Cohesive Bulk Research Articles

Fracture Resistance and Failure Mode of Monolithic Zirconia, Veneered Zirconia, and Metal-Ceramic Full-Coverage Restorations: A Comparative In Vitro Study.

To evaluate and compare the fracture mode and strength of monolithic zirconia to veneered zirconia and metal-ceramic full-coverage restorations following artificial aging and to test the performance of translucent zirconia in terms of load-bearing capacity. Two mandibular first molars were prepared and scanned for their respective groups of full-coverage restorations. A total of 75 full-coverage restorations were fabricated and divided into five groups: two groups for monolithic zirconia, two groups for veneered zirconia, and one group for metal-ceramic. Then, 75 light-cured hybrid composite resin dies were fabricated to serve as abutments. Before cementation, all full-coverage restorations were subjected to accelerated aging. After cementation, all full-coverage restorations were subjected to compressive loading until fracture in an electromechanical universal testing machine. A two-way nested ANOVA and Tukey test were used to analyze the results with 95% confidence levels. Monolithic zirconia full-coverage restorations showed the highest mean fracture resistance of 4,201.0 N, followed by metalceramic full-coverage restorations of 3,609.3 N, and veneered zirconia full-coverage restorations showed the lowest mean fracture resistance of 2,524.6 N. The main mode of failure was cohesive bulk fracture for the monolithic zirconia group, cohesive/adhesive failure with infrastructure damage for the veneered zirconia group, and cohesive/adhesive failure without infrastructure damage for the metal-ceramic group. Monolithic zirconia full-coverage restorations showed superior resistance to fracture compared to metal-ceramic full-coverage restorations and are highly reliable in terms of load-bearing capacity within the posterior regions of the mouth.

Homogenization increases stickiness of concentrated pea protein - apple pectin mixtures

Food binders must be sticky to glue different components together. This study evaluated the impact of homogenization of pea proteins on the stickiness of pea protein – apple pectin mixtures. A reduction in protein particle size was suggested to improve adhesion and cohesion by increasing reactive surface area, resulting in a sticky material. In addition, impact of concentrating methods, namely freeze-drying and vacuum evaporation, on concentrating the homogenized pea protein dispersions was investigated. A reduced particle size of pea proteins upon homogenization led to an increased stickiness of the biopolymer matrix when the proteins were vacuum evaporated as demonstrated by a tack test. This was attributed to a firmer and more cohesive bulk structure demonstrated by rheological, confocal laser scanning microscopy and visual observations. Vacuum evaporation prevented re-aggregation of protein particles compared to freeze-drying since less water was expelled during vacuum evaporation. These results provide new insights into tuning stickiness of biopolymer mixtures. • Homogenization of pea proteins reduced their particle size. • Vacuum evaporation maintained reduced particle size better than freeze-drying. • Pea protein-apple pectin blends with small particles were more cohesive & stickier.

The axial force model optimization and dynamic characteristics of shear-friction force in screw

The cohesive bulk materials are easily compressed and agglomerated when employing a screw feeder, affecting the subsequent handling procedure. A mechanistic model is developed to predict the axial force characteristics and understand the compression behaviors of the powder in the screw. The vertical stress in silo, the forces on the shaft and tailing side of flight surfaces are optimized on the axial force. The axial forces exerted on the powder at four boundaries are indicated by the different material properties and geometric parameters. The axial shear-friction force is examined directly using gravity balance by ropes method in conveying process, which validates the axial force model in screw. The dynamic characteristic, variation pattern and evolution mechanism of the axial shear-friction force are obtained and discussion. The weak correlation between rotational speed and the forces on the material further demonstrates the model reliability. The effect of dimensionless particle size on axial force is quantified and adopted to optimize the axial force model.

Characterization and growth mechanisms of adhesion‐induced microcavities during debonding of softened glass

AbstractThe glass/mold interaction is crucial for controlling the surface quality of high‐precision glass products and elongating the lifespan of precious molds in hot forming techniques. Here we employ the probe tack test to separate a typical glass molding interface composed of N‐BK7 glass and tungsten carbide molds at different temperatures from 655 to 690°C. The macroscale debonding behavior translates from interfacial fracture to cohesive bulk deformation as temperature increases. The glass surfaces after debonding are covered by numerous randomly distributed cavities in micrometer. With temperature increasing, the maximum depth of microcavities greatly increases from less than 0.5 to over 10 μm; the area fraction overall increases and reaches 15% at maximum. These microcavities could result from the development of localized deformation at the gas‐trapping spots, due to the separation of the adhesive glass/mold interface. A large‐sized cavity evolves from the cyclic growth and coalescence of small cavities. For the interfacial fracture cases, cavities mainly propagate as cracks along the interface, and thus develop into shallow disc‐like shapes. However, for the cohesive cases, cavities prefer to grow in the bulk. The growth bifurcation could be governed by the competition between strain energy release rate and viscoelastic complex modulus.

Fast Orbital-Free Full-Potential Calculations for Large Nano Objects: C, Al and Ti

In the context of a full-potential orbital-free approach for the modeling of multi-atomic systems we investigated the dependence of the cohesive energies and bulk elastic modules of the large nanosystems Cn (n is up to 4096 atoms), Aln (n is up to 23,328 atoms) and tin (n is up to 2160 atoms). It was shown that the cohesive energies and elastic modules tend towards bulk crystal values at n ≈ 3000 for Cn systems, at n ≈ 1500 for Tin and at n ≈ 20,000 for Aln. The execution time for one energy iteration for Ti23328 was only 23 min.

ENERGETICS AND ELASTIC PROPERTIES OF LARGE NANO-OBJECTS: ORBITAL-FREE APPROACH ON THE BASIS OF THE DENSITY FUNCTIONAL THEORY

Cohesive energy Ecoh and bulk modulus B of large nanosystems Cn, Sin, Aln и Tin, were calculated in the workframe of the all-electron version of the orbital-free approach on the basis of the density functional theory: number of atoms n was varied up to 4096 for carbon and silicon, 23 328 for aluminum, and 2662 for titanium. Nanosystems were taken as fragments of corresponding crystals. It was found that Ecoh and B tend to their values known for bulk materials. Therefore, it was convincingly shown that our orbital-free approach could be used successfully for study mechanical properties of large nanosystems.

Bulk Nanostructured Metal from Multiply-Twinned Nanowires.

Using chemically synthesized silver nanowires with 5-fold twinning planes as a model system, a bottom-up process to generate a bulk nanostructured metal has been demonstrated. Although the nanowires would be shortened and deformed during densification, they are chosen as a model system because they are currently the most scalable and convenient way to obtain Ag particles with high twinning densities. Direct cold pressing of a silver nanowire filter cake did not generate a sufficiently cohesive sample, while hot pressing at 190 °C for 8 h resulted in extensive sintering, eliminating the nanowire morphology. Copper was then electroplated on the silver nanowires as a binder and filler to increase the densification upon hot pressing; despite nonuniform plating across the thickness of the filter cake, the thermal stability of the nanowires was increased, allowing hot pressing at 390 °C. Finally, a uniform copper coating on silver nanowires was achieved by electroless plating, leading to cohesive bulk metal after hot pressing.

Robust Performance of Spider Viscid Silk on Hairy and Smooth Insect Substrates.

Spider viscid silk adheres to insects in orb webs and is a "smart-adhesive" that quickly changes droplet size, viscosity, and adhesiveness in response to atmospheric humidity. Different species of spiders "tune" water uptake to match the humidity of their foraging environments, achieving a similar "universal" viscosity that optimizes tradeoffs in spreading versus cohesive bulk energy needed to enhance adhesion. Too much water lowers viscosity so that the glue spreads well, but cohesive failure occurs easily, generating poor adhesion. However, the optimal viscosity model of adhesion is based on experiments using smooth glass. Here we test the hypothesis that a less viscous, "over-lubricated" glue, which shows poor adhesion on smooth glass, will be stickier on hairy insects because of its greater ability to spread across three-dimensional rough surfaces. We ran adhesion tests of the furrow spider (Larinioides cornutus [Clerck 1757]) viscid silk on honey bee (Apis mellifera) thorax, with and without hairs, in either high or medium humidity. Our results show that "over-lubricated" glue increases adhesion on hairy surfaces, performing equally as well as an optimally viscous glue.

Temperature and rate dependent debonding behaviors of precision glass molding interface

AbstractThe glass‐to‐mold adhesion in precision glass molding could severely degrade the quality of molded optics and shorten the lifespan of the precious molds. Since the consequences of adhesion take effect during the separation between glass and molds, it is important to investigate the debonding behaviors of a typical glass molding interface. To this end, here we perform a probe tack test procedure for borosilicate glass BK7, where debonding is conducted at molding temperature and specific velocity. We fully characterize the debonding behaviors using the peak adhesion stress σmax and the work of debonding Wdeb. Experiments show that when temperature is decreased from 690°C to 655°C at 10 μm/s, σmax continuously increases, while Wdeb first increases but then sharply decreases. When the debonding velocity is increased from 10 to 50 μm/s at 680°C, σmax also increases while Wdeb overall decreases. Therefore, the debonding behaviors are highly temperature and rate dependent. More importantly, depending on the debonding conditions, three debonding types are identified, that is, the cohesive bulk deformation, the cohesive‐interfacial transition and the interfacial fracture. The cohesive type can be converted into the interfacial fracture, by either decreasing temperature or increasing the debonding velocity. Based on the Wdeb criterion, the three debonding regimes can be clearly distinguished. Finally, analyses on the temperature and velocity experimental results are unified by incorporating the reduced crack velocity aTvc. The dependences of both viscoelasticity and Wdeb on aTvc qualitatively explain the transition condition for different debonding types. Concerning these findings, the work of debonding not only supplements the characterization of adhesion strength, but also throws insightful light on revealing the debonding mechanisms.

Mathematical modeling of axisymmetric flow of granular materials

A mathematical model of the axisymmetric flow of cohesive granular materials is considered. A plastically compressible material is considered as a mathematical model. This material has an additional characteristic - its own rotation of particles. The existence of this characteristic leads to the asymmetry of the stress tensor and the presence of torque stresses. Asymmetric components of the stress tensor are balanced by normal pressure due to the presence of rolling friction. It is shown that the distinctive characteristic of the axisymmetric state is the layering of its flow. The condition for the flow of cohesive bulk material is obtained.

DEM calibration of cohesive material in the ring shear test by applying a genetic algorithm framework

This research demonstrates capturing different stress states and history dependency in a cohesive bulk material by DEM simulations. An automated calibration procedure, based on the Non-dominated Sorting Genetic Algorithm, is applied. It searches for the appropriate simulation parameters of an Elasto-Plastic Adhesive contact model such that its response is best fitted to the shear stress measured in experiments. Using this calibration procedure, the optimal set of DEM input parameters are successfully found to reproduce the measured shear stresses of the cohesive coal sample in two different pre-consolidation levels. The calibrated simulation resembles the stress history dependent values of shear stress, bulk density and wall friction. Through the case study of the ring shear tester, this research demonstrates the robustness and accuracy of the calibration framework using multi-objective optimization on multi-variable calibration problems irrespective of the chosen contact model.

Nonstationary 1D Dynamics Problems for Heteromodular Elasticity with Piecewise-Linear Approximation of Boundary Conditions

The paper provides the investigation of a heteromodular elastic medium under dynamic loading. The heteromodularity (when the stress - strain relation depends on the deformation direction) is a distinctive feature of many natural and structural materials: rocks, porous and cohesive bulk media, fibrous and granular composites, some metal alloys, etc. The fact that the listed materials show the heteromodular property at the stage of elastic deformation should be especially taken into account when solving problems of their shock dynamics. To describe the heteromodular behavior of an elastic medium in terms of small strains we use the physically nonlinear model of V.P. Myasnikov. The accepted assumption about the one-dimensional straining reduces the nonlinear relationship of stresses and small strains to piecewise linear equations. In the case of dynamic shock deformation, the initial nonlinearity of the model is concentrated in the equations which define the velocity of the shock wave abruptly transforming the heteromodular medium from a stretched to a compressed state. In this paper we investigate the processes of generation, motion, and possible interactions of plane one-dimensional deformation waves (including shock ones) in a heteromodular elastic half-space. The points of the half-space boundary undergo one-dimensional motions according to a given non-linear law corresponding to the “stretching-compression” mode. We suggest replacing the nonstationary boundary condition of the problem by its piecewise linear approximation and constructing a connected sequence of analytical solutions with a linear boundary condition at each local time interval. The proposed approach is the basis of the numerical solving algorithm for a boundary value problem with a given nonlinear condition. It is shown that the general solution behind the shock wave consists of several local layers, which number is related to the quantity of nodes in the piecewise linear decomposition of the boundary condition. In these layers, the compression deformation is defined by the relevant part of the boundary condition and simultaneously “stores” information on the preliminary tension, which should be considered an important feature of the heteromodular medium dynamics.

Modelling of the Flow of Streams of Cohesionless and Cohesive Bulk Materials in a Conveyor Discharge Point with a Flat Conveyor Belt

The paper presents the analysis of flow conditions of cohesive and cohesionless bulk materials in a conveyor discharge point of a flat conveyor belt. The analysis was carried out for stationary flows at high velocities. It presents mathematical methods for the description of the velocity of a material leaving a throwing point of a flat conveyor belt as well as final equations which enable the determination of velocity of the material after it has left the throwing point (with the accuracy sufficient for practical use). Next, the velocity calculated for the proposed mathematical description (for selected material groups) has been compared with the velocity obtained from mathematical relations commonly used by engineers. The proposed equations for determining the velocity of the material beyond the point have proved useful, since they enable excluding the indirect equations. Finally, the difference between the values of the velocity obtained with the proposed and indirect equations have been determined and the relative error for the proposed method has been calculated.

A modified flowability classification model for moist and cohesive bulk solids

The Jenike flowability classification is designed to indicate the handleability of bulk solids in material handling processes. However, for moist, cohesive bulk solids, it may fail to represent the true flowability/handleability at high consolidation stresses. The study presented in this paper introduces a modified flowability classification model aiming to improve the flowability/handleability prediction across a wide range of consolidation pressures. The evolution of the flow function of bulk materials under increasing moistures is initially discussed, based on which the Jenike flowability classification is reviewed. The modified flowability classification is then derived by considering the flow/no flow condition for mass flow hopper designs under both low and high consolidation stress states. The influence of the bulk material type and general operational conditions are also taken in account in deriving the modified classification method. A suite of bulk solids with a wide range of material properties are selected in an experimental program to validate the modified flowability classification. Results suggested the modified flowability classification is able to more accurately indicate the overall flowability/handleability of the bulk solids independent of the applied major consolidation stress.

Simulation study of the discharge characteristics of silos with cohesive particles

In many industrial applications the silo for bulk materials is an important part of an overall process. Silos are used for instance to buffer intermediate products to ensure a continuous supply for the next process step. This study deals with the discharging behaviour of silos containing cohesive bulk solids with particle sizes in the range of 100-500 μm. In this contribution the TOMAS [1,2] model developed for stationary and non-stationary discharging of a convergent hopper is verified with experiments and simulations using the Discrete Element Method. Moreover the influence of the cohesion of the bulk solids on the discharge behaviour is analysed by the simulation. The simulation results showed a qualitative agreement with the analytical model of TOMAS.

Contact properties determination of macroscopic fine disperse glass particles via compression tests under cyclic loading/unloading

To investigate the permanent compressive stress of single particles in a cohesive bulk solid during handling or transport, uniaxial compression tests were carried out with cyclic loading/unloading. This involves a continuation of our previous studies (contact properties determination of macroscopic fine disperse glass particles via compression tests in normal direction). The experiments were implemented by means of a home-built compression tester. By repeated loading/unloading cycles with spherical, dry and smooth soda lime glass particles with a mean diameter of d50,3=513.20μm, a set of micromechanical contact properties was determined. It is a model-based back-calculation of experimentally generated normal force-displacement curves of the elastic-plastic sample material. For this purpose, the contact model ‘stiff particles with soft contacts’, which was developed by Tomas, is used. In order to influence the flowability of soda lime glass particles and to verify the effect of the characteristic adhesion force on the elastic-plastic contact, the particles were altered to be hydrophilic and hydrophobic.Finally, the effect of the number of cyclic loading/unloading tests on the contact stiffness, the effective modulus of elasticity and the resulting contact radius was explained and discussed. According to this, there is an increase of the elastic-plastic contact stiffness up to a saturation of the plastic deformation during the repeated loading of the contact with a constant load rate of FN,max=45N.The determined residual plastic deformations in cyclic compression tests are directly correlated to the densification in the contact area of soda lime glass particles due to the absent long-range order in the glass structure.

Binary mixtures of ethylene containing copolymers and low molecular weight resins: A new approach towards specifically tuned art conservation products

Painted face artworks often require adhesives during their conservation treatment. Damage or ageing processes that result in cracking, powdering, lifting or similar phenomena can occur, at the interface between two paint layers or between a paint layer and the support. Adhesives need to be applied without compromising the original appearance of the object. Those products can either be formulated to provide structural strength at the interface or be applied more diluted, so that they can penetrate into a weakened porous material to improve cohesive bulk properties strength. In the latter case the adhesive is typically referred to as a consolidant.Since this is a niche sector, only a few formulations are tailored specifically for restoration, while almost all products used are transposed from various commercial fields. Furthermore the notion of reversibility limits the range of polymeric materials for conservation to chemically stable thermoplastics, because they can be safely removed.This paper introduces the results of an international project which aims at characterizing the behavior of new adhesive formulations with tuned physicochemical properties and enhanced stability. These are binary mixtures, based on ethylene copolymers and four classes of tackifier. On the contrary of urea-aldehyde and hydrogenated hydrocarbon resins, ketone and rosin ester resins showed good miscibility with the polymeric component. Nevertheless, all tackifiers investigated were able to modify copolymers properties.

Modelling of the Flow of Streams of Cohesionless and Cohesive Bulk Materials in a Conveyor Discharge Point of a Descending Belt Conveyor

The paper presents the analysis of the condition of the flow of cohesionless and cohesive bulk solids in a conveyor discharge point of a descending belt. The analysis was carried out for stationary flows at high velocities. It presents mathematical methods of description of the velocity of materials leaving a conveyor discharge point with a descending conveyor belt, as well as final equations which enable the determination of the velocity of material after it has left the discharge point, with the accuracy sufficient for practical use. Next, the obtained mathematical descriptions for the mentioned material groups have been compared. The usefulness of the equations in determining the velocity of the material beyond the point, which enable to omit the indirect equations, has been demonstrated. Finally, the difference between the values of the velocities obtained with the proposed and indirect equations as well as the relative error for the proposed method have been calculated.

Anisotropic behaviour of bulk solids and its effect on silo design

The anisotropic material behaviour is a characteristic property of bulk solids. Prior research work proved that the measured unconfined yield strength of cohesive bulk solids is lower when the direction of the collapsing stress and the consolidation stress does not coincide. A new powder tester has been developed to measure both the unconfined yield strength in the uniaxial direction, which provides consolidation and collapse stresses in the same direction, as well as the anisotropic unconfined yield strength, which means consolidation and collapse stresses are in orthogonal directions. The friction between the walls and the bulk solid is minimised by the use of rubber membranes which deform uniformly with the bulk sample. The uniaxial and the anisotropic unconfined yield strengths have been measured for a limestone powder at different consolidation stresses. Experimental results as well as their application on the design for funnel flow silos are discussed. To avoid piping two design criteria exist for the design of a funnel flow silo: Jenike assumes a passive state of stress to prevail close to the silo outlet (Jenike, 1961 [1]); Johanson assumes an active state of stress (Johanson, 1969 [2]). While the approach of Jenike leads to good results, the estimation of the critical outlet diameter in a silo with an active state of stress is not satisfying. The pipe dimensions for an active state of stress were measured using a silo centrifuge. The application of the design criteria gives reasonable and safe design value only if the anisotropic behaviour of the bulk solid is considered which was measured by the new powder tester.

Sampling of cohesive bulk materials by falling stream cutters

DEM has proved to be a useful method for analysing sample bias for falling stream cutters. It provides detailed information on bias and size-dependant extraction ratios based on closely matched reference and actual samples taken from the same simulations. It has allowed the identification of operational and design parameters which influence sample cutter performance and bias. To date, only cohesionless bulk materials have been considered. Wet bulk materials typically become sticky and this strongly influences their flow behaviour and particularly the mobility of fine particles and the overall flowability of the material. The inclusion of cohesion in DEM simulation of flow from a head pulley shows that the falling stream breaks up into large fragments that move as rigid bodies. The size of these fragments depends on the degree of cohesion. The effect of cohesion on sampler performance is explored. No statistically significant bias was observed with increasing cohesion level even though the extraction ratio was observed to decline. The effect of cohesion for different aperture sizes of the cutter was also investigated. As the cutter becomes narrower the extraction ratio declines sharply. However, even for a cutter of aperture twice the particle top size (which would be highly biased for a cohesionless material) no bias was detectable. For cohesive materials the extraction ratio no longer correlates with sample bias. Cohesion improves sample cutter performance by restricting the ability of different size fractions to move independently in the congested flow region that forms at the cutter opening.