Void Surface Research Articles

Tensile mechanical performance of Ni-Co alloy nanowires by molecular dynamics simulation.

In this present contribution, tensile mechanical properties of Ni–Co alloy nanowires with Co content from 0 to 20% were studied by molecular dynamics. The simulation results show the alloy nanowire with the Co content of 5% has the highest yield value of 9.72 GPa. In addition, more Frank dislocations were generated during the loading process to improve the performance of the alloy nanowire. The Young's modulus increases little by little from 105.68 to 179.78 GPa with the increase of Co content. Secondly, with the increase of temperature, the yield strength gradually decreases to 2.13 GPa. Young's modulus tends to decrease linearly from 170.7 GPa to 48.21 GPa. At the temperatures of 500 K and 700 K, it is easier to form Frank dislocation and Hirth dislocation, respectively, in the loading process. The peak value of the radial distribution function decreases and the number of peaks decreases, indicating the disappearance of the ordered structure. Finally, after the introduction of the surface and inner void, the yield strength of the nanowire drops about to 8.97 and 6.6 GPa, respectively, and the yield strains drop to 0.056 and 0.043. In the case of the existence of internal void, perfect dislocation and Hirth dislocation can be observed in the structure.

Phase field study the effects of interfacial energy anisotropy on the thermal migration of voids

While it is well known that inclusions (gaseous, fluid or solid) in a solid matrix would migrate under the driving force of a temperature gradient and the inclusion morphology is strongly affected by the material anisotropy, the influence of material anisotropy on the thermal migration of inclusions is still not clear. Via including an interfacial energy anisotropy into a phase field model of void migration driven by a temperature gradient, here we show that the interfacial energy anisotropy brings nontrivial effects on the void migration behavior. On the one hand, the void deformation during migration depends on the orientation of the anisotropy. On the other hand, a transverse velocity of the void appears if there is a misalignment between the symmetry axis of the anisotropy and the global temperature gradient direction. Such a transverse motion is due to the fact that the thermal diffusion flow becomes asymmetrically distributed around the void surface if the temperature gradient direction does not coincide with the symmetry axis of the anisotropy. The effects of the void size, the anisotropy parameter, and the thermal conductivity difference between the void and the matrix are discussed.

Analyzing effects of surface roughness, voids, and particle–matrix interfacial bonding on the failure response of a heterogeneous adhesive

An automated computational framework is introduced to link the microstructure to the micromechanical behavior of a heterogeneous adhesive composed of an epoxy matrix and embedded silica particles. A new reconstruction algorithm is employed to synthesize 3D periodic microstructural models of this materials system based on morphological and statistical data extracted from micro-computed tomography images. A non-iterative mesh generation algorithm is implemented in parallel to transform resulting virtual microstructures into finite element (FE) models. The continuum ductile and cohesive damage models used for simulating the adhesive’s failure response are calibrated with experimental data and the statistical representative volume element (SRVE) is identified. We have then carried out several high-fidelity FE simulations to investigate the effects of pre-existing voids in the adhesive layer, surface roughness of adherends, and the bonding strength along particles–matrix interfaces on the failure response of the adhesive subject to tensile, compressive, and shear loads.

Size-dependent surface energy of Ar and Si nanovoids

A simple model for the size dependence of liquid–vapor surface energy of a liquid drop, free of any adjustable parameter, has been extended for the size dependence of the surface energy of nanovoids in this work. Considering nanovoids, there is a negative curvature on the void surface, which is completely different from nanoparticles or liquid drops, which have positive curvature surfaces. Thus curvature has an important effect on material properties. Moreover, by introducing negative curvature into the model for deduction of liquid–vapor surface energy, a successful prediction for the behaviour of argon (Ar) and silicon (Si) surface energy on the surface of nanovoids are obtained, which further confirms the role of curvature. Furthermore, it is shown that the surface energy of nanovoids increases as void size is reduced, suggesting a greater difference in energy between surface atoms and interior atoms, especially when void size is smaller than 4 nm. This is owing to the fact that the surface energy gradually approaches that of the bulk when the void size is larger than 4 nm. Thus, the effect of size on the surface energy of voids is significant when the voids are of small dimensions.

Comparative investigation on physical and mechanical properties of water hyacinth and cattail fiber reinforced epoxy hybrid composites

In this research, the study to investigate and compare the physical and mechanical properties of water hyacinth and cattail fiber reinforced epoxy hybrid composites. The composites were fabricated by hand lay-up process. The effect of investigation was analyzed via water absorption, microstructure, tensile properties, flexural properties and impact strength tests for total fiber contents, 15 wt% and different water hyacinth- cattail fiber ratios (10/0, 8/2, 6/4, 4/6, 2/8 and 0/10). The results showed that the addition of water hyacinth and cattail fiber in epoxy, improves tensile properties, flexural properties and impact properties, but decrease water absorption. The analysis of the microstructure found that surface fracture behavior and void between the fiber and matrix of the composites using scanning electron microscope (SEM).

Preparation and Electrochemical Properties of the Spongelike Melamine Formaldehyde-Poly(vinyl alcohol)/LiFePO4 Porous Composite as the Lithium-Battery Cathode

The electrospun-based porous nitrogen-enriched carbon-coated olivine LiFePO4 nanocomposites (MF-PVA/LFP) were prepared by employing the practical electrospin technique and using affordable polymers, melamine-formaldehyde (MF) with poly(vinyl alcohol) (PVA), as the nitrogen-containing carbon precursor, followed by carbonization at various temperatures. Uniform and good interfiber separation of electrospun MF-PVA/LFP composite nanofibers with a average diameter of about 150–200 nm were initially obtained and then transformed into the spongelike porous structure after carbonization. The observed conductive pyridinic-N and pyrrolic-N nitrogen functional groups within the carbon coating material combining with the enhanced specific surface and high void volume of the porous structure played a great impact on the rate capability and cycling performance under high current density cycles. The highest obtained discharging capacity of the porous MF-PVA/LFP composite was 105 mAh g–1 at 0.1C. This promising concept c...

3D SnS2 hierarchical micro-flowers synthesized by ZnSn(OH)6 for ultra-sensitive NH3 sensor

3D SnS2 hierarchical micro-flowers were successfully synthesized by a hydrothermal method with ZnSn(OH)6 hollow nanospheres as precursor. The 3D hierarchical microstructure with large specific surface area was obtained by self-assembling individual 2D nanometer sized sub-units. The architectures, morphologies and gas-sensing performances of the products were characterized and it was found that sensors based on SnS2 micro-flowers had a relatively high response value to 50 ppm ammonia gas at 160 °C, with response and recovery times of approximately 4 s and 15 s, respectively. The good gas-performance to low-concentration of ammonia is attributed to the structure of SnS2 which possessed a large surface area and void.

Imaging Material Texture of As-Deposited Selective Laser Melted Parts Using Spatially Resolved Acoustic Spectroscopy

Additive manufacturing (AM) is a production technology where material is accumulated to create a structure, often through added shaped layers. The major advantage of additive manufacturing is in creating unique and complex parts for use in areas where conventional manufacturing reaches its limitations. However, the current class of AM systems produce parts that contain structural defects (e.g., cracks and pores) which is not compatible with certification in high value industries. The probable complexity of an AM design increases the difficulty of using many non-destructive evaluation (NDE) techniques to inspect AM parts—however, a unique opportunity exists to interrogate a part during production using a rapid surface based technique. Spatially resolved acoustic spectroscopy (SRAS) is a laser ultrasound inspection technique used to image material microstructure of metals and alloys. SRAS generates and detects `controlled’ surface acoustic waves (SAWs) using lasers, which makes it a non-contact and non-destructive technique. The technique is also sensitive to surface and subsurface voids. Work until now has been on imaging the texture information of selective laser melted (SLM) parts once prepared (i.e., polished with R a < 0.1 μ m)—the challenge for performing laser ultrasonics in-process is measuring waves on the rough surfaces present on as-deposited parts. This paper presents the results of a prototype SRAS system, developed using the rough surface ultrasound detector known as speckle knife edge detector (SKED)—texture images using this setup of an as-deposited Ti64 SLM sample, with a surface roughness of S a ≈ 6 μ m, were obtained.

Cellulose porosity improves its dissolution by facilitating solvent diffusion

This paper reports on the effect of improved porous characteristics of cellulose on its solubility in DMAc/LiCl. The use of freeze-drying (FD) treatment led to higher surface area and void fraction of volume for high-molecular-weight (HMW) cotton cellulose (DP~5000). Improvement in porous characteristics of cellulose did not change the chemical and crystalline structures of cellulose, as compared to hot-drying (HD) treatment. However, significant improvement in the dissolution of FD-HMW cellulose in DMAc/LiCl was observed under relatively low temperature (80 °C). The relationship between the solubility and the porous characteristics of cellulose was discussed using Stokes-Einstein Equation and effective diffusion coefficient equation. It was concluded that the increase in the diffusion coefficient of the solvent and the improvement in the porous characteristics of cellulose played key roles to enhance the diffusion rate of the solvent through the cellulose molecular network.

Nondestructive evaluation of sintering and degradation for rotational molded polyethylene

Developments in new sensor technologies and data processing are helping to increase the number of applications of nondestructive characterization methods. In this study, two major physiochemical phenomena affecting product quality of rotationally molded polyethylene parts, namely sintering and degradation, were evaluated using both traditional characterization techniques and a newer alternative ultrasonic-based method. Oven temperature and heating cycle time were controlled to produce six different process conditions for rotomolding. Increasing peak internal air temperature (PIAT) inside the mold produced a reduction in surface voids (pitting) and increased the impact strength for produced parts, which can be related to greater densification during sintering. Contrary to these characterizations denoting improved part quality, degradation was detected for PIAT above 220 °C by an increase in surface carbonyl groups by Fourier-transform infrared spectroscopy (FT-IR) and an increase in zero-shear viscosity, both relatable to thermo-oxidative free radical reactions. The newly proposed monitoring technique applying propagating ultrasonic guided waves showed that its data-rich spectral features based on harmonic frequencies were positively correlated to the same sintering and degradation properties observed above. Coupled with multivariate statistical analysis, the nondestructive ultrasonic technique shows great promise for combining multiple analyses in a single sensor technology, making it well suited to the implementation of advanced manufacturing methodologies in polymer processing practices.

Modelling Shear Behaviour of Joint Based on Joint Surface Degradation During Shearing

An empirical model of the shear behaviour of rock joints is suggested based on a recently proposed joint surface degradation investigation method. Plaster joint replicas were tested with various surface roughnesses, normal stresses, and shear displacements. Joint surfaces with identical joint surface roughness were tested under the same normal stress and the tests were stopped at different shear displacements to investigate the joint surface degradation during shearing. Moreover, the joint surfaces before and after the shear tests were digitized and the digital surfaces were aligned with the help of reference points attached to the sides of the specimens. As a result, the joint surface deformation and void space during shearing could be determined. An empirical model was built based on analysis of the failure mode and failure process of the joint asperities under shearing; the model also takes into account that the basic friction angle varies with normal stress acting on the contact area. The concept of the equivalent effective asperity dipping angle, de, which can be regarded as the representative angle along which the two surfaces slide, was proposed and employed in the derivation of the joint shear and dilation model. Additional shear tests were conducted to verify the proposed model by comparing it with the modified Barton model.

Effect of concrete rheological properties on quality of formed surfaces cast with self-consolidating concrete and superworkable concrete

An experimental program was undertaken to evaluate the effect of rheology of self-consolidating concrete and superworkable concrete on formed surface quality. In total, 31 mixtures with different workability and rheological properties were cast in a specially designed Z-shaped column without any mechanical consolidation. Surface defects, including surface air voids, signs of bleeding, segregation, and low filling ability were evaluated using a proposed image analysis methodology. The proposed method was successfully compared to other approaches that mainly target the detection of surface voids. Statistical models were developed between surface defect characteristics of formed surfaces cast with self-consolidating concrete and superworkable concrete and the rheological properties of the concrete. Concrete mixtures with yield stress lower than 25 Pa were found to develop superior surface finish. It was also observed that a prolonged delay in cement hydration of mixtures with yield stress lower than 50 Pa could lead to surface defects associated with bleeding. Mixtures with yield stress greater than 100 Pa exhibited considerable surface defects caused by insufficient filling ability of the concrete in the absence of mechanical consolidation. Finally, surface defects resulting from segregation were found with flowable concrete with plastic viscosity lower than 10 Pa s and yield stress lower than 100 Pa.

MD-based characterization of plastic deformation in Cu/Ag nanocomposites via dislocation extraction analysis: Effects of nanosized surface porosities and voids

Recently, copper-silver nanocomposites (NCs) have been utilized in medical instruments owing to their ability in destroying the bacterial cell wall, which prevents inflammation of the body tissue. It has been revealed that introducing nanosized porosities in their structure can lead to an increase in the interfacial surface area with the tissue promoting the quality of treatment. However, since yielding and occurrence of plastic deformation are not acceptable in medicine, analyzing the mechanical behavior of these NCs having nanopores is an important challenge. Therefore, the focus of this study is to assess the role of porosities on the deformation mechanism of Cu/Ag NCs under uniaxial tensile loading conditions. Accordingly, several perfect and defected samples are systematically studied through molecular dynamics simulation. It is observed that plastic deformation of perfect sample occurs through twinning. For samples with surface voids, this is happened as a result of perfect dislocations gliding. Meanwhile, for volumetric porosities, the deteriorating effect is stopped passing the critical void content. This is ascribed to the formation of many motionless dislocations such as stair-rod, Hirth and Frank as confirmed via the dislocation extraction analysis. Consequently, it is demonstrated that presence of surface voids can be more destructive.

Experimental and thermomechanical analysis of the effect of tool pin profile on the friction stir welding of poly(methyl methacrylate) sheets

Effects of frustum, cubic and triangle tool pin profiles on the friction stir welding (FSW) of poly(methyl methacrylate) (PMMA) were studied by experimental analysis and thermomechanical simulations. It is shown that employing pins with a low contact surface area form macrocracks and voids along the joint line. Edged-type pins locally stir the polymer and cause tunnel defects which are formed at the root of the joint. In contrast, pins with relatively smooth and large surface area would yield a defect-free weldment if an appropriate processing condition is employed. Mechanical examinations in longitudinal (LS) and transverse (TS) directions indicate that the highest tensile strength (LS = 59 MPa and TS = 43 MPa) of the joint is attained for the frustum pin. The result of Izod test also determines a higher impact energy of the joint for the frustum pin as well. Fractographic studies indicate that cracks are initiated from internal defects and voids, propagating through the material to cause brittle rupture. Interestingly, the effect of pin geometry on the tensile strength and impact energy is different than that hardness (in ShoreD scale).

A Pore-Skeleton-Based Method for Calculating Permeability and Capillary Pressure

We have developed a new method for calculating permeability and capillary pressure from the pore skeleton that is extracted from a fractured rock model, which might comprises medial axes of matrix pores and/or medial surfaces of fracture voids. Such a skeleton, therefore, is able to encapsulate the total connected fluid flow paths in the pore-void space. To do pore-network flow simulations, the pore skeleton needs to be further “discretised” into a network of interconnected nodes and bonds to capture local pore morphology. Jiang et al. (Adv Water Resour 107:280–289, 2017) developed a method to extract pore skeletons of this type and a discretisation to construct a pore-network model that is optimal in many aspects. In this work, we develop a new in-place discretisation method, by simply inserting a virtual link, a bond, between every pair of skeleton voxels, nodes, which are either face or only edge adjacent under certain conditions. This new method results in a simpler pore-network model, i.e. a virtual network, in which each node or bond is assumed as either a cylinder or a tiny fracture, as well as prescribed with length and inscribed radius/aperture only. As a result, a simpler pore-network simulator is also developed using improved formulae of conductance and capillary pressure according to where each virtual link falls, appropriately distinguishing every local configuration within matrixes or fractures. We verify our methods by comparing the simulation results against with those of lattice Boltzmann methods and a laboratory flooding experiment and demonstrate the accuracy and efficiency of our methods with sensitivity analysis.

Diffusion-limited oxidation of polyamide: Three stages of fracture behavior

Polyamides (PAs) frequently experience diffusion-limited oxidation (DLO) under elevated temperatures due to their combination of relatively high oxygen barrier properties and high susceptibility to, and rate of, oxidation; under DLO conditions, oxidation is uneven and limited to a thin surface layer. In this study, the reduced extensibility/embrittlement of unstabilized PA6 under DLO conditions was understood by revealing DLO-induced fracture behavior. The DLO was induced by thermally ageing PA6 samples at 180 °C; the built-up of the thin oxidized layer by ageing was revealed by infrared microscopy. Notably, the formation of the thin oxidized layer significantly reduced the strain-at-break. Depending on whether the oxidized layer was brittle, two types of surface behavior (voiding and cracking) occurred during the tensile tests, which in turn lead to three types (stages) of tensile fracture behavior. In particular, in the early stage (Stage I) of ageing, the fracture was caused by a long crack formed by the coalescence of adjacent surface voids, leading to a decrease in the strain-at-break from 300% to 30%. In Stage II, multiple surface cracks, which initiated in the oxidized layer, was arrested by the interface between the oxidized and unoxidized material, leading to an almost constant strain-at-break (at or close to the necking strain). Maximum brittleness occurred in Stage III, where a more extensive oxidation of the oxidized layer initiated cracks with high propagation rate, causing the interface to be unable to arrest the cracks.

Phase field study of surface-induced melting and solidification from a nanovoid: Effect of dimensionless width of void surface and void size

The size effect and the effects of a finite-width surface on barrierless transformations between the solid (S), surface melt (SM), and melt (M) from a spherical nanovoid are studied using a phase field approach. Melting (SM → M and S → M) from the nanovoid occurs at temperatures which are significantly greater than the solid-melt equilibrium temperature θe but well below the critical temperature for solid instability. The relationships between the SM and M temperatures and the ratio of the void surface width and width of the solid-melt interface, Δ¯, are found for the nanovoids of different sizes. Below a critical ratio Δ¯*, the melting occurs via SM and the melting temperature slightly reduces with an increase in Δ¯. Both S → SM and SM → M transformations have a jump-like character (excluding the case with the sharp void surface), causing small temperature hysteresis. However, the solid melts without SM for Δ¯>Δ¯*, and the melting temperature significantly increases with increasing Δ¯. The results for a nanovoid are compared with the melting/solidification of a nanoparticle, for which the melting temperatures, in contrast, are much lower than θe. A linear dependency of the melting temperatures with the inverse of the void radius is shown. The present study shows an unexplored way to control the melting from nanovoids by controlling the void size and the width and energy of the surface.

Small-angle x-ray scattering in amorphous silicon: A computational study

We present a computational study of small-angle X-ray scattering (SAXS) in amorphous silicon ($a$-Si) with particular emphasis on the morphology and microstructure of voids. The relationship between the scattering intensity in SAXS and the three-dimensional structure of nanoscale inhomogeneities or voids is addressed by generating ultra-large high-quality $a$-Si networks with 0.1-0.3% volume concentration of voids, as observed in experiments using SAXS and positron annihilation spectroscopy. A systematic study of the variation of the scattering intensity in the small-angle scattering region with the size, shape, number density, and the spatial distribution of the voids in the networks is presented. Our results suggest that the scattering intensity in the small-angle region is particularly sensitive to the size and the total volume-fraction of the voids, but the effect of the geometry or shape of the voids is less pronounced in the intensity profiles. A comparison of the average size of the voids obtained from the simulated values of the intensity, using the Guinier approximation and Kratky plots, with those from the spatial distribution of the atoms in the vicinity of void surfaces is presented.

Rapid consolidation and curing of advanced composites using electron beam irradiation

A low-cost, low-waste manufacturing method for advanced thermoset composite parts could improve market penetration of composites compared to other engineering materials such as aluminum or steel. Such a method could combine some of the new trends in composites manufacturing such as resin infusion (eliminates need for prepreg), out-of-autoclave consolidation, and snap curing. The feasibility of a hybrid process with these characteristics has been demonstrated by uniting liquid composite molding, resin curing by electron beam irradiation, and high pressure consolidation with specialized elastomeric tooling. To demonstrate feasibility, a mold set was designed to make flat, square four-ply woven carbon fiber parts by (1) vacuum-infusing dry preforms with an electron beam–curable epoxy resin in minutes, (2) applying 690 kPa of uniform pressure and consolidating in seconds using an elastomer-faced specialized elastomeric tooling tool and simple hydraulic press, and (3) curing in seconds using a 3 MeV electron beam source. To better understand how various process parameters affect part performance, parameters are varied in a simple design of experiments, and flexural strength and stiffness, thickness distribution, fiber and void volume fractions, surface roughness, and cross-sectional characteristics (via microscopy) are measured and compared.

Research on degradation mechanisms of recycled building gypsum

In order to provide a solid theoretical foundation for the efficient reuse of waste gypsum, the research on the degradation mechanisms of recycled gypsum mechanical performance was carried out in this paper. The recycled gypsum was prepared with natural gypsum ores in the laboratory and three rational comparative methods were proposed through constraining the conditions: the preparation process, the particle size distribution and the water/gypsum ratio. The results showed that the recycled dihydrate gypsum was much easier to grind than the natural gypsum ores, resulting in deteriorated particle size distribution, large specific surface area and void fraction, poor sphericity, which remarkably increased the water requirement. The increase in porosity and larger pore size due to excessive water evaporation was the determining factor leading to a significant decrease in the strength of the recycled gypsum. However, the short columnar shape crystals seemly would only have a slight disadvantageous on the flexural strength. Moreover, the crystal morphological characteristics of dihydrate gypsum had a decisive influence on the sphericity of recycled hemihydrate particles.