Superplastic flow in a nanostructured aluminum alloy produced using high-pressure torsion

Samples of a spray-cast Al-7034 alloy were processed by high-pressure torsion (HPT) at temperatures of 293 or 473 K using an imposed pressure of 4 GPa and torsional straining through five revolutions. Processing by HPT produced significant grain refinement with grain sizes of 60 and 85 nm at the edges of the disks for the two processing temperatures. In tensile testing at room temperature, the alloy processed by HPT exhibited higher strength and lower ductility than the unprocessed material. Good superplastic properties were achieved in tensile testing at elevated temperatures with a maximum elongation of 750% for the sample processed at 473 K and tested in tension at 703 K under an initial strain rate of 1.0 × 10⁻² s⁻¹. The measured superplastic elongations are lower than in samples prepared by equal-channel angular pressing because of the use of very thin disks in the HPT processing.     

Significance of temperature increase in processing by high-pressure torsion

Experiments and finite element simulations were conducted to measure the temperature increase in processing disc samples by high-pressure torsion. Aluminum, copper, iron and molybdenum were selected as model materials. The temperature increases at the early stages of straining but saturates to steady-state levels at large strains. The increase of temperature is proportional to the hardness and rotation speed and is higher at higher imposed pressures and is somewhat higher at larger distances from the disc center.     

Allotropic phase transformation of pure zirconium by high-pressure torsion

Pure Zr is processed by high-pressure torsion (HPT) at pressures in the range of 1–40 GPa. A phase transformation occurs from α to ω phase during HPT at pressures above 4 GPa while the total fraction of ω phase increases with straining and saturates to a constant level at higher strain. This phase transformation leads to microstructural refinement, hardness and strength enhancement and ductility reduction. Lattice parameter measurements confirm that c for α phase is expanded about 0.6% by the presence of ω phase. The temperature for reverse transformation from ω to α phase increases with straining and thus, straining under high pressure increases thermal stability of ω phase. The ω phase obtained by HPT is stable for more than 400 days at room temperature.     

Using high-pressure torsion for the cold-consolidation of copper chips produced by machining

High-pressure torsion (HPT) was used to consolidate chips machined from coarse-grained copper and from copper processed by ECAP. The results are compared with discs prepared by HPT and with discs processed by a combination of equal-channel angular pressing (ECAP) and HPT. It is demonstrated that the consolidated discs have exceptionally fine microstructures and high hardness. The results suggest the possibility of a saturation in grain refinement which may be related to a release in heat during high-pressure straining.     

An experimental verification of the finite element modelling of equal channel angular pressing

Strain hardening of pure copper and friction between the sample and die channels is considered for finite element modelling. To validate the FEM results, the FEM calculated effective strain variations were compared with the hardness measurements. Simulated load–stroke curve and peak load calculations were also compared with the experimentally recorded load–stroke curve and peak load. Different stages of the load–stroke curve of the ECAP process was explained in detail. In over all, good conformity is observed between the FEM calculations and experimental results.     

Parametric study of stress state development during twinning using 3D finite element modeling

A deformation twinning model which simulates the characteristic twin shear and corresponding grain reorientation has been developed using a 3D finite element method. This model has been used to study how twinning affects the stress state in both the parent grain and twin, and the stress states that are energetically favorable for twinning. The component of shear stress on the twin plane and direction is primarily responsible not only for whether twinning can occur, but also the energetically favorable twin volume fraction. A map predicting twin volume fraction as a function of parent grain deviatoric stress has been developed.     

Seat-cushion and soft-tissue material modeling and a finite element investigation of the seating comfort for passenger-vehicle occupants

Improved seating comfort is an important factor that most car manufacturers use to distinguish their products from those of their competitors. In today’s automotive engineering practice, however, design and development of new, more comfortable car seats is based almost entirely on empiricism, legacy knowledge and extensive, time-consuming and costly prototyping and experimental/field testing. To help accelerate and economize the design/development process of more-comfortable car seats, more extensive use of various computer aided engineering (CAE) tools will be necessary. However, before the CAE tools can be used more successfully by car-seat manufacturers, issues associated with the availability of realistic computer models for the seated human, the seat and the seated-human/seat interactions as well as with the establishment of objective seating-comfort quantifying parameters must be resolved.In the present work, detailed finite element models of a prototypical car seat and of a seated human are developed and used in the investigation of seated-human/seat interactions and the resulting seating comfort. To obtain a fairly realistic model for the human, a moderately detailed skeletal model containing 16 bone assemblies and 15 joints has been combined with an equally detailed “skin” model of the human. The intersection between the two models was then used to define the muscular portion of the human. Special attention in the present work has been given to realistically representing/modeling the materials present in different sections of the car seat and the seated human. The models developed in the present work are validated by comparing the computational results related to the pressure distribution over the seated-human/seat interface with their open-literature counterparts obtained in experimental studies involving human subjects.     

Random response of a rotating composite blade with flexure–torsion coupling effect by the finite element method

A finite element model is employed to investigate the mean square response of a damped rotating composite blade with flexure–torsion interaction under stationary or non-stationary random excitation. The effects of transverse shear deformation and rotary inertia are considered. The finite element model can satisfy all the geometric and natural boundary conditions of a thick blade. The blade is considered to be subjected to white noise, band-limited white noise or filtered white noise excitation. The numerical results indicate that the increment of rotational speed will reduce the mean square response. It is also found that the mean square response decreases when the low natural frequency of base decreases. Inversely, the mean square response increases when the high natural frequency of base decreases. It is also shown that the fiber orientations have a significant effect on the mean square response of an orthotropic blade under random excitations. Moreover, the flexure–torsion coupling effect on the mean square response is changed by different fiber orientations.     

Finite element analysis of the plastic deformation in tandem process of simple shear extrusion and twist extrusion

Recently, simple shear extrusion (SSE) and twist extrusion (TE) are introduced to fabricate ultrafine grained bulk rod metallic materials. The SSE and TE processes generate significant deformation inhomogeneity, with higher and lower strains in the center, respectively, which easily causes mechanical instability of the materials. In this study, to overcome this deformation inhomogeneity problem in SSE and TE, a tandem process of SSE and TE (TST) is suggested. The finite element method is applied for plastic deformation behavior during the TST process. The results demonstrate that the TST process can produce relatively homogeneously deformed materials. In particular, the effects of back pressure and processing order on the plastic deformation behaviors in the TST process are systematically analyzed.     

Deformation behavior in Simple Shear Extrusion (SSE) as a new severe plastic deformation technique

A new method for severe plastic deformation is proposed herein, entitled as Simple Shear Extrusion (SSE) due to the manner in which specimen's cross-section shape changes. This method is based on pressing material through a specially designed direct extrusion die. The process was investigated experimentally on commercially pure aluminum. Additionally, simulation of the process was also carried out, using the commercial finite element code ABAQUS/Explicit. Moreover, effect of back-pressure on the results was studied. Results show that SSE method is capable of imposing high strain values via strain accumulation during repeating the process, which is of great importance in producing ultrafine-grained or nanostructured materials.     

Optimal tool angles for equal channel angular extrusion of strain hardening materials by finite element analysis

Equal channel angular extrusion (ECAE) is a novel deformation process capable of imparting a large amount of plastic strain to bulk material through the application of uniform simple shear. ECAE die geometry, material properties and process conditions influence the shear deformation behavior during extrusion that in turn governs the microstructure and mechanical properties of the extruded materials. Finite element analysis, the most appropriate technique was used to analyze the deformation behavior of extruded materials without neglecting important and realistic factors like strain hardening behavior of the material, frictional conditions and speed of the process. In this study the deformation behavior of material, dead zone/corner gap formation and strain homogeneity achieved in the samples during ECAE were studied by using commercial finite element code Abaqus/Explicit. The influence of tool angles, strain hardening behavior of material and friction between the billet and die was considered for simulations. Results showed that the optimal strain homogeneity in the sample with lower dead zone formation, without involving any detrimental effects, can be achieved with channel angle of 90° and outer corner angle of 10°.     

Multiscale finite element modeling of failure process of composite laminates

A multiscale nonlinear finite element modeling technique is developed in this paper to predict the progressive failure process for composite laminates. A micromechanical elastic–plastic bridging constitutive model, which considers the nonlinear material properties of the constituent fiber and matrix materials and their interaction and the damage and failure in fibrous composites at the fiber and matrix level, is proposed to represent the material behavior of fiber-reinforced composite laminates. The micromechanics constitutive model is employed in the macroscale finite element analysis of structural behavior especially progressive failure process of the fiber-reinforced composites based on a 4-node 24-DOF shear-locking free rectangular composite plate element.     

Developments in ultrasonic modeling with finite element analysis

An overview is given of finite element analysis and its application to the modeling of ultrasonic nondestructive evaluation phenomena. Following a discussion of the underlying weighted residual methodology, a mass-lumping technique is described which results in an efficient computer implementation for 2D geometries. Code predictions are compared with both analytical and experimental results, and data from studies of attenuation, anisotropy, defect interactions, and surface waves are given. Initial results from a full 3D formulation are also shown.     

Multiscale finite element modeling of superelasticity in Nitinol polycrystals

An efficient method is proposed for modeling superelastic polycrystalline NiTi by solving a two-scale problem. The RVE size of the fine scale is determined using a statistics-based approach. Both problems are discretized in space using the finite element method and their communication is effected using MPI. Representative simulations illustrate the modeling capabilities of the proposed approach.     

Finite element analysis of the effect of the inner corner angle in equal channel angular pressing

The inner corner angle (ICA) is one of the major factors affecting deformation homogeneity in workpieces during equal channel angular pressing (ECAP). In this study, the effect of the ICA on the plastic deformation behavior in ECAP was investigated using the finite element method. A round ICA induces highly inhomogeneous deformation in the head, tail, top and bottom regions of the workpiece due to increasing compressive and decreasing shear deformation components. It was found that a round inner corner with an angle up to 9° is acceptable in finite element simulations for reproducing a sharp inner corner. These results can serve as a design guide for processing and dies of ECAP.     

Micromechanical formulation and 3D finite element modeling of microinstabilities in composites

A 3D micromechanical formulation and a FE-model of fiber micro-buckling in materials with isotropic and transversal isotropic fibers in compression is presented. Three variants of geometrical modeling of the characteristic cell are proposed and compared. An appropriate one is then selected. An eigenvalue analysis of a characteristic cell is performed. The results show that the fiber anisotropy reduces significantly the critical loads and must be taken into account.     

Finite element analysis of rotary-die equal channel angular pressing

In this paper, the finite element method (FEM) was applied to analyze the plastic flow and strain hardening behavior of pure copper, subjected to rotary-die equal channel angular pressing (RD-ECAP) up to four passes. The die was rotated 90° counter clockwise between the passes in the simulation. The effective strain distribution and load–stroke curves were investigated. The load was increased with the number of rotary-die equal channel angular pressing passes. The results show that, plastic deformation becomes inhomogeneous with the number of passes due to an end effect, which was not found seriously in conventional equal channel angular pressing (ECAP). Especially, decreasing corner gap with increasing the number of passes was observed and explained by the strain hardening effect.     

Finite element modeling of GTA weld surfacing applied to hot-work tooling

A finite element modeling of GTA weld-surfacing process is performed using computer code. The model is used to optimize welding parameters by analyzing temperature transients during welding, as well as deformation and residual stresses as a result of repair-weld surfacing of complex-geometry H13 tooling. In addition to computational analysis, an extensive experimental study is performed. Both remelting and surfacing are performed by GTA welding. A series of welds are prepared using different welding parameters. The temperature transients are measured at a number of positions in the material and macrosections of the welds are prepared. The data are used to develop a relationship between the welding parameters and characteristic weld dimensions, to select the most appropriate geometry of the heat source, and finally to verify the model. The model developed is applied to predict deformation and residual stresses and detect areas critical to cracking at repair-welding of complex-geometry tooling. Understanding these parameters is significant for quality improvement of weld-surfaced tooling and thus essential for extension of in-service life of refurbished tooling.     

An augmented finite element method for modeling arbitrary discontinuities in composite materials

An augmented finite element method (“A-FEM”) is presented that is a variant of the method of Hansbo and Hansbo (Comput Methods Appl Mech Eng, 193: 3523–3540, 2004), which can fully account for arbitrary discontinuities that traverse the interior of elements. Like the method of Hansbo and Hansbo, the A-FEM preserves elemental locality, because element augmentation is implemented within single elements and involves nodal information from the modified element only. The A-FEM offers the additional convenience that the augmentation is implemented via separable mathematical elements that employ standard finite element nodal interpolation only. Thus, the formulation is fully compatible with standard commercial finite element packages and can be incorporated as a user element without access to the source code. Because possible discontinuities include both elastic heterogeneity and cracks, the A-FEM is ideally suited to modeling damage evolution in structural or biological materials with complex morphology. Elements of a multi-scale approach to analyzing damage mechanisms in laminated or woven textile composites are used to validate the A-FEM and illustrate its possible uses. Key capabilities of the formulation include the use of meshes that need not conform to the surfaces of heterogeneities; the ability to apply the augmented element recursively, enabling modeling of multiple discontinuities arising on different, possibly intersecting surfaces within an element; and the ease with which cohesive zone models of nonlinear fracture can be incorporated.     

In-plane shear investigation of biaxial carbon non-crimp fabrics with experimental tests and finite element modeling

In-plane shear is one of the basic deformation mechanisms in forming fabrics on complicated shapes. In this paper, the in-plane shear behavior of non-crimp fabrics (NCFs), including NCFs based on T300 carbon fibers with chain or tricot-chain stitches, was characterized by picture frame and bias extension tests. It was found that the stitching yarns’ strained condition depending on loading direction influences the shear behavior of NCFs. Further, the results of these two tests were compared by normalizing the shear force. It was observed that the normalized results of these two tests for shear force are consistent with each other in the direction of shear of the stitching, while deviations in other directions are attributed to the different strain mechanisms, as a result of the clamping way of the sample in the test. Finally, a mesoscopic finite element (FE) model was established to simulate the picture frame and bias extension tests for the selected T300 NCF with chain stitches. The model’s validity was checked by comparing the simulated results with the experimental ones. Although some improvements are still needed, the model provides encouraging results and good foundations to predict the shear behavior for NCFs’ forming.