Analysis of tests on impact strength of concrete armor blocks

This paper summarizes a series of tests on the impact strength of large concrete breakwater cubes. A simple, design-oriented, analytical model was established on the basis of the continuum damage theory in order to interpret the data collected during a rather ambitious and expensive experimental program involving a large number of cubes of different sizes subjected to impacts of varying intensity. The suggested model involves only one additional material parameter related to the static strength of the concrete specimen in uniaxial compression. A rather surprising accuracy of this fairly crude model is further encouragement for the use of continuum damage mechanics for concrete.     

Experimental limit on spontaneous parametric up conversion

Abstract

We present a first experimental test of the existence of spontaneus parametric up conversion predicted in Marshall and co-workers [2000, J. mod. Optics, 47, 1273]. The measurement has been made using a ccd camera on the emission of a LiIO₃ crystal pumped by a 351 nm and/or a 789 nm laser. We obtain an upper limit of 160 times on the ratio between the intensity of spontaneous parametric up conversion and spontaneous parametric down conversion.     

基于多维性能极限状态的概率地震需求分析

基于概率地震需求分析(PSDA),分别采用增量动力和非线性时程分析,得到某框架结构的最大层间漂移比和最大加速度响应,通过定义多维性能极限状态的性能水准,计算该结构的多维地震易损性,联合地震动危险性曲线,建立了年平均超越概率的三重积分公式,采用梯形法求得50年内地震需求(漂移)危险性曲线。在此基础上,进行了同时考虑性能极限状态的随机性和相关性对结构需求危险性的敏感性分析。在性能极限状态不确定性中选择适当的变异系数(cidrcpfa)及相互作用因子NID R,能够使年平均超越概率增加;相比单一极限状态,考虑二维极限状态的年平均超越概率也将提高。研究结果表明,所提方法可描述对多维响应参数敏感的结构破坏行为,可获得设计基准期内更加符合实际的结构需求危险性曲线,为震后损失估计提供可靠的理论依据。     

Effects of alloying in the dilute limit on the quantum states of electrons in magnesium

The electron quantum interference phenomenon was used to determine the effects of substitutional alloying upon the quantum-state lifetime and band gapsE _g at the Fermi energy in single crystals of pure Mg. Vapor-grown alloys containing either Zn or Cd in concentrationsC0.3–15 ppm were studied. The magnetic field dependence of the interference oscillation amplitudes for these samples indicates reductions in and significant increases inE _g relative to pure Mg (impurity concentration 10^–8). Within experimental accuracy the observed quantum-state lifetime satisfies the relation ^–1=C, with 2.7×10¹⁰ sec^–1 per ppm of Cd and 2×10¹¹ sec^–1 per ppm of Zn. The band gap corresponding to Bragg reflection from the (0001) plane of the hcp structure (which in pure Mg arises solely from spin-orbit coupling) was found to increase by more than a factor of two upon the addition of only 10 ppm Cd to Mg. These results are discussed within the framework of the pseudopotential theory of alloying. It is shown that there are discrepancies of more than three orders of magnitude between the experimental and theoretical values forE _g and of about one order of magnitude for in these dilute-limit alloys.Work supported by the National Science Foundation.Submitted to the Department of Physics, the University of Chicago in partial fulfillment of the requirements for the degree of Doctor of Philosophy.Fannie and John Hertz Foundation Fellow.     

小量块国际比对及能力验证

按照国际比对组织机构提供的比对样品及技术指标,采用比较法比对测量原理,完成了国际间小量块比对任务.建立的测量不确定度分析模型,其扩展不确定度置信概率按国际间要求统一选取95%,包含因子选用2的原则,较以前国内扩展不确定度评定范围有所缩小,并增加了对测量环境和测量设备的技术要求.此次国际比对证明我们的检测数据全部有效、可信,其评定方法得到了APLAC组织的认可.     

Experimental and analytical studies on the prediction of forming limit diagrams

Metal forming processes are widely used in industrial productions, automobile bodies, food industries, oil refineries, and liquid and gas transmission systems. Analyzing these processes is very important to reduce wastes and optimize the processes. Study of some main factors such as physical and mechanical properties of material and its formability, die geometry, die material, lubrication and pressing speed has been the topic of many research projects. In this paper, forming limit diagrams (FLDs) for LC and ULC steels and the effect of different parameters like the work-hardening exponent, n, and the plastic strain ratio, r, on these diagrams have been evaluated and simulated using ABAQUS/Standard. In this case, Hill’s quadratic anisotropy function is assumed to be the yield function and the Atkins criterion is used as the failure criterion.     

Monte Carlo homogenized limit analysis model for randomly assembled blocks in-plane loaded

A simple rigid-plastic homogenization model for the limit analysis of masonry walls in-plane loaded and constituted by the random assemblage of blocks with variable dimensions is proposed. In the model, blocks constituting a masonry wall are supposed infinitely resistant with a Gaussian distribution of height and length, whereas joints are reduced to interfaces with frictional behavior and limited tensile and compressive strength. Block by block, a representative element of volume (REV) is considered, constituted by a central block interconnected with its neighbors by means of rigid-plastic interfaces. The model is characterized by a few material parameters, is numerically inexpensive and very stable. A sub-class of elementary deformation modes is a-priori chosen in the REV, mimicking typical failures due to joints cracking and crushing. Masonry strength domains are obtained equating the power dissipated in the heterogeneous model with the power dissipated by a fictitious homogeneous macroscopic plate. Due to the inexpensiveness of the approach proposed, Monte Carlo simulations can be repeated on the REV in order to have a stochastic estimation of in-plane masonry strength at different orientations of the bed joints with respect to external loads accounting for the geometrical statistical variability of blocks dimensions. Two cases are discussed, the former consisting on full stochastic REV assemblages (obtained considering a random variability of both blocks height an length) and the latter assuming the presence of a horizontal alignment along bed joints, i.e. allowing blocks height variability only row by row. The case of deterministic blocks height (quasi-periodic texture) can be obtained as a subclass of this latter case. Masonry homogenized failure surfaces are finally implemented in an upper bound FE limit analysis code for the analysis at collapse of entire walls in-plane loaded. Two cases of engineering practice, consisting on the prediction of the failure load of a deep beam and a shear wall arranged with random texture are critically discussed. In particular, homogenization results are compared with those provided by a heterogeneous approach. Good agreement is found both on the failure mechanism and on the distribution of the collapse load.     

Exciton-like trap states limit electron mobility in TiO2 nanotubes

Nanoparticle films have become a promising low-cost, high-surface-area electrode material for solar cells and solar fuel production. Compared to sintered nanoparticle films, oriented polycrystalline titania nanotubes offer the advantage of directed electron transport, and are expected to have higher electron mobility. However, macroscopic measurements have revealed their electron mobility to be as low as that of nanoparticle films. Here, we show, through time-resolved terahertz spectroscopy, that low mobility in polycrystalline TiO(2) nanotubes is not due to scattering from grain boundaries or disorder-induced localization as in other nanomaterials, but instead results from a single sharp resonance arising from exciton-like trap states. If the number of these states can be lowered, this could lead to improved electron transport in titania nanotubes and significantly better solar cell performance.     

Fatigue limit and life evaluation formulae for compressive mean stress states

Considering the difference of stress concentration behaviour near micro-defects in materials, a unified fatigue limit evaluation formula has been developed, which can be applied to compressive mean stress states without the necessity of introducing additional material’s properties except for the modification parameter of mean stress. A generalised life evaluation formula has also been proposed and the dependence of fatigue life on mean stress can be simply expressed by the effect of fatigue limit corresponding to the mean stress. By considering the effect of fatigue limit corresponding to the compressive mean stress, the fatigue life can be evaluated by the same generalised formula as that developed for tensile mean stress state, without the necessity of carrying out additional S–N curve fatigue tests.     

Evaluating small failure probabilities of multiple limit states by parallel subset simulation

A novel subset simulation algorithm, called the parallel subset simulation, is proposed to estimate small failure probabilities of multiple limit states with only a single subset simulation analysis. As well known, crude Monte Carlo simulation is inefficient in estimating small probabilities but is applicable to multiple limit states, while the ordinary subset simulation is efficient in estimating small probabilities but can only handle a single limit state. The proposed novel stochastic simulation approach combines the advantages of the two simulation methods: it is not only efficient in estimating small probabilities but also applicable to multiple limit states. The key idea is to introduce a “principal variable” which is correlated with all performance functions. The failure probabilities of all limit states therefore could be evaluated simultaneously when subset simulation algorithm generates the principal variable samples. The statistical properties of the failure probability estimators are also derived. Two examples are presented to demonstrate the effectiveness of the new approach and to compare with crude Monte Carlo and ordinary subset simulation methods.     

Experimental comparison of two methods for the measurement of long gauge blocks

A comparison of two methods of measurement of long gauge blocks is performed. It is shown that the relative divergence of the measurement results obtained by a method involving counting of the interference bands between the centers of gravity of the achromatic bands and a method involving matching fractional parts of the orders of interference amounts to less than 10⁻⁸. __________ Translated from Izmeritel’naya Tekhnika, No. 7, pp. 20–23, July, 2007.     

Experimental bond tests on masonry panels strengthened by FRP

The results of an experimental campaign on bond between Glass Fiber Reinforced Polymer (GFRP) sheets and single clay brick or masonry panel is presented. Four different types of clay bricks (new and ancient) are considered, where the difference between bricks is not only due to their mechanical properties but also to their surface texture. Another focus point of the experimental campaign is the effect of mortar joints on the GFRP-masonry panel bond. Moreover, the effects of different surface preparations on the debonding load were investigated, concerning both bricks and masonry panels. A total number of 38 specimens was tested and results in terms of debonding force, strain along the GFRP and failure modes are here reported. The experimental results were also compared to design formula proposed by the new version of Italian Guidelines. Furthermore, in order to numerically describe the bond behaviour of the specimens tested, non-linear interface laws were calibrated starting from the debonding load and the measured strains along the GFRP for various loading levels.     

多维性能极限状态下基于模糊失效准则的结构概率地震风险分析EI北大核心CSCD

考虑了结构多维性能极限状态的模糊不确定性,建立基于模糊失效准则的概率地震风险分析方法。综合考虑结构和非结构构件的抗震性能,建立多维性能极限状态方程。选择三种隶属函数描述极限状态的模糊性,包括降半梯形分布、降岭型分布和二次抛物型分布,基于地震烈度概率模型,建立基于模糊失效准则的概率地震风险分析表达式。基于SAP2000建立RC框剪结构模型进行分析。研究表明:基于二次抛物型分布的年平均超越概率会随着隶属区间的扩大而减小,而基于降半梯形分布和降岭型分布的年平均超越概率会随着隶属区间的扩大而增大;若隶属区间相同,三种隶属函数的年平均超越概率,从大到小依次为降半梯形分布、降岭型分布、二次抛物型分布;忽略不同工程需求参数性能极限状态的相关性会使年平均超越概率偏低。     

Experimental study of transport in nanoscale planar MOSFETs near the ballistic limit

Using standard planar 22-nm-nMOS devices featuring a typical metalurgical length lower than 10 nm, we investigated the impact of nanoscale phenomena on device behavior. The data show that direct source-drain tunneling dominates the device leakage below 250 K and that carrier transport approaches the ballistic limit. The significant contribution of access resistance is revealed by low-temperature measurements below the superconducting transition of the cobalt disilicide contacts.     

多维性能极限状态下概率地震需求分析的多元相关核密度估计法

本文不对工程需求参数（EDP）的分布类型进行人为假定,提出基于多元相关核密度估计的概率地震需求分析法。在带宽矩阵和多元高斯核函数中分别引入相关系数,并分别采用三种相关系数描述相关性,将传统核密度估计拓展到可以考虑随机变量相关性的多元核密度估计。基于SAP2000建立某钢筋混凝土框剪结构,选择最大层间位移角、最大层加速度衡量多维性能极限状态。在不同峰值地面加速度（PGA）下建立基于多元相关核密度估计的概率地震需求模型,并给出基于蒙特卡洛（MC）模拟的地震需求公式,得到结构需求的年平均超越概率。采用传统基于多维对数正态分布假定的地震风险概率法和不考虑EDP相关性的核密度估计进行对比,研究表明：与传统的多维对数正态分布假定相比,基于相关多元核密度估计的结构需求年平均超越概率偏大,而不考虑相关性的多元核密度估计所得年平均超越概率偏小;不同相关系数会影响到年平均超越概率的大小,其中Pearson相关系数影响最大,Spearman相关系数次之,Kendall相关系数最小。     

Experimental identification and mechanical interpretation of the interaction behaviour between concrete paving blocks

In recent years, concrete block (CB) pavements have become a favourite alternative to asphalt pavements, mainly in intra-urban regions due to their architectural design possibilities. Unfortunately, this trend is restrained by a lack of adequate design methods to assess the load capacity and durability of such pavements. Especially, the mechanical performance of the vertical joints between CBs is often not depicted realistically enough. For this reason, in this work three new experiments are proposed to determine the mechanical behaviour of the joints between the CBs, and thus the load transmission capability of different joint formations. Mechanical models and the corresponding material parameters to describe the joint behaviour are identified from the experimental results. Finally, performance optimisation of block pavements with respect to their jointing behaviour should become possible.     

Estimation of the endurance fatigue limit for structural steel in load increasing tests at low temperature

The endurance fatigue strength of structural steel S355 was investigated in fatigue tests according to the method of increasing stress amplitude. The so‐called ‘load increasing test’ is based on the direct correlation between the fatigue limit and the temperature changes caused by local plastic deformation ahead of the tip of a micro crack, which was initiated as a result of cyclic loading. In the present work the fatigue limit for testing temperatures 40°C and ?20°C was estimated not only from the temperature measurements but also from the electrical potential measurements. Further, the obtained results were validated in standard fatigue tests with constant stress amplitude and a very good agreement was found.     

Simulation of thermally stressed state and evaluation of the fire-resistance limit for a wall of “Besser” concrete blocks

We carry out a numerical investigation of the thermally stressed state and make a theoretical evaluation of the fire-resistance limit for a wall of vibrocompressed concrete blocks. We perform calculations on the basis of the method of finite elements. Different variants of thermal loading under ordinary fire conditions are considered. Conclusions are drawn concerning the possibility of using blocks for constructing three-layered walls. Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 70, No. 3, pp. 480–486, May–June, 1997.