Internal loops in pseudoelastic behaviour of Ti-Ni shape memory alloys: Experiment and modelling

The stress-strain isothermal hysteresis loops due to the incomplete martensitic transformation are analysed for Ti-Ni shape memory alloys. Experiments show the existence of two distinct yield lines for phase transition; one for the forward transformation austenitemartensite (AM), the other for the reverse transformation MA. The tensile behaviour of single crystals with only one yield line (AM) [1] can be considered as an ideal case. An extension of a thermodynamic model for pseudoelasticity [2] allows these two yield lines to be taken into account.

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Sommario Per leghe Ti-Ni con memoria di forma vengono analizzati i cicli di isteresi isotermici tensione-deformazione prodotti da una incompleta trasformazione martensitica. Gli esperimenti mostrano l'esistenza di due distinte linee di snervamento per la transizione di fase, una verso la trasformazione austenitemartensite (AM), l'altra per la trasformazione inversa MA. Il comportamento a trazione di un singolo cristallo con una sola linea di snervamento (AM) [1], può essere considerato un caso ideale. L'estensione ad un modello termodinamico pseudo-elastico [2] consente di analizzare queste due linee di snervamento.

     

Holomorphic Flows on Simply Connected Regions Have No Limit Cycles

The dynamical system or flow = f(z), where f is holomorphic on C, is considered. The behavior of the flow at critical points coincides with the behavior of the linearization when the critical points are non-degenerate: there is no center-focus dichotomy. Periodic orbits about a center have the same period and form an open subset. The flow has no limit cycles in simply connected regions. The advance mapping is holomorphic where the flow is complete. The structure of the separatrices bounding the orbits surrounding a center is determined. Some examples are given including the following: if a quartic polynomial system has four distinct centers, then they are collinear.     

CO2 will probably not help to save Venice from the sea

It has been proposed to rise Venice by injecting CO₂ into an aquifer lying 600–800 m below its lagoon. It is shown that because of the prevailing ambient conditions in the aquifer phase change of CO₂ cannot be avoided. The ensuing change of specific volume and compressibility will inhibit a uniform uplift pattern which is needed to avoid cracks in historical buildings. Further, both liquid and gaseous CO₂ produce capillary effects at the contact surface with water in the voids of the reservoir rock and this affects its mechanical response. Hence further non homogeneity in the uplift pattern will be produced. This prevents the use of CO₂ in the proposed conditions.     

Bubble clustering and trapping in large vortices. Part 1: Triggered bubbly jets investigated by phase-averaging

The characteristics of large coherent structures in turbulent, periodically-excited, as well as naturally-developing bubbly jets are investigated, and interactions between bubbles and large-scale vortices are studied quantitatively.     

等达因图象的自动采集与识别

文中提出了一种机机械扫描系统．小功率激光器、图象板、相移元件、微机和程序软件组成的等达因图象自动采集与处理系统，用灰度值比较的思想实现了等达因图象的跟踪采集。提出了四幅图象相移法实现等达因图象的自动识别。系统能快速、实时、精确和全自动地实现等达因图象的记录和分析。给出与物体内各点应力状态一一对应的位相图。并用来研究了其他实验方法困难的局部三维效应问题。     

Transport of salts and micron-sized particles entrained from a boiling water pool

The entrainment of soluble (KI, CsI) and non-soluble (Al₂O₃) substances through droplets, which are produced by disintegrating steam bubbles at the surface of a boiling water pool, is determined in a pilot-scale facility. Integral measurements are conducted at steady-state conditions in an atmosphere of either pure steam or an air–steam mixture. The ratio of the entrained liquid mass flow and the gas mass flow through the pool, the entrainment factor, is determined for air–steam ratios between 0 and 0.47 kg/kg in the gas atmosphere and at constant total pressures between 2 and 6 bar. The influence of the vertical temperature profile in the gas atmosphere on the convective velocity field is demonstrated by phase Doppler anemometry and particle image velocimetry measurements at a location 2.1 m above the pool surface. The influences of nucleation and natural convection are demonstrated during slow de-pressurization of the facility at rates below 420 Pa/s.     

Rain-wind-induced vibrations of a simple oscillator

In this paper a relatively simple mechanical oscillator which may be used to study rain-wind-induced vibrations of stay cables of cable-stayed bridges is considered. In recent publications, mention is made of vibrations of (inclined) stay cables which are excited by a wind field containing rain drops. The rain drops that hit the cables generate a rivulet on the surface of the cable. The presence of flowing water on the cable changes the cross section of the cable experienced by the wind field. A symmetric flow pattern around the cable with circular cross section may become asymmetric due to the presence of the rivulet and may consequently induce a lift force as a mechanism for vibration. During the motion of the cable the position of rivulet(s) may vary as the motion of the cable induces an additional varying aerodynamic force perpendicular to the direction of the wind field. It seems not too easy to model this phenomenon, several author state that there is no model available yet.The idea to model this problem is to consider a horizontal cylinder supported by springs in such a way that only one degree of freedom, i.e. vertical vibration is possible. We consider a ridge on the surface of the cylinder parallel to the axis of the cylinder. Additionally, let the cylinder with ridge be able to oscillate, with small amplitude, around the axis such that the oscillations are excited by an external force.It may be clear that the small amplitude oscillations of the cylinder and hence of the ridge induce a varying lift and drag force. In this approach it is assumed that the motion of the ridge models the dynamics of the rivulet(s) on the cable. By using a quasi-steady approach to model the aerodynamic forces, one arrives at a non-linear second-order equation displaying three different kinds of excitation mechanisms: self-excitation, parametric excitation and ordinary forcing. The first results of the analysis of the equation of motion show that even in a linear approximation for certain values of the parameters involved, stable periodic motions are possible. In the relevant cases where in linear approximation unstable periodic motions are found, results of an analysis of the non-linear equation are presented.     

Crack growth resistance of shape memory alloys by means of a cohesive zone model

Crack growth resistance of shape memory alloys (SMAs) is dominated by the transformation zone in the vicinity of the crack tip. In this study, the transformation toughening behavior of a slowly propagating crack in an SMA under plane strain conditions and mode I deformation is numerically investigated. A small-scale transformation zone is assumed. A cohesive zone model is implemented to simulate crack growth within a finite element scheme. Resistance curves are obtained for a range of parameters that specify the cohesive traction-separation constitutive law. It is found that the choice of the cohesive strength t₀ has a great influence on the toughening behavior of the material. Moreover, the reversibility of the transformation can significantly reduce the toughening of the alloy. The shape of the initial transformation zone, as well as that of a growing crack is determined. The effect of the Young's moduli ratio of the martensite and austenite phases is examined.     

Phase-Transformation Fronts Evolution for Stress- and Strain-Controlled Tension Tests in TiNi Shape Memory Alloy

Nucleation and development of phase transformation fronts in TiNi shape memory alloy subjected to the stress- and strain-controlled tension tests were investigated. A thermovision camera was applied to register the distribution of infrared radiation emitted by the specimen and to find its temperature variations. During the loading, narrow bands of considerably higher temperature corresponding to the martensitic phase, starting from the central part of the specimen and developing towards the specimen grips, under both approaches, were registered. The inclined bands of heterogeneous temperature distribution were observed also during the unloading process of the SMA, while the reverse transformation accompanied by temperature decrease took place. Thermomechanical aspects of martensitic and reverse transformations for various strain rates were analyzed under both stress- and strain-controlled tests.     

(Liquid + liquid), (solid + liquid), and (solid + liquid + liquid) equilibria of systems containing cyclic ether (tetrahydrofuran or 1,3-dioxolane), water,and a biological buffer MOPS

Two liquid phases were formed as the addition of a certain amount of biological buffer 3-(N-morpholino)propane sulfonic acid (MOPS) in the aqueous solutions of tetrahydrofuran (THF) or 1,3-dioxolane. To evaluate the feasibility of recovering the cyclic ethers from their aqueous solutions with the aid of MOPS, we determined experimentally the phase diagrams of the ternary systems of {cyclic ether (THF or 1,3-dioxolane) + water + MOPS} at T = 298.15 K under atmospheric pressure. In this study, the solubility data of MOPS in water and in the mixed solvents of water/cyclic ethers were obtained from the results of a series of density measurements, while the (liquid + liquid) and the (solid + liquid + liquid) phase boundaries were determined by visually inspection. Additionally, the tie-line results for (liquid + liquid) equilibrium (LLE) and for (solid + liquid + liquid) equilibrium (SLLE) were measured using an analytical method. The reliability of the experimental LLE tie-line results data was validated by using the Othmer–Tobias correlation. These LLE tie-line values were correlated well with the NRTL model. The phase diagrams obtained from this study reveal that MOPS is a feasible green auxiliary agent to recover the cyclic ethers from their aqueous solutions, especially for 1,3-dioxolane.