The effects of a finite pulse time in the flash thermal diffusivity method

After a brief review of the finite pulse time effects in flash thermal diffusivity measurements, an analytical expression for an exponential shape pulse was determined using the Green function method. The results were compared with those obtained by Larson and Koyama. It was found that, using the Larson and Koyama equation, when the dimensionless time is equal to zero, the dimensionless temperature rise V cannot reach zero, and when _p, the time characterizing the dimensionless pulse, approaches 1/n ² (n=1,2,3,...), a large error of _1/2 will result. These contradictions have been resolved by the present work. In other respects, both sets of results concurred. The results are compared with the triangular pulse and are discussed.     

Measurement of thermal diffusivity by the flash method for a two-layer composite sample in the case of triangular pulse

A method for measuring thermal diffusivity in one of the layers of a two-layer composite sample has been described. The heat transfer problem of a two-layer sample associated with pulse thermal diffusivity measurements has been analyzed for two cases: exponential and square-wave pulses. According to our measurements, a triangular heat-pulse function approximates reasonably well the output of the Nd-glass laser. In this paper, an expression is derived for the temperature transient at the rear face of two-layer sample being subjected to a triangular heat-pulse input on the front face. The analytical solution of the problem forms the basis of our method of data reduction. This solution has been programmed for computer processing of the data. The method described here has been successfully tested by limited measurements on copper and iron.Paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

The errors in measuring thermal diffusivity by a pulsed method due to random fluctuations of the form of the laser pulse

An experimental estimate is given of the errors in measuring thermal diffusitivity by a pulsed method, due to random changes in the time distribution of the radiation of the pulsed laser. __________ Translated from Izmeritel’naya Tekhnika, No. 11, pp. 37–39, November, 2006.     

Measurement of the thermal characteristics of materials without the action of heat

The theoretical principles of a qualitatively new method for the express measurement of the thermal characteristics of materials (the thermal conductivity and the thermal diffusivity) are presented. The operating principle and the basic technical data of an instrument for realizing the proposed method are described. __________ Translated from Izmeritel’naya Tekhnika, No. 9, pp. 44–47, September, 2007.     

Experimental investigation of nonuniform heating and heat loss from a specimen for the measurement of thermal diffusivity by the laser pulse heating method

Nonuniform heating effect and heat loss effect from the specimen in the measurement of thermal diffusivity by the laser pulse heating method have been experimentally investigated using an axially symmetric Gaussian laser beam and a laser beam homogenized with an optical filter. The degree of error is theoretically estimated based on the solution of the two-dimensional heat conduction equation under the boundary condition of heat loss from the surface of the specimen in the axial direction and the initial conditions of axially symmetric nonuniform and uniform heating. A correction factor, which is determined by comparison of the entire experimental and the theoretical history curves, is introduced to correct the values obtained by the conventionalt _1,2 method. The applicability of this modified curve-fitting method has been experimentally tested using materials in the thermal diffusivity range 10⁻³ to 1 cm²·s⁻¹. The experimental error due to the nonuniform heating and heat loss was reduced to approximately 3%.     

A mixed finite volume formulation for determining the small strain deformation of incompressible materials

A finite volume formulation for determining small strain deformations in incompressible materials is presented in detail. The formulation includes displacement and hydrostatic pressure variables. The displacement field varies linearly along and across each cell face. The hydrostatic pressure field associated with each face is uniform. The cells that discretize the structure are geometrically unrestricted, each cell can have an arbitrary number of faces. The formulation is tested on a number of linear elastic plane strain benchmark problems. This testing reveals that when meshes of multifaceted cells are employed to represent the structure then locking behaviour is exhibited, but when triangular cells are used then accurate predictions of the displacement and stress fields are produced. Copyright © 1999 John Wiley & Sons, Ltd.     

Simultaneous measurement of thermal diffusivity and specific heat at high temperatures by a single rectangular pulse heating method

A method for the simultaneous measurement of thermal diffusivity and specific heat by a single rectangular heating pulse on a finite cylindrical specimen is described. The method takes into account radiation losses from all the surfaces of the specimen. The theoretical principle of the technique was studied by solving the transient heat conduction equation for a finite disk heated on the front surface by a single rectangular radiant energy pulse. An apparatus was constructed to comply with the theoretical conditions and was connected to a personal computer. Thermal diffusivity and specific heat were determined from the data obtained on the temperature response of the back surface of the specimen and from the theoretical results. This method can be applied to materials having a wide range of thermal conductivity values and has a good accuracy at high temperatures. Examples of the measurements are presented.Invited paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

A noncontact method for measuring thermal conductivity and thermal diffusivity of anisotropic materials

A noncontact method for measuring the thermal conductivity and thermal diffusivity of anisotropic materials is proposed. This method is based on the fact that the surface temperature variation with time depends on the thermal properties of the material when its surface is heated locally. The three-dimensional transient heat conduction equation in the material is solved numerically. The dimensionless average surface temperature variations are obtained along each principal axis: that is, thex andy axes. The relation between the dimensionless temperature and the Fourier number is expressed by a polynomial equation and used as a master plot, which is a basic relation to be compared with measured temperature variation. In the experiments, the material surface is heated with a laser beam and the surface temperature profiles are measured by an infrared thermometer. The measured temperature variations with time are compared with the master plots to yield the thermal conductivity λ _x and thermal diffusivityx _v in thex direction and the thermal conductivity ratioE _xy (=λ _y λ _x ) simultaneously. To confirm the applicability and the accuracy of the present method, measurements were performed on multilayered kent-paper, vinyl chloride, and polyethylene resin film, whose thermal properties are known. From numerical simulations, it is found that the present method can measure the thermophysical properties λ _x , α _x andE _xy within errors of ±6, ±22, and ±5%, respectively, when the measuring errors of the peak heat flux, the heating radius, and the surface temperature rise are assumed to be within ±2, ±3%, and ±0.2 K, respectively. This method could be applied to the measurement of thermophysical properties of biological materials.     

The thermal inertia of materials heated with a laser pulse faster than relaxation time

The nonlocal hyperbolic heat conduction equation is used to describe the thermal inertia of thin metal films (TMF) heated with femtosecond laser pulses. It is shown that for TMF the signatures of thermal inertia are (i) the delay of the heating process and (ii) the strong localization of the thermal energy in TMF.Paper presented at the Fourth International Workshop on Subsecond Thermophysics, June 27–29, 1995, Köln, Germany.     

Enriching the finite element method with meshfree particles in structural mechanics

The automatic generation of meshes for the finite element (FE) method can be an expensive computational burden, especially in structural problems with localized stress peaks. The use of meshless methods can address such an issue, as these techniques do not require the existence of an underlying connection among the particles selected in a general domain. This study advances a numerical strategy that blends the FE method with the meshless local Petrov–Galerkin technique in structural mechanics, with the aim at exploiting the most attractive features of each procedure. The idea relies on the use of FEs to compute a background solution that is locally improved by enriching the approximation space with the basis functions associated to a few meshless points, thus taking advantage of the flexibility ensured by the use of particles disconnected from an underlying grid. Adding the meshless particles only where needed avoids the cost of mesh refining, or even of remeshing, without the prohibitive computational cost of a thoroughly meshfree approach. In the present implementation, an efficient integration strategy for the computation of the coefficients taking into account the mutual FE–meshless local Petrov–Galerkin interactions is introduced. Moreover, essential boundary conditions are enforced separately on both FEs and meshless particles, thus allowing for an overall accuracy improvement also when the enriched region is close to the domain boundary. Numerical examples in structural problems show that the proposed approach can significantly improve the solution accuracy at a local level, with no remeshing effort, and at a low computational cost. Copyright © 2016 John Wiley & Sons, Ltd.     

材料热扩散率的非稳态测量与应用

热扩散率是材料热物性中一个非常重要的参数。在材料热扩散率的测试中,主要是利用非稳态方法进行测量。非稳态测量方法具有测量周期短、测试方便、结果准确等优点。主要对常用的5种利用非稳态测试材料热扩散率的方法进行了阐述,详细介绍了它们的工作原理、方法特点以及近几年来的科研成果。同时,列举了目前热扩散率测试的一些常用产品和相关测试标准。最后,对热扩散率测试的未来发展趋势进行了展望。     

Suggestions regarding thermal diffusivity measurements on pyrolytic graphite and pyrolytic boron nitride by the laser pulse method

The laser pulse method can be successfully applied to the measurement of thermal diffusivity of isotropic materials subject to some assumptions. For anisotropic materials, this method is applicable to the measurement of principal thermal diffusivity only on the condition that there is no difference in direction between the principal axis and that of the temperature gradient. After analyzing the heat conduction process in an anisotropic solid, it has been shown that large errors in the measurement of thermal diffusivity would exist if the direction of the principal axis deviates inconspicuously from that of the temperature gradient. The experimental results of thermal diffusivity of highly oriented pyrolytic graphite (HOPG) samples with various deviation angles have been compared with the analytical results. The laser pulse method is not applicable to measurements on semitransparent pyrolytic boron nitride (PBN). We adopted a two-layer composite sample to measure the thermal diffusivity of PBN in the c direction and a particular graphite-PBN composite sample has been prepared which has a very low thermal resistance at the interface. The thermal diffusivity and thermal conductivity of PG (below 2300°C) and PBN (below 1000°C) are given.Invited paper presented at the Ninth Symposium on Thermophysical Properties, June 24–27, 1985, Boulder, Colorado, U.S.A.     

晶体材料导热性质的经验方程与预测方法

依据现有相关的功能晶体材料导热系数实验数据，寻求了晶体材料导热系数与平均原子密度和平均原子量之间关联的经验方程，并对Ⅲ-Ⅴ族化合物半导体和碳化物晶体材料给出了经验方程中的常数，从而为这些晶体材料导热性质的预测提供了一种可靠的方法。     

A method and instrument for measuring the thermal properties of samples of a regenerative product in a matrix when they are heated by a constant heat flux

A mathematical model of a method of measuring the thermal properties of new regenerative products is developed. The construction of a measuring instrument is proposed. Experimental data on the thermal conductivity and volume heat capacity of regenerative products in a matrix are obtained. Translated from Izmeritel’naya Tekhnika, No. 5, pp. 49–53, May, 2009.     

基于有限元法的周期压电智能梁滤波特性分析

周期结构具有通频和禁频特性,使其在动态载荷的滤波器、具有主动控制功能的结构研究中得到了重要应用。基于Timoshenko梁理论,考虑基梁和压电片的转动惯量和剪切效应,采用有限元法和传递矩阵法推导了波在周期性地粘贴压电片的Timoshenko梁中的传播模型,分析了几何尺寸和材料特性对其频带性质的影响,并与Bernoulli-Euler梁理论得到的结果进行了对比。研究表明,当基梁与压电层厚度比达到40时,禁带带宽减小了54%,因此对于周期结构中的深梁,应舍弃Bernoulli-Euler梁理论而采用Timoshenko梁理论建立的模型;对于不同尺寸和材料特性的压电周期结构,频带性质会有很大不同,可以通过调整结构的参数来改变其频带性质,从而改变波动在结构中的传播特性。     

On a Method of Defining the Plastic-Strain Zone

We propose a new contactless method of defining the plastic-strain zone in tensile testing of structural materials. The method is based on measuring the dimensions of a thermal field appearing during deformation by means of a thermal imager.