The Effects of Stacking Sequence on Dynamic Mechanical Properties and Thermal Degradation of Kenaf/Jute Hybrid Composites

This research focused on the dynamic mechanical and thermal properties of woven mat jute/kenaf/jute(J/K/J)and kenaf/jute/kenaf(K/J/K)hybrid composites.Dynamic mechanical analysis(DMA)and Thermo-gravimetric Analysis(TGA)were used to study the effect of layering sequence on the thermal properties of kenaf/jute hybrid composites.The DMA results;it was found that the differences in the stacking sequence between the kenaf/jute composites do not affect their storage modulus,loss modulus and damping factor.From the TGA and DMA results,it has been shown that stacking sequence has given positive effect to the kenaf/jute hybrid composite compared to pure epoxy composite.This is because kenaf and jute fibre has increased the Tg values of the composites,thus affect the thermal degradation.Results showed that the storage modulus for kenaf/jute hybrid composites increased compared with pure epoxy composites with increasing temperature and the values of remained almost the same at glass transition temperature(Tg),the hybrid composite perhaps due to the improved fibre/matrix interface bonding.The preliminary analysis could provide a new direction for the creation of a novel hybrid composite which offers unique properties which cannot be accomplished in a single material system.

Dynamic Mechanical and Thermal Properties of Cross-linked Polystyrene/Glass Fiber Composites

Cross-linked polystyrene/glass fiber composites were fabricated using cross-linked polystyrene （CLPS） as matrix and E-glass fiber as the reinforcement. Surfaces of E-glass fibers were modified by vinyl triethoxysilane （VTES）, vinyl trimethoxysilane （VTMS） and γ-methacryloylpropyl trimethoxysilane （MPS）. The treated glass fibers were analyzed by fourier transform infrared spectroscopy （FTIR）. Dynamic mechanical thermal analysis （DMTA） and thermo-gravimetric analysis （TGA） were employed to investigate the effect of glass fibers surface modification on viscoelastic behavior and thermal properties. The morphology of fracture surfaces of various composites was observed by scanning electron microscopy （SEM）. The results revealed that these coupling agents were connected to the surfaces of the fibers by chemical bonding. Dynamic mechanical properties as well as thermal stability of the composites were improved considerablely, but to varying degrees depending on the fiber modification. The diversities of improvement of properties were attributed to the different interfacial adhesion between CLPS matrix and the glass fibers.

Temperature Dependency of Dynamic Mechanical Properties of Cement Asphalt Paste by DMTA Method

We investigated the temperature dependency of the dynamic mechanical properties of cement asphalt paste by the dynamic mechanical thermal analysis（DMTA） method. The experimental results show that the dynamic mechanical properties of cement asphalt pastes are sensitive to temperature due to the inclusion of asphalt, and may go through different states within a temperature range of-40 ℃ to 60 ℃, which is different from that of pure cement and asphalt. As the temperature of the cement asphalt paste increases, a considerable change of dynamic mechanical properties, including storage modulus（E＇）, loss modulus（E＇＇） and loss factor（tand） is observed. Moreover, the influence of asphalt to cement（A/C） ratio on the temperature sensitivity of the dynamic mechanical properties of cement asphalt composites was investigated. The temperature dependency of cement asphalt composites is ascribed to the temperature dependency of the asphalt and its interaction with cement paste. A simple fractional model is proposed to describe the viscoelastic behavior of cement asphalt composites.

Analysis and Design of Thermally Actuated Micro-Cantilevers for High Frequency Vibrations Using Finite Element Method

Vibrational behavior of thermally actuated cantilever micro-beams and their mechanical response at moderately high frequency under a non-harmonic periodic loading is studied in this paper. Two different configurations are considered: 1) a straight beam with two actuation layers on top and bottom which utilizes the bimorph effect to induce bending;2) a uniform beam with base excitation, where the beam is mounted on an actuator which moves it periodically at its base perpendicular to its axis. Generally, vibrating micro-cantilevers are required to oscillate at a specified frequency. In order to increase the efficiency of the system, and achieve deflections with low power consumption, geometrical features of the beams can be quantified so that the required vibrating frequency matches the natural frequencies of the beam. A parametric modal analysis is conducted on two configurations of micro-cantilever and the first natural frequency of the cantilevers as a function of geometrical parameters is extracted. To evaluate vibrational behavior and thermo-mechanical efficiency of micro-cantilevers as a function of their geometrical parameters and input power, a case study with a specified vibrating frequency is considered. Due to significant complexities in the loading conditions and thermo-mechanical behavior, this task can only be tackled via numerical methods. Selecting the geometrical parameters in order to induce resonance at the nominal frequency, non-linear time-history (transient) thermo-mechanical finite element analysis (using ANSYS) is run on each configuration to study its response to the periodic heating input. Approaches to improve the effectiveness of actuators in each configuration based on their implementation are investigated.

SiC nanowire-based SU-8 with enhanced mechanical properties for MEMS structural layer design

In addition to being used for pattern transfer,the negative photoresist SU-8 iswidely used as a structural material in microelectromechanical systems(MEMS).Due to its good photopatternability,SU-8 has lower manufacturing costs than many other materials,but its mechanical properties are relatively weak to some extent,which limits its performance.The mechanical properties of epoxy-like SU-8 can be enhanced by addingmicro-or nano-fillers such as carbon nanotube,clay,and SiC nanowire,which have superior elastic modulus.In this study,SiC nanowires were used to improve the mechanical properties of SU-8 while the SU-8 retains its photopatternability.The SiC nanowires were uniformly dispersed in SU-8 by stirring and ultrasonication.SU-8 materials with different SiC nanowire contents were fabricated into dog bone samples by lithography.The elastic modulus,storage modulus,and damping factor of the samples were measured by the Dynamic mechanical analysis(DMA)Q800.The experiment result shows that the rigidity and toughness increased,and the damping reduced.The 2 wt%SiC nanowires-reinforced SU-8 had a 73.88%increase in elastic modulus and a 103.4%increase in elongation at break.Furthermore,a spring component made by SiC-doped SU-8 could withstand greater acceleration.The SiC nanowires-reinforced SU-8 has the potential tomeet higher requirements in the design andmanufacture of MEMS and greatly reduce the manufacturing costs of MEMS devices.

磁场辐射下TNT晶体性能的变化特性

为了解强磁场辐射下三硝基甲苯(TNT)晶体性能的变化特性,借助扫描电子显微镜仪研究了磁场辐射(0,6 T)下TNT晶体形貌,利用X射线衍射仪和拉曼光谱分析了磁场辐射后TNT晶格的变化,采用差示扫描量热仪(DSC)获得了磁场辐射下TNT的热分解特性。基于分子动力学模拟,探讨了磁场辐射下(0,3,6 T和8 T)TNT的晶格常数、微观分布、力学性能和撞击感度变化情况。结果表明:TNT晶体受到6 T磁场辐射后,微观形貌由辐射前的鳞片针状结构转变为不规则的块状结构,热分解放热峰温度由289.6℃升高到304.1℃,但TNT的晶体相结构和晶格常数未发生明显改变。TNT晶格常数不受磁场辐射的影响,但磁场会改变TNT分子在晶体内部的分布。8 T磁场辐射能显著改善TNT晶体的延展性。通过比较最长引发键的占比,发现8 T磁场会增加TNT晶体的撞击感度。

基于DMA方法的沥青胶浆微观结构

采用动态力学分析(DMA)方法对沥青胶浆的微观结构进行了研究,通过对沥青胶浆的动态黏弹性能的变化规律以及Han曲线的温度依赖性进行分析。比较了沥青胶浆和改性沥青胶浆的相态结构的区别,并且根据沥青及沥青矿粉胶浆的损耗因子tanδ计算出参数A,用来评价沥青与胶浆的界面粘结程度。研究结果表明了用DMA方法可以很好地研究沥青胶浆的微观结构,从而为更好地改善沥青胶浆的路用性能提供了理论依据。

基于DMA方法的沥青砂浆动态弯拉黏弹特性

为了研究沥青砂浆的动态弯拉黏弹特性及其影响因素,提出了一种基于动态力学分析方法(DMA)的沥青砂浆动态弯拉试验方法,并对7种沥青砂浆进行了不同条件下的动态应变和频率扫描试验,分析加载频率、温度、油石比、沥青种类和矿料级配对沥青砂浆线黏弹性应变范围及黏弹性能的影响。基于时温等效原理,采用非线性最小二乘法绘制参考温度为20℃的沥青砂浆动态弯拉模量和相位角主曲线。研究结果表明:基于DMA方法的动态弯拉扫描试验,是一种能够有效、可靠评价沥青砂浆的动态弯拉黏弹特性的测试方法;沥青砂浆的弯拉线黏弹性应变范围与加载频率成正比,与温度成反比;增加油石比,会显著降低沥青砂浆的弯拉线黏弹性应变范围和高温低频条件下的动态模量,增大相位角峰值及温度敏感性,而对低温高频条件下的动态模量与相位角影响较小。相比于普通基质沥青,采用SBS改性沥青和橡胶沥青,均能增加沥青砂浆的弯拉线黏弹性应变范围与动态模量,减小其相位角峰值与温度敏感性,大幅改善沥青砂浆在高温低频条件下的动态弯拉黏弹特性。矿料级配是影响沥青砂浆体积参数与动态黏弹特性的重要因素,较粗的矿料级配会显著降低沥青砂浆的弯拉线黏弹性应变范围和动态模量,增大相位角峰值和温度敏感性。

TATB基高聚物粘结炸药及其粘结剂的玻璃化温度与DMA测量频率关系的研究

用动态力学分析 (DMA)方法 ,研究了四种TATB基PBX及其粘结剂 (氟聚物 )的测量频率与玻璃化温度的关系。结果显示 :在这四种氟聚物F2 314 、F2 311、F2 4 63 和F2 60 3中 ,只有F2 314 存在明显的二次转变 ,这是由于F2 314 分子中具有较多的C—Cl键所引起的。从测量频率与玻璃化温度之间的关系 ,得到PBX及其粘结剂的活化能数据。经比较发现 :TATB基PBX中 ,极高填充量的TATB颗粒的存在会对粘结剂链段的构象运动产生明显的限制作用。

FRW阻燃胶合板的DMA分析

The thermodynamic parameters of FRW fire-retardant plywood at different temperatures were measured by dynamic mechanical analysis (DMA) equipment such as storage modulus, loss modulus and loss angle tangent. The influences on the thermodynamic properties of FRW fire-retardant plywood by the FRW fire retardant were analyzed between the FRW fire-retardant plywood and the untreated plywood. The results showed that the transition temperature of FRW fire-retardant plywood’s storage modulus was delayed than the untreated plywood. Its value was also higher than the untreated plywood. Meanwhile the glass transition temperature of plywood treated by FRW fire retardant was raised. FRW fire-retardant plywood could keep the better mechanical performances and ability of deformation-resistance at higher temperature and during longer period than the untreated plywood.

DMA在沥青流变性能研究中的应用(英文)

沥青具有显著的流变性,动态力学分析(Dynamic Mechanical Analysis,简称DMA)恰是用流变学指标来评价材料的各种性能。简要阐述了DMA的原理,论述了用DMA方法进行沥青材料性能研究的优越性,结合具体实例阐述该方法在研究沥青及沥青结合料的粘弹性、高温性能、疲劳性能和低温性能等方面的应用,以及DMA方法用于测定沥青零剪切粘度的方法介绍。结果表明,动态力学分析方法可以在相对较短的时间内获得较丰富的粘弹信息,是全面研究沥青性能的有效方法。

DMA技术在木材科学中的应用及展望

介绍了动态力学实验研究及动态热机械分析(DMA)在木材科学中的应用及其发展方向。

氟聚物的玻璃化温度与DMA测量频率关系的研究

采用动态力学分析 (Dynamic mechanical analysis,DMA)方法 ,研究了 F2 314、F2 311、F2 6 0 3、F2 46 3等四种氟聚物的动态力学的频率与玻璃化温度关系。并用最小二乘法对相关频率与玻璃化温度进行线性回归 ,得到了这四种氟聚物的动态力学活化能数据。这些活化能在一定程度上反映了氟聚物链段构象运动的受阻性。

热分析技术在弹性体材料中的应用

热分析技术是评价弹性体材料热行为的主要手段之一。通过热分析技术全面深入地研究弹性体的物化特性,对改善和提高弹性体的应用性能具有重要的现实意义和价值。本文介绍了常见的热分析方法,包括:差示扫描量热法(DSC)、热机械分析法(TMA)、动态热机械分析(DMA)、热重分析(TGA)技术及热分析联用技术在弹性体材料中的应用研究进展。综述了热分析技术表征弹性体材料的特点及优势,同时展望了热分析技术的应用前景。

DMA法评价微胶囊-环氧树脂复合材料自修复率

以脲醛树脂(urea-formaldehydem,UF)为囊壁,正丁基缩水甘油醚改性环氧树脂的修复剂为囊芯,采用原位聚合法合成了UF自修复微胶囊,并以4',4'-二氨基二苯基甲烷(4',4'-diaminodiphenylmethane,DDM)作为微胶囊固化剂一起埋植到环氧树脂基体中,制成微胶囊-环氧树脂自修复材料.该微胶囊的囊芯质量分数为71.02%,平均粒径和囊壁厚度分别是105和3μm,囊壁表面粗糙而致密,与基体的界面结合较好,微胶囊在环氧树脂中分布均匀.UF微胶囊-环氧树脂复合材料经过恒应变损伤后产生裂纹,并在材料内部破裂,导致改性的环氧树脂修复剂从破裂的微胶囊流出至裂纹表面,改性的环氧树脂与设计的固化剂接触,在高温下发生有效的固化交联反应自动修复裂纹.热处理条件(60℃,2 h)可消除材料基体内残留的环氧活性中心与未反应的基体固化剂二乙烯三胺,在高于玻璃化转变温度tg的区域发生后固化反应,是测量UF微胶囊-环氧树脂复合材料自修复率的重要条件.采用动态力学分析(dynamic mechanical analysis,DMA)法测定的复合材料自修复率为103.03%,该方法便速、无损,具有良好的重复性,可用于表征材料自修复率.

基于空间设计的5089结构胶改性制备与应用策略研究

基于环境空间设计需要,对空间设计用5089胶粘剂进行了改性处理,对比分析了3种不同改性胶粘剂的动态热机械分析曲线和玻璃化转变温度-固化温度关系曲线。结果表明,与改性胶粘剂A相比,相同固化温度下改性胶粘剂B和改性胶粘剂C的储能模量都相对较高,且在固化温度为100~130℃时,改性胶粘剂A、改性胶粘剂B和改性胶粘剂C都兼具弹性和粘性;耐热性从高至低顺序依次为改性胶粘剂B、改性胶粘剂C、改性胶粘剂A,且改性胶粘剂B在固化温度100、110和130℃时的玻璃化转变温度都高于原始5089胶粘剂,表明改性胶粘剂B在此温度下的胶接接头耐热性能都优于原始5089胶粘剂。

金沙江下游高温天气的环流分型和诊断

金沙江流域作为我国重要的生态屏障和清洁能源输出地,其下游干热河谷地段是高温频发地区,针对该区域开展高温天气环流特征和诊断对提高高温灾害预报水平和提高该区域能源调度效率具有重要意义。利用1981—2020年逐日最高气温资料和欧洲中期天气预报中心(the European Centre for Medium-Range Weather Forecasts,ECMWF)第五代大气再分析数据集ERA5(Reanalysis v5),通过K-means聚类分析法对金沙江下游高温天气环流形势进行分型,并对各类高温天气的动力和热力因子进行诊断,提炼出各型高温的物理量阈值。结果表明:金沙江下游高温天气的环流背景可分为暖脊型、副热带高压-青藏高压型和青藏高压型3种类型。高温期间中高层均有≥0.25 Pa·s^(-1)的下沉气流,天空晴朗无云,晴空辐射强,且近地面受到≤800 hPa强度的热低压直接加热作用。热力诊断发现金沙江下游高温主要由非绝热加热引起,温度平流对局地升温的贡献为负,垂直绝热变化贡献接近于0,但下沉气流有利于地面接收更多太阳辐射,使近地层非绝热加热增加,从而导致金沙江下游出现高温天气。

DMA法研究不同粒径胶粉改性沥青黏弹性能

采用动态力学分析(DMA)方法,在动态剪切流变仪(DSR)上进行频率扫描实验,并利用广义Maxwell模型对实验结果进行拟合,得到胶粉改性沥青的离散松弛时间谱和零剪切黏度,并分析了胶粉粒径对胶粉改性沥青黏弹性能的影响。结果表明,在测定频率范围内,同一温度下改性沥青的损失模量(G″)大于储存模量(G′),且低频范围内两者差值较大;温度升高,G″和G′差别变大。在测定粒径范围内,G′和G″随胶粉粒径增加而增大,但粒径变化对胶粉改性沥青常规指标的影响较小。频率扫描结果与广义Maxwell模型拟合结果一致,计算出的零剪切黏度(η0)随胶粉粒径的增加而增加。胶粉以条状或棒状结构分散在沥青中,胶粉粒径减小,条状结构变小且分散更密集。

典型舰用复合材料DMA动态力学性能试验研究

典型舰用复合材料体系以玻璃纤维(SW220(S2))、碳纤维(T700)等为增强纤维,乙烯基类(430LV、3201)树脂和环氧类(350)树脂等为基体,以其为研究对象,通过开展不同纤维和基体组成的纤维增强复合材料(FRP)动态热机械分析(DMA)试验,得到以上材料体系在不同温域和频域内的阻尼特性,并对树脂基体种类、铺层角度和增强纤维类型的影响规律进行分析。研究结果表明:树脂基体是影响FRP阻尼特性的主要因素,三种树脂基体中,tanδ_(max(430LV))≈0.88,tanδ_(max(3201))≈0.8,tanδ_(max(350))≈0.7,常温段(0℃~30℃)损耗因子基本相同,约为0.05,加入纤维后,FRP的损耗因子下降至0.035左右,但储能模量提高了7倍~8倍;常温段,与0°/90°铺层方向相比,±45°铺层复合材料的损耗因子提高约0.025,储能模量下降6 GPa^8GPa;0℃~30℃,与碳纤维增强复合材料(CFRP)相比,玻纤增强复合材料(GFRP)损耗因子是其两倍,但储能模量只有其1/3。实验频率从1 Hz增加到1000 Hz,FRP的损耗因子降低45%~70%,储能模量上升10%以内。

动态力学分析技术(DMA)在食品研究中的优势及应用进展

动态力学分析(DMA)是研究高分子材料在各种环境中对于外部施加应力反馈的一种重要技术。近年来,随着学科交叉研究热度的不断上升,DMA在表征食品高分子材料中的力学特征研究中表现出极大的应用价值。因此,本文系统综述了DMA技术的工作原理、对食品材料的测定优势及在食品研究领域的具体应用,该技术在食品材料储能模量和损耗模量测定、α-弛豫监控及食品玻璃化转变温度定位等方面体现出灵敏度高、稳定性好和准确性高的特点,为食品动态力学物理性质测定提供了更科学的评测手段。在未来,DMA技术不仅可以通过DMA-振动光谱联用技术分析分子变化规律,从而开发食品级绿色清洁材料,还可以作为传统热力学技术辅助验证技术,对食品热物性继续深入研究。