锆基非晶合金的动态弛豫机制和高温流变行为

非晶合金的动态弛豫机制对于理解其塑性变形,玻璃转变行为,扩散机制以及晶化行为都至关重要.非晶合金的力学性能与动态弛豫机制的本征关联是该领域当前重要科学问题之一.本文借助于动态力学分析(DMA),探索了Zr₅₀Cu₄₀Al₁₀块体非晶合金从室温到过冷液相区宽温度范围内的动态力学行为.通过单轴拉伸实验,研究了玻璃转变温度附近的高温流变行为.基于准点缺陷理论(quasi-point defects theory),对两种力学行为的适用性以及宏观力学行为变化过程中微观结构的演化规律进行描述.研究结果表明,准点缺陷理论可以很好地描述非晶合金损耗模量α弛豫的主曲线.基于非晶合金的内耗行为,玻璃转变温度以下原子运动的激活能U_β为0.63 eV.与准点缺陷浓度对应的关联因子χ在玻璃转变温度以下约为0.38,而在玻璃转变温度以上则线性增大.Zr₅₀Cu₄₀Al₁₀块体非晶合金在玻璃转变温度附近,随温度和应变速率的不同而在拉伸实验中显示出均匀的或不均匀的...     

Elastic properties of polyethylene melts at high shear rates with respect to extrusion

At high shear rates a steady state of shear flow with constant shear rate, constant shear stress, and constant recoverable shear strain is observed in the short-time sandwich rheometer after some few shear units already. The melt exhibits rather high elastic shear deformations and the recovery occurs at much higher speed than it is observed in the newtonian range. The ratio of first normal stress difference and twice the shear stress, being equal to the recoverable strain in the second-order fluid limit, significantly underestimates the true elastic shear strains at high shear rates. The observed shear rate dependence of shear stress and first normal stress difference as well as of the (constrained) elastic shear strain is correctly described on the basis of a discrete relaxation time spectrum. In simple shear a stick-slip transition at the metal walls is found. Necessary for the onset of slip is a critical value of shear stress and a certain amount of elastic shear deformation or orientation of the melt.     

Extensional behavior influence on viscoelastic turbulent channel flow

In this work we use in the simulation of a viscoelastic turbulent channel flow a modification of the finitely extensible of non-linear elastic dumbbells with the Peterlin approximation (FENE-P) constitutive model for dilute polymer solutions, applicable to high extensional deformations. The new feature introduced by this modification is that the free energy of the polymer (since it is assumed to be entirely entropically driven) remains always bounded (FENE-PB). The characteristics of the model under steady shear flow, pure elongational flow and transient extensional behavior are presented. It is found that the FENE-PB model is more shear thinning than FENE-P. Most importantly, it also shows a higher extensional viscosity than the FENE-P model. Although the steady-state Trouton ratio asymptotically reaches at high extensional rates the same limit as the FENE-P model, the transition from the Newtonian value is sharper and faster. We use the FENE-PB model in direct numerical simulations (DNS) of viscoelastic turbulent channel flow using spectral approximations. The results for various statistics of the flow and the polymer conformation, when compared against those obtained with the original FENE-P model and the same rheological parameters, show an enhanced polymer-induced drag reduction effect and enhanced deformation of the polymer molecules. This indicates that it is not only the asymptotic but also details from the extensional rheological behavior that matter in quantitatively specifying turbulent viscoelastic flow behavior.     

Phenomenological modeling of the response of a dense colloidal suspension under dynamic squeezing flow

The split Hopkinson pressure bar experimental technique is used to evaluate the squeezing flow response of a concentrated, discontinuously thickening colloidal suspension of spherical silica particles loaded at high stresses/strain rates. These results provide insight into the transitional behavior of these materials, as well as the post-transitional response under compressive loading. A method of analyzing the strain and strain rate dependent behavior is presented to identify modes of material response (viscous, elastic, etc.). Experimental results are presented as stress–strain–strain rate plots and a surface fitting approach is used to develop a phenomenological model describing the overall response. From this model, it is possible to identify regions of elastic and viscous behavior using a gradient analysis approach. It was found that, after an initial period of viscous deformation, the suspension behaves like a viscoelastic material – this regime corresponds well with transition in which large clusters of particles percolate. This is followed by a third, viscous regime in which the material undergoes viscous deformation. At the highest stresses, a plateau region of plastic deformation has been identified. This approach and the conditions under which it may be applied are described in detail in the paper.     

Analytic derivation of the Cox�CMerz rule using the MLD ��toy�� model for polydisperse linear polymers

A general constitutive formalism, the ??na?ve?? polydisperse MLD model, has been developed by Mead et al. (Macromolecules 31:7895?C7914, 1998) and Mead (Rheol Acta 46:369?C395, 2007) at both the tube coordinate level and the mathematically simplified ??toy?? level independent of the tube coordinate. The model includes constraint release generated by convection-driven chain retraction (which is equivalent to ??convective constraint release?? (CCR)), reptation, and tube contour length fluctuations. The properties of the mathematically simplified na?ve polydisperse ??toy?? MLD model are explored in linear and nonlinear steady shear flows where we analytically derive the Cox?CMerz rule relating the steady shear viscosity to the modulus of the linear viscoelastic dynamic viscosity. The Cox?CMerz rule relating the linear viscoelastic material properties and the nonlinear material properties is shown to be a direct consequence of convective constraint release. The specific feature of CCR that leads to this result is that the relaxation rate due to convective constraint release is proportional to the shear rate, $\dot{{\gamma }}$ , independent of molecular weight. The viability of this well-known empirical relationship is a direct consequence of a coincidence in the mathematical structure of the linear viscoelastic material properties and convective constraint release. There is no physical analogy or relationship between the molecular relaxation mechanisms operative in linear (diffusive relaxation) and nonlinear (convective relaxation) flow regimes. The polydisperse MLD model predictions of the individual molecular weight component contributions to the flow curve, and interpretations thereof, are effectively identical to those first postulated by Bersted (J Appl Polym Sci 19:2167?C2177, 1975, J Appl Polym Sci 20:2705?C2714, 1976). Following the theoretical developments, a limited experimental study is executed with a commercial polydisperse polystyrene melt. Nearly quantitative agreement between the polydisperse MLD theory and experimental measurements of steady-shear viscosity and dynamic moduli is achieved over a wide range of shear rates.     

Non-Newtonian viscous oscillating free surface jets, and a new strain-rate dependent viscosity form for flows experiencing low strain rates

A model for oscillating free surface jet flow of a fluid from an elliptical orifice, together with experimental measurements, can be exploited to characterize the elongational viscosity of non-Newtonian inelastic fluids. The oscillating jet flow is predominantly elongational, with a small strain that oscillates rapidly between large and zero strain rates. We find that to reproduce the experimentally observed steady oscillating jet flow in model simulations, the assumed form of the non-Newtonian viscosity as a function of strain rate must have zero gradient, i.e., be Newtonian, at zero strain rate (a behavior exhibited, in general, by real inelastic fluids). We demonstrate that the Cross, Carreau, Prandtl-Eyring, and Powell-Eyring forms, although they have finite viscosity at zero strain rate, have either nonzero or even unbounded gradient at zero, and hence are unable to model oscillating jet behavior. We propose a new non-Newtonian viscous form which has all of the desirable features of existing forms (high and low strain rate plateaus, with adjustable location and steepness of the transition) and the additional feature of Newtonian behavior at low strain rates. Received: 7 February 2000 Accepted: 31 October 2000     

The reptation model with segmental stretch

The Doi-Edwards model with segmental stretch and a non-linear finitely extensible spring law is described and examined in simple flow situations where analytic results are derivable; namely oscillatory flow and steady state flow at high deformation rates. The model is shown to be consistent with the Bueche-Ferry hypothesis in fast large strain unidirectional flows but to violate this rule in small strain reversing flows. The discrepancy is identified with a preaveraging approximation used to describe the relative tube-chain velocity. Experimentally verifiable scaling rule for the birefringence as a universal function of a planar flow-type parameter and deformation rate are identified. Sensitivity to the extensional flow character, absent in the original tube model, manifests itself with the introduction of segmental stretch. Although the model generates a non-separable memory function kernel the deformation dependence of the memory function is quantitatively shown to have negligible impact on the predicted theological properties relative to the original Doi-Edwards model. With this simplification, relatively uncomplicated approximations to the segmental stretch model can be deduced.     

The solid–fluid transition in a yield stress shear thinning physical gel

We present an experimental investigation of the solid–fluid transition in a yield stress shear thinning physical gel (Carbopol^® 940) under shear. Upon a gradual increase of the external forcing, we observe three distinct deformation regimes: an elastic solid-like regime (characterized by a linear stress–strain dependence), a solid–fluid phase coexistence regime (characterized by a competition between destruction and reformation of the gel), and a purely viscous regime (characterized by a power law stress-rate of strain dependence). The competition between destruction and reformation of the gel is investigated via both systematic measurements of the dynamic elastic moduli (as a function of stress, the amplitude, and temperature) and unsteady flow ramps. The transition from solid behavior to fluid behavior displays a clear hysteresis upon increasing and decreasing values of the external forcing. We find that the deformation power corresponding to the hysteresis region scales linearly with the rate at which the material is being forced (the degree of flow unsteadiness). In the asymptotic limit of small forcing rates, our results agree well with previous steady state investigations of the yielding transition. Based on these experimental findings, we suggest an analogy between the solid–fluid transition and a first-order phase transition, e.g., the magnetization of a ferro-magnet where irreversibility and hysteresis emerge as a consequence of a phase coexistence regime. In order to get further insight into the solid–fluid transition, our experimental findings are complemented by a simple kinetic model that qualitatively describes the structural hysteresis observed in our rheological experiments. The model is fairly well validated against oscillatory flow data by a partial reconstruction of the Pipkin space of the material’s response and its nonlinear spectral behavior.     

A finite deformation fractional viscoplastic model for the glass transition behavior of amorphous polymers

The stress response of amorphous polymers exhibits tremendous change during the glass transition region, from soft viscoelastic response to stiff viscoplastic response. In order to describe the temperature-dependent and rate-dependent stress response of amorphous polymers, we extend the one-dimensional small strain fractional Zener model to the three-dimensional finite deformation model. The Eyring model is adopted to represent the stress-activated viscous flow. A phenomenological evolution equation of yield strength is used to describe the strain softening behaviors. We demonstrate that the stress response predicted by the three-dimensional model is consistent with that of one-dimensional model under uniaxial deformation, which confirms the validity of the extension. The model is then applied to describe the stress response of an amorphous thermoset at various temperatures and strain rates, which shows good agreement between experiments and simulation. We further perform a parameter study to investigate the influence of the model parameters on the stress response. The results show that a smaller fractional order results in a larger yield strain while has little effect on the yield stress when the temperature is below the glass transition temperature. For the stress relaxation tests, a smaller fractional order leads to a slower relaxation rate.     

Thickening behaviour of dilute polymer solutions in non-inertial elongational flows

A molecular interpretation is proposed to interpret the thickening behaviour of dilute solutions of high molecular weight flexible polymers in non-intertial flows having an elongational character. A set of new results has been gathered showing that the onset of very high end-pressure losses at high deformation rates in capillary flow can be explained by a flow-induced coil-stretch transition of macromolecules in solution. When a high degree of elongation is achieved a marked increase in viscous dissipation occurs in elongational flows.     

非均匀颗粒材料的类固-液相变行为及本构方程

以非均匀颗粒介质为研究对象,采用三维离散元方法对其在不同密集度和剪切速率下的动 力过程进行了数值模拟,分析了其在由瞬时接触的快速流动向持续接触的准静态流动的转变 过程及其行为特点. 通过对不同材料性质下相变过渡区内颗粒材料的宏观应力、接触时间数、 配位数、团聚颗粒数量、有效摩擦系数等参量的计算,更加全面地描述了非均匀颗粒材料在 类固-液相变过程中的基本特征. 基于以上数值计算结果,建立了一个适用于颗粒材料 类固态、类液态以及其相变过程的本构方程,并通过剪切室实验结果验证了它的合理性.     

Transitions in Poiseuille flow of nematic liquid crystal

Recent experiments by Sengupta et al. (Phys. Rev. Lett. 2013) [9] revealed interesting transitions that can occur in flow of nematic liquid crystal under carefully controlled conditions within a long microfluidic channel of width much larger than height, and homeotropic anchoring at the walls. At low flow rates the director field of the nematic adopts a configuration that is dominated by the surface anchoring, being nearly parallel to the channel height direction over most of the cross-section; but at high flow rates there is a transition to a flow-dominated state, where the director configuration at the channel centerline is aligned with the flow (perpendicular to the channel height direction). We analyze simple channel-flow solutions to the Leslie–Ericksen model for nematics. We demonstrate that two solutions exist, at all flow rates, but that there is a transition between the elastic free energies of these solutions: the anchoring-dominated solution has the lowest energy at low flow rates, and the flow-dominated solution has lowest energy at high flow rates.     

Rheology of oil-in-water emulsions

The effect of interfacial tension on the steady-flow and dynamic viscoelastic behavior of emulsions are studied experimentally. At very low inter-facial tensions and low volume fractions, the viscosity decreases with increasing shear rate and becomes constant at high shear rates. The high-shear-rate Newtonian viscosity is not affected by interfacial tension, but the transition from pseudoplastic to Newtonian flow shifts to lower shear rates as the interfacial tension decreases. At an interfacial tension of 5 × 10^–3 Nm^–1, the viscosity decreases, passes through a minimum, and then increases as the shear rate is increased. The dilatant behavior may be attributed to elastic responses of interfaces during collision of drops. At high volume fractions, the emulsions show remarkable elasticity resulting from the interfacial energy associated with deformation of liquid films. The modulus and viscosity are proportional to interfacial tension and inversely proportional to drop size.     

Incorporation of yield surface distortion in finite deformation constitutive modeling of rigid-plastic hardening materials based on the Hencky logarithmic strain

In this paper a constitutive model for rigid-plastic hardening materials based on the Hencky logarithmic strain tensor and its corotational rates is introduced. The distortional hardening is incorporated in the model using a distortional yield function. The flow rule of this model relates the corotational rate of the logarithmic strain to the difference of the Cauchy stress and the back stress tensors employing deformation-induced anisotropy tensor. Based on the Armstrong–Fredrick evolution equation the kinematic hardening constitutive equation of the proposed model expresses the corotational rate of the back stress tensor in terms of the same corotational rate of the logarithmic strain. Using logarithmic, Green–Naghdi and Jaumann corotational rates in the proposed constitutive model, the Cauchy and back stress tensors as well as subsequent yield surfaces are determined for rigid-plastic kinematic, isotropic and distortional hardening materials in the simple shear deformation. The ability of the model to properly represent the sign and magnitude of the normal stress in the simple shear deformation as well as the flattening of yield surface at the loading point and its orientation towards the loading direction are investigated. It is shown that among the different cases of using corotational rates and plastic deformation parameters in the constitutive equations, the results of the model based on the logarithmic rate and accumulated logarithmic strain are in good agreement with anticipated response of the simple shear deformation.     

高速冲击表面处理对金属材料力学性能和组织结构的影响

高速冲击表面处理过程中的应变率对金属材料的宏观力学性能和微观组织结构都具有重要影响。根据当前应变率效应的研究成果,从宏观与微观相结合的角度出发,综述了高速冲击表面处理过程中应变率对金属材料强度和塑性的影响规律,并重点阐述了不同应变率下金属材料内部微观组织结构的演变规律,主要包括晶粒结构、绝热剪切带、相变、位错组态和析出相以及变形孪晶等。此外,还分析了组织结构随应变率的演化和微观变形机制的转变对材料力学性能的强化和弱化机理。最后,对高速冲击表面处理梯度组织的变形特点进行了总结。提出了不同组织结构对材料性能影响的综合效应模型,以期为应变率效应的深入研究奠定基础。     

颗粒介质固-流态转变的理论分析及实验研究

颗粒介质由大量离散的颗粒聚集而成,因而与传统固体和流体不同,运动过程中的颗粒介质中可能同时存在多种流态及其相互间复杂的转换过程. 颗粒介质弹性失稳机理、不可恢复应变量化是研究颗粒介质固态和流态及固-流态转变的关键. 在前期建立的双颗粒温度热力学(two-granular-temperature, TGT) 理论基础上,确定了颗粒介质的弹性稳定性条件,建立了不可恢复应变流动法则,搭建了描述颗粒固态-液态及其相互转化的简单模型. 颗粒堆积体坍塌过程是典型的颗粒介质固态和流态及其转变过程,因此本文首先开展了25 167 个陶颗粒堆积体坍塌过程的实验研究,并使用基于TGT 理论的物质点方法和离散元方法对物理实验进行了模拟. 结果表明,模型数值结果与物理实验在颗粒堆坍塌过程中的形态、速度分布等细节上吻合很好,同时也发现了现阶段所使用的物质点方法和TGT 理论的不足. 初步说明TGT 理论可以实现颗粒介质固态和流态,以及状态转变的描述.      

Fibre spinning of a weakly elastic liquid

This paper presents a study of a silicone oil (poly(dimethyl siloxane)) in extensional deformation using an instrument developed recently by the authors. Data from steady shear and low amplitude sinusoidal deformation of this liquid clearly establish that it is weakly elastic. The viscometric data, for shear rates less than 100 s ⁻¹, are best represented by either the Maxwell model or the Jeffrey's model, the latter being marginally superior. The extensional data show that at low deformation rates, this fluid exhibits a Newtonian behavior with an apparent extensional viscosity equal to three times the shear viscosity. Under these conditions the velocity profiles along the spinline are also well represented by the Newtonian model. However, at higher deformation rates better predictions of the velocity profiles are obtained from the Jeffrey's and Maxwell models. At deformation rates above 100 s ⁻¹ none of these simple models is adequate. Under the conditions used in these experiments, the fractional increase in tensile stress along the fiber is shown both theoretically and experimentally to be a unique function of the total strain. Furthermore, the apparent extensional viscosity at any point on the spinline can be calculated from steady state expressions if allowance is made for the variation of stretch rates by defining a time averaged stretch rate.The results obtained here show that elasticity must be considered if these model liquids are used to conduct rheological experiments at high deformation rates. Additionally, it is found that elastic effects in extension can be predicted using simple constitutive equations provided viscometric data can be represented properly in the deformation rate range of interest. Finally, the present research further substantiates the utility of the extensional viscometer developed by the authors.     

Flow-induced polymer degradation probed by a high throughput microfluidic set-up

A complex fluid submitted to strong flows can endure irreversible changes in its structure. This is the case for long chain polymer additives that are commonly used as viscosity enhancers in industry, notably for oil recovery. These polymers break in solution when submitted to high deformation rates, eventually causing a serious viscosity loss. This problem of practical importance is though difficult to handle from a fundamental point of view given its complexity. We introduce here a new tool, based on microfluidic technology, for the screening of the degradation of solutions in the model situation of the flow through a constriction. We integrate two functions in a single set-up, a micro-fabricated constriction and an on-chip viscosimeter. This tool enables us to probe rapidly the viscosity loss imparted by flowing through the constriction at a given flow rate. Thanks to microfluidics, the sample preparation and measurement time are significantly lower than those implied by classical measurement protocols (reduction by up to two orders of magnitude). In addition, confinement provides control of the flow in terms of inertia. To illustrate the potential of this approach in a screening perspective, we use this tool to study the degradation of a series of semi-dilute aqueous solutions of PEO of varying molecular weights and concentrations. For each solution we identify a threshold flow rate for polymer degradation. The corresponding critical deformation rate decreases with molecular weight and concentration, as expected (the mass dependence is in line with previous reports and theories for dilute solutions). In addition we characterize the viscosity loss for larger deformation rates and find that, despite the polydispersity of our solutions, the observations for the various solutions can be roughly recast on a master curve by renormalization of the imposed deformation rates according to a law $M_w^{?1.7\pm 03}{c}^{?0.7\pm 03}$.     

Thermally and flow induced crystallization of polymers at low shear rate

In semi-crystalline thermoplastic products, final properties are strongly dependent on the thermo-mechanical history experienced by the polymer melt during processing. More precisely, structural heterogeneities such as rigidity gradients and shrinkage anisotropy are directly related to the crystalline microstructure. Therefore, accurate prediction of part properties by a processing computer simulation code requires the implementation of an appropriate crystallization kinetics model, including both the effects of thermally and flow induced structure development. One issue is the necessity to improve the modeling of shear/extensional experimental data by relating the crystallization accelerating factors to an easily accessible material related variable. Several authors modeled the effect of the flow on the crystallization kinetics by using the isokinetic approach of Nakamura. Often, the resulting kinetic equations of these models account only for the evolution of the crystallinity fraction α leaving the influence of crystalline morphology aside. We may quote the work of Guo and Narh [1], which connects the flow influence on the crystallization rate to the increase in the thermodynamic melting temperature in the Nakamura model. In 2005, R.I. Tanner presented a comparison of some models describing the polymer crystallization at low shear deformation rates under isothermal conditions. Based on Tanner's study, we developed a model of crystallization at low shearing, applied to non-isothermal flows, using only macroscopic measurable parameters. The key features of the concentrated suspension theory were used to characterize the effect of crystallization on the viscosity. In addition, we assumed that the flow generates additional crystallization nuclei via a parameter which combines the deformation and the deformation rate. The concept of germination-growth is introduced using the fundamentals of the Avrami–Kolmogorov theory, coupled with a modified Schneider's approach. The model is applied to a polypropylene, in a cooled Couette flow configuration. The results show the enhancement of the crystallization kinetics due to the shearing. The definition of global parameters simplifies the type and the number of experiments needed for the model parameter identification. The use of Schneider's approach leads to a new way of discriminating the relative roles of the flow and the temperature on the crystallization phenomenon. The competition between the two driving causes is presented and discussed: at low cooling rate or at high temperature, the shearing effect predominates. Other interesting results show the size distribution of the spherulites as well as the volume proportion for each crystalline size in the polymer.