Showing papers on "Elastic modulus published in 2019"

Soft and elastic hydrogel-based microelectronics for localized low-voltage neuromodulation

Abstract: Narrowing the mechanical mismatch between tissue and implantable microelectronics is essential for reducing immune responses and for accommodating body movement. However, the design of implantable soft electronics (on the order of 10 kPa in modulus) remains a challenge because of the limited availability of suitable electronic materials. Here, we report electrically conductive hydrogel-based elastic microelectronics with Young’s modulus values in the kilopascal range. The system consists of a highly conductive soft hydrogel as a conductor and an elastic fluorinated photoresist as the passivation insulation layer. Owing to the high volumetric capacitance and the passivation layer of the hydrogel, electrode arrays of the thin-film hydrogel ‘elastronics’, 20 μm in feature size, show a significantly reduced interfacial impedance with tissue, a current-injection density that is ~30 times higher than that of platinum electrodes, and stable electrical performance under strain. We demonstrate the use of the soft elastronic arrays for localized low-voltage electrical stimulation of the sciatic nerve in live mice. Conductive and elastic hydrogel-based microelectronic arrays with high current-injection density and low interfacial impedance with tissue enable the localized low-voltage electrical stimulation of the sciatic nerve in live mice.

Stretchable materials of high toughness and low hysteresis.

Abstract: In materials of all types, hysteresis and toughness are usually correlated. For example, a highly stretchable elastomer or hydrogel of a single polymer network has low hysteresis and low toughness. The single network is commonly toughened by introducing sacrificial bonds, but breaking and possibly reforming the sacrificial bonds causes pronounced hysteresis. In this paper, we describe a principle of stretchable materials that disrupt the toughness-hysteresis correlation, achieving both high toughness and low hysteresis. We demonstrate the principle by fabricating a composite of two constituents: a matrix of low elastic modulus, and fibers of high elastic modulus, with strong adhesion between the matrix and the fibers, but with no sacrificial bonds. Both constituents have low hysteresis (5%) and low toughness (300 J/m2), whereas the composite retains the low hysteresis but achieves high toughness (10,000 J/m2). Both constituents are prone to fatigue fracture, whereas the composite is highly fatigue resistant. We conduct experiment and computation to ascertain that the large modulus contrast alleviates stress concentration at the crack front, and that strong adhesion binds the fibers and the matrix and suppresses sliding between them. Stretchable materials of high toughness and low hysteresis provide opportunities to the creation of high-cycle and low-dissipation soft robots and soft human-machine interfaces.

Odd elasticity

Abstract: Hooke's law states that the forces or stresses experienced by an elastic object are proportional to the applied deformations or strains. The number of coefficients of proportionality between stress and strain, i.e., the elastic moduli, is constrained by energy conservation. In this Letter, we lift this restriction and generalize linear elasticity to active media with non-conservative microscopic interactions that violate mechanical reciprocity. This generalized framework, which we dub odd elasticity, reveals that two additional moduli can exist in a two-dimensional isotropic solid with active bonds. Such an odd-elastic solid can be regarded as a distributed engine: work is locally extracted, or injected, during quasi-static cycles of deformation. Using continuum equations, coarse-grained microscopic models, and numerical simulations, we uncover phenomena ranging from activity-induced auxetic behavior to wave propagation powered by self-sustained active elastic cycles. Besides providing insights beyond existing hydrodynamic theories of active solids, odd elasticity suggests design principles for emergent autonomous materials.

Effect of interphase, curvature and agglomeration of SWCNTs on mechanical properties of polymer-based nanocomposites: Experimental and numerical investigations

Abstract: In this study, the elastic modulus of single-walled carbon nanotubes (SWCNTs)/epoxy nanocomposite was studied using the 3D finite element method and compared with experimental results to investigate the effect of SWCNTs interphase, curvature, and agglomeration on the prediction of the elastic modulus. Nanocomposite specimens containing 0.1, 0.3, and 0.5 wt% SWCNTs were fabricated to obtain SWCNTs/epoxy elastic modulus. The elastic modulus increased until SWCNTs was incorporated up to 0.3 wt% and after that, the trend of increasing elastic modulus declined. TEM images showed that in higher contents of filler, there were some local SWCNTs agglomerations within the composites which caused a dropped in elastic modulus of specimens containing 0.5 wt% SWCNTs. Also, six different 3D representative volume element (RVE) of SWCNTs/epoxy including incorporated cylindrical, cylindrical with agglomeration, curved-cylindrical, cylindrical with interphase, cylindrical with interphase and agglomeration and curved-cylindrical with agglomeration SWCNTs in the epoxy matrix have been generated using Digimat-FE and their elastic modulus evaluated by Digimat-FE solver. The numerical results cleared that the simplest cylindrical RVE has the greatest discrepancy with experimental results which showed the necessity of consideration of three important parameters including SWCNTs interphase, curvature, and agglomerations. By considering SWCNTs interphase and agglomeration the difference of numerical and experimental results decreased so that in specimens containing 0.1 wt% SWCNTs the error was only 6.8%. Also, the best results obtained from RVE of curved-cylindrical with agglomeration in specimen containing 0.1 wt% SWCNTs with only 4.1% error which showed the importance of considering SWCNTs agglomeration and curvature for modeling of nanocomposites.

Comparison of Mechanical Properties and Energy Absorption of Sheet-Based and Strut-Based Gyroid Cellular Structures with Graded Densities

Abstract: Bio-inspired functionally graded cellular materials (FGCM) have improved performance in energy absorption compared with a uniform cellular material (UCM). In this work, sheet-based and strut-based gyroid cellular structures with graded densities are designed and manufactured by stereo-lithography (SLA). For comparison, uniform structures are also designed and manufactured, and the graded structures are generated with different gradients. The mechanical behaviors of these structures under compressive loads are investigated. Furthermore, the anisotropy and effective elastic modulus of sheet-based and strut-based unit gyroid cellular structures are estimated by a numerical homogenization method. On the one hand, it is found from the numerical results that the sheet-based gyroid tends to be isotropic, and the elastic modulus of sheet-based gyroid is larger than the strut-based gyroid at the same volume fraction. On the other hand, the graded cellular structure has novel deformation and mechanical behavior. The uniform structure exhibits overall deformation and collapse behavior, whereas the graded cellular structure shows layer-by-layer deformation and collapse behavior. Furthermore, the uniform sheet-based gyroid is not only stiffer but also better in energy absorption capacity than the uniform strut-based gyroid structure. Moreover, the graded cellular structures have better energy absorption capacity than the uniform structures. These significant findings indicate that sheet-based gyroid cellular structure with graded densities have potential applications in various industrial applications, such as in crashworthiness.

Microstructural, thermal and mechanical characterization of TiB2–SiC composites doped with short carbon fibers

Abstract: Spark plasma sintering method, at the temperature of 1800 °C under the pressure of 40 MPa for 7 min, was employed for fabrication of TiB2–SiC-based composites. The influences of short carbon fiber (Cf) addition (2 wt%) on microstructural, mechanical and thermal properties of TiB2–SiC ceramics were studied. Carbon fiber addition increased the relative density of sintered composite which observed to have direct effect on mechanical and thermal properties. The mechanical properties of composites were measured by nanoindentation method. Hardness and elastic modulus of TiB2/SiC interfaces in carbon fiber doped composite were measured 27.1 GPa and 445 GPa, respectively, while these values were obtained 24.2 GPa and 422 GPa for carbon-free sample. The thermal diffusivity of samples was measured by laser flash technique (LFT). It was found that TiB2–SiC–Cf composite has a higher thermal conductivity (55 w/m.K) compared to TiB2–SiC ceramic with a value of 54.8 w/m.K.

Effect of Bi2O3 content on mechanical and nuclear radiation shielding properties of Bi2O3-MoO3-B2O3-SiO2-Na2O-Fe2O3 glass system

Abstract: The melt quench method has been used to prepare the xBi2O3-(30-x)MoO3-40B2O3-20SiO2-9Na2O-1Fe2O3 glass system, where x = 15, 17.5, 20, 25 and 30 mol. %. In the energy range of 0.015–15 MeV, the mass attenuation coefficient (μm), half value layer (HVL) and effective atomic number (Zeff) values have been calculated using MCNPX code for prepared glasses. Using the Geometric progression (G–P) fitting method the exposure buildup factor (EBF) and energy absorption buildup factor (EABF) have been computed. In addition, the removal cross-section (ΣR) values for fast neutrons have been evaluated. Moreover, the mass attenuation coefficient for different compositions glasses have been calculated using XMuDat program and XCOM and have been compared with MCNPX values. The μm, HVL and ΣR values of the investigated glass samples have been compared with the different types of concretes in other glass materials. Using the pulse-echo technique, the elastic moduli for glass samples have been measured. The glass sample 30Bi2O3-40B2O3-20SiO2-9Na2O-1Fe2O3 shows the highest values of μm and lowest values of HVL, which indicate that this glass composition is the most effective at radiation shielding.

Effect of High Temperature on Deformation Failure Behavior of Granite Specimen Containing a Single Fissure Under Uniaxial Compression

Abstract: To investigate the role of fissure angle and heat treatment temperature on the mechanical properties and deformation failure behavior, uniaxial compression tests were carried out on granite specimens containing a single fissure. Using stress–strain curves, the peak strength, peak strain, and elastic modulus of the one-fissured granite specimens were analyzed in detail. The mechanical parameters are closely related to the fissure angle and the high temperature. As the fissure angle increases from 0° to 90°, the peak strength and elastic modulus first decrease and then increase, while the peak strain increases slowly. However, the peak strength and elastic modulus first increase and then decrease, while the peak strain first decreases and then increases with increasing treatment temperature. During the experiments, the crack evolution process and acoustic emission (AE) counts were obtained using real-time photography and the AE monitoring technique. In the granite specimens containing a pre-existing fissure, large AE counts are clearly observed before the peak strength, which indicates crack initiation and propagation. The accumulated AE count first increases slowly, but is followed by a sharp increase, with increasing deformation. The AE events in the one-fissured specimen also depend on the heat treatment temperature. As the temperature increases, the rate of increase of the accumulated AE count curve is reduced. Finally, using a digital image correlation method, the full fields of surface deformation were obtained for the entire testing process. In addition, the local strain around the pre-existing fissure was measured using strain gauges. The full strain field and local strain concentration are discussed to describe the fracture mechanism of brittle granite.

Creep behavior of concrete containing glass powder

Abstract: Glass powder (GP) is a solid waste with increasing reserves. GP can be used as a supplementary cementitious material (SCM) to produce concrete in order to effectively save resources and solve environmental pollution problems. The effect of GP to replace cement partially by weight on the compressive strength, elastic modulus and creep of concrete has experimentally been studied, and the internal microstructure is also determined by mercury intrusion porosimetry, scanning electron microscope and nanoindentation techniques. The results show that the use of GP reduces the compressive strengths and elastic modulus at the early ages, but the use of GP content less than 20% increases the compressive strengths and elastic modulus at the later ages. The use of GP content less than 20% can obviously reduce the creep and the use of 20% GP content seems to be the best in terms of the reduction of the creep. The use of GP with the appropriate content can effectively improve the internal microstructure of concrete and increase the content of high density calcium silicate hydrate at the later ages which is helpful in reducing the creep. This may be attributed to the pozzolanic reaction and microfiller effect of GP.

Optimization of SLM Process Parameters for Ti6Al4V Medical Implants

Abstract: Ti6Al4V alloy has received a great deal of attention in medical applications due to its biomechanical compatibility. However, the human bone stiffness is between 10 and 30 GPa while solid Ti6Al4V is several times stiffer, which would cause stress shielding with the surrounding bone, which can lead to implant and/or the surrounding bone’s failure.,In this work, the effect of selective laser melting (SLM) process parameters on the characteristics of Ti6Al4V samples, such as porosity level, surface roughness, elastic modulus and compressive strength (UCS), has been investigated using response surface method. The examined ranges of process parameters were 35-50 W for laser power, 100-400 mm/s for scan speed and 35-120 µm for hatch spacing. The process parameters have been optimized to obtain structures with properties very close to that in human bones.,The results showed that the porosity percentage of a SLM component could be increased by reducing the laser power and/or increasing the scan speed and hatch spacing. It was also shown that there was a reverse relationship between the porosity level and both the modulus of elasticity and UCS of the SLM part. In addition, the increased laser power was resulted into a substantial decrease of the surface roughness of SLM parts. Results from the optimization study revealed that the interaction between laser process parameters (i.e. laser power, laser speed, and the laser spacing) have the most significant influence on the mechanical properties of fabricated samples. The optimized values for the manufacturing of medical implants were 49 W, 400 mm/s and 99 µm for the laser power, laser speed and laser spacing, respectively. The corresponding porosity, surface roughness, modulus of elasticity and UCS were 23.62 per cent, 8.68 µm, 30 GPa and 522 MPa, respectively.,Previous investigations related to additive manufacturing of Ti alloys have focused on producing fully dense and high-integrity structures. There is a clear gap in literature regarding the simultaneous enhancement and adjustment of pore fraction, surface and mechanical properties of Ti6Al4V SLM components toward biomedical implants. This was the objective of the current study.

Dynamic mechanical and fracture behaviour of sandstone under multiaxial loads using a triaxial hopkinson bar

Abstract: Variations in stress conditions of rocks have been observed during blasting for excavation or large-scale seismic events such as an earthquake. A triaxial Hopkinson bar is developed to apply initial pre-stresses achieving various in situ stress conditions, including uniaxial (principal stresses σ1 > σ2 = σ3 = 0), biaxial (σ1 ≥ σ2 > σ3 = 0) and triaxial (σ1 ≥ σ2 ≥ σ3 ≠ 0) confinements, and then to determine properties of materials under multiaxial pre-stress states at high strain rate. A series of tests was conducted on sandstone specimens to investigate dynamic responses under multiaxial pre-stress states. A high-speed camera at the frame rate of 200,000 fps with a resolution of 256 × 256 pixels was used to capture the fracture characteristics rocks under biaxial compression tests. Experiments show that under the same impact velocity, dynamic properties (e.g. dynamic strength, elastic modulus, fracture modes) of sandstone exhibit confinement dependence. Dynamic strength decreases with increasing axial pre-stress σ1 along the impact direction, while it increases with the increase of lateral pre-stresses σ2 and σ3. The elastic modulus increases with the confinement varying from uniaxial, biaxial to triaxial compression. Rocks are pulverised into powder under uniaxial pre-stress impact, and fragments are ejected from the free face under biaxial compression, while they show slightly damaged or a macroscopic shear fracture under triaxial compression. The 3D imaging of fracture networks in the damaged/fractured specimens was acquired via the X-ray computed tomography system.

Investigation on the orientation dependence of elastic response in Gyroid cellular structures.

Abstract: Materials used for hard tissue replacement should match the elastic properties of human bone tissue. Therefore, cellular structures are more favourable for the use of implants than solid materials for their custom-designed mechanical properties. The superimposed load from various directions in vivo makes uniaxial compression testing insufficient for describing the mechanical responses. In this paper, the rotational symmetry of Gyroid cellular structure (GCS) was discussed. An approach using structural simplification and analytical solution was presented to investigate the relationship between Young's modulus and volume fraction, as well as the orientation dependence of the mechanical responses for GCS loaded in various orientations. It is concluded that the analytical solution is reasonable for a low volume fraction, through the comparison between analytical results, finite element (FE) and experimental data. Gained polar diagrams illustrate the anisotropic property of GCS and also confirm the superiority for their stable mechanical responses of diverse loading directions.

Deformation failure characteristics of coal–rock combined body under uniaxial compression: experimental and numerical investigations

Abstract: The deformation and failure behaviour of coal–rock combined body under uniaxial compression were investigated experimentally and numerically. The mechanical parameters, including the uniaxial compressive strength (UCS), elastic modulus and full-scale stress–strain curves, were obtained. A detailed analysis of the evolution of the internal cracks based on X-ray computed tomography (CT) observations and acoustic emission (AE) locations is presented. The experimental results show that the mechanical properties and deformation failure characteristics of the coal–rock combined body were governed mainly by the coal. The UCS and elastic modulus of the coal–rock combined body were slightly larger than those of the coal and most of the cracks occurring in the coal were a result of the uniaxial compression. Furthermore, a numerical simulation was conducted to validate the experimental evidence. Finally, based on this understanding, a constitutive relationship was proposed using the natural strain described in Hooke’s law for accurate modelling of the deformation of the coal–rock body. A good agreement was obtained between the numerical results and experimental data during the pre-peak regime.

Remarkably Distinct Mechanical Flexibility in Three Structurally Similar Semiconducting Organic Crystals Studied by Nanoindentation and Molecular Dynamics

Abstract: Distinct macroscopic mechanical responses of the three crystals of naphthalene diimide derivatives, 1Me, 1Et, and 1nPr, studied here are very intriguing because their molecular structures are very similar, with the difference only in the alkyl chain length. Among the three crystals examined, 1Me shows highly plastic bending nature, 1Et shows elastic flexibility, and 1nPr is brittle. A detailed investigation by nanoindentation and molecular dynamics (MD) simulations allowed us to correlate their distinct mechanical responses with the way the weak interactions pack in crystal structures. The elastic modulus (E) of 1Me is nearly an order of magnitude lower than that of 1Et, whereas hardness (H) is less than half. The low values of E and H of 1Me indicate that these crystals are highly compliant and offer a low resistance to plastic flow. As the knowledge of hardness and elastic modulus of molecular crystals alone is insufficient to capture their macroscopic mechanical deformation nature, that is, elastic, br...

Engineering hydrogel viscoelasticity.

Abstract: The aim of this study was to identify a method for modifying the time-dependent viscoelastic properties of gels without altering the elastic component. To this end, two hydrogels commonly used in biomedical applications, agarose and acrylamide, were prepared in aqueous solutions of dextran with increasing concentrations (0%, 2% and 5% w/v) and hence increasing viscosities. Commercial polyurethane sponges soaked in the same solutions were used as controls, since, unlike in hydrogels, the liquid in these sponge systems is poorly bound to the polymer network. Sample viscoelastic properties were characterised using the epsilon-dot method, based on compression tests at different constant strain-rates. Experimental data were fitted to a standard linear solid model. While increasing the liquid viscosity in the controls resulted in a significant increase of the characteristic relaxation time (τ), both the instantaneous (Einst) and the equilibrium (Eeq) elastic moduli remained almost constant. However, in the hydrogels a significant reduction of both Einst and τ was observed. On the other hand, as expected, Eeq - an indicator of the equilibrium elastic behaviour after the occurrence of viscoelastic relaxation dynamics - was found to be independent of the liquid phase viscosity. Therefore, although the elastic and viscous components of hydrogels cannot be completely decoupled due to the interaction of the liquid and solid phases, we show that their viscoelastic behaviour can be modulated by varying the viscosity of the aqueous phase. This simple-yet-effective strategy could be useful in the field of mechanobiology, particularly for studying cell response to substrate viscoelasticity while keeping the elastic cue (i.e. equilibrium modulus, or quasi-static stiffness) constant.

Bio-inspired low elastic biodegradable Mg-Zn-Mn-Si-HA alloy fabricated by spark plasma sintering

Abstract: In this paper, biodegradable low elastic Mg-Zn-Mn-Si-HA alloys have been synthesized by element-alloying assisted spark plasma sintering (SPS) process. The main concern of the current investigation...