Showing papers by "Vadim V. Silberschmidt published in 2011"

Analysis of anisotropic viscoelastoplastic properties of cortical bone tissues.

Abstract: Bone fractures affect the health of many people and have a significant social and economic effect. Often, bones fracture due to impacts, sudden falls or trauma. In order to numerically model the fracture of a cortical bone tissue caused by an impact it is important to know parameters characterising its viscoelastoplastic behaviour. These parameters should be measured for various orientations in a bone tissue to assess bone’s anisotropy linked to its microstructure. So, the first part of this study was focused on quantification of elastic–plastic behaviour of cortical bone using specimens cut along different directions with regard to the bone axis—longitudinal (axial) and transverse. Due to pronounced non-linearity of the elastic–plastic behaviour of the tissue, cyclic loading–unloading uniaxial tension tests were performed to obtain the magnitudes of elastic moduli not only from the initial loading part of the cycle but also from its unloading part. Additional tests were performed with different deformation rates to study the bone’s strain-rate sensitivity. The second part of this study covered creep and relaxation properties of cortical bone for two directions and four different anatomical positions–anterior, posterior, medial and lateral–to study the variability of bone’s properties. Since viscoelastoplasticity of cortical bone affects its damping properties due to energy dissipation, the Dynamic Mechanical Analysis (DMA) technique was used in the last part of our study to obtain magnitudes of storage and loss moduli for various frequencies. Based on analysis of elastic–plastic behaviour of the bovine cortical bone tissue, it was found that magnitudes of the longitudinal Young’s modulus for four cortical positions were in the range of 15–24 GPa, while the transversal modulus was lower — between 10 and 15 GPa. Axial strength for various anatomical positions was also higher than transversal strength with significant differences in magnitudes for those positions. Quantitative data obtained in creep and relaxation tests exhibited no significant position-specific differences. DMA results demonstrated relatively low energy-loss capability due to viscosity of bovine cortical bone that has a loss factor in the range of 0.035–0.1.

Computational Study of Ultrasonically-Assisted Turning of Ti Alloys

Abstract: 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.

Numerical modelling of impact fracture of cortical bone tissue using X-FEM

Abstract: A cortical bone tissue is susceptible to fracture that can be caused by events, such as traumatic falls, sports injuries and traffic accidents. A proper treatment of bones and prevention of their fracture can be supported by in-depth understanding of deformation and fracture behaviour of this tissue in such dynamic events. Parameters such as damage initiation under impact, damage progression and impact strength can help to achieve this goal. In this paper, Extended Finite-Element Method (X-FEM) implemented into the commercial finite-element software Abaqus is used to simulate the actual crack initiation and growth in a cantilever beam of cortical bone exposed to quasi-static and impact loading using the Izod loading scheme. Izod tests were performed on notched bone specimens of bovine femur to measure its impact strength and to validate simulations. The simulation results show a good agreement with the experimental data.

Non-uniformity of deformation in low-density thermally point bonded non-woven material: effect of microstructure

Abstract: One of the most important characteristic features of a low-density thermally bonded non-woven material is its discontinuous and non-uniform microstructure, resulting in a complicated and unstable deformation mechanism of the material. In order to estimate the effects of such microstructure on the overall mechanical properties of the non-woven material, tensile tests are carried out for specimens with different systems of marks for both two principle directions—machine direction and cross direction—with images being captured with high-speed camera. The non-uniform strain fields are analysed based on the obtained images. Discontinuous finite-element models are developed to study the deformation mechanism of non-woven specimens in both principle directions, and the effects of the discontinuous and non-uniform fibrous network and different arrangements of bond points are analysed numerically.

Analysis of forces in vibro-impact and hot vibro-impact turning of advanced alloys

Abstract: 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.

Effect of Cutting Conditions on Temperature Generated in Drilling Process: a FEA Approach

Abstract: Heat generated during a drilling process has a major influence on the tool life and the workpiece material behaviour that are significantly affected by cutting conditions (cutting speed, feed rate). In this paper, the effect of cutting conditions on temperature generated in drilling process is investigated by means of finite element (FE) simulations using commercially available code MSC MARC MENTAT. A Johnson Cook material model is used to describe elasto-plastic deformation behaviour. The updated Lagrangian procedure is used to implement the transient analysis for the elasto-plastic material in the model. A modified shear friction model is employed to model friction at the tool tip-workpiece interface. The effect of friction on chip shape is investigated with FE simulations. Experiments were carried out to verify the FE results.

Energy-Based Analysis of Ultrasonically Assisted Turning

Abstract: The process of ultrasonically-assisted turning (UAT) is a superposition of vibration of a cutting tool on its standard movement in conventional turning (CT) The former technique has several advantages compared with the latter, one of the main being a significant decrease in the level of cutting forces I n this paper the effects observed in UAT are analysed employing ideas of dynamic fracture mechanics The active stage of loading duration depends heavily on ultrasonic frequency and the cutting speed; he application of the fracture criterion based on the notion of incubation time makes it possible to calculate a dependence of this duration on its threshold amplitude An estimation of energy, necessary to create a threshold pulse in the materi al, is made by solving the contact Hertz problem The obtained time dependence of energy has a marked minimum Thus, the existence of energy-efficient loading duration is demonstrated This ex plains the decrease in the cutting force resulting from supe rimposed ultrasonic vibration The obtained results are in agreement with experiments on ultrasonic assisted machining of aluminium and Inconel 718 alloy

Interaction of Matrix Cracking and Delamination in Cross-ply Laminates: Simulations with Stochastic Cohesive Zone Elements

Abstract: Two main damage mechanisms of laminates—matrix cracking and inter-ply delaminationare closely linked together (Joshi and Sun 1). This paper is focussed on interaction between matrix cracking and delamination failure mechanisms in CFRP cross-ply laminates under quasi-static tensile loading. In the first part of the work, a transverse crack is introduced in 90o layers of the cross-ply laminate [01/904/01], and the stresses and strains that arise due to tensile loading are analyzed. In the second part, the cohesive zone modelling approach where the constitutive behaviour of the cohesive elements is governed by traction-displacement relationship is employed to deal with the problem of delamination initiation from the matrix crack introduced in the 90o layers of the laminate specimen. Additionally, the effect of microstructural randomness, exhibited by CFRP laminates on the damage behaviour of these laminates is also accounted for in simulations. This effect is studied in numerical finite-element simulations by introducing stochastic cohesive zone elements. The proposed damage modelling effectively simulated the interaction between the matrix crack and delamination and the variations in the stresses, damage and crack lengths of the laminate specimen due to the microstructural randomness.

Effect of ultrasonic energy on nanoscale interfacial structure in copper wire bonding on aluminium pads

Abstract: The effect of ultrasonic vibration on nanoscale interfacial structure of thermosonic copper wire bonding on aluminium pads was investigated. It was found that bonding strength was determined by the extent of fragmentation of a native aluminium oxide overlayer (5–10 nm thick) on aluminium pads, forming paths for formation of intermetallic compound CuAl2 in areas of direct contact of bonded metal surfaces. The degree of fracture of the oxide layer was strongly affected by a level of ultrasonic power.

Nonlinear compression behavior of warp-knitted spacer fabric : effect of sandwich structure

Abstract: Compressibility of warp-knitted spacer fabrics is one of their important mechanical properties with regard to many special applications such as body protection, cushion and mattresses Due to specific structural features of the fabric and a non-linear mechanical behavior of monofilaments, the compression properties of this kind of fabrics are very complicated Although several studies have been performed to investigate their compression behavior, its mechanism has not well been understood yet This work is concerned with a study of compression mechanism of a selected warp-knitted spacer fabric with a given sandwich structure Both experimental and numerical methods are used to study the effect of the material's structure on the overall compression mechanism Compression tests are conducted to obtain force-displacement relationships of the fabric A micro-computed tomography system is used to analyze specimens under different levels of compression displacement to investigate the change in material's structure during the compression process At the same time, finite element models are developed separately to simulate the initial geometric structure and the compression behavior of the fabric Three finite element models based on beam elements are firstly developed to simulate the effect of manufacturing process on shapes of monofilaments within the fabric and to determine their morphologies, which are used to assemble a geometry part of the finite element model of the overall fabric Then the finite-element model is developed using beam and shell elements to describe the compression behavior of the fabric by introducing the effect of its complex microstructure and real non-linear mechanical properties of the monofilaments A comparison of the obtained experimental and CT data, and results of simulation is carried out, demonstrating a good agreement With this study, a compression mechanism of the warp-knitted spacer fabric can be better understood

Analysis of nonlinear shear deformations in CFRP and GFRP textile laminates

Abstract: Carbon fibre-reinforced polymer (CFRP) and glass fibre-reinforced polymer (GFRP) woven composites are widely used in aerospace, automotive and construction components and structures thanks to their lower production costs, higher delamination and impact strengths. They can also be used in various products in sports industry. These products are exposed to different in-service conditions such as large tensile and bending deformations. Composite materials, especially ±45° symmetric laminates subjected to tensile loads, exhibit significant material as well as geometric non-linearity before damage initiation, particularly with respect to shear deformations. Such a nonlinear response needs adequate means of analysis and investigation, the major tools being experimental tests and numerical simulations. This research deals with modelling the nonlinear deformation behaviour of CFRP and GFRP woven laminates subjected to in-plane tensile loads. The mechanical behaviour of woven laminates is modelled using nonlinear elasto-plastic as well as material models for fabrics in commercial finite-element code Abaqus. A series of tensile tests is carried out to obtain an in-plane full-field strain response of [+45/-45]2s CFRP and GFRP laminates using digital image correlation technique according to ASTM D3518/D3518M-94. The obtained results of simulations are in good agreement with experimental data.

Analysis of Forces and Temperatures in Conventional and Ultrasonically-Assisted Cutting of Bone

Abstract: Bone cutting is a frequently used procedure in orthopaedic and neuro surgeries. Current research on bone cutting is concerned with the efforts to decrease the forces generated during cutting the bone as well as temperature to avoid mechanical and thermal damage (bone necrosis) induced by surgical tools. The paper presents results of finite-element simulations of conventional cutting (CC) and ultrasonically-assisted cutting (UAC) of bone in order to understand thermomechanics of the process. The study was aimed at investigating the levels of tool-penetration force and temperatures induced in the bone when a hard cutting tool penetrates into it in both types of cutting. The models allow the quantitative analysis of forces and temperatures produced during the cutting process. The use of UAC reduces the tool penetration force and temperature in the cutting region

Analysis of nonlinear deformations and damage in CFRP textile laminates

Abstract: Carbon fibre-reinforced polymer (CFRP) textile composites are widely used in aerospace, automotive and construction components and structures thanks to their relatively low production costs, higher delamination and impact strength. They can also be used in various products in sports industry. These products are usually exposed to different in-service conditions such as large bending deformation and multiple impacts. Composite materials usually demonstrate multiple modes of damage and fracture due to their heterogeneity and microstructure, in contrast to more traditional homogeneous structural materials like metals and alloys. Damage evolution affects both their in-service properties and performance that can deteriorate with time. These damage modes need adequate means of analysis and investigation, the major approaches being experimental characterisation, numerical simulations and microtomography analysis. This research deals with a deformation behaviour and damage in composite laminates linked to their quasi-static bending. Experimental tests are carried out to characterise the behaviour of woven CFRP material under large-deflection bending. Two-dimensional finite element (FE) models are implemented in the commercial code Abaqus/Explicit to study the deformation behaviour and damage in woven CFRP laminates. Multiple layers of bilinear cohesive-zone elements are employed to model the onset and progression of inter-ply delamination process. X-ray Micro-Computed Tomography (MicroCT) analysis is carried out to investigate internal damage mechanisms such as cracking and delaminations. The obtained results of simulations are in agreement with experimental data and MicroCT scans.

Analysis of Creep Behavior of Polypropylene Fibers

Abstract: Time and stress-dependent viscoelastic properties of polypropylene fibers are determined by means of a series of creep tests and data analysis technique. Relaxation tests are carried out to verify and update the suggested model. The tests are performed for single polypropylene fibers that form a thermally bonded low-density nonwoven. The obtained properties are implemented into the finite element analysis software MSC. Marc - Mentat. Tensile tests and simulations are performed in order to demonstrate suitability of the developed creep model. The obtained results are discussed and further recommendations are given.

Dynamic response of thermally bonded bicomponent fibre nonwovens

Abstract: Having a unique microstructure, nonwoven fabrics possess distinct mechanical properties, dissimilar to those of woven fabrics and composites. This paper aims to introduce a methodology for simulating a dynamic response of core/sheath-type thermally bonded bicomponent fibre nonwovens. The simulated nonwoven fabric is treated as an assembly of two regions with distinct mechanical properties. One region - the fibre matrix – is composed of non-uniformly oriented core/sheath fibres acting as link between bond points. Non-uniform orientation of individual fibres is introduced into the model in terms of the orientation distribution function in order to calculate the structure’s anisotropy. Another region – bond points – is treated in simulations as a deformable bicomponent composite material, composed of the sheath material as its matrix and the core material as reinforcing fibres with random orientations. Time-dependent anisotropic mechanical properties of these regions are assessed based on fibre characteristics and manufacturing parameters such as the planar density, core/sheath ratio, fibre diameter etc. Having distinct anisotropic mechanical properties for two regions, dynamic response of the fabric is modelled in the finite element software with shell elements with thicknesses identical to those of the bond points and fibre matrix.

Crack Propagation in a Toughened Epoxy Adhesive under Repeated Impacts

Abstract: Adhesives are being increasingly used in structural applications, especially in aerospace, automotive and naval structures, making their structural integrity an important issue. In-service loading histories of such structures usually contain low-energy impacts, repetition of which can significantly affect their performance. This paper deals with the behaviour of the toughened epoxy adhesive FM73 under repeated impacts, known as impact fatigue. Izod impact fatigue tests were performed on FM73 specimens in order to study the evolution of damage and to characterise this via measurable parameters, such as the maximum force and the contact time. A finite element model was developed to simulate the impact tests and this was used to calculate the dynamic strain energy release rate, which was compared with that determined using a simple analytical method. A relationship between the maximum dynamic strain energy release rate and impact fatigue crack growth rate was established that was used as the basis of an impact fatigue crack growth law.

Finite-Element Simulations of Split Hopkinson Test of Ti-Based Alloy

Abstract: Ti-based alloys are extensively used in aerospace and other advanced engineering fields due to their high strength and toughness, light weight, excellent corrosion resistance and ability to withstand extreme temperatures. Since these alloys are hard to machine, there is an obvious demand to develop simulation tools in order to analyse the material's behaviour during machining and thus optimise the entire machining process. The deformation processes in machining of Ti-alloys are typically characterized by high strains and temperatures. A Split Hopkinson Pressure Bar (SHPB) technique is a commonly used experimental method to characterize a material`s behaviour at high strain rates; a stress-strain relation of the material is derived from the obtained experimental data. A computational study on a three-dimensional finite element model of the SHPB experiment is performed to assess various features of the underlying mechanics of deformation processes at high-strain and -strain-rate regimes. In the numerical analysis, an inhomogeneous deformation behaviour is observed in the workpiece at the initial stages of compression contrary to a standard assumption of stress and strain homogeneity in the specimen.

Void growth in thermosonic copper/gold wire bonding on aluminum pads

Abstract: Void growth in wire bonds significantly influences the reliability of electronic devices. This paper found that, in the as-bonded state, a few voids nucleate in Au-Al bonds, while they are absent in Cu-Al bonds. Voids grow much faster in Au-Al bonds than in Cu-Al bonds during thermal annealing. It is proposed that void growth in Au-Al bonds due to the oxidation of IMCs and volumetric shrinkage resulted from the growth of Au 8 Al 3 and Au 4 Al, but not due to the Kirkendall effect. Large voids up to 10μm exist after extended annealing, which is also attributed to the outward diffusion of Au to react with Al in the area beyond the perimeter of the bonds. In Cu-Al bonds, the void growth rate is low, and only a few voids of 9 Al 4 . The oxidation of Cu-Al IMCs and outward diffusion of Cu is insignificant; therefore the void growth rate in Cu-Al bonds is low.

Dynamic Properties of Cortical Bone Tissue: Impact Tests and Numerical Study

Abstract: Bone is the principal structural component of a skeleton: it assists the load-bearing framework of a living body. Structural integrity of this component is important; understanding of its mechanical behaviour up to failure is necessary for prevention and diagnostic of trauma. Bone fractures occur in both low-energy trauma, such as falls and sports injury, and high-energy trauma, such as car crash and cycling accidents. By developing adequate numerical models to predict and describe the deformation and fracture behaviour up to fracture of a cortical bone tissue, a detailed study of reasons for, and ways to prevent or treatment methods of, bone fracture could be implemented. This study deals with both experimental analysis and numerical simulations of this tissue and its response to impact dynamic loading. Two areas are covered: Izod tests for quantifying a bone’s behaviour under impact loading, and a 3D finite-element model simulating these tests. In the first part, properties of cortical bone tissue were investigated under impact loading condition. In the second part, a 3D numerical model for the Izod test was developed using the Abaqus/Explicit finite-element software. Bone has time-dependent properties – viscoelastic – that were assigned to the specimen to simulate the short term event, impact. The developed numerical model was capable of capturing the behaviour of the hammer-specimen interaction correctly. A good agreement between the experimental and numerical data was found.

Vibro-Impact Response of a Cracked Bar

Abstract: The presence of a crack in a structure affects its dynamic behaviour under working conditions. Cracks introduce nonlinearities into the system; the use of such nonlinearit ies for damage detection should be investigated. A model of a one- dimensional cracked cantilever bar subjected to longitudinal harmonic excitation is used to analyse a vibro-impact response as a way to monitor structural health. The effect of contact nonlinearity due to crack's faces interaction is considered . This nonlinear information is obtained based on a combination of the analytical technique and the Matlab-Simulink computation. The procedure uses a numerical approximation for dynamic compliance operators and a nonlinear model of contact faces interaction implemented numerically as a nonlinear feedback. Nonlinear resonant phenomena due to vibro-impact interaction in the cracked bar are obtained and analysed. A distribution of the higher harmonics along the bar length, generated due to the nonlinear response of the crack, is revealed as a function of the distan ce from the crack. Recommendations on structural health monitoring of cracked bars due to contact nonlinearity are presented