A Resonant Cavity Approach for the Nondestructive Determination of Complex Permittivity at Microwave Frequencies

A nondestructive microwave cavity approach for measuring complex permittivities of materials in sheet form is described. The resonant cavity is a section of a rectangular waveguide terminated by a thin rigid and large flange containing a small rectangular iris opening. The iris is placed in firm contact with one side of the dielectric sample while the other side is backed with a highly conducting plate. Variations of the cavity resonant frequency and Q-factor caused by the dielectric can be related to its complex permittivity through the consideration of equivalent admittance of this open-ended dielectricloaded aperture at resonance. Experimental determination of aperture admittance of a loaded iris is made and the results compared with theoretical calculations. The validity of this technique is confirmed by evaluating the resonant cavity characteristics by loading it with dielectrics of known permittivities and comparing the results with theoretical results. The permittivity of a lossy dielectric slab is measured and the value obtained by this method is compared with those found by other techniques. In all these cases the agreement between theory and measurements is satisfactory. Utility of this technique in evaluating the local inhomogeneities of permittivity of sheets is demonstrated. Measurement errors and limitations of this technique are pointed out.     

水泥混凝土路面板早龄期翘曲行为数值分析研究

以1/4面板结构模型程序为基础,考虑基层与面板的摩擦界面作用,开展了足尺路面板结构的三维早龄期翘曲有限元数值模拟研究,并与现场实验进行校验。对比显示,相比1/4面板模型,考虑基层摩擦接触界面的全面板模型程序板角位移计算值减小,日变化量级、趋势与试验监测结果更为接近,面板-基层界面参数对计算迭代收敛性影响较大。不同的工况组合下,由于早龄期的历程效应路面板会产生很多复杂的行为特征,计算仿真显示面板早龄期存在板角-1/4板中-板中的翘曲变形迁移运动,不同条件的边界约束导致演化形成不对称面板翘曲。在夏季28 d的记忆-松弛-记忆多次叠加历程作用下,路面板一般将产生正的固化温度差性状。由于混凝土早龄期行为的复杂性,早龄期数值模拟技术仍需研究模型优化、材料早龄期参数特性以及施工工艺的影响。     

温度与荷载耦合下农村公路混凝土路面板结构尺寸优化

为优化农村公路混凝土路面板结构设计以减小路面板厚度和节约成本,对板尺寸和板厚进行研究,分析正负温度梯度下交通荷载的最不利位置,建立文克尔地基上的板模型进行计算。研究结果表明：相比于板厚,板平面尺寸对板中最大拉应力的影响更大,因此可通过减小板尺寸来减小板厚;将只承受轮载作用的板称为小尺寸板,其临界板尺寸为2.0 m×2.0 m;路基反应模量对小尺寸路面板的板中最大拉应力影响不大;考虑疲劳损伤的小尺寸板在正、负温度梯度(15℃和-15℃)下的临界板厚分别为19 cm和16 cm。研究结果可以为节约成本为目的农村公路路面板结构尺寸优化设计提供参考。     

Calibration and measurement of dielectric properties of finitethickness composite sheets with open-ended coaxial sensors

The application of open-ended coaxial sensors for dielectric measurement of finite thickness composite sheets is studied. Expressions for calculation of the complex aperture admittance for two geometries are presented. These expressions are used to calculate the dielectric constant of infinite half-space as well as finite thickness slabs. A more efficient method of such calculations, using a personal computer, for low to medium loss dielectrics is demonstrated. The question of when a dielectric layer may be considered as infinitely thick is also addressed, and examples are presented. A different calibration technique (compared to the conventional ones) is described and successfully implemented. This calibration technique utilizes a dielectric sheet with known dielectric properties and thickness. Measurements for different airgaps between the open-ended coaxial line and the dielectric sheet are used to perform and enhance the calibration. The results of this calibration technique and several subsequent measurements are presented and discussed     

A five-region model for the edge-guided mode ferrite-loaded stripline isolator

A five-region model for the inhomogeneously ferrite-loaded strip/microstrip isolator with loss loading on one side of the ferrite slab is presented. A ferrite slab between two infinite perfectly conducting planes is sided by a lossy sheet and a dielectric slab. The bias magnetization is taken to be perpendicular to the ground planes. The problem is solved approximately by replacing the lossy film by suitable impedance boundary conditions.     

A general formulation for admittance of an open-ended rectangular waveguide radiating into stratified dielectrics

A general approach is presented for calculating the aperture admittance of a rectangular waveguide radiating into layered dielectric media. The two specific geometries of stratified, lossy dielectric media that are addressed terminate into either an infinite half-space or a perfectly conducting surface. The geometries describe two prevalent categories of layered dielectric composites and coatings that often are encountered in practical microwave nondestructive evaluation applications. Solutions are found initially by constructing a complete set of field components and subsequently enforcing the continuity of power flow across the aperture interface of the waveguide. Final results are presented as a superposition of transverse electric and magnetic components of the aperture admittance. The solutions presented allow the systematic calculation of admittance in the presence of arbitrary multilayer media, which in turn may be related to experimentaly measurable quantities of interest. With the practical assumption of dominant mode incidence on the aperture, the final expressions may be implemented without intense computational power, which often is desirable in practice where inaccuracies due to random errors and instrumentation sensitivity render incorporation of more rigorous solutions inefficient. Numerically simulated data also are presented to verify and interpret the results.     

模板支撑薄弱层对现浇钢筋混凝土建筑结构施工荷载分布的影响

对现浇钢筋混凝土建筑结构施工期间,模板支撑系统架设中可能存在的薄弱层对施工荷载分布的影响,提出了简化分析方法,并用工程实测结果进行了验证,表明模型计算结果的正确性.分析表明,模板支撑薄弱层将导致模板支撑薄弱层的上部楼板承担的施工荷载增大,而模板支撑薄弱层下部楼板承担的施工荷载减小;实测表明,模板支撑薄弱层上的楼板承担的施工荷载同比增大约4.3%,薄弱层下的楼板承担的施工荷载减小约3.9%.施工期间应避免模板支撑薄弱层的出现,以防模板支撑薄弱层可能带来的风险.     

四边简支钢筋桁架混凝土叠合板抗火性能试验研究

钢筋桁架混凝土叠合楼板由预制混凝土底板、钢筋桁架及现浇混凝土后浇层组成。预制板块间设置混凝土现浇带,受力钢筋在现浇带处搭接连接,形成整体楼板。通过4块四边简支钢筋桁架混凝土叠合板抗火试验,研究了叠合板在火和荷载耦合作用下的火灾行为,试验中考虑了不同后浇叠合层厚度和不同预制板拼装方式。结果表明:钢筋桁架混凝土叠合板在高温-荷载耦合作用下,预制底板与叠合层未产生明显脱离现象,火灾时叠合板仍具有较好的整体工作性能;板上表面沿现浇带与预制板结合面及钢筋桁架将产生纵向裂缝,最终结合面形成贯通裂缝,楼板被烧穿达到耐火极限,试验板的耐火极限均达到3 h;火灾时现浇板带板底混凝土爆裂严重,现浇带处采用搭接方式连接的钢筋外露,失去作用,双向板受力机理发生变化,形成由预制板和现浇带组成的沿预制板跨方向的单向板带,承载力大幅降低,火灾时板的剩余承载力宜按单向板计算;后浇层厚度小,预制板宽大、拼缝少,耐火极限短;钢筋桁架对保证火灾时叠合层与预制层共同工作起到关键作用。     

Behaviour of surface oxide scale before roll bite in hot rolling of steel

Abstract

Hot stalling rolling tests using mild steel slabs with thin and thick scale layers, and cold stalling rolling simulation tests using lead slabs coated with brittle lacquer layers, have been conducted to investigate the behaviour of the surface oxide scales before hot rolling. The effects of rolling reduction, temperature, scale thickness and structure, and slab geometry were examined. The experimental results indicate that the oxide scale can either adhere and deform with the parent steel, or delaminate from the parent steel, or suffer from cracking before hot rolling, depending on the hot strength of the scale and the stress status in the scale, which depend on rolling reduction and temperature, and scale thickness and structure. Therefore, the cracks observed in the oxide scales on the steel slab after hot rolling are generally the combined results of cracking before rolling and further cracking in the roll gap. On the basis of comparative analysis, the prerolling behaviour of the oxide scales in a laboratory scale rolling mill and in an industrial hot strip mill is discussed.     

CFRP增强工程水泥基复合材料桥面连接板的结构和性能

桥梁面板的伸缩缝连接装置是现今桥梁施工与维护的难题之一。桥面无缝连接板是将相邻的桥面板连接在桥墩上,形成桥跨间的连续板。结合碳纤维增强聚合物基复合材料（CFRP）良好的耐腐蚀性与工程水泥基复合材料（Engineered Cementitious Composites,ECC）的高延性,对CFRP材料增强ECC桥面连接板进行研究。对比了CFRP软质格栅、CFRP筋材和纯ECC桥面连接板的荷载-位移曲线、裂缝分布、应变以及变形,分析三种连接板的连接性能。试验结果表明,三种桥面连接板能满足工程变形与开裂要求。CFRP软质格栅削弱连接板材料界面,降低板多裂缝开展,对增强桥面连接板作用较小。相对其他ECC连接板构件,CFRP筋材对ECC连接板增强作用明显,其弯曲变形能力显著提高,裂缝均匀且分布广泛,整体工作性能良好。      

预应力混凝土叠合空心楼板的受剪性能试验研究

结合现浇空心楼盖与叠合楼盖的优点,提出在一种新型预应力叠合楼板中布置筒芯内模以形成叠合空心楼板。进行了3组共10块模型板带的板端抗剪试验,根据实测试件的荷载位移曲线、受剪承载力及板中箍筋应变变化情况,研究了筒芯内模布置方式、预制底板形式、抗剪箍筋、肋梁及顺筒肋对叠合空心板受剪性能和自然粗糙叠合面粘结强度的影响。研究结果表明:顺筒布置的叠合空心板抗剪承载力高于横筒布置的对比试件;预制带肋底板的预制纵肋能有效提高叠合空心试件的抗剪性能和叠合面水平抗剪强度;无论筒芯布置方式和预制底板形式如何变化,叠合层混凝土与预制底板的叠合面粘结强度均满足水平抗剪要求;依据试验结果,提出了考虑不同混凝土强度等级、叠合面高度、肋梁和顺筒肋影响的叠合空心板受剪承载力计算方法。     

隔声板应用于混凝土阳台栏板隔声改造可行性分析

考虑既有住宅建筑阳台栏板为薄混凝土板可能存在隔声性能不足的风险,在降噪隔声改造过程中,为有效避免资源浪费及增加建筑垃圾,探索采取增设隔声板提升阳台栏板整体隔声性能的可行性。实验设计了3种不同厚度的混凝土板,实验室内测试其增设隔声板前后的隔声量。研究结果显示,随着厚度增加,混凝土板的隔声量有所提高,60~70mm厚混凝土板的交通噪声频谱修正后隔声量达到40dB以上;混凝土板临界频率位于160~250Hz频率范围,在临界频率以上范围基本符合质量定律;增设隔声板可一定程度提高混凝土板的隔声性能,但提升幅度差异性较大,一定范围内,增加混凝土板厚度对隔声性能提升效果较增设隔声板更有效。     

预制UHPC模板及采用预制模板的RC板受力性能及承载力分析

采用超高性能混凝土（UHPC）制作厚度为10 mm的模板,对UHPC模板进行施工阶段加载试验,评估其作为现浇楼板的模板的可行性;以UHPC模板作为底模,分别制作了6个简支单向板和1个两跨连续单向板,对其进行静力加载试验。结果表明,厚度为10 mm的UHPC板作为建筑模板时,模板下的支撑间距可取0.5 m,模板处于弹性状态下可施加的均布荷载为6 kN/m²,约为施工均布活荷载设计值的1.7倍。以UHPC作为模板的钢筋混凝土（RC）单向板,在板破坏时,UHPC模板与后浇混凝土界面未出现肉眼可见的粘结滑移现象;其受弯承载力约为相同截面尺寸及配筋的RC板的2倍。考虑UHPC模板及受拉区混凝土的受拉作用,建立了这种RC单向板的受弯承载力计算模型,模型的计算值与试验值符合较好。     

钢筋混凝土楼板火灾反应数值计算模型

基于退化壳原理,考虑火灾下板壳截面的不均匀温度场分布而引入分层模型,同时在每分层上考虑材料在不同温度下的热力弹塑性本构关系,建立了火灾下钢筋混凝土板壳结构的有限元数值计算模型。另外,通过全拉格朗日方法考虑了大位移的几何非线性影响。最后通过一钢筋混凝土板在高温下的试验进行了验证,并分析了配筋率和保护层厚度的影响。结果表明:提出的火灾下钢筋混凝土壳单元数值计算模型的计算结果与试验结果吻合较好,可以用来分析火灾下钢筋混凝土框架结构楼板的反应。     

Inhomogeneous Dielectric Slabs with Analytic Frequency-dependent Permittivities

We consider a general class of inhomogeneous dielectric slabs, where the dielectric permittivity (l) is assumed to depend on the frequency (y) at each position, through the Kramers-Krönig dispersion relation. The physical basis for this assumption is discussed and the following general consequences of it are established. First, using integral relations for propagation modes, we prove the absence of the latter for physically unwanted values of y and of the propagation constant. Secondly, we treat the transmission and reflection of electromagnetic waves by the slab through rigorous eikonal methods, and prove that improvement of convergence does occur at high y. Thirdly, we derive analytic representations for the phases of the transmission and reflection amplitudes in terms of their moduli and their complex zeros in the y -plane, at normal incidence. Finally, we present a formal construction of l at high y, in terms of the reflection amplitude at normal incidence, thus giving a partial mathematical solution for the inverse scattering problem. Most of the previous results break down if l is y -independent. We also study the dispersion relation for the propagation mode, when the slab is a monomode optical waveguide.     

分离式叠合板组合梁抗火性能研究与数值分析

为了研究分离式叠合板组合梁的火灾行为,对四块足尺分离式叠合板组合梁进行了均布荷载作用下的火灾试验。考虑了栓钉间距、后浇层厚度、预制板在钢梁上翼缘搭接长度的影响因素,研究了组合梁在受火过程中沿板厚混凝土温度场分布规律、混凝土板中钢筋温度、沿钢梁高度温度场分布规律、抗弯刚度以及叠合板与钢梁的整体工作性。并利用ABAQUS对叠合板组合梁在热力耦合作用下的温度场和变形行为进行了数值模拟分析。结果表明：升温过程中,后浇层与预制底板结合界面未发生明显破坏,仍能共同承受荷载;但由于结合界面和预制板拼缝的存在,组合梁的整体抗弯刚度明显降低;热力耦合作用下,预制板在钢梁上翼缘搭接长度对组合梁的变形能力影响显著;对比试验研究结果与数值分析结果,二者吻合良好,验证了数值分析模型的有效性与可行性。     

混合结构房屋现浇楼板裂缝的有限元数值分析

现浇钢筋混凝土楼板开裂是混合结构房屋常遇到的问题之一。在对若干实际工程调查和模型试验的基础上,用有限元程序建立了两种结构形式的有限元模型,进行了非线性有限元分析。有限元分析结果包括混凝土主拉应力及应变、楼板裂缝和钢筋应力的分布和发展,其与试验结果符合程度较好。研究结论给出了楼板开裂的主要原因和影响因素、裂缝形态及范围、控制裂缝的关键措施和楼板钢筋合理的布置方式。     

声子晶体平板组合成像特性分析

用多重散射方法分析了声子晶体单平板和多平板的成像特性。发现：在特定频率范围内,引入等效负折射率后,单板负折射成像可用反Snell定律描述;多层声子晶体平板成像服从依次成像规律,可类比传统几何光学中共轴球面系统的成像,引入实物实像、虚物虚像等概念;平板透镜不改变出射波的出射方向,仅使出射位置产生平移;多平板透镜的累加成像与等厚度的单一平板透镜成像具有相同的效果。平板透镜的这些成像特点,使其在成像质量、加工方法、系统组合等方面比球面透镜更具优势。     

Influence of snowpack layering on human-triggered snow slab avalanche release

Dry-snow slab avalanches involve the release of a cohesive slab over an extended plane of weakness. In most fatal avalanches, the triggering of the initial failure occurred by localized rapid near-surface loading by people — followed by fracture propagation. Whereas a limit-equilibrium (LE) approach to snow slope failure only takes into account slab depth, slab density and weak layer strength, it omits properties such as the stiffness of adjacent layers and the fracture propagation process. Nevertheless, LE has been applied with some success to the frequency of skier triggering, suggesting that it is relevant to failure initiation. Since field studies have shown that, for a given slab thickness, stiffer slabs are less likely to be triggered, slab properties influence failure initiation, fracture propagation or both. A highly simplified finite element (FE) model of static skier loading was used to assess the effect of slab and substratum properties on skier-induced stresses in the weak layer. Compared to a uniform slab, the skier-induced stress at the depth of the weak layer varied by a factor of 2 due to layering. In particular, the simplified FE model suggests that while stiffer layers in the slab will reduce the skier-induced stress in the weak layer, stiff layers just below the weak layer can increase the shear stress. These results were incorporated into a modified stability index and compared to stability test results. However, by taking into account snowpack layering the correlation between the modified stability index and stability test results did not improve. While our simulations suggest that less stress penetrates through stiffer slabs and thus fracture initiation is less likely, other studies show that, once initiated, fractures under stiffer slabs have high propagation propensity.