Anish Roy

Size effects and idealized dislocation microstructure at small scales: Predictions of a Phenomenological model of Mesoscopic Field Dislocation Mechanics: Part II

In Part I of this set of two papers, a model of mesoscopic plasticity is developed for studying initial-boundary value problems of small scale plasticity. Here we make qualitative, finite element method-based computational predictions of the theory. We demonstrate size effects and the development of strong inhomogeneity in simple shearing of plastically-constrained grains. Nonlocality in elastic straining leading to a strong Bauschinger effect is analyzed. Stability of the time dependent, spatially homogeneous, simple shearing solution of PMFDM is studied. Results from thermal cycling of small scale beams/films with different degrees of constraint to plastic flow are presented showing size effects and reciprocal-film-thickness scaling of dislocation density boundary layer width.

Enhanced ultrasonically assisted turning of a β-titanium alloy

Although titanium alloys have outstanding mechanical properties such as high hot hardness, a good strength-to-weight ratio and high corrosion resistance; their low thermal conductivity, high chemical affinity to tool materials severely impair their machinability. Ultrasonically assisted machining (UAM) is an advanced machining technique, which has been shown to improve machinability of a β-titanium alloy, namely, Ti-15-3-3-3, when compared to conventional turning processes.

Strength prediction for bi-axial braided composites by a multi-scale modelling approach

Braided textile-reinforced composites have become increasingly attractive as protection materials thanks to their unique inter-weaving structures and excellent energy-absorption capacity. However, development of adequate models for simulation of failure processes in them remains a challenge. In this study, tensile strength and progressive damage behaviour of braided textile composites are predicted by a multi-scale modelling approach. First, a micro-scale model with hexagonal arrays of fibres was built to compute effective elastic constants and yarn strength under different loading conditions. Instead of using cited values, the input data for this micro-scale model were obtained experimentally. Subsequently, the results generated by this model were used as input for a meso-scale model. At meso-scale, Hashin’s 3D with Stassi’s failure criteria and a modified Murakami-type stiffness-degradation scheme was employed in a user-defined subroutine developed in the general-purpose finite-element software Abaqus/Standard. An overall stress–strain curve of a meso-scale representative unit cell was verified with the experimental data. Numerical studies show that bias yarns suffer continuous damage during an axial tension test. The magnitudes of ultimate strengths and Young’s moduli of the studied braided composites decreased with an increase in the braiding angle.

Turning of Advanced Alloys with Vibrating Cutting Tool

A demand for high-strength alloys in aerospace, marine and off-shore industries has stimulated development of new and efficient machining techniques. In the recent past, a novel machining technique known as ultrasonically assisted turning (UAT) has been introduced; in it low-energy ultrasonic vibration is superimposed on movement of a cutting tool. In the present work, a comparative study of machining of two advanced alloys - Ti15V3Cr3Al3Sn and Inconel 718 - is carried out numerically by developing a two-dimensional finite-element model of the turning process. A non-linear material description is used in the FE model to incorporate plastic deformation behaviour of the high-strength alloys. The model is employed to investigate the effect of tool geometry and contact conditions on cutting forces, temperature of the cutting region and the chip shape in orthogonal turning of modern alloys.

Analysis of Forces in Vibro-Impact and Hot Vibro-Impact Turning of Advanced Alloys

Analysis of the cutting process in machining of advanced alloys, which are typically difficult-to-machine materials, is a challenge that needs to be addressed. In a machining operation, cutting forces causes severe deformations in the proximity of the cutting edge, producing high stresses, strain, strain-rates and temperatures in the workpiece that ultimately affect the quality of the machined surface. In the present work, cutting forces generated in a vibro-impact and hot vibro-impact machining process of Ti-based alloy, using an in-house Ultrasonically Assisted Turning (UAT) setup, are studied. A three-dimensional, thermo-mechanically coupled, finite element model was developed to study the thermal and mechanical processes in the cutting zone for the various machining processes. Several advantages of ultrasonically assisted turning and hot ultrasonically assisted turning are demonstrated when compared to conventional turning.

Cutting forces in ultrasonically assisted drilling of carbon fibre-reinforced plastics

Ultrasonically assisted drilling (UAD) is a non-traditional hybrid machining process, which combines features of conventional drilling and vibratory machining techniques to obtain remarkable improvements in machinability of advanced materials. The experiments are conducted on commercially available samples of a carbon fibre-reinforced plastic (CFRP) at a feed rate of 16 mm/min. In this study, a thrust force reduction in excess of 60% is observed in UAD when compared to conventional drilling (CD). Lower delamination was observed when compared to CD techniques. Optical microscopy revealed that the material is removed as a continuous chip in UAD whereas in case of CD we observe powdered dust. Light and scanning electron microscopy of CFRP chips obtained in drilling elucidate fundamental differences in the underlying machining processes in UAD of CFRP.

Analysis of Machinability of Ti- and Ni-Based Alloys

Efficient machining of advanced Ti- and Ni-based alloys, which are typically difficult-to-machine, is a challenge that needs to be addressed by the industry. During a typical machining operation of such alloys, high cutting forces imposed by a tool on the work-piece material lead to severe deformations in the process zone, along with high stresses, strains and temperatures in the material, eventually affecting the quality of finished work-piece. Conventional machining (CT) of Ti- and Ni-based alloys is typically characterized by low depths of cuts and relatively low feed rates, thus adversely affecting the material removal rates (MRR) in the machining process. In the present work, a novel machining technique, known as Ultrasonically Assisted Turning (UAT) is shown to dramatically improve machining of these intractable alloys. The developed machining process is capable of high MRR with an improved surface quality of the turned work-piece. Average cutting forces are significantly lower in UAT when compared to those in traditional turning techniques at the same machining parameters, demonstrating the capability of vibration-assisted machining as a viable machining method for the future.

Finite element analysis of drilling in carbon fiber reinforced polymer composites

Carbon fiber reinforced polymer composite (CFRP) laminates are attractive for many applications in the aerospace industry especially as aircraft structural components due to their superior properties. Usually drilling is an important final machining process for components made of composite laminates. In drilling of CFRP, it is an imperative task to determine the maximum critical thrust forces that trigger inter-laminar and intra-laminar damage modes owing to highly anisotropic fibrous media; and negotiate integrity of composite structures. In this paper, a 3D finite element (FE) model of drilling in CFRP composite laminate is developed, which accurately takes into account the dynamic characteristics involved in the process along with the accurate geometrical considerations. A user defined material model is developed to account for accurate though thickness response of composite laminates. The average critical thrust forces and torques obtained using FE analysis, for a set of machining parameters are found to be in good agreement with the experimental results from literature.

Computational Study of Ultrasonically-Assisted Turning of Ti Alloys

Industrial applications of titanium 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. The machinability of these difficult-to-cut metallic materials with conventional turning (CT) techniques has seen a limited improvement over the years. Ultrasonically-assisted turnning (UAT) is an advanced machining process, which has shown to have specific advantages, especially in the machining of high-strength alloys. In this study a three-dimensional finite element model of ultrasonically-assisted oblique cutting of a Ti-based super-alloy is developed. The nonlinear temperature-sensitive material behaviour is incorporated in our numerical simulations based on results obtained with split-Hopkinson pressure bar tests. Various contact conditions are considered at the tool tip-workpiece interface to get an in-depth understanding of the mechanism influencing cutting parameters. The simulation results obtained are compared for both CT and UAT conditions to elucidate main deformation mechanisms responsible for the observed changes in the material’s responses to cutting techniques.

Ultrasonically assisted turning of Ti-6Al-2Sn-4Zr-6Mo

Titanium alloys are widely used in the aerospace and offshore industries due to their high strength-to-weight ratio sustained at elevated temperatures, their fracture-resistance features and exceptionally good corrosion-resistance properties. However, poor thermal conductivity and high chemical affinity of these alloys to tool materials severely impair their machinability. As a result the machining processes of titanium alloys are typically characterized by low cutting feeds and speeds making production of components uneconomical. Recently, a non-conventional hybrid machining technique, namely, ultrasonically assisted turning has been shown to significantly improve the machinability of intractable alloys with a concomitant improvement in material removal rates, thus improving machining economics. In the current work, a 3D finite element model of turning of Ti-6Al-2Sn-4Zr-6Mo is developed in the commercial software, MSC Marc/Mentat. A constitutive behaviour of the workpiece material under large deformations and elevated temperatures is adequately represented by a Johnson-Cook material model. For validation of the developed numerical model, experimental tests were carried out. The numerical and experimental results were found to be in good agreement.