Cavity Volume Fraction Research Articles

Coarse-grained molecular dynamics simulations of the tensile behavior of a thermosetting polymer.

Using a previously developed coarse-grained model, we conducted large-scale (∼ 85 × 85 × 85 nm(3)) molecular dynamics simulations of uniaxial-strain deformation to study the tensile behavior of an epoxy molding compound, epoxy phenol novolacs (EPN) bisphenol A (BPA). Under the uniaxial-strain deformation, the material is found to exhibit cavity nucleation and growth, followed by stretching of the ligaments separated by the cavities, until the ultimate failure through ligament scissions. The nucleation sites of cavities are rather random and the subsequent cavity growth accounts for much (87%) of the volumetric change during the uniaxial-strain deformation. Ultimate failure of the materials occurs when the cavity volume fraction reaches ∼ 60%. During the entire deformation process, polymer strands in the network are continuously extended to their linear states and broken in the postyielding strain hardening stage. When most of the strands are stretched to their taut configurations, rapid scission of a large number of strands occurs within a small strain increment, which eventually leads to fracture. Finally, through extensive numerical simulations of various loading conditions in addition to uniaxial strain, we find that yielding of the EPN-BPA can be described by the pressure-modified von Mises yield criterion.

Experimental Studies of a Rectangular Cup Formation of Al 7075 Alloy in Superplastic Forming Process

Superplastic behaviour of certain metals and alloys having very fine grains, very large tensile elongations are obtained within certain temperature ranges at low strain rates. In this work, Al-alloy superplastic sheet material was considered. The AA 7075 sheets were subjected to the modified thermomechanical treatment, to obtain the average grain size of 10μm in less processing time. These alloys can be formed into complex shapes by superplastic forming, a process that employs common metalworking techniques. This paper aims to study the formability, thickness distribution, cavitation effect and microstructure feature for aluminium material by considering variable parameters such as forming pressure from 0.2 MPa, 0.3 MPa and 0.4 MPa and the sheet thickness of 2 and 1.5 mm. The 2 mm sheet gives better results of 0.4 MPa for both high thinning factor of 0.9466 and less cavity volume fraction of 4.4%.

Multi-dome forming test for determining the strain rate sensitivity index of a superplastic 7075Al alloy sheet

Abstract Superplasticity is a phenomenon indicating exceptional ductility that certain metals and alloys exhibit, when deformed under specific conditions. The tensile elongation of superplastic materials typically exceed 400%, elongations exceeding 5000% having been achieved. Superplasticity in sheet metals offers unique advantages for forming complex shapes easily, as a regular production process. In pressure thermo forming, the applied gas pressure deforms the sheet continuously, until it matches the shapes of the female die. The present study is about a technique to assess the strain rate sensitivity of a 7075 Al alloy sheet by a multi-dome forming test, under different forming pressures, temperatures and annealing times. This study considers mainly the thickness at dome apex, cavity volume fraction and assessment of the strain-rate sensitivity index. It was found that the material after 30 min annealing acquired a higher strain rate sensitivity of 0.49 and a pole thickness of 1.33 mm in diameter of a 15 mm dome, under the forming parameters of 0.5 MPa pressure, a forming time of 60 min and temperature of 530 °C.

Investigation of design parameters in ultrasound reactors with confined channels

This paper presents a three-dimensional numercial simulation of sonochemical degradation upon cavitational activity. The model relates the simulation of the acoustic pressure distribution to the sonochemical reaction rate. As a case study, the thermal degradation of carbon tetrachloride during sonication is studied in a tubular milliscale reactor. The model is used to optimize the reactor diameter, ultrasound frequency and power dissipated to the ultrasound transducers. The results indicate that multiple transducers at a moderate power level are more efficient than one transducer with high power level. Furthermore, the average cavity volume fraction is proposed as a reaction independent parameter to estimate the optimal reactor design. Within the results obtained in this paper, it appears possible to optimise reactor design based on this parameter.

Popcorning-type cracking failure in thermohyperelastic packaging materials in the presence of “wet” and “dry” cavities

Thermal cracking occurs in the plastic packaging materials due to the presence of moisturized micro-cavities in the material. The moisture resident in the micro-cavities gives rise to the internal vapor pressure that drives the thermal expansion of micro- cavities as temperature rises. The plastic packaging materials are considered a class of thermo-hyperelastic materials, thus allowing the micro-cavities to thermally expand to the substantial extent before the cracking failure. The micro-cavities can be moisture-abundant (i.e., wet) or substantially dry when cracking occurs. Cracking appears to be almost certain in the presence of wet cavities. The possibility of cracking in dry cavities turns to be two-sided: when the initial volume fraction of the micro- cavities is relatively small, cracking cannot occur in the dry cavities regardless of the phase transition temperature; when the initial cavity volume fraction is relatively large, cracking tends to occur in the dry cavities especially when the phase transition temperature is large. Because of the two-sided cracking possibility, the dry-cavity cracking mode presents a scenario that might reveal the mechanism of popcorning-type cracking failure in plastic packaging materials.

Simulation of incoherent and coherent backscattered wave fields from cavities in a solid matrix

This paper reports a study of the backscattered ultrasonic signal from a solid layer containing spherical cavities, to determine the conditions in which an effective medium model is a valid description of the response. The work is motivated by the need to model the response of porous composite materials for ultrasonic non-destructive evaluation (NDE) techniques. The numerical simulation predicts the response of a layer containing cavities at a single set of random locations, and compares it to the predicted response from a homogeneous layer with ensemble-averaged material properties (effective medium model). The study investigates the conditions in which the coherent (ensemble-averaged) response is obtained even from a single configuration of scatterers. Simulations are carried out for a range of cavity sizes and volume fractions. The deviation of the response from effective medium behavior is modeled, along with the trends as a function of cavity radius, volume fraction, and frequency, in order to establish an acceptability criterion for application of an effective medium model.

Geometric optimization of a convective T-shaped cavity on the basis of constructal theory

The aim of this paper is to optimize, by means of Bejan’s theory, the geometry of a T-shaped cavity that intrudes into a solid conducting wall. One direction in which the optimization of intrusion geometry can be pursued is that of increasing the complexity of the growing structures. When the heat generated by the volume is removed through one port, and when the smallest volume element size is fixed, the optimization of geometry generates a tree-shaped flow structure. The simplest tree-shaped structure is a ‘first construct’, or an optimized assembly of elemental volumes. The simplest first construct is the T-shaped tree here treated. The cavity is cooled by a steady stream of convection while the solid generates heat uniformly and it is insulated on the external perimeter. The structure has four degrees of freedom: L0/L1 (ratio between the lengths of the stem and bifurcated branches), H1/L1 (ratio between the thickness and length of the bifurcated branches), H0/L0 (ratio between the thickness and length of the stem) and H/L (ratio between the height and length of the conducting solid wall) and one restriction, the ratio between the cavity volume and solid volume (ϕ). The purpose of the numerical investigation is to minimize the maximal dimensionless excess of temperature between the solid and the cavity. The simulations were performed for the following values of the cavity volume fractions (after having fixed H/L=1): ϕ=0.05, 0.1, 0.2 and 0.3. The first optimization, referred to the degree of freedom L0/L1, highlighted one “intermediate” optimal shape, i.e., the best geometry was not obtained for the extremes values of L0/L1. As for the degree of freedom H1/L1, the optimal geometries were obtained for lowest ratio of H1/L1. Finally, when compared to the C-shaped cavity, i.e. the basic configuration, the T-shaped cavity performs approximately 45% better under the same thermal and geometric conditions.

Material characterization of Inconel 718 from free bulging test at high temperature

Macroscopic superplastic behavior of metallic or non metallic materials is usually represented by the strain-rate sensitivity, and it can be determined by tensile tests in uniaxial stress state and bulging tests in multi axial stress state, which is the actual hot forming process. And macroscopic behavior of Non-SPF grade materials could be described in a similar way as that of superplastic materials, including strain hardening, cavity and so on. In this study, the material characterization of non-SPF grade Inconel 718 has been carried out to determine the material parameters for flow stress throughout free bulging test under constant temperature. The measured height of bulged plate during the test was used for estimation of strain-rate sensitivity, strain-hardening index and cavity volume fraction with the help of numerical analysis. The bulged height obtained from the simulation showed good agreement with the experimental findings. The effects of strain-hardening and cavity volume fraction factor for flow stress were also compared.

Fluid flow and heat transfer in a latent thermal energy unit with different phase change material (PCM) cavity volume fractions

Abstract In present work, the effects of different cavity volume fractions of phase change material (PCM) on fluid flow and heat transfer behavior in a latent thermal unit are studied numerically. The commercial Computational Fluid Dynamics (CFD) package, Fluent, is used for the numerical solution based on transient conjugate heat transfer. The numerical results have been verified and validated against numerical and experimental data available in published literature. The volume expansion ratio, the time of complete thermal storage, heat flux, liquid fraction, velocity and temperature fields are investigated for the range of PCM cavity volume fractions ( ϕ max ) from 35% to 95%. It is noted that a vortex (as a heat transfer enhancer) is present near the heating plate wall for the PCM cavity volume equal to 85%. It is found that the volume expansion ratio decreases as ϕ max increasing, whereas the time for complete energy storage increases. Further, the correlations of the volume expansion ratio and the time of complete thermal storage are developed as a function of ϕ max. The detailed knowledge regarding interface heat transfer rate provides a deeper understanding the heat transfer mechanisms.

Heat transfer augmentation of magnetohydrodynamics natural convection in L-shaped cavities utilizing nanofluids

A numerical study of natural convection heat transfer through an alumina-water nanofluid inside L-shaped cavities in the presence of an external magnetic field is performed. The study has been carried out for a wide range of important parame?ters such as Rayleigh number, Hartmann number, aspect ratio of the cavity and solid volume fraction of the nanofluid. The influence of the nanoparticle, buoyancy force and the magnetic field on the flow and temperature fields have been plotted and discussed. The results show that after a critical Rayleigh number depending on the aspect ratio, the heat transfer in the cavity rises abruptly due to some significant changes in flow field. It is also found that the heat transfer enhances in the presence of the nanoparticles and increases with solid volume fraction of the nanofluid. In addition, the performance of the nanofluid utilization is more effective at high Ray?leigh numbers. The influence of the magnetic field has been also studied and de?duced that it has a remarkable effect on the heat transfer and flow field in the cavity that as the Hartmann number increases the overall Nusselt number is significantly decreased specially at high Rayleigh numbers.

Impact of left ventricular trabeculations and papillary muscles on measures of cavity volume and ejection fraction

Left ventricular (LV) trabeculations are typically considered LV cavity volume (i.e. ignored) when analyzing cardiac magnetic resonance (CMR) images as they are difficult to manually segment, but they are not actually part of the LV bloodpool. The treatment of papillary muscles is more variable, but they too have often been considered bloodpool.

Superplastic behavior of an Al–Mg–Li alloy

A novel thermomechanical processing has been developed for producing fine-grained Al–Mg–Li alloy 1420 sheet for superplasticity. The refined microstructure had layers of different grain structure through the normal direction of the sheet. The surface layer (SL) had fine, nearly equiaxed grains whereas the center layer (CL) had coarse, elongated grains. The superplastic properties and microstructural evolution of alloy 1420 with such microstructure were investigated using uniaxial tensile tests. It was shown that the refined alloy exhibited considerable elongations of 580–915% when deformed at 510–540 °C and initial strain rates from 5 × 10 −4 to 1 × 10 −3 s −1. Microstructure observation revealed that the alloy 1420 exhibited unusual cavitation behavior and extensive GBS occurred when tested at 510 °C and above, however, the occurrence of GBS was not uniform. The effects of temperature and strain on grain size and shape, cavity size and volume fraction in the SL and CL were determined, respectively.

Damage Evolution Equation of AZ31B Magnesium Alloy during Superplastic Deformation

After superplastic tensile tests and quantitative analysis of cavity volume fraction, the damage evolution equation based on the law of the micro-damage evolution and statistical mechanics was derived out. The characteristic parameters of damage evolvement and critical value of damage variables are achieved from the experiments.

Deformation characteristics and cavitation during multiaxial blow forming in superplastic 8090 alloy

This work examined the effect of multiaxial stress on deformation characteristics of a superplastic aluminum alloy 8090 by deforming the sheet into a die with a cylindrical cavity. Several interrupted tests were performed to bulge the sheets to various depths for different strain rates, the formed parts were utilized to evaluate the deformation status, thickness distribution, local strain states, and cavitation. It was found that evolution of cavity volume fraction with forming time could be related to the thinning behavior of the deformed sheet during forming. Decrease in cavity volume fraction at the central region was observed in the later stage of forming as the thickness of the deformed sheet remained constant for all test forming rates.

Study on Cavity Growth in Uniaxial Tension of ZK60 Magnesium Alloy

Cavity growth is a typical microstructure feature in superplastic forming (SPF) of materials. Substantial growth and interlink of cavities in superplastic deformation usually lead to reduction in elongation, even to failure. Consequently, it is necessary to investigate the mechanism and model of cavity growth. In this paper, experimental studies on cavity growth were carried out by means of superplastic tension of ZK60 magnesium alloys. Scanning electronic microscope (SEM) was employed for observation of fractography. Experimental cavity radius and volume fraction were determined by optical microscopy and corresponding picture-based analysis software. It is found that, the fractured surfaces after a superplastic elongation have a mixed characteristic of intergranular cavities and dimples. Further, the cavity growth is identified to follow a exponentially increasing mode.

Mechanical properties and texture of a superplastically deformed AZ31 magnesium alloy

Room temperature tensile properties of a superplastic AZ31 magnesium alloy prior to and following high temperature deformation at 673 and 723 K were examined. The material behaved in a superplastic manner at low strain rates, and dislocation creep dominated deformation at high strain rates. The specimens after high temperature deformation exhibited higher ductility and lower strength as the strain rate during high temperature deformation decreased. Pre-existing basal texture changed gradually toward random during high temperature deformation at low strain rates, whereas it was strengthened at high strain rates. Both the basal texture weakening and lower cavity volume fraction brought about the higher ductility following superplastic deformation at lower strain rates.

Effect of lubrication on deformation characteristics of a superplastic 5083 Al alloy during bi-axial deformation

Effect of lubrication on deformation behavior of a superplastic material has relatively been less examined, though it is important for industrial application. In this paper, a superplastic 5083 Al alloy under bi-axial deformation was investigated by deforming the sheet into a rectangular die cavity with and without lubrication. It was found that reducing the interfacial friction by use of a lubricant altered the metal flow after the deformed sheet had made contact with the die surface. Changes of the metal flow during forming not only developed a better thickness distribution of the formed part, but also improved cavitation distribution. The cavitation levels could be effectively reduced for forming with lubrication at a higher strain rate; the maximum cavity volume fraction decreased from 6.75 to 3.33% for forming at a strain rate of 1 × 10 −3 s −1.

Cell cavities increase tortuosity in brain extracellular space

Brain extracellular space (ECS) forms hindered pathways for molecular diffusion in chemical signaling and drug delivery. Hindrance is quantified by the tortuosity λ ; the tortuosity obtained from simulations using uniformly spaced convex cells is significantly lower than that measured experimentally. To attempt to account for the difference in results, this study employed a variety of ECS models based on an array of cubic cells containing open rectangular cavities that provided the ECS with dead-space microdomains. Monte Carlo simulations demonstrated that, in such ECS models, λ can equal or exceed the typical experimental value of about 1.6. The simulations further revealed that λ is relatively independent of cavity shape and the number of cavities per cell. It mainly depends on the total ECS volume fraction α , the cavity volume fraction α c , and whether the cavity is located at the center of a cell face or formed at the junction of multiple cells. To describe the results from the different ECS models, an expression was obtained that related λ to α , α c , and an empirical exit factor β that correlated with the ease with which a molecule could leave a cavity and its vicinity.

Insights into positron annihilation lifetime spectroscopy by molecular dynamics simulations

The relationship between free-volume properties measured from positron annihilation lifetime spectroscopy (PALS) and calculated from molecular dynamics simulations has been investigated for glassy and liquid glycerol in the temperature range 150–400 K. A virtual probing procedure has been developed to retrieve information on the basic free-volume properties of the simulated microstructures, i.e. mean cavity volume and free-volume cavity fractions. Our data leads us to infer on the occurrence of experimentally non-detectable small cavities with mean equivalent radius of 1.8–1.9 A between 250 and 275 K. The size of these limiting cavities is found to be temperature dependent, being smaller at low temperatures. At high temperatures, above a characteristic PALS temperature Tb2L , the formation of very large cavities is predicted. This finding suggests that, when the dimension of the holes in the system exceeds a given value, the PALS measurements become unable to catch the complete structural information and phenomena of dynamical origin enter into play in the PALS signal decay. The calculated number of cavities is found to be almost independent on the temperature from the glassy up to the liquid phase, thus furnishing a certain support to theoretical models proposed to evaluate the free-volume cavity fractions.

Superplastic Behavior and Cavitation for WC-Co Cemented Carbides

Superplastic behavior and cavitation were investigated for WC-15 mass % Co cemented carbides with the WC grain sizes of 0.7 µm (A) and 5.2 µm (B), WC-10 mass % Co cemented carbide with the WC grain size of 1.5 µm (C) and WC-5 mass % Co cemented carbides with the WC grain sizes of 0.5 µm (D) and 2.5 µm (E) by tensile tests at 1473 K. WC contiguity were 0.51, 0.31, 0.27, 0.56 and 0.49, respectively. The large elongations about 200 % were obtained for the B and the C having smaller values of WC contiguity compared to the other cemented carbides. The values of cavity volume fraction for them were less for the other cemented carbides, furthermore, cavities formed at WC/WC interfaces. Therefore, it is noted that the distribution of the Co phase is important for superplasticity of the cemented carbides.