A NOVEL METHOD FOR MEASURING AND CHARACTERIZING DYNAMIC FRACTURE-INITIATION TOUGHNESS OF ELASTIC-PLASTIC MATERIALS

The characterization and testing methods of the dynamic fractureinitiation toughness of elas- tic-plastic materials under tensileimpact are studied. By using the self-designed bar-bar tensile impactappa- ratus, a novel test method for studying dynamicfracture-initiation ahs been proposed based on the one-di- mensionaltest principle. The curve of average load v. s. displacement (P-δ)is smooth until unstable crack propagation, and the kinetic energywhich does not contribute to the crack growth can be removed fromtotal work done by external-force to the specimen.

CRACK INITIATION AND STEADY PROPAGATION OF ELASTIC-PLASTIC MATERIALS IN PLANE STRESS CONDITION

The crack tip stress-strain fields of the elastic-plastic cracked specimens have been analyzed using finite element calculations.The crack initiation and steady propagation behaviours have also been investigated by means of slip line pattern etching technique and mechanical tests. The results show that there are HRR near field and distant field in the crack tip region,and the later depends on the specimen configuration.The crack initiation behaviour is controlled by a single parameter J.In contrast,the steady crack propagation is affected by the distant strain field and can not be described by single parameter only.

On the yield functions in Lagrangian stress space of elastic-plastic materials with internal constraints

In the present paper it is shown that the elastic range in the second Piola-Kirchhoff stress space can be chosen in a hyperplane which is through the origin of Lagrangian stress space and perpendicular to the normal of the constraint manifold at the plastic configuration, if the determinate stress response of the elastic-plastic material with simple internal constraints with some condition is correctly chosen, otherwise, it is in general in a hypersurface and the normal flow rule by Ⅱ yushin＇s postulate will have an indeterminate part. The choice of deterrninate stress response is probable because of its indeterminacy. Therefore the yield function should be a function of the second Piola-Kirchhoff stress lying in the hyperplane so that it is more simple and the back stress as the geometric center of the elastic range in general is inside the elastic range. Finally some examples are concerned.

The Life Predictions in Whole Process Realized with Different Variables and Conventional Materials Constants for Elastic-Plastic Materials Behaviors under Unsymmetrical Cycle Loading

Author researches a lot of the mathematical models and the related conventional material constants in the traditional and the modem mechanics; to adopt two types of variables a and D, for the fatigue-damage-fracture behaviors to elastic-plastic steels contained flaws, to put forward several calculation models, which are the driving force and the life prediction expressions at each stage and in whole process; for the key parameters .A1 and ,A2 in two stages, there are functional relation with other conventional material constants σF,m1 and M2,λ2, they are defined as the new calculable comprehensive material constants, and indicate their physical and geometrical meanings. In addition, for conversion methods between two types of variables, relevant calculating example is provided. Thereby, make a linking between the fracture mechanics and the damage mechanics, communicating their relationships. This works for saving man powers and funds on fatigue-damage-fracture testing that will be having practical significance.

AN INVESTIGATION ON ASYMPTOTIC NEAR-TIP FIELDS OF DYNAMIC CRACK IN AN ELASTIC-PERFECTLY PLASTIC COMPRESSIBLE MATERIAL

The stress and deformation fields near the tip of a mode-I dynamic crack steadily propagating in an elastic-perfectly plastic compressible material are considered under plane strain conditions. Within the framework of infinitesimal displacement gradient theory, the material is characterized by the Von Mises yield criterion and the associated J(2) flow theory of plasticity. Through rigorous mathematical analysis, this paper eliminates the possibilities of elastic unloading and continuous asymptotic fields with singular deformation, and then constructs a fully continuous and bounded asymptotic stress and strain field. It is found that in this solution there exists a parameter phi(0) which cannot be determined by asymptotic analysis but may characterize the effect of the far field. Lastly the variations of continuous stresses, velocities and strains around the crack tip are given numerically for different values of phi(0).

Mechanical behavior and deformation mechanism of ballast bed with various fouling materials

In order to study the interaction between various fouling particles and ballast,a multi-layer and multi-scale discrete element model(DEM)including the sleeper,ballast bed and the surface layer of subgrade was developed.Two typical fouling particles,the hard particles(sand)and soft ones(coal fines),are considered.A support stiffness test of the ballast bed under various fouling conditions was conducted to calibrate the microscopic parameters of the contact model.With the model,the influence of fouling particles on the mechanical behavior and deformation of the ballast bed was analyzed from macro and micro perspectives.The results show that the increase in the strength of the fouling particles enlarges the stiffness of the ballast bed.Hard particles increase the uniformity coefficient of the contact force bondγof ballast by 50.4%.Fouling particles increase the average stress in the subgrade,soft particles by 2 kPa and hard particles by 1 kPa.Hard particles can reduce the elasticity,plastic deformation and energy dissipation in the track structure.As the fouling particle changes from hard to soft,the proportion of the settlement in ballast bed increases to 40.5%and surface layer of swbgrade settlement decreases to 59.5%.Thus,the influence of fouling particles should be considered carefully in railway design and maintenance.

STEADY GROWTH OF CRACKS IN COMPRESSIBLE ELASTIC-PLASTIC MEDIA

Based on the theoretical framework for crack growth analysis provided by Gao and Hwang, the 5-sector soiution of near-tip fields of mode-I cracks growing quasi-statically and steadily in compressible elastic perfectly plastic materials is obtained.As Poisson's ratio v tends to 1/2,the 5-sector solution degener- ates to the 4-sector solution of near-tip fields of crack growth in incompressible elastic perfectly plastic materials.

Higher order implicit CNDG-PML algorithm for left-handed materials

By incorporating the higher order concept,the piecewise linear recursive convolution(PLRC)method and CrankNicolson Douglas-Gunn(CNDG)algorithm,the unconditionally stable complex frequency shifted nearly perfectly matched layer(CFS-NPML)is proposed to terminate the left-handed material(LHM)domain.The proposed scheme takes advantages of CFSNPML formulation,the higher order concept PLRC method and the unconditionally stable CNDG algorithm in terms of absorbing performance,computational efficiency,calculation accuracy and convenient implementation.A numerical example is carried out to demonstrate the effectiveness and efficiency of the proposed scheme.The results indicate that the proposed scheme can not only have considerable absorbing performance but also maintain the unconditional stability of the algorithm with the enlargement of time steps.

Optical Properties and Structural Perfection of Chicago Sky Blue 6B-doped KDP Crystals

KDP crystals doped with Chicago Sky Blue 6B（CSB-6B） were grown by traditional lowering temperature method.The optical properties and structural perfection of KDP crystals were investigated by transmittance spectra and high-resolution X-ray diffraction,respectively.The results indicate that CSB-6B tends to be incorporated into the pyramidal sector of KDP crystals（PyS-KDP） and lead to inclusions parallel to（101） face.Additionally,the transmittance of as-grown KDP crystals decreases as the amount of CSB-6B increases. Moreover,the rocking curves of PyS-KDP suggest that CSB-6B can deteriorate the structural perfection of PyS-KDP.

CORRECT FORMULATION AND SOLUTION OF NEAR-TIP FIELD PROBLEM FOR CRACK GROWING IN COMPRESSIBLE ELASTIC-PLASTIC MATERIALS

In the 5-sector solution by Drugan et al. for near-tip field of plane strain crack growing in compressible materials, the plastic flow rule is violated in the plastic zone (called the nonsingular zone by Rice) between the central fan sector and the elastic unloading sector. Based on the theoretical framework provided by Rice and supplemented by the "unloading boundary theorem", the correct formulation of the problem is given and the correct asymptotic solution for the near-tip field of the crack growth is obtained.

NEAR-TIP FIELDS FOR A CRACK GROWING ALONG AN ELASTIC PERFECTLY PLASTIC INTERFACE

The stress and deformation fields near the tip of an anti-plane crack growing quasi-statically along an interface of elastic perfectly plastic materials are given in this paper. A family of solutions for the growing crack fields is found covering all admissible crack line shear stress ratios.

基于塔姆等离激元的硼烯近完美吸收器(特邀)

针对单层硼烯吸光效率低的难题,提出了一种基于塔姆等离激元的硼烯近完美吸收器件,通过入射光直接激发高度局域的塔姆等离激元模式,深入研究了硼烯的近红外光学响应。仿真结果表明,所设计的硼烯/一维光子晶体结构能够有效提高硼烯的吸光效率,相比单层硼烯提升了一个数量级。同时器件在不同面内晶体方向上表现出强烈的各向异性。通过改变硼烯狄拉克电子密度,将器件在x和y偏振方向上的吸光效率提高至95.52%和96.63%,对应共振吸收波长为1550 nm和1607 nm。此外,通过改变一维光子晶体结构、氧化铝间隔层厚度及单层硼烯在间隔层中的位置等参数能够实现对吸收效率及吸收波长的灵活调控。该器件具有吸收效率高、工作波段可调等显著优点,可为发展下一代片上集成光电器件提供机遇。

Visible-near infrared ultra-broadband polarization-independent metamaterial perfect absorber involving phase-change materials

We numerically demonstrate a novel ultra-broadband polarization-independent metamaterial perfect absorber in the visible and near-infrared region involving the phase-change material Ge_2Sb_2Te_5(GST).The novel perfect absorber scheme consists of an array of high-index strong-absorbance GST square resonators separated from a continuous Au substrate by a low-index lossless dielectric layer(silica)and a high-index GST planar cavity.Three absorption peaks with the maximal absorbance up to 99.94% are achieved,owing to the excitation of plasmon-like dipolar or quadrupole resonances from the high-index GST resonators and cavity resonances generated by the GST planar cavity.The intensities and positions of the absorption peaks show strong dependence on structural parameters.A heat transfer model is used to investigate the temporal variation of temperature within the GST region.The results show that the temperature of amorphous GST can reach up to 433 K of the phase transition temperature from room temperature in just 0.37 ns with a relatively low incident light intensity of 1.11×10~8W∕m^2,due to the enhanced ultra-broadband light absorbance through strong plasmon resonances and cavity resonance in the absorber.The study suggests a feasible means to lower the power requirements for photonic devices based on a thermal phase change via engineering ultra-broadband light absorbers.

Tunable near-infrared plasmonic perfect absorber based on phase-change materials

A tunable plasmonic perfect absorber with a tuning range of 650 nm is realized by introducing a 20 nm thick phase-change material Ge2Sb2Te5 layer into the metal–dielectric–metal configuration.The absorption at the plasmonic resonance is kept above 0.96 across the whole tuning range.In this work we study this extraordinary optical response numerically and reveal the geometric conditions which support this phenomenon.This work shows a promising route to achieve tunable plasmonic devices for multi-band optical modulation,communication,and thermal imaging.

A large scale perfect absorber and optical switch based on phase change material(Ge_2Sb_2Te_5) thin film

In the conventional optical coating, the minimum thickness required to achieve anti-reflection should be quarter wavelength(λ/4n, where n is the refractive index). However, it was demonstrated that a lossy thin film with the thickness below quarter wavelength can also sustain the interference effect leading to a high absorption close to unity. In this paper we show that by depositing a phase change material Ge2Sb2Te5(GST) thin film(amorphous) on a metal reflector, unity absorption is attainable. We attribute the high absorption to the interference effect within the GST thin film even though its thickness is less than a quarter wavelength. The wavelength of the absorption peak can be tuned by adjusting the GST film thickness. In addition, the performance of this perfect absorber is insensitive to the incidence angle variation. Relying on the fact that GST is a phase change material, the absorption band can be tuned by inducing the phase transition of GST. The huge difference of the reflectivity between amorphous and crystalline phase leads to a high optical contrast ratio as high as 400 at the specific wavelength, suggesting the potential in the application of optical switch and rewritable data storage.

Perfect invisibility concentrator with simplified material parameters

We present a series of invisibility concentrators with simplified material parameters beyond trans- formation optics. One of them can achieve the perfect invisible effect at frequencies of Fabry-Perot resonances, while others have very small scattering. The required materials are feasible in practice. Analytical calculations and numerical simulations confirm the functionalities of these devices.

Angle-selective perfect absorption with two-dimensional materials

Two-dimensional(2D)materials have great potential in photonic and optoelectronic devices.However,the relatively weak light absorption in 2D materials hinders their application in practical devices.Here,we propose a general approach to achieve angleselective perfect light absorption in 2D materials.As a demonstration of the concept,we experimentally show giant light absorption by placing large-area single-layer graphene on a structure consisting of a chalcogenide layer atop a mirror and achieving a total absorption of 77.6%in the mid-infrared wavelength range(~13μm),where the graphene contributes a record-high 47.2%absorptivity of mid-infrared light.Construction of such an angle-selective thin optical element is important for solar and thermal energy harvesting,photo-detection and sensing applications.Our study points to a new opportunity to combine 2D materials with photonic structures to enable novel device applications.

左手介质组成的完美透镜的两种不同的理论分析

在分析了两种不同的左手介质组成的基础上,根据电磁理论,从传统透镜的理论缺陷入手,利用传统光学性质和传输线理论两种不同的方法来分析,利用倏逝波在左手介质中被放大的性质,解析地研究由左手介质组成的完美透镜以及完美透镜成像的原理.

塑性本构理论与工程材料塑性本构关系

研究了材料的塑性本构理论,从理论上建立了严密的塑性本构方程,为建立工程材料塑性本构关系提供了理论基础,此后将理论应用于3种工程材料.依据材料的性质以及工程的要求,通过简化得出满足工程计算精度要求的岩土类摩擦材料、金属类晶体材料的塑性本构关系;对强度控制的工程问题如有充分塑性变形条件则可将材料视作理想塑性材料,应用屈服条件和极限分析条件,采用传统的或数值的极限分析方法,求得工程安全系数或极限承载力.

多资源复杂网络的应急调度研究

以一次性消耗系统为背景,提出了多应急点多出救点以及多资源的复杂网络应急调度模型。以应急点的损失最小和出救点的个数最少为目标,将损失量大小转化为应急时间长短进行衡量,运用理想点法求解。最后构造算例,运用本文提出的方法对模型进行计算,验证了所建立模型的合理性及理想点法的可行性和有效性。