Dynamic buckling and plastic collapse of rectangular strips under axial slamming impact

The dynamic buckling and plastic collapse of elastic-plastic rectangular strips under axial slamming impact are investigated experimentally. The dynamic response of the specimens is measured by several back-to-back paris of strain gages located at different positions. According to the experimental records, the compressive and bending motions of the rectangular strips are analyzed. The strips exhibit three different critical dynamic conditions: buckling, plastic incipience and plastic collapse. Based on the response characters, three criteria are proposed which completely define the elastic-plastic dynamic behavior of rectangular strips under axial slamming impact with loading durations ranging from 14 to 18 milliseconds. These conditions are estimated by introducing three critical axial compressive strains. Moreover, the effect of geometric imperfection on the dynamic behavior of the strips is discussed.     

A suppression method for thermomechanical stresses during the laser processing of structural elements

A method to reduce thermomechanical stresses during the laser processing of a beam-like specimen along its side surface and a thin disk along its central circular hole is considered. Some analytical temperature expressions are obtained in both these cases. The problems of determining thermal stresses during the heating of specimens with consideration of heat transfer on their surfaces are solved. The solutions obtained with heat transfer and without it are compared. It is shown that the side surface blowing can be used to suppress the fracture of heated specimens.     

Effect of density ratio on critical heat flux in closed end vertical tubes

An experimental study was conducted to measure the critical axial heat flux in countercurrent two phase flow of liquid and its vapour in a closed-end vertical tube. The experimental results for four different fluids; carbon-tetrachloride, normal hexane, ethyl alcohol and water, were reduced to give a correlation for evaluation of the flooding critical heat flux. Results of other investigators on vertical heated tubes and vertical thermosyphons were also reduced and compared with the present experimental results. The effect of the density ratio of liquid to its vapour on the critical heat flux was shown. The length to diameter ratio of the test section was shown to have an influence on the flooding critical heat flux and was included in the correlation obtained.     

加热压电纤维复合材料圆板的横向自由振动

基于经典薄板理论和极正交各向异性材料的本构理论,建立了加热压电纤维复合材料圆板的线性振动控制微分方程。采用打靶法分别获得了加热压电纤维复合材料圆板在周边固支和简支情况下,无量纲固有频率随温度和电场强度变化的关系曲线,并分析了压电纤维体积分数、刚度参数、电场强度和温度变化对压电纤维复合材料圆板无量纲固有频率的影响。结果表明,一定体积分数或者电场强度下,压电纤维复合材料圆板的无量纲固有频率都随温度的升高而单调下降;同一温度下,刚度参数越小,无量纲固有频率越低;电场强度越大,无量纲固有频率越高。     

Mixed Convection in a Darcy–Brinkman Porous Medium with a Constant Convective Thermal Boundary Condition

The characteristics of the boundary layer flow past a plane surface adjacent to a saturated Darcy–Brinkman porous medium are investigated in this paper. The flow is driven by an external free stream moving with constant velocity. The surface is heated with a convective boundary condition with constant heat transfer coefficient. The problem is non-similar and is investigated numerically by a finite difference method. The problem is governed by four non-dimensional parameters, that is, the convective Darcy number, the convective Grashof number, the Prandtl number, and the axial distance along the plate. The influence of these parameters on the results is investigated, and the results are presented in tables and figures. The Darcy term and the Grashof term in the momentum equation contradict each other and this contradiction makes the problem complicated. However, the wall shear stress and the wall temperature increase continuously along the plate and the wall temperature always tends to 1.     

Analytical series solution for the fully developed forced convection duct flow with frictional heating and variable viscosity

Laminar forced convection flow of a liquid in the fully developed region of a circular duct with isothermal wall is analyzed. The effects of viscous dissipation as well as of temperature dependent viscosity are taken into account. The coupled momentum and energy equations are solved analytically by means of a power series method. Then, reference is made to the Poiseuille model for the temperature change of viscosity. For a fixed value of the axial pressure gradient along the duct, dual solutions are found for the velocity and temperature fields. Although dual solutions correspond to the same value of the axial pressure gradient, they lead in general to different values of the average fluid velocity, of the average fluid temperature and of the wall heat flux. It is shown that, for a given fluid and for a fixed duct radius, the absolute value of the axial pressure gradient has an upper bound above which no steady laminar solution can exist.     

Temperature and stress analysis of an elastic circular cylinder in contact with heated rigid stamps

Summary  The present paper discusses a plane strain problem of transient thermoelasticity in a circular cylinder which is in partial contact with two heated rigid stamps, in the case where the coefficient of relative heat transfer on the contact surface of the cylinder is different from that on the traction-free surface. A finite difference method with respect to the time variable and Airy's thermal stress function is employed to analyze the temperature and thermoelastic fields. The problem is formulated in terms of two dual-series equations derived not only from the thermal boundary conditions but also from the mechanical boundary conditions. Since the radial, hoop and axial stresses have singularities at the end of the contact surface of the cylinder, the stress singularity coefficients are defined and then the relationship among these three coefficients is also obtained. Finally, numerical results are illustrated graphically. Received 3 March 2000; accepted for publication 12 July 2000     

弹体侵彻过程中装药温升的近似分析

对侵彻过程中内部装药轴向应力和变形进行理论分析,并考虑装药的弹塑性变形,得到轴向应力 和形变量沿着装药轴向分布函数,以内壁与装药摩擦生热和热传导为基础,得到装药温度的分布曲线、最高温 度在装药中的相对位置。计算结果表明,装药最高温升以及最高温升在装药中的位置与炸药的物性参数、装 药长度和过载最大值相关,临界装药长度可作为弹体装药设计的参考,计算结果为弹体侵彻过程中的装药设 计和安全性分析提供理论依据。     

THERMAL POST-BUCKLING OF FUNCTIONALLY GRADED MATERIAL TIMOSHENKO BEAMS

Analysis of thermal post-buckling of FGM (Functionally Graded Material) Timoshenko beams subjected to transversely non-uniform temperature rise is presented. By accurately considering the axial extension and transverse shear deformation in the sense of theory of Timoshenko beam, geometrical nonlinear governing equations including seven basic unknown functions for functionally graded beams subjected to mechanical and thermal loads were formulated. In the analysis, it was assumed that the material properties of the beam vary continuously as a power function of the thickness coordinate. By using a shooting method, the obtained nonlinear boundary value problem was numerically solved and thermal buckling and post-buckling response of transversely non-uniformly heated FGM Timoshenko beams with fixed-fixed edges were obtained. Characteristic curves of the buckling deformation of the beam varying with thermal load and the power law index are plotted. The effects of material gradient property on the buckling deformation and critical temperature of beam were discussed in details. The results show that there exists the tension-bend coupling deformation in the uniformly heated beam because of the transversely non-uniform characteristic of materials.     

The buckling behavior of uniformly heated thin circular cylindrical shells

The results of buckling tests on uniformly heated, clamped, thin circular cylindrical shells are presented and discussed. Particular attention is paid to both the actual buckling process and the ensuing post-buckling behavior. Load vs. end-shortening curves are included. The possibility of “snap-through” buckling which occurs at a value of end shortening greater than that corresponding to the maximum supported load is experimentally verified. A comparison of the present experimental results with available theory is made. It is observed that the experimental values of the buckling temperature can be substantially greater than the temperatures calculated by linear theory from the experimental buckling loads; however, the buckling stresses are the same whether the loading is thermal or mechanical.     

Influence of strain rate on a premixed turbulent flame stabilized in a stagnating flow

In order to describe the influence of strain rate on the behaviour and on the characteristics of premixed turbulent combustion, a methane-air flame stabilized by a stagnation plate is studied experimentally. The plate is set at a fixed distance from the nozzle and the strain is varied by changing the exit velocity at the nozzle. At low strain rates, the evolution of profiles of mean axial velocity along the centreline agrees with classical results, and these results are used to characterise the flame. The variation of these characteristics with parameters such as plate temperature, equivalence ratio and strain rate is investigated. At the highest strain rates, the shape of the axial velocity profiles along the stagnation line is modified. This change emphasises a critical strain rate K _C that has to be considered as well as the extinction strain rate K _EX. Measurements also demonstrate the existence of a virtual stagnation point that moves towards the plate as the strain rate increases. The axial and transverse fluctuating components of the velocity are analyzed along the centreline and very close to the wall. The results show the importance of the critical strain rate K _C , which is linked to a drastic change in the evolution of the axial and transverse velocity fluctuations. Received: 15 January 1998/Accepted: 7 February 1999     

A numerical simulation of combined radiation and natural convection heat transfer in a square enclosure heated by a centric circular cylinder

A numerical simulation of combined natural convection and radiation in a square enclosure heated by a centric circular cylinder and filled with absorbing-emitting medium is presented. The ideal gas law and the discrete ordinates method are used to model the density changes due to temperature differences and the radiation heat transfer correspondingly. The influence of Rayleigh number, optical thickness and temperature difference on flow and temperature fields along with the natural convection, radiation and total Nusselt number at the source surfaces is studied. The results reveal that the radiation heat transfer as well as the optical thickness of the fluid has a distinct effect on the fluid flow phenomena, especially at high Rayleigh number. The heat transfer and so the Nusselt number decreases with increase in optical thickness, while increases greatly with increase in temperature difference. The variation in radiation heat transfer with optical thickness and temperature difference is much more obvious as comparison with convection heat transfer.     

Experimental studies on dynamic plastic buckling of circular cylindrical shells under axial impact

In the present paper, experimental studies on dynamic plasticbuckling of circular cylindrical shells under axial impact are carried out. Hopkinson bar and drop hammer apparatus are used for dynamic loading. Three groups of circular cylindrical shells made of copper are tested under axial impact. From the experiments, the first critical velocity corresponding to the axi-symmetric buckling mode and the second critical velocity corresponding to the non-axisymmetric buckling mode are determined. The present results come close to those of second critical velocity given by Wang Ren^[4–6]. Two different kinds of non-axisymmetric buckling modes oval-shaped and triangle shaped are founded. The buckling modes under two loading cases, viz. with small mass but high velocity and with large mass and low velocity using Hopkinson bar and drop hammer, are different. Their critical energies are also discussed. The project is supported by the National Natural Science Foundation of China (19672039) and the Foundation for Returned Scholar from Abroad of Shanxi Province     

Forced convection performance of a MEPCM suspension through an iso-flux heated circular tube: an experimental study

The paper examines experimentally forced convection performance of a microencapsulated phase change material (MEPCM) suspension through an iso-flux heated circular tube. Forced convection experiments have been undertaken using the pure water or MEPCM suspensions as the working fluid. The heat transfer performance of the MEPCM suspension was gauged in terms of local/average heat transfer coefficients and temperature control effectiveness along the tube wall compared with that obtained for the pure water.     

Size-dependent axial buckling analysis of functionally graded circular cylindrical microshells based on the modified strain gradient elasticity theory

In this paper, a size-dependent first-order shear deformable shell model is developed based upon the modified strain gradient theory (MSGT) for the axial buckling analysis of functionally graded (FG) circular cylindrical microshells. It is assumed that the material properties of FG materials, which obey a simple power-law distribution, vary through the thickness direction. The principle of virtual work is utilized to formulate the governing equations and corresponding boundary conditions. Numerical results are presented for the axial buckling of FG circular cylindrical microshells subject to simply-supported end conditions and the effects of material length scale parameter, material property gradient index, length-to-radius ratio and circumferential mode number on the size-dependent critical buckling load are extensively studied. For comparison purpose, the critical buckling loads predicted by modified couple stress theory (MCST) and classical theory (CT) are also presented. Results show that the size effect plays an important role for lower values of dimensionless length scale parameter. Moreover, it is observed that the critical buckling loads obtained based on MSGT are greater than those obtained based on MCST and CT.     

On the Interaction of Closely Spaced Circular Openings in a Transversely Isotropic Shallow Cylindrical Shell

A transversally isotropic shallow cylindrical shell with two different circular openings under axial tension is examined. The centers of the openings are aligned along a generatrix. Stresses are evaluated by finding an approximate analytical solution to differential equations of a two-dimensional shell theory that takes transverse shears into account. The results are compared with those obtained by the finite-element method     

黏弹性层合圆柱壳的临界轴压分析

基于经典屈曲理论,研究了轴向受压黏弹性复合材料层合圆柱壳的临界屈曲载荷. 利用Boltzmann线性积分型本构关系描述铺设单层的各向异性黏弹性行为. 结合解析与数值 方法,由Donnell型屈曲控制方程以及边界条件的Laplace变换导出相空间的特征方程,根 据Laplace逆变换的极值定理,获得层合圆柱壳的瞬时弹性临界载荷与持久临界载荷. 针对 多组铺设方式,通过数值算例重点分析了临界载荷随铺设角的变化特征,两种临界载荷的峰 值点差异程度与铺设方式、几何参数以及材料类型的关系,得到了一些对黏弹性层合圆柱壳 的优化设计有参考价值的结论.     

Mechanical buckling of a functionally graded cylindrical shell with axial and circumferential stiffeners using the third-order shear deformation theory

This paper deals with an analytical approach of the buckling behavior of a functionally graded circular cylindrical shell under axial pressure with external axial and circumferential stiffeners. The shell properties are assumed to vary continuously through the thickness direction. Fundamental relations and equilibrium and stability equations are derived using the third-order shear deformation theory. The resulting equations are employed to obtain the closed-form solution for the critical buckling loads. A simply supported boundary condition is considered for both edges of the shell. The comparison of the results of this study with those in the literature validates the present analysis. The effects of material composition (volume fraction exponent), of the number of stiffeners and of shell geometry parameters on the characteristics of the critical buckling load are described. The analytical results are compared and validated using the finite-element method. The results show that the inhomogeneity parameter, the geometry of the shell and the number of stiffeners considerably affect the critical buckling loads.     

Instability of a Longitudinal Movement along the Cylindrical Surface for a Thermoelastic Web Simulated by Stretched and Heated String

On the basis of classical methods for mathematical physics and mechanics, the stability problem of a thermoelastic web moving at a constant speed without friction along a cylindrical surface is investigated. The web is modeled by a stretched and heated string. At a sufficiently high speed and heating of a string, a loss ofmotion stability and the stringmovement in a direction normal to the cylindrical surface occur. To study the instability, a static method based on the consideration of stationary nontrivial modes of stability loss, that is, on the study of the problem for bifurcation of solutions (eigenvalue problem) for the corresponding differential equations is used. The case of the web motion along the circular cylinder is separately considered and an expression for the critical velocity leading to the instability is found.     

Thermomechanical postbuckling of unilaterally constrained shear deformable laminated plates with temperature-dependent properties

This paper presents a study on the postbuckling responses of shear deformable laminated plates resting on a tensionless foundation of the Pasternak-type and subjected to combined axial and thermal loads. Two different postbuckling cases are considered, namely (1) the compressive postbuckling of initially heated plates and (2) the thermal postbuckling of initially compressed plates. The postbuckling analysis of laminated plates is based on the higher order shear deformation plate theory with a von Kármán-type of kinematic non-linearity. It is assumed that the foundation reacts in compression only. The thermal effects are also included and the material properties are assumed to be temperature dependent. The initial geometric imperfection of the plates is taken into account. The analysis uses a two-step perturbation technique to determine the postbuckling response of the plates. An iterative scheme is developed to obtain numerical results without using any assumption on the shape of the contact region. Numerical solutions are presented in tabular and graphical forms to study the postbuckling behavior of antisymmetric angle-ply and symmetric cross-ply laminated plates resting on tensionless elastic foundations of the Pasternak-type, from which results for conventional elastic foundations are obtained as comparators. The results reveal that the unilateral constraint has a significant effect on the postbuckling response of the plates subjected to combined axial and thermal loads when the foundation stiffness is sufficiently large. The results also confirm that the postbuckling responses are significantly influenced by temperature dependency and initial membrane stress as well as initial thermal stress.