Showing papers on "Elastic modulus published in 2020"

Mechanical and fracture properties of ultra-high performance geopolymer concrete: Effects of steel fiber and silica fume

Abstract: This study investigates the effects of steel fiber and silica fume on the mechanical and fracture properties of ultra-high performance geopolymer concrete (UHPGC). Four volume fractions of steel fiber (0%, 1%, 2% and 3%) and four contents of silica fume by the mass of total binders (5%, 10%, 20% and 30%) were used. The mechanical and fracture properties evaluated include the compressive, splitting tensile and ultimate flexural strengths, modulus of elasticity, flexural behavior, fracture energy and stress intensity factor. In addition, the correlations among the compressive and splitting tensile strengths, and compressive strength and elastic modulus were studied. The results indicated the increase of steel fiber dosage resulted in the decrease of the workability, but the continuous improvement of mechanical and fracture performance of UHPGC. The empirical equations for predicting elastic modulus of conventional ultra-high performance concrete overestimated the elastic modulus of UHPGC, however some prediction formulas for the splitting tensile strength of PC-based concretes could be applied for UHPGC. Silica fume had a complicated influence on workability and hardened properties of UHPGC, which is strongly dependent on its amount. The inclusion of 10% silica fume induced the increase of the flowability, but the sharp degradation of the mechanical performance, while the specimens with 20% and 30% silica fume possessed the superior mechanical characteristic to that with 5% silica fume. The steel fiber dosage could be decreased without sacrificing the mechanical and fracture performance of UHPGC, via the increase of silica fume content.

Dynamic instability of functionally graded porous arches reinforced by graphene platelets

Abstract: This paper investigates the dynamic instability of a functionally graded porous arch reinforced with uniformly distributed graphene platelets (GPLs) under the combined action of a static force and a dynamic uniform pressure in the radial direction. The relationship between the elastic modulus and mass density of the material is determined by the closed-cell cellular solids under Gaussian Random Field scheme. The governing equation is derived based on classical Euler-Bernoulli theory. Galerkin approach is used to derive the Mathieu-Hill equation from which the dynamic unstable region is obtained using Bolotin method. A comprehensive parametric study is conducted to examine the effects of GPL weight fraction and dimensions, porosity distribution, pore size, static force, and arch geometry and size on the dynamic stability characteristics of the arch. Numerical results show that the porous arch's resistance against dynamic instability can be considerably improved by using symmetrically non-uniform porosity distribution and the addition of a small amount of GPLs.

Extraordinary reinforcing effect of carbon nanotubes in aluminium matrix composites assisted by in-situ alumina nanoparticles

Abstract: Existing aluminium metal matrix composites (Al MMCs) can attain both high tensile yield strength (>500 MPa) and high elastic modulus (>90 GPa) but usually at the expense of tensile strain-to-fracture (~5% or less). Here we report the development of a novel class of Al MMCs that can offer tensile yield strength of 515 ± 17 MPa, elastic modulus of 95.6 ± 1.7 GPa and tensile strain-to-fracture of 10.4 ± 0.8%. Our design hypothesis is to reinforce the Al matrix with ex situ introduced carbon nanotubes (CNTs) for primary strengthening but at the same time we craft a high number density of in situ formed ultrafine γ-Al2O3 nanoparticles to improve dimple fracture. Together they act in concert to render outstanding tensile properties. The strengthening and failure mechanisms of the as-fabricated Al-CNTs-γ-Al2O3 MMCs are characterized in detail. The design concept proposed and validated in this study can be informative for the fabrication of other high-performance carbon-reinforced MMCs.

Temperature dependence of elastic and plastic deformation behavior of a refractory high-entropy alloy.

Abstract: Single-phase solid-solution refractory high-entropy alloys (HEAs) show remarkable mechanical properties, such as their high yield strength and substantial softening resistance at elevated temperatures. Hence, the in-depth study of the deformation behavior for body-centered cubic (BCC) refractory HEAs is a critical issue to explore the uncovered/unique deformation mechanisms. We have investigated the elastic and plastic deformation behaviors of a single BCC NbTaTiV refractory HEA at elevated temperatures using integrated experimental efforts and theoretical calculations. The in situ neutron diffraction results reveal a temperature-dependent elastic anisotropic deformation behavior. The single-crystal elastic moduli and macroscopic Young’s, shear, and bulk moduli were determined from the in situ neutron diffraction, showing great agreement with first-principles calculations, machine learning, and resonant ultrasound spectroscopy results. Furthermore, the edge dislocation–dominant plastic deformation behaviors, which are different from conventional BCC alloys, were quantitatively described by the Williamson-Hall plot profile modeling and high-angle annular dark-field scanning transmission electron microscopy.

Effect of Slot Inclination Angle and Borehole-Slot Ratio on Mechanical Property of Pre-cracked Coal: Implications for ECBM Recovery Using Hydraulic Slotting

Abstract: Low permeability is the main constraint on the high-efficiency coalbed methane recovery in deep coal seams. Hydraulic slotting has been proved to be a favorable method to stimulate low-permeability coal seams. In this paper, the coal samples with various slot inclination angles and borehole-slot ratios were used to investigate the weakening effect of slot inclination angle and borehole-slot ratio on the mechanical property of the pre-cracked coal. Besides, the crack patterns of the slotted coal specimens were identified to reveal the slot weakening mechanism. It is revealed that the variations in compression strength, elastic modulus and Poisson’s ratio with the slot inclination angle generally conform to a Boltzmann function, logistic function and quadratic function, respectively. With the increase in the borehole-slot ratio, the curve clusters of compression strength and elastic modulus show the horizontal “V” with left opening, and the curve clusters of Poisson’s ratio show the trend of rapid increase after slow increase. Compared with elastic modulus, the slot weakening degrees of compression strength and Poisson’s ratio are more significant. Moreover, the tensile and shear cracks mainly appear in the coal samples with small and large slot inclination angles, respectively, which verify the fact that the slot weakening effect on mechanical property of the slotted coal samples with small slot inclination angles is more significant. The research achievements are attributed to the improvement in the efficiency of the hydraulic slotting-based enhanced coalbed methane recovery.

Mechanical and radiation-shielding properties of B 2 O 3 –P 2 O 5 –Li 2 O–MoO 3 glasses

Abstract: Amassed borophosphate lithium molybdate samples were synthesized by the melting classical technique. To check the conditions of these synthesized glasses, the X-ray diffractometer technique was applied. The density value of these samples was increased as well as the molar volume decreased. The mechanical characteristics were linked with Fourier-transform infrared spectrum results. The addition of MoO3 increases the polymerization degree and changes the basic boring units from BO3 to BO4, increases the polymerization degree of phosphate network and changes the negative non-bridging oxygen to positive charges. Ultrasonic velocities and elastic modulus (experimental and theoretical) are increased. This behaviour is correlated to the substitution of Li–O with Mo–O linkages. Mass attenuation coefficient of the prepared samples decreased with the energy increase and with the addition of MoO3. With the increase in photon energy and MoO3 half value layer, the tenth value layer and the mean free path values increase. Hence, the increase of MoO3 leads to a better attenuation of gamma radiation. Therefore, the glass under investigation had superior characteristics for radiation protection applications.

PolyJet 3D Printing of Composite Materials: Experimental and Modelling Approach

Abstract: Multi-material 3D printing has created a wide range of applications because of its high-resolution and multi-functional capabilities. It is important to understand the interaction of the materials both macroscopically and microscopically. This article investigates the mechanical responses of rigid-rubbery polymeric material fabricated using the PolyJet technique as an individual constituent and as an integrated composite unit cell. A series of experiments were conducted to obtain the mechanical responses for individual VeroMagentaV (rigid) and Agilus30 (flexible) polymers with different shore-hardness levels. Tensile results show that the interface of the dual material is strong enough to withstand the stretching during the tensile experiment. The interfacial hardness and local elastic modulus in dual-material parts investigated using nano-indentation and visual inspections showed distinct transition properties. The introduction of different types of rigid reinforcement particles of the 3D-printed composite has been demonstrated and quantified. A numerical model is developed, and the results show good agreement with the compression experiments.

Size-dependent stochastic tensile properties in additively manufactured 316L stainless steel

Abstract: Recent work in metal additive manufacturing (AM) suggests that mechanical properties may vary with feature size; however, these studies do not provide a statistically robust description of this phenomenon, nor do they provide a clear causal mechanism. Because of the huge design freedom afforded by 3D printing, AM parts typically contain a range of feature sizes, with particular interest in smaller features, so the size effect must be well understood in order to make informed design decisions. This work investigates the effect of feature size on the stochastic mechanical performance of laser powder bed fusion tensile specimens. A high-throughput tensile testing method was used to characterize the effect of specimen size on strength, elastic modulus and elongation in a statistically meaningful way. The effective yield strength, ultimate tensile strength and modulus decreased strongly with decreasing specimen size: all three properties were reduced by nearly a factor of two as feature dimensions were scaled down from 6.25 mm to 0.4 mm. Hardness and microstructural observations indicate that this size dependence was not due to an intrinsic change in material properties, but instead the effects of surface roughness on the geometry of the specimens. Finite element analysis using explicit representations of surface topography shows the critical role surface features play in creating stress concentrations that trigger deformation and subsequent fracture. The experimental and finite element results provide the tools needed to make corrections in the design process to more accurately predict the performance of AM components.

Bulk high-entropy nitrides and carbonitrides.

Abstract: High-entropy ceramics have potential to improve the mechanical properties and high-temperature stability over traditional ceramics, and high entropy nitrides and carbonitrides (HENs and HECNs) are particularly attractive for high temperature and high hardness applications. The synthesis of 5 bulk HENs and 4 bulk HECNs forming single-phase materials is reported herein among 11 samples prepared. The hardness of HENs and HECNs increased by an average of 22% and 39%, respectively, over the rule-of-mixtures average of their monocarbide and mononitride precursors. Similarly, elastic modulus values increased by an average of 17% in nitrides and 31% in carbonitrides over their rule-of-mixtures values. The enhancement in mechanical properties is tied to an increase in the configurational entropy and a decrease in the valence electron concentration, providing parameters for tuning mechanical properties of high-entropy ceramics.

Towards brain-tissue-like biomaterials.

Abstract: Many biomaterials have been developed which aim to match the elastic modulus of the brain for improved interfacing. However, other properties such as ultimate toughness, tensile strength, poroviscoelastic responses, energy dissipation, conductivity, and mass diffusivity also need to be considered.

The influence of swelling on elastic properties of polyacrylamide hydrogels

Abstract: Polyacrylamide (PAM) hydrogels are commonly used as substrates for cell mechanical and mechanobiological studies because of their tunable stiffness and ease of handling. The dependence of bulk rheological and local elastic properties (assessed by Atomic Force Microscopy, or AFM) of PAM hydrogels on its composition and polymerization temperature has been extensively studied. PAM hydrogels swell when immersed in media, but the influence of swelling on local elastic properties is poorly characterized. Direct measurements of the effect of swelling on PAM elastic properties are scarce. We report here, for the first time, the direct measurements of volumetric swelling and local elastic properties of PAM gels throughout the post-polymerization swelling process until equilibrium. First, local and global elastic properties (measured by rheology), were obtained during polymerization in the absence of swelling, and showed good agreement with each other. Four PAM hydrogel compositions were characterized thus, with corresponding storage shear moduli (as measured immediately after polymerization) of 4,530 Pa (termed stiffest), 2,900 Pa (stiff), 538 Pa (soft), and 260 Pa (softest). Next, all compositions were subjected to swelling in phosphate buffered saline. Swelling ratios and local elastic moduli were measured at 0, 3, 6, 9, 12, and 24 h post-polymerization for the soft and softest compositions, and once daily till 6 days post-polymerization for all four compositions. For the stiffest and stiff gels, swelling ratio, and local elastic modulus changed negligibly with time, while for the soft and softest gels, substantial changes between Day 0 and Day 1 were found for both swelling ratio (increased by 21.6 and 133%, respectively), and local elastic modulus decreased (by 33.7 and 33.3%, respectively), substantially. Experimental data were analyzed by a model that combined ideal elastomer mechanics and poroelastic swelling kinetics model. Model predictions confirmed the validity of present measurements with respect to past studies where swelling and elastic properties were not measured simultaneously. The present study underlines the important effect swelling can have on PAM elastic properties and provides detailed quantitative data to guide the duration taken to reach equilibrium—a useful information for cell mechanics experiments. In addition, the simultaneous measurements of swelling and local elastic moduli provide novel data for the validation of theoretical models.

Stability and Dynamics of Viscoelastic Moving Rayleigh Beams with an Asymmetrical Distribution of Material Parameters

Abstract: In this article, vibration of viscoelastic axially functionally graded (AFG) moving Rayleigh and Euler–Bernoulli (EB) beams are investigated and compared, aiming at a performance improvement of translating systems. Additionally, a detailed study is performed to elucidate the influence of various factors, such as the rotary inertia factor and axial gradation of material on the stability borders of the system. The material properties of the beam are distributed linearly or exponentially in the longitudinal direction. The Galerkin procedure and eigenvalue analysis are adopted to acquire the natural frequencies, dynamic configuration, and instability thresholds of the system. Furthermore, an exact analytical expression for the critical velocity of the AFG moving Rayleigh beams is presented. The stability maps and critical velocity contours for various material distributions are examined. In the case of variable density and elastic modulus, it is demonstrated that linear and exponential distributions provide a more stable system, respectively. Furthermore, the results revealed that the decrease of density gradient parameter and the increase of the elastic modulus gradient parameter enhance the natural frequencies and enlarge the instability threshold of the system. Hence, the density and elastic modulus gradients play opposite roles in the dynamic behavior of the system.