颗粒包覆厚度对磁流变液性能的影响

为了研究磁性颗粒包覆厚度对磁流变液性能的影响,通过改变水溶液中聚乙烯醇的浓度来获取不同包覆厚度的磁性颗粒,以此配制磁流变液;对包覆颗粒的表面形貌、粒径及静磁性能进行表征,并测试不同包覆厚度的磁性颗粒制备的磁流变液的零场黏度、剪切屈服应力和沉降稳定性性能。结果表明:颗粒包覆厚度对磁流变液的零场黏度影响不大,对剪切屈服应力影响作用由弱到强再减弱;包覆层厚度增大,大颗粒团聚物更易形成,导致磁流变液沉降稳定性降低。     

考虑磁性颗粒不均匀分布的磁流变液修正微观力学模型及试验验证

为了研究磁性颗粒在磁场作用下的不均匀分布对磁流变液力学性能的影响,通过卡方分布来模拟磁性颗粒的间距分布,对现有的磁流变液微观力学模型进行修正,并通过磁流变阻尼器的力学性能试验验证了模型的有效性。在磁流变液双链微观力学模型的基础上,修正相邻磁性颗粒的间距完全相等且不随磁感应强度而变化的假设,采用卡方分布来表征磁性颗粒间距的不均匀分布,并引入分布参数来描述磁性颗粒间距随磁感应强度的变化关系,推导了考虑磁性颗粒不均匀分布的磁流变液修正微观力学模型;基于修正的微观力学模型,分析了分布参数对磁流变液剪切屈服应力的影响;将该文提出的磁流变液修正微观力学模型带入到磁流变阻尼器的准静态模型中,可以得到不同电流下的阻尼器最大出力,并与磁流变阻尼器力学性能试验数据进行对比来验证所提模型的有效性。结果表明,考虑了磁性颗粒不均匀分布的磁流变液修正微观力学模型可以更加精确地预测磁流变液在不同磁感应强度下的剪切屈服应力,尤其是在低磁感应强度情况下可以改善现有微观力学模型放大了磁流变液剪切屈服应力的缺点。     

单一粒径与两种粒径羰基铁粉磁流变液特性的实验比较研究

通过控制球磨时间,用球磨机球磨出不同粒径的羰基铁粉,制得单一粒径磁流变液8组和两种粒径磁流变液6组。采用旋转流变仪对磁流变液的零场粘度和剪切强度进行了测试。结果表明,在相同磁场强度下,两种粒径磁流变液的剪切强度比单一粒径磁流变液的有明显提升,提升效果与颗粒的成份密切相关,且其零场粘度和沉降稳定性与单一粒径的差别很小。     

磁流变液的特性研究

对以金属铁颗粒为主体的磁流变液的一般特性进行了研究。详细研究了材料的颗粒大小、形状、比例以及温度等对磁流变液特性的影响。研究了流体的屈服应力与磁场关系,剪切应力与磁场以及剪切速率的关系。实验发现,磁流变液出现磁饱和现象不仅与材料特性有关,而且与配比比例有关;剪切应力不仅与材料的颗粒大小有关,还与材料的制备工艺有关。     

稠化剂含量对磁流变脂流变行为的影响

稠化剂含量是决定磁流变脂骨架结构强度的重要因素,对磁流变脂的实际应用具有较大的影响。实验制备了羰基铁粉质量百分数含量为30%、不同稠化剂含量的5种磁流变脂,考察了不同稠化剂含量对磁流变脂屈服应力、储能模量、损耗模量及磁流变效应的影响。结果表明,磁流变脂表现出剪切稀化的假塑性流体特征,不同稠化剂含量磁流变脂的屈服应力与磁场强度呈现出幂律关系,在适宜稠化剂含量时,幂律指数达到最佳值。提出评价磁流变脂的磁流变效应、交联结构体系强度和稳定性的参数指标,通过调节稠化剂的含量可改变非磁性骨架结构的密度,优化磁性部分对磁流变脂结构强度的贡献,改善磁流变脂的磁流变效应,拓宽磁流变脂的储能模量在磁场下的可控范围。     

杆形颗粒磁流变液的剪切屈服应力模型

杆形颗粒磁流变液是新近出现的新型磁流变液,它比传统磁流变液具有更好的性能.基于磁力学理论,通过分析磁化链中杆形颗粒的受力,包括磁力、压力、摩擦力及磁场对颗粒的力矩等,建立了磁流变液的剪切屈服应力模型,并和球形颗粒磁流变液加以对比,发现杆形颗粒磁流变液具有更高的屈服应力.计算结果和实验数据的比较表明,该模型能描述不同磁场...     

超细Co-Ni合金复合磁流变液的制备及流变性质研究

利用多元醇软化学方法制备了Co-Ni超细颗粒,将Co-Ni磁性颗粒与微孔材料通过粘结剂混合研磨,制得复合磁性颗粒,为了克服磁流变液的沉降稳定性,令复合磁性颗粒的密度等于载液的密度,各组分按此比例制备的MRF具有较好的稳定性。利用旋转粘度计测试表明,无外场时,磁流变液的剪切应力τ随剪切速率γ变化的本构关系为：τ＝11．4γ^0.7,为一非牛顿液体。探讨了剪切应力和表观粘度系数随外加磁场的变化关系,表明研制的MRF的流变性质具有较高的低磁场敏感性质。     

磁流变脂材料及其应用研究进展

智能材料是指在磁场、电场、应力等环境变量作用下性能可控的新型功能材料,是继天然材料、合成高分子材料、人工设计材料之后的第四代材料.磁流变材料是一种重要的场响应型智能材料,主要成分为磁性颗粒和非磁性基载液,其物理和流变特性受磁场影响,并且具有优异的可控性,这种特性使磁流变材料受到学者的广泛关注,在科学与工程领域拥有良好的应用前景.近年来出现的磁流变材料包括磁流变液、磁流变脂和磁流变弹性体等,目前磁流变液的研究与应用较为广泛,磁流变液可通过外部磁场进行调控,流变性质在几个数量级上表现出阶跃性和可调性的变化,可以在几毫秒内从类流体转变为类固体状态.在零磁场强度下,磁流变液的行为类似于非牛顿流体,具体特性取决于磁流变液的自身组分,通常表现出具有屈服应力的宾汉塑性流体行为.磁流变液可以在阻尼器、减震器和离合器等领域提供半主动控制,但沉降问题一直制约着磁流变液的发展.磁流变脂以润滑脂为基载液,润滑脂的特殊结构能够有效改善磁流体的沉降稳定性,因此在磁流变器件应用领域展现出显著优势.本文从磁流变脂制备、流变特性、磁流变脂器件及其潜在应用这四个方面综述了磁流变脂材料及其应用研究进展,针对磁流变脂的材料特性和实际应用展开讨论,与磁流变液进行相应的比较,并对磁流变脂作为润滑材料的发展趋势进行了展望.     

磁流变液的屈服应力与温度效应

通常磁流变液的屈服应力大约比电流变液大一个数量级，磁流变学液由于具有较强的屈服应力而受到广泛的重视。我们用多畴的软磁材料同连续相一起充分混事获得的磁流变液，在３×１０＾５Ａ／ｍ的场强下静态屈服应力达到６０ｋＰａ以上，而其零场时的粘度仅为１５ｍＰａ．ｓ，本文报道了匀们研制的拖板式磁流变仪的性能。同时，我们比较了几种磁流变液在零场下的沉淀性，以及外加磁场，屈服应力，温度，永磁材料，表面处理剂，添加剂和     

磁流变酯机理模拟研究

磁流变液已成为智能材料研究领域的一个重要分支.但磁流变液本身的沉降问题仍然没有得到很好地解决.因此,一种新型的无沉降的磁流变材料--磁流变脂应运而生.建立了磁流变脂的剪切应力描述模型,并根据铁磁颗粒的浓度建立了体心立方柱状模型来分析磁流变脂机理及其特性.磁流变脂总的剪切应力可以表达为粘性载液产生的剪应力与颗粒产生的剪应...     

部分凝固合金的等温流变特性

文章研究了用同轴双筒流变仪测定在 Couette 剪切流动中Sn-15％Pb 合金部分凝固时的流交性质。结果表明:当固相分数接近极限固相分数时,半固态浆液具有宾汉流体的流变学特征:宾汉屈服现象和剪切应力与切变速率曲线线性化。这是由于固相颗粒形成了不连续网。而当浆液中固相分数较低时,它的流变性更象假塑性流体。研究发现:在一定的剪切条件下,固相颗粒不是彼此分离的,而是存在聚集。在等温剪切流动中固相颗粒群的形成与崩溃,对部分凝固合金的流变性质有重要影响。     

Ductile shear failure or plug failure of spot welds modelled by modified Gurson model

For resistance spot welded shear-lab specimens, interfacial failure under ductile shearing or ductile plug failure are analyzed numerically, using a shear modified Gurson model. The interfacial shear failure occurs under very low stress triaxiality, where the original Gurson model would predict void nucleation and very limited void growth. Void coalescence would therefore be largely postponed. However, using the shear modification of the Gurson model, recently introduced by Nahshon and Hutchinson (2008) [1], failure prediction is possible at zero or even negative mean stress. Since, this shear modification has too large effect in some cases where the stress triaxiality is rather high, an extension is proposed in the present study to better represent the damage development at moderate to high stress triaxiality, which is known to be well described by the Gurson model. Failure prediction and tensile response curves for an interfacial shear failure or a ductile plug failure, are here compared when using either the original Gurson model, the shear modified model, or the extension to the shear modified model. The suggested extension makes it possible to use the shear modified model as a simple way of accounting for damage development under low triaxiality shearing, without further increasing the damage rate in regions of moderate to high stress triaxiality.     

Study of shear behavior of granular materials by 3D DEM simulation of the triaxial test in the membrane boundary condition

This paper studies the shear behavior of granular materials by using a three-dimensional (3D) discrete element method (DEM) simulation of the triaxial test. The experimental triaxial tests were conducted on glass beads samples for verification. DEM simulations of the triaxial test were carried out in the membrane boundary condition consisting of 37,989 membrane particles. A new method that divides the irregular sample shape into two parts of cones and parts of three-dimensional simplexes is used to follow the volume change of irregular deformation of samples. The free rotatable upper platen is considered during the shearing process, which influences the shear behavior of samples especially in the residual stage and formations of a single shear band or X-shape shear band. The confining pressures have been demonstrated to influence the rotation angle and angular velocity of the upper platen. Moreover, the timing of replacing a rigid wall boundary condition with the membrane boundary condition is investigated, which affects the porosity of samples before shearing and the mechanical strength. The DEM model in the membrane boundary condition reflects well the evolution of irregular sample deformation and shear band in the shearing process. From the perspective of micro structures, the normal force decreases and the tangential stress increases during the shearing stage. This study greatly improves the accuracy of DEM simulations of the triaxial test in the membrane boundary condition.     

基于修正GTN模型的不锈钢管剪切过程韧性断裂准则研究

为了改善修正Gurson-Tvergaard-Needleman模型(修正GTN模型)无法准确模拟不锈钢管剪切过程的缺陷,该文在原模型基础上,构造了一个与位移相关的损伤函数作为新的断裂准则。同时,在ABAQUS中采用隐式向后Euler应力更新算法编制了改进后模型的VUMAT用户子程序,实现了其数值求解。最后进行了SUS304不锈钢管剪切试验,通过分析对比试验数据与改进后模型的模拟结果,验证了该文所提出的方法的准确性。     

中厚板圆盘剪剪切力能参数测试及最佳剪刃间隙数学模型的建立

针对中厚板圆盘剪剪切质量差、剪切力理论计算偏差大等问题开展了生产实测、数据采集、回归拟合处理,得出现有的剪切力理论公式计算结果和实测结果相比误差在-9.56%～+56.5%之间.这些理论公式求解中未考虑随着厚度的增加剪切抗力和抗剪面积的非线性变化,导致剪切力计算结果大幅减小.这些公式适宜于薄规格钢板的剪切力计算,但用于厚板剪切力计算还要进一步完善.同时,建立了一个综合剪切钢板厚度、材料属性、累积剪切面积的圆盘剪剪刃间隙调整模型,并且考虑了刀盘装置在剪切受力过程中的变形对剪刃间隙影响.因此,针对不同的圆盘剪设备,要在该模型中考虑刀轴载荷-变形值的影响,对模型进行修正完善,就可获得适合于自己的间隙调整工艺参数.     

Simulation of mechanical response in crystallizing polymers: crystallization under a constant shear force

Summary The mechanical response of crystallizing polymers under shearing controlled by a constant load is simulated by the model developed by Negahban et al. [10]. It is shown that under a constant shear load the shear strain increases with crystallization. The history of this shear strain directly affects the residual shape of the material after unloading. The shear modulus and elastic moduli are calculated as functions of the shear load and the degree of crystallization. Comparison with simulations based on a constant shear strain shows that some material moduli have the same functional relations and others are different. Specific simulations based on parameters selected for natural rubber are presented and compared to the response under constant shear strain.     

论岩土塑性体应变与剪应变的相互作用原理

对岩土塑性变形过程中体应变与剪应变的相互作用原理的内涵做了深入的阐述，特别对塑性体应变和剪应变之间的相互作用方式，塑性体应变对抗剪能力的直接控制作用，剪缩和剪胀发生的条件，应力路径相关性是这种相互作用的综合体现及实例验证，临界状态是塑性和弹性体应变都保持不变的纯粹剪切变形过程等方面进行了论证；根据这个相互作用原理，在理论上证明了空间临界状态线的存在性和唯一性以及它与应力是无关的。     

磁场作用下磁流变液剪切性能的实验分析

本文研究了磁流变液的力学性能,制备了羰基铁粉体积分数为20%的硅油基磁流变液,观察了磁流变液在磁场作用下的微观变化,测试了磁流变液的剪切性能。实验表明,无磁场作用下,磁流变液为牛顿流体;在磁场作用下,随着剪切速率的增加,剪切应力趋于稳定,表观粘度呈现指数式下降,磁流变液具有剪切稀化效应,符合广义Bingham模型。磁流变液剪切应力和外加电流的依赖关系为:在电流较小时,剪切应力表现为指数增长,指数值约为1.5;随着外加电流的增大,剪切应力表现为线性增长,最终达到稳定值。     

DEM simulation of shear vibrational fluidization of granular material

Fluidization of dry granular material is the transition from a solid state to a liquid state when sufficient energy is applied during vibration. This behavior is important because it is closely related to deformations of geotechnical structures during an earthquake. The scientific challenge lies in the understanding on how strain localization is related to the fluidization zone during the entire shearing process. Despite the importance of the mechanical behavior of granular material during fluidization, it cannot be easily characterized using traditional direct shear test. In this paper, 2D DEM model is firstly conduct, shear vibrational fluidization is defined for dry granular material, and the discrete element method has been used to simulate the direct shear test on granular material under vibrational loading during shearing. The peak, residual and vibro-residual shear strength envelopes have been obtained from the numerical simulations. Three distinct zones have been identified in the upper shear box based on the observed changes in volumetric strain before vibration. During vibration, fluidization occurs in the three zones with the characteristics that the shear stress, porosity, volumetric strain, and the coordination number drop to relatively lower values. During vibration, material becomes denser than the critical state, and strain localization has been relieved. Densification of the material at the shear zone leads to a strengthening of the material which increases the shearing resistance after vibration. Furthermore, a comparison of the 2D and 3D simulations is performed. Results reveal that the motion of particles in the out-of-plane direction in the 3D simulations lead to smoother shear stress and more consistent with the experimental result.     

Contribution to critical shear stress of nanocomposites produced by interaction of screw dislocation with nanoscale inclusion

The contribution to the critical shear stress of nanocomposites caused by the interaction between screw dislocations and nanoscale circular cylindrical inclusions with interface stresses is derived by means of the Mott and Nabarro's model. The effect of the radius of the nanoscale inclusion and the volume fraction of inclusions as well as the interface stress on the critical shear stress is examined. The most important finding is that, when the negative interface stress is considered, a critical value of the radius of the nanoscale inclusion or the volume fraction of inclusions may exist to gain the best strengthening effects for the second-phase strengthened composites which differs from the classical solution without considering the interface stress under same external conditions.