Showing papers by "Vadim V. Silberschmidt published in 2015"

Metamaterials with Negative Poisson’s Ratio: A Review of Mechanical Properties and Deformation Mechanisms

Abstract: Compared to conventional materials, materials with a negative Poisson’s ratio are endowed with many specific mechanical features; consequently, there are many potential applications for them For the last two decades, many efforts have been made on this sort of metamaterial both experimentally and theoretically This paper provides a brief review of those studies with a focus on mechanical properties and deformation mechanisms of the metamaterials The latter are explained using a structure of a multi-phase metamaterial system for a more comprehensive understanding and as an inspiration for future works Additionally, respective manufacturing methods and applications are also summarised

Preparation, characterization and properties of polycaprolactone diol-functionalized multi-walled carbon nanotube/thermoplastic polyurethane composite

Abstract: Multi-walled carbon nanotubes (MWCNTs) were chemically functionalized to prepare thermoplastic polyurethane (PU) composites with enhanced properties. In order to achieve a high compatibility of functionalized MWCNTs with the PU matrix, polycaprolactone diol (PCL), as one of PU’s monomers, was selectively grafted on the surface of MWCNTs (MWCNT–PCL), while carboxylic acid groups functionalized MWCNTs (MWCNT–COOH) and raw MWCNTs served as control. Both MWCNT–COOH and MWCNT–PCL improved the dispersion of MWCNTs in the PU matrix and interfacial bonding between them at 1 wt% loading fraction. The MWCNT–PCL/PU composite showed the greatest extent of improvement, where the tensile strength and modulus were 51.2% and 33.5% higher than those of pure PU respectively, without sacrificing the elongation at break. The considerable improvement in both mechanical properties and thermal stability of MWCNT–PCL/PU composite should result from the homogeneous dispersion of MWCNT–PCL in the PU matrix and strong interfacial bonding between them.

Characterisation of mechanical behaviour and damage analysis of 2D woven composites under bending

Abstract: In this paper, flexural loading of woven carbon fabric-reinforced polymer laminates is studied using a combination of experimental material characterisation, microscopic damage analysis and numerical simulations. Mechanical behaviour of these materials was quantified by carrying out tensile and large-deflection bending tests. A substantial difference was found between the materials' tensile and flexural properties due to a size effect and stress stiffening of thin laminates. A digital image-correlation technique capable of full-field strain-measurement was used to determine in-plane shear properties of the studied materials. Optical microscopy and micro-computed tomography were employed to investigate deformation and damage mechanisms in the specimens fractured in bending. Various damage modes such as matrix cracking, delaminations, tow debonding and fibre fracture were observed in these microstructural studies. A two-dimensional finite-element (FE) model was developed to analyse the onset and propagation of inter-ply delamination and intra-ply fabric fracture as well as their coupling in the fractured specimen. The developed FE model provided a correct prediction of the material's flexural response and successfully simulated the sequence and interaction of damage modes observed experimentally.

Properties and application of polyimide-based composites by blending surface functionalized boron nitride nanoplates

Abstract: Properties and application of polyimide-based composites by blending surface functionalized boron nitride nanoplates

Mechanical analysis of bi-component-fibre nonwovens: Finite-element strategy

Abstract: In thermally bonded bi-component fibre nonwovens, a significant contribution is made by bond points in defining their mechanical behaviour formed as a result of their manufacture. Bond points are composite regions with a sheath material reinforced by a network of fibres’ cores. These composite regions are connected by bi-component fibres — a discontinuous domain of the material. Microstructural and mechanical characterization of this material was carried out with experimental and numerical modelling techniques. Two numerical modelling strategies were implemented: (i) traditional finite element (FE) and (ii) a new parametric discrete phase FE model to elucidate the mechanical behaviour and underlying mechanisms involved in deformation of these materials. In FE models the studied nonwoven material was treated as an assembly of two regions having distinct microstructure and mechanical properties: fibre matrix and bond points. The former is composed of randomly oriented core/sheath fibres acting as load-transfer link between composite bond points. Randomness of material’s microstructure was introduced in terms of orientation distribution function (ODF). The ODF was obtained by analysing the data acquired with scanning electron microscopy (SEM) and X-ray micro computed tomography (CT). Bond points were treated as a deformable two-phase composite. An in-house algorithm was used to calculate anisotropic material properties of composite bond points based on properties of constituent fibres and manufacturing parameters such as the planar density, core/sheath ratio and fibre diameter. Individual fibres connecting the composite bond points were modelled in the discrete phase model directly according to their orientation distribution. The developed models were validated by comparing numerical results with experimental tensile test data, demonstrating that the proposed approach is highly suitable for prediction of complex deformation mechanisms, mechanical performance and structure-properties relationships of composites.

In-vitro experimental analysis and numerical study of temperature in bone drilling

Abstract: BACKGROUND: Bone drilling is a common practice of surgical treatments in orthopaedics and traumatology. Penetration of a high-speed drill into bone tissue is accompanied by generation of a significant amount of heat. Cooling of the drilling region is necessary to avoid potential risk of thermal damage to bone. OBJECTIVE: The purpose of this study was to measure and predict bone temperature by conducting experiments and numer- ical simulations using cooling by means of irrigation at two different temperatures. METHODS: A series of experiments and numerical studies were performed to investigate the effect of cooling conditions on the rise in bone temperature in drilling. The temperature increase in bone was assessed for different drilling speeds and feed rates in the presence irrigation at 5◦C and 25◦ C. RESULTS: Bone temperature was found to be strongly affected by the drilling parameters and cooling conditions. Irrigation with water at 5◦C kept bone temperature well below the thermal threshold level. CONCLUSION: This study strongly recommends the use of irrigation at lower temperature for safe surgical incision.

Numerical analysis of the interactive damage mechanisms in two-dimensional carbon fabric-reinforced thermoplastic composites under low velocity impacts

Abstract: Damage and failure in carbon fabric-reinforced polymer (CFRP) composites under low velocity impacts is investigated using explicit finite element analyses. Three-dimensional finite element models are developed to simulate the deformation behaviour of, and damage evolution in CFRP laminates under such loading conditions. In these simulations, the onset and growth of inter-ply delamination is captured by bilinear cohesive zone elements inserted between plies of the composite. Intra-ply fabric fracture is simulated by defining a transverse layer of cohesive elements at the specimen fracture location. The energies associated with dynamic propagation of these damage mechanisms are also captured by the numerical simulations, demonstrating their potential to model the damage modes as well as their interaction. In this study, a novel damage modelling technique based on the cohesive zone method is proposed for interaction of various damage modes, which is more efficient for coupling between failure modes than a co...

Viscoelasticity of periodontal ligament: an analytical model

Abstract: Understanding of viscoelastic behaviour of a periodontal membrane under physiological conditions is important for many orthodontic problems. A new analytic model of a nearly incompressible viscoelastic periodontal ligament is suggested, employing symmetrical paraboloids to describe its internal and external surfaces. In the model, a tooth root is assumed to be a rigid body, with perfect bonding between its external surface and an internal surface of the ligament. An assumption of almost incompressible material is used to formulate kinematic relationships for a periodontal ligament; a viscoelastic constitutive equation with a fractional exponential kernel is suggested for its description. Translational and rotational equations of motion are derived for ligament’s points and special cases of translational displacements of the tooth root are analysed. Material parameters of the fractional viscoelastic function are assessed on the basis of experimental data for response of the periodontal ligament to tooth translation. A character of distribution of hydrostatic stresses in the ligament caused by vertical and horizontal translations of the tooth root is defined. The proposed model allows generalization of the known analytical models of the viscoelastic periodontal ligament by introduction of instantaneous and relaxed elastic moduli, as well as the fractional parameter. The latter makes it possible to take into account different behaviours of the periodontal tissue under short- and long-term loads. The obtained results can be used to determine loads required for orthodontic tooth movements corresponding to optimal stresses, as well as to simulate bone remodelling on the basis of changes in stresses and strains in the periodontal ligament caused by such movements.

Effect of micromorphology of cortical bone tissue on crack propagation under dynamic loading

Abstract: Structural integrity of bone tissue plays an important role in daily activities of humans. However, traumatic incidents such as sports injuries, collisions and falls can cause bone fracture, servere pain and mobility loss. In addition, ageing and degenerative bone diseases such as osteoporosis can increase the risk of fracture [1]. As a composite-like material, a cortical bone tissue is capable of tolerating moderate fracture/cracks without complete failure. The key to this is its heterogeneously distributed microstructural constituents providing both intrinsic and extrinsic toughening mechanisms. At micro-scale level, cortical bone can be considered as a four-phase composite material consisting of osteons, Haversian canals, cement lines and interstitial matrix. These microstructural constituents can directly affect local distributions of stresses and strains, and, hence, crack initiation and propagation. Therefore, understanding the effect of micromorphology of cortical bone on crack initiation and propagation, especially under dynamic loading regimes is of great importance for fracture risk evaluation. In this study, random microstructures of a cortical bone tissue were modelled with finite elements for four groups: healthy (control), young age, osteoporosis and bisphosphonate-treated, based on osteonal morphometric parameters measured from microscopic images for these groups. The developed models were loaded under the same dynamic loading conditions, representing a direct impact incident, resulting in progressive crack propagation. An extended finite-element method (X-FEM) was implemented to realize solution-dependent crack propagation within the microstructured cortical bone tissues. The obtained simulation results demonstrate significant differences due to micromorphology of cortical bone, in terms of crack propagation characteristics for different groups, with the young group showing highest fracture resistance and the senior group the lowest.

Analysis of impact induced damage in composites for wind turbine blades

Abstract: Glass fabric-reinforced polymer (GFRP) composites used in wind turbine blades are usually exposed to large-deflection bending impacts caused by wind storms, heavy rainfall, water splashes and hailstones in the offshore; and sand and dust impingement in the desert environments. Such loadings can cause deterioration of structural integrity and load-bearing capacity of the blade structure due to induced damage in the form of matrix cracking, delamination and fibre fracture. These types of damage mechanisms become more detrimental and pose a threat to the fatigue life of the turbine blades. In this work, first the load-bearing and energy absorbing capability of woven GFRP laminates is investigated under impact loading. Experimental tests are conducted to characterise the behaviour of GFRP composites under large-deflection dynamic bending in Izod type impact tests using Resil impactor. Impact tests are performed at various energy levels to determine the ultimate fracture toughness of the laminates. In these tests, the material demonstrated interply delamination damage due to weaker matrix at low energy levels. At higher impact energies, apart from delamination, the material also exhibited permanent deflection instead of catastrophic fabric fracture. The latter was due to the visco-elasto-plastic nature of the glass fibres apart from the thermoplastic matrix. The deformation behaviour and delamination damage ensued by dynamic loading is also studied by developing three-dimensional finite element (FE) model in Abaqus/Explicit commercial package. In FE model, multiple layers of bilinear cohesive-zone elements are defined at the damage locations. Stress-based criteria and fracture-mechanics techniques are used to assess damage initiation and its progression, respectively. Numerical results gave good correlation when compared to the dynamic response observed in experiments. The methodology developed here can be employed in damage tolerant design of wind turbine composite blades subjected to similar impact loading conditions.

A numerical study on indentation properties of cortical bone tissue: influence of anisotropy.

Abstract: Purpose The purpose of this study is to investigate the effect of anisotropy of cortical bone tissue on measurement of properties such as direction-dependent moduli and hardness. Methods An advanced three-dimensional finite element model of microindentation was developed. Different modelling schemes were considered to account for anisotropy of elastic or/and plastic regimes. The elastic anisotropic behaviour was modelled employing an elasticity tensor, and Hill's criteria were used to represent the direction-dependent post-yield behaviour. The Oliver-Pharr method was used in the data analysis. Results A decrease in the value of the transverse elasticity modulus resulted in the increased material's indentation modulus measured in the longitudinal direction and a decreased one in the transverse direction, while they were insensitive to the anisotropy in post-elastic regime. On the other hand, an increase in plastic anisotropy led to a decrease in measured hardness for both directions, but by a larger amount in the transverse one. The size effect phenomenon was found to be also sensitive to anisotropy. Conclusions The undertaken analysis suggests that the Oliver-Pharr method is a useful tool for first-order approximations in the analysis of mechanical properties of anisotropic materials similar to cortical bone, but not necessarily for the materials with low hardening reserves in the plastic regime.

Copper coin-embedded printed circuit board for heat dissipation: manufacture, thermal simulation and reliability

Abstract: Purpose – The purpose of this paper is to form copper coin-embedded printed circuit board (PCB) for high heat dissipation. Design/methodology/approach – Manufacturing optimization of copper coin-embedded PCB involved in the design and treatment of copper coin, resin flush removal and flatness control. Thermal simulation was used to investigate the effect of copper coin on heat dissipation of PCB products. Lead-free reflow soldering and thrust tests were used to characterize the reliable performance of copper coin-embedded PCB. Findings – The copper coin-embedded PCB had good agreement with resin flush removal and flatness control. Thermal simulation results indicated that copper coin could significantly enhance the heat-dissipation rate by means of a direct contact with the high-power integrated circuit chip. The copper coin-embedded PCB exhibited a reliable structure capable of withstanding high-temperature reflow soldering and high thrust testing. Originality/value – The use of a copper coin-embedded PCB could lead to higher heat dissipation for the stable performance of high-power electronic components. The copper coin-embedded method could have important potential for improving the design for heat dissipation in the PCB industry.

Ballistic damage in hybrid composite laminates

Abstract: Ballistic damage of hybrid woven-fabric composites made of plain-weave E-glass- fabric/epoxy and 8H satin-weave T300 carbon-fabric/epoxy is studied using a combination of experimental tests, microstructural studies and finite-element (FE) analysis. Ballistic tests were conducted with a single-stage gas gun. Fibre damage and delamination were observed to be dominating failure modes. A ply-level FE model was developed, with a fabric-reinforced ply modelled as a homogeneous orthotropic material with capacity to sustain progressive stiffness degradation due to fibre/matrix cracking, fibre breaking and plastic deformation under shear loading. Simulated damage patterns on the front and back faces of fabric-reinforced composite plates provided an insight into their damage mechanisms under ballistic loading.

Damage mechanisms of random fibrous networks

Abstract: Fibrous networks are ubiquitous: they can be found in various engineering applications as well as in biological tissues. Due to complexity of their random microstructure, anisotropic properties and large deformation, their modelling is challenging. Though, there are numerous studies in literature focusing either on numerical simulations of fibrous networks or explaining their damage mechanisms at micro or meso-scale, the respective models usually do not include actual random microstructure and failure mechanisms. The microstructure of fibrous networks, together with highly non-linear mechanical behaviourof their fibres, is a key to initiation of damage, its spatial localization and ultimate failure [1]. Numerical models available in literature are not capable of elucidating actual microstructure of the material and, hence, its influence on damage processes in fibrous networks. To emulate a real-life microstructure in a developed finite-element model, an orientation distribution function for fibresobtained from X-ray micro computed-tomography images was considered to provide actual alignment of fibres. To validate the suggested model, notched and unnotched rectangular specimens were experimentally tested. A good correlation between the experimental data and simulation results was observed. This study revealed a significant effect of a notch on damage evolution.

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

Abstract: 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.

Fracture processes in cortical bone: effect of microstructure

Abstract: Understanding of bone fracture can improve medical and surgical procedures. Therefore, investigation of the effect of bone’s microstructure and properties as well as loading conditions on crack initiation and propagation is of great importance. In this paper, several modelling approaches are used to study fracture of cortical bone tissue at various length scales and different types of loading. Two major problems are tackled: crack propagation under impact loading and bone cutting in surgical procedures. In the former case, a micro-scale finite-element (FE) fracture model was suggested, accounting for bone’s microstructure and using X-FEM for crack-propagation analysis [1, 2]. The cortical bone tissue was modelled as four-component heterogeneous materials. The morphology of a transverse-radial cross section captured with optical microscopy was used to generate FE models; extensive experimental studies provided necessary mechanical input data [3]. The problem of bone cutting was treated within the framework of tool-bone interaction analysis [4, 5]. A two-domain approach was used, with a process zone simulated using a smooth-particle hydrodynamics method. This zone was embedded in a continuum domain with macroscopic anisotropic properties obtained in experiments. This study is supported by analysis of damage induced by interaction between the cutting tool and the bone tissue using wedge-indentation tests and considering also the anisotropic behaviour of the bone.

Bulk Metallic Glasses: Mechanical Properties and Performance

Abstract: In this paper, a history of development of bulk metallic glasses (BMGs) was presented, followed by a review of fundamental mechanisms of their deformation and fracture. In this study, observations of fracture surfaces of the Zr-Cu-based BMG exposed to a 3-point test revealed features that are different from those observed in crystalline materials. Indentation techniques were extensively used to characterise elastic deformation of the studied BMG alloy, followed by a systematic analysis of initiation and evolution of shear-band localisation in the indented material. Our results, obtained with the suggested wedge-indentation technique, demonstrated initiation of shear bands in the material volume. This technique can be particularly useful for development of appropriate constitutive models to analyse plastic events in amorphous materials in the small-length scale. A current state of constitutive models of deformation and fracture behaviour of BMGs are presented together with modelling challenges. Simulation of simple tensile and compressive tests were conducted with JH-2, JHB and Drucker-Prager constitutive models by employing identical boundary conditions, type of element and specimen’s geometry. Based on the obtained simulation results, the JH-2 model was considered as not suitable for quasi-static analysis due to ambiguity of the data produced with it for uniaxial tensile and compressive conditions. However, it is concluded that the extended Drucker-Prager and JHB models can be used to study deformation modes in BMGs.

Fracture of Cortical Bone Tissue

Abstract: In this chapter, mechanical behaviours of a unique type of composite material—cortical bone tissue—are considered for different length scales. Both experimental and computational approaches are discussed in this study to evaluate the effects of mechanical anisotropy and structural heterogeneity on the fracture process of cortical bone. First, variability and anisotropic mechanical behaviour of cortical bone tissue are characterised and analysed experimentally for different loading conditions and orientations. Then, results from the experimental studies are used to develop finite-element models across different length-scales to elucidate mechanical and structural mechanisms underpinning the anisotropic and non-linear fracture processes of cortical bone.

Dynamic fractures of adhesively bonded carbon fibre-reinforced polymeric joints

Abstract: The main aim of this chapter is to investigate the behaviour of adhesive joints exposed to repeated low-velocity impact, that is, impact fatigue (IF), and to compare this loading regime with standard fatigue (SF), that is, nonimpacting, constant amplitude, sinusoidal loading conditions. Fatigue crack growth using bonded carbon fibre-reinforced polymeric lap strap joints is conducted. It is seen that IF had the potential to initiate a crack and to cause its rapid propagation at levels of loading that were significantly lower than quasistatic and dynamic strengths and even the fatigue durability of joints. Differences between IF and SF are seen in mixed-mechanism failure and also with regard to the crack speed. It is found that in the initial stages of the crack propagation, the crack rate is 10 times higher in IF than in SF. It is found that the introduction of a relatively small number of in-plane impacts between blocks of SF drastically changed the dynamics of fracture in the specimen, with the IF blocks having a damage accelerating effect. The fatigue crack growth rate curve is seen to be a valid representation of fatigue propagation under SF and IF, in which a combination of experimental data and finite element analysis enables the curve to be constructed. It is seen that the curve shows a normal fatigue relation shape with clearly distinction of a linear and critical regions. It is concluded that this curve can be used to analyse crack propagation in IF and also in SF. Changes of the fracture mechanism in specimens are modelled by the mixed-mechanism fracture model, which represented the experimentally observed acceleration of fatigue crack growth rate when the crack path changes.

Crystalline Deformation in the Small Scale

Abstract: In the last two decades, experimental observations demonstrated—and numerical simulations confirmed—that plastic deformation in the small scale, i.e. at the micron or sub-micron scales, is different from that at the macro-scale; this phenomenon is known as size effect. It was observed mostly in indentation, torsion and bending experiments, being ascribed to strong gradients of strain in such deformation processes. The size effect was also reported in uniaxial micro- and nano-pillar compression experiments in spite of their inherent lack of (or limited) macroscopic strain gradients. In the present study, we first review some critical and essential experimental studies that were conducted over the years to analyse various mechanisms that govern deformation in the small scale. In the second part, different modelling approaches describing this phenomenon are briefly reviewed.