高温结构陶瓷的高温蠕变

高温蠕变是高温结构陶瓷材料一项非常重要的性能指标。现代耐高温结构材料要求有很高的抗蠕变性能。本文从理论上描述了蠕变及其机理，分析了影响高温结构陶瓷蠕变的因素，并结合实际提出了提高高温结构陶瓷抗蠕变性的措施。     

焊接结构高温强度设计的研究进展

综述了高温下焊接结构强度及寿命评价研究的最新进展，介绍了以结构蠕变破断方程为基础确定焊缝强度减弱系数的方法。还给出了一系列焊接接头的蠕变强度减弱系数及寿命减弱系数。     

一种压力容器常用钢疲劳及疲劳蠕变寿命预测方法

基于应变能密度理论,并以半寿命等效应变能密度作为控制参量,提出一种新的应力控制寿命预测方法。该方法不仅考虑了疲劳诱导的累积塑性损伤,同时考虑了循环蠕变和静蠕变诱导的延性耗竭损伤。通过三种低合金钢室温及高温、连续疲劳及带保载时间的疲劳蠕变试验验证,预测效果都较好,寿命预测精度基本在±2倍比例因子以内,且能定量反映疲劳、循环蠕变和静蠕变对材料损伤的贡献,此方法适用于应力控制模式不同条件下的疲劳及疲劳蠕变交互作用寿命预测。     

基于有限元计算方法的制氢转化炉管蠕变寿命分析

以制氢转化炉为研究对象,通过炉管材料高温蠕变实验,确定了蠕变方程中相关系数。采用有限元计算方法,建立了制氢转化炉整体数值计算模型。考虑高温蠕变效应,对其服役6 a后各管系的蠕变应力进行了计算与强度评定。结合拉森-米勒尔曲线,通过对其不同操作周期寿命分数的计算,最终得到了该转化炉剩余寿命为6.24 a。     

高温构件的概率寿命预测与可靠性

通过应用高温构件蠕变损伤的随机方程及一次二阶矩理论和方法 ,对高温构件的概率寿命预测进行了研究。还应用高温构件寿命的概率计算方法 ,考虑服役高温构件的实际运行工况 ,得到了高温构件寿命与可靠性的关系。由于充分考虑了各参数的随机性和相关性 ,因而大大提高了计算的准确性     

基于Omega方法的蠕变强度评定方法

现有压力容器标准对蠕变工况下工作的设备提供了按单向等应力实验数据和安全系数为1.5确定许用应力进行设计的方法,但针对工程中遇到的多种蠕变工况无法用该方法进行处理,如温度、压力变化的工况,蠕变温度下使用时间超过105 h的设备,在役设备的蠕变损伤和剩余寿命的估计,等等。Omega方法认为可将材料的蠕变应变速率作为材料蠕变损伤的表征,其与材料蠕变损伤的关系为指数关系,从而可得到用以估计材料蠕变损伤、蠕变寿命的计算式。分析了该数学模型,将Omega方法与现有标准提供的方法所得到的结果进行了比较,说明了两种方法在工程应用中均能确保安全的理由以及各个方法的推荐应用场合。     

Y—TZP陶瓷的蠕变活化能

本文研究了一种氧化钇稳定四方氧化锆多晶体陶瓷的高温蠕变活有以及应变速率、晶粒大小和致密度等因素对其蠕变活化能的影响，讨论了材料的形变机理。     

钛化物陶瓷的高温蠕变行为与失效机理

分析了原位生成含钛复相陶瓷TiB2-TiC(TT），TiB2-TiCxN1-x(TTT）及TiB2-TiC-SiC（TTS）的高温蠕变性能和高温氧化行为，考察在恒定载荷不同温度下，材料挠度随时间的变化，探讨了其高温失效机理，并给出了材料抗三点弯曲高温蠕变方程-挠度/时间方程，结果表明，含钛复相陶瓷TT，TTT及TTS在1100℃时均出现较大的蠕变，因此，上述3种材料不宜在1100℃及以上的温度下使用，指出失效的原因之一是TiB2陶瓷在800℃时的高温氧化及1100℃时的高温热分解。     

氧化铝、氮化硅和碳化硅的疲劳特性与寿命预测

分析了几种典型的结构陶瓷在长期载荷下的失效特性和差别。认为长期失效的本质是强度衰减,建立了一个疲劳失效的强度衰减模型和提出了寿命预测方法。分别研究了氧化铝、氮化硅和碳化硅几种常用工程陶瓷在常温和高温下的疲劳特性和差异。采用三点弯曲的受力方式测试了不同载荷水平下的断裂时间。结果表明：碳化硅的疲劳门榄值超过强度的80％,而且受温度影响最小;氧化铝的静疲劳受微小裂纹扩展控制;氮化硅的高温疲劳主要是蠕变机制导致强度衰减,疲劳门槛值不超过强度50％。由实验研究了氮化硅的高温静疲劳、动疲劳和循环疲劳三者在相同温度和相同应力峰值变化下的寿命关系,结果与计算一致。     

高温下焊接结构的强度设计及寿命评价的研究进展

综述了高温下焊接结构强度及寿命评价研究的最新进展，着重介绍了以结构蠕变破断方程为基础确定焊缝强度减弱系数的方法，探讨了如何计及多轴应力因素的影响和由于焊缝的引入结构寿命的减少等问题．     

Tensile Creep Behavior of a Gas-Pressure-Sintered Silicon Nitride Containing Silicon Carbide

The tensile creep behavior of a gas-pressure-sintered silicon nitride containing silicon carbide was characterized at temperatures between 1375° and 1450°C with applied stresses between 50 and 250 MPa. Individual specimens were tested at fixed temperatures and applied loads. Each specimen was pin-loaded within the hot zone of a split-tube furnace through silicon carbide rods connected outside the furnace to a pneumatic cylinder. The gauge length was measured by laser extensometry, using gauge markers attached to the specimen. Secondary creep rates ranged from 0.54 to 270 Gs⁻¹, and the creep tests lasted from 6.7 to 1005 h. Exponential functions of stress and temperature were fitted to represent the secondary creep rate and the creep lifetime. This material was found to be more creep resistant than two other silicon nitride ceramics that had been characterized earlier by the same method of measurement as viable candidates for high-temperature service.     

Application of Hertzian Tests to Measure Stress–Strain Characteristics of Ceramics at Elevated Temperatures

A new method for evaluating the elastic–plastic properties of ceramics from room temperature up to the onset of creep based on Hertzian indentation testing is proposed. Indentation stress–strain curves are compiled for representative alumina and zirconia ceramics at prescribed temperatures. Deconvolution of the indentation stress–strain curves for each material provides a measure of Young's modulus, yield stress, and work-hardening coefficient as a function of temperature, enabling construction of true stress–strain curves. The stress–strain curves flatten out with increasing temperature, in accordance with an expected increased plastic response at elevated temperatures.     

Estimating tensile creep rate of ceramics from flexure data

A new model of ceramic creep in four-point bending is proposed to determine the creep rate that corresponds to tensile creep at an elevated temperature. Based on the assumption that ceramics creep only in tension and there is no creep in compression, the tensile creep rate which is invariant with time in the secondary mode is calculated in a simple way. Since the initially applied maximum tensile stress does not correspond to the stress at the secondary creep range, the creep-induced stress at the time of measurement is calculated based on beam deflection. Then, the calculated tensile creep rates from fourpoint bending data are compared with observed tensile creep rates for both an alumina ceramic at 1000 °C and a silicon nitride ceramic at 1200 °C. This study shows the usefulness of flexural creep tests not only to verify the accuracy of tensile creep tests, but also to obtain the tensile creep data in a less expensive and easier way.     

Creep and recrystallization of sintered scandium oxide at high temperatures

Conclusions A study was made of the creep and recrystallization of sintered scandium oxide at elevated temperatures. We calculated the effective diffusion coefficient on the basis of creep data for scandium oxide at 1700–1900°C. The low creep rate and recrystallization of scandium oxide compared with aluminum, magnesium, and zirconium oxides and in combination with other positive properties studied previously permits us to consider ceramics based on scandium oxide as valuable construction material suitable for prolonged operation in equipment operating at elevated temperatures.Translated from Ogneupory, No. 5, pp. 40–43, May, 1973.     

Development of a creep‐free stress‐strain law for fire analysis of steel structures

This paper presents a practical procedure for obtaining creep‐free stress‐strain laws for steel exposed to fire, on the basis of codified stress‐strain laws that consider creep implicitly. The applicability of the proposed procedure has been tested on two commonly used stress‐strain laws for steel at elevated temperature, the Eurocode 3 law and a Ramberg–Osgood model, both of which have implicit consideration of creep. The simulation of two published steel coupon experiments on steel of grades S275 and S355 shows that both the Eurocode and Ramberg–Osgood stress‐strain laws produce inaccurate predictions of creep in fire at elevated temperatures. The proposed procedure was thereby used to extract the implicit creep according to the heating rates of the transient coupon tests and to derive the creep‐free stress‐strain laws. It has been shown that, by combining the creep‐free stress strain law obtained by the proposed methodology with an explicit creep model, a more realistic prediction of steel behaviour in the selected coupon test studies can be achieved. Copyright © 2015 John Wiley & Sons, Ltd.     

Study of some properties of quartz ceramics in a wide temperature range

Conclusions The strength of quartz ceramics determined during short-term loading gradually increases with temperature rise to 1100°C. Differences in the short-term strength connected with porosity at elevated temperatures are preserved.The creep of quartz ceramics obeys an empirical rule for the creep in the range 1100–1300°C. At higher temperatures the process sharply deviates from the normal rule in connection with the cristobalite formation in the material. The rate of deformation of the porous materials compared with dense materials increases by an order of magnitude.The gas permeability of the quartz ceramic with an apparent porosity of up to 8% is very slight. However, the specimens are not vacuum tight.Marked volatilization of quartz ceramics occurs in vacuum only at temperatures above 1200–1300°C.The electric conductivity of quartz ceramics is the same as that of fused quartz, and hardly changes with density changes in the porosity range 1–10%.Translated from Ogneupory, No. 4, pp. 45–51, April, 1971.     

Accelerated characterization of a chopped fiber composite using a strain energy failure criterion

The creep and creep rupture response of a chopped fiber composite material (SMC-R50) were investigated experimentally and analytically. The goal of this research was to use the short time laboratory data to predict long time creep and creep rupture behavior. The creep response data up to 200 min duration were obtained at various constant temperature and stress levels. The short time creep data were then modeled using a modified power law equation. The modified power law equation contains the parameters of the so-called accelerated characterization procedure. Using this power law equation, the short time creep response at the elevated temperatures were able to successfully predict the long time creep response at a lower temperature and stress level. To predict the creep rupture behavior, the modified power law equation was then coupled with a strain energy based failure criterion. It was found that the same parameters that were used in the prediction of the long-time creep response can also be used to predict the creep rupture. At a given temperature level, the strain energy density related to creep rupture was found to be a constant. Furthermore, this strain energy density was found to increase with an increase in temperature. With a limited amount of data, it was found that the strain energy based failure criterion coupled with the modified power law equation can be used to predict long time creep rupture behavior.     

基于十字交叉法的陶瓷胶粘剂剪切蠕变性能研究

本研究设计了“十字交叉法”陶瓷胶粘剂剪切蠕变试验装置,选取刚性环氧树脂及柔性硅酮结构胶进行剪切蠕变试验,研究了环境温度、剪切应力、粘结面积等因素对胶粘剂剪切蠕变的影响,通过模型拟合对胶粘剂的剪切蠕变行为进行了分析和预测,探究了两种胶粘剂的蠕变破坏模式。结果表明:采用十字交叉法能够准确便捷地测试陶瓷胶粘剂的蠕变性能。增大胶粘层柔性、提高环境温度、增大剪切应力都会加速蠕变的发展,但粘结面积对蠕变速率无明显影响。刚性环氧树脂胶粘剂试样的蠕变失效形式为粘结层内聚破坏及界面脱粘,符合时间硬化模型;柔性硅酮结构胶试样失效形式为粘结层内聚破坏,符合Burgers模型。     

High-Temperature Behavior of Indent and Creep-Nucleated Cracks in Vitreous-Bonded Alumina

Crack behavior was studied at elevated temperatures in a commercial vitreous-bonded alumina for two types of cracks: one introduced by indentation at room temperature and the other by the creep process. Indentation cracks with relatively small initial size grew progressively longer during creep before they became blunt and arrested; however, they continued to widen throughout the creep process. Larger indentation cracks under high stress condition continued to grow until failure. The evolution of creep-nucleated cracks was so fast that they were observed only in their arrested state. Once observed, their length remained essentially constant, but they did grow in width. The crack-opening displacement rates of both types of cracks were linearly related to the creep rate as predicted by fracture mechanics for stationary cracks. All but the specimens with the largest indentation crack exhibited flaw tolerance in that they failed by the coalescence of creep-nucleated cracks instead of the growth of a single crack. The results illustrate the crack behavior in the brittle-to-ductile transition regime for ceramics that deform by grain boundary sliding.