Microstructure variability and macroscopic composite properties of high performance fiber reinforced cementitious composites

High performance fiber reinforced cementitious composites have made major advances in recent years, to the point where they are being adopted in building and bridge constructions. The most significant advantage of HPFRCC over conventional concrete is their high tensile ductility. However, the tensile strain capacity has been observed to vary, most likely as a result of the variability of the microstructure derived from the processing of these materials.This paper describes the composite property variability, as well as the variability of the material microstructure. Scale linkage is discussed. In particular, the tensile stress–strain curves, and the crack pattern on uniaxially loaded specimens are presented. The treatment of random fibers in micromechanical models, and tailoring of matrix flaw size distribution for saturated multiple cracking are examined. It is suggested that robust composite properties can be achieved by deliberate control of microstructure variability. Some open issues concerning the randomness of microstructures and possibly related macroscopic behavior are also identified. Further gains in composite property control may be expected from improvements in characterization and modeling of the microstructure randomness.     

掺杂钨丝低温延性的影响因素及改进措施

根据掺杂钨丝的实际生产情况．对所用原料纯度、附加剂含量以及工艺条件等影响其低温延性的因素进行了分析。并提出了相应的改进措施。     

水胶比对超高性能混凝土施工与力学性能的影响

兼顾优异施工及力学性能超高性能混凝土（UHPC）研制的重点之一是确定合适的水胶比。为此,以150 MPa级UHPC的原材料和配合比均不变（除用水量外）为前提,研究水胶比（0.15~0.2）对UHPC施工与力学性能的影响规律。通过相关试验得到了UHPC不同湿拌时间的扩展度、充分湿拌时间、静停一段时间的扩展度、抗压、抗折强度、四点弯曲应力挠度曲线、四点弯曲出现可视裂纹时下缘等效拉应力;对抗压、抗折强度进行了变异性分析;利用应力挠度曲线,基于规范法得到了弯曲韧性指标,并提出了其改进方法。结果表明：UHPC达最佳扩展度所需湿拌时间（充分湿拌时间）为6 min;水胶比由0.16增至0.19时UHPC扩展度基本呈线性增长,水胶比每增加0.01,UHPC扩展度平均增幅为109 mm;当水胶比、静停时间分别为0.19、4 h时,扩展度损失40 mm,损失率仅5.9%;UHPC抗压强度、抗折强度及弯曲韧性指标随水胶比的增大皆呈先增后减趋势,当水胶比分别为0.18、0.16和0.16时达到最优;应力峰值前的应力挠度曲线并不是典型全凸形曲线;可视初裂抗折强度约为抗折强度的0.85倍;UHPC试件的弯曲韧性较好,宜采用可视初裂挠度作为初始变形参考进行韧性指标计算;为保证UHPC兼顾良好施工与力学性能,建议湿拌时间、水胶比分别为6 min、0.18或0.185。     

先张法预应力钢绞线高强混凝土大直径管桩受弯性能研究

为研究大直径钢绞线管桩的受弯性能,对3根不同直径钢绞线桩进行足尺试验、理论分析和有限元模拟,对比研究了大直径钢绞线桩的开裂弯矩、裂缝开展及分布、受弯承载力、变形延性和破坏特征等。结果表明：大直径钢绞线桩受弯破坏时受压区高强混凝土压溃,截面中心轴以下钢绞线都达到屈服强度,具有较高的受弯承载力和良好的变形延性;理论分析方法能够较好地预测大直径钢绞线桩的受弯承载力;建立的有限元模型可以合理地模拟大直径钢绞线桩的受弯性能,模型计算结果与试验结果吻合较好。     

基于测力延度试验的SBS改性沥青老化特征评价指标

采用不同时程的室内薄膜烘箱试验模拟SBS改性沥青的老化过程,对不同老化时间的SBS改性沥青进行常规试验、动态剪切流变试验及测力延度试验,分析针入度、软化点、车辙因子及测力延度相关指标与SBS改性沥青老化时间的相关性,同时分析这些指标与SBS改性沥青中SBS含量的相关性.试验与分析结果表明：针入度、软化点及车辙因子等指标与SBS改性沥青中SBS含量及老化时间的相关关系具有一致性,无法有效地区分SBS含量和老化时间对SBS改性沥青性能的影响;测力延度试验中的韧性指标与SBS含量正相关,且与SBS改性沥青老化时间负相关,是判别SBS改性沥青老化特征的有效指标.     

基于ANSYS的CFRP加固钢筋混凝土柱的轴压比分析

轴压比是影响钢筋混凝土柱结构受力性能的重要因素,本文主要研究了这种因素对钢筋混凝土柱以及CFRP加固后钢筋混凝土柱的力学性能的影响.通过用ANSYS有限元分析软件模拟低周反复位移加载,分析在不同轴压比条件下试件的力学性能.通过研究,本文建议在地震区钢筋混凝土柱的轴压比不大于0.6,这样就可以保证在钢筋混凝土柱的极限承载力得到一定程度提高的同时,其延性不会降低太多;在使用CFRP“围束条带两层＋纵向条带两层＋末端两层锚固”的方式加固后,其力学性能得到了不同程度的改善,轴压比的增大虽然使柱的变形能力减弱,但柱的水平承载力得到了显著提高,并进一步激发了纤维布对混凝土的约束作用,提高了纤维布的加固效果.     

Study on preparation and mechanical property of nanocrystalline NiAl intermetallic

Nanocrystalline NiAl materials were fabricated using mechanical alloying and hot-pressing sintering technique. The crystal structural and microstructure of milled powders during mechanical alloying, and the microstructure and mechanical properties of bulk NiAl intermetallic were characterized. The results show that B2 ordered nanocrystalline NiAl powders were successfully synthesized by solid-state diffusion via the gradual exothermic reaction mechanism during mechanical alloying. Scanning electron microscope image confirmed that the powder particles were flat and flake shape in the early stage of milling, but changed to a spherical shape with the crystallite size about 30 nm after the milling. After sintering, the crystal structure of nanocrystalline NiAl intermetallic was assigned to B2 order NiAl phase with the average crystallite size about 100 nm. The nanocrystalline NiAl intermetallic exhibited prominent room temperature compressive properties, such as the true ultimate compressive strength and the fracture strain were 2143 MPa and 32.2%, respectively. The appearances of vein-like patterns on the fracture surface of NiAl intermetallic materials indicated that the fracture mechanism could be characterized as ductile fracture. It can be concluded that higher sintering density and nanocrystalline of NiAl intermetallic were benefited for the improvement of mechanical properties.     

ZL101铝合金轮毂的时效处理

按正常工艺热处理的ZL101铝合金轮毂,强度达到了要求,但由于塑性较差,落锤冲击试验时往往开裂。不改变固溶处理工艺和时效温度,适当缩短时效时间,结果,塑性有所提高,而强度仍符合要求,轮毂冲击试验合格率大大提高。     

不同冷速下36Mn2V钢坯高温塑性及碳氮化物的析出

采用Gleeble-1500热模拟试验机研究了微合金钢36Mn2V连铸坯试样在冷速为3、8和15℃/s,变形温度为650～1000℃/s之间的高温塑性,并通过试样的断口形貌、显微组织和微合金析出相等的变化情况分析其脆化机理。结果表明：实验钢的第Ⅲ脆性温度区为950～650℃/s,800℃以上脆化是由碳氮化物单一原因造成,800℃以下脆化的原因则是碳氮化物的析出和沿晶界铁素体膜的形成;冷速过快加剧900～700℃温度区间的脆化;900℃已有大量碳氮化物的析出,随变形温度的降低,析出量减少,750℃仍然存在碳氮化物的析出。