Thermal stresses in functionally graded materials caused by a laser thermal shock

 Mathematical simulation of a thermal shock method for reliability testing of functionally graded material (FGM) is performed with the end to determine operating parameters of the testing device (power of a laser, laser beam radius, duration of heating) and to investigate the effect of the composition of FGM on a magnitude of thermal stresses in a coating. An analytical method for solution of the thermal elasticity problem is developed whereby the approach of a multilayer plate is used for determining temperature and thermal stresses distributions in a coating. We considered the limiting case of the obtained solution when the thickness of a layer is infinitesimally small and the number of layers tends to infinity. This procedure allowed us to obtain the thermal stresses distribution in a FGM coating. The results for the FGM coating composed of WC (tungsten carbide) ceramics and HS-steel are presented. It is showed that variation of the volume content of ceramics strongly affects thermal stresses in a coating and they decrease significantly in the case of the uniform spatial distribution of ceramics. Received on 21 November 2000 / Published online: 29 November 2001     

碳纤维布加固钢筋混凝土梁受力性能的试验研究

通过对8根矩形截面钢筋混凝土梁的静力加载试验,研究了抗弯和抗剪两种加固方式对钢筋混凝土梁的破坏形态和力学性能的影响,对比分析了碳纤维加固率、试验梁的状态、混凝土强度对加固效果的影响。结果表明：经碳纤维抗弯哥口抗剪加固后加圉梁相应的力学性能指标值明显提高;混凝土裂缝扩展得到了有效延缓,加固效果明显;抗弯加固后可显著提高混凝土梁的承载力和延性,但提高程度并不与加固率成正比;预裂粱抗弯加固后承载力和延性得到提高,但与其相同加固参数的梁相比,其承载力和延性有所降低;抗剪加固时混凝土强度越高,加固后混凝土梁的承载力和变形能力提高幅度越大,其中混凝土梁的变形能力受混凝土强度的影响较大。最后,对加固梁的极限承载力进行了理论分析与计算,建立了实用的理论计算公式,计算结果与试验结果吻合较好。     

The dynamic thermal stresses in hollow cylinder by thermal shock simultaneous with mechanical shock acting on the inner movable boundary

The transient temperature fields and dynamic thermal stress fields in gun barrel are studied in this paper, which are problems of interior ballistics.The high blast temperature and pressure act simultaneously on the inner surface of the barrel behind projectile moving in the barrel during shooting. Under this boundary condition, the governing equations of the thermoelastic problems of hollow cylinder are given, the corresponding functional is studied and the numerical results calculated by the finite element method are obtained.     

Interaction of thin perforated plates and weak shock waves

The paper presents experimental results on elastic deformation of perforated thin plates made of glass-fiber laminate and subjected to weak shock waves. The characteristics of the dynamic process in the plates are determined __________ Translated from Prikladnaya Mekhanika, Vol. 42, No. 11, pp. 126–130, November 2006.     

Improving small laser light sheets by means of a diffractive optical element

Conventional optical elements like lenses and stops for the generation of light sheets in experimental fluid mechanics are challenged by modern diffractive optical elements (DOE) made to the specifications of the user. We report on a first application in connection with droplet investigations suggesting an enormous potential of the new technique.     

Heat transfer and thermal performance characteristics of heat exchanger tube fitted with perforated twisted-tapes

Twisted tape insert was applied as a swirling flow generator for the passive heat transfer enhancement in the present work. The influences of the perforated twisted tapes (PTs) on the heat transfer, pressure loss and thermal performance characteristics were investigated experimentally. The experiments were performed under uniform wall heat flux condition by using PTs with y/W?=?3, 4 and 5, d/W?=?0.11, 0.14 and 0.17 and s/W?=?0.4, 0.6 and 0.8 where y is a twist length, d is a perforation hole diameter, s is a spacing between holes (pitch) and W is a tape width. The experimental results reveal that Nusselt number increased with decreasing s/W and y/W and increasing d/W. For the present range, the maximum heat transfer was obtained by utilizing the tape with s/W?=?0.4, d/W?=?0.17 and y/W?=?3, which is higher than those obtained from the plain tube with and without typical twisted tape by around 27.4 and 86.7%, respectively. In addition, the empirical correlations for Nusselt number, friction factor and thermal performance are also proposed in the present paper.     

Shape of a shock wave front diffracting on a perforated wall

 The shape of a shock wave front diffracting on a perforated wall is determined by comparing numerical data and experimental findings. Experiments were conducted in a 60 mm×150 mm cross sectional area shock tube equipped with a double-exposure holographic interferometer. The numerical simulation was conducted using a TVD upwind finite difference scheme. First, a discharge coefficient for the mass flow through the perforations was determined by comparing the numerical results with those obtained using a simplified quasi-one-dimensional analysis. This value agreed well with the experimentally obtained value. Finally, the shape of a backward inclined incident shock wave over a perforated wall was successfully determined by employing this discharge coefficient and the numerical result. Received: 17 March 1995/Accepted: 13 August 1997     

经典梁热过屈曲问题的解析解

基于非线性经典梁理论,建立了控制轴向和横向变形的基本方程,将两个非线性方程化简为一个关于横向挠度的四阶非线性积分-微分方程。对于本文所考虑的三类边界条件,该方程与相应的边界条件构成了微分特征值问题;直接求解该问题,得到热过屈曲构形的解析解,该解是外加热载荷的函数。为考察热载荷以及边界条件的影响,根据得到的解析解给出了一些数值算例,讨论了梁过屈曲行为的性质。本文得到的解析解可用于验证或改进各类近似理论和数值方法。     

Orthotropic semi-infinite medium with a crack under thermal shock

A cracked orthotropic semi-infinite plate under thermal shock is investigated. The thermal stresses are generated due to sudden cooling of the boundary by ramp function temperature change. The superposition technique is used to solve the problem. The crack problem is formulated by applying the thermal stresses obtained from the uncracked plate with opposite sign to be the only external loads on the crack surfaces as the crack surface tractions. The Fourier transform technique is used to solve the problem leading to a singular equation of the Cauchy type. The singular integral equation is solved numerically using the expansion method. The influence of the material orthotropy on the stress intensity factors is shown by comparing the results obtained for different orthotropic materials and isotropic materials in the case of plane stress. The numerical results of the stress intensity factors are demonstrated as a function of time, crack length, location of the crack and the duration of the cooling rate.     

Performance of metallic honeycomb-core sandwich beams under shock loading

Stainless steel square honeycomb core sandwich and solid monolithic beams have been subjected to high-pressure, short-duration impulses using a shock simulation technique involving high-speed impact of Al foam projectiles. The experiments have been designed to achieve two objectives: (i) to demonstrate the benefits of sandwich construction, and (ii) to assess the fidelity of dynamic finite element calculations in simulating the structural response. The results affirm that, when subjected to impulse levels representative of those associated with nearby explosions, the sandwich beams exhibit smaller displacements than the solid beams at equivalent weight. The benefit is especially large at lower impulses where the effective dynamic strength of the honeycomb core prevents crushing. The measurements and finite element simulations having greatest relevance to the shock resistance are found to correspond closely, particularly the displacements and the core crushing strains. One implication is that the dynamic finite element model has the requisite fidelity at impulse levels of interest.     

Generalized solutions of transient thermal shock problem with bounded boundaries

In this work, the generalized thermoelastic solutions with bounded boundaries for the transient shock problem are proposed by an asymptotic method. The governing equations are taken in the context of the generalized thermoelasticity with one relaxation time (L–S theory). The general solutions for any set of boundary conditions are obtained in the physical domain by the Laplace transform techniques. The corresponding asymptotic solutions for a thin plate with finite thickness, subjected to different sudden temperature rises in its two boundaries, are obtained by means of the limit theorem of Laplace transform. In the context of these asymptotic solutions, two specific problems with different boundary conditions have been conducted. The distributions of displacement, temperature and stresses, as well as the propagations, intersections and reflections of two elastic waves, named as thermoelastic wave and thermal wave separately, are obtained and plotted. These results are agreed with the results obtained in the existing literatures.     

Energy conditions of gas laser cutting of thick steel sheets

Results of experimental determination of the energy balance in oxygen laser cutting of sheets 5–16 mm thick by a CO ₂ laser in the operation regime of high-quality cutting with the minimum possible degree of surface roughness are presented. Oxygen is used as an assisting gas. For sheets 5, 10, and 16 mm thick, the energy power fluxes, which are involved into the energy balance equation, normalized to the unit thickness of the sheet are found to be independent of the latter and to have close values.     

Prevention of thermal bending of multilayered beams and plates

A method is described to prevent bending in multilayered beams and plates of different isotropic materials with uniform and nonuniform temperature distribution through the thickness. The method involved the addition of an extra layer to the multilayered beams or plates. With the proper selection of the thermoelastic properties, the added layer would eliminate the curvature produced prior to this addition. A complete analysis for the determination of the various thermoelastic parameters of the extra layer was made. In addition, to ensure that the multilayered beams and plates actually remained straight, a thermoelastic analysis was performed for the solution of thermal stresses and strains in the laminate. The results gave assurance to the straightness of the laminate since the calculated strains have the same value throughout the thickness. The solutions are valid for any given uniform temperature change and for any given nonlinear temperature distribution through the thickness of the multilayered beams and plates. Several numerical examples are presented that illustrate the application of the method for various temperature distributions. A simple experiment was conducted that showed the validity of the analytical method. A brass strip was added to a bimetalic strip made of aluminum and steel at room temperature. The thickness of the brass strip was calculated from the theory to prevent bending. The trimetal strip was placed in a furnace and, as expected, it remained straight for varying temperatures.     

The action of intense light beams on metal surfaces

Studies of the effect of lasers on metals have shown that the action of intense light beams leads to the formation of craters on metal surfaces. A considerable increase in hardness is often observed in the region of such craters; the hardness of low-carbon steels may reach 700 kg/mm², which is considerably higher than that produced by other heat and mechanical treatments.In conclusion, the authors would like to thank R. V. Khokhlov for providing the facilities for the experimental work, and G. I. Barenblatt and I. M. Gryuznov for their assistance in interpreting the results.     

Crater formation in a metal by means of a high-enthalpy plasma shock

Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, Vol. 30, No. 6, pp. 19–22, November–December, 1989.