Dislocation Velocity Exponent and the Strain Rate

The process of dislocation multiplication has been described hv chaos theory, trying to reveal the connection between the microstructures on the mesoscopic scale and the mechanical properties of material on the macroscopic scale. The relationship between the dislocation velocity exponent and the maximum of strain rate is given. The results obtained from logistic equation with exponent and the dislocation multiplication dynamic equation are compared. A scale law in one-dimension-map model with exponent is shown when the exponents of equations are changed.     

Effect of Erodent Particle Hardness on Velocity Exponent in Erosion of Steels and Cast Irons

This article reports the effect of hardness of erodent particles on velocity exponent of some weld deposited alloys. Three steels and two alloy cast irons were selected for the present investigation. The bulk hardness of the alloys was in the range of 300 to 800 HV, whereas erodnet particles were having hardness in the range of 400 to 1875 HV. Erosion tests were conducted with 125-150 µm cement clinker, 125-150 µm blast furnace sinter, 100-150 µm silica sand, and 125-150 µm alumina particles and at impingement angles of 30° and 90° and with impingement velocities in the range of 25 to 120 m sec^-1. The erosion rate showed power-law dependence on impingement velocity, E = kVⁿ, where kis a constant and nis the velocity exponent. The velocity exponents obtained in the present work were in the range of 1.91 to 2.52. The velocity exponent showed an increasing trend with increasing hardness of the alloys irrespective of the hardness of the erodent particles and the impingement angle. The velocity exponent increased with increasing hardness of erodent particles, irrespective of the impingement angle and hardness of the alloys. The velocity exponents obtained in the present work were rationalized with respect to erodent particle properties, material properties and erosion mechanisms.     

Dislocation Behaviour near the Crack Tip in Bulk Fe-3% Si Single Crystal

An investigation has been made of the disloca-tion distribution and dislocation free zone near thecrack tip in bulk Fe-3% Si single crystal duringdeformation in SEM.It has been found that anumber of dislocations were emitted from the cracktip during deformation.After that,the dislocationsmoved rapidly away from the crack tip,which indi-cated that they were strongly repelled by the stressfield at the crack tip.Between the crack tip and theplastic zone there is a region of dislocation-free,which is referred to as dislocation-free zone (DFZ).The length of DFZs is roughly estimated 100μm which is much longer than that found in thinfoil specimen.The variation of dislocation densityas a function of the distance from the crack tip wasmeasured,which showed that the dislocations areinversely piled up in the plastic zone.The length ofDFZs increased with both the length of pre-crackand the amplitude of applied stress.     

In-situ TEM Observation of Dislocation Emission, Multiplication and Motion Induced by Anodic Dissolution in Type 310 Stainless Steel

A special constant deflection device for TEM has been designed and then the dislocation configuration change ahead of a loaded crack tip after corrosion or anodic dissolution for some time but before the initiation of SCC can be observed in TEM. The results showed that anodic dissolution during SCC of a type 310 stainless steel in deionized water could promote dislocation emission,multiplication and motion before the initiation of SCC.     

Dislocation Model and Morphology Simulation of bcc fcc Martensitic Transformation

By using molecular dynamics computer simulation at atomic level, the effects of single dislocation and dipole dislocations on nucleation and growth of martensitic transformation have been studied. It was found that only the location of tension or compression stress fields of the dislocations are favorable for martensite nucleation in NiAl alloy and the dislocations can move to accommodate partly the transformation strain during the nucleation and growth of martensite. Combined with the molecular dynamics simulation, a two dimensional simulation for martensite morphology based on a dislocation model has been performed. Many factors related to martensitic transformation were considered, such as supercooling, interface energy, shear strain, normal strain and hydrostatic pressure. Different morphologies of martensites, similar to lath, lenticular, thin plate, couple-plate and lenticular couple-plate martensites observed in Fe-C and Fe-Ni-C alloys, were obtained.     

High Velocity Impact Behaviour of Layered Steel Fibre Reinforced Cementitious Composite (SFRCC) Panels

Behaviour of layered steel fibre reinforced cementitious composite (SFRCC) panels is studied under high velocity impact of short projectiles. The panels consist of slurry infiltrated fibre concrete (SIFCON) layers in external faces and an intermediate (core) layer of latex modified concrete (LMC) and steel wire mesh embedded in cement sand slurry. In order to minimize acoustic impedance mismatch at the interfaces, judiciously selected materials are provided in the layers with appropriate lay-up sequences. For relative evaluation of high velocity impact performances of these panels', impact experiments are conducted in controlled environment. Two most commonly used types of short projectiles having calibre diameter of 5.56 mm and 7.62 mm are used in this study. Various important response parameters like depth of penetration (DOP), crater size, spalling, and cracking in the panels are considered for the performance evaluation. This paper presents the results of experimental study conducted on SFRCC panels. Considering the results obtained from experimental study, relative assessment of impact performances of SFRCC panels is carried out with respect to the materials of core layer. Influence of steel fibre volume on impact performance of the panels is also investigated and expressions are proposed based on regression analysis. In order to determine the residual impact resistance of the SFRCC panel after first hit, the same panel was impacted consecutively two to three times, generally within the damage zone of the first hit. Promising potential to resist high velocity impact is exhibited by the SFRCC panels. The structural integrity of the SFRCC panels is found intact even under multiple hits.     

基于RVM的爆破振动速度预测模型

为提高对爆破振动速度预测的准确性,提出一种基于相关向量机(RVM)的爆破振动速度预测模型.利用该模型建立爆破振动速度与其影响因素之间的非线性映射关系,通过选取影响爆破振动速度的3个主要因素(炸药用量、距离、高程差),并对这3个主要影响因素产生的36组数据进行拟合训练,根据这36组训练样本来对剩余5组样本进行精准预测.将...     

Theoretical Analysis of Isotope Effect Exponent of Diboride and Iron-Based Superconductors: Three-Square-Well Model Approach

Analytical calculations of isotope effect exponent of both s-wave and π-phase-shifted s-wave two-band superconductors are presented within the framework of Bogoliubov–Valatin formalism using three-square-well model approach. The effective pairing interaction consists of contributions from the attractive acoustic electron–phonon, optical electron–phonon contributions in addition to repulsive electron–electron or antiferromagnetic fluctuation mediated pairing contributions for the diboride or pnictide superconductors, respectively. These are used to obtain analytical results that are applied in studying the isotope effect exponent in the diboride and pnictide superconductors, and the results are compared with experimental data.     

薄板优化设计的二次性态模型法

 在结构优化设计过程中,结构重分析的数值计算常常是主要的工作量.为了提高优化效率,必须减少结构重分析次数.采用一种称为“性态模型法”的近似重分析法时,完整的结构重分析次数将显著减少,重分析次数仅与变量数n成线性关系.此外,以往的薄板优化得到的是阶梯形的板厚分布,利用虚拟层合板单元的有限元计算得到的是连续性的板厚分布,因此更具合理性.几个经典算例说明了性态模型法在优化设计中的有效性和实用性.     

适于水工物理模型的新型表面流速测量技术

针对水工物理模型表面流速测量方法的技术现状,提出了一种不对流场造成破坏的新型光电非接触式表面流速测量方法.对流速测量原理进行推导,并对影响测量精度的因素进行了分析.搭建了实验系统,在6种不同标定流速下,对0、25 kg/m~3及50 kg/m~3三种含沙量情况下的流速进行了测量.实验结果表明,在三种不同含沙量情况下,实测流速与标定流速的一致性较好,证明理论的可行性.实验还发现,水体适量含沙能提高流速测量精度,流速相对误差能控制在8%以下.     

Dislocation etching in molybdenite

An etchant to reveal the non-basal dislocations on the (0001) faces of MoS₂ single crystals has been established. Evidence concerning the ability of this etchant to reveal the sites of non-basal screw dislocations is also given.     

A Non-Linear Viscoelastic Viscoplastic Model for the Behaviour of Polyester Fibres

In this study the macroscopic analysis proposed by Schapery has been applied to model the non-linear viscoelastic and viscoplastic behaviour of polyester fibres. First, a special non-contact extensometry method has been developed in order to measure the fibre response without affecting the behaviour of these extremely anisotropic fibres. An accelerated identification method has then been developed. This enables a unique and independent set of parameters to be determined, which separate the reversible and non-reversible mechanisms responsible for measured strains. The model prediction for an arbitrary loading sequence corresponds closely to the measured strains. The model has been applied here at the yarn level but can equally well be used to predict rope behaviour, either by including the identified parameters in a rope structure analysis or by identifying parameters at the rope level.     

爆破震动速度峰值预测模型的比较和讨论

传统的爆破震动峰值质点速度(PPV)预测公式已不能适应现代爆破安全的要求。基于人工神经网络(ANN)原理,建立了结构为2-5-1的BP神经网络预测模型,以青藏铁路关角隧道爆破震动测试数据为样本,对人工神经网络模型和几种传统公式的爆破震动速度预测结果进行比较。通过对实测结果与预测结果相关性系数(COD)、平均绝对误差(MAE)和相对误差(RE)的分析,表明:在传统预测模型中USBM预测公式精度最高,而人工神经网络模型较传统模型更接近实际值。     

基于BP神经网络的爆破振速峰值预测

在分析爆破振动主要影响因素的基础上,建立了对爆破振动质点速度峰值进行预测的BP神经网络模型.将某露天铁矿的爆破振动监测数据分别与模型预测结果和应用传统经验公式进行预测的结果比较,发现由于每次爆破的地震波传播不同,直接应用萨氏公式的K、α值进行预测会引起较大误差,相比之下,BP网络模型的预测结果更为准确和可靠.     

Multiplication factors in ethylene-filled proportional counters

Gas multiplication factors were measured in a proportional counter filled with high-purity ethylene in the pressure range 10-40 Torr. The multiplication factors were measured as a function of the reduced electric field S(a). The results show that, within the range of the high values of reduced electric field applied in this work (2648 < or = S(a) < or = 6455 V cm(-1) Torr(-1)), the reduced first Townsend coefficient alpha/P for ethylene may still be considered a function of S(a) alone. Good agreement was found with a model proposed by Akande.     

Dislocation dynamics in confined geometry

A simulation of dislocation dynamics has been used to calculate the critical stress for a threading dislocation moving in a confined geometry. The optimum conditions for conducting simulations in systems of various sizes, down to the nanometer range, are defined. The results are critically compared with the available theoretical and numerical estimates for the problem of dislocation motion in capped layers. This revised version was published online in July 2006 with corrections to the Cover Date.     

泡沫铝孔单元结构模型及压缩特性研究

泡沫金属是近年发展起来的一种新型结构功能材料.对熔体发泡法制备的泡沫铝进行压缩实验,测试了其压缩应力-应变曲线,探讨了压缩变形过程的机理.结果表明,泡沫铝压缩过程具有明显的三阶段变形特征,即弹性段、塑性变形平台段和压实段.压缩性能显示随泡沫铝孔隙率和孔径升高,屈服强度和压实强度下降,且随着颗粒增强剂SiCp的加入,泡沫铝强度得到较大提高.同时,初步建立起理想的孔单元结构模型,推导出孔隙率与孔结构(孔径、壁厚、孔分布)的关系.