Fatigue life of polymethyl methacrylate in stationary and stepped nonisothermal cyclic loading regimes

Rotating cantilever experiments have revealed the heating characteristics of PMMA in multicycle fatigue tests. The effect of heating and damage on the fatigue life under stationary and single-step loading is considered. The statistical characteristics of the fatigue life are presented.     

Minimax optimal control for atmospheric fly-through trajectories

Necessary conditions for minimax problems with isolated or flat maxima are presented. Some relevant properties concerning the peak heating rate and the peak deceleration during atmospheric entry are discussed. As application of the theory, the problem of minimizing the peak heating rate of a skip trajectory is solved with special emphasis on the discussion of the continuity of the life control at the point where the maximum occurs along the trajectory.This research was supported by the Jet Propulsion Laboratory, Contract No. 916416, with Dr. K. D. Mease as project manager.     

地源热泵全寿命周期节能减排技术经济模型及应用

地源热泵是一种可再生清洁能源供暖技术,本文旨在综合分析其对于节能环保以及居民技术经济方面产生的影响,强调热泵技术的适用性,鼓励将该设备作为取代冬季居民锅炉取暖的技术之一。以北京市居民住宅投资使用地源热泵为背景,与锅炉取暖进行对比,构建地源热泵全寿命周期内的节能减排、技术经济效益模型,通过实际算例进行计算,分析二者之间的节能减排差异效果,综合考虑用户的技术经济效果。采用敏感性分析法,确定影响居民住宅地源热泵全寿命周期环境和技术经济效果的敏感性因素及其影响程度。最后提出推广热泵的合理化措施,并就北京地区地源热泵的推广效果进行了预测,分析其对北京“十二五”节能环保规划的贡献程度。     

地源热泵全寿命周期节能减排技术经济模型及应用

地源热泵是一种可再生清洁能源供暖技术,本文旨在综合分析其对于节能环保以及居民技术经济方面产生的影响,强调热泵技术的适用性,鼓励将该设备作为取代冬季居民锅炉取暖的技术之一。以北京市居民住宅投资使用地源热泵为背景,与锅炉取暖进行对比,构建地源热泵全寿命周期内的节能减排、技术经济效益模型,通过实际算例进行计算,分析二者之间的节能减排差异效果,综合考虑用户的技术经济效果。采用敏感性分析法,确定影响居民住宅地源热泵全寿命周期环境和技术经济效果的敏感性因素及其影响程度。最后提出推广热泵的合理化措施,并就北京地区地源热泵的推广效果进行了预测,分析其对北京“十二五”节能环保规划的贡献程度。     

Polymer fatigue from the standpoint of the kinetic theory of fracture

A comparison of the static and cyclic lives of various polymers shows that, whatever the loading regime, fracture may be regarded from the standpoint of the thermal-fluctuation theory. Under comparable test conditions the lifetimes in different loading regimes coincide. The experimentally observed cases of a reduced cyclic as compared with static life do not conflict with the kinetic theory of fracture and can be attributed to hysteresis heating effects or to differences in the structural changes that take place in the test material in different loading regimes.From the standpoint of the kinetic theory, fracture is regarded as a certain process that develops in a body under load and not as a critical event that occurs when a critical stress — ultimate strength — is reached.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 1, pp. 70–96, January–February, 1969.     

Growth of arterial cracks in polymers subject to static and fatigue loading

The growth kinetics of artificial and natural cracks (the former arising from notches) are studied in viscose, di- and triacetyl cellulose, nitrocellulose, polymethylmethacrylate, polystyrene, polypropylene, and carpon films by a micro-motion-picture method over a wide temperature range under both static and cyclic (fatigue) loading. In all the cellulose materials studied, the time required to form the nuclei of visible cracks is much shorter than the total life of the sample. The initial rate of crack growth depends exponentially on the applied stress and test temperature, so that the equation for the growth rate of an arterial crack is analogous to the general life equation. The different ways in which stress and test temperature affect the parameters of the equation describing the kinetics of crack growth enable us to distinguish the effect of local heating and that of the relaxation processes in the fatigue problem.A. F. Ioffe Physicotechnical Institute, Academy of Sciences of the USSR, Leningrad. Translated from Mekhanika Polimerov, No. 2, pp. 253–265, March–April, 1970.     

Taking account of heat transfer in locally heated thermoelastic plates made out of composite materials

Conclusions The proposed approach to the determination of the thermal stress state of thin plates made out of composite materials permits taking into account the effect of the coefficient of heat transfer from a region of local heating outside of it. The significant effect of heat transfer from the heating zone on the temperature field and the stresses indicates the necessity of taking it into account in connection with calculations on the strength of composite plates subjected to local heating.Translated from Mekhanika Kompozitnykh Materialov, No. 6, pp. 1027–1030, November–December, 1979.     

Thermal deformation of oriented irradiated polyethylene and ethylene-propylene copolymer under uniaxial tensile load

The deformation of preoriented irradiated polyethylene and ethylenepropylene copolymer has been investigated under conditions of multiple heating above the melting point of the crystals and subsequent cooling with application of a constant uniaxial tensile load. The specimens are found to shorten during heating and to lengthen during cooling. However, this process is not reversible and irreversible elongation of the specimen occurs during each temperature cycle (heating and cooling).Mekhanika Polimerov. Vol. 3, No. 3, pp. 392–394, 1967     

Modelling of ultrasound therapeutic heating and numerical study of the dynamics of the induced heat shock response

In this presentation we consider hyperthermia, a procedure of raising the temperature above 43 °C, as a treatment modality. To this purpose, a numerical model of in vivo soft tissue ultrasound heating is proposed by extending a previously presented in vitro model. Based on the numerical simulations, a heating scheme satisfying some constraints related to potential clinical applications is established, and the resulting temperature time-course profile is composed with the temperature-dependent protein denaturation formula of a recently published mathematical model for the eukaryotic heat shock response. The obtained simulation results of the combined models are discussed in view of potential application of ultrasound soft tissue heating in clinical treatment.     

Optimal laser heating of plasmas with constant density

The laser heating of a plasma with constant density is analyzed using optimal control theory. Heating strategies that minimize the total energy spent, the heating time, or a linear combination of the two, for several values of weighting coefficients, are obtained by determining the optimal laser intensity associated with each point of the phase plane. A numerical example is used to illustrate the application of the theory. In this particular example, savings in the energy spent up to 75%, compared with the energy required using a constant laser pulse, are obtained when minimum energy trajectories are implemented. Strategies that minimize the heating time, however, did not yield a significant reduction in the heating time. Numerical results may depend strongly on the initial state of the system as well as on the final ion temperature of the plasma.Based on a thesis submitted as partial fulfillment for a Ph.D. degree at the University of Michigan. Financial support, provided to the first author by Consejo Nacional de Ciencia y Tecnología, Universidad Nacional Autónoma de México, by the National Science Foundation, and by the Michigan Memorial Phoenix Project, is deeply appreciated.     

Thermomechanical behavior of liquid-crystal polyesters

We have used the thermomechanical method to investigate the processes of self-elongation and shrinkage in uniaxially oriented liquid-crystal (LC) polyesters (CPET — the copolyester of terephthalic acid, phenylhydroquinone, and p-hydroxybenzoic acid; Ultrax — the copolyester of terephthalic acid, isophthalic acid, and bisphenol) and the injection-molded copolyester of hydroxybenzoic and hydroxynaphthoic acids Vectra A900. A universal tensile tester and a thermomechanical tester were used for the investigations. We studied the deformation behavior for the first and second heating under negligible applied stress. We prepared CPET samples in the form of as-spun and annealed fibers, Vectra samples in the form of cubes cut from the nonoriented part of the molded piece (VI) and from the longitudinally oriented central part (VII), Ultrax samples in the form of strands (noncrystalline UI and crystalline UII). We observed a permanent length increase for the as-spun CPET (Fig. 1) and shrinkage in the glass-transition region for the annealed CPET (Fig. 2) for the first and second heating. Note that the as-spun CPET is a smectic polymer, and the annealed CPET contains smectic and crystalline phases. The thermomechanical curves of VI seem to be typical of any molded polymer (Fig. 3). In contrast, for the oriented VII we see spontaneous elongation upon heating in the direction of orientation, with pronounced transverse shrinkage (Fig. 4). The length increase for the noncrystalline UI is quite considerable for the first heating. We observed both spontaneous elongation below the glass-transition temperature and subsequent shrinkage for crystalline UII for the first heating. All the Ultrax curves are much smoother for the second heating (Fig. 5). Spontaneous elongation upon heating appears to be a characteristic feature of oriented LC polymers, the polymer being either fully oriented or oriented within some part of the sample. Shrinkage of LC polyesters upon heating is also possible, as a result of both processing factors and crystallinity. There are two types of restructurization processes: orientation and crystallization (possible for LC polymers capable of forming a crystalline phase).A. V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, Moscow, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 32, No. 1, pp. 98–105, January–February, 1996.     

The elastoplastic stressed state of a half-plane under local nonstationary heating

We solve the thermoplastic problem for a semi-infinite plate under local nonstationary heating by heat sources. The physical equations are taken to be the relations of the nonisothermic theory of plastic flow associated with the Mises fluidity condition. The solution of the problem is constructed by the method of integral equations and the self-correcting method of sequential loading, where time is taken as the loading parameter. We carry out numerical computations of the stresses in the case of heating a plate with heat output by normal-circular heat sources. We study the problem of optimization of heating regimes in order to introduce favorable residual compressive stresses (from the point of view of hardness) in a given region of a half-plane. Two figures.Translated fromMatematicheskie Metody i Fiziko-Mekhanicheskie Polya, Issue 27, 1988, pp. 29–34.     

Sufficient conditions for time optimal heating of rigid bodies by internal heat sources

We derive sufficient conditions for the time optimality of the control of heating of rigid bodies by internal heat sources under constraints on phase coordinates. We numerically solve the optimal-control problem in the case of heating of an unbounded plats with constraints on temperature and thermoelastic stresses.Translated from Matematicheskie Metody i Fiziko-Mekhanicheskie Polya, No. 25, pp. 56–60, 1987.     

Optimization of the heating of a heat-sensitive layer under constraints on the stresses in the plastic region of deformation of the material

We study the problem of optimal control for rapidity of the heating of a heat-sensitive layer under constraints on the control (the temperature of the heating medium or the heat flux) and maximal values of the stress intensity in the plastic region of deformation of the material. We propose an algorithm for solving the problem that presumes it has been reduced to the inverse problem of thermoplasticity. For the case of one-sided heating we give a numerical analysis of the direct and inverse problems of thermoplasticity. Translated fromMatematichni Metody i Fiziko-Mekhanichni Polya, Vol. 38, 1995.     

侧向局部加热对流的周期性

通过流体力学方程组的数值模拟,研究了侧向局部加热条件下Prandtl数Pr=0.0272时流体对流的周期性.结果表明:随着Grashof数Gr的增加,对流按稳态对流、单局部周期对流、双局部周期对流、准周期对流的顺序发展.当Gr<3.6×103时,对流为稳态;在3.6×103相似文献   

Stress-optimal local heating during welding of unlike plates with variable thicknesses

A method is proposed and is used to solve the problem of determining the stress-optimal temperature fields of the local heating during welding of unlike plates with variable thicknesses. Computational modelling was carried out for plates of M-40 material and St. 3 steel, with thickened edges, depending on the width of the heating zone and the beginning of the region of elastic deformation.Translated from Matematicheskie Metody i Fiziko-Mekhanicheskie Polya, No. 25, pp. 53–56, 1987.     

Electromagnetic and thermal modelling of axisymmetric induction furnaces

A numerical method to describe the thermo-electric behavior of an induction heating furnace is introduced. It is obtained by using an enthalpy formulation concerning the thermal model and an integral representation of the electromagnetic potential in an unbounded domain. A BEM-FEM method is used and an iterative algorithm together with numerical results for an industrial heating system are presented. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Modeling the heating of biological tissue based on the hyperbolic heat transfer equation

In modern surgery, a multitude of minimally intrusive operational techniques are used which are based on the point heating of target zones of human tissue via laser or radiofrequency currents. Traditionally, these processes are modeled by the bioheat equation introduced by Pennes, who considers Fourier’s theory of heat conduction. We present an alternative and more realistic model established using the hyperbolic equation of heat transfer. To demonstrate some features and advantages of our proposed method, we apply the results obtained to different types of tissue heating with high energy fluxes, in particular radiofrequency heating and pulsed laser treatment of the cornea to correct refractive errors. We hope that the results from our approach will help with refining surgical interventions in this novel field of medical treatment.     

Stability of axially compressed cylindrical composite shells in the presence of nonstationary heating

The problem of smooth cylindrical composite shells uniformly compressed in the axial direction and subjected to nonstationary heating is solved in the linear quasi-static formulation. Expressions are obtained for the critical loads and their regions of application are determined. The calculations are compared with experimental data obtained by linearly heating the outer surface of axially compressed smooth cylindrical shells of glass-reinforced plastic based on phenol-formaldehyde resin.Zhukovskii Central Aerohydrodynamic Institute, Moscow Region. Translated from Mekhanika Polimerov, No. 2, pp. 289–297, March–April, 1973.