Showing papers by "Vadim V. Silberschmidt published in 2019"

When superhydrophobic coatings are icephobic : role of surface topology

Abstract: Among different types of anti-icing coatings, superhydrophobic coatings have attracted considerable attention due to their water repellency and low heat-transfer rate. However, condensation on superhydrophobic surfaces at low temperatures usually causes an increase in ice adhesion because of the induced wetting of micro- and nanostructures. By tuning the weight ratio of surface-modified nanoparticles to unmodified ones, five superhydrophobic coatings with different structural features at the microscale were developed. Ice-adhesion strength and ice-nucleation temperature were studied, together with the effect of moisture condensation on ice adhesion. It was found that the ice-adhesion strength and icing temperature of these coatings do not necessarily follow the same order among these surfaces because of different mechanisms involved. Surface roughness is inadequate to describe the necessary surface features that critically affect the anti-icing behavior of the coatings. Detailed topology/geometry has to be considered when designing icephobic coatings. Superhydrophobic coatings can be adopted for icephobic applications once the surface topology is carefully designed.

Machinability of natural-fibre-reinforced polymer composites: conventional vs ultrasonically-assisted machining

Abstract: Natural-fibre-reinforced polymer (NFRP) composites are becoming a viable alternative to synthetic fibre based composites in many industrial applications. Machining is often necessary to facilitate assembly of parts in a final product. This study focuses on a comparative experimental analysis of the effects of conventional drilling (CD) and a hybrid ultrasonically-assisted drilling (UAD) of a hemp fibre-reinforced vinyl ester composite laminate. The results obtained indicate that UAD is more efficient when compared to CD for a range of drilling conditions. It yields lower cutting forces and energy resulting in reduced machining-induced damage in the composite, including diminished burr formation and fibre pull-outs. The holes drilled with UAD exhibit improved surface finish and hole quality when compared to those produced with CD. The study demonstrates the applicability of UAD as a viable machining process for improved machinability of heterogeneous NFRP composite materials.

In-situ SEM study of slip-controlled short-crack growth in single-crystal nickel superalloy

Abstract: Initiation and growth of short cracks in a nickel-based single crystal were studied by carrying out in-situ fatigue experiments within a scanning electron microscope (SEM). Specimens with two different crystallographic orientations, i.e., [001] and [111], were tested under load-controlled tension fatigue in vacuum. Slip-caused crack initiation was identified at room temperature while initiation of a mode-I crack was observed at 650 °C. Slip traces continuously developed ahead of the crack tip once initiated and acted as nuclei for early-stage crack growth at both room and high temperature (650 °C). These slip traces were caused by accumulated shear deformation of activated octahedral slip systems, which were specifically identified by analysing the surface slip traces and crack-propagation planes. The crack-growth rates were evaluated against stress intensity factor range, revealing the anomaly of slip-controlled short-crack growth. The effects of crystallographic orientations and temperature on fatigue crack growth were subsequently analysed and discussed, including the influence of microstructural features such as carbides and pores.

Low-cycle fatigue of single crystal nickel-based superalloy mechanical testing and TEM characterisation

Abstract: Low-cycle fatigue (LCF) is studied for a nickel-based single-crystal superalloy in this paper, with a focus on the effect of crystal orientation and temperature. Specifically, cyclic deformation of the alloy was compared for [001]- and [111]-oriented samples tested under strain-controlled conditions at room temperature and 825 °C. Either cyclic hardening or softening was observed during the LCF process, depending on the strain amplitude, crystallographic orientation and temperature. LCF life was also reduced significantly by changing loading orientation from [001] to [111] or increasing temperature to 825 °C. Employing a comprehensive study with transmission electron microscopy (TEM), a connection between microstructure and mechanical behaviour of the alloy is discussed. It was found that the processes of γ′-precipitate dissolution and dislocation recovery were responsible for cyclic softening. Alignments and pile-ups of dislocations in the γ matrix, which prohibited their movement and reduced the interaction of dislocations on different slip systems, contributed to cyclic hardening.

Improvements of machinability of aerospace-grade Inconel alloys with ultrasonically assisted hybrid machining

Abstract: Aerospace-grade Ni-based alloys such as Inconel 718 and 625 are widely used in the airspace industry thanks to their excellent mechanical properties at high temperatures. However, these materials are classified as ‘difficult-to-machine’ because of their high shear strength, low thermal conductivity, tendency to work-harden and presence of carbide particles in their microstructure, which lead to rapid tool wear. Machining-induced residual stresses in a machined part is an important parameter which is assessed since it can be used to evaluate overall structural resilience of the component and its propensity to fatigue failure in-service. Ultrasonically assisted turning (UAT) is a hybrid machining technique, in which tool-workpiece contact conditions are altered by imposing ultrasonic vibration (typical frequency ~ 20 kHz) on a tool’s movement in a cutting process. Several studies demonstrated successfully the resulting improvements in cutting forces and surface topography. However, a thorough study of UAT-induced residual stresses is missing. In this study, experimental results are presented for machining Inconel 718 and 625 using both conventional turning (CT) and UAT with different machining parameters to investigate the effect on cutting forces, surface roughness and residual stresses in the machined parts. The study indicates that UAT leads to significant cutting force reductions and improved surface roughness in comparison to CT for cutting speeds below a critical level. The residual stresses in machined workpiece show that UAT generates more compressive stresses when compared to those in CT. Thus, UAT demonstrates an overall improvement in machinability of Inconel alloys.

Polydimethylsiloxane and poly(ether) ether ketone functionally graded composites for biomedical applications.

Abstract: Functionally graded materials (FGMs), with varying spatial, chemical and mechanical gradients (continuous or stepwise), have the potential to mimic heterogenous properties found across biological tissues. They can prevent stress concentrations and retain healthy cellular functions. Here, we show for the first time the fabrication of polydimethylsiloxane and poly(ether) ether ketone (PDMS-PEEK) composites. These were successfully manufactured as a bulk material and functionally graded (stepwise) without the use of hazardous solvents or the need of additives. Chemical, irreversible adhesion between layers (for the FGMs) was achieved without the formation of hard, boundary interfaces. The mechanical properties of PDMS-PEEK FGMs are proven to be further tailorable across the entirety of the build volume, mimicking the transition from soft to harder tissues. The introduction of 20 wt% PEEK particles into the PDMS matrix resulted in significant rises in the elastic modulus under tensile and compressive loading. Biological and thermal screenings suggested that these composites cause no adverse effects to human fibroblast cell lines and can retain physical state and mass at body temperature, which could make the composites suitable for a range of biomedical applications such as maxillofacial prosthetics, artificial blood vessels and articular cartilage replacement.

Tensile properties of 3D multi-layer wrapping braided composite: Progressive damage analysis

Abstract: An experimental and numerical study on mechanical properties and damage behavior of 3D multi-layer wrapping braided composite under axial tensile load is presented. The braiding process of this material is introduced and its tensile properties are obtained in tensile tests. Numerical simulations employ periodical boundary conditions, with interface elements between yarns and matrix added to improve the accuracy of prediction. 3D Hashin-type criteria and Von-Mises stress criterion are employed as damage initiation criteria for yarns and matrix, respectively. The obtained numerical results show a good agreement with the experimental data. The load-bearing capacity and failure mechanisms of 3D multi-layer wrapping braided composites under axial tensile loading are also discussed. A stress distribution shows that the axial yarns are the main load-bearing component of the composite. The main failure mode of the yarns is the yarn-matrix tensile cracking in the width direction, followed by the yarn-matrix tensile cracking in the thickness direction and fibre tensile failure. When the fibres in axial yarns begin to break, the material loses its load-bearing capacity.

Multi-objective optimization of ultrasonic-assisted magnetic abrasive finishing process

Abstract: Ultrasonic-assisted magnetic abrasive finishing (UAMAF) is an advanced abrasive finishing process that finishes a workpiece surface effectually when compared to a traditional magnetic abrasive finishing process in the order of nanometer. A change of surface roughness and material removal rate are two important factors determining the efficacy of the process. These two factors affect the surface quality and production time and, thereby, a total production cost. The finishing performed at higher material removal rates leads to a loss in shape/form accuracy of the surface. At the same time, increasing the rate of change of surface roughness increases loss of material. For an optimized finishing process, a compromise has to be made between the change of surface roughness and the material removal (loss). In this work, a multi-objective optimization technique based on genetic algorithm is used to optimize the finishing parameters in the UAMAF processes. A fuzzy-set-based strategy for a higher level decision is also discussed. The results of the optimization based on a mathematical model of the process are validated with the experimental results and are found to be in compliance.

A review: microstructure and properties of tin-silver-copper lead-free solder series for the applications of electronics

Abstract: Purpose The research on lead-free solder alloys has increased in past decades due to awareness of the environmental impact of lead contents in soldering alloys. This has led to the introduction and development of different grades of lead-free solder alloys in the global market. Tin-silver-copper is a lead-free alloy which has been acknowledged by different consortia as a good alternative to conventional tin-lead alloy. The purpose of this paper is to provide comprehensive knowledge about the tin-silver-copper series. Design/methodology/approach The approach of this study reviews the microstructure and some other properties of tin-silver-copper series after the addition of indium, titanium, iron, zinc, zirconium, bismuth, nickel, antimony, gallium, aluminium, cerium, lanthanum, yttrium, erbium, praseodymium, neodymium, ytterbium, nanoparticles of nickel, cobalt, silicon carbide, aluminium oxide, zinc oxide, titanium dioxide, cerium oxide, zirconium oxide and titanium diboride, as well as carbon nanotubes, nickel-coated carbon nanotubes, single-walled carbon nanotubes and graphene-nano-sheets. Findings The current paper presents a comprehensive review of the tin-silver-copper solder series with possible solutions for improving their microstructure, melting point, mechanical properties and wettability through the addition of different elements/nanoparticles and other materials. Originality/value This paper summarises the useful findings of the tin-silver-copper series comprehensively. This information will assist in future work for the design and development of novel lead-free solder alloys.

SPH-BEM simulation of underwater explosion and bubble dynamics near rigid wall

Abstract: A process of underwater explosion of a charge near a rigid wall includes three main stages: charge detonation, bubble pulsation and jet formation. A smoothed particle hydrodynamics (SPH) method has natural advantages in solving problems with large deformations and is suitable for simulation of processes of charge detonation and jet formation. On the other hand, a boundary element method (BEM) is highly efficient for modelling of the bubble pulsation process. In this paper, a hybrid algorithm, fully utilizing advantages of both SPH and BEM, was applied to simulate the entire process of free and near-field underwater explosions. First, a numerical model of the free-field underwater explosion was developed, and the entire explosion process– from the charge detonation to the jet formation–was analysed. Second, the obtained numerical results were compared with the original experimental data in order to verify the validity of the presented method. Third, a SPH model of underwater explosion for a column charge near a rigid wall was developed to simulate the detonation process. The results for propagation of a shock wave are in good accordance with the physical observations. After that, the SPH results were employed as initial conditions for the BEM to simulate the bubble pulsation. The obtained numerical results show that the bubble expanded at first and then shrunk due to a differences of pressure levels inside and outside it. Here, a good agreement between the numerical and experimental results for the shapes, the maximum radius and the movement of the bubble proved the effectiveness of the developed numerical model. Finally, the BEM results for a stage when an initial jet was formed were used as initial conditions for the SPH method to simulate the process of jet formation and its impact on the rigid wall. The numerical results agreed well with the experimental data, verifying the feasibility and suitability of the hybrid algorithm. Besides, the results show that, due to the effect of the Bjerknes force, a jet with a high speed was formed that may cause local damage to underwater structures.

Modeling of finishing force and torque in ultrasonic-assisted magnetic abrasive finishing process:

Abstract: A new finishing technique called ultrasonic-assisted magnetic abrasive finishing integrates ultrasonic vibration with magnetic abrasive finishing process for finishing of workpiece surface more efficiently as compared to magnetic abrasive finishing in the nanometer range. During finishing, two types of forces are generated in ultrasonic-assisted magnetic abrasive finishing, namely, a normal force (indentation force) and a tangential force (cutting force) that produces a torque. The finishing forces have direct control on the rate of change of surface roughness and material removal rate of the workpiece surface. This article deals with the theoretical modeling of the normal force and the finishing torque based on the process physics. In this work, finite element simulations of the electromagnet were performed to calculate a magnetic flux density in the working zone; they were also used to evaluate the normal force on the workpiece surface. The theory of friction for the abrasion of metals was applied together with the effect of ultrasonic vibration to calculate the finishing torque. The developed model predicts the normal force and finishing torque in ultrasonic-assisted magnetic abrasive finishing as functions of the supply voltage, working gap and concentration of abrasive particles in a flexible magnetic abrasive brush. A comparison of theoretical and experimental results is performed to validate the proposed model.

Mechanical and chemical characterisation of bioresorbable polymeric stent over two-year in vitro degradation:

Abstract: Polymeric stent is a temporary cardiovascular scaffold, made of biodegradable poly (l-lactic) acid, to treat coronary artery stenosis, with expected resorption by the human body over two to three y...

Theoretical Analysis on Needle-Punched Carbon/Carbon Composites

Abstract: Needle-punched carbon/carbon composites (NP-C/Cs) are advanced materials widely used in aerospace applications. The needle-punching technique improves the integrality of carbon-fibre plies, however, it also introduces many defects, affecting the mechanical behavior of NP-C/Cs. A theoretical model of irregular beams is suggested to investigate the mechanical behavior of unidirectional needle-punched carbon/carbon composites. Stress distributions in punched and squeezed fibres and an effect of the needle-punching technology are assessed.

Characterisation of Additively Manufactured Metallic Stents

Abstract: This paper focuses on microstructural characterisation of metallic stents produced with additive manufacturing, a promising technique to deliver patient-specific stents. A 316L stainless steel tube, manufactured by selective laser melting (SLM), and a 316L stainless steel stent were investigated. Specimens were prepared for microstructural studies through sectioning, mounting, grinding and metallurgical polishing procedures. Microstructures were examined employing a JEOL 7100F scanning electron microscope, with simultaneous elemental analysis using energy dispersive x-ray spectroscopy (EDS) and orientation analysis with electron backscatter diffraction. The obtained results showed that a center of the selective laser melted (SLMed) tube had a columnar and coarse grain microstructure, with high-angle grain boundaries. The EDS analysis confirmed that the composition of the SLMed tube were similar to those of commercial stent, but with some differences in weight fractions of alloy elements.

Nonwovens—Structure-process-property relationships

Abstract: The definition of nonwovens is even more complicated. The term nonwoven refers to web-like assemblages of fibers wherein fiber-to-fiber bonding replaces twisting and interlacing. We define a nonwoven as an engineered fabric structure that may contain fibrous and nonfibrous elements and that is often manufactured directly from fibers or filaments and may incorporate other types of fabrics. The difference primarily between a nonwoven and its more traditional counterparts (woven, knitted, and braided structures) is the structure. The fibers or filaments in a nonwoven are not interlaced or interlooped and are somewhat random layered assemblies of fibers held together by a variety of different means. The structure of a nonwoven is defined, therefore, as its fiber orientation distribution function (ODF). Another structural aspect important to consider is the basis weight (mass per unit area—g/m2 or more commonly referred to as gsm) and its uniformity. While ODF may dictate behavior, basis weight uniformity dictates failure. The structure-property relationships in a nonwoven cannot be decoupled from the process utilized to form the nonwoven. Therefore, below, we present a short review of the processes employed in the making of nonwovens followed by a discussion of the structure-process-property relationships and will make an attempt to describe the mechanical properties of one class of nonwovens.

Deformation response and microstructural evolution of as-cast Mg alloys AM30 and AM50 during hot compression

Abstract: The present study deals with the hot deformation behaviour of as-cast Mg alloys AM30 and AM50. The alloys have been investigated for mechanical and microstructural evolution in the tempera...

Experimental study of synthesized co-polymer for stent application

Abstract: Poly-L-lactic acid (PLLA), predominantly used for manufacturing bioresorbable polymeric stents, is brittle and has a considerable degradation time (over 2~3 years). To address its deficiencies, one of the employed techniques is to blend it with elastomers, typically polymers with rubber-like behaviour. The aim of this paper is to assess a co-polymer developed for potential cardiovascular application to address the limitations of PLLA. The focus is on characterisation of mechanical properties of the co-polymer using a spherical nanoindentation technique, in comparison with those of PLLA. The copolymer is a blended poly(L-lactide-co-e-caprolactone)-poly (ethylene glycol) (PLA-(PCL-PEG)), with weight ratios of 60% and 40% for PLA and PCL-PEG, respectively. The novelty of this copolymer is the addition of phosphate-glass particles (less than 2 μm in size and 10 wt%), aiming to toughen the material. The material was supplied in the form of tubing. The obtained nanoindentation results, together with the data of tensile testing, showed that the novel co-polymer (PLA-(PCL-PEG)) did not perform well in comparison to PLLA, and, hence, required further improvements in composition and processing.

Mechanical Performance of Self-expandable Nitinol Stent with Lesion-specific Design

Abstract: This paper aims to assess the performance of a self-expandable nitinol stent, with lesion-specific design, using a finite-element (FE) method. A superelastic model was adopted to describe the superelasticity of nitinol. Hyperelastic models with damage, calibrated against experimental results, were used to describe the stress-stretch responses of arterial layers and plaque. Abaqus CAE was used to create FE models for a femoral artery with non-uniform diffusive stenosis and a nitinol stent with a lesion-specific design. In numerical simulations, an elastic tube was used to crimp and release the self-expandable stent in the diseased artery. The effect of this lesion-specific design on lumen gain was investigated by employing FE results for a commercial stent with a uniform design as a reference. The obtained results showed that the lesion-specific stent achieved larger lumen area in the artery with diffusive lesions.

Small-Scale Machining Simulations

Abstract: Molecular dynamics (MD) and single-crystal plasticity finite-element method (CP FEM) are approaches used to simulate the micro-machining process. At such small-length scales, anisotropic behaviour of material becomes important; the two methods can essentially capture it. Therefore, it is important to understand the fundamental principle behind these methods as well as their capabilities and limitations in order to select the scheme to simulate the micro-machining process. In this paper, the fundamentals of MD and CP FEM are introduced in brief. The applicability of the respective method is further illustrated with the help of examples from the literature. Thereafter, the two methods are compared and discussed in terms of their various aspects and capabilities. This discussion should enrich the reader and assist their choice of an appropriate simulation method.

Finite Element Modelling of Stent Deployment in a Patient-specific Coronary Artery

Abstract: This paper aims to investigate the outcomes of stent deployment in a patient-specific coronary artery using finite-element (FE) method. Hyperelastic models with damage, verified against experimental results, were used to describe stress-stretch responses of arterial layers and plaque. Based on intravital optical coherency tomography imaging of a specific patient’s coronary artery, three-dimensional model of a diseased artery was developed, with two constituent layers (media and adventitia) and plaque, using Mimics. The model was meshed using 3-Matics and then exported to FE software Abaqus. Abaqus CAE was used to create the models for off-the-shelf Resolute Integrity™ drug-eluting coronary stents and tri-folded expansion balloons. The effect of percutaneous coronary intervention on a lumen gain was investigated by simulating the deployment of stent, in comparison with the clinical result.

Quantifying the mechanical properties of polymeric tubing and scaffold using atomic force microscopy and nanoindentation

Abstract: Measurement of mechanical parameters of polymeric scaffolds presents a significant challenge due to their intricate shape and small characteristics dimensions of their elements – around 100μm. In this study, mechanical properties of polymeric tubing and scaffold, made of biodegradable poly (l-lactic) acid (PLLA), were characterised using atomic force microscopy (AFM) and nanoindentation, complemented with tensile testing. AFM was employed to assess the properties of the tube and scaffold locally, whilst nanoindentation produced results with a dependency on the depth of indentation. As a result, the AFM-measured elastic modulus differs from the nanoindentation data due to a substantial difference in indentation depth between the two methods. With AFM, a modulus between 2 and 2.5 GPa was measured, while a wide range was obtained from nanoindentation on both the tube and scaffold, depending on the indentation scale. Changes in the elastic modulus with in-vitro degradation and ageing were observed over the one-year period. To complement the indentation measurements, tensile testing was used to study the structural behaviour of the tube, demonstrating the yielding, hardening and fracture properties of the material.