Boundary Effect

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Chunlei Wang - One of the best experts on this subject based on the ideXlab platform.

  • grain Boundary Effect on the dielectric properties of cacu3ti4o12 ceramics
    Journal of Physics D, 2005
    Co-Authors: Guozhong Zang, Jialiang Zhang, Peng Zheng, Jinfeng Wang, Chunlei Wang
    Abstract:

    The electrical properties of CaCu3Ti4O12 ceramic materials, showing an enormously large dielectric constant, were investigated. It was found that the grain Boundary plays an important role in the giant dielectric behaviour of these ceramics. Measurement of the electrical current density (J) versus the electrical field (E) was carried out. A good linear relationship between lnJ and E1/2 was found, which demonstrates that the Schottky barrier should exist at the grain Boundary. A double Schottky barrier model composed of a depletion layer and a negative charge sheet was proposed, analogous to the barrier model for ZnO varistors. An activation energy value of about 0.6 eV was obtained from the data of the characteristic frequency corresponding to the peak of the imaginary part of the dielectric permittivity versus temperature, which may be attributed to the activation of to in the depletion layer.

Guozhong Zang - One of the best experts on this subject based on the ideXlab platform.

  • grain Boundary Effect on the dielectric properties of cacu3ti4o12 ceramics
    Journal of Physics D, 2005
    Co-Authors: Guozhong Zang, Jialiang Zhang, Peng Zheng, Jinfeng Wang, Chunlei Wang
    Abstract:

    The electrical properties of CaCu3Ti4O12 ceramic materials, showing an enormously large dielectric constant, were investigated. It was found that the grain Boundary plays an important role in the giant dielectric behaviour of these ceramics. Measurement of the electrical current density (J) versus the electrical field (E) was carried out. A good linear relationship between lnJ and E1/2 was found, which demonstrates that the Schottky barrier should exist at the grain Boundary. A double Schottky barrier model composed of a depletion layer and a negative charge sheet was proposed, analogous to the barrier model for ZnO varistors. An activation energy value of about 0.6 eV was obtained from the data of the characteristic frequency corresponding to the peak of the imaginary part of the dielectric permittivity versus temperature, which may be attributed to the activation of to in the depletion layer.

Xiaozhi Hu - One of the best experts on this subject based on the ideXlab platform.

  • Scaling Of Specimen Boundary Effect On Quasi-Brittle Fracture
    2020
    Co-Authors: Kai Duan, Xiaozhi Hu
    Abstract:

    The size Effect on quasi-brittle fracture is modelled by considering the distance of crack-tip to specimen Boundary. It is shown that the tensile strength criterion applies if specimen Boundary is very close to the crack-tip, and the fracture toughness criterion applies if specimen Boundary is far away from the crack-tip. The specimen Boundary influence captures the mechanism of size Effect, i.e. the interaction of the crack-tip fracture process zone with specimen Boundary. The Boundary Effect model proposed in the study is then compared with the common size Effect model, which emphasizes the specimen size influence, by analysing the same experimental results.

  • Asymptotic Analysis of Size Dependent Fracture Strength Using Boundary Effect Concept
    Advanced Materials Research, 2008
    Co-Authors: Kai Duan, Xiaozhi Hu
    Abstract:

    In this paper, the extensively-reported “size Effect” phenomena in fracture mechanics tests are explained using the Boundary Effect concept. It is pointed out that the widely-observed size Effect in fracture, including the dependence of the fracture energy on ligament, strength and fracture toughness on crack and/or ligament and the strength of geometrically similar specimens on characteristic size, is in fact, due to the Boundary influence on the crack tip damage zone. Furthermore, the recently-developed asymptotic model is used to demonstrate that the dependence of strength on crack and ligament lengths as well as on the characteristic size of geometrically similar specimens is a result of the dominance of the distance of the crack tip to specimen boundaries on the specimen failure mode. To verify further the Boundary Effect concept, the asymptotic model is also applied to two sets of selected experimental data available in the literature, and the implications are discussed.

  • Boundary Effect on concrete fracture and non constant fracture energy distribution
    Engineering Fracture Mechanics, 2003
    Co-Authors: Kai Duan, Xiaozhi Hu, Folker H Wittmann
    Abstract:

    Abstract This paper extends the local fracture energy concept of Hu and Wittmann [29] , [30] , and proposes a bilinear model for Boundary or size Effect on the fracture properties of cementitious materials. The bilinear function used to approximate the non-constant local fracture energy distribution along a ligament is based on the assumption of the proportionality of the local fracture energy to the fracture process zone (FPZ) height and characterises the FPZ height reduction when approaching a specimen back Boundary. The bilinear function consists of a horizontal straight line of the intrinsic fracture energy GF and a declining straight line that reduces to zero at the back Boundary. It is demonstrated that using the bilinear model, the size-independent fracture energy GF can be estimated from the fracture energy data measured on laboratory-size specimens, and the intersection of these two linear functions, defined as the transition ligament, represents the influence of the back Boundary on the fracture properties. It is also demonstrated that the specimen size alone is not sufficient to characterise the size Effect in the fracture properties observed on laboratory-size specimens.

Cyrille Chazallon - One of the best experts on this subject based on the ideXlab platform.

  • Determination of Fracture Properties of Concrete Using Size and Boundary Effect Models
    Applied Sciences, 2019
    Co-Authors: Ning Yang, Georg Koval, Yu Qiao, Yu Hu, Qingbin Li, Cyrille Chazallon
    Abstract:

    Tensile strength and fracture toughness are two essential material parameters for the study of concrete fracture. The experimental procedures to measure these two fracture parameters might be complicated due to their dependence on the specimen size or test method. Alternatively, based on the fracture test results only, size and Boundary Effect models can determine both parameters simultaneously. In this study, different versions of Boundary Effect models developed by Hu et al. were summarized, and a modified Hu-Guan’s Boundary Effect model with a more appropriate equivalent crack length definition is proposed. The proposed model can correctly combine the contributions of material strength and linear elastic fracture mechanics on the failure of concrete material with any maximum aggregate size. Another size and Boundary model developed based on the local energy concept is also introduced, and its capability to predict the fracture parameters from the fracture test results of wedge-splitting and compact tension specimens is first validated. In addition, the classical Bažant’s Type 2 size Effect law is transformed to its Boundary Effect shape with the same equivalent crack length as Koval-Gao’s size and Boundary Effect model. This improvement could extend the applicability of the model to infer the material parameters from the test results of different types of specimens, including the geometrically similar specimens with constant crack-length-to-height ratios and specimens with different initial crack-length-to-height ratios. The test results of different types of specimens are adopted to verify the applicability of different size and Boundary Effect models for the determination of fracture toughness and tensile strength of concrete material. The quality of the extrapolated fracture parameters of the different models are compared and discussed in detail, and the corresponding recommendations for predicting the fracture parameters for dam concrete are proposed.

  • A Size and Boundary Effects Model for Quasi-Brittle Fracture
    Materials, 2016
    Co-Authors: Xiaofeng Gao, Georg Koval, Cyrille Chazallon
    Abstract:

    The fracture behaviors of quasi-brittle materials are commonly specimen size (size Effect) and crack size (Boundary Effect) dependent. In this study, a new failure model is developed for characterizing the size and Boundary Effects. The derivative of the energy release rate is firstly introduced to predict the nominal strength dominated by the strength mechanism. Combined with the energy criterion for the energy mechanism, an asymptotic model is developed to capture the Effect of any crack size on the nominal strength, and its expression for geometrically similar specimens is also established, which is able to characterize the size Effect. Detailed comparisons of the proposed model with the size Effect law and the Boundary Effect model are performed, respectively. The nominal strength predictions based on the proposed model are validated with the experimental results of cracked three-point bending beam specimens made of concrete, of limestone and of hardened cement paste and compared with the model predictions given by the size Effect law and the Boundary Effect model.

Ye Fang-hui - One of the best experts on this subject based on the ideXlab platform.

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