Showing papers by "Vadim V. Silberschmidt published in 2017"

Experimental and computational studies of poly-L-lactic acid for cardiovascular applications: recent progress

Abstract: Stents are commonly used in medical procedures to alleviate the symptoms of coronary heart disease, a prevalent modern society disease. These structures are employed to maintain vessel patency and restore blood flow. Traditionally stents are made of metals such as stainless steel or cobalt chromium; however, these scaffolds have known disadvantages. An emergence of transient scaffolds is gaining popularity, with the structure engaged for a required period whilst healing of the diseased arterial wall occurs. Polymers dominate a medical device sector, with incorporation in sutures, scaffolds and screws. Thanks to their good mechanical and biological properties and their ability to degrade naturally. Polylactic acid is an extremely versatile polymer, with its properties easily tailored to applications. Its dominance in the stenting field increases continually, with the first polymer scaffold gaining FDA approval in 2016. Still some challenges with PLLA bioresorbable materials remain, especially with regard to understanding their mechanical response, assessment of its changes with degradation and comparison of their performance with that of metallic drug-eluting stent. Currently, there is still a lack of works on evaluating both the pre-degradation properties and degradation performance of these scaffolds. Additionally, there are no established material models incorporating non-linear viscoelastic behaviour of PLLA and its evolution with in-service degradation. Assessing these features through experimental analysis accompanied by analytical and numerical studies will provide powerful tools for design and optimisation of these structures endorsing their broader use in stenting. This overview assesses the recent studies investigating mechanical and computational performance of poly(l-lactic) acid and its use in stenting applications.

Incorporation and evolution of ZrO2 nano-particles in Pt-modified aluminide coating for high temperature applications

Abstract: ZrO2 nano-particles were incorporated into electro-deposited PtAl coatings in an attempt to enhance their performance by exploiting the effect of reactive element oxides. PtAl coatings with and without ZrO2 particles were deposited onto three commercially available Ni-based superalloys: Mar-M247, Mar-M246 and Inconel 718. After aluminising and annealing, thermal cycling oxidation tests were carried out to evaluate the influence of ZrO2 addition and substrate composition. Cross-sectional SEM images were obtained to characterise the coatings after deposition, after heat treatment and after 200 thermal cycles. The addition of ZrO2 particles to PtAl coatings on Mar-M-246 and Inconel 718 appeared to increase the growth of thermally grown oxide and reduce its rumpling. However, such effects were not observed for the addition of ZrO2 particles to the PtAl coatings on Mar-M247. The analysis of the coatings on different substrates revealed and elucidated the interactions between Hf, Al and ZrO2, providing better understanding of reactions of ZrO2 and the influence of the substrate on bond coat behaviour.

Modelling of Damage Evolution in Braided Composites: Recent Developments

Abstract: Composites reinforced with woven or braided textiles exhibit high structural stability and excellent damage tolerance thanks to yarn interlacing. With their high stiffness-to-weight and strength-to-weight ratios, braided composites are attractive for aerospace and automotive components as well as sports protective equipment. In these potential applications, components are typically subjected to multi-directional static, impact and fatigue loadings. To enhance material analysis and design for such applications, understanding mechanical behaviour of braided composites and development of predictive capabilities becomes crucial. Significant progress has been made in recent years in development of new modelling techniques allowing elucidation of static and dynamic responses of braided composites. However, because of their unique interlacing geometric structure and complicated failure modes, prediction of damage initiation and its evolution in components is still a challenge. Therefore, a comprehensive literature analysis is presented in this work focused on a review of the state-of-the-art progressive damage analysis of braided composites with finite-element simulations. Recently models employed in the studies on mechanical behaviour, impact response and fatigue analyses of braided composites are presented systematically. This review highlights the importance, advantages and limitations of as-applied failure criteria and damage evolution laws for yarns and composite unit cells. In addition, this work provides a good reference for future research on FE simulations of braided composites.

Low Cycle Fatigue of a Directionally Solidified Nickel-Based Superalloy: Testing, Characterisation and Modelling

Abstract: Low cycle fatigue (LCF) of a low-carbon (LC) directionally-solidified (DS) nickel-base superalloy, CM247 LC DS, was investigated using both experimental and computational methods. Strain-controlled LCF tests were conducted at 850 °C, with a loading direction either parallel or perpendicular to the solidification direction. Trapezoidal loading-waveforms with 2 s and 200 s dwell times imposed at the minimum and the maximum strains were adopted for the testing. A constant strain range of 2% was maintained throughout the fully-reversed loading conditions (strain ratio R = −1). The observed fatigue life was shorter when the loading direction was perpendicular to the solidification one, indicating an anisotropic material response. It was found that the stress amplitude remained almost constant until final fracture, suggesting limited cyclic hardening/softening. Also, stress relaxation was clearly observed during the dwell period. Scanning Electron Microscopy fractographic analyses showed evidence of similar failure modes in all the specimens. To understand deformation at grain level, crystal plasticity finite element modelling was carried out based on grain textures measured with EBSD. The model simulated the full history of cyclic stress-strain responses. It was particularly revealed that the misorientations between columnar grains resulted in heterogeneous deformation and localised stress concentrations, which became more severe when the loading direction was normal to a solidification direction, explaining the shorter fatigue life observed.

Numerical Representation of Multiple Premature Failures in Steel-Plated RC Beams

Abstract: Realizing the importance of widely used technique of plating for flexural retrofitting of reinforced concrete (RC) beams and its drawbacks due to premature failure(s), present work concentrates in developing a finite element tool model capable of successfully capturing multiple premature failure modes and their corresponding behaviors. The model is simple but focused; the capability and accuracy of the results have been validated through test literature, particularly focusing on the load capacities of beams at progressive stages of failure modes; which is from crack initiation through to complete failure, such as the load of crack initiation, first crack and complete failure. Acceptable accuracy is shown in terms of crack type(s), crack patterns, sequence, location and direction of propagation through the innovative use of cohesive zone model (CZM). The model clearly explains that debonding and peeling, although originating from a same location for most cases, are extensions of different types of cracks.

Numerical Simulation of Bone Cutting: Hybrid SPH-FE Approach

Abstract: Anisotropic mechanical behavior of cortical bone and its intrinsic hierarchical microstructure act as protective mechanisms to prevent catastrophic failure due to natural loading conditions; however, they increase the extent of complexity of a cutting process in the case of orthopedic surgery. Experimental results available in the literature suggest that bone tissue deforms significantly prior to the separation in the vicinity of the cutting zone, which is challenge to model because of large deformation and damage process. In addition, existing models of bone cutting do not account for material anisotropy or the effect of damage mechanisms. In this chapter, a novel smoothed particle hydrodynamic (SPH) domain embedded into a continuum finite element (FE) model (SPH + FE scheme) was developed to characterize anisotropic deformation and damage behavior of cortical bone under a cutting process. A set of tool-penetration experiments was performed in directions parallel and perpendicular to bone axis. Distinct deformation and damage mechanisms linked to different microstructure orientations were captured using a microlens high-speed camera. The FE model reflected adequately the experimental results and demonstrated a clear advantage in accounting for its anisotropy and damage mechanisms when modeling hard biological tissues such as cortical bone.

Effect of micro-morphology of cortical bone tissue on fracture toughness and crack propagation

Abstract: Specific features of crack propagation in human cortical bone depend on many factors; bone micro-morphology is one of the main features. A bone compact-tension simulation model with zero-thickness cohesive element is employed in this study to investigate the effect of micro-morphology of cortical bone on fracture toughness and crack propagation. Various groups of bone sample – from young, senior, diseased and treated patients – were studied. It was found that the young group has the best performance in terms of fracture resistance, with the initiation fracture toughness (K0) and slope of 1.45 MPa(m)1/2 and 1.16 MPa(m)1/2/mm, respectively. The cracks in this group propagate mostly along the cement line to protect osteons from crack penetration.

Computational modelling of wounded tissue subject to negative pressure wound therapy following trans-femoral amputation.

Abstract: Proof-of-concept computational models were developed and applied as tools to gain insights into biomechanical interactions and variations of oxygen gradients of wounded tissue subject to negative pressure wound therapy (NPWT), following trans-femoral amputation A macro-scale finite-element model of a lower limb was first developed based on computed tomography data, and distributions of maximum and minimum principal stress values we calculated for a region of interest (ROI) Then, the obtained results were applied iteratively as new sets of boundary conditions for a specific spatial position in a capillary sub-model Data from coupled capillary stress and mass- diffusion sub-models were transferred to the macro-scale model to map the spatial changes of tissue oxygen gradients in the ROI The −70 mmHg NPWT resulted in a dramatic change of a wound surface area and the greatest relative contraction was observed at −150 mmHg Tissue lateral to the depth of the wound cavity revealed homogenous patterns of decrease in oxygenation area and the extent of such decrease was dependent on the distance from the wound surface However, tissue lateral to the width of the wound demonstrated heterogeneous patterns of change, as evidenced by both gradual increase and decrease in the oxygenation area The multiscale models developed in the current study showed a significant influence of NPWT on both macro-deformations and changes of tissue oxygenation The patterns of changes depended on the depth of the tissue, the geometry of the wound, and also the location of tissue plane

Estimation of cutting forces in conventional and ultrasonic-vibration assisted turning using inverse modelling

Abstract: In this work, cutting forces in conventional and ultrasonic-vibration assisted turning are estimated using an inverse method to evaluate the velocity-dependent friction and fracture toughness based on a few tests in conventional turning. The inverse methodology requires the data on cutting and feed forces at two specified cutting speeds. Analytical expressions are employed to estimate the cutting forces in conventional as well as ultrasonic-vibration assisted turning. The suggested method was verified with experimental data. The validation of the direct model with the finite-element results available in the literature was also carried out. A sensitivity analysis revealed a significant effect of friction on cutting forces. Thanks to its simplicity, the proposed procedure may find a good application in industrial practice.

Enhanced gradient crystal-plasticity study of size effects in a β-titanium alloy

Abstract: A calibrated model of enhanced strain-gradient crystal plasticity is proposed, which is shown to characterize adequately deformation behaviour of b.c.c. single crystals of a β-Ti alloy (Ti-15-3-3-3). In this model, in addition to strain gradients evolving in the course of deformation, incipient strain gradients, related to a component's surface-to-volume ratio, is accounted for. Predictive capabilities of the model in characterizing a size effect in an initial yield and a work-hardening rate in small-scale components is demonstrated. The characteristic length-scale, i.e. the component's dimensions below which the size effect is observed, was found to depend on densities of polar and statistical dislocations and interaction between them.

Retardation effects due to overloads in aluminium-alloy aeronautical components

Abstract: © 2017 Wiley Publishing Ltd.Fatigue data are generally derived under constant-amplitude loading conditions, but aircraft components are subjected to variable-amplitude loading. Without interaction effects, caused by overloads and underloads intermingled in a loading sequence, it could be relatively easy to establish a crack growth curve by means of a cycle-by-cycle integration. However, load-spectrum effects largely complicate a crack growth under variable-amplitude cycling. In this paper, fatigue crack growth behaviour of aeronautical aluminium alloy 2024-T3 was studied. Effects of various loading conditions such as stress ratio and amplitude loadings were investigated. In particular, the effect of different overloads on the fatigue crack growth was simulated using Zencrack code. Preliminary analyses on Compact Tension (CT) specimens proved that the numerical results generated were in agreement with the results provided by an afgrow code for the same conditions. A case study was carried out on a helicopter component, undergoing repeated overloads, to compare numerical results obtained implementing yield zone models in Zencrack.

Analysis of pulsed electroplasticity in metals

Abstract: Application of high-intensity electric fields and/or currents is known to enhance materials' deformability For instance, their continuous or in short-pulses application on metals and ceramics may significantly affect their deformation response to external loads This phenomenon is commonly referred to as electroplasticity (EP) or electroplastic effect In the present preliminary study, mechanical tensile experiments were carried out in combination with the application of short duration high-intensity electric currents in copper samples Our study captures the enhanced plasticity induced in the metal due to EP Postevent microstructural studies highlighted the effect of high-intensity electric current on the material

Dynamic damage in woven carbon/epoxy composites due to air blast

Abstract: In this study, the dynamic response to air blast observed in a flat plate specimen of 2 x 2 twill weave T300 carbon fibre/epoxy composite is examined, using a combination of non-invasive analysis techniques. The study investigates deformation and damage following air blasts with incident pressures of 0.4 MPa, 0.6 MPa and 0.8 MPa, with wave speeds of between 650 m/s and 950 m/s. Digital image correlation was employed to obtain displacement data from the rear surfaces of the specimens during each experiment, then used to assess the effect of air blast magnitude on the specimen’s response. 3D x-ray tomography was employed to assess the resultant internal damage within the samples, allowing for a damage cloud to be visualized for each specimen. It was demonstrated that the global deformation and transitions in curvature of each specimen appeared to be very similar for all damage cases and that only the out-of-plane displacement increased showing that the pressure magnitude had no effect on the curvature or modes during deformation. Damage was found to propagate from the rear surface of the specimens towards the front surface as the air blast magnitude increased. A finite-element model based on a phenomenological continuum damage approach was also developed and validated against the experimental data.