Showing papers on "Elastic modulus published in 2017"

High thermal conductivity in soft elastomers with elongated liquid metal inclusions

Abstract: Soft dielectric materials typically exhibit poor heat transfer properties due to the dynamics of phonon transport, which constrain thermal conductivity (k) to decrease monotonically with decreasing elastic modulus (E). This thermal-mechanical trade-off is limiting for wearable computing, soft robotics, and other emerging applications that require materials with both high thermal conductivity and low mechanical stiffness. Here, we overcome this constraint with an electrically insulating composite that exhibits an unprecedented combination of metal-like thermal conductivity, an elastic compliance similar to soft biological tissue (Young's modulus 600% strain). By incorporating liquid metal (LM) microdroplets into a soft elastomer, we achieve a ∼25× increase in thermal conductivity (4.7 ± 0.2 W⋅m-1⋅K-1) over the base polymer (0.20 ± 0.01 W⋅m-1·K-1) under stress-free conditions and a ∼50× increase (9.8 ± 0.8 W⋅m-1·K-1) when strained. This exceptional combination of thermal and mechanical properties is enabled by a unique thermal-mechanical coupling that exploits the deformability of the LM inclusions to create thermally conductive pathways in situ. Moreover, these materials offer possibilities for passive heat exchange in stretchable electronics and bioinspired robotics, which we demonstrate through the rapid heat dissipation of an elastomer-mounted extreme high-power LED lamp and a swimming soft robot.

Temperature dependent mechanical properties of graphene reinforced polymer nanocomposites – A molecular dynamics simulation

Abstract: This paper investigates the mechanical properties of graphene/PMMA nanocomposite system by using the molecular dynamics simulations. The graphene nanoplates are assumed to be fully exfoliated in the PMMA matrix and are all planar orientated, which are similar to the ones assembled using layer-by-layer technique. The Young's modulus and shear modulus of the composites with different graphene volume fractions under different temperatures are simulated and discussed. The results show that the Young's and shear moduli increase with the increase of graphene volume fraction and decrease as the temperature rises from 300 K to 500 K, while the efficiency of the reinforcement is reduced as the graphene content becomes higher. Simulations of single layer graphene under uniaxial tension, in-plane pure shear and uniformly distributed transverse load are performed and the effective thickness and the elastic moduli of graphene are subsequently determined uniquely. The obtained stiffnesses of graphene are then substituted into the simple rule of mixture to predict the overall mechanical properties of the composite. Large discrepancies between the results from the MD simulations and the rule of mixture are observed.

Structure-property relationships from universal signatures of plasticity in disordered solids

Abstract: When deformed beyond their elastic limits, crystalline solids flow plastically via particle rearrangements localized around structural defects. Disordered solids also flow, but without obvious structural defects. We link structure to plasticity in disordered solids via a microscopic structural quantity, “softness,” designed by machine learning to be maximally predictive of rearrangements. Experimental results and computations enabled us to measure the spatial correlations and strain response of softness, as well as two measures of plasticity: the size of rearrangements and the yield strain. All four quantities maintained remarkable commonality in their values for disordered packings of objects ranging from atoms to grains, spanning seven orders of magnitude in diameter and 13 orders of magnitude in elastic modulus. These commonalities link the spatial correlations and strain response of softness to rearrangement size and yield strain, respectively.

Nanoindentation and wear properties of Ti and Ti-TiB composite materials produced by selective laser melting

Abstract: Ti and Ti-TiB composite materials were produced by selective laser melting (SLM). Ti showed an α΄ microstructure, whereas the Ti-TiB composite revealed a distribution of needle-like TiB particles across an α-Ti matrix. Hardness (H) and reduced elastic modulus (Er) were investigated by nanoindentation using loads of 2, 5 and 10 mN. The results showed higher H and Er values for the Ti-TiB than Ti due to the hardening and stiffening effects of the TiB reinforcements. On increasing the nanoindentation load, H and Er were decreased. Comparison of the nanoindentation results with those derived from conventional hardness and compression tests indicated that 5 mN is the most suitable nanoindentation load to assess the elastic modulus and hardness properties. The wear resistance of the samples was related to their corresponding H/Er and H3/Er2 ratios obtained by nanoindentation. These investigations showed that there is a high degree of consistency between the characterization using nanoindentation and the wear evaluation from conventional wear tests.

Standardized Nanomechanical Atomic Force Microscopy Procedure (SNAP) for Measuring Soft and Biological Samples.

Abstract: We present a procedure that allows a reliable determination of the elastic (Young's) modulus of soft samples, including living cells, by atomic force microscopy (AFM). The standardized nanomechanical AFM procedure (SNAP) ensures the precise adjustment of the AFM optical lever system, a prerequisite for all kinds of force spectroscopy methods, to obtain reliable values independent of the instrument, laboratory and operator. Measurements of soft hydrogel samples with a well-defined elastic modulus using different AFMs revealed that the uncertainties in the determination of the deflection sensitivity and subsequently cantilever's spring constant were the main sources of error. SNAP eliminates those errors by calculating the correct deflection sensitivity based on spring constants determined with a vibrometer. The procedure was validated within a large network of European laboratories by measuring the elastic properties of gels and living cells, showing that its application reduces the variability in elastic moduli of hydrogels down to 1%, and increased the consistency of living cells elasticity measurements by a factor of two. The high reproducibility of elasticity measurements provided by SNAP could improve significantly the applicability of cell mechanics as a quantitative marker to discriminate between cell types and conditions.

Topology optimization of anisotropic broadband double-negative elastic metamaterials

Abstract: As the counterpart of electromagnetic and acoustic metamaterials, elastic metamaterials are artificial periodic elastic composite materials offering the possibility to manipulate elastic wave propagation in the subwavelength scale through different mechanisms. For the promising superlensing in the medical ultrasonic detection, double-negative metamaterials possessing the negative effective mass density and elastic modulus simultaneously can be utilized as the ideal superlens for breaking the diffraction limit. In this paper, we present a topology optimization scheme to design the two-dimensional (2D) single-phase anisotropic elastic metamaterials with broadband double-negative effective material properties and demonstrate the superlensing effect at the deep-subwavelength scale. We also discuss the impact of several design parameters adopted in the objective function and constraints on the optimized results. Unlike all previously reported mechanisms, the present optimized structures exhibit the novel quadrupolar or multipolar resonances for the negative effective mass density and negative effective elastic modulus. In addition, negative refraction of the transverse waves in a single-phase material is observed. Most optimized structures in this paper can serve as the anisotropic zero-index metamaterials for the longitudinal or transverse waves at a certain frequency. The cloaking effect is demonstrated for both the longitudinal and transverse waves. Moreover, with the particular constraints in the optimization procedure, a super-anisotropic metamaterial exhibiting the double-negative and hyperbolic dispersions in two principal directions within two different frequency ranges is obtained. The developed optimization scheme provides a robust computational tool for negative-index engineering of elastic metamaterials and may guide the design and optimization of other types of metamaterials, including the electromagnetic and acoustic metamaterials. The unusual properties of our optimized structures can inspire new ideas and novel applications including the low-frequency vibration attenuation, flat lens and ultrasonography for elastic waves.

Finite element analysis of mechanical behavior, permeability and fluid induced wall shear stress of high porosity scaffolds with gyroid and lattice-based architectures.

Abstract: Scaffold design necessitates the consideration of mechanical properties and fluid flow dynamics as the main factors in the development of such materials. The mechanical behavior of bone scaffolds is characterized by properties such as elastic modulus and compressive strength. In terms of fluid flow dynamics, within bone scaffolds, permeability is an important parameter that affects cells' biological activities, and flow-induced shear stress is used as a mechanical stimulant of cell growth. In this study, two scaffold architectures with gyroid and lattice-based rectangular unit cells were designed to analysis the effective elastic moduli, compressive strength, permeability and fluid flow-induced wall shear stress as functions of porosity. Six levels of porosity (65%, 70%, 75%, 80%, 85% and 90%) were assigned to the scaffold architectures, and 12 models were developed. Scaffold deformation under static loading, compressive strength based on von Mises criteria, pressure drop, and fluid flow-induced wall shear stress in the scaffolds were then determined by finite element analysis. In both the scaffold types, models with higher porosity exhibited lower mechanical properties. Under the same porosity, the lattice-based scaffolds exhibited a Young's modulus and a compressive strength higher than those achieved by the gyroid scaffolds. With reference to geometrical parameters and the derived pressure drop from the computational fluid dynamics (CFD) analysis, scaffolds permeability was calculated using Darcy's law. In both the scaffold architectures, high porosity increased permeability and decreased wall shear stress. In the same porosity, the lattice-based models exhibited higher permeability and lower wall shear stress than did the gyroid models. On the basis of the results on elastic modulus and permeability, the models that most effectively mimic the properties of cancellous bones were identified.

Effects of hot isostatic pressing on the elastic modulus and tensile properties of 316L parts made by powder bed laser fusion

Abstract: The microstructure and mechanical properties of 316L steel have been examined for parts built by a powder bed laser fusion process, which uses a laser to melt and build parts additively on a layer by layer basis. Relative density and porosity determined using various experimental techniques were correlated against laser energy density. Based on porosity sizes, morphology and distributions, the porosity was seen to transition between an irregular, highly directional porosity at the low laser energy density and a smaller, more rounded and randomly distributed porosity at higher laser energy density, thought to be caused by keyhole melting. In both cases, the porosity was reduced by hot isostatic pressing (HIP). High throughput ultrasound based measurements were used to calculate elasticity properties and show that the lower porosities from builds with higher energy densities have higher elasticity moduli in accordance with empirical relationships, and hot isostatic pressing improves the elasticity properties to levels associated with wrought/rolled 316L. However, even with hot isostatic pressing the best properties were obtained from samples with the lowest porosity in the as-built condition. A finite element stress analysis based on the porosity microstructures was undertaken, to understand the effect of pore size distributions and morphology on the Young's modulus. Over 1–5% porosity range angular porosity was found to reduce the Young's modulus by 5% more than rounded porosity. Experimentally measured Young's moduli for samples treated by HIP were closer to the rounded trends than the as-built samples, which were closer to angular trends. Tensile tests on specimens produced at optimised machine parameters displayed a high degree of anisotropy in the build direction and test variability for as-built parts, especially between vertical and horizontal build directions. The as-built properties were generally found to have a higher yield stress, but lower upper tensile strength and elongation than published data for wrought/hot-rolled plate 316L. The hot isostatically pressed parts showed a homogenisation of the properties across build directions and properties much more akin to those of wrought/hot-rolled 316L, with an increase in elongation and upper tensile strength, and a reduction in yield over the as-built samples.

3D Printing of a Double Network Hydrogel with a Compression Strength and Elastic Modulus Greater than those of Cartilage

Abstract: This article demonstrates a two-step method to 3D print double network hydrogels at room temperature with a low-cost ($300) 3D printer. A first network precursor solution was made 3D printable via extrusion from a nozzle by adding a layered silicate to make it shear-thinning. After printing and UV-curing, objects were soaked in a second network precursor solution and UV-cured again to create interpenetrating networks of poly(2-acrylamido-2-methylpropanesulfonate) and polyacrylamide. By varying the ratio of polyacrylamide to cross-linker, the trade-off between stiffness and maximum elongation of the gel can be tuned to yield a compression strength and elastic modulus of 61.9 and 0.44 MPa, respectively, values that are greater than those reported for bovine cartilage. The maximum compressive (93.5 MPa) and tensile (1.4 MPa) strengths of the gel are twice that of previous 3D printed gels, and the gel does not deform after it is soaked in water. By 3D printing a synthetic meniscus from an X-ray computed tomog...

Widely tunable and anisotropic charge carrier mobility in monolayer tin(II) selenide using biaxial strain: a first-principles study

Abstract: Two dimensional (2D) materials are promising candidates for developing next-generation electronics. Monolayer tin(II) selenide (SnSe), which can be obtained by exfoliating bulk SnSe crystals at a low cleavage energy, is shown to be a nearly direct band gap semiconductor using first-principles calculations. By incorporating the anisotropic characteristics of effective masses, elastic modulus, and deformation potential with the longitudinal acoustic deformation potential scattering mechanism, we demonstrate that the charge carrier mobilities of monolayer SnSe strongly depend on the carrier type, valley index, transport direction, and biaxial strain. In particular, electron mobility is generally higher than hole mobility, and exhibits anisotropy of up to 241%. With increasing biaxial tensile strain, both electron mobility and hole mobility decline gradually, which is mainly attributed to a strain-induced heavier effective mass. Surprisingly, it is found that carrier mobilities can be enhanced by about 147% (electrons) and 968% (holes) via a small biaxial compressive strain, which effectively produces smaller effective masses of carriers and larger elastic modulus, suggesting the possibility of achieving high mobility of monolayer SnSe over a large number of substrates. Our results highlight a new promising 2D tin-based chalcogenide material with high thermal stability and controllable superior carrier mobility, having great potential in future flexible nano-electronic devices.

Computationally designed lattices with tuned properties for tissue engineering using 3D printing.

Abstract: Tissue scaffolds provide structural support while facilitating tissue growth, but are challenging to design due to diverse property trade-offs. Here, a computational approach was developed for modeling scaffolds with lattice structures of eight different topologies and assessing properties relevant to bone tissue engineering applications. Evaluated properties include porosity, pore size, surface-volume ratio, elastic modulus, shear modulus, and permeability. Lattice topologies were generated by patterning beam-based unit cells, with design parameters for beam diameter and unit cell length. Finite element simulations were conducted for each topology and quantified how elastic modulus and shear modulus scale with porosity, and how permeability scales with porosity cubed over surface-volume ratio squared. Lattices were compared with controlled properties related to porosity and pore size. Relative comparisons suggest that lattice topology leads to specializations in achievable properties. For instance, Cube topologies tend to have high elastic and low shear moduli while Octet topologies have high shear moduli and surface-volume ratios but low permeability. The developed method was utilized to analyze property trade-offs as beam diameter was altered for a given topology, and used to prototype a 3D printed lattice embedded in an interbody cage for spinal fusion treatments. Findings provide a basis for modeling and understanding relative differences among beam-based lattices designed to facilitate bone tissue growth.

Comprehensive study on physical, elastic and shielding properties of lead zinc phosphate glasses

Abstract: A series of ternary phosphate glasses in the form of (PbO)x(ZnO)60-x(P2O5)40 where x = 0–60 mol%, have been successfully prepared by conventional melt-quenching technique. The physical and elastic properties of the glasses have been investigated using pulse echo technique. The longitudinal and shear velocity of the glasses were measured using the MBS8000 ultrasonic data acquisition system at 10 MHz frequency in room temperature. The density, ultrasonic velocity and elastic moduli are found to be composition dependent and the correlation between the elastic moduli with the atomic packing density is discussed in detailed. The shielding parameters, mass attenuation coefficients, half value layers and exposure buildup factor (EBF) values have been computed using WinXCom program with the use of GP fitting method, and variation of shielding parameters are discussed for the effect of PbO addition into the glasses and photon energy. An increase in the density of the glasses results in a change in crosslink density. The sound velocity and elastic properties increased with PbO content and increase in Poisson's ratio trend suggests that the rigidity of the glasses has decreased. Besides, the replacement of ZnO by PbO causes an increase in mass attenuation coefficient, while the half value layer and the exposure buildup factor were decreased and these glasses has been potentially used as shielding material.

Effect of Elevated Temperature on Mechanical Properties of Limestone, Quartzite and Granite Concrete

Abstract: Although concrete is a noncombustible material, high temperatures such as those experienced during a fire have a negative effect on the mechanical properties. This paper studies the effect of elevated temperatures on the mechanical properties of limestone, quartzite and granite concrete. Samples from three different concrete mixes with limestone, quartzite and granite coarse aggregates were prepared. The test samples were subjected to temperatures ranging from 25 to 650 °C for a duration of 2 h. Mechanical properties of concrete including the compressive and tensile strength, modulus of elasticity, and ultimate strain in compression were obtained. Effects of temperature on resistance to degradation, thermal expansion and phase compositions of the aggregates were investigated. The results indicated that the mechanical properties of concrete are largely affected from elevated temperatures and the type of coarse aggregate used. The compressive and split tensile strength, and modulus of elasticity decreased with increasing temperature, while the ultimate strain in compression increased. Concrete made of granite coarse aggregate showed higher mechanical properties at all temperatures, followed by quartzite and limestone concretes. In addition to decomposition of cement paste, the imparity in thermal expansion behavior between cement paste and aggregates, and degradation and phase decomposition (and/or transition) of aggregates under high temperature were considered as main factors impacting the mechanical properties of concrete. The novelty of this research stems from the fact that three different aggregate types are comparatively evaluated, mechanisms are systemically analyzed, and empirical relationships are established to predict the residual compressive and tensile strength, elastic modulus, and ultimate compressive strain for concretes subjected to high temperatures.