Effect of fiber-reinforcement on the mechanical behavior of sand approaching the critical state

Several types of ground improvement methods that employ fiber-reinforcement have been developed in recent years. A series of consolidated drained triaxial compression tests has been conducted here to examine the effect of short fibers on the mechanical properties of Toyoura sand. Sand with 0%, 0.2%, 0.4%, and 1% fiber contents, prepared to yield random distribution, was sheared under several confining pressures and controlled via their initial relative densities. The test results showed that the maximum and residual deviatoric stresses increased, whereas the volumetric expansion decreased with an increase in fiber content. Although the stress ratio η (=q/p′) and specific volume changed depending on the fiber content and confining pressure with shear progression, they each reached the same values for a definite fiber content at the end of shearing, independent of initial relative density. In other words, the unique critical state line can be found for a definite fiber content. Moreover, the greater the fiber content, the larger the slope of the critical state line at the end of shearing. Additionally, as the length of fibers shortened with the same percentage of fiber inclusions in sand, the deviatoric stress and the stress ratio decreased, approaching the shear-strain-volumetric response of unreinforced sand.     

吹砂对DD6单晶高温合金表面完整性和高周疲劳强度的影响EI北大核心CSCD

将标准热处理的试样分别采用粒径为150、124和100μm白刚玉砂在0.5 MPa压力下吹砂,研究吹砂对第二代单晶高温合金DD6表面完整性的影响;对未吹砂和粒径150μm吹砂试样分别进行760和980℃旋转弯曲高周疲劳性能测试,研究吹砂对DD6合金疲劳性能的影响。结果表明:吹砂会破坏单晶高温合金的表面完整性,使表面出现砂粒切削造成的不规则凹坑,改变表面形貌;砂粒粒径增加,表面粗糙度和显微硬度均增大;吹砂使大量位错在γ相通道中滑移,靠近表面区域位错密度较大;并且,大量位错剪切γ’相,形成反相畴界和层错;吹砂造成形变强化、引入残余应力;150μm、0.5 MPa吹砂对DD6合金760℃旋转弯曲疲劳性能基本无影响,但会降低合金980℃疲劳性能,对低应力幅区疲劳寿命影响较大,使疲劳强度下降约7.3%。缺口效应、氧化损伤、形变强化和残余压应力的耦合作用导致吹砂与不吹砂试样疲劳寿命产生差异。     

Simulation of Binder Infiltration in Additive Manufacturing of Sand Molds

This article proposes a computational fluid dynamics approach to simulate binder infiltration in 3D printing of sand molds using OpenFOAM facilitating the identification of suitable levers for application-specific material and process developments. A method for randomly generating powder bulks of designated powder size distributions (PSD) and procedures for automated analysis of the infiltration profile and volume are introduced. Simulation is utilized to investigate binder infiltration using different droplet spacings, representing different printheads’ resolutions. The apparent particle size at the exact location of the droplets’ impact, the droplets’ landing position in relation to the respective surface topography, and thus the statistical appearance of particle formations appear to be influencing the infiltration profile. High-speed camera observations show the plausibility of the predicted infiltration kinetics. An exemplary use case compares the predicted infiltration profiles to the compressive strength of specimens printed from silica sand with low binder contents. Simulation predicts an average infiltration of 250 μm that presumably achieves reliable bonding for layer thicknesses up to 365 μm. A decrease in strength with increasing layer thickness at constant binder contents can be found in the experiment – at layer thicknesses above 350 μm, only minor strengths are achieved.     

陕西榆林风沙滩区全新世气候和环境变迁

榆林长城沿线处于环境变化敏感带,其风沙滩区沉积剖面的磁化率和粒度分析结果显示,该区全新世早期11500～10000aBP随着全球气候的逐渐转暖,磁化率、黏粒和粉沙含量逐渐增高;全新世中期的10000～4500aBP气候相对比较湿润,植被覆盖度增大,风沙活动减弱,地表沉积物当中粉沙和黏土成分含量显著增大,风化成壤作用增强,地表发育沙质弱古土壤;全新世中期后段的4500～3000aBP期间,气候以比较温和湿润为主,但在4200～4000aBP出现一个气候干旱事件,风沙活动强烈;近3000a以来,气候显著干旱化,波动频繁,地表风沙沉积物显著变粗,表现出有5次沙漠扩张。该区生态环境演化、人文历史变迁基本上是自然因素与人为活动综合作用的结果,以自然演化为主,近代人为因素对环境变化起着加强和放大作用。     

筋材-砂-黏土层状体系接触面剪切特性试验研究

以筋材-砂-黏土层状体系为研究对象, 采用界面剪切试验, 对筋材-黏土间夹砂层后改善筋土之间相互作用效果进行试验研究。借助直接剪切试验和拉拔试验, 通过改变砂层的粒度成分(粗砂和细砂)和砂层厚度(1～3 cm), 分析该层状体系的具体影响。试验结果表明, 尽管在筋材-黏土间夹砂层可有效提高界面的抗剪强度, 但在25~75 kPa法向应力下, 砂的组构(粗砂和细砂)和砂层厚度对该体系的影响都不明显。     

选用细砂充填砂袋的围堰体稳定性验算

澳门大学新校区专用隧道跨越500m的十字门水道,采用两侧临时围堰形式进行明挖干法施工,围堰稳定性对隧道施工非常关键,文中主要对细砂作为充填砂袋材料的围堰体在加载完成后及堰内抽水后的工况进行稳定性验算分析,为类似工程实施与监控提供数据参考。     

石屑砂在砂浆中的应用研究

研究了石屑砂和汾砂对砌筑砂浆性能的影响。针对太原地区汾砂的特点,通过在汾砂中掺入50%的石屑砂配制砂浆,明显提高了砂浆中细骨料的质量,使砂浆的各项性能均优于单独使用汾砂的砂浆性能。     

Experimental study on the behavior of eccentrically loaded circular footing model resting on reinforced sand

In this study, an experimental investigation has been conducted on a circular footing model subjected to eccentric load resting on the geonet-reinforced sand. To this end, five series of tests were carried out in order to evaluate the effect of reinforcement dimension and eccentricity on the bearing capacity, settlement, and rotation of the footing. Results show that the bearing capacity ratio (BCR) is in direct relationship with eccentricity and the impact of soil reinforcement at low settlements is much more significant in the case of eccentric loading. Additionally, the bearing capacity interaction diagram and variation in the position of rotation line at different load levels for reinforced and unreinforced conditions are presented.     

气流紧实软复盖砂型的试验研究

本文试验研究了在砂型顶面用不同方法(盖塑料膜、喷水、喷粘土浆、撒粘土粉和细砂等)施加软复盖对砂型气流紧实效果的影响。试验表明:只要在砂型顶面形成一层不透气的软隔膜,均可提高气流紧实的效果;并改善砂型背面的松散状态。其中以复盖塑料膜最为简单可靠,软复盖的气流紧实还显著地提高低矮砂型的紧实效果。     

用纵波首波幅度识别砂岩气层的方法

从纯砂岩、泥质砂岩(人工岩样)与其含气情况下的纵波首波幅度比与含气饱和度的实验发现,当砂岩内含气饱和度大于20%时,纵波首波幅度的衰减可达到最大值。而且,纵波首波幅度对砂岩内的天然气响应比纵波首波频率和速度敏感。基于这一实验结果,本文提出了用纵波首波幅度识别砂岩气层的方法。目的层岩石的纵波首波幅度可由声波波形分析图求取,而该目的层岩石的纵波首波幅度背景值,可用邻近含水层或含油层(轻质油层除外)岩石的纵波首波幅度代替。这样,当目的层岩石的纵波首波幅度小于该层的幅度背景值30%±10%时,则指示此目的层岩石泥浆侵入带内存在天然气。据此,可识别此处目的层是气层。另外,文中还给出了描述含气层岩石和含水层岩石纵波首波幅度的数学模型及应用纵波首波幅度识别砂岩气层的两个实例。