Creep-rupture lifetime simulation of unidirectional metal matrix composites with and without time-dependent fiber breakage

A numerical simulation for predicting the axial creep-rupture lifetime of continuous fiber-reinforced metal matrix composites is proposed, based on the finite element method. The simulation model is composed of line elements representing the fibers and four-node isoparametric plane elements representing the matrix. While the fibers behave as an elastic body at all times, the matrix behaves as an elasto-plastic body at the loading process and an elasto-plastic creep body at the creep process. It is further assumed in the simulation that the fibers are fractured not only in stress criterion but time-dependently with random nature. Simulation results were compared with the creep-rupture lifetime data of a boron-aluminum composite with 10% fiber volume fraction experimentally obtained. The simulated creep-rupture lifetimes agreed well with the averages of the experimental data. The proposed simulation is further carried out to predict a possibility of creep-rupture for the composite without time-dependent fiber breakage. It is finally concluded that the creep-rupture of a boron-aluminum composite is closely related with the shear stress relaxation occurring in the matrix as well as time-dependent fiber breakage.     

Limit state of structural elements during inelastic deformation

It is shown that, in the case of an axisymmetric stress state, the solution of the statically definable boundary-value problem for an ideal rigid-plastic body using the Mises-Schleicher strength criterion is extended to the rigid-creep model with any specified creep-rupture strength and corresponds to the limit state of a real creeping body.     

Creep rupture of polymer-matrix composites

An accelerated characterization method for resin-matrix composites is reviewed. Methods for determining modulus and strength master curves are given. Creep-rupture analytical models are discussed as applied to polymers and polymer-matrix composites. Comparisons between creep-rupture experiments and analytical models are presented.     

Determining the Thermoviscoplastic State of Shells of Revolution Subject to Creep Damage

A technique is developed for determining the thermoviscoplastic state of shells of revolution with allowance for creep damage. The technique is based on the hypotheses of rectilinear element and the theory of deformation along paths of small curvature. The equivalent stress appearing in the kinetic equations of damage and creep is determined using a creep-rupture criterion that accounts for the stress mode and the level of irreversible strains. The technique is tested by determining the thermoviscoplastic state and time to failure of tubular specimens under a tensile force and a torque     

Results of studying creep and long-term strength of metals at the Institute of Mechanics at the Lomonosov Moscow State University (To Yu. N. Rabotnov’s Anniversary)

Basic results of experimental and theoretical research of creep processes and long-term strength of metals obtained by researchers of the Institute of Mechanics at the Lomonosov Moscow State University are presented. These results further develop and refine the kinetic theory of creep and long-duration strength proposed by Yu. N. Rabotnov. Some problems arising in formulating various types of kinetic equations and describing experimental data for materials that can be considered as statically homogeneous materials (in studying the process of deformation and rupture of such materials, there is no need to study the evolution of individual cracks) are considered. The main specific features of metal creep models at constant and variable stresses, in uniaxial and complex stress states, and with allowance for one or two damage parameters are described. Criterial and kinetic approaches used to determine long-term strength under conditions of a complex stress state are considered. Methods of modeling the metal behavior in an aggressive medium are described. A possibility of using these models for solving engineering problems is demonstrated.     

Semi-theoretical analyses of the concrete plate perforated by a rigid projectile

Based on the three-stage perforation model, a semi-theoretical analysis is conducted for the ballistic performances of a rigid kinetic projectile impacting on concrete plates. By introducing the projectile resistance coefficients, dimensionless formulae are proposed for depth of penetration (DOP), perforation limit thickness, ballistic limit velocity, residual velocity and perforation ratio, with the projectile nosed geometries and projectile-target interfacial friction taken into account. Based on the proposed formula for DOP and lots of penetration tests data of normal and high strength concrete targets, a new expression is obtained for target strength parameter. By comparisons between the results of the proposed formulae and existing empirical formulae and large amount of projectile penetration or perforation tests data for monolithic and segmented concrete targets, the validations of the proposed formulae are verified. It is found that the projectile-target interfacial friction can be neglected in the predictions of characteristic ballistic parameters. The dimensionless DOP for low-to-mid speed impacts of non-flat nosed projectiles increases almost linearly with the impact factor by a coefficient of 2/(πS). The anti-perforation ability of the multilayered concrete plates is dependent on both the target plate thickness and the projectile impact velocity. The variation range of the perforation ratio is 1–3.5 for concrete targets.     

塑料SHPB实验中的端面摩擦效应

基于H.Meng的动摩擦实验结果,建立了一个端面动摩擦模型,以描述端面动摩擦因数随SHPB塑料试样与钢压杆接触界面间最大径向相对滑动速度的关系。在SHPB实验的数值模拟中,分别考虑了该端面动摩擦模型和以往被广泛采用的常摩擦因数模型,结果发现,选取不同的端面摩擦模型不影响通过塑料SHPB实验的数值模拟获得的转折应变率(量级为102s-1),然而当应变率超出转折应变率以上时,不同的端面摩擦模型会对塑料SHPB实验的数值模拟结果产生显著影响,使用提出的端面动摩擦模型可得到更准确的塑料SHPB实验的数值模拟结果。     

A new thixotropic model for waxy crude

Waxy crude oil shows thixotropic behavior below the gelation temperature. The mostly used thixotropic model for waxy crude is the model proposed by Houska. One problem of Houska’s model is that after the stepwise change in shear rate, the predicted shear stress decreases to its equilibrium value more quickly than the measured data. To address this problem, a new viscoplastic thixotropic model is proposed. The evolution of structural parameter is described by a new kinetic equation. In the kinetic equation, a new pre-factor with shear strain as variable is introduced for the buildup and breakdown terms, and the breakdown term is assumed to be dependent on energy dissipation rate rather than on shear rate. The proposed model was validated by the stepwise shear rate test and hysteresis loop test. And the results showed that the new model’s fitting and predictive capability is satisfactory.     

Statistics and Modelling of Turbulent Kinetic Energy Transport in Different Regimes of Premixed Combustion

The statistical behaviour of turbulent kinetic energy transport in turbulent premixed flames is analysed using data from three-dimensional Direct Numerical Simulation (DNS) of freely propagating turbulent premixed flames under decaying turbulence. For flames within the corrugated flamelets regime, it is observed that turbulent kinetic energy is generated within the flame brush. By contrast, for flames within the thin reaction zones regime it has been found that the turbulent kinetic energy decays monotonically through the flame brush. Similar trends are observed also for the dissipation rate of turbulent kinetic energy. Within the corrugated flamelets regime, it is demonstrated that the effects of the mean pressure gradient and pressure dilatation within the flame are sufficient to overcome the effects of viscous dissipation and are responsible for the observed augmentation of turbulent kinetic energy in the flame brush. In the thin reaction zones regime, the effects of the mean pressure gradient and pressure dilatation terms are relatively much weaker than those of viscous dissipation, resulting in a monotonic decay of turbulent kinetic energy across the flame brush. The modelling of the various unclosed terms of the turbulent kinetic energy transport equation has been analysed in detail. The predictions of existing models are compared with corresponding quantities extracted from DNS data. Based on this a-priori DNS assessment, either appropriate models are identified or new models are proposed where necessary. It is shown that the turbulent flux of turbulent kinetic energy exhibits counter-gradient (gradient) transport wherever the turbulent scalar flux is counter-gradient (gradient) in nature. A new model has been proposed for the turbulent flux of turbulent kinetic energy, and is found to capture the qualitative and quantitative behaviour obtained from DNS data for both the corrugated flamelets and thin reaction zones regimes without the need to adjust any of the model constants.     

硅锰合金扭簧应力松弛行为研究

研制了矩形截面扭簧应力松弛试验装置并进行了多组高温实验;提出了可动位错密度函数,结合现有应力松弛位错理论提出了新的硅锰合金弹簧钢扭转应力松弛动力学方程;利用ABAQUS二次开发接口CREEP子程序,实现了将新的应力松弛动力学方程转化为基于隐式有限元法的修正时间硬化蠕变模型,对400℃下的硅锰合金弹簧钢扭转应力松弛行为进行了数值模拟;模拟结果表明,修正的时间硬化蠕变模型与试验结果吻合,扭簧表面的Von Mises应力随时间变化规律与试验中的扭矩随时间变化规律一致.     

Predicting crossflow induced transition with laminar kinetic energy transition model

The hypersonic laminar kinetic energy transition model is developed to be appropriate for crossflow induced boundary layer transition prediction. A crossflow timescale is constructed and incorporated in the k_T-k_L-ω transition model to reflect crossflow effect during three-dimensional boundary layer transition. The stream-wise vorticity is selected as the indicator of crossflow strength. Regarding the inviscid unstable characteristic of crossflow instability, the crossflow timescale is constructed by reference to the second mode timescale. To eliminate inappropriate development of the crossflow timescale where the effective length scale is large enough while the crossflow strength remains at a quite low level, a crossflow velocity limit function is proposed. The revised k_T-k_L-ω transition model has been applied to HIFiRE-5 and blunt cone with 1°angle of attack test cases. Results show good correspondence with the experimental data and DNS data, which demonstrates that the constructed crossflow timescale makes the revised transition model capable of reproducing crossflow induced transition behavior with a reasonable degree of accuracy.     

Prediction of the particle-laden jet with a two-equation turbulence model

A two-equation turbulence model for two-phase flows has recently been proposed by Elghobashi & Abou-Arab (1983). They derived the exact equations of the kinetic energy of turbulence and the rate of dissipation of that energy, and modeled the turbulent correlations, resulting from time-averaging, up to third order. In order to validate the proposed model, a turbulent axisymmetric gaseous jet laden with spherical uniform-size solid particles is studied here. The predictions of the mean flow properties of the two-phases and the turbulence kinetic energy and shear stress of the carrier phase show good agreement with the experimental data.     

Steady-shear viscosity and transient stress response for elasto-thixotropic fluids

A phenomenological model for dispersed systems which exhibit complex rheological behaviour such as shear and time-dependent viscosity, yield stress, and elasticity is proposed. The model extends the Quemeda model to describe the viscosity function with a structural parameter λ which varies according to different kinetic orders of particle aggregation and segregation. The transient stress response is obtained by solving an instantaneous Maxwell model with an assumed shear modulus functionG of the same form as the viscosity function η. Accuracy of the proposed model is verified experimentally with the results obtained for two oil (creosote)/water emulsions. The model that gives the best fit of experimental data appears to be the one with kinetic ordersn=m=2.     

Separation-induced boundary layer transition: Modeling with a non-linear eddy-viscosity model coupled with the laminar kinetic energy equation

We present an effort to model the separation-induced transition on a flat plate with a semi-circular leading edge, using a cubic non-linear eddy-viscosity model combined with the laminar kinetic energy. A non-linear model, compared to a linear one, has the advantage to resolve the anisotropic behavior of the Reynolds-stresses in the near-wall region and it provides a more accurate expression for the generation of turbulence in the transport equation of the turbulence kinetic energy. Although in its original formulation the model is not able to accurately predict the separation-induced transition, the inclusion of the laminar kinetic energy increases its accuracy. The adoption of the laminar kinetic energy by the non-linear model is presented in detail, together with some additional modifications required for the adaption of the laminar kinetic energy into the basic concepts of the non-linear eddy-viscosity model. The computational results using the proposed combined model are shown together with the ones obtained using an isotropic linear eddy-viscosity model, which adopts also the laminar kinetic energy concept and in comparison with the existing experimental data.     

Implementation of physical boundary conditions into computational domain in modelling of oscillatory bottom boundary layers

This paper discusses the importance of realistic implementation of the physical boundary conditions into computational domain for the simulation of the oscillatory turbulent boundary layer flow over smooth and rough flat beds. A mathematical model composed of the Reynolds averaged Navier–Stokes equation, turbulent kinetic energy (k) and dissipation rate of the turbulent kinetic energy (ε) has been developed. Control‐volume approach is used to discretize the governing equations to facilitate the numerical solution. Non‐slip condition is imposed on the bottom surface, and irrotational main flow properties are applied to the upper boundary. The turbulent kinetic energy is zero at the bottom, whereas the dissipation rate is approaching to a constant value, which is proportional to the kinematic viscosity times the second derivative of the turbulent kinetic energy. The output of the model is compared with the available experimental studies conducted in oscillatory tunnels and wave flume. It is observed that the irrotational flow assumption at the upper boundary is not realistic in case of water tunnels. Therefore, new upper boundary conditions are proposed for oscillatory tunnels. The data of wave flume show good agreement with the proposed numerical model. Additionally, several factors such as grid aspect ratio, staggered grid arrangement, time‐marching scheme and convergence criteria that are important to obtain a robust, realistic and stable code are discussed. Copyright © 2009 John Wiley & Sons, Ltd.     

软体连续机械臂动力学建模与仿真

由于软体机械臂的质量是沿臂的长度连续分布,因此采用拉格朗日方法建立软体机械臂的动力学模型时,涉及计算复杂的积分运算,采用离散化的集中质量模型降低了计算的复杂性,但准确性不足.为了提高软体机械臂动力学建模与仿真的准确度和计算效率,本文采用模态方法对软体机械臂进行运动学描述,并从能量的角度分析软体机械臂动力学特性,研究发现...     

A new dynamic subgrid-scale model using artificial neural network for compressible flow

The subgrid-scale (SGS) kinetic energy has been used to predict the SGS stress in compressible flow and it was resolved through the SGS kinetic energy transport equation in past studies. In this paper, a new SGS eddy-viscosity model is proposed using artificial neural network to obtain the SGS kinetic energy precisely, instead of using the SGS kinetic energy equation. Using the infinite series expansion and reserving the first term of the expanded term, we obtain an approximated SGS kinetic energy, which has a high correlation with the real SGS kinetic energy. Then, the coefficient of the modelled SGS kinetic energy is resolved by the artificial neural network and the modelled SGS kinetic energy is more accurate through this method compared to the SGS kinetic energy obtained from the SGS kinetic energy equation. The coefficients of the SGS stress and SGS heat flux terms are determined by the dynamic procedure. The new model is tested in the compressible turbulent channel flow. From the a posterior tests, we know that the new model can precisely predict the mean velocity, the Reynolds stress, the mean temperature and turbulence intensities, etc.     

A linearization of Mieussens's discrete velocity model for kinetic equations

A linearization is developed for Mieussens's discrete velocity model (see, e.g., [L. Mieussens, Discrete-velocity models and numerical schemes for the Boltzmann-BGK equation in plane and axisymmetric geometries, J. Comput. Phys. 162 (2000) 429–466]) for kinetic equations. The basic idea is to use a linearized expression of the reference distribution function in the kinetic equation, instead of its exact expression, in the numerical scheme. This modified scheme is applied to various kinetic models, which include the BGK model, the ES-BGK model, the BGK model with velocity-dependent collision frequency, and the recently proposed ES-BGK model with velocity-dependent collision frequency. One-dimensional stationary shock waves and stationary planar Couette flow, which are two benchmark problems for rarefied gas flows, are chosen as test examples. Molecules are modeled as Maxwell molecules and hard sphere molecules. It is found that results from the modified scheme are very similar to results from the original Mieussens's numerical scheme for various kinetic equations in almost all tests we did, while, depending on the test case, 20–40 percent of computational time can be saved. The application of the method is not affected by the Knudsen number and molecular models, but is restricted to lower Mach numbers for the BGK (or the ES-BGK) model with velocity-dependent collision frequency.