Improved correlations for computations of liquid helium two phase flow in cryogenic transfer lines

The available experimental data, Vishnev et al. [6], implies that the widely used homogenous equilibrium model and the Martinelli–Nelson equation do not map correctly the pressure drop data for horizontal liquid helium two phase flow particularly in cases for high vapour fraction. Hence, considering the empirical nature of Martinelli–Nelson equation, suitable parameters and terms have been suggested so that, the pressure drop can be better predicted. The proposed equation being a third order in ‘x’ (vapour fraction) is more accurate to predict the characteristic behavior of liquid helium two phase flow as observed in experiments [6], Mamedov et al. [7], and Deev et al. [8], which the available equations are unable to predict.     

On the phenomenon of curved microcracks in [(S)/90n]s laminates: Their shapes, initiation angles and locations

A variational analysis of the stress state in microcracked cross-ply laminates has been used to investigate the phenomenon of curved microcracking in [(S)/90_n]_s laminates. Previous investigators proposed that the initiation and orientation of curved microcracks are controlled by local maxima and stress trajectories of the principal stresses. We have implemented a principal stress model using a variational mechanics stress analysis and we were able to make predictions about curved microcracks. The predictions agree well with experimental observations and therefore support the assertion that the variational analysis gives an accurate stress state that is useful for modeling the microcracking properties of cross-ply laminates. An important prediction about curved microcracks is that they are a late stage of microcracking damage. They occur only when the crack density of straight microcracks exceeds some critical value-the critical crack density for curved microcracking. The predicted critical crack density for curved microcracking agrees well with experimental observations.     

Elastic anisotropy and micro-damage processes in polycrystalline ice Part I: Theoretical formulation

A microstructural damage model is developed for polycrystalline ice deforming at the high end of the quasi-static domain of loading. The formation (nucleation) of microcracks is attributed to the extension of grain boundary precursors under the influence of the applied stresses and microstructural stresses resulting from the elastic anisotropy mechanism. In a compressive stress field, a growing population of stable cracks leads to progressive damage in the material. Nucleation, and hence damage accumulation, is influenced by three random variables-the precursor orientation, the basal plane orientation of the grains adjoining the precursor of interest, and the grain size. Model predictions consist of the following steps: (a) computation of microstructural stresses using a first-order approximation of the Eshelby procedure, (b) analysis of nucleation using a mixed-mode fracture criterion, and (c) computation of the elastic compliance using the self-consistent method. When coupled to a creep model, the relative contribution of microcracking and creep to the total deformation can be delineated.     

Critical plane approach with two families of microcracks for modelling of unilateral fatigue damage

New multiaxial fatigue damage model based on the critical plane approach is proposed. Two different physical mechanisms of the fatigue damage development on each potential failure plane (critical plane) are considered. In general, each critical plane contains two families of a parallel microcracks. The proposed model reproduces simultaneously fatigue damage induced anisotropy, the influence of positive and negative mean stresses, unilateral fatigue damage, microcrack closure effect and fatigue behaviour under variable amplitude loading. The expression for the equivalent stress in the damage evolution equation includes the stress intensity for the amplitudes as well as joint invariants for the mean values of the stress tensor and for the vectors associated with the directions of microcracks. The theoretical predictions are compared with experimental data under uniaxial cyclic loading of brass specimens. The influence of positive and negative mean stresses on the fatigue life of brass is investigated.     

Three-dimensional finite element analysis of finite deformation micromorphic linear isotropic elasticity

A finite deformation micromorphic materially linear isotropic elastic model is formulated and implemented for three dimensional finite element analysis. The model is based on the kinematics, balance equations and thermodynamic equations proposed by Eringen and Suhubi (1964). The constitutive equations are calculated in the reference configuration, and the resulting stresses are mapped to the current configuration. The balance of linear momentum and the balance of first moment of momentum are linearized to construct the consistent tangent for three dimensional finite element implementation for solution by the Newton–Raphson method. Three dimensional numerical examples are analyzed to demonstrate preliminarily the implementation.     

Interfacial chemistry of oxides on InxGa(1−x)As and implications for MOSFET applications

The prospect of enhanced device performance from III–V materials has been recognized for at least 50 years, and yet, relative to the phenomenal size of the Si-based IC industry, these materials fulfilled only specific niches and were often referred to as “the material of the future” [1]. A key restriction enabling widespread use of III–V materials is the lack of a high quality, natural insulator for III–V substrates like that available for the SiO₂/Si materials system [2]. The prospect of impending scaling challenges for technologies based on silicon metal oxide semiconductor field effect transistor (MOSFET) devices has brought renewed focus on the use of alternate surface channel materials from the III–V compound semiconductor family. The performance of the traditional MOSFET device structure is dominated by defects at the semiconductor/oxide interface, which in turn requires a high quality semiconductor surface. In this review, reflecting the authors’ current opinion, the recent progress in the understanding of the dielectric/III–V interface is summarized, particularly in regard to the interfacial chemistry that impacts the resultant electrical behavior observed. The first section summarizes the nature of the oxidation states of surface oxides on In_xGa_1−xAs. Then the atomic layer deposition of such oxides on the In_xGa_1−xAs surface is summarized in view of the interfacial chemical reactions employed. Finally the resultant electrical properties observed are examined, including the effects of substrate orientation. Portions of this review have been published previously [3] and [4].     

Numerical simulation of residual stresses fields of DD6 blade during laser shock processing

The object of this paper was to address the effect of laser shock processing (LSP) with single and multiple impacts on the residual stresses of aeroengine blades manufactured by a type of thick DD6 alloy of [0 0 1] orientation at 980 °C. The finite element method (FEM) model of the DD6 blade was established during LSP with round laser spot, and LS-DYNA and ANSYS are employed to simulate the residual stresses fields of the DD6 blade by numerical computation. The first four modal shapes of the DD6 blade of [0 0 1] orientation at 980 °C were given. Moreover, the validity of the model was verified by numerical computation and LSP experiments. As a result, the distribution rules of the compressive residual stress with different impacts multiplicity were described on the basis of discussing the measurement method of peak pressure. Results showed that the impacts number corresponding to the state of uniform stress was not the same as that related to the maximum compressive residual stress which might occur at lower number of shots. For the DD6 blade of [0 0 1] orientation at 980 °C, the best compressive residual stress could be achieved by three impacts.     

基于微缺陷成核序列的岩石微裂纹生长和损伤演化模型

岩石在承载之初,由于微缺陷无序成核和有限生长,在材料内部形成大量分布性微裂纹。在该文中,这种演化机制被归结为:微缺陷随机、孤立成核生成最小微裂纹和微缺陷无重叠聚集成核、排列生长形成大尺度微裂纹,裂纹尺度生长是微缺陷成核数的函数。利用微裂纹尺度-频数分布分形以及微裂纹粗糙表面分形,建立基于微缺陷累计成核数序列的裂纹尺度生长模型和损伤演化模型。通过对二维岩石试件破坏过程的微裂纹尺度统计和损伤测试表明,模型的预测结果与观测值符合较好。由于微缺陷成核与声发射源机制具有相似性以及微缺陷成核数序列与声发射数序列具有相似性,所以该模型可用于通过声发射参数序列跟踪微裂纹生长和损伤演化。裂纹尺度生长对于完整认识材料宏观力学性质演化和预测材料灾变具有重要意义。     

Effects of crystallographic orientations and dwell types on low cycle fatigue and life modeling of a SC superalloy

Effects of temperature (760 °C and 980 °C), crystallographic orientation ([0 0 1], [0 1 1] and [1 1 1]) and dwell types (tensile, compressive and balanced dwell type) on low cycle fatigue (LCF) of a Ni-based single crystal (SC) superalloy are experimentally investigated and modeled. Since the LCF behavior shows strong crystallographic orientation and dwell type dependences, corresponding accurate life models are needed for safe application in gas turbine components. The feasibility of stress-based, strain-based and energy based models on anisotropic fatigue behavior was evaluated. A modified Cyclic damage accumulation (CDA) method combined with critical slip plane concept is developed to correlate the influence of orientation and dwell type on LCF data.     

A three dimensional computational model of the mechanical response of a dual-phase ceramic

This paper presents a constitutive model of a fired ceramic consisting of two phases each with a different thermal expansion coefficient. Each phase is linear elastic but the stiffness of one of the phases is progressively reduced anisotropically to simulate the growth of microcracks under tension. This damage, in any orientation, is proportional to the normal tensile traction in that direction and no damage is produced by compressive stresses. The damage is recorded as a damage orientation function held numerically at 20 orientations.

The model is specifically designed to be used in the pre-peak region of the stress–strain curve and to be robust in use in finite element programs. To this end, while the stresses in the phases are determined assuming that the strains in each phase are equal, the overall compliance of the composite is calculated from the complementary energy.

The model reproduces non-linear stress–strain behaviour when loaded in tension and, because of the thermal mismatch, also shows permanent deformation on the removal of the load. A user defined subroutine has been written for the commercial finite element program ABAQUS and used to simulate an experimental test on a commercial refractory and the results compared with experimental data. The UMAT is also used to simulate a hollow axisymmetric cylinder in generalised plane strain subject to moderately rapid heating on the inner surface; and the results for linear elasticity and the damaging model are compared.     

Prediction of condensation in steam ejector for a refrigeration system

An experimental refrigeration system based on a two-stage steam ejector was set-up in the Thermodynamics and Heat Transfer Laboratory of our Department. The system optimization and realization have been described elsewhere ( [Grazzini and Mariani, 1998] and [Grazzini and Rocchetti, 2008] ). In both stages, primary flows are highly supersonic and reach low pressure and temperature levels. As usual in the literature, an ideal gas model was used during the design process. This paper is intended to check the validity of this assumption. In order to understand the actual working condition of our system, several models have been compared. The presence of high flow speed suggests the existence of metastable conditions. To set the border for the metastable region, the spinodal curve has been drawn. Isentropic expansion of vapour through the nozzle, modelled as ideal gas, seems well within the metastable zone. However, the Classic Nucleation Theory shows that the Wilson line is crossed at the nozzle throat. Condensation produces a marked difference in the conditions at the nozzle exit. Results coming from the present analysis will be used in further optimization of the experimental ejector design.     

Characterization of matrix damage in metal matrix composites under transverse loads

A volume integral equation method is used to investigate the mechanics of damage evolution in a unidirectional SiC/Ti composite under transverse loading. It is shown that the most likely mechanism of the damage is the initiation of partial fiber debonding followed by transverse cracking (in brittle matrix composites, e.g., SiC/Ti₃Al) or plastic yielding (in ductile matrix composites, e.g., SiC/Ti-15-3). The matrix damage has been observed to occur at extremely low transverse loads and a rational explanation of this phenomenon does not appear to have been given previously in the literature. Our results indicate that the initiation of matrix cracking or yielding can be explained if microcracks are present in the fiber-matrix interface zone. In absence of the microcracks the stresses in the matrix are too low to cause any damage.     

A simple damage model for concrete considering irreversible mode‐II microcracks

A simple damage model with the concept of mode‐II microcracks on crack wall contributing to the irreversible strains for concrete is developed. By applying the micromechanics method, a microcell‐based damage model is introduced to understand the damage behaviour. Further, by introducing the physical interpretation of the damage variable using thermodynamic method, a novel damage variable (irreversible‐damage variable) is proposed to describe the irrecoverable strains generated by both mode‐II microcracks and irreversible‐frictional sliding. With this methodology, a simple continuum damage mechanics model is developed in which both elastic and irreversible damages are considered. As demonstrated by the comparison with experimental results, the proposed model is characterized by accuracy of solutions, sufficiency of physical sense and convenience of implementation.     

Quench protection tests of a cryocooler cooled 6 T NbTi superconducting magnet by an active power method

When a quench occurs in a superconducting magnet, excessive joule heating may damage the magnet. We have presented the quench protection system based on an active power method. Our previous quench protection tests have been carried out for small superconducting magnets whose self inductances are less than several hundred mH to verify principles of our proposed system. In this paper, we present experimental results of quench protection tests of a cryocooler cooled 6 T NbTi superconducting magnet (self inductance 15.5 H), which is a commercial size magnet made by Tamakawa Co., Ltd. We confirmed that our proposed system could inhibit the maximum temperature of the superconducting magnet (initial temperature 4.3 K) after the quench to less than about 44 K at operation magnetic field 5.5 T. Experimental results suggest that our proposed system is useful for practical used superconducting magnets.     

Effect of mechanical treatment on the endurance of ceramic materials

The effect of microcracks arising during abrasive treatment on the short-term and long-term strength of specimens prepared from magnesium-aluminum ferrite with a spinel structure has been studied. With the aim of orientating surface microcracks grinding of specimens has been carried out parallel, perpendicular, and at an angle of 45° to their axes; testing was performed by a four-point bending scheme. It has been shown that in order to improve the reliability of ceramic articles whose production process includes abrasive treatment a scheme should considered for applying the maximum tensile stresses during operation of an article and orientation of microcracks contributed by the abrasive treatment in relation to these stresses.Leningrad. Institute of Strength Problems, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Problemy Prochnosti, No. 9, pp. 31–36, September, 1989.     

Micromechanics modeling of plastic yielding in a solid containing mode III cohesive cracks

A new micromechanics damage model is proposed by averaging distributed microcracks with cohesive zones in a two dimensional representative volume element. The cohesive microcracks are mode-III Dugdale-Bilby-Cottrell-Swinden (Dugdale-BCS) crack. The damage model may be used to construct plasticity potentials that take into account the presence of such microcracks.     

Determination for the time-to-fracture of solids

A method to determine the time to fracture taking into account the physical mechanisms of microcracks and crack formation is developed on the basis of the fractal model of fracture. The fractal dimension of a crack at different stages of its growth is determined theoretically. The damage evolution law which allows for the kinetic and microstructural properties of a material is obtained on the basis of the kinetic theory of strength. Conditions at which the microcracks accumulation gives way to the propagation of a large crack are determined with the use of the percolation theory. It is shown that the fractal dimension of the initial part of a crack is much more than the fractal dimension of the rest of the crack.     

Resistance to delamination of 3D woven textile composites evaluated using End Notch Flexure (ENF) tests: Cohesive zone based computational results

The flexural response of 3D woven textile composite panels containing an edge crack is evaluated using the End Notch Flexure (ENF) test. In doing so, the effectiveness of 3D reinforcement in increasing and/or eliminating delamination is demonstrated. A finite element model of the ENF configuration using the Discrete Cohesive Zone Model (DCZM) was used to evaluate the deformation response and fracture properties corresponding to the experimental results presented in Pankow et al. (2011) [1]. A modified trapezoidal traction law was used in the DCZM to computationally evaluate the ENF test results. Good agreement between experimental results and predictions are reported, up to the point at which the crack reaches under the loading roller and damage begins to occur locally under the roller.