增铺马赛克对约束阻尼结构阻尼性能的影响

为了提高刚度较小材料作为约束层的约束阻尼结构的阻尼性能,通过单点锤击试验研究了增铺马赛克约束阻尼结构的传递函数曲线和复合损耗因子等特性,探讨了增铺马赛克的方式以及层数对结构的单位力振动响应、复合损耗因子等阻尼性能的影响规律。结果表明:相较于传统的约束阻尼结构,正铺马赛克约束阻尼结构的单位力振动响应值为5.69 m/(s~2·N),降低了27.42%,复合损耗因子为0.1215,提高了29.81%,铺设2层马赛克约束阻尼结构的振动响应值仅为4.05 m/(s~2·N),复合损耗因子达0.2223,减振性能进一步提升,在两层马赛克之间增铺1 mm的阻尼层,振动响应值与复合损耗因子的变化均不明显。     

粘弹性约束阻尼层结构的动力学性能研究

研究粘弹性约束阻尼层结构（简称约束阻尼层结构）的动力学性能。结果表明：丁基橡胶可以较好地应用于约束阻尼层结构的阻尼层材料;约束阻尼层结构可以有效降低50～2 050 Hz范围内共振频率处的振动,同时共振频率向低频移动;随着阻尼层材料的损耗因子、储能模量和厚度的增大,约束阻尼层结构的结构损耗因子增大,振动衰减效果明显;约束阻尼层结构的振动衰减的仿真结果与试验结果一致。本研究可为约束阻尼层结构的设计和优化提供参考。     

阻尼层厚度对嵌入式共固化复合材料阻尼结构损耗因子的影响

用Ritz法研究了各应力分量的应变能,并计算了嵌入式共固化复合材料阻尼结构的损耗因子,得到了损耗因子随阻尼层厚度变化的规律。结果表明:当薄板总厚度不变,阻尼层厚度由2 mm增加到5.5 mm时,损耗因子随着阻尼层厚度的增大而增大,阻尼层较厚时,损耗因子对阻尼层厚度的变化不再敏感;当复合材料层厚度不变时,增加阻尼层的厚度可以使损耗因子增大,阻尼层厚度较厚时,阻尼层厚度的变化对损耗因子的影响较小;与复合材料层厚度不变时相比,薄板总厚度不变时,阻尼层较厚时对损耗因子的影响更小。     

聚氨酯泡沫层对隔离复合阻尼材料减振性能影响

以不同密度的硬质聚氨酯泡沫塑料作为隔离层,以D-803-Z型丁腈橡胶为阻尼层,制成一系列用于控制结构振动的聚氨酯-橡胶隔离复合阻尼材料。采用动态热机械分析法(DMA)分析了聚氨酯硬泡和橡胶材料的动态力学性能,并通过锤击实验从幅频曲线、复合损耗因子、模态频率等方面将敷设隔离复合阻尼材料的悬臂梁与橡胶自由阻尼悬臂梁进行了对比分析,探讨了4种不同密度的聚氨酯泡沫对隔离复合阻尼悬臂梁减振性能的影响。结果表明:聚氨酯泡沫损耗因子相对较小,其主要作用是扩大阻尼层的形变;相比于自由阻尼悬臂梁,隔离复合阻尼悬臂梁的前三阶模态振动响应降低了8%～52%,复合损耗因子提高了2～3倍;随着聚氨酯泡沫密度的增大,各阶模态的振动响应持续降低。聚氨酯-橡胶隔离复合阻尼材料能够有效降低结构的振动,适当提高聚氨酯泡沫层的密度有助于进一步改善材料的减振性能。     

改性空心玻璃微珠对约束层轻量化影响研究

为制备低密度、高模量的轻量化约束阻尼材料,使用A187对空心玻璃微珠表面进行改性处理,通过扫描电镜观察改性结果。进一步将改性后的空心玻璃微珠加入到约束阻尼涂料中,通过力学性能、极限氧指数、导热系数、复合损耗因子等测试,考察改性后的空心玻璃微珠对约束层各项性能的影响。结果表明：A187可以成功完成表面改性,随着空心玻璃微珠用量的增加,约束层密度降低、模量降低、黏度先降低后升高、氧指数提高、导热系数降低。添加6%改性空心玻璃微珠后,弯曲模量保持率为90.2%,密度降低16.67%,阻尼损耗因子整体温域仅少量降低,降低约束层密度的同时,保证了阻尼材料的降噪特性。     

高阻尼性涂层用环氧-丙烯酸酯复合乳液的合成

采用分步乳液聚合法合成了一种具有核-壳结构的环氧-丙烯酸酯复合乳液,通过核层和壳层之间的化学交联使涂膜具有较高的阻尼性能。通过透射电镜观察到乳胶粒子呈均匀的核-壳球形结构,红外光谱(FTIR)结果表明复合乳液涂膜实现了环氧与丙烯酸树脂的交联反应。动态热机械分析(DMA)显示该乳液涂膜具有较高的损耗因子和较宽的阻尼温域,加热成膜的最大损耗因子(tanδ)达到2.12,明显高于传统阻尼涂料,在tanδ0.3的阻尼温域为40℃,在金属阻尼涂层材料领域具有良好的应用潜能。     

高阻尼性涂层用环氧-丙烯酸酯复合乳液的合成

采用分步乳液聚合法合成了一种具有核-壳结构的环氧-丙烯酸酯复合乳液,通过核层和壳层之间的化学交联使涂膜具有较高的阻尼性能。通过透射电镜观察到乳胶粒子呈均匀的核-壳球形结构,红外光谱（FTIR）结果表明复合乳液涂膜实现了环氧与丙烯酸树脂的交联反应。动态热机械分析（DMA）显示该乳液涂膜具有较高的损耗因子和较宽的阻尼温域,加热成膜的最大损耗因子（tan δ）达到2.12,明显高于传统阻尼涂料,在tan δ >0.3的阻尼温域为40℃,在金属阻尼涂层材料领域具有良好的应用潜能。     

聚氨酯灌注量对蜂窝垫高自由阻尼结构减振性能影响

介绍了聚氨酯粘弹阻尼材料Qtech T501的力学性能。通过时域分析、传递函数分析等分析方法研究Qtech T501灌注量对蜂窝垫高自由阻尼结构(复合减振结构)的时域波形、各阶模态频率及损耗因子等减振性能的影响。结果表明,Qtech T501可以提高复合减振结构在350 Hz以下频段的减振性能,其二阶损耗因子提高61. 6%;增加灌注量会使上述复合减振结构模态频率向低频段移动;增加灌注量对结构高频段减振效果明显,但在低频段减振性能会出现小幅度下降。     

阻尼结构对复合结构阻尼性能的影响

对比自由阻尼结构和约束阻尼结构的阻尼性能,研究基板、阻尼层、约束层对约束结构阻尼性能的影响。结果表明：约束结构阻尼性能相较于自由结构更好;约束阻尼结构中,基板越薄,阻尼层和约束层越厚,复合结构阻尼性能越优异。     

阻尼结构对复合结构阻尼性能的影响

对比自由阻尼结构和约束阻尼结构的阻尼性能,研究基板、阻尼层、约束层对约束结构阻尼性能的影响。结果表明:约束结构阻尼性能相较于自由结构更好;约束阻尼结构中,基板越薄,阻尼层和约束层越厚,复合结构阻尼性能越优异。     

聚氨酯阻尼材料及其约束阻尼结构动态性能

分别采用聚己二酸新戊二醇酯–2,4-甲苯二异氰酸酯(TDI)–3,3′-二氯-4,4′-二氨基二苯甲烷(MOCA),聚己二酸新戊二醇酯–异弗尔酮二异氰酸酯(IPDI)–MOCA,聚丁二酸1115酯酯–TDI–MOCA体系合成3种聚氨酯阻尼材料——PU–1,PU–2,PU–3,并制备了3种四片型约束阻尼结构,进行动态性能试验,考察了加载频率、激励振幅、试验温度对约束阻尼结构的水平等效刚度、等效阻尼比的影响。结果表明,3种聚氨酯阻尼材料的损耗因子(tanδ)峰值均在0.8以上,其中PU–3的最高,为1.04,tanδ0.3的有效阻尼温域都在60℃以上;随激励振幅和试验温度的上升,3种聚氨酯约束阻尼结构的水平等效刚度均下降,随加载频率的增加,PU–2体系的水平等效刚度升高,而其它两种体系均下降;随激励振幅的上升,PU–2体系的等效阻尼比基本不变,而其它两种体系均先升高后下降,在试验温度范围内3种约束阻尼结构的等效阻尼比均在0.20以上。综合分析,以聚丁二酸1115酯酯–TDI–MOCA合成的聚氨酯阻尼材料及其约束阻尼结构综合性能最优。     

Expanded graphite–epoxy composites with high dielectric constant

Composites of an expanded graphite/diglycidyl ether of bisphenol A (DGEBA) were prepared by a simple melt blending method, and their dielectric and mechanical properties were investigated. During observations of fractured surfaces of the composites, the graphite sheets were seen to be homogeneously dispersed in the epoxy matrix. Moreover, the composites presented an enhanced dielectric constant (～ 180) and a low loss factor (～ 0.05) at 50 Hz, suggesting their potential suitability for embedded dielectric applications. The enhanced dielectric constant can be explained by the percolation theory and the relatively low loss factor was attributed to strong interfacial interactions between the polymer molecules and the ? OH/? COOH groups of the expanded graphite, which constrained the orientational polarization of the polarons. Furthermore, dynamic mechanical analysis of the composites showed a restricted macromolecular relaxation and improved mechanical properties. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

改性氧化石墨烯/丁基橡胶复合材料的制备和隔声性能研究

通过Hummers法并以硅烷偶联剂KH-550为改性剂制备改性氧化石墨烯(GO),以改性GO作为补强填料、丁基橡胶(IIR)为橡胶基体制备改性GO/IIR复合材料,并对其性能进行研究。结果表明:加入改性GO制备的复合材料的拉伸强度先增大后减小,损耗因子峰值增大,阻尼温域扩宽,隔声性能略有提升;复合材料可以有效抑制单层镀锌钢板的共振和吻合效应,基于其阻尼特性制备的约束阻尼隔声板隔声性能明显提高。     

Hydrodynamics of constrained inverse fludization and semifluidization in a gas-liquid-solid system

Experiments were conducted to study the hydrodynamic behavior of the constrained inverse gas-liquid-solid fluidized bed and the semifluidized bed where the liquid is the continuous phase. Also included in the experiments was a study of the hydrodynamic behavior of countercurrent flow of gas and liquid in a packed bed. This part of the study simulates the packed section of a constrained inverse semifluidized bed. A mathematical model is developed to account for the friction factor between the liquid and solid in the packed bed. The gas hold-up and friction factor of the packed bed are analyzed and empirically correlated.Wallis' drift flux model is modified to describe the gas hold-up in a constrained inverse fluidized bed. For the constrained inverse semifluidized bed,     

Burning rate of solid wood measured in a heat release rate calorimeter

Burning rate is a key factor in modeling fire growth and fire endurance of wood structures. This study investigated the burning rate of selected wood materials as determined by heat release, mass, loss and charring rates. Thick samples of redwood, southern pine, red oak and basswood were tested in a heat release rate calorimeter. Results on ignitability and average heat release, mass loss and charring rates are reported for a heat flux range between 15 and 55 kw m^?2. In this range, burning rate increased linearly with heat flux. Burning rate was very species dependent. Heat release rate was related to mass loss by effective heat of combustion, which also increased with heat flux. Charring rate was related to mass loss rate and original wood density. Important char property data such as yield, density and contraction are reported. A simplified calculation method is proposed for calculating mass loss rate and charring rate based on heat release rate.     

Molecular Origin of the Influence of the Temperature on the Loss Factor of a Solid Propellant

This study focuses on the viscoelastic behavior of an industrial hydroxyl‐terminated polybutadiene (HTPB) based solid propellant. The analysis of the loss factor as function of temperature enables the investigation of the molecular mechanisms participating in the nonlinear viscoelastic behavior. A design of experiments determines the influences of the filler fraction, of the NCO/OH ratio, of the plasticizer content, and of the presence or absence of filler‐binder bonding agents. For all the tested materials, the loss factor as function of temperature exhibits two distinct peaks when measured by Dynamic Mechanical Analysis. Exponentially modified Gaussian distributions are applied on each peak to characterize the behavior. While the first peak is commonly associated with the rubber‐glass transition of the material, the second peak has not been clearly associated with a molecular mechanism. This study shows that the second peak of the loss factor in HTPB‐based solid propellants originates from the flow of free polymer chains in the polymer network with a reptation mechanism. The sol polymer fraction controls the area of the second peak, whereas its temperature at the maximum corresponds to an activation temperature determined by the molar masses of the sol polymer. Finally, when the propellant is stretched, a decrease in area and an increase in the temperature of the peak show that the reptation of the sol polymer chains is constrained by the network.     

Dynamic mechanical analysis of fibers. II. Estimation of fiber orientation and characterization of heat set PET yarns

A drawn PET yarn was heat set (annealed) at temperatures between 110°C and 245°C in an unconstrained mode and the samples characterized by dynamic mechanical analysis (DMA). A method for estimating the fiber orientation factor (α) is proposed using DMA and shown to be more sensitive than the crystalline orientation (x-ray diffraction) or total orientation (birefringence) measurements of the heat set yarns. The extension/shrinkage behavior of the heat set yarns has been discussed in the light of morphological changes, e.g., degree of orientation and the micro-crystallite formation. Unlike in the unconstrained mode, heat setting under constraint does not lead to the formation of micro-crystallites as revealed by differential scanning calorimetry. As a consequence, although the modulus and degree of orientation increase upon constrained annealing, the thermal stability, i.e., loss of orientation (reflected by shrinkage) could not be improved.     

Effect of fly ash and nanosilica on compressive strength of concrete at early age

The replacement of cement by mineral admixtures in concrete has been of increasing interest in the construction industry. Nevertheless, several of the potential replacements, such as fly ash class F, lower the compressive strength of concrete at early age. This project investigates the use of nanosilica to compensate for such loss of compressive strength. A statistical experimental design involving mixtures of Portland cement, fly ash and nanosilica, in addition to water/binder ratio as an external factor, is proposed to study their combined effect on the compressive strength of concrete. This design allows estimating a cubic regression model that properly accounts for the effects of the mixture components within a constrained experimental region. The range of each factor was selected according to levels normally used in the industry. Finally, an optimisation strategy permits to recommend the use of nanosilica when high percentages of cement replacement by fly ash are present.     

Comparison of guarded and unguarded linear polarization CCD devices with weight loss measurements

Two types of linear polarization devices may be used to measure the corrosion current density (CCD) to assess the degree of active corrosion in concrete structures. The devices, with and without a guard electrode, provide significantly different CCD values for the same conditions. The 5-year study reported herein presents a comparison of the two devices over a range of CCD values from passive to highly active corrosion conditions for outdoor exposure slabs. The results demonstrate that both devices are able to qualitatively rank the instantaneous corrosion conditions in structures. The guarded electrode device measurements were less quantitatively precise than the unguarded electrode device. The guarded electrode device underestimated the amount of metal loss by a factor of 4-6. The unguarded electrode device overestimate the metal loss by a factor of about 1.5 when compared to weight loss measurements and adjusted for concrete temperature and resistance.     

Direct measurement of the constrained polymer region in polyamide/clay nanocomposites and the implications for gas diffusion

The recognition that polymer/clay nanocomposites impart improved barrier properties related to gas transport has prompted much discussion and research into the mechanism of this improvement. The plate like morphology of the clays was the attribute first seized upon as the dominant factor in improving barrier properties. A simple two dimensional model was proposed in which the clay plates acted as barriers to the gas diffusion thus making the effective path length longer. This model has been given the name of the “Tortuous Path Model”. Many polymer nanocomposites appear to follow this model simple model reasonably well. There are, however, many examples that deviate substantially from the model. A more complex model involving a constrained polymer region was proposed to accommodate the deviations from the “Tortuous Path Model”. This model fits much of the data in the literature but includes variables that have not been measured. These variables include the size and shape of the constrained region and the magnitude of the diffusion coefficient in the constrained region. This paper reports Atomic Force Microscopy work that has directly measured the size and shape of the constrained polymer region. The measured size of the constrained region leads to a method of identifying the diffusion coefficient for the constrained region from experimental permeability data. The implications for wider applications of the model are discussed.