Parametric studies of failure mechanisms in elastic EB-PVD thermal barrier coatings using FEM

Catastrophic failure of thermal barrier coatings (TBCs), usually occurs due to large scale buckling and spallation, primarily originating at the bond coat and TGO interface. Spallation in TBCs is preceded by a competition between buckling and interface delamination that is stimulated by the waviness of the interface. In the presence of thermal loading, the waviness is responsible for growth of interfacial delamination. In this paper, a finite element model of the two and three layer TBC’s is developed in the commercial code ANSYS to investigate the buckle and interface delamination mechanisms and develop a simplified parametric understanding of these mechanisms. The models for simulation are validated with analytical and experimental results. Parametric relations, in terms of geometric and material parameters representing constituents of the TBC, are developed in this paper for critical stresses and energies causing buckling and debonding initiated instabilities. Through these relations, critical parameters that control failure mechanics are identified for a fail-safe design space.     

Dynamic response of clamped sandwich beams: analytical modeling

An improved analytical model is developed to predict the dynamic response of clamped lightweight sandwich beams with cellular cores subjected to shock loading over the entire span.The clamped face sheets are simplified as a single-degree-of-freedom(SDOF) system, and the core is idealized using the rigid-perfectly-plastic-locking(RPPL) model. Reflection of incident shock wave is considered by incorporating the bending/stretching resistance of the front face sheet and compaction of the core. The model is validated with existing analytical predictions and FE simulation results, with good agreement achieved. Compared with existing analytical models, the proposed model exhibits superiority in two aspects: the deformation resistance of front face sheet during shock wave reflection is taken into account; the effect of pulse shape is considered. The practical application range of the proposed model is therefore wider.     

Comparison of different models for non-equilibrium CO2 flows in a shock layer near a blunt body

The paper presents results of a numerical simulation of a supersonic two-dimensional (2D) viscous flow containing CO₂ molecules near a spacecraft entering the Mars atmosphere. The gas–dynamic equations in the shock layer are coupled to the equations of non-equilibrium vibrational and chemical kinetics in the five-component mixture CO₂/CO/O₂/C/O. Transport and relaxation processes in the flow are studied on the basis of the rigorous kinetic theory methods; the developed transport algorithms are incorporated in the numerical scheme. The influence of the vibrational excitation of CO₂ and chemical reactions on the gas flow parameters and heat transfer is analyzed. The obtained results are compared with those found using two simplified models based on the two-temperature and one-temperature vibrational distributions in CO₂. The accuracy of the simplified models and the limits of their validity within the shock layer are evaluated. The effect of bulk viscosity in a flow near a re-entry body is discussed. The role of different diffusion processes, chemical reactions, and surface catalytic properties in a flow of the considered mixture in the shock layer is estimated.     

Interaction of two Mach reflections over concave double wedges-analytical model

A simplified analytical model for solving the wave configuration resulting when two triple points join together over a concave double wedge is developed. The model equations are solved analytically. Predictions of the model are compared to experimental results and fairly good agreement is obtained. It is believed that the present model can be used to better understand complex wave interaction phenomena. Received 2 May 1995 / Accepted 18 December 1997     

刚性弹丸侵彻钢筋混凝土的实验和简化分析模型

通过Φ57mm半穿甲弹对钢筋混凝土的垂直侵彻实验,得到了弹丸的撞靶速度、成坑深度、最大侵彻深度以及过载时程曲线等实验数据.对实验后钢筋的断裂特征进行分析,得到钢筋的典型破坏模式.将钢筋的破坏简化为弯曲+剪切断裂和弯曲+拉伸断裂这两种模式.根据混凝土侵彻模型和梁断裂失效理论,建立了刚性弹丸垂直侵彻钢筋混凝土的简化分析模型.将理论计算得到的侵彻深度、速度与过载时间历程分别与实验数据进行对比,结果表明两者吻合较好.研究表明,钢筋只对弹体侵彻过程产生局部影响,混凝土的抗侵彻阻力仍是钢筋混凝土抗侵彻阻力的主要组成部分.     

无基底焦平面阵列结构优化设计及性能分析

基于光学读出非制冷红外成像技术,本文作者提出并成功设计制作了无基底焦平面阵列结构(Focal Plane Array,FPA),不仅简化了制作工艺,而且大幅提升了成像性能。但由于其为单层膜结构,固有频率值低,可能会引起热机械振动噪声(NETDTM),导致噪声等效温度差(NETD)上升。为解决这一问题,本文提出两种改进的加强梁结构,借助有限元简化模型和激光位移传感器,通过数值模拟和实验验证,证实这两种结构可以大幅度提高FPA固有频率值(分别从74Hz提升至835Hz和404Hz),NETDTM降低约两个数量级,从而实现了对FPA的优化设计。     

A new description for dispersion

Chang and Slattery (1986) introduced a simplified model for dispersion that contains only two empirical parameters, both of which can be determined in one-dimensional experiments. The traditional model for dispersion (Nikolaevskii, 1959; Scheidegger, 1961; de Josselin de Jong and Bossen, 1961; Bear, 1961a; Peaceman, 1966; Bear, 1972) has three empirical parameters, two of which can be measured in one-dimensional experiments while the third, the transverse dispersivity, must be measured in experiments in which a two-dimensional concentration profile develops. For the common one-dimensional experiment in which the signs of the concentration gradient and of the velocity field are different, the simplified model and the traditional model give identical results. For a one-dimensional experiment in which the signs of the concentration gradient and of the velocity field are at least sometimes the same and for two- and three-dimensional flows, the simplified model of Chang and Slattery (1986) gives results that can differ from those predicted using the traditional model.Only the experimental data of Bear (1961b) and of Yule and Gardner (1978) are sufficiently complete to permit a comparison of the two models. Considering the quality of the experimental data, we can not distinguish between the predicted profiles based upon the simplified model and those based upon the traditional model.     

Further analytical considerations of weak planar shock wave reflections over a concave wedge

A simplified analytical model of the Mach reflection of a planar shock wave over a concave cylindrical wedge has been used to predict the triple point trajectory and the triple point trajectory angle at glancing incidence.Comparison with experimental results suggests that the simplified approach for predicting the triple point trajectory is good for an incident shock wave Mach number, M_i, less than 1.1. However, the prediction of the triple point trajectory angle at glancing incidence is good for any value of M_i as long as the assumption of a perfect gas is valid.     

Simulation of mass transfer during osmotic dehydration of apple: a power law approximation method

In this study, unsteady one-dimensional mass transfer during osmotic dehydration of apple was modeled using an approximate mathematical model. The mathematical model has been developed based on a power law profile approximation for moisture and solute concentrations in the spatial direction. The proposed model was validated by the experimental water loss and solute gain data, obtained from osmotic dehydration of infinite slab and cylindrical shape samples of apple in sucrose solutions (30, 40 and 50 % w/w), at different temperatures (30, 40 and 50 °C). The proposed model’s predictions were also compared with the exact analytical and also a parabolic approximation model’s predictions. The values of mean relative errors respect to the experimental data were estimated between 4.5 and 8.1 %, 6.5 and 10.2 %, and 15.0 and 19.1 %, for exact analytical, power law and parabolic approximation methods, respectively. Although the parabolic approximation leads to simpler relations, the power law approximation method results in higher accuracy of average concentrations over the whole domain of dehydration time. Considering both simplicity and precision of the mathematical models, the power law model for short dehydration times and the simplified exact analytical model for long dehydration times could be used for explanation of the variations of the average water loss and solute gain in the whole domain of dimensionless times.     

两异径湿球颗粒间静态液桥力的近似解析公式及数值评测

针对异径球形湿颗粒间液桥力,以Young-Laplace公式为基础,结合环形近似法并引入等效半径,在宽松设定的条件下推导出了简化的近似解析公式。此公式形式简洁,且当两球半径相等时,可回归到Pitois等提出的径球液桥力公式。本文对此近似解析公式进行参数敏感度分析,发现随着颗粒间距的增大,液桥力对颗粒半径的敏感度降低。此外,本文近似解析公式与其他实验和理论结果进行对比发现,该近似解析公式与上述实验和数值结果相当吻合,表明该近似解析公式可以较准确地计算接触角较小时的异径球颗粒间的液桥力,因而适于湿颗粒系统的数值模拟。     

Local effects in plates - theoretical and practical consequences

The term “local effect” denotes local, pronounced, three-dimensional stress states which occur primarily at the boundaries of flat beams and stretched plates made of homogeneous or composite materials, particularly in the regions of notches, cracks, local mechanical and thermal loads, etc., but which may also occur in any other region. These effects are still ignored in common procedures of analytical mechanics and in traditional procedures of experimental stress analysis, despite the fact that they often increase drastically the maximal stress values occurring in critical regions of structures made of homogeneous and composite materials.The most consistent and rapid response to the needs of modern engineering design with respect to local effects has developed within the field of analytical mechanics. The emphasis has been placed on mathematically and physically admissible analytical solutions. The problem is that the developed analytical models and solutions must be tested experimentally and that this task cannot be reliably accomplished by using experimental procedures based on the concept of plane stress state.This paper presents three new analytical/experimental groups of methods developed by the author and his co-workers: isodyne methods, strain gradient methods and thermoelastic methods. The first two methods have been developed to analyze local effects: static and dynamic. The third method was developed to study dynamic strain/stress fields. The presented empirical evidence shows that the isodyne techniques and strain gradient techniques can be used to evaluate major stress components of three-dimensional stress states in plates and beams caused by local effects. This evidence can be used either directly in engineering design, or as a measure to test the predictive power of analytical models and solutions, and in particular, to quantify errors caused by the simplifying assumptions accepted in analytical solutions.     

Nonlinear Dynamics of Floating Cranes

The nonlinear dynamic responses of moored crane vessels to regular wavesare investigated experimentally and theoretically. The main subject ofinterest are nonlinear phenomena like bifurcations and the existence ofmultiple attractors. In the experimental part of the work, a mooredmodel of a crane vessel has been excited by regular waves in a wavetank. A special mechanism has been developed to model the nonlinearbehavior of real mooring systems. The theoretical part of the workconcerns the mathematical modeling of the floating cranes. Twomathematical models of different levels of complexity are presented. Twodifferent tools are used to systematically determine the responses ofthe systems to periodic forcing of waves. Firstly, the path-followingtechniques in combination with numerical integration of equations ofmotion applied to a full nonlinear model give insight into the dynamicsin time domain. Secondly, the multiple scales method allows for ananalytical investigation of simplified nonlinear models in frequencydomain. Many results of computations for two crane vessels, barge andship, are presented. Special attention is paid to oscillations near thefrequencies of primary resonances and to subharmonic motions. Anexcellent agreement is found between the results of time-domain andfrequency-domain analysis. The computational examples chosen correspondto the models used not only in the present experiments but in theexperiments of others as well. The results presented in the work allow usto draw several important conclusions concerning the dynamic behavior offloating cranes during offshore operations. Both the developed modelsand the analytical tools can be used to identify the limits of theoperating range of floating cranes.     

Quantitative comparison of Taylor flow simulations based on sharp‐interface and diffuse‐interface models

A pressure‐driven flow of elongated bullet‐shaped bubbles in a narrow channel is known as Taylor flow or bubble‐train flow. This process is of relevance in various applications of chemical engineering. In this paper, we describe a typical simplified experimental setting, with surface tension, density and viscosity as prescribed input parameters. We compare a sharp‐interface model based on a moving grid aligned with the bubble boundary (ALE coordinates) and a diffuse‐interface model where the bubble shape is implicitly given by a phase‐field function. Four independent implementations based on the two modeling approaches are introduced and described briefly. Besides the simulation of the bubble shapes, we compare some resulting quantities such as pressure difference and film widths within the implementations and to existing analytical and experimental results. The simulations were conducted in 2D and 3D (rotationally symmetric). Good accordance of the results indicate the applicability and the usability of all approaches. Differences between the models and their implementations are visible but in no contradiction to theoretical results. Copyright © 2013 John Wiley & Sons, Ltd.     

FINITE ELEMENT MODELING AND ROBUST VIBRATION CONTROL OF TWO-HINGED PLATE USING BONDED PIEZOELECTRIC SENSORS AND ACTUATORS

Active vibration control for a kind of two-hinged plate is developed in this paper. A finite element model for the hinged plate integrated with distributed piezoelectric sensors and actuators is derived, including bending and torsional modes of vibration. In this model, the hinges are simplified as regular plate elements to facilitate operation. The state space representations for bending and torsional vibrations are obtained. Based on two low-order models of the bending and torsional motion, two H ∞ robust controllers are designed for suppressing the vibrations of the bending and torsional modes, respectively. The simulation results indicate the effectiveness and feasibility of the designed H ∞ controllers. The vibration magnitudes of the low-order modes can be reduced without affecting the high frequency modes.     

Experimental stress analysis of a nuclear-reactor pressure vessel

The stresses from internal pressure in a nuclear-reactor pressure vessel were investigated using both strain-gaged and photoelastic models. The methods used in obtaining complete stress distributions along the nozzles from strain-gage data are described, and comparisons are made between the distributions obtained from the photoelastic model and those derived from strain-gage data. The effects of nonradial attachment of a nozzle to a spherical shell upon the stresses in the nozzle and in the shell are shown. Finally, comprisons are made between the experimental results and those for simplified analytical models.     

Alternative state space formulations for magnetoelectric thermoelasticity with transverse isotropy and the application to bending analysis of nonhomogeneous plates

By introducing two displacement functions and two stress functions, the governing equations of the linear theory of magneto-electro-thermo-elasticity with transverse isotropy are simplified. On selecting certain physical quantities as the basic unknowns, two new state equations are established. Each of them is order reduced when compared with the one reported recently in literature, leading to a higher numerical efficiency. The material inhomogeneity along the axis of symmetry (z-direction) can be taken into account and an approximate laminate model is employed to facilitate deriving analytical solutions. The validity of new formulations is examined by considering a laminated magneto-electro-elastic rectangular plate and good agreement is obtained with existent results. A plate with a functionally graded property is then analyzed. The effect of magnetoelectric coupling in a BaTiO₃–CoFe₂O₄ composite predicted from the micromechanics simulation is studied quantitatively.     

Turbulent mixing,induced by the Richtmyer-Meshkov instability

Turbulent mixing development, induced by passing of shock waves through the interface, has been studied on the basis of two semi-empirical models. The process of transition from random initial perturbations to turbulence is a rather complicated one, but it proceeds in a finite time. Here it is assumed that passing of shock waves through the initial zone of roughness leads to the instant development of mixing. The expression for the turbulent kinetic energy, transfered by the arriving shock wave, has been derived. The value of this energy is determined by the value of the non-dimensional parameter which is equivalent to the Richardson number. The analytical solution describing the turbulent kinetic energy decay and the mixing zone width change in time, has been obtained.The analytical form of constructed solutions allows to perform comparison between the models and to make the choice of constants on the basis of comparison with the known experiments. Comparison of the obtained theoretical results with Zaitzev's experimental data is given for the impulsive acceleration.This article was processed using Springer-Verlag T_EX Shock Waves macro package 1.0 and the AMS fonts, developed by the American Mathematical Society.     

Calculation procedure to determine average mass transfer coefficients in packed columns from experimental data for ammonia–water absorption refrigeration systems

The ammonia purification process is critical in ammonia–water absorption refrigeration systems. In this paper, a detailed and a simplified analytical model are presented to characterize the performance of the ammonia rectification process in packed columns. The detailed model is based on mass and energy balances and simultaneous heat and mass transfer equations. The simplified model is derived and compared with the detailed model. The range of applicability of the simplified model is determined. A calculation procedure based on the simplified model is developed to determine the volumetric mass transfer coefficients in the vapour phase from experimental data. Finally, the proposed model and other simple calculation methods found in the general literature are compared.     

Mechanical model for staggered bio-structure

A generic mechanical model for bio-composites, including stiff platelets arranged in a staggered order inside a homogeneous soft matrix, is proposed. Equations are formulated in terms of displacements and are characterized by a set of non-dimensional parameters. The displacements, stress fields and effective modulus of the composite are formulated. Two analytical models are proposed, one which includes the shear deformations along the entire medium and another simplified model, which is applicable to a slender geometry and yields a compact expression for the effective modulus. The results from the models are validated by numerical finite element simulations and found to be compatible with each other for a wide range of geometrical and material properties. Finally, the models are solved for two bio-structures, nacre and a collagen fibril, and their solutions are discussed.     

Numerical investigation of parametric resonance due to hydrodynamic coupling in a realistic wave energy converter

Representative models of the nonlinear behavior of floating platforms are essential for their successful design, especially in the emerging field of wave energy conversion where nonlinear dynamics can have substantially detrimental effects on the converter efficiency. The spar buoy, commonly used for deep-water drilling, oil and natural gas extraction and storage, as well as offshore wind and wave energy generation, is known to be prone to experience parametric resonance. In the vast majority of cases, parametric resonance is studied by means of simplified analytical models, considering only two degrees of freedom (DoFs) of archetypical geometries, while neglecting collateral complexity of ancillary systems. On the contrary, this paper implements a representative 7-DoF nonlinear hydrodynamic model of the full complexity of a realistic spar buoy wave energy converter, which is used to verify the likelihood of parametric instability, quantify the severity of the parametrically excited response and evaluate its consequences on power conversion efficiency. It is found that the numerical model agrees with expected conditions for parametric instability from simplified analytical models. The model is then used as a design tool to determine the best ballast configuration, limiting detrimental effects of parametric resonance while maximizing power conversion efficiency.