation and Relaxation Processes of Polymeric Materials

The problems of the reliability of the prediction of the relaxation and deformation properties of polymer materials in the region of action of nondestructive loads close to the conditions of their operation are considered on the basis of mathematical modeling of the relaxation and creep processes. For practical evaluation of the reliability of this prediction, integral criteria are obtained on the basis of the determining equations of deformation and relaxation processes of these materials.

A new numerical modelling method for deformation behaviour of metallic porous materials using X-ray computed microtomography

Microtomographic devices have limited imaging accuracy and are often insufficient for proper mapping of small details of real objects (e.g. elements of material mesostructures). This paper describes a new method developed to compensate the effect of X-ray computed microtomography (micro-CT) inaccuracy in numerical modelling of the deformation process of porous sintered 316L steel. The method involves modification of microtomographic images where the pore shapes are separated. The modification consists of the reconstruction of fissures and small pores omitted by micro-CT scanning due to the limited accuracy of the measuring device. It enables proper modelling of the tensile deformation process of porous materials. In addition, the proposed approach is compared to methods described in the available literature. As a result of numerical calculations, stress and strain distributions were obtained in deformed sintered 316L steel. Based on the results, macroscopic stress-strain curves were received. Maximum principal stress distributions obtained by the proposed calculation model, indicated specific locations, where the stress reached a critical value, and fracture initiation occurred. These are bridges with small cross sections and notches in the shape of pores. Based on calculation results, the influence of the deformation mechanism of the material porous mesostructures on their properties at the macroscale is described.

Numerical solutions of metal plastic flow in compression tests with sinusoidal profile of die roughness

During metal bulk-forming in microscale the so-called size effect, making changes in the material deformation process should be taken into account. One of the factors having a significant influence on the flow mechanics is a die roughness. Paper contains results of numerical analysis of the cylindrical sample compression between anvils with sinusoidal profile of roughness and with zero friction factor at the tool-workpiece interface in dry conditions. Two models of dies were used: parallel and conical one. Obtained data were compared to analogous, conventional models of compression tests with flat dies and with constant friction factor – equivalent to sinusoidal wave. Sinusoidal curve modeling the die roughness was determined from the surface roughness profile and using the motifs method. Presented analysis revealed increase in value and in variability of stress and strain in layers flowing in the vicinity of the wave – size effect. No impact of the wave presence on the compression forces values was obtained. DOI: http://dx.doi.org/10.5755/j01.mech.22.6.13574

Investigation on size effect of specific cutting energy in mechanical micro-cutting

Mechanical micro-cutting is one of advanced processes to manufacture the miniature parts. The uncut chip thickness is one of the two important parameters in mechanical cutting. The uncut chip thickness and the cutting edge radius are at the same length scale, and the cutting tool extrudes and shears the workpiece with the round cutting edge during micro-cutting process. Size effect of specific cutting energy in micro-cutting is significant in comparison with that in macro cutting. A slip-field model is proposed in this paper to analyze the deformation process of workpiece material in micro-cutting. Johnson–Cook constitutive model, which includes shear strain hardening, shear strain rate hardening, shear temperature softening, and plasticity strain gradient (PSG) effects, is applied to the calculation of the shear flow stress at the primary shear zone (PSZ). Predicted cutting force using the shear flow stress is validated by the micro-orthogonal cutting experiments. The specific cutting energy is also calculated. It shows that the size effect on specific cutting energy in micro-cutting is influenced greatly by the shear strain hardening, shear strain rate hardening, shear temperature softening, and relative cutting length due to the ratio of uncut chip thickness to cutting edge radius. The plasticity strain gradient has effect on the specific cutting energy when the uncut chip thickness is smaller than 25 μm for micro-orthogonal cutting AISI 1045 steel.

Cracks Propagation as a Function of Grain Size Variants on Nanocrystalline Materials’ Yield Stress Produced by Accumulative Roll-Bonding

Cracks are usually observed at the edge of materials deformed by accumulative roll bonding from conventional materials to nanostructure materials. The observed cracks then propagate in the materials during grain refinement. The cracks propagation affects the yield stress and the effective fracture energy of nanocrystalline materials. In this study, the impacts of crack propagation when measured as a function of grain size variants on nanocrystalline materials’ yield stress are investigated for a material deformed by accumulative roll-bonding. The study employs experimental data and theoretical concepts of severe plastic deformation and cracks processes in nanocrystalline materials. The current studies also focus on nano-cracks that will not lead to rapid materials failure during grain refinement. The study revealed that crack propagation varied as a function of grain size variants during grain refinement. The study also revealed that nano-crack increased during the deformation of nanostructured materials. The study also revealed that the effective fracture energy decreased as grain refinement took place. The study revealed that nanomaterials yield stress decreased with the increase in effective fracture energy. The current study suggests a theoretical model that shows the generation of nanomaterials cracks during grain refinement as a function of grain size variants. In the model, the cracks propagate on nanocrystalline materials due to the compressive load applied to a material. The model predicts that the generation of cracks as functions of grain size variants impacts the energy level in nanocrystalline materials.

High-speed X-ray PCI and XRD During Dynamic Fracture

When a crack or a shear band propagates in a ductile material under impact loading, there is a severe stress concentration and/or strain localization near the tip of the discontinuity. It has been understood that the locally severe and fast deformation can lead to phase transformation and heat generation. The adiabatic conditions cause the local temperature to rise. However, it has been a challenge to experimentally measure the microstructural evolution, such as crystal d-spacing, texture evolution and material phase transitions, associated with the dynamic deformation and fracture process of ductile metallic materials. It is also a challenge for optical high-speed imaging techniques to image the details of the advancing crack tip with sufficient spatial resolution. In the dynamic experiments presented in this article, we used Kolsky bars integrated with a simultaneous X-ray phase contrast imaging (PCI) and diffraction (XRD) technique to study the dynamic deformation and fracture processes during high-rate loading. In such a Kolsky bar experiment, high-speed imaging of the specimen deformation and fracture processes and high-speed X-ray diffraction are recorded simultaneously in real time. The experimental setups and results on the dynamic fracture of an aluminum alloy and a bovine tibia bone are presented and discussed.

Single-point incremental forming of sheet metals: Experimental study and numerical simulation

In this article, the single-point incremental forming of sheet metals made of micro-alloyed steel and Al alloy is investigated by combining the results of numerical simulation and experimental characterization, performed during the process, as well as on the final product. A finite element model was developed to perform the process simulation, based on an explicit dynamic time integration scheme. The finite element outcomes were validated by comparison with experimental results. In particular, forming forces during the process, as well as the final shape and strain distribution on the finished component, were measured. The obtained results showed the capability of the finite element modelling to predict the material deformation process. This can be considered as a starting point for the reliable definition of the single-point incremental forming process parameters, thus avoiding expensive trial-and-error approaches, based on extensive experimental campaigns, with beneficial effects on production time.

Glassy features of crystal plasticity

Crystal plasticity occurs by deformation bursts due to the avalanche-like motion of dislocations. Here we perform extensive numerical simulations of a three-dimensional dislocation dynamics model under quasistatic stress-controlled loading. Our results show that avalanches are power-law distributed, and display peculiar stress and sample size dependence: The average avalanche size grows exponentially with the applied stress, and the amount of slip increases with the system size. These results suggest that intermittent deformation processes in crystalline materials exhibit an extended critical-like phase in analogy to glassy systems, instead of originating from a non-equilibrium phase transition critical point.

Ambiguity of the critical load for spherical shells with shear damageability of the material

The structural-probabilistic approach to the modeling of combined crack formation and material deformation processes is used to develop a technique for solving bifurcation stability problems for thin-walled structural members made of damageable materials under single and repeated loadings. The example of a uniformly compressed spherical shell is used to show that, under repeated loading, thin-walled structural members made of shear damageable materials can lose stability under loads smaller than the upper critical loads. The ambiguity of the critical loads for various damage accumulation processes in the material of thin-walled structures depends on the level and character of loading. This phenomenon can be one possible cause of the experimental data spread and the discrepancy between theoretical and experimental results used to determine the critical loads for spherical and cylindrical shells.

Evaluating scintillator performance in time-resolved hard X-ray studies at synchrotron light sources.

The short pulse duration, small effective source size and high flux of synchrotron radiation is ideally suited for probing a wide range of transient deformation processes in materials under extreme conditions. In this paper, the challenges of high-resolution time-resolved indirect X-ray detection are reviewed in the context of dynamic synchrotron experiments. In particular, the discussion is targeted at two-dimensional integrating detector methods, such as those focused on dynamic radiography and diffraction experiments. The response of a scintillator to periodic synchrotron X-ray excitation is modelled and validated against experimental data collected at the Diamond Light Source (DLS) and European Synchrotron Radiation Facility (ESRF). An upper bound on the dynamic range accessible in a time-resolved experiment for a given bunch separation is calculated for a range of scintillators. New bunch structures are suggested for DLS and ESRF using the highest-performing commercially available crystal LYSO:Ce, allowing time-resolved experiments with an interframe time of 189 ns and a maximum dynamic range of 98 (6.6 bits).

Numerical study on the fracture characteristics of projectile material under impact loading

As one of the most common material used for projectile, the numerical simulation about high strength steel about its fracture characteristics under impact loading has always been emphasized. These kinds of problem often involved in the non-linear dynamic response and fracture of multi-material at high temperatures and pressures and strain rate. We put forwards a kind of mapping algorithm about local marked particles through coupling Lagrangian particles and Eulerian meshes. It is obtained by adding marked particles with regular hexahedral influence domain. According to the topological relation between particles and cells, the physical variables of cell are mapped to particles by weighted influence domain. Based on MPI standards, 3D parallel multi-material program with particles was developed. Besides, Johnson-Cook constitutive equation and fracture criterion about effective strain. A series of penetration in different working conditions were simulated and compared with experiment. It is found that the algorithm, which combines the best feature of Eulerian method and Lagrangian method, could be applied in simulating large deformation and fracture process of metal material. At the same time, the computational efficiency and accuracy could also be guaranteed. It is concluded that the improvement could make the algorithm more suitable for the simulation of various impact dynamic problems.

Discrete element analysis for shear band modes of granular materials in triaxial tests

ABSTRACTThe discrete element method (DEM) is adopted to simulate the triaxial tests of granular materials in this study. In the DEM simulations, two different membrane-forming methods are used to generate triaxial samples. One method is to pack the internal particles first, then to generate the enclosed membrane; the other is to generate the internal particles and the enclosed membrane together. A definition of the effective strain, which combines microscopic numerical results with macroscopic expression in three-dimensional space, is presented to describe the macroscopic deformation process of granular materials. With these two membrane generation methods, the effective strain distributions in longitudinal section and transverse section of the triaxial sample are described to investigate the progressive failure and the evolution of the shear bands in granular materials. Two typical shear band failure modes in triaxial tests are observed in the DEM simulations with different membrane-forming methods. One is a single shear band like a scraper bowl, and the other is an axial symmetric shear band like two hoppers stacking as the shape of rotational “X” in triaxial sample. The characteristics of the shear bands during the failure processes are discussed in detail based on the DEM simulations.

A new configuration in a prosthetic knee using of hybrid concept of an MR brake with a T-shaped drum incorporating an arc form surface

This paper focuses on developing a new configuration on magnetorheological (MR) brake damper as prosthetic knee. Prosthetic knee uses magnetic fields to vary the viscosity of the MR fluid, and thereby its flexion resistance. Exerted transmissibility torque of the knee greatly depends on the magnetic field intensity in the MR fluid. In this study a rotary damper using MR fluid is addressed in which a single rotary disc will act as a brake while MR fluid is activated by magnetic field in different walking gait. The main objective of this study is to investigate a prosthetic knee with one activating rotary disc to accomplish necessary braking torque in walking gait via T-shaped drum with arc surface boundary and implementing of Newton's equation of motion to derive generated torque at the inner surface of the rotary drum. For this purpose a novel configuration of a T-shaped drum based on the effects of a material deformation process is proposed. In this new design, the T-shaped disc will increase the effective areas of influences in between drum and MR fluid together and the arc wall crushes the particles chains (fibrils) of the MR fluid together instead of breaking them via strain in a conventional MR brake. To verify the proposed MR brake, results of the proposed and conventional MR brakes are compared together and demonstrated that the resisting torque of the proposed MR brake is almost two times greater than that of the conventional brake.

On Modelling of Creep and Plasticity in a Problem of Viscosimetric Flow of a Material

This paper considers a deformation process of an incompressible elastoviscoplastic material placed between two coaxial rigid cylinders. The material is deformed when the internal cylinder rotates around its axis while the external one is fixed. The adhesion conditions are set on both boundary surfaces. The theory of large elastoviscoplastic deformations is used to solve the problem. The mathematical model is constructed in cylindrical coordinates.

Interactive Rheological Modeling in Elasto-visco-plastic Finite Element Analysis

The elasto-visco-plastic structural analysis with adaptive correction of material model during computation is considered. Several aspects of classification criteria and general principles of the complex rheological models formation are discussed for description of irreversible rate-dependent deformation processes in materials applied in civil and mechanical engineering.

Исследование параметров холодной прокатки ультрамелкозернистого титана

The objective of the work was achieving fatigue and strength properties of commercial titanium VT1-0 by hardening in cold working process. Cold rolling is a common method of hardening of materials and alloys. The interest to cold rolling as a deformation scheme is due to the fact that rolling process is fully explored and widely used in practice. Ultra-fine grained blanks (diameter 40 mm and above) received by severe plastic deformation process were rolled on CNC (computerized numerical control) cluster mill. Standard cylindrical samples for determining tensile and compression properties and fatigue-strength test were made by rolling. Experimental data show an increase of strength characteristics of nanostructural titanium as a result of cold rolling. Samples were also investigated with the CNC-2 plastometer by the method developed by the Machines and Technology of Material Deformation Processes Department of the South Ural State University. After plastometric research the samples were tested to determine deformation resistance, tensile strength and Brinell hardness. Divisibility of deformation increases yield point for all stressing degrees. Increase of σ 0,2 and hardness is a witness of grain refinement. The plasticity research of commercial titanium under divisional stress shows that plasticity of titanium increases with increasing number of stress degrees. Tests showed no significant effect of tempering at intermediate stress degrees on tensile strength and hardness of titanium.

Rotary draw bending of rectangular tubes using a novel parallelepiped elastic mandrel

Rotary draw bending is usually applied to round tubes, since their axisymmetric cross section helps reduce any type of distortion. In the present paper, we investigate rotary draw bending of rectangular tubes employing a novel elastic parallelepiped mandrel. For this purpose, tubes, manufactured by cold forming flat rolled sheets of steel S235JR into rectangular tubular shapes and electric resistance welded into solid wall tubing, were bent, both the “hard way” and the “easy way,” to form a nominal angle of 90°. A finite element (FE) model was developed to perform the process simulation based on an explicit dynamic time integration scheme using the commercial code ABAQUS. The FE outcomes have been validated by comparison with experimental results. Specifically, a coordinate measuring machine was used to determine the degree of bend as well as the radius, the profile, and the wall thickness of the bent sections. The obtained results showed the capability of the FE modeling to predict the material deformation process. Implementing the stiffness of the welding seam into the FE model of the tube dramatically increases the accuracy of the numerical simulations. The validated model is adopted for investigating the effect of mandrel position, clearance, and friction and how they can be set to minimize the tube distortions.