Showing papers by "Vadim V. Silberschmidt published in 2014"

Thermally enhanced ultrasonically assisted machining of Ti alloy

Abstract: A B S T R A C T Recently, a non-conventional machining technique known as ultrasonically assisted turning (UAT) was introduced to machine modern alloys, in which low-energy, high-frequency vibration is superimposed on the movement of a cutting tool during a conventional cutting process. This novel machining technique results in a multi-fold decrease in the level of cutting forces with a concomitant improvement in surface finish of machined modern alloys. Also, since the late 20th century, machining of wear resistant materials that soften when heated has been carried out with hot machining techniques. In this paper, a new hybrid machining technique called hot ultrasonically assisted turning (HUAT) is introduced for the processing of a Ti-based alloy. In this technique, UAT is combined with a traditional hot machining technique to gain combined advantages of both schemes for machining of intractable alloys. HUAT of the Ti alloy was analysed experimentally and numerically to demonstrate the benefits in terms of reduction in the cutting forces and improvement in surface roughness over a wide range of industrially relevant speed-feed combinations for titanium alloys. 2014 CIRP.

Analysis of temperature in conventional and ultrasonically-assisted drilling of cortical bone with infrared thermography

Abstract: BACKGROUND: Bone drilling is widely used in orthopaedics, dental and neurosurgeries for repair and fixation purposes. One of the major concerns in drilling of bone is thermal necrosis that may seriously affect healing at interfaces with fixtures and implants. Ultrasonically-assisted drilling (UAD) is recently introduced as alternative to conventional drilling (CD) to minimize invasiveness of the procedure. OBJECTIVE: This paper studies temperature rise in bovine cortical bone drilled with CD and UAD techniques and their comparison using infrared thermography. METHODS: A parametric investigation was carried out to evaluate effects of drilling conditions (drilling speed and feed rate) and parameters of ultrasonic vibration (frequency and amplitude) on the temperature elevation in bone. RESULTS: Higher levels of the drilling speed and feed rate were found responsible for generating temperatures above a thermal threshold level in both types of drilling. UAD with frequency below 20 kHz resulted in lower temperature compared to CD with the same drilling parameters. The temperatures generated in cases with vibration frequency exceeding 20 kHz were significantly higher than those in CD for the range of drilling speeds and feed rates. The amplitude of vibration was found to have no significant effect on bone temperature. CONCLUSIONS: UAD may be investigated further to explore its benefits over the existing CD techniques.

3D Finite-Element Modelling of Drilling Cortical Bone: Temperature Analysis

Abstract: Bone drilling is a key part of major orthopaedic surgeries for fixing fractured bones and replacing damaged joints. One of the main problems in bone drilling is thermal necrosis of tissue, which can occur due to elevated temperatures in the drilling zone. Investigation of the temperatures arising in bone drilling is necessary to analyse the extent of bone necrosis. This paper presents a three-dimensional thermo-mechanical finite-element model of a bone-drilling process to study the effect of drilling parameters (cutting speed and feed rate) and cooling conditions (air and saline solution) on the temperature in drilled bone. The drilling speed was found to have a higher effect compared to that of the feed rate in inducing thermal necrosis in bone for the tested cooling environments. The level of necrosis penetration into bone was strongly affected by the drilling speed and the application of saline cooling (irrigation) in the drilling zone. A considerable extent of necrosis was predicted even at lower drilling speeds when no cooling was used. Drilling experiments were performed on real cortical bone to measure temperatures near the immediate vicinity of the drill. Calculated temperatures were compared with experimental values and were found to be in good agreement with them.

Numerical analysis of composite structure with in-plane isotropic negative Poisson’s ratio: Effects of materials properties and geometry features of inclusions

Abstract: A novel composite structure with isotropic negative Poisson’s ratio has been presented in our previous paper [1]. However, the previous study has only focused on the effects of random inclusions. In this work, an extended numerical study on the effects of materials properties and geometry features of inclusions is conducted in order to better understand the deformation mechanism and mechanical properties of this kind of composites. Using finite element method, the overall negative Poisson’s ratio effects and mechanical properties of the composites are analyzed in terms of different factors including thickness and stiffness of inclusions with uniform and non-uniform struts. The structure optimization of the composites with better negative Poisson’s ratio effect and mechanical performance are discussed based on the numerical calculated results. The study shows that the Poisson’s ratio effects and mechanical properties of the composites are significantly affected by geometrical features, stiffness and boundary conditions of inclusions.

Comparative study of conventional and ultrasonically-assisted bone drilling

Abstract: BACKGROUND Bone drilling is a well-known surgical procedure in orthopaedics and dentistry for fracture treatment and reconstruction. Advanced understanding of the mechanics of the drill-bone interaction is necessary to overcome challenges associated with the process and related postoperative complications. OBJECTIVE The aim of this study was to explore the benefits of a novel drilling technique, ultrasonically-assisted drilling (UAD), and its possible utilization in orthopaedic surgeries. METHODS The study was performed by conducting experiments to understand the basic mechanics of the drilling process using high speed filming of the drilling zone followed by measurements to quantify thrust force, surface roughness and cracking of the bone near the immediate vicinity of the hole with and without ultrasonic assistance. RESULTS Compared to the spiral chips produced during conventional drilling (CD), UAD was found to break the chips in small pieces which facilitated their fast evacuation from the cutting region. In UAD, lower drilling force and better surface roughness was measured in drilling in the radial and longitudinal axis of the bone. UAD produced crack-free holes which will enhance postoperative performance of fixative devices anchoring the bone. CONCLUSIONS UAD may be used as a possible substitute for CD in orthopaedic clinics.

Influence of strain gradients on lattice rotation in nano-indentation experiments: a numerical study

Abstract: In this paper the texture evolution in nano-indentation experiments was investigated numerically. To achieve this, a three-dimensional implicit finite-element model incorporating a strain-gradient crystal-plasticity theory was developed to represent accurately the deformation of a body-centred cubic metallic material. A hardening model was implemented to account for strain hardening of the involved slip systems. The surface topography around indents in different crystallographic orientations was compared to corresponding lattice rotations. The influence of strain gradients on the prediction of lattice rotations in nano-indentation was critically assessed.

Assessment of structural integrity of subsea wellhead system: analytical and numerical study

Abstract: Subsea wellhead systems exposed to severe fatigue loading are becoming increasingly a significant problem in offshore drilling operations due to their applications in wells with higher levels of pressure and temperature, situated at larger depths and in harsher environments. This has led to a substantial increase in the weight and size of offshore equipment, which, in combination with different loading conditions related to the environmental factors acting on the vessel and riser, has greatly increased the loads acting on subsea well systems. In particular, severe fatigue loading acting on the subsea wellhead system was detected. For this reason, a combined analytical and numerical study investigating the critical effect of crack depth on the overall structural integrity of subsea wellhead systems under cyclic loading was carried out. The study is based on a Linear Elastic Fracture Mechanics (LEFM) approach.

Ti Alloy with Enhanced Machinability in UAT Turning

Abstract: Metastable β-titanium alloys such as Ti 15V 3Al 3Cr 3Sn are of great technological interest thanks to their high fatigue strength-to-density ratio. However, their high hardness and poor machinability increase machining costs. Additionally, formation of undesirable long chips increases the machining time. To address those issues, a metastable β-titanium alloy (Ti 15V 3Al 3Cr 2Zr 0.9La) with enhanced machinability was developed to produce short chips even at low cutting speeds. A hybrid ultrasonically assisted machining technique, known to reduce cutting forces, was employed in this study. Cutting force components and surface quality of the finished work-pieces were analyzed for a range of cutting speeds in comparison with those for more traditional Ti 15V 3Al 3Cr 3Sn. The novel alloy demonstrated slightly improved machining characteristics at higher cutting speeds and is now ready for industrial applications.

Ultrasonically Assisted Machining of Titanium Alloys

Abstract: In this chapter we discuss the nuances of a non-conventional machining technique known as ultrasonically assisted machining, which has been used to demonstrate tractable benefits in the machining of titanium alloys. We also demonstrate how further improvements may be achieved by combining this machining technique with the well known advantages of hot machining in metals and alloys.

Large deformation of thermally bonded random fibrous networks: microstructural changes and damage

Abstract: A mechanical behaviour of random fibrous networks is predominantly governed by their microstructure. This study examines the effect of microstructure on macroscopic deformation and failure behaviour of random fibrous networks and its practical implication for optimisation of its structure by using finite-element simulations. A subroutine-based parametric modelling approach—a tool to develop and characterise random fibrous networks—is also presented. Here, a thermally bonded polypropylene nonwoven fabric is used as a model system. Its microstructure is incorporated into the model by explicit introduction of fibres according to their orientation distribution in the fabric. The model accounts for main deformation and damage mechanisms experimentally observed and provides the meso- and macro-level responses of the fabric. The suggested microstructure-based approach identifies and quantifies the spread of stresses and strains in fibres of the network as well as its structural evolution during deformation and damage. Its simulations also predict a continuous shift in the distribution of stresses due to structural evolution and progressive failure of fibres.

Modelling fracture processes in bones

Abstract: This chapter discusses numerical simulations of fracture and deformation of a cortical bone tissue employing an extended finiteelement method (X-FEM). The chapter first reviews the literature on available finite-element models for both macroscopic and microscopic fracture behaviours of the cortical bone tissue. It then discusses the formulation and analysis of a number of finite-element models investigating deformation and fracture of cortical bone using X-FEM at different length scales and for various loading conditions.

Three-Dimensional Analysis of the Effect of Material Randomness on the Damage Behaviour of CFRP Laminates with Stochastic Cohesive-Zone Elements

Abstract: Laminated carbon fibre-reinforced polymer (CFRP) composites are already well established in structural applications where high specific strength and stiffness are required. Damage in these laminates is usually localised and may involve numerous mechanisms, such as matrix cracking, laminate delamination, fibre de-bonding or fibre breakage. Microstructures in CFRPs are non-uniform and irregular, resulting in an element of randomness in the localised damage. This may in turn affect the global properties and failure parameters of components made of CFRPs. This raises the question of whether the inherent stochasticity of localised damage is of significance in terms of the global properties and design methods for such materials. This paper presents a numerical modelling based analysis of the effect of material randomness on delamination damage in CFRP materials by the implementation of a stochastic cohesive-zone model (CZM) within the framework of the finite-element (FE) method. The initiation and propagation of delamination in a unidirectional CFRP double-cantilever beam (DCB) specimen loaded under mode-I was analyzed, accounting for the inherent microstructural stochasticity exhibited by such laminates via the stochastic CZM. Various statistical realizations for a half-scatter of 50 % of fracture energy were performed, with a probability distribution based on Weibull’s two-parameter probability density function. The damaged area and the crack lengths in laminates were analyzed, and the results showed higher values of those parameters for random realizations compared to the uniform case for the same levels of applied displacement. This indicates that deterministic analysis of composites using average properties may be non-conservative and a method based on probability may be more appropriate.

Tailoring structure of inclusion with strain-induced closure to reduce Poisson's ratio of composite materials

Abstract: A novel 3D continuum shell structure is introduced as inclusion for composite materials with special mechanical properties in this paper. Its geometry is based on a hollow re-entrant tetrahedron. In a composite, such an inclusion can demonstrate a closure effect induced by external compression. Its specific deformation mechanism results in a special character of deformation and affects effective (global) mechanical properties of the composite. A finite-element method is used to explore quantitatively and qualitatively the deformation mechanism of the suggested inclusion and its effect on the overall mechanical performance of the composite. In this study, geometrical features of the inclusion are used as parameters. The obtained results demonstrate that this kind of inclusion could reduce the composite's Poisson's ratio; moreover, its magnitude is adjustable by changing geometrical parameters of the inclusion. Besides, an overall hardening effect is achieved for the composite, with the magnitude of global stiffness also significantly affected by geometrical features of the inclusion. Thus, the developed inclusion actually provides a potential to develop new composites with a tunable Poisson's ratio and enhanced mechanical properties.

Damage analysis of carbon fabric-reinforced composites under dynamic bending

Abstract: Fabric-reinforced polymer composites used in various applications can be subjected to dynamic loading such as impacts causing bending deformations. Under such loading scenarios, composite structures demonstrate multiple modes of damage and fracture if compared with more traditional, macroscopically homogeneous, structural materials such as metals and alloys. Among damage and fracture modes are fibre breaking, transverse matrix cracking, debonding between fibres and matrix and delamination. Damage evolution affects both their in-service properties and performance that can deteriorate with time. These failure modes need adequate means of analysis and investigation, the major approaches being experimental characterization and numerical simulations. This study deals with analysis of damage in carbon fabric-reinforced polymers (CFRP) under dynamic bending. The properties of, and damage evolution in, the composite laminates were analysed using a combination of mechanical testing and microstructural damage analysis using optical microscopy. Experimental tests are carried out to characterize the behavior of CFRP composites under large-deflection dynamic bending in Izod type impact tests using Resil Impactor. A series of impact tests is carried out at various energy levels to obtain the force-time diagrams and absorbed energy profiles for laminates. Three-dimensional finite element (FE) models are implemented in the commercial code Abaqus/Explicit to study the deformation behavior and damage in composites for cases of dynamic bending. In these models, multiple layers of bilinear cohesive-zone elements are placed at the damage locations identified in microscopic study. Initiation and progression of inter-ply delamination at the impact and bending locations is studied numerically by employing cohesive-zone elements between each ply of the composite. Stress-based criteria are used for damage initiation, and fracture-mechanics techniques to capture its progression in composite laminates. The developed numerical models are capable to simulate these damage mechanisms as well as their subsequent interaction observed in tests and microscopy. Simulations results showed a good agreement when compared to experimentally obtained transient response of the woven laminates.

Flexural properties of polyamides: influence of strain rate, friction and moulding-induced anisotropy

Abstract: In recent years, advances in material testing equipment caused the determination of mechanical properties by means of three-point bending tests to lose ground in detriment to more accurate tensile tests. However, if components undergo bending deformation in service, the identification of the materials flexural behaviour is essential. The investigated material is a thermoplastic polymer, test specimens being cut in prismatic shapes from injected sheets, which present a variation in properties due to cooling conditions. This paper presents results of three-point bending tests with emphasis on the influence of strain rate and anisotropy on flexural strength and chord modulus. Results show an increase in flexural properties with strain rate and a considerable influence of anisotropy on mechanical properties.

Damage in fibre-reinforced laminates under dynamic loading

Abstract: Fibre-reinforced polymers (FRPs) became one of the most important structural materials in various industries due to their unique combination of properties such as excellent stiffness, high strength-to-weight ratio, and ease to manufacture shapes tailored for applications. Hence, they are now broadly used in aerospace and naval structures as well as in automotive, construction and energy industry; there is an increasing use of them in sports products. In service, components and structures, containing composites, can be exposed to different loading conditions including dynamic events, e.g. impacts. Such loads can cause deterioration of their structural integrity and load-bearing capacity due to induced damage. Because of their heterogeneity and microstructure, composite laminates usually demonstrate multiple modes of damage and fracture if compared with more traditional, macroscopically homogeneous, structural materials such as metals and alloys. This study deals with analysis of damage in two types of fibre-reinforced polymers - cross-ply and woven laminates - under impact loading. The first type of FRP is exposed to conditions of impact fatigue (IF). IF can be defined as a repetition of low-energy impacts with energy amplitudes insufficient to cause a total failure of a component in a single impact. Another type of laminate - reinforced with 2/2 twill fabric - was loaded in various modes. The properties of, and damage evolution in, the studied two types of laminates were analysed using a combination of mechanical testing and microstructural and damage studies using optical microscopy and X-ray micro computed tomography. Dynamic mechanical tests on cross-ply laminates were implemented using a uni-axial tensile impact loading. Advanced FE models were developed in Abaqus/Explicit to characterise the response of FRP laminates to impact loading conditions in order to elucidate their dynamical mechanical behaviour. A 3d finite-element model for uni-axial tensional impact loading of tested samples of FRP cross-ply laminates was developed with a hammer-specimen interaction simulated directly to obtain detailed information about impact conditions. The obtained results are compared with experimental data.

Nonlinear dynamics of structures with propagating cracks

Abstract: The aim of this paper is to study the evolution of nonlinear dynamics of structures with a propagating crack. A method of simulation used for analysis of dynamics of the cracked structure is based on a combination of an analytical technique and Matlab-Simulink-based simulations. As an example, a model of a cracked bar subjected to longitudinal excitation is used to analyse its nonlinear response as a way to monitor the structural health as crack propagates.