Showing papers by "Vadim V. Silberschmidt published in 2020"

Printed hydrogel nanocomposites: fine-tuning nanostructure for anisotropic mechanical and conductive properties

Abstract: Additive manufacturing of composites offers a potential for a new level of control over a material’s structure at the microscale. The focus of this work is a 2-hydroxyethyl methacrylate (HEMA)–based gelation system with orderly distributed carbon nanotubes (CNTs). CNTs undergo shear-induced alignment during printing process, and retain their orientation after the polymerisation of HEMA monomers, thereby, forming a nanocomposite with anisotropic mechanical and electrical properties. It is characterised with an intensive programme of mechanical tests including quasistatic uniaxial stretching, and dynamic cyclic loadings, as well as its four-terminal sensing of conductive characteristics. A coupling effect of mechanical and electrical properties is also studied. The experimental findings are discussed in detail and demonstrate that the orientation of CNTs affects both the mechanical and electrical conductive properties of the nanocomposites in terms of its ultimate strength, resistivity, and a piezoresistive coefficient. Understanding of anisotropic electromechanical properties of printed PHEMA-CNT hydrogel nanocomposite will ultimately underpin the development of smart soft materials for diverse applications, such as biomimetic nucleus pulposus or flexible electronics.

Interlayer bonding has bulk-material strength in extrusion additive manufacturing: New understanding of anisotropy

Abstract: This study demonstrates that the interface between layers in 3D-printed polylactide has strength of the bulk filament. Specially designed 3D-printed tensile specimens were developed to test mechanical properties in the direction of the extruded filament (F specimens), representing bulk material properties, and normal to the interface between 3D-printed layers (Z specimens). A wide range of cross-sectional aspect ratios for extruded-filament geometries were considered by printing with five different layer heights and five different extruded-filament widths. Both F and Z specimens demonstrated bulk material strength. In contrast, strain-at-fracture, specific load-bearing capacity, and toughness were found to be lower in Z specimens due to the presence of filament-scale geometric features (grooves between extruded filaments). The different trends for strength as compared to other mechanical properties were evaluated with finite-element analysis. It was found that anisotropy was caused by the extruded-filament geometry and localised strain (as opposed to assumed incomplete bonding of the polymer across the interlayer interface). Additionally, effects of variation in print speed and layer time were studied and found to have no influence on interlayer bond strength. The relevance of the results to other materials, toolpath design, industrial applications, and future research is discussed. The potential to use this new understanding to interpret historic and future research studies is also demonstrated.

Effect of environment on mechanical properties of 3D printed polylactide for biomedical applications.

Abstract: © 2019 Elsevier Ltd In this study, the importance of the testing environment for correct assessment of tensile strength of polylactide (PLA) is investigated. A novel design of tensile specimen was developed to test the anisotropic mechanical properties of additively manufactured specimens. The effects of three environmental factors were considered: physiological temperature (37 °C), hydration (specimens stored in solution for 48 h) and in-aqua testing (specimens submerged in solution). For the first time, these factors were studied both individually and combined, and were evaluated against a control point (non-hydrated specimens tested in air at room temperature). The tensile strength and elastic modulus of hydrated specimens tested submerged at 37 °C were reduced by 50.1% and 20.3%, respectively, versus the control. In contrast, testing the hydrated polymer in air at room temperature, which is commonly used to refer to wet strength in literature, only showed an 18.3% reduction in tensile strength with a negligible change in elastic modulus. To assess transferability of the results, additively manufactured specimens were also tested normal to the interface between 3D printed layers, and they demonstrated similar reductions in strengths and moduli. The results demonstrate the importance of using an appropriate methodology for tensile testing; otherwise, mechanical properties may be overestimated by two-fold.

Intelligent manipulator with flexible link and joint: modeling and vibration control

Abstract: This paper presents a finite-element (FE) model of a manipulator with a flexible link and flexible joint as well as embedded PZT actuators and proposes a corrected rebuilt reduced model (CRRM) to make its dynamic characteristics more consistent with reality and facilitate control design. The CRRM considers the holding torque of the manipulator driving motor and eliminates the response divergence induced by a fault of the mass matrix of the FE model. In order to reduce the dimensions and maintain the precision of the model, an iterated improved reduction system (IIRS) method is adopted. Additionally, a LQR controller is designed based on the output function of the improved model. The simulation results demonstrate that the CRRM is consistent with reality and the active controller has good performance in suppressing vibration of the manipulator with both the flexible link and the flexible joint.

Mechanism of material removal in orthogonal cutting of cortical bone

Abstract: Analysis of a mechanism of bone cutting has an important theoretical and practical significance for orthopaedic surgeries. In this study, the mechanism of material removal in orthogonal cutting of cortical bone is investigated. Chip morphology and crack propagation in cortical bone for various cutting directions and depth-of-cut (DOC) levels are analysed, with consideration of microstructural and sub-microstructural features and material anisotropy. Effects of different material properties of osteons, interstitial matrix and cement lines on chip morphology and crack propagation are elucidated for different cutting directions. This study revealed that differences in chip morphology for various DOCs were due to comparable sizes of the osteons, lamellae and DOC. Acquired force signals and recorded high-speed videos revealed the reasons of fluctuations of dynamic components in tests. Meanwhile, a frequency-domain analysis of force signals showed a frequency difference between formation of a bulk fractured chip and small debris for different cutting directions. In addition, SEM images of the top and side surfaces of the machined bone were obtained. Thus, the analysis of the cutting force and surface damage validated the character of chip formation and explained the material-removal mechanism. This study reveals the mechanism of chip formation in the orthogonal cutting of the cortical bone, demonstrating importance of the correlation between the chip morphologies, the depth of cut and the microstructure and sub-microstructure of the cortical bone. For the first time, it assessed the fluctuations of cutting forces, accompanying chip formation, in time and frequency domains. These findings provide fundamental information important for analysis of cutting-induced damage of the bone tissue, optimization of the cutting process and clinical applications of orthopaedic instruments.

Mechanistic evaluation of long-term in-stent restenosis based on models of tissue damage and growth

Abstract: Development and application of advanced mechanical models of soft tissues and their growth represent one of the main directions in modern mechanics of solids. Such models are increasingly used to deal with complex biomedical problems. Prediction of in-stent restenosis for patients treated with coronary stents remains a highly challenging task. Using a finite element method, this paper presents a mechanistic approach to evaluate the development of in-stent restenosis in an artery following stent implantation. Hyperelastic models with damage, verified with experimental results, are used to describe the level of tissue damage in arterial layers and plaque caused by such intervention. A tissue-growth model, associated with vessel damage, is adopted to describe the growth behaviour of a media layer after stent implantation. Narrowing of lumen diameter with time is used to quantify the development of in-stent restenosis in the vessel after stenting. It is demonstrated that stent designs and materials strongly affect the stenting-induced damage in the media layer and the subsequent development of in-stent restenosis. The larger the artery expansion achieved during balloon inflation, the higher the damage introduced to the media layer, leading to an increased level of in-stent restenosis. In addition, the development of in-stent restenosis is directly correlated with the artery expansion during the stent deployment. The correlation is further used to predict the effect of a complex clinical procedure, such as stent overlapping, on the level of in-stent restenosis developed after percutaneous coronary intervention.

Dry vs. wet: Properties and performance of collagen films. Part I. Mechanical behaviour and strain-rate effect.

Abstract: Collagen forms one-third of the body proteome and has emerged as an important biomaterial for tissue engineering and wound healing. Collagen films are used in tissue regeneration, wound treatment, dural substitute etc. as well as in flexible electronics. Thus, the mechanical behaviour of collagen should be studied under different environmental conditions and strain rates relevant for potential applications. This study's aim is to assess the mechanical behaviour of collagen films under different environmental conditions (hydration, submersion and physiological temperature (37 °C)) and strain rates. The combination of all three environment factors (hydration, submersion and physiological temperature (37 °C)) resulted in a drop of tensile strength of the collagen film by some 90% compared to that of dry samples, while the strain at failure increased to about 145%. For the first time, collagen films were subjected to different strain rates ranging from quasi-static (0.0001 s−1) to intermediate (0.001 s−1, 0.01 s−1) to dynamic (0.1 s−1, 1 s−1) conditions, with the strain-rate-sensitivity exponent ( m ) reported. It was found that collagen exhibited a strain-rate-sensitive hardening behaviour with increasing strain rate. The exponent m ranged from 0.02-0.2, with a tendency to approach zero at intermediate strain rate (0.01 s−1), indicating that collagen may be strain-rate insensitive in this regime. From the identification of hyperelastic parameter of collagen film, it was found that the Ogden Model provides realistic results for future simulations.

Finite element evaluation of artery damage in deployment of polymeric stent with pre- and post-dilation.

Abstract: Using finite element method, this paper evaluates damage in an arterial wall and plaque caused by percutaneous coronary intervention. Hyperelastic damage models, calibrated with experimental results, are used to describe stress-stretch responses of arterial layers and plaque; these models are capable to simulate softening behaviour of the tissue due to damage. Abaqus CAE is employed to create the finite element models for the artery wall (with media and adventitia layers), a symmetric uniform plaque, a bioresorbable polymeric stent and a tri-folded expansion balloon. The effect of percutaneous coronary intervention on vessel damage is investigated by simulating the processes of vessel pre-dilation, stent deployment and post-stenting dilation. Energy dissipation density is used to assess the extent of damage in the tissue. Softening of the plaque and the artery, due to the pre-dilation-induced damage, can facilitate the subsequent stent deployment process. The plaque and the artery experienced heterogeneous damage behaviour after the stent deployment, caused by non-uniform deformation. The post-stenting dilation was effective to achieve a full expansion of the stent, but caused additional damage to the artery. The continuous and discontinuous damage models yielded similar results in the percutaneous coronary intervention simulations, while the incorporation of plaque rupture affected the simulated outcomes of stent deployment. The computational evaluation of the artery damage can be potentially used to assess the risk of in-stent restenosis after percutaneous coronary intervention.

Dry vs. wet: Properties and performance of collagen films. Part II. Cyclic and time-dependent behaviours.

Abstract: Collagen constitutes one-third of human-body proteins, providing mechanical strength and structural stability. Films of collagen are widely used in tissue engineering as scaffolds for wound healing and corneal implants, among other applications, presupposing the investigation of their mechanical properties and performance under various loading and environmental conditions. Part I of this research (Bose et al., 2020) demonstrated a drastic change in the mechanical response of collagen films under in-aqua conditions when compared to dry specimens. It was also observed that collagen films exhibited a strain-rate-dependent hardening behaviour with a strain-rate-sensitivity exponent ranging from 0.02 to 0.2. In Part II, the cyclic and time-dependent behaviours of collagen films were analysed under different loading and environmental conditions. Strain ratchetting was observed for collagen subjected to cyclic loading under various stress levels and environmental (in-air and in-aqua) conditions, while the in-aqua samples demonstrated an increase in the stiffness (50% in the first cycle), which may be referred to as cyclic stiffening. In contrast, the dry samples showed a drop in the modulus after the first cycle, without any subsequent changes. Additionally, time-dependent viscoelastic properties were analysed, using dynamic mechanical analysis as well as creep and stress-relaxation techniques. Tan δ values for dry samples ranged from 0.05 to 0.075, while for hydrated ones it varied from 0.12 to 0.24. Collagen films exhibited primary and secondary creep stages, while the initial stress-relaxation was fast followed by a monotonous decay. The stress-strain-time data obtained from experiments were fitted in Prony series to estimate the relaxation moduli and times.

Patient-specific modelling of stent overlap: Lumen gain, tissue damage and in-stent restenosis.

Abstract: This paper investigates the effects of multiple stents, with and without overlap, on the outcome of stent deployment in a patient-specific coronary artery using the finite element method. Specifically, the objective of this study is to reveal the effect of stent overlap on lumen gain, tissue damage and in-stent restenosis in percutaneous coronary intervention. Based on intravital optical coherency tomography imaging, three-dimensional model of a specific patient's coronary artery was developed, with two constituent layers (media and adventitia) and plaque, using Mimics. Hyperelastic models with damage, verified against experimental results, were used to describe stress-stretch responses of arterial layers and plaque. Abaqus CAE was used to create the models for Resolute Integrity™ drug-eluting stents and tri-folded expansion balloons. The results showed that lumen gain was improved by the overlapping stents than a single stent after deployment; however, damage to the media layer was greater, promoting a higher rate of in-stent restenosis. Meanwhile, the lumen gain achieved with the non-overlapping stents was smaller than that with the overlapping ones, due to an increased recoiling effect. Also, non-overlapping stents induced more tissue damage and higher rate of in-stent restenosis than overlapping stents. With respect to long-term clinical outcomes, the study recommended the use of a single stent where possible or multiple stents with minimal overlaps to treat long or angulated lesions.

A brief review on the mechanical behavior of nonwoven fabrics

Abstract: Although nonwovens used in many areas from civil and mechanical engineering to consumer products, research on their mechanical behavior are rather limited in the literature. In this study, a brief ...

Modeling of friction in manufacturing processes

Abstract: Determination of friction in manufacturing processes forms an important part of the manufacturing process modeling. Friction is dependent on the material properties, roughness of contacting surfaces, sliding length, sliding speed, temperature, lubricant characteristics, and wear during the manufacturing. In this chapter, the commonly used friction models in manufacturing processes are reviewed. An asperity-based friction model available in the literature is chosen and modified to include the effect of temperature. A MATLAB code is developed and a parametric study is carried out to assess the influence of material properties and surface parameters on the coefficient of friction. It is observed that the friction factor is the most influencing parameter on the coefficient of friction compared to other material properties and surface parameters. An inverse method is employed to determine the friction factor as a function of the material and process parameters based on the experimental results. Application of various friction models in different manufacturing processes is reviewed, highlighting that there is gap available between friction models and applications. Some directions for further research are provided.

Size effect in flexural behaviour of unidirectional GFRP composites

Abstract: In this paper, the size effect in unidirectional (UD) laminated glass-fibre reinforced plastic (GFRP) composites subjected to quasi-static bending loading was investigated: the sensitivity of a specimen’s mechanical behaviour and failure mechanism to its geometry was studied. Composite beams with different numbers of 0° unidirectional plies were tested and their post-deformation structures were analysed microscopically. In the subsequent simulations, the intraply damage was modelled using continuum damage mechanics, implemented as a user-defined VUMAT subroutine in ABAQUS/Explicit, while cohesive zone elements were employed to characterize the delamination between different plies. It was observed that the flexural failure triggered the multiple delaminations; their location was studied. The influence of the size effect on the bending response of the UD composite beams was analysed in depth. The findings of the current study can be used to design modern structures made of composite materials.

Damage in extrusion additive manufactured parts: Effect of environment and cyclic loading

Abstract: With a general cautious attitude regarding the anisotropic properties of upright 3D-printed parts, there is a lack of fundamental understanding of behavior of 3D-printed polymers under cyclic loading condition, which is more representative of real-life applications including biomedical ones. To this date, no study considered the multi cyclic testing of an interface bond between layers. So, to examine this, specially designed specimens were developed with the filament widths varied as printed normal to the direction of printing in order to produce dogbone specimens for cyclic tensile testing with two key aims: (i) to characterise the accumulation of damage adjacent extruded filaments; and (ii) to investigate the effect of testing environment on the degradation of mechanical properties. It was found that cyclic loading of 3D-printed polylactide (PLA) specimens resulted in the accumulation of plastic strain, lowering the ultimate strength and strain at break by less than 10% compared to non-cyclic testing. The strength of specimens tested submerged at 37°C were 50% lower than that of tested in air. PLA was plasticised by water, which increased the strain at fracture by approximately 40%. Incremental loading of specimens increased the energy dissipation as approaching the yield point of the material for both testing environments. Meanwhile, damage estimation from the slope of unloading curves indicated that plasticised polymer accumulated 18.1% more damage at lower strain compared to that of tested in air. Specimens tested in air failed in a brittle manner, while, submerged cyclic testing resulted in an intermediate brittle-ductile fracture by formation of apparent shear lips and striation along the fracture plane. The results of this study provide new understanding of the material behavior under condition close to in-vivo environment.

Fracture Behaviour of Collagen: Effect of Environment

Abstract: Collagen forms one-third of the human-body proteome and finds a wide range of applications in a biomedical field thanks to its mechanical stability, biocompatibility and biodegradability. Collagen can be produced in a form of films suitable for scaffolds, tissue regeneration, flexible electronics etc. significant differences in the mechanical properties were observed for collagen films tested in-aqua environment. Considering this and potential biomedical applications of collagen films, their mechanical testing should be performed in aqua to mimic the in-vivo conditions. Hence, this study reported the fracture behaviour of collagen in-aqua compared with that at ambient (in-air) loading conditions. Single-edged notched tension (SENT) specimens of collagen films demonstrated completely different stress-strain curves in-aqua conditions. A reduction in their tensile strength (by 90%) and fracture energy (by 40%) accompanied with an increase in the failure strain (by 1600%) was observed for such conditions. Crack propagation was rapid for in-air specimens, with a brittle failure, while for in-aqua specimens the crack opening was rather slow and accompanied with by crack blunting, leading to large plastic deformation (ductile failure). These behaviours encouraged the quantification of the fracture toughness of collagen films using different fracture toughness parameters: KIC (linear elastic fracture mechanics) for in-air specimens and JC-integral (elastic-plastic fracture mechanics) for in-aqua specimens.

Modelling strain localization in Ti-6Al-4V at high loading rate: a phenomenological approach

Abstract: A phenomenological approach, based on a combination of a damage mechanism and a crystal plasticity model, is proposed to model a process of strain localization in Ti6AI4V at a high strain rate of 1...

Theory of dynamic mode-II delamination in end-loaded split tests

Abstract: The dynamic mode-II energy release rate (ERR) of the end-loaded split (ELS) test with applied time-dependent displacement is derived for the first time with the effect of vibration included. Dynamic Euler-Bernoulli beams are used together with a deflection condition to simulate contact. To understand the dynamic effect and the relative dynamic contribution from each vibration mode, a ‘dynamic factor’ and a ‘spatial factor’ are defined. It is found that the contribution of the ith vibration mode is dependent on the spatial factor, which is a function of the delamination length and the total length of the ELS specimen. Certain vibration modes are dominant dependent on the spatial factor. In addition, for a given spatial factor, there may be a certain vibration mode, which makes approximately zero contribution to the ERR. The developed theory is verified against results from finite-element method (FEM) simulations and it is in excellent agreement. This work now allows the loading rate-dependent mode-II delamination toughness of materials to be determined by using ELS tests. In addition, it provides understanding of the structural dynamic response in the presence of mode-II delamination and can guide the design of structures to mitigate against vibration-driven delamination.

Fracture of 3D-printed micro-tensile specimens: filament-scale geometry-induced anisotropy

Abstract: The interlayer interface was widely considered as the reason for anisotropic mechanical properties in 3D-printed parts produced by material extrusion additive manufacturing (MEAM). Still, the cause has remained widely debated. Utilising a specially developed micro-tensile specimen formed by single filaments, this study examines the roles of their orientation and filament-scale geometric features on mechanical performance. The specimens were loaded in two directions: (i) longitudinal (F), coinciding with the main axis of extruded filaments, and (ii) transverse (Z), normal to the interface between layers. To replicate the geometrical groove features found at the interlayer interfaces in Z specimens, some of the F specimens were scored manually perpendicular to the load prior to tensile testing to produce similar filament-scale features. Tensile testing of all specimens with microscopic characterisation showed that both F specimens (with and without manual grooves) and Z specimens shared very similar strength characteristics, close to those of bulk polylactide (PLA). Manually grooved F specimens demonstrated significantly reduced plasticity, strain-at-fracture and load-bearing area - very close to the Z specimen’s characteristics indicating that the presence of natural grooves in Z specimens is the predominant cause of mechanical anisotropy in MEAM as opposed to commonly assumed poor interlayer molecular diffusion.