Effect of high shear rate on solidification microstructure of semisolid AZ91D alloy

To investigate the effects of rotation speed and shearing time on morphology of semisolid AZ91D alloy,experimental work was undertaken using a twin-screw slurry maker.The results show that increasing the rotation speed and reasonable time can give rise to substantial grain refinement during continuous shearing stage,which can be attributed to the increasing of effective nucleation rate caused by the extremely uniform temperature due to high shear rate and high degree of turbulence.Comparing with low rotation speed at the same thermal condition,the analysis indicates that the microstructures obtained at high rotation speed are homogenous spherical and fine grains instead of dendritic or rosette and exhibits uniform distribution in the eutectic matrix.

Specific inhibiting effects of Ilexonin A on von Willebrand factor-dependent platelet aggregation under high shear rate

Background llexonin A (IA), purified from the Chinese herbal medicine Maodongqing (Ilex pubescens Hook, et Am) has been commonly used in south China to treat thrombotic disorders. In this study, we aimed to study the inhibiting effects and mechanism of IA on von Willebrand factor (vWF)-dependent high shear-induced platelet aggregation.Methods vWF-dependent high shear (10 800 s-1) induced aggregation of platelets obtained from normal donors in the presence or absence of IA was measured by a modified cone-plate viscometer and shear-induced vWF binding was measured by quantitative flowcytometry with monoclonal antibody known to bind exclusively to the C-terminal domain of vWF (LJ-C3) directly labeled with fluorescein isothiocyanate (FITC). P-selectin surface expression was also measured by a similar method with FITC conjugated anti-P-selectin monoclonal antibody (WGA1).Results Shear-induced platelet aggregation was inhibited by IA in a dose-dependent manner. The extent of aggregation decreased from (78. 6 ±4. 6)% in the absence of IA to (36. 5 ±2.1)% in the presence of IA (3. 3 mmol/L) (P<0. 0001, n =9) with a high shear rate of 10800 s-1. vWF binding and P-selectin expression were also inhibited by IA in a dose dependent manner. The number of binding FITC-LJ-C3 molecules increased after exposure of platelet-rich plasma to a high shear rate of 10800 s-1 for 6 minutes, but this shear-induced increased binding platelet surface vWF molecules and P-selectin expression can be decreased in the presence of IA.Conclusion vWF binding and vWF mediated platelet activation, aggregation occurring under high shear rate were inhibited by IA. IA may be a unique antithrombotic drug inhibiting the vWF-GP Ibα interaction, and may thus facilitate drug design targeting arterial thrombosis.

Hot Spot in Materials with Structural Defects under High Shear Loading Rates

The response of three-dimensional sample of Al, containing vacancy complex, under shear loading was simulated. The molecular dynamics method was used and interaction between atoms was described on the base of pseudopotential theory Solitary waves were generated in the sample under mechanical loading. Their interaction with the vacancy complexes was shown to be able to initiate hot spot in that local region of the complexes. Some parameters of the hot spot as well as solitary waves were calculated. The initiation of the hot spot is accompanied with sufficient local structural relaxation.

Electrical Resistivity Contrasts and High Flow Rates Discontinuous Aquifers Identification in a Sheared Crystallophyllian Basement Zone at Boniérédougou (North-Central Côte d’Ivoire)

This study proposes an alternative approach to the investigation of high flow hydrogeological fractures within the basement in the Dabakala region of north-central Côte d’Ivoire. The used approach consists of exploring the subsurface by measuring electrical resistivity contrasts along the main shear direction within crystallophyllian rocks. Electrical resistivity profiling and vertical electrical sounding techniques, coupled with boreholes monitoring, have identified fractured aquifers whose best flow rates are around 96 and 116 m3/h. These aquifers mostly hosted in granodiorite have an average strength of 10 meters and are located at depth of around 100 meters. They are associated with open fractures created by tangential shear stresses that have affected the Dabakala volcano-sedimentary trench formations. The search for fractured aquifers along the main shear direction offers great perspective for obtaining high flow rates.

高应变速率下TB6钛合金在绝热剪切带内的晶粒瞬间细化机制

采用分离式霍普金森压杆装置对TB6钛合金帽型试样进行室温动态加载试验,通过微观组织表征技术对绝热剪切带内的微观结构特征进行观察,研究高应变速率下TB6钛合金绝热剪切带内的晶粒瞬间细化机制。TEM观察结果表明:绝热剪切带中晶粒的瞬时细化是三种机制共同作用的结果。第一种机制是由于位错的快速运动、增殖以及塞积促使晶粒内部产生应力集中现象,最终在此现象的作用下形成裂纹、断裂以及细晶;第二种机制是通过增大晶粒内部的颈缩程度迫使细晶形成;在转动态再结晶机制的作用下,绝热剪切带的中心区域形成100nm的细小等轴晶。

不同条件下高速剪切均质和高压均质对红景天复合饮料的影响

基于CiteSpace软件可视化分析及结合实际情况下复合饮料极易产生沉淀问题,该研究发现高速剪切均质和高压均质均能降低饮料的沉淀,但不同的均质方式对饮料的影响不同。以粒径、沉淀率、稳定系数、色差值、L^(*)和感官评分为指标,在不同参数条件下采用高压均质和高速剪切均质对复合饮料进行均质,选取最优的均质方法。结果表明,虽然高压均质和高速剪切均质均能明显提高饮料的稳定性,但经高压均质后的饮料色泽过于暗沉。而采用均质转速为12500 r/min,均质次数为2次的高速剪切均质的方式对复合饮料进行均质后,饮料的稳定性升高的同时,色泽鲜艳明亮,呈鲜艳的暖橙色,使人食欲感增强,感官评分也较高。故均质转速为12500 r/min,均质次数为2次的高速剪切均质的方式更适用于复合饮料,其作用效果也较好。研究结果可为后续饮料均质提供一定的参考。

预制UHPC永久模板钢筋混凝土组合梁抗剪性能有限元分析

为了研究预制超高性能混凝土永久模板钢筋混凝土组合梁抗剪性能,选取合适的U型UHPC预制单元本构关系,建立了钢筋混凝土梁的有限元永久模型,并与已有的试验数据进行比较,验证了模型的可靠性,并分析配箍率、剪跨比等参数对超高性能混凝土组合梁抗剪性能的影响。结果显示:超高性能混凝土预制单元可显著提高斜向开裂荷载和极限荷载;预制单元会限制箍筋的作用,所以配箍率对组合梁影响小;剪跨比对组合梁抗剪性能影响大,但是和配箍率一样都会影响其延性。

冷轧压下率对高强无取向电工钢变形组织和磁性能的影响

借助场发射扫描电镜(SEM+EBSD)、快速升温管式炉和交流磁性能测量仪研究高强无取向电工钢冷轧变形组织和磁性能随冷轧压下率的变化规律。结果表明:冷轧高强无取向电工钢中形成含有大量亚结构的粗糙条带和变形量较小的光滑条带。粗糙条带具有γ取向,主要为{111}〈110〉,光滑条带具有{112}〈110〉和{001}〈110〉取向。粗糙条带中存在大量宽度约为2~3μm的剪切带,与轧向呈20°~35°夹角,且随压下率增大,数量逐渐增多。剪切带与基体间具有一定取向差,随压下率增大,剪切带逐渐从{111}〈110〉取向转向{223}〈110〉取向,与基体间取向差逐渐增大。退火后Goss和{111}面织构增多,{001}面织构减少;轧向磁感应强度增大,横向磁感应强度减小,各向异性显著,铁损下降。

Ti6321钛合金焊接接头高温-中应变率力学性能及微观组织

采用Gleeble热模拟试验机对焊接接头不同区域进行了高温-中应变率压缩试验,研究了焊接接头不同区域组织对其力学性能的影响,分析了温度和应变率对其变形过程的影响。结果表明,在高温-中应变率压缩时,热影响区强度最高,母材区较差,焊缝区强度最差。高温下(350℃和550℃),随应变率增加(1s^(-1)和10s^(-1)),变形主导因素由应变硬化效应转变为热软化效应。母材绝热剪切敏感性较低,在高温-中应变率压缩变形时沿剪应力最大方向发生大范围晶粒协调变形。由于热影响区组织均匀,应变率的增加相对于温度的增加,更能促进热影响区绝热剪切带的生成。焊缝区压缩时,应力易于在晶界处集中,此时温度对其压缩变形时绝热剪切带的生成更为有利。

高剪切调浆预处理对煤泥浮选动力学的影响研究

为明确高剪切调浆预处理对煤泥浮选动力学的影响,基于对煤样的可浮性和粒度组成分析,通过改变调浆转速和调浆时间,以浮选精煤产率、灰分和浮选速率常数等作为试验指标进行了实验室单元浮选试验。结果表明:高剪切调浆可有效提高浮选速度并缩短浮选时间;适度的调浆转速和调浆时间可同步提高浮选精煤的产率和浮选过程的选择性;对于可浮性较好的细粒煤需要的调浆时间较短,对于可浮性较差的细粒煤则需适度延长调浆时间;相较于经典一级浮选动力学模型,KELSALL分速动力学模型更符合细粒煤可浮性分布的非均一性特点。研究结果对提高现有煤泥浮选工艺效果具有一定帮助。

Study on Rheological Characteristics of a Grease Used in High Speed Bearing

On a self-made super-high shear strain rate rheometer, the rheological characteristics and apparent viscosity curves of a high-speed bearing grease were obtained under different working conditions. A new grease rheological model suited to a shear strain rate range of 0—3.5×10~6s^(-1) was presented. The results showed that the shear stress increased linearly at first and then increased nonlinearly with the increase in shear strain rate up to 1.5×10~6s^(-1), and finally the shear stress decreased slightly with the successive increase in shear strain rate. The shear stress increased with a decreasing rolling speed and an increasing contact pressure. The apparent viscosity decreased rapidly with the increase of shear strain rate at beginning and could approach the viscosity of the base oil if the shear strain rate surpassed 1.5×10~6s^(-1). The fits between the test data and the predicted values by the new model were fairly good.

Analysis of Adiabatic Shearing Failure Mechanism for Aluminum Matrix Composites Based on Experimental and Numerical Simulation

Adiabatic shear behavior and the corresponding mechanism of TiB2/Al composites were researched by split Hopkinson pressure bar （SHPB）.Results show that the flow stresses of the TiB2/Al composites exhibit softening tendency with the increasing of strain rates. All the composites fail in splitting and cutting with a 45 degree, and the phase transformed bands of molten aluminum are found on the adiabatic shear layers. The deformation behavior and shear localization of the TiB2/Al composites specimens were simulated by finite element code MSC.Marc. The Johnson-Cook model was used to describe the thermo-viscoplastic response of the specimen material. There was unanimous between the numerical result and the experimental result on the location of the adiabatic shear band. From the numerical simulation and experiment, it was concluded that the instantaneous failure of the composite was ascribed due to the local low strength area where the formation of adiabatic shear band was, and the stress condition had significant effect on the initiation and propagation of adiabatic shear band （ASB）.

Microstructural Characterization of the Shear Bands in Fe-Cr-Ni Single Crystal by EBSD

An investigation has been made into the microstructural characterization of the shear bands generated under high-strain rate （≈10^4 s^-1） deformation in Fe-15%Cr-15%Ni single crystal by EBSD-SEM （electron backscatter diffraction-scanning electron microscopy）, TEM （transmission electron in microscopy） and HREM （high- resolution electron microscopy）. The results reveal that the propagation of the shear band exhibits an asymmetrical behavior arising from inhomogenous distribution in plasticity in the bands because of different resistance to the collapse in different crystallographic directions; The γ-ε-α′phase transformations may take place inside and outside the bands, and these martensitic phases currently nucleate at intersections either between the twins and deformation bands or between the twins and ε-sheet. Investigation by EBSD shows that recrystallization can occur in the bands with a grain size of an average of 0.2μm in diameter. These nano-grains are proposed to attribute to the results of either dynamic or static recrystallization, which can be described by the rotational recrystallization mechanism. Calculation and analysis indicate that the strain rate inside the shear band can reach 2.50×10^6 s^-1, which is higher, by two or three orders of magnitude, than that exerted dynamically on the specimen tested.

An approach to measure infill matric suction of irregular infilled rock joints under constant normal stiffness shearing

Rock joints infilled with sediments can strongly influence the strength of rock mass. As infilled joints often exist under unsaturated condition, this study investigated the influence of matric suction of infill on the overall joint shear strength. A novel technique that allows direct measurement of matric suction of infill using high capacity tensiometers(HCTs) during direct shear of infilled joints under constant normal stiffness(CNS) is described. The CNS apparatus was modified to accommodate the HCT and the procedure is explained in detail. Joint specimens were simulated by gypsum plaster using threedimensional(3D) printed surface moulds, and filled with kaolin and sand mixture prepared at different water contents. Shear behaviours of both planar infilled joints and rough joints having joint roughness coefficients(JRCs) of 8-10 and 18-20 with the ratios of infill thickness to asperity height(t/a)equal to 0.5 were investigated. Matric suction shows predominantly unimodal behaviour during shearing of both planar and rough joints, which is closely associated with the variation of unloading rate and volumetric changes of the infill material. As expected, two-peak behaviour was observed for the rough joints and both peaks increased with the increase of infill matric suction. The results suggest that the contribution of matric suction of infill on the joint peak normalised shear stress is relatively independent of the joint roughness.

应变速率对Al_(0.4)CoCrFeNi高熵合金绝热剪切敏感性的影响

在室温下利用分离式霍普金森压杆对Al_(0.4)CoCrFeNi高熵合金帽型试样进行动态加载,研究了不同应变速率下Al_(0.4)CoCrFeNi高熵合金的绝热剪切敏感性。结果表明,动态加载前后Al_(0.4)CoCrFeNi高熵合金的晶粒尺寸分别约为100μm、100 nm,相差约3个数量级,动态加载后细小的晶粒尺寸使Al_(0.4)CoCrFeNi高熵合金具有更低的绝热剪切敏感性;Al_(0.4)CoCrFeNi高熵合金绝热剪切敏感性随应变速率增加而增加,在实验范围内,应变速率3 360 s-1时绝热剪切敏感性最高,产生了与动态加载方向成45°、宽约2μm的绝热剪切带,此时临界应变值和单位体积绝热剪切形成能也最小。Al_(0.4)CoCrFeNi高熵合金在高应变率变形过程中晶粒发生明显细化。Al_(0.4)CoCrFeNi高熵合金在动态加载下的绝热剪切归结为材料的热-黏塑性本构失稳。

长焰煤泥高剪切油团聚分选提质的试验研究

针对我国低阶煤煤泥含水率高、表面含氧官能团含量高、孔隙发达等使得其亲水而可浮性差,难以通过浮选方法有效分选提质的问题,采用接触角测量仪、傅里叶红外光谱仪、X射线衍射仪等手段对陕西某长焰煤泥性质进行了分析,并采用高剪切油团聚法对难以采用浮选法有效分选的长焰煤泥进行了分选提质的试验研究。通过单因素探索试验,得到了矿浆质量分数为10%、搅拌速率为2200 r/min、团聚油柴油用量为干矿量的40%、搅拌时间为20 min等最佳参数,在最佳参数条件下,分选效果为精煤产率13.51%,可燃体回收率71.04%。

应变速率对HCP/FCC双相CoCrFeNiNb_(0.5)高熵合金力学性能的影响

利用分离式霍普金斯压杆、扫描电子显微镜和透射电子显微镜研究应变速率对铸态CoCrFeNiNb_(0.5)高熵合金力学行为的影响。CoCrFeNiNb_(0.5)高熵合金由先共析密排六方(HCP)相和共析组织构成,合金的共析结构为片层状的HCP相和片层状的面心立方(FCC)相。当应变率从1×10^(-4)s^(-1)增加至6×10^(3)s^(-1)时,合金的屈服强度不会显著增加;然而,合金的破坏应变剧烈降低。片层状HCP相的准静态塑性变形机理为缠结状位错的增殖,而其动态塑性变形机理转变为剪切。片层状HCP相的剪切变形能够导致CoCrFeNiNb_(0.5)高熵合金在低压缩应变(ε=0.1)下形成微裂纹。由于片层状HCP相中微裂纹的数量随着动态压缩应变的提高而增加,微裂纹的扩展引发材料的雪崩式断裂。因此,铸造态CoCrFeNiNb_(0.5)高熵合金表现出显著的应变速率诱发脆性特性。

高温高应变率加载下Ti-5553钛合金动态力学性能研究

钛合金是遥感类航天器推进系统重要的结构支撑材料,面对空间碎片日益增多的威胁,探究新型钛合金材料的动态力学性能是航天器空间防护的重要技术之一。本文利用高温分离式霍普金森杆(SHPB)对新型Ti-5553钛合金的等轴组织(R1)和双态组织(R2)材料进行动态力学测试,测试温度分别设定为200℃、400℃和600℃,应变率为1200s^(-1)和2500s^(-1)。试验结果表明:在高温高应变率加载条件下,Ti-5553钛合金双态与等轴组织,随着温度的升高,流变应力均下降,而应变变化不明显,即2种组织高温动态压缩力学行为相似;环境温度低于200℃时,双态组织的温度敏感性高于等轴组织,而塑性吸收功小于等轴组织,表现出较高的绝热敏感性;环境温度高于200℃时,等轴组织的温度敏感性高于双态组织,而塑性吸收功小于双态组织,表现出较高的绝热敏感性;Ti-5553钛合金2种组织均在高温高应变率下发生了剪切破坏,靠近剪切带的α相沿着剪切方向被拉长细化。本文获取的新型Ti-5553钛合金动态力学规律可为航天器空间防护结构设计提供基础数据。

金属材料动态剪切测试方法

材料的动态剪切性能参数是进行弹箭及装甲防护系统设计的重要基础。由于目前所采用的拉伸剪切试样在应力均匀性及受力状态方面仍存在问题,给测试结果的准确性带来影响。采用数值模拟对5种常见拉伸剪切试样进行了对比分析,并针对现有问题设计出一种新型拉伸剪切试样。该试样在准静态、动态加载下应力分布均匀而且接近纯剪切的应力状态,适合在大应变率、大应变范围内对材料的剪切力学性能进行准确测试。通过与压缩加载下双剪切试样的测试结果进行对比,发现两种试样获得的应力-应变曲线吻合较好,而且新型试样具有更长的塑性流动段。采用有限元软件对TC4钛合金试样的变形及断裂过程进行了模拟,并通过与高速摄影结果进行对比发现两者剪切区的变形及裂纹扩展情况相互吻合,从而验证了试样设计及数值模拟结果的有效性。

TC17和TC4合金锻件的动态响应及绝热剪切行为

利用SHPB装置研究高应变速率下TC17和TC4合金锻件的动态力学性能,并利用OM、SEM和EBSD分析两种合金的绝热剪切行为。结果表明:随着应变速率的增加,两种合金的强度均呈现出升高趋势,表现出应变速率强化效应;与TC17合金相比,TC4合金在相同应变速率下具有更大的塑性应变和动态吸收能;TC17合金经β锻造后获得网篮组织,板条状α相和残余β相形成了大量的相界面,增加了绝热剪切带(ASB)的形成位置,且ASB在扩展过程中容易出现分叉现象;TC4合金经α+β锻造后具有双态组织,等轴状初生α相具有较好的延展性,提高了合金的动态塑性变形能力,合金中排列较规则的片层状次生α相导致相界面减少,ASB的数目较少且难以分叉;在动态压缩中断条件下,TC17合金中ASB的萌生时刻早,萌生孕育能低;TC17合金的绝热剪切敏感性高于TC4合金,且两种合金的绝热剪切敏感性均随着应变速率增加而升高。