Model of thermal destruction of material subjected to one-sided heating

The article presents a model of destruction establishing a correlation between the temperature field and the rate of destruction of the material.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 52, No. 1, pp. 33–37, January, 1987.     

Calculation of the state of thermal stress and of endurance of a plate subjected to cyclic surface heating

The article explains the results of the calculation of the state of thermal stress of a plate which is heated on one side and cooled on the other. An attempt is made to estimate its endurance with the use of the well-known Coffin-Manson relations for different years.Translated from Problemy Prochnosti, No. 7, pp. 119–124, July, 1990.     

Probes to monitor in-plant material degradation

Abstract

Fireside corrosion in coal fired boilers has been well-investigated. The main causes of water wall fireside corrosion are: (1) impurities in the fuel, such as sulphur alkali metals and chlorine; (2) the lack of control of the combustion process resulting in a reducing gaseous environment at the tube surface; (3) flame impingement; and (4) overtemperature of tube metal.

Co-firing secondary fuels in coal fired boilers is becoming common practice in many power stations in Europe. Secondary fuels like wood, refuse derived fuels, meat and bone meal, straw, poultry litter or mixtures of several secondary fuels are co-fired up to 20-wt%.

Most of these biomass fuels contain high concentrations of alkali chlorides. Considering the composition of these fuels, limitations on the maximum amount of secondary fuels to be co-fired in coal fired boilers are expected.

In addition to the environmental benefits from biomass fired power plants, co-firing can result in “green” power labelling and governmental subsidy. Also savings on fuel costs may be a driving force for an increase of the amount of biomass or secondary fuels to be co-fired.

However, without corrosion monitoring, short-term policies concerning co-firing secondary fuels in large volumes can lead to high costs in the medium or long term. These costs can be due to corrosion damage both in the furnace and superheater sections and penalties due to unplanned outages in a highly competitive electricity market.

This paper summarizes practical experiences from corrosion monitoring programs with KEMA corrosion probes. The first prototype was successfully tested in 1997 at the Hemweg Unit 8 coal fired power plant of Reliant Energy in Amsterdam, the Netherlands. Other corrosion monitoring programs were carried out at coal fired power plants and at a waste incineration plant.

At present a large-scale corrosion monitoring and material testing program is in progress at the Maasvlakte power station Unit 1 near Rotterdam, the Netherlands. In this 520 MWe power plant of E.on Benelux more than 10-wt% of mixtures of secondary fuels are directly co-fired.

In addition to aspects such as emissions, fuel handling and fuel cost savings, co-firing secondary fuels requires corrosion monitoring to check the tolerance to different fuel types of coal fired boilers.     

Dynamic material properties of wood subjected to low-velocity impact

This article studied the effects of low-velocity impact on the failure stresses and stiffness using a pendulum test. The specimens were of variable depth (20, 30, and 40 mm), a width of 50 mm, length of 650 mm, and span-length of 480 mm. The smallest specimen depth was similar to specimen sizes tested in the literature used to create the duration of load curve, while the largest specimen depth are considered structural size specimens. The impact was predicted using a numerical approach with Euler–Bernoulli beam, as well as Timoshenko beam theory, with a plastic contact law. The models were validated for impact from a low release-angle (where the beam remained elastic), but could use improvement for the force prediction at a high incidence velocity. The measured force signals were used as forcing functions to obtain the dynamic failure stresses for all of the evaluated specimens, and the Timoshenko–Goens–Hearmon Method to derive the dynamic E. The resulting strain rates ranged from 9.11?×?10^?5 s^?1 for the quasi-static specimens up to 25 s^?1 for the greatest incidence velocity. The results from this study suggest different duration of load factors than the Madison Curve, influencing the design of structures subjected to dynamic loading.     

Self-oscillatory system subjected to an external thermal agent

The conditions for the onset of oscillations in a system are analyzed for the case in which the only external agent is a thermal agent. The stability boundary and the dependence of the oscillation frequency on the parameters of the system are evaluated. An experimental apparatus is described, and the results found in experiments with moist water vapor are described.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 28, No. 1, pp. 96–102, January, 1975.     

Fly ash concrete subjected to thermal cyclic loads

The present study describes the behaviour of concrete as well as fly ash concrete when subjected to varying number of high temperature heating cycles. A Concrete mix (1:2.37:2.98) with 340 kg/m³ cement and w/cm ratio 0.45 was prepared. Cement was replaced by varying percentages (0%, 20%, 40%, 50% and 60%) of fly ash by weight of cement. The concrete was subjected to a constant temperature of 200°C for 7, 14, 21 and 28 heating cycles. One heating cycle corresponds to 8 h heating and subsequent cooling in 24 h. Subsequently the effect of temperature on the properties of the concrete was investigated and compared with that of the properties of unheated concrete. The compressive strength of plain as well as fly ash concrete increased when it was subjected to thermal cyclic loads. Moreover, the compressive strength increased with an increase in number of heating cycles. Thermal conductivity of concrete was found to decrease with an increase in the fly ash content.     

Resistance to crack propagation in ceramics subjected to thermal shock

It is shown that the resistance to crack propagation in alumina, zirconia and porcelain, when subjected to thermal shock, is high when the initial strength is low and, vice versa when the initial strength is high. It is proposed that the high resistance to crack propagation results from discrete microcracking in the highly stressed zone ahead of the major crack, whereas the low resistance results from the unrestrained propagation of major cracks. Evidences of the crack patterns in alumina and zirconia substantiate this hypothesis.     

Applications of nonlinear ultrasonics to the NDE of material degradation

Nonlinear ultrasonics is suggested as a new approach for the effective evaluation of material degradation. As its quantification, the parameter beta is introduced on the basis of nonlinear elasticity, and a new method to measure the parameter beta using bispectral analysis is proposed. Then, the correlation between beta and material degradation is investigated. From the results for several mild steel (SS41, SS45) specimens that were degraded by stretching and cyclic loads, it was confirmed that the parameter beta has a strong correlation with material degradation. As another practical application, the evaluation of the aging degradation in a high temperature material is tried. For this, Cr-Mo-V specimens that are generally used in turbine rotors in power plants were prepared, and the variation of beta caused by aging time was investigated. For comparison, the fracture appearance transition temperature (FATT) of the specimen was measured, and its behavior showed good agreement with beta. In addition, for all of the experiments, no noticeable change in attenuation and sound velocity in the same specimens with change of degradation were observed. From these results, it may be concluded that nonlinear ultrasonics could be applied to the quantitative evaluation of material degradation.     

Study of the stress state of ribbed shells subjected to local loading

The methods of photoelasticity and extensometry are used to study the stress state of cylindrical shells provided with stringers and rings. The results of the study are compared with theoretical data. It is found that the experimental and theoretical results agree satisfactorily.Translated from Problemy Prochnosti, No. 2, pp. 71–74, February, 1990.     

热冲击作用下的陶瓷材料破裂过程数值分析

运用热传导和热2力耦合的相关理论 , 借助统计分布来考虑陶瓷中存在的微孔洞和微裂隙 ; 建立了一种可以模拟陶瓷遭受热冲击作用下的裂纹萌生、 扩展过程的数值模拟方法 , 并通过材料破坏过程分析系统 (RF2PA , Realistic Failure Process Analysis) 加以实施。该数值方法基于细观非均匀性假设 , 突破了以往连续介质力学视陶瓷为均匀介质的假设 , 并从细观损伤角度考虑陶瓷热冲击破坏演化的过程。运用该方法对三面绝热、 一面受热冲击的平板状陶瓷材料的破裂过程进行了数值试验。结果表明 : 起始裂纹发端于受热冲击表面 , 且在初始的裂纹萌生阶段 , 在受热冲击表面产生一系列无序的裂纹 ; 但随着时间的延续 , 裂纹逐渐演变成多条近乎平行的、沿受冲击表面内法向方向扩展的主裂纹 , 其中一些裂纹的发展受到了屏蔽 , 这一结果与试验结果吻合较好。本数值方法为相关研究提供了新的思路。      

Thermal stress analysis of cordierite materials subjected to thermal shock

Cordierite disks were obtained by slip casting and sintering at 1,450 °C for 2 h. Sintered disks were indented at the center of one flat surface. The fracture strength of indented disks was measured in biaxial flexure. The critical temperature differential was determined by sudden cooling of the center of the indented surface at high temperature, using a high-velocity air jet at room temperature. The initial temperature was successively increased by 10 °C until crack propagation was detected with the naked eye. Temperature and stress distributions during air impinging were calculated by a finite element analysis. The heat transfer coefficient was estimated by fitting experimental temperature profiles. A radial variation of the coefficient that considers its increment beyond the stagnation region and a diminution toward the disk periphery was assumed. The calculated cordierite thermal stresses were analysed in relation to experimental data. Also, the comparison with alumina tested in similar conditions was included.