Size Effect Theory Research Articles

Compression test and size effect study of defective wood

ABSTRACT Wood, as a natural material, inevitably contains initial defects such as knots, cracks, and decay, which are important factors affecting its mechanical properties and cannot be ignored. In order to investigate the influence of initial defects on the compressive mechanical properties of wood, Norway spruce was selected as the research object. Wood specimens without and with initial defects were prepared, and uniaxial compression tests were conducted. Combined with Digital Image Correlation (DIC) and finite element analysis, the compressive mechanical properties of defective wood were studied. The results showed that compared to defect-free specimens, specimens with defects exhibited a shortened elastic stage in the load-displacement curve, and the rate of decrease in load after reaching the peak was lower than that of defect-free specimens. For specimens of the same size, the compressive strength of specimens with defects was lower than that of defect-free specimens. The established finite element model could effectively simulate the compressive failure process of wood with initial defects, validating the effectiveness of numerical simulation. Bažant's size effect theory can be used for size effect analysis of the compressive strength of wood with initial defects in the longitudinal direction.

Experimental research on PPF anti-cracking effect in scale model of hollow rigid frame bridge

The hollow rigid frame bridge has many advantages, such as being lightweight, having strong spanning ability, and having slight deflection in span. This type of bridge has a broad potential for application in regions of many canyons and hills. While there has been less research into the application of this type of bridge, some of the bridges in early service have cracking problems. To satisfy the demand for cracking resistance of the Yunnanzhuang large-span hollow rigid frame bridge, our group proposed using Polypropylene Fibers (PPF) to enhance the strength of concrete against cracking. After analyzing the structure of the whole bridge, we gave the critical sections of the bridge and carried out a series of tests to give the PPF dosage that applies to the project. This paper focuses on the use of reduced-size bridge model tests to investigate the effect of PPF on the cracking resistance of hollow rigid frame bridges under bending.In this paper, the similarity criterion between the actual bridge and the model was calculated by the method of magnitude analysis, according to which scale-down model beams of the upper chord S4 segment and mid-span jointing segment of Yunnanzhuang Bridge were designed under the geometry of 1:5. We respectively made these models using the same C55 concrete as the actual bridge and Polypropylene Fibre Reinforced Concrete (PPFRC) with 0.3 % volumetric admixture of PPF in the same mix ratio, and three-point bending static load tests were performed on these models. To make up for the insufficiency of test groups, we performed a finite element analysis using ABAQUS and the CDP constitution, which led us to some reliable conclusions.Considering that concrete materials have size effect, directly using the results of model tests to predict the cracking stresses of the actual bridge is not accurate. In this paper, some theories on size effects were examined. Finally, the Size Effect Model (SEM), Boundary Effects Model (BEM), and Weibull's brittle strength theory were selected to correct the test results.The results show that PPF can increase the cracking load of the test beams, ranging from 11.22 % to 13.49 %. After using selected size effect theories to predict the cracking stresses in actual bridges, the results show that PPF can enhance the cracking strength of C55 concrete, with an increase ranging from 9.15 % to 18.75 %. Combined with the results of the structural analysis of the whole bridge, the critical section has a cracking stress between 2.6 MPa and 3.5 MPa when using C55 concrete, which has a certain risk of cracking. However, when using PPFRC, the cracking stress ranges from 3.1 MPa to 4.2 MPa, with no risk of cracking in most unfavorable cases, which can satisfy the demand for concrete cracking resistance of Yunnanzhuang Bridge.

Boron nitride nanosheet @ quantum‐sized diamond nanocrystal complex/epoxy nanocomposites with enhancing thermal conductivity and sufficient dielectric breakdown strength

AbstractWith the development of high‐voltage and high‐power devices in the direction of compactness, lightness and extreme working environments, the contradiction between thermal conductivity and dielectric breakdown strength is becoming more and more obvious. In this work, a novel boron nitride nanosheet with uniformly dispersed diamond nanocrystalline (abbreviated as BA@DN) was prepared via facile acidizing hydroxylation, in situ copolymerization self‐assembly of dopamine and blending process. The obtained hybrid filler was employed to embed into epoxy matrix, showing enhanced thermal conductivity and sufficient dielectric breakdown strength. For example, the through‐plane and in‐plane thermal conductivity of epoxy composite with 5% BA@DN‐2 reaches to 0.56 and 3.824 W/m K, respectively. The study proposes a modified plane thermal conductivity model. Based on calculations, it was found that the modified thermal conductivity of 5% BA@DN‐2 is 3.47 W/m K, which is significantly higher than that of pure EP, specifically 17.35 times higher. These findings are significant in terms of materials science. while its alternating current and direct current dielectric breakdown strength are 48.84 and 136.31 kV/mm, about 85.68% and 99.1% of pure epoxy, respectively. The mechanism of outstanding dielectric breakdown strength was analyzed via dielectric characteristic, partial discharge performance and the theory of quantum size effect. In addition, a modified multi‐core model with a multilevel overlapping interface is presented.Highlights The acidified diamond nanocrystals (DNs) were attached to dopamine‐coated boron nitride nanocrystals by a blending process. The small size effect of DNs allows the epoxy film to have both high thermal conductivity and high insulation properties. A thermal conductivity correction model is proposed to eliminate the effect of graphite layer on the film.

Investigation of thermal contact conductance of bare copper-copper and aluminum 1070-aluminum 1070 interfaces at low temperatures between 10 and 70 K

The prediction model of thermal contact conductance (TCC) for bare OFHC-OFHC and Al 1070-Al 1070 flat interfaces is presented. OFHC (Oxygen free high conductivity copper) and Aluminum are widely used in cryogenic systems. Since the thermal conductivity of pure copper or aluminum materials has a highly non-linear characteristic with its peak value at around 20 K, it is difficult to predict TCC of those materials at this temperature range. The TCC model in this paper proposes the dimensionless correlation through 16 experiments considering various conditions such as surface roughness, materials, pressure, and temperature. The model has been established by the modification on the previous CMY (Cooper, Mikic, Yovanovich) model with additional parameters. The first important feature of the model is to adopt the contact thermal conductivity (kc). It can be calculated by using the size effect theory at the micro-scale contact single spot. The second feature is the dimensionless temperature (T/TD). An experimental setup is constructed inside of the conduction-cooled cryostat to measure the thermal conductivity of bulk material and the TCC simultaneously for the temperature range between 10 K and 70 K. OFHC and Al 1070 specimens with various surface roughness are prepared and tested with contact pressure of 6 to 13.5 MPa in vacuum environment. The predicted TCC by the proposed model shows a reasonable agreement with the experimental results in low temperature.

Functionalization of ITO thin films for VO2 based thermochromic devices

ITO thin films are the most commonly used transparent conductors in a wide variety of devices. However, in their fully optimized state the ITO films are not compatible with other types of devices exhibiting optical change in the near infrared region. One such class are the thermochromic devices based on vanadium dioxide (VO2) thin films. The VO2 films exhibit a very effective optical modulation in the near infrared region where the ITO films are not transparent. The aim of the present work is to functionalize the ITO films and to render them compatible for use with the VO2 based thermochromic devices. The thickness dependence of the optical and electrical properties of these functionalized ITO films is also studied. The dependence of the electrical properties on film thickness has been examined in the light of the existing size effect theories for the first time. Near-infrared transparent ITO films with adequate electrical properties have been achieved for the required application.

Analysis of dynamic size effect of recycled concrete using base force element method

AbstractA 2D meso models of recycled concrete with different sizes is established using the base force element method. The specimens are loaded with different strain rates. The meso damage of recycled concrete is analyzed. In addition, the structural performance of specimens under the joint actions with strain rate and size are also studied. The simulation results find that the compressive strength has little difference when the values of strain rates are lower than 0.1 s−1. The greater the strain rates, the greater the difference of compressive strength between different sizes when strain rates are larger than 0.1 s−1. In addition, the Bažant statistical theory can be used to analysis the mechanical properties of recycled concrete. Besides, a modified formula in calculating the dynamic size effect of recycled concrete is also given based on the Bažant statistical size effect theory.

Size effect of ultra-high-performance concrete under compression: effects of steel fiber characteristics and water-to-binder ratio

Understanding the size effect of UHPC under compression is fundamental and essential for component and/or structure design and analysis. This paper investigated the size effect of UHPC cubes (40, 70.7, 100, and 150 mm) under compression through considering the steel fiber characteristics (1%, 2%, 3% short straight, short hooked, and long hooked steel fibers) and water-to-binder ratios (0.16, 0.18, 0.2 and 0.22). Bažant size effect law (SEL) and Weibull statistical theory of size effect were employed to analyze the influence of those factors on the size effect of compressive behavior of UHPC. Results showed the size effect of compressive strength of UHPC increased with the increase of short straight fiber content, but decreased with the increase of w/b ratio. Given the fiber content, UHPC with short hooked fiber showed the greatest size effect, while that with long hooked fiber exhibited the lowest size effect. SEL was more robust and reliable in predicting the change in compressive strength of UHPC with specimen size. The fitted compressive strength from SEL was more accurate with the increase in fiber content and the decrease in w/b ratio.

A new approach for determining GND and SSD densities based on indentation size effect: An application to additive-manufactured Hastelloy X

• A new approach to determine GND and SSD densities for FCC metals was established. • The new approach was based on indentation size effect and classical strengthening theories. • Results of new approach were verified by conventional Hough-based EBSD and XRD CMWP methods. • Due to low SFE of L-PBF Hastelloy X, edge-GNDs contribute greatly the increment of strengthening. • Dislocation distribution shows grain orientation-dependent: low in large <101> grains, high in fine <001> grains. • This method may construct a firm basis for future quantitative work on BCC and HCP metals. Dislocation plays a crucial role in controlling the strength and plasticity of bulk materials. However, determining the densities of geometrically necessary dislocations (GNDs) and statistically stored dislocations (SSDs) is one of the classical problems in material research for several decades. Here, we proposed a new approach based on indentation size effect (ISE) and strengthening theories. This approach was performed on a laser powder bed fused (L-PBF) Hastelloy X (HX), and the results were verified by the Hough-based EBSD and modified Williamson–Hall (m-WH) methods. Furthermore, to better understand the new approach and essential mechanisms, an in-depth investigation of the microstructure was conducted. The distribution of dislocations shows a clear grain orientation-dependent: low density in large <101> preferentially orientated grains while high density in fine <001> orientated grains. The increment of strengthening in L-PBF HX is attributed to a huge amount of edge-GNDs. Planar slip is the main operative deformation mechanism during indentation tests, and the slip step patterns depend mostly on grain orientations and stacking fault energy. This study provides quantitative results of GND and SSD density for L-PBF HX, which constructs a firm basis for future quantitative work on other metals with different crystal structures.

Size effect on compressive behaviors of waste fiber-reinforced recycled aggregate concrete

The compressive size effect of conventional concrete is well known, but there is limited knowledge of the fiber-reinforced recycled concrete. A total of 270 specimens with the different cube and prism sizes, water-cement ratios, compressive strength grades, the replacement rates of recycled aggregates, and volume fractions of waste fibers were tested until failure to study the size effect behaviors of waste fiber-reinforced recycled concrete subjected to axial compression. The results showed that as the replacement rate of recycled aggregates increases, the size effect degree of the cube and axial compressive strength increases. With the increase of the volume fraction of waste fibers, the size effect degree of compressive strength decreases first and then increases, with the optimum volume incorporation of 0.12%. The higher the strength grade of concrete is, the more obvious the size effect of strength is. For large-scale components, no incision correction of Bažant and Carpineri’s fractal size effect theory is more suitable for the analysis of the size effect of waste fiber recycled concrete. It is essential to understand the compression size effect of waste fiber-reinforced recycled concrete so that the laboratory investigation results can be applied to real full-scale structures subjected to compressive loading.

Modification of the Constitutive Model of Concrete Material Considering the Effect of Structural Size

As the size of the structure increases, the size effect on the mechanical properties of the structure cannot be ignored. During the service period, the reinforced concrete structure will be affected by the size effect. At present, there is no item concerning material size effect in the current regulations in my country. This paper summarizes the existing size effect theory and finds a function that expresses the size effect of concrete. The peak strength size effect coefficients of domestic and foreign researchers are analyzed, and the logarithmic function form is used to calculate the peak strength size effect coefficients obtained by various researchers. The average value was fitted. We found the size effect coefficient of the ultimate strain of concrete based on the test results, and took the size effect coefficient of the ultimate strain of the concrete with a cube side length of 150 mm as one for analysis. Finally we revised the constitutive relationship of concrete.

Theoretical Study on the Size Effect of the Rate Characteristics of Concrete Cubes Compressive Strength

In order to study the effect of size effect and rate correlation on the mechanical properties of concrete, a total of 228 concrete cube test blocks of three sizes (70, 100 and 150mm) were tested and studied under four strain rates (10−5,10−4,10−3,10−2/s). The test results show that under the same strain rate, the peak stress of concrete decreases with the increase of the size. Under the same size, the peak stress of concrete increases with the increase of strain rate. Experimental data were used to verify and modify Weibull size effect equation based on statistics and Bazant size effect equation based on fracture mechanics. The experimental data fit well with the equation, and the size effect theory of concrete strength with correlation of holdup is established.

Adoption of size effect combined with Winkler elastic foundation beam theory in section shield tunnel under underground space development

It aims to discuss the adoption value of combining the size effect and Winkler elastic foundation beam theory in the large-section shield tunnel under the background of underground space development. Two kinds of soil layers of silty clay and sand pebble are simulated. The difference of settlement, longitudinal deformation, and earth pressure of large-section shield tunnel in the simulated soil layers is analyzed. The longitudinal design model of shield tunnel based on large-section and compound stratum is constructed according to Winkler’s elastic foundation beam theory and the longitudinal equivalent continuous model of shield tunnel. The results show that the tunnel diameter has an exponential relationship with the effective rate of longitudinal stiffness and equivalent longitudinal stiffness. The overall longitudinal settlement curve of the tunnel in the compound stratum presents the “S” shape. The maximum differential settlement of the tunnel is −55 mm. The soil pressure around the tunnel from high to low is arch bottom, left arch waist, right arch waist, and arch top. The maximum settlement of the tunnel in the silty clay and sand pebble is −6.08 mm and −15.85 mm, respectively; the maximum uplift of the arch bottom is 11.65 mm and 17.74 mm, respectively; the earth pressure around the arch top is 2.184 MPa and 4.22 MPa, respectively; and the earth pressure around the arch bottom is 2.236 MPa and 3.88 MPa, respectively. At the boundary of the two strata, the maximum earth pressure around the top of the arch is 4.56 MPa, and the maximum earth pressure around the bottom of the arch is 4.17 MPa. It suggests that the longitudinal design model of shield tunnel with large-section and compound stratum is successfully constructed according to the size effect combined with Winkler elastic foundation beam theory, which provides a reference method for the design of the tunnel structure with large cross-section.

Size Effect and Influence Factors of Asphalt Mixture Based on Particle Discrete Element

To explore the size effect of asphalt mixture, asphalt mastic asphalt mixture was taken as the research object, and the uniaxial compressive strength was used as the evaluation index. The applicability of the three size effect theories of Bazant, Carpinteri, and Weibull were evaluated, and the corresponding size effect law parameters were calculated. The critical dimensions and critical strength of the asphalt mixture were determined, and the effects of coarse aggregate properties, maximum nominal particle size, and loading rate on the size effect of asphalt mixture were analyzed. Results show that the asphalt mixture has an obvious size effect phenomenon, and the uniaxial compressive stress-strain curves of asphalt samples with different sizes have the characteristics of compaction stage, elastic stage, and transition from plastic deformation to failure stage. The uniaxial compressive strength of the asphalt mixture is negatively correlated with the sample size, and as the sample size increases, the strength reduction decreases gradually. With the increase in the elastic modulus of the coarse aggregate, the critical strength value increases continuously, and the critical dimension decreases. As the maximum nominal particle size increases, the critical dimension and critical strength increase gradually. With the increase in the loading rate, the critical strength continues to increase, and the critical dimension gradually decreases. The Bazant and Carpinteri size effect laws are more suitable for the analysis of the size effect of asphalt mixture, while the Weibull size effect law has a large error. When SMA is used as the pavement structural layer, the ratio S of the structural layer thickness to the maximum nominal particle size calculated by the Bazant size effect law is 2.6 to 3.5, and the S range calculated by the Carpinteri size effect law is 2.6 to 4.0.

SIZE EFFECT LAW FOR NOMINAL STRENGTH OF STIRRUP-CONFINED CIRCULAR CONCRETE COLUMNS UNDER AXIAL COMPRESSION

The failure modes and mechanisms of reinforced concrete (RC) members are far different from those of the concrete material. It is therefore debatable to use the size effect theory and formula based on the concrete material to describe the size effect of RC members. Moreover, the size effect formula of the concrete material is difficult to describe the influence of other important structural parameters on the size effect of RC structural members. Taking circular concrete columns confined by stirrups for the research objects, combined with the physical test and numerical results, the main parameter of stirrup ratio that affects the size effect of nominal compressive strength was revealed. Furthermore, combined with the influence mechanism of the confining effect on the axial compressive strength of RC columns and based on the basic theory of the Size Effect Law (SEL) proposed by Bažant, a semi-theoretical and semi-empirical formula for the size effect of the uniaxial compressive strength of stirrup-confined RC columns was established. The experimental and numerical results in the present study as well as other experimental data verify the accuracy and rationality of thesemi-theoretical and semiempirical formula of the size effect on the RC member level.

Failure behavior and size effect theory of circular concrete-filled steel tubular columns under axial compression

Failure behavior and size effect theory of circular concrete-filled steel tubular columns under axial compression

Study on pore size effect of low permeability clay seepage

Low-permeability clays are characterized by a large amount of clay minerals and a special distribution of pore sizes, so their seepage behavior shows an obvious pore size effect. Therefore, conventional seepage theories show deviations in describing the seepage behavior of low-permeability clays. This paper analyzes two reasons for the pore size effect in low-permeability clay seepage: the pore distribution property and the electric double layer effect of the clay surface. Considering these two factors, the seepage theory of the pore size effect based on the microscale seepage of the circular tube model is proposed. The rationality of this theory is tested by the seepage experiments of natural undisturbed clay and artificial clay, and from these experiments, three beneficial conclusions are drawn: (1) The results of the pore size effect seepage theory are in good agreement with the experimental results and are better than the modified Kozeny-Carman equation; (2) The properties of the electric double layer have a certain regular influence on the seepage behavior of the clay; (3) Considering the properties of the soil pore size distribution, the results obtained by using the pore size effect theory are much closer to the experimental results than the results calculated using the average pore diameter of the clay.

Особенности проявления квантового размерного эффекта в явлениях переноса в тонких пленках висмута на подложках из слюды

The dependences of the resistivity, magnetoresistance,and Hall coefficient on the sample thickness were studied forthin bismuth films on mica at 77 K. We discovered quantum sizeoscillations of electrical and galvanomagnetic properties for filmswith the thickness less than 50 nm. The charge carrier free path isestimated. The reasons for deviation of the observed experimentaldependences from the simple quantum size effect theory in thesemimetal films are discussed.

Finite-size effects for spiking neural networks with spatially dependent coupling.

We study finite-size fluctuations in a network of spiking deterministic neurons coupled with nonuniform synaptic coupling. We generalize a previously developed theory of finite-size effects for globally coupled neurons with a uniform coupling function. In the uniform coupling case, mean-field theory is well defined by averaging over the network as the number of neurons in the network goes to infinity. However, for nonuniform coupling it is no longer possible to average over the entire network if we are interested in fluctuations at a particular location within the network. We show that if the coupling function approaches a continuous function in the infinite system size limit, then an average over a local neighborhood can be defined such that mean-field theory is well defined for a spatially dependent field. We then use a path-integral formalism to derive a perturbation expansion in the inverse system size around the mean-field limit for the covariance of the input to a neuron (synaptic drive) and firing rate fluctuations due to dynamical deterministic finite-size effects.

El sentimiento de los inversores sobre el tamaño en los múltiplos de valoración europeos

El objetivo de este documento es aportar una nueva perspectiva sobre los factores de control, diferentes a sector o industria, necesarios para el correcto uso de los múltiplos de valoración: MARKET-EBITDA, MARKET-SALES y MARKET-BOOK en el mercado europeo. Analizamos el papel del tamaño y sus interacciones con el resto de las variables, en especial aquellas relativas a rentabilidad y riesgo. Se ha desarrollado un modelo de valoración basado en datos de panel para empresas cotizadas europeas durante el período 2002-2013. Descubrimos que el tamaño incluye aspectos económicos de las empresas, pero también capta otros aspectos diferenciadores, demostrando que los inversores pagan diferente en cada ratio la misma magnitud económica en función del tamaño de las empresas. Por todo ello el tamaño es un factor de control importante, junto al resto de variables, para seleccionar el grupo de comparación y averiguar si una empresa está cara o barata frente a otras. Lo descrito ayuda a comprender por qué las empresas grandes evolucionan de un modo diferente a comparación de otras más pequeñas ante las mismas condiciones económicas o ciclos. El artículo está en consonancia con el marco teórico de valoración por múltiplos, así como con la teoría del efecto tamaño.

Phase-Modificate Defects Engineering CdS Sphalerite-Wurtzite System for Efficient Photocatalytic H2 Evolution under Visible Light Irradiation

According to quantum size effect and interface defects theory, the phase-modificate defects engineering photocatalyst MoS2/CdS-1/CdS-2 with smaller lattice mismatch degree was successfully designed...