Strain-gradient crystal-plasticity modelling of micro-cutting of b.c.c. single crystal

In recent years thanks to enhancements in design of advanced machines, laser metrology and computer control, ultra-precision machining has become increasingly important. In micromachining of metals the depth of cut is usually less than the average grain size of a polycrystalline aggregate; hence, a cutting process can occur entirely within a single crystal. The respective effect of crystallographic anisotropy requires development of machining models that incorporate crystal plasticity for an accurate prediction of micro-scale material removal under such conditions. To achieve this, a 3D finite-element model of orthogonal micro-cutting of a single crystal of b.c.c. brass was implemented in a commercial software ABAQUS/Explicit using a user-defined subroutine VUMAT. Strain-gradient crystal-plasticity theories were used to demonstrate the influence of evolved strain gradients on the cutting process for different cutting directions

Temperature-dependent crystal-plasticity model for magnesium: a bottom-up approach

A crystal-plasticity model is developed to account for temperature-dependent mechanical behaviour of magnesium in this paper. The constitutive description of plastic deformation accounts for crystalline slip and twining as well as their interactions. The temperature dependence is incorporated into the constitutive equations for both slip and twin modes based on experimental observations. A bottom-up computational modelling framework is proposed to validate the developed constitutive model. First, the crystal-plasticity model is calibrated with experimental results for plane compression at micro-scale. At meso-scale, a three-dimensional representative element volume was adopted to represent the microstructure of polycrystalline magnesium. In the combination with the proposed constitutive theory, the effects of temperature on mechanical response and evolution of twins and texture in polycrystalline magnesium were predicted. Comprehensive experimental validations at meso-scale were performed to consolidate further the developed crystal-plasticity model incorporating temperature dependence in terms of stress-strain curves, the Hall-Petch relationship and texture evolution. This work provides a useful modelling tool for understanding temperature-dependent behaviour of magnesium, which could be used to improve the formability of this family of materials

Dynamic behavior of advanced Ti alloy under impact loading: experimental and numerical analysis

Industrial applications of Ti-based alloys, especially in aerospace, marine and offshore industries, have grown significantly over the years primarily due to their high strength, light weight as well as good fatigue and corrosion-resistance properties. A combination of experimental and numerical studies is necessary to predict a material behavior of such alloys under high strain-rate conditions characterized also by a high level of strains accompanied by high temperatures. A Split Hopkinson Pressure Bar (SHPB) technique is a commonly used experimental method to characterize a dynamic stress-strain response of materials at high strain rates. In a SHPB test, the striker bar is shot against the free end of the incident stress bar, which on impact generates a stress pulse propagating in the incident bar towards the specimen sandwiched between the incident and transmitted bars. An experimental study and a numerical analysis based on a three-dimensional finite element model of the SHPB experiment are performed in this study to assess various features of the underlying mechanics of deformation processes of the alloy tested at high-strain and -strain-rate regimes

A crystal-plasticity model of extruded AM30 magnesium alloy

An enhanced crystal-plasticity finite-element model is developed to model the effects of texture, grain size and loading direction on asymmetrical tension-compression behaviour of AM30 magnesium alloy. A constitutive description of plastic deformation in the suggested scheme accounts for contributions from deformation slip and twinning. The calibrated model was employed to investigate the effects of texture and grain size on the yield stress and strain- - hardening behaviour of AM30 magnesium alloy at room temperatures under various loading conditions. The study reveals that grain refinement and initial texture significantly influence the mechanical behaviour of AM30. Results show that the key factor controlling the tension compression asymmetry is deformation twinning. Two techniques, which could be used to reduce this asymmetry, are grain refinement and weakening of the initial texture in extruded AM30

Ultrasonically-assisted polymer molding: an evaluation

Energy reduction in extrusion and injection molding processes can be achieved by the introduction of ultrasonic energy. Polymer flow can be enhanced on application of ultrasonic vibration, which can reduce the thermal and pressure input requirements to produce the same molding; higher productivity may also be achieved. In this paper, a design of an ultrasound–assisted injection mold machine is explored. An extrusion-die design was augmented with a commercial 1.5 kW ultrasonic transducer and sonotrode designed to resonate close to 20 kHz with up to 100 ȝPY vibration amplitude. The design was evaluated with modal and thermal analysis using finite-element analysis software. The use of numerical techniques, including computational fluid dynamics, fluid-structure interaction and coupled Lagrangian-Eulerian method, to predict the effect of ultrasound on polymer flow was considered. A sonotrode design utilizing ceramic to enhance thermal isolation was also explored

Hybrid cutting of bio-tissues

© 2016 The Authors.Modern-day histology of bio-tissues requires high-precision cutting to ensure high quality thin specimens used in analysis. The cutting quality is significantly affected by a variety of soft and hard tissues in the samples. The paper deals with the next step of microtome development employing controlled ultrasonic vibration to realise a hybrid cutting process of bio-tissues. The study is based on a numerical (finite-element) analysis of multi-body dynamics of a cutting system. Conventional and ultrasonically assisted cutting processes of bio-tissues were simulated using material models representing cancellous bone and incorporating an estimation of friction conditions between a cutting blade and the material to be cut. The models allow adjustments of a section thickness, cutting speed and amplitude of ultrasonic vibration. The efficiency and quality of cutting was dependent on cutting forces, which were compared for both conventional and ultrasonically assisted cutting processes

Indentation studies in b.c.c. crystals with enhanced model of strain-gradient crystal plasticity

An enhanced model of strain-gradient crystal plasticity is used to study the deformation behaviour of b.c.c. single crystals of a β-Ti alloy under indentation. In this model, both incipient strain gradients, linked to a component’s surface-to-volume ratio, and strain gradients evolving in the course of deformation were characterized. The results of numerical simulations are in a good agreement with the obtained experimental data demonstrating an anisotropic nature of surface topographies around the indents performed in different crystallographic orientations. The influence of evolved strain gradients on the surface profile of indents is demonstrated

Indentation study of mechanical behaviour of Zr-Cu-based metallic glass

It has been well known that plastic deformation of bulk metallic glasses (BMGs) is localised in thin shear bands. So, initiation of shear bands and related deformation should be studied for comprehensive understanding of deformation mechanisms of BMGs. In this paper, indentation techniques are extensively used to characterise elastic deformation of Zr-Cu-based metallic glass, followed by a systematic analysis of initiation and evolution of shear bands in the indented materials. Our results, obtained with a suggested wedge-indentation technique, demonstrated initiation of shear bands in materials volume

Mechanical behaviour of silicon carbide under static and dynamic compression

This paper compared the mechanical behaviour of 6H SiC under quasi-static and dynamic compression. Rectangle specimens with a dimension of 3×3×6 mm3 were used for quasi-static compression tests under three different loading rates (i.e., 10-5/s, 10-4/s and 10-3/s). Stress-strain response showed purely brittle behavior of the material which was further confirmed by SEM/TEM examinations of fractured fragments. For dynamic compression, split Hopkinson pressure bar (SHPB) tests were carried out for cubic specimens with a dimension of 6×6×4 mm3. Stress-strain curves confirmed the occurrence of plastic deformation under dynamic compression, and dislocations were identified from TEM studies of fractured pieces. Furthermore, JH2 model was used to simulate SHPB tests, with parameters calibrated against the experimental results. The model was subsequently used to predict strength and plasticity-related damage under various dynamic loading conditions. This study concluded that, under high loading rate, SiC can deform plastically as evidenced by the development of non-linear stressstrain response and also the evolution of dislocations. These findings can be explored to control the brittle behaviour of SiC and benefit end users in relevant industries

Size-dependent crystal plasticity: from micro-pillar compression to bending

Size-dependent crystal plasticity of metal single crystals is investigated using finite-element method based on a phenomenological crystal-plasticity model, incorporating both first-order and second-order effects. The first-order effect is independent of the nature of the loading state, and described by three phenomenological relationships based on experimental results. The second-order effect is considered in terms of storage of geometrically necessary dislocations, affected significantly by the loading state. The modelling approach is shown to capture the influence of loading conditions on the sample size effect observed in compression and bending experiments. A modelling study demonstrates the subtleness and importance of accounting for first-order and second-order effects in modelling crystalline materials in small length-scales