Viskosität und Struktur von basischen Aluminiumchlorid-Lösungen

Zusammenfassung Es wird der Einfluß von Konzentration, Temperatur und Schubspannung auf die effektive Viskosität der Lösungen von basischem Aluminiumchlorid (BACh) der Formel m Al₂O₃ · m HCl · 55,5 H₂O ([m] = mol Al₂O₃/1000 g H₂O) untersucht. Dabei werden Lösungen betrachtet, für die M = m/m = 1 ist. Je nach der Konzentration zeigen die BACh-Salze entweder ein newtonsches oder aber ein nicht-newtonsches Fließverhalten. Aus der Aktivierungsenergie, der Fließenthalpie und der Fließentropie und deren Abhängigkeit von der Konzentration und der Schubspannung kann man Schlüsse betreffend die Struktur der Lösungen ziehen. Es wird gezeigt, daß hierfür Wasserstoffbrückenbindungen eine entscheidende Rolle spielen.

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The influence of concentration, temperature and shear stress on the effective viscosity of basic aluminium chloride (BACh) solutions with the formula m Al₂O₃ · m HCl · 55.5 H₂O ([m] = mol Al₂O₃/1000 g H₂O) has been investigated for solutions with M = m/m = 1. Depending on the concentration these solutions show rheological behaviour of either Newtonian or non-Newtonian type. Conclusions about the solution's structure can be obtained from the activation energy, enthalpy and entropy of the flow and their dependence on the concentration and the shear stress. It is shown that hydrogenbond networks play a decisive role in the formation of the solution structure.

     

Stress optical measurement of the third normal stress difference in polymer melts under oscillatory shear

Results are reported for the dynamic moduli,G andG, measured mechanically, and the dynamic third normal stress difference, measured optically, of a series bidisperse linear polymer melts under oscillatory shear. Nearly monodisperse hydrogenated polyisoprenes of molecular weights 53000 and 370000 were used to prepare blends with a volume fraction of long polymer, _L, of 0.10, 0.20, 0.30, 0.50, and 0.75. The results demonstrate the applicability of birefringence measurements to solve the longstanding problem of measuring the third normal stress difference in oscillatory flow. The relationship between the third normal stress difference and the shear stress observed for these entangled polymer melts is in agreement with a widely predicted constitutive relationship: the relationship between the first normal stress difference and the shear stress is that of a simple fluid, and the second normal stress difference is proportional to the first. These results demonstrate the potential use of 1,3-birefringence to measure the third normal stress difference in oscillatory flow. Further, the general constitutive equation supported by the present results may be used to determine the dynamic moduli from the measured third normal stress difference in small amplitude oscillatory shear. Directions for future research, including the use of birefringence measurements to determineN ₂/N ₁ in oscillatory shear, are described.     

Influence of superimposed steady shear flow on the dynamic properties of non-Newtonian fluids

Summary Experiments in which an oscillatory shear flow is superimposed on a steady-state circular shear flow between a cone and a plate were performed on non-Newtonian solutions by means of aWeissenberg Rheogoniometer. The steady-state shear stress and in a first approximation also the normal stress difference arising from the steady shear flow appear not to be influenced by the superimposed oscillatory flow. On the other hand, the dynamic moduli as obtained from the oscillatory parts of shear stress and shear flow are highly dependent on the superimposed steady rate of shear. The absolute value of the complex shear modulus decreases and the phase difference between oscillatory shear stress and shear flow increases in all cases and for all frequencies if the superimposed shear rate is increased. Consequently, this phase difference can become equal to and even larger than /2. Between the angular frequency ₀ at which the phase difference is /2 and the steady shear rateq the relation ₀= 1/2,q was experimentally found to exist in most cases. These dynamic results cannot be described by the current theories of viscoelasticity. The large and fast deformations imposed on the material should explicitly be taken into account.     

Yield stress: A time-dependent property and how to measure it

This paper reviews the different aspects of the yield stress phenomenon and attempts a synthesis of knowledge. Yield stress can be probed using constant shear stress or shear rate. The magnitude of the result depends on the time allowed to determine whether the sample has developed continuous flow or has ceased flowing. It is closely associated with creep, stress growth and thixotropic breakdown and recovery, and the characteristic times of these transient responses play a part in yield stress measurement. In thixotropic fluids, yield stress is a function of structure and hence of time. In simple thixotropy, the yield stress derived from the equilibrium flow curve is the same as that for the fully built-up structure. But in many materials, the static yield stress obtained after prolonged rest is much higher than the dynamic yield stress from the equilibrium flow curve. This is associated with the phenomenon in which the equilibrium flow curve bends upwards as the shear rate is reduced to very low values. The paper also reviews the many methods that can be used to measure yield stress. It is pointed out that the choice of observation time or shear rate to use should be related to the characteristic time of the flow process to which the result is to be applied. Examples discussed are solids suspension capability of fluids, levelling and sagging, pipeline flow and start-up power requirement of mixers. CS constant structure - D diameter of Gun Rheometer tube - EFC equilibrium flow curve - G measured torque - L length of Gun Rheometer tube - P _min minimum pressure to cause flow - t time - form factor for shear stress - - _y - shear rate - a particular value of shear rate - reference shear rate - test shear rate - shear stress - _y yield stress - _yd dynamic yield stress - _ys static yield stress - 0 initial value after speed change - e equilibrium Paper presented at the British Society of Rheology Conference on New Techniques in Experimental Rheology, University of Reading, 9–11 September 1985.     

Experimental validation of steady shear and dynamic viscosity relation for yield stress fluids

A common problem when studying yield stress fluids under steady shear in rotating rheometry is that of sample fracture. It is therefore preferable to work with oscillating shear, where fracture is limited. Doraiswamy et al. (1991) proposed a model for yield stress fluids that predicts the relation: ^*( _m ) = (y) between the viscosity in steady shear and the complex viscosity in dynamic shear. The present study validates this relation experimentally with both controlled stress and controlled strain, and demonstrates its limitations. Three yield stress fluids were used: a lubricating grease with lithium based soap, a thixotropic dispersion of colloidal silica in a polymer solution and a non-thixotropic aqueous gel.     

Electrorheological response of SnO<Subscript>2</Subscript> and Y<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles in silicon oil

The electrorheological properties (ER) of some fluids containing particles change extensively under the external electrical field. This phenomenon is applicable in many industries and equipments, such as clutches and motor driven rotor, which would transfer the spin to a drive shaft through a thin layer of electrorheological fluid. In this investigation, the effects of external electrical field on ER properties of non-Newtonian fluids (silicon oil) with the addition of SnO₂ and Y₂O₃ nanoparticles were studied. The ER properties were measured for a wide range of SnO₂ and Y₂O₃ nanoparticle concentrations and DC electrical voltages using concentric cylinder rotary rheometer. Based on the results, ER properties of nanofluids, e.g., apparent viscosity, shear stress, and yield stress, were enhanced by applying electrical field and increasing SnO₂ and Y₂O₃ concentrations.

Shear orientation of a lamellar lyotropic liquid crystal

Shear orientation of a lyotropic lamellar liquid crystalline phase of tetra ethyleneglycol mono dodecylether, C₁₂A₄ in water was studied by combined rheo-small-angle light scattering. Shear thinning was observed with a sample of 500 m thickness. The scattering patterns showed that domains were aligned and stretched in flow direction. Shear thinning was also observed with a thinner sample, but caused a strong change in light-scattering pattern. A maximum of scattering intensity was observed at finite scattering vector and shifted to a higher scattering vector with increasing shear stress. This observation can be explained by a fragmentation of domains at high shear stress and is supported by a mosaic texture observed in optical microscopy.     

Novel material tailoring method for internally pressurized FG spherical and cylindrical vessels

A new material tailoring method for spherical and cylindrical vessels made of functionally graded materials (FGMs) is presented. It is assumed that the FG material is composed of an Al–SiC metallic-matrix composite. A uniform ratio of in-plane shear stress to yield strength [\(\varphi \left( r \right) \)] is used as the design criterion to utilize the maximum capacity of the vessel. The aim is to find a distribution of SiC particles in the radial direction, i.e., \(f\left( r \right) \), that achieves a uniform index \(\varphi \left( r \right) =\hbox {const}.\) through the wall thickness of the internally pressurized spherical or cylindrical vessel. Both the Mori–Tanaka and rule-of-mixtures homogenization schemes are used to express the effective elastic module and Poisson’s ratio. Moreover, the strength of the composite is expressed based on the rule of mixtures. Besides, finite element simulation is carried out to verify the accuracy of the analytical solution. The effects of input parameters such as the internal pressure, strength of the SiC particles, ratio of in-plane shear stress to effective yield strength, and choice of homogenization scheme on the tailored distribution of the SiC volume fraction in the radial direction are also investigated.     

高速车削300M超高强度钢时的Al2O3 基陶瓷刀具磨损机理研究

使用Al2O3基陶瓷刀具对300M超高强度钢进行了干切削试验,采用电子扫描显微镜（SEM）观察刀具的磨损形貌,并通过能谱分析仪（EDS）测量了陶瓷刀具磨损微区的各元素含量,分析了陶瓷刀具的主要磨损机理.结果表明：陶瓷刀具磨损的主要机理为粘结磨损、磨粒磨损和氧化磨损.粘结磨损主要发生在前刀面上,且受刀具材料和工件材料接触点应力状态的影响.刀具前、后刀面的磨损边缘区易发生氧化磨损.     

Analysis of a permeable interface crack in elastic dielectric/piezoelectric bimaterials

A permeable interface crack between elastic dielectric material and piezoelectric material is studied based on the extended Stroh’s formalism. Motivated by strong engineering demands to design new composite materials, the authors perform numerical analysis of interface crack tip singularities and the crack tip energy release rates for 35 types of dissimilar bimaterials, respectively, which are constructed by five kinds of elastic dielectric materials: Epoxy, Polymer, Al₂O₃, SiC, and Si₃N₄ and seven kinds of practical piezoelectric ceramics: PZT-4, BaTiO₃, PZT-5H, PZT-6B, PZT-7A, P-7, and PZT-PIC 151, respectively. The elastic dielectric material with much smaller permittivity than commercial piezoelectric ceramics is treated as a special transversely isotropic piezoelectric material with extremely small piezoelectricity. The present investigation shows that the structure of the singular field near the permeable interface crack tip consists of three singularities: and , which is quite different from that in the impermeable interface crack. It can be concluded that different far field loading cases have significant influence on the near-tip fracture behaviors of the permeable interface crack. Based on the present theoretical treatment and numerical analysis, the electric field induced crack growth is well explained, which provides a better understanding of the failure mechanism induced from interface crack growth in elastic dielectric/piezoelectric bimaterials. The project supported by the National Natural Science Foundation of China (10572110), Doctor Foundation of the Chinese Education Ministry and Doctorate Foundation of Xi’an Jiaotong University. The English text was polished by Yunming Chen.     

Dynamic effective shear strength of saturated sand

The dynamic effective shear strength of saturated sand under cyclic loading is discussed in this paper. The discussion includes the transient time dependency behaviors based on the analysis of the results obtained in conventional cyclic triaxial tests and cyclic torsional shear triaxial tests. It has been found that the dynamic effective shear strength is composed of effective frictional resistance and viscous resistance, which are characterized by the strain rate dependent feature of strength magnitude, the coupling of consolidation stress with cyclic stress and the dependency of time needed to make the soil strength sufficiently mobilized, and can also be expressed by the extended Mohr-Coulomb's law. The two strength parameters of the dynamic effective internal frictional angle φ_d and the dynamic viscosity coefficient η are determined. The former is unvaried for different number of cyclic loading, dynamic stress form and consolidation stress ratio. And the later is unvaried for the different dynamic shear strain rate developed during the sand liquefaction, but increases with the increase of initial density of sand. The generalization of dynamic effective stress strength criterion in the 3-dimensional effective stress space is studied in detail for the purpose of its practical use. The project supported by the National Natural Science Foundation of China (10172070)     

A direct comparison of non-destructive techniques for determining bridging stress distributions

Crack bridging is an important source of crack propagation resistance in many materials and the bridging stress distribution as a function of crack opening displacement is widely believed to represent a true material property uninfluenced by sample geometry, loading conditions, and other extrinsic factors. Accordingly, accurate measurement of the bridging stress distribution is needed and many non-destructive methods have been developed. However, there are many challenges to accurately determining bridging stresses. A comparison of bridging stresses measured using R-curve, crack opening displacement (COD), and spectroscopy methods has been made using two bridging ceramics, Y₂O₃ and MgO doped Si₃N₄ and 99.5% pure Al₂O₃. The COD method is surface sensitive and gives a lower peak bridging stress compared to the R-curve technique which samples through the entire material thickness. This is attributed to a more compliant near surface bridging zone. Conversely, when R-curves rise steeply over the first few micrometers of growth from a notch, an effect of negative T-stress is expected to raise the R-curve determined peak bridging stress. Spectroscopy methods were only found to yield reliable bridging stress results if a reasonable through thickness volume of material is sampled. It was found that 2.5% of the specimen thickness achieved using fluorescence spectroscopy appears adequate for Al₂O₃ while 0.1–0.2% of the sample thickness achieved using Raman spectroscopy for Si₃N₄ appears inadequate. Overall, it is concluded that in the absence of T-stresses a bridging distribution can be determined that is a true material property. Also, a new method is proposed for determining the bridging stresses of fatigue cracks from (1) the bridging stress distribution for monotonically loaded cracks and (2) experimental fatigue data.     

准三维针刺C_f/SiC复合材料室温层向动态压缩力学行为的实验研究

为了研究应变率对准三维针刺碳纤维增韧的碳化硅复合材料(Cf/SiC)层向压缩力学性能的影响,本文利用分离式Hopkinson压杆装置对三维针刺Cf/SiC复合材料进行了应变率为10-4至6.5×103s-1的单轴压缩力学性能测试。实验结果表明,由于材料缺陷,其动态压缩强度分布遵循Weibull分布。破坏时,材料并未表现出典型的脆性破坏,而是在应力达到压缩强度后经历了较大的伪塑性变形才最终破坏。这表明三维针刺Cf/SiC复合材料沿厚度方向针刺的碳纤维有助于提高材料的韧性。同时,材料的压缩强度随应变率的升高显著增大,并与对数应变率近似成线性关系。借助光学显微镜和扫描电镜对压缩断口的观察表明:材料的失效模式随着应变率变化而发生改变。在准静态下,材料主要表现为剪切和分层破坏,而在高应变率下,则主要表现为劈裂。     

The wedge subjected to tractions: a paradox resolved

The classical two-dimensional solution provided by Lévy for the stress distribution in an elastic wedge, loaded by a uniform pressure on one face, becomes infinite when the opening angle 2 of the wedge satisfies the equation tan 2 = 2. Such pathological behavior prompted the investigation in this paper of the stresses and displacements that are induced by tractions of O(r ^–) as r0. The key point is to choose an Airy stress function which generates stresses capable of accommodating unrestricted loading. Fortunately conditions can be derived which pre-determine the form of the necessary Airy stress function. The results show that inhomogeneous boundary conditions can induce stresses of O(r ^–), O(r ^– ln r), or O(r ^– ln² r) as r0, depending on which conditions are satisfied. The stress function used by Williams is sufficient only if the induced stress and displacement behavior is of the power type. The wedge loaded by uniform antisymmetric shear tractions is shown in this paper to exhibit stresses of O(ln r) as r0 for the half-plane or crack geometry. At the critical opening angle 2, uniform antisymmetric normal and symmetric shear tractions also induce the above type of stress singularity. No anticipating such stresses, Lévy used an insufficiently general Airy stress function that led to the observed pathological behavior at 2.     

On-line measurement of elasticity and viscosity in flowing polymeric liquids

A new slit-die rheometer (the Stressmeter) for on-line and sample measurement of the viscosity, , and the first normal stress difference, N ₁, in steady shear flow for molten polymers and other high-viscosity liquids is described. Two liquid-filled transverse slots, located in one die wall near the center station, give pressures P ₂ and P ₃ from whose difference the wall shear stress is calculated. In the other die wall at a location opposite the center of the P ₂ slot is a flush-mounted transducer, giving a pressure P ₁. N ₁ is calculated from the hole pressure P ^* = P ₁–P ₂. A metering pump, used to measure the flow rate Q, is supplied with melt either from an extruder (online mode) or from a pressurized sample cylinder (sample mode). The wall shear rate is calculated from Q and ; the Weissenberg-Rabinowitsch correction and a new small-viscous-heating-correction algorithm (affecting ) are used. Viscous heating corrections are small; entrance and exit errors are negligible. The instrument is tested by comparing its results with those obtained from cone-plate and capillary rheometers. Measurement ranges extend to = 200 kPa, = 3000 s^–1, and temperature = 250°C.Dedicated to Prof. Dr. J. Meissner on the occasion of his retirement from the chair of Polymer Physics at the Eidgenössische Technische Hochschule (ETH) Zürich, Switzerland     

Microhardness indentation application and limitation in fracture-toughness evaluation of ceramics

The use of a microhardness indentation as a controlled flaw in a flexural specimen for measuring fracture toughness in advanced ceramics is critically evaluated. From a review of fracture-toughness testing methodologies for ceramics it is shown that currently there are not any standardized or widely agreed uponK _Ic test procedures. The problems associated with the controlled flaw test are discussed and analyzed. An experimental program is used to investigate the effects of residual stress on the measurement of fracture toughness in a wide variety of sintered and hot-pressed advanced ceramics including TiB₂, Al₂O₃, SiC, ZrO₂, Si₃N₄. The validity of the test procedure for evaluatingK _Ic of advanced ceramics in general is put forth.     

花岗岩动静态压剪复合加载实验研究

岩石为天然含有微裂纹微孔洞等缺陷的复杂介质,其在复杂应力状态,尤其是在压剪应力状态下的性能研究,对工程应用有着十分重要的意义。本文利用双斜面垫块改进现有装置对花岗岩进行了动静态下的压剪复合加载实验,其中静态实验基于MTS材料试验机,动态实验基于分离式Hopkinson压杆（以下简称SHPB）,由此得到不同压剪载荷联合作用下花岗岩的静态和动态力学性能。通过改变倾斜端面的角度就能够得到岩石在不同压剪加载路径下的力学性能,由此可以得到岩石的静动态破坏面。结果表明:花岗岩的破坏强度具有明显的加载速率效应。本研究为探讨岩石在复杂应力状态下的力学性能提供了新的方法。     

The “Twin capillary” a simple device to separate shear- and slip-flow of fluids

When the flow behaviour of fluids is investigated with capillary-or rotational rheometers, adhesion of the fluid to the wall is normally one of the boundary conditions. For many fluids, especially for suspensions, this assumption is not valid. These fluids tend to slip at the wall. Therefore the normal evaluation of rheometer measurements leads to apparent but not compatible flow functions. The flow behaviour of these fluids can be characterized with two material functions which describe separately slipping in the boundary layer and shearing within the fluid. Only if both functions are known, correct predictions of flow processes are possible. A simple equipment to separate the shear function and the slip function is described.List of symbols Y^* apparent shear rate - Y _w ^* apparent wall shear rate - Y_w wall shear rate corrected with Rabinowitsch and Weissenberg correction - Y_s reduced shear rate (slip corrected) - Y_ws reduced wall shear rate (slip corrected) - * (r) velocity distribution in a capillary - _G slip velocity (at the wall) - * (r) velocity distribution in a capillary (without slip) - shear stress - _w wall shear stress - V_S total volume rate - V_G shear volume rate - V_G slip volume rate - p ₁ pressure in the reservoir channel of the capillary rheometer - p ₀ athmospheric pressure - L capillary length - R capillary radius     

Impact failure of granular materials – Non-equilibrium multiscale simulations and high-speed experiments

We present results of high-speed impact experiments on aluminum oxide (Al₂O₃) and silicon carbide (SiC) ceramics and propose a mesoscopic way to model the fracture behavior of these brittle materials based on discrete particles. The two-dimensional model used here has only three adjustable parameters, but is able of reproducing many salient features of the investigated ceramics under compressive, tensile and shock impact load. We discuss our particle model in detail and then consider strain and shear load simulations. In particular, we model explicitly the macroscopic experimental set-up of the edge-on impact experiment and show that the experimentally observed crack patterns can in principle be explained by the random distribution of particle overlaps and the thereby generated differences in the local strength of the material.