Size Effects In Materials Research Articles

Analysis of Plane Displacement of Pressure Vessels with Defects

Aiming at the problem that the internal defects of the pressure vessel are not easy to detect, the out-of-plane displacement method is used to study it. The out-of-plane displacement field of circular flat plate heads with defects is obtained by the method of elastic theory analysis and compared with the finite element method to verify the accuracy of the finite element method. Based on the finite element method, the effects of vessel material, pressure load, defect shape, and defect size on the first-order out-of-plane displacement derivative are studied. The results show that the material of the container has little effect on the out-of-plane displacement. The first derivative of out-of-plane displacement increases linearly with the increase of pressure, and this relationship will not change with the increase of pressure. With the increase of defect aspect ratio, the maximum value of the first derivative of out-of-plane displacement increases gradually. When δ ˃ 9, the defect is detectable. The research results lay a theoretical foundation for the internal defect detection of pressure vessels based on digital speckle shearing interferometry.

Nanosized Complex of Metal–Ion-Exchanger Composites in the Oxygen Electrochemical Reduction

The role of primary (the particles’ size) and secondary (the particles’ content in the material) size effects of metal–ion-exchanger composites in the oxygen electrochemical reduction is elucidated. To this purpose, metal–ion-exchanger particulate composites with different grain size and the metal (Cu) particles’ content are prepared as spherical grains, based on macroporous sulfonic cation-exchange matrix (Lewatit K 2620). X-ray diffraction analysis showed the deposited metal basic particles to be nanosized. A special feature of the metal particles is that during the repeated cycles of their chemical deposition into the ion-exchange matrix pores both the capacity ε, and the particles’ radius r0 increased. On this reason, the primary and secondary size effects appeared being interconnected in a common nanosized complex f=ε/r0. With the increasing of the capacity the complex increased up to certain limiting value, which is connected with percolation transition from separate metal clusters to collective associates. Correspondingly, the reduced oxygen specific amount also reached its constant value. The oxygen electroreduction process reached quasi-steady-state regime.

Enhancing performance of ceramic membranes for recovering water and heat from flue gas

Transport membrane condenser can efficiently recover water and heat from wet flue gas. Ceramic membrane is the heat and mass transfer medium in the process. However, the relationship between membrane property and the recovery performance is only partially reported based on current research state. This work focuses on the effects of permeability, material, and ceramic particle size of membrane on the water and heat recovery. Membranes were prepared from three different sizes of coal fly ash and an alumina powder. Water and heat recovery performances of the membranes are experimentally compared. Results indicate that increasing membrane permeability improves water and heat recovery because the resistance of heat and mass transfer is reduced. The alumina membrane performs 3–10% more water recovery than the coal fly ash-based membrane while thermal conductivity of the alumina membrane was 11.5 times that of the coal fly ash-based membrane. Increasing ceramic particle size shows two opposite effects on the water and heat recovery, promotion because of the permeability rising, and undermine because of the thermal conductivity reduction. Low-cost membranes with high permeability and high thermal conductivity are recommended in transport membrane condenser application.

Indentation size effect and hardness of materials

In recent decades, the Indentation Size Effect (ISE), which manifests itself as a change in fixed hardness depending on indentation conditions, has become one of the most urgent problems of modern materials science. Research efforts are aimed at identifying and eliminating the causes of ISE, because the discrepancy between actual and true hardness values greatly complicates the task of finding the optimal material for the manufacture of parts. The proposed paper gives a brief review of the theories explaining ISE and its relationship with hardness of materials. A more detailed analysis of the effect of ISE on hardness is presented using an energy model based on the determination of the specific work of plastic deformation during indenter indentation. In all cases of ISE manifestation, a proportional relationship was established between the fixed hardness and the specific work of plastic deformation. Thus, the main cause of ISE is the hardening of the material in the centre of deformation under the indenter. The degree of influence of ISE on the fixable hardness as a function of material pre-hardening was determined. At a high level of pre-hardening (at ε>0.25) the influence of the ISE becomes insignificant, so it cannot be of practical importance. The proposed method of hardness determination will allow to assign material for manufacturing of parts to a greater extent corresponding to the operating conditions.

Discrete modeling of concrete failure and size-effect

Size-effect in concrete and other quasi-brittle materials defines the relation between the nominal strength and structural size when material fractures. The main cause of size-effect is the so-called energetic size-effect which results from the release of the stored energy in the structure into the fracture front. In quasi-brittle materials and in contrast to brittle materials, the size of the fracture process zone is non-negligible compared to the structural size. As a consequence, the resulting size-effect law is non-linear and deviates from the response predicted by linear elastic fracture mechanics. In order to simulate the size-effect, one needs to rely on numerical modeling to describe the formation, development and propagation of the fracture process zone. Although a number of models have been proposed over the years, it transpires that a correct description of the fracture and size-effect which accounts for boundary effects and varying structural geometry remains challenging. In this study, the Lattice Discrete Particle Model (LDPM) is proposed to investigate the effects of structural dimension and geometry on the nominal strength and fracturing process in concrete. LDPM simulates concrete at the aggregate level and has shown superior capabilities in simulating complex cracking mechanisms thanks to the inherent discrete nature of the model. In order to evaluate concrete size-effect and provide a solid validation of LDPM, one of the most complete experimental data set available in the literature was considered and includes three-point bending tests on notched and unnotched beams. The model parameters were first calibrated on a single size notched beam under three-point bending and on the mechanical response under unconfined compression. LDPM was then used to perform blind predictions on the load-crack mouth opening displacement curves of different beam sizes and notch lengths. Splitting test results on cylinders were also predicted. The results show a very good agreement with the experimental data. The quality of the predictions was quantitatively assessed. In addition, a discussion on the fracturing process and dissipated energy is provided. Last but not least, the Universal Size-Effect Law proposed by Bažant and coworkers was used to estimate concrete fracture parameters based on experimental and numerical data.

Interpretation of micromorphic constitutive relations for porous materials at the microscale via harmonic decomposition

Micromorphic theories became an established tool to model size effects in materials like dispersion, localization phenomena or (apparently) size dependent properties. However, the formulation of adequate constitutive relations with its large number of constitutive relations and respective parameters hinders the usage of the full micromorphic theory, which has 18 constitutive parameters already in the isotropic linear elastic case. Although it is clear that these parameters are related to predicted size effects, the individual meaning of single parameters has been rather unclear.The present work tries to elucidate the interpretation of the constitutive relations and their parameters. For this purpose, a harmonic decomposition is applied to the governing equations of micromorphic theory. The harmonic modes are interpreted at the microscale using a homogenization method for a simple volume element with spherical pore. The resulting boundary-value problem at the microscale is solved analytically for the linear-elastic case using spherical harmonics resulting in closed-form expressions for all of the elastic 18 parameters. These values are used to predict the size effect in torsion of slender foam specimens. The predictions are compared with respective experimental results from literature.

Investigation on minimum uncut chip thickness and size effect in micro milling of glow discharge polymer (GDP)

Glow discharge polymer (GDP) is used as the ablator material in inertial confinement fusion (ICF) applications. Its machining surface quality and geometric accuracy directly affect its high energy performance. Minimum uncut chip thickness (MUCT) and size effect are the keys to obtain higher surface quality during micro cutting. In this work, a hybrid analytical model of MUCT using cutting forces and effective rake angle based on minimum energy method (Model A), and micro milling experiment are applied to determine the MUCT of GDP. Furthermore, a specific cutting energy-based model (Model B), considering the mechanical properties, strain evolution, and tool geometry, is established to verify model A. Additionally, the material removal mechanism and size effect are also studied. It is found that the MUCT value of GDP determined by model A is between 0.268 and 0.532 μm, and the normalized MUCT value is between 0.148 and 0.295. These results are verified by the specific cutting energy obtained from model B. It is also proved that model B could preliminarily evaluate the MUCT value of GDP without conducting milling experiment. Moreover, below the MUCT threshold, the severe size effect and ploughing effect cause nonlinearly increase of specific cutting energy, and the random variation of cutting force and surface roughness. When the ratio of feed per tooth to cutting edge radius is larger than 0.61, the size effect could be ignored since the cutting mode is shearing.

Effect of bed material size on gas-solid flow characteristics in a CFB at low solid recirculation rates

The gas–solid flow characteristics greatly influence the heat transfer, combustion and pollutant formation processes in a circulating fluidized bed (CFB) boiler. The effects of particle size on the gas–solid flow characteristics, including flow structure, axial pressure and bed density profiles were studied in a two-dimensional CFB cold apparatus under low solid circulation rates (Gs’s). At the same time, an optical fiber probe was used to assess the particle size effect on the formation of particle clusters and their properties under different operating conditions. Results revealed that when the averaged size of bed materials decreased from ∼200 μm to ∼100 μm, at the same operating condition, the bottom dense bed became remarkably higher, and the upper dilute zone had stronger pressure fluctuation, more even axial and horizontal particle distributions, and higher solid density. Gs was more sensitive to the variation of the superficial gas velocity than the bed inventory. In addition, the standard deviation and power spectrum density of instantaneous pressure decreased in the dense bed and increased in the dilute zone. The particle velocity was slightly lower in the lower section of the riser but noticeably greater in the upper riser section. In addition, the clusters formed more frequently and lasted significantly longer. The study showed that flow characteristics in the bed qualitatively changed from that of Type B particles to that Type A particles in the Geldart’s classification, even at a small Gs, and previous studies with Type B particles could very likely not apply in the CFB boilers operating with fine bed materials.

Influence of silicide and texture on the room temperature deformation size effect of Ti65 alloy sheets/foils and its constitutive model modification

In micro-scaled plastic deformation process such as micro-forming, material size effect is challenging to investigate and reveal using traditional material models. Since high-temperature titanium alloys have become the critical materials of micro-parts depending on outstanding mechanical properties, studying the effect of texture and silicide precipitate on size effect in microtensile deformation of a high-temperature titanium alloy is of great significance because the size effect of highly alloyed titanium alloys has been less studied. In the present study, various heating treatments were performed to regulate grain size and texture and obtain silicide precipitate. The room temperature tensile tests were carried out to investigate the effect of the texture and silicide precipitate on the size effect of plastic deformation behaviour. It is revealed that silicide precipitation in the heat-treated specimens can lead to a more obvious “smaller is weaker” phenomenon and the specimens with transverse textures show a better strength–ductility match. In addition, considering solution strengthening, grain boundary strengthening and precipitation strengthening, the material constitutive model is modified by the effect of silicide and texture on the size effect, allowing for an accurate description of the tensile deformation characteristics at the microscopic scale. The results of the physical experiments and the hybrid constitutive model provide a basis for understanding and further exploration of the microscale plastic deformation behaviour of high-temperature titanium alloys.

Size effect of mortar specimens under uniaxial compression

This paper investigates the phenomenon of the size effect of quasi-brittle lime-based material. This research aims to confirm the existence of a size effect on a homogeneous material - test specimens of mortar, which are subjected to uniaxial compression and provide additional knowledge to explain the causes of the size effect. This paper deals with the analysis of the results of experimental tests of mortar specimens without inclusion and with one centrally located steel inclusion. Two series of tests with different slenderness and size were prepared. The results of the experiment showed that the compressive strength of the test specimens decreased with increasing specimen size. The larger interface area between the inclusion and the mortar matrix negatively affected the compression strength.

Nanoarchitected metal/ceramic interpenetrating phase composites.

Architected metals and ceramics with nanoscale cellular designs, e.g., nanolattices, are currently subject of extensive investigation. By harnessing extreme material size effects, nanolattices demonstrated classically inaccessible properties at low density, with exceptional potential for superior lightweight materials. This study expands the concept of nanoarchitecture to dense metal/ceramic composites, presenting co-continuous architectures of three-dimensional printed pyrolytic carbon shell reinforcements and electrodeposited nickel matrices. We demonstrate ductile compressive deformability with elongated ultrahigh strength plateaus, resulting in an extremely high combination of compressive strength and strain energy absorption. Simultaneously, property-to-weight ratios outperform those of lightweight nanolattices. Superior to cellular nanoarchitectures, interpenetrating nanocomposites may combine multiple size-dependent characteristics, whether mechanical or functional, which are radically antagonistic in existing materials. This provides a pathway toward previously unobtainable multifunctionality, extending far beyond lightweight structure applications.

Determining the fracture strength and toughness of polarized GaN ceramics through the mean size of aggregates

The fracture strength and service life of ceramic components are closely related to their microstructures and intrinsic defects. In this paper, instead of pure data or curve fitting, the tensile strength and fracture toughness of GaN piezoelectric semi-conductive ceramics (PSCs) have been investigated based on the boundary effect model for describing the size effect of quasi-brittle materials. It is shown that the fracture toughness of polarized or depolarized GaN PSCs can be fully determined by the tensile strength and the mean size of their aggregates which follow a normal distribution. The fracture strength of polarized GaN PSCs is higher than that of depolarized ones due to the refining of aggregates. It implies that polarization can be well applied to improve the fracture resistance and durability of GaN and other PSCs.

Temperature-dependent hardness model of high-temperature materials can account for indentation size effect

Indentation size effect has been found in the micro-nano hardness tests of both ceramic materials and metallic materials, even at high temperature. However, there are few temperature-dependent hardness theoretical models that can quantitatively characterize the indentation size effect of materials. This work introduces the indentation size effect into a novel temperature-dependent hardness model of materials by using energy method. The model takes into account the Young's modulus, indentation depth, melting point, and constant pressure heat capacity of materials without any fitting parameters. Further, for solving the problem of the difficulty to obtain the heat capacity of some materials, a simpler temperature-dependent model that just includes the Young's modulus, indentation depth and melting point is proposed. The remarkably excellent agreements between predicted and measured data of both ceramic materials and metallic materials verify the currency of the models. In addition, the effects of indentation depth and Young's modulus on the hardness of materials and their evolution with temperature are studied.

Discrete modeling of deterministic size effect of normal-strength and ultra-high performance concrete under compression

Size effect of heterogeneous granular materials under tensile loading is a well known phenomenon and the subsequent size effect laws are widely available in the scientific literature. On the contrary, size effect in compression is scarcely studied, which, especially for concrete, is more significant for its brittle failure mode in practical applications. This paper investigates the deterministic size effect in compression of concretes including a normal strength concrete (NSC) and an ultra high performance concrete (UHPC). The Lattice Discrete Particle Model (LDPM) is selected to determine the compressive strength of a broad range of sizes utilizing experimental-based calibrated data sets. The simulated tests include both cylinders and prisms with the diameter/width range from 150 mm to 600 mm for normal strength concrete and from 50 mm to 200 mm for UHPC. The investigated aspect ratio ranges from 1 to 16. It is found that the simulated strengths show no size effect under low friction loading, and the magnitude of size effect decreases with rising aspect ratio under high friction loading. The size effect laws available in the literature are not able to fit the obtained compressive strength data and a new compressive size effect law formula named ComSEL3D is proposed in terms of diameter and aspect ratio. The correlation by ComSEL3D for the simulated strengths is above 0.99, which shows its excellent ability in describing and predicting the deterministic size effect behavior in compression for both concretes in cylindrical and prism shapes. The ComSEL3D also outperforms the available formulas in the literature in matching the experimental results of various concretes under compression.

An examination of the size effect in quasi-brittle materials using a bond-based peridynamic model

This paper examines the size effect in quasi-brittle materials using a three-dimensional bond-based peridynamic model. This is the first time that the capability of a peridynamic model to capture the size effect in quasi-brittle materials has been examined. Correctly reproducing the size effect is an essential check on the validity of any computational model and it is demonstrated that a bond-based peridynamic model can accurately capture the failure stress of geometrically identical structures over a range of sizes. A systematic examination of geometrically similar notched and unnotched beams provides new insights into the predictive capabilities of the model, and mode I and mixed-mode problems are considered to examine the generality of the model. The model is validated using published experimental data, and the predictive accuracy is equivalent, if not superior, to well-established numerical methods whilst offering several benefits that justify further research and development. This study provides evidence that the length scale in a peridynamic model is a numerical parameter and not a material property.

Thickness-dependent neutralization of low-energy alkali-metal ions scattering on graphene

A challenging effort is under way to understand the essence of the quantum size effect of two-dimensional materials in order to achieve the ultimate goal of arbitrarily tailoring their properties in the near future. Here we present experimental and theoretical study of resonant neutralization of low-energy alkali-metal ions on clean and graphene-covered polycrystalline copper surfaces. The ion neutralization strongly depends on the number of graphene layers, and it gradually saturates for three to five layers of graphene. This result is consistent with that for the graphite surface. The neutral fraction for the clean polycrystalline copper surface is found to be significantly higher than that for the graphene-covered surface, which is not consistent with known regularities. We quantitatively explain those observations through the small energy level width of resonant electron transfer that is determined by the special electronic structure of graphene layers. This finding indicates that resonant neutralization spectroscopy has promising applications in the detection of the quantum size effects of two-dimensional materials at an atomic layer level.

Experimental study of rheological behavior of foam flow in capillary tubes

• Rheological behavior of the pre-generated foam was studied in capillary tubes • Tube materials were: PTFE and FEP (hydrophobic) and glass tubes (hydrophilic) • Foam behaves as shear-thinning fluid in all capillary tubes • Transition foam quality in tubes depends on the material type and tube diameter • Foam's apparent viscosity is higher in hydrophobic tubes than in hydrophilic tube due to wall-slip velocity • Wall-slip velocity depends on the tube material type and diameter • Foam texture in tubes is highly dependent on foam quality and flow rate Studies of foam flow in highly permeable porous media are still limited due to foam's complex behavior and discrepancies in foam research. Specifically, it is still unclear how foam flows in capillary tubes and what the effects of material and tube diameter are. We have investigated the rheology of pre-generated foam in capillary tubes. Experiments were carried out using two types of capillary tubes: hydrophobic (PTFE – polytetrafluoroethylene; FEP – fluorinated ethylene propylene) and hydrophilic (GT – glass tubes). The foam was previously formed in the sand-pack by co-injecting a surfactant solution and nitrogen gas. We investigated the effect of material and tube size on foam rheology versus gas fraction (for a fixed flow rate) and flow rate (for a fixed gas fraction). A three-parameter Herschel-Bulkley model was used to describe foam rheology in capillary tubes. Pictures of foam flow in GT and FEP tubes were taken to determine the mean bubble area using image analysis. We estimated wall-slip velocity in capillary tubes and compared the results with the bulk-foam rheology using an analytical expression of the Herschel-Bulkley model for volumetric flow through the circular tubes. We observed shear-thinning behavior in all capillary tubes (FEP, PTFE, GT), and the Herschel-Bulkley model successfully fitted its behavior. The foam in PTFE tubes behaved as a yield-stress fluid, while yield stress was not observed in GT and FEP tubes. We also found that transition foam quality depends on the material type and tube diameter. The results show that foam's apparent viscosity is higher in hydrophobic tubes (FEP and PTFE tubes) than in hydrophilic glass tubes. This was explained by the wall-slip velocity being higher in glass tubes than in FEP and PTFE tubes because of the difference in surface roughness. The corrected flow rate without wall slip matches the flow rate calculated using the measured bulk foam viscosity better. Therefore we conclude that considering wall-slip velocity is important when studying foam flow in porous media.

Catalyzed frontal polymerization-aided 3D printing of epoxy thermosets

• Tailoring frontal polymerization of epoxy resin with carbon materials • Revealing the size and geometric effect of carbon materials on the catalyzed frontal polymerization of epoxy resin • New method in lowering frontal temperature while maintaining the frontal polymerization rate • 3D printing of epoxy thermoset with self-propagated in-situ curing Frontal polymerization is a self-propagating exothermic reaction and provides a rapid and energy-efficient way to manufacture thermosets. A critical issue for frontal polymerization is to concurrently maintain a low frontal temperature and a self-sustained frontal propagation, which significantly depends on the frontal velocity. In this work, carbon nanotubes (CNTs), graphene oxide(GO) and discontinuous carbon fibers (d-CFs) were incorporated into epoxy thermosets to tune the frontal polymerization. Their catalytic effects on frontal temperature and frontal velocity were studied. Both CNTs and GO were found to significantly reduce the activation energy of frontal polymerization, whereas d-CFs did not show any obvious effect on the activation energy. The real-time non-destructive characterization showed that 1wt % CNTs incorporation reduced the frontal temperature from 240 to 227°C while the front velocity remained the same (6.5 cm min −1 ), indicating effective decoupling frontal temperature from frontal velocity. The frontal temperature could be further reduced to 220°C or lower at an increasing loading of CNTs while the frontal velocity remained the same. In contrast, 1wt% GO incorporation reduced the frontal temperature from 240 to 220°C, but also decreased the frontal velocity from 6.5 to 5.1 cm min −1 (21.5% reduction). In addition, as-prepared CNTs-incorporated epoxy resins were used in the 3D printing process via frontal polymerization and their printability were demonstrated. This discovery opens a new pathway for additive manufacturing through catalyzed in-situ frontal polymerization.

Ultrafast Generation of Coherent Phonons in Two-Dimensional Bismuthene.

Coherent phonons generated through regulation of lattice oscillation via ultrafast laser pulses or X-rays have been desired in various fields, including optoelectronics, thermal and quantum information, and communications. Phonon coherence of two-dimensional (2D) materials is particularly attractive as it enables controllable information transmission but is challenging as the weak interplanar coupling makes phonon excitation extremely difficult. Herein we managed to generate size-dependent phonon coherence from bulk Bi to few-layer bismuthene by an ultrafast femtosecond laser pulse and made a systematic comparison thorough a combination of computation, transient absorption, and reflectance spectroscopic methods. The results witnessed the A1g phonon excitation in all of the three Bi materials with distinct thicknesses, and the quantum size effect of 2D materials caused phonon confinement and eventual bond softening manifested as a red-shifted vibration frequency and shortened decoherence time compared with those of their bulk counterpart. This study offers new perspectives for tailoring coherent phonons in 2D materials for quantum science and technology including quantum communication, computing, and design of novel quantum devices, etc.

Defect tolerant fatigue assessment of AM materials: Size effect and probabilistic prospects

Despite the significant advantages such as rapid prototyping of complex structures, engineering application of Additive Manufacturing (AM) technique has been limited due to the weak fatigue performance and lacking of fatigue design rules of AM parts. Particularly, this application relies on the improvement of AM processing technique and integrity assessment of AM parts, the latter usually demands costly and time-consuming fatigue tests, especially for full-scale tests. Accordingly, the size effect and fatigue performance scatter in AM materials are explored on basis of defect data scanned by CT and experimental data in this work. Firstly, the distribution of maximum defects of AM materials under size effect is extrapolated via extreme value statistics theory; then a strength altering factor is proposed to characterize the effect of defects on fatigue scatter and size effect. From essential structure of fatigue curves, the relationship among defects, fatigue strength and fatigue curves are quantitatively analysed to extrapolate the probabilistic fatigue curves for AM materials; finally, three series of AlSi10Mg specimens with different processing parameters and gauge volumes prove the method effectiveness that the “location” and “scale” of fatigue curves is successfully predicted, and defect-tolerant assessment for AM parts is performed by employing the predicted fatigue curves.