Thermal shock of a cracked strip: Effect of temperature-dependent material properties

The problem of a strip containing an edge crack and an interior crack subjected to a thermal shock on one edge and insulated on the other is solved in order to analyze the difference between using temperature-dependent material properties and constant ones. For this purpose two brittle materials, ceramics and glass, are each subjected to a thermal shock. The results show that in general, using constant material properties over large temperature ranges can lead to considerable underestimation of the maximum stress intensity factors. The difference in the results is dependent on the variation of the thermal diffusivity and the thermal expansion coefficient with temperature for a given material. Also, this difference varies for different crack lengths and different thermal shock conditions at the boundary.     

Investigation of cyclic thermal shock behaviour of fibre reinforced glass matrix composites using non-destructive forced resonance technique

Abstract

A non-destructive forced resonance technique was used to assess the damage development in SiC fibre reinforced glass matrix composite materials subjected to cyclic thermal shock. Both elastic modulus and internal friction measurements were conducted. The thermal shock tests involved quenching the specimens from high temperatures (590–710°C) to room temperature in a water bath. Damage in theform of matrix microcracks was induced by quenchingfrom 620 and 660°C, and the extent of damage increased with the number of thermal shock cycles. After a certain number of shocks, this damage was detected by a decrease in the Youngs modulus and a simultaneous increase in the internal friction. The non-destructive dynamic forced mechanical resonance technique employed was shown to be more sensitive than a destructive three point flexural technique for detecting crack development in the early stages of thermal shock damage. The technique was also used to confirm the occurrence of a crack healing process in the thermally shocked specimens: after an annealing heat treatment for 12 h at 550°C, the initial values of Young's modulus and internal friction were recovered. This was attributed to crack closure due to viscous flow of the glass matrix.     

基于第二抗热震因子的BNNTs/Si3N4复合材料抗热震性能评价

利用Kingery抗热震断裂理论构建了氮化硼纳米管（BNNTs）强韧化陶瓷复合材料的第二抗热震因子模型,通过真空热压烧结法制备了BNNTs质量分数分别为0.5wt%、1.0wt%、1.5wt%和2.0wt%的BNNTs/Si₃N₄复合材料,并采用预制裂纹法测试了复合材料的抗热震性能,测试结果证实了在平稳状态下模型的正确性。结果表明,BNNTs的存在使复合材料第二抗热震因子增大,抗热震性能提升。分布在晶界上的BNNTs起到裂纹钉扎、桥联和裂纹偏转作用,增加了裂纹扩展的阻力,从而有效提高了BNNTs/Si₃N₄复合材料抗热震断裂能力。     

Erhöhung der Thermoschockbeständigkeit von Sinterglas und Keramik über das Verbundwerkstoffkonzept

Increasing the Thermal Shock Resistance of Sintered Glass and Ceramics by the Composite Materials Concept The thermal shock resistance of brittle materials such as glass and ceramics is one of their weaknesses. Pores and above all incorporated second phases in these materials alter these properties which are decisive for thermal shock behavior, and may therefore increase this behavior in a precalculable manner. The present paper will first theoretically demonstrate when and why porosity leads to an improvement in thermal shock resistance. The thermal shock resistance for porous borosilicate sintered glass and porous eutectic calcium titanate ceramic are calculated and compared to experimental values. They confirm

• that low porosities lead to an improvement in thermal shock resistance
• that the thermal shock resistance has a maximum at a certain porosity and
• that above certain porosities the presence of pores deteriorates the thermal shock resistance.

If one considers porous materials as a special case of composite materials then relations valid for composite materials can be transferred to porous materials (“composite material concept”) and viceversa. This is investigated using the examples of borosilicate sintered glass with incorporated antimony particles and eutectic calcium titanate ceramic with incorporated paladium particles. In the case of the glass-antimony composite material, improvements in thermal shock resistance of about 15% with 10 vol% antimony incorporation were calculated and confirmed experimentally, while for calcium titanate-paladium composite materials a 15% improvement in thermal shock resistance was already achieved with about 5 vol% of the metallic phase.     

A probabilistic approach for thermal shock fatigue life of glass

This paper presents a probabilistic approach for predicting fatigue life of glass subjected to near‐ΔT_C (critical temperature difference) thermal shock which exhibits little subcritical crack extension. First, thermal shock fatigue life N_f was derived as a function of temperature difference ΔT, fracture probability F and Biot's modulus β from the slow crack growth concept in conjunction with the Weibull distribution model. Next, thermal shock fatigue tests as well as flexural tests were performed for borosilicate glass to measure ΔT_C and N_f versus ΔT. The parameters associated with slow crack growth were then determined from the experimental results while the heat transfer coefficient h or β was obtained with the aid of finite element analysis. Thirdly, the thermal shock fatigue diagram (ΔT?N_f curves) was depicted for various values of β. Finally, crack length was simulated on the basis of the present approach.     

Statistical study of the effect of subcritical crack growth on thermal shock resistance

An experimental study of the effects of subcritical crack growth on thermal shock damage is presented, based on a statistical analysis of the retained strength distribution. Single-quench thermal shock and thermal shock fatigue tests were performed in a room-temperature distilled water bath on glass microscope slides. Experimental results indicate that subcritical crack growth effects are observable in the shock testing of glass slides in terms of systematic shifts in the retained strength distribution.     

Virtual crack extension methods for non-linear materials

The virtual crack extension technique is a very efficient and accurate approach to fracture mechanics calculations in the numerical analysis of bodies containing cracks using the finite element method. A few variations of the technique have been described in the literature, and have been extensively used in linear elastic fracture mechanics, where good validation has frequently been available with accepted alternative solutions for standard tests. However, for non-linear materials extra complications arise in the technique, particularly in describing material response in a compatible manner. It is shown that, using few assumptions, a very competitive virtual crack extension technique based, on a direct minimization of potential energy is available for elastic non-linear elastic materials. Such materials can be closely approximated to elastic–plastic behaviour for monotonically increasing loads including mechanical, thermal and body force forms. The technique is described and demonstrated via examples to be in good agreement with alternative fracture parameter evaluations when evaluated in the same computer system, BERSAFE.     

Fracture Mechanics Analysis Model for Functionally Graded Materials with Arbitrarily Distributed Properties

Functionally Graded Materials (FGMs) have been developed as super-resistant materials for propulsion systems and airframe of space-planes in order to decrease thermal stresses and to increase the effect of protection from heat. It has been experimentally observed that surface cracking in FGMs is the most common failure mode of a metal-ceramic FGM when it is subjected to a thermal shock. Therefore, it is very important to consider the thermally induced fracture behaviors of FGMs. In this paper, a functionally graded material strip containing an embedded or a surface crack perpendicular to its boundaries is considered. The graded region is treated as a large number of single layers, with each layer being homogeneous material. The problem is reduced to an integral equation and is solved numerically. Unlike most of the existing researches, which considered only certain assumed material distributions, the method developed in this paper can be used to investigate functionally graded materials with arbitrarily varied material properties.     

Theoretical and experimental considerations on the thermal shock resistance of sintered glasses and ceramics using modelled microstructure-property correlations

The thermal shock resistance of brittle materials such as glass and ceramics is one of their weaknesses. Pores and other incorporated second phases in these materials alter these properties which are decisive for thermal shock behaviour, and may therefore increase this behaviour in a precalculable manner. It has been theoretically demonstrated when and why porosity leads to an improvement in thermal shock resistance. The thermal shock resistance for porous borosilicate sintered glass and porous eutectic calcium titanate ceramic have been calculated and compared to experimental values. The results confirm that low porosities lead to an improvement in thermal shock resistance, that the thermal shock resistance has a maximum at a certain porosity, and that above certain porosities, the presence of pores deteriorates the thermal shock resistance. If porous materials are considered as a special case of composite materials, then relations valid for porous materials can be transferred to composite materials and vice versa (composite concept). This has been investigated using the examples of borosilicate sintered glass with incorporated antimony particles and eutectic calcium titanate ceramic with incorporated paladium particles. In the case of the glass-antimony composite material, improvements in thermal shock resistance of about 15% with 10 vol % antimony incorporation, were calculated and confirmed experimentally, while for calcium titanate-palladium composite materials, a 15% improvement in thermal shock resistance was already achieved with about 5 vol % metallic phase.Deceased.     

Controlled Crack Propagation Experiments with a Novel Alumina‐Based Refractory

Innovative, carbon‐reduced and carbon‐free refractory materials are currently under development within the framework of the DFG priority program “FIRE”. Among various novel material solutions an alumina‐based refractory with titania and zirconia additives (AZT) has gained special interest for application in high temperature processes under thermal shock conditions. The resistance against fracture of the AZT material and, for comparison, of a pure alumina refractory was examined by controlled crack propagation experiments. Wedge splitting and compact tension tests with in situ crack growth observation, partially on microstructural level, have been performed for both materials. Based on the measured room temperature values of dissipated energy, refractory stiffness and fracture stress, the Hasselman thermal shock parameter R″″ was determined. The results allow to predict that AZT is less prone to scale thermal shock damage than pure alumina. The microstructural observations reveal that growth and opening displacement of the main crack is accompanied in AZT by pronounced microcracking, branching and bridging processes. First efforts are also directed towards a mechanical quantification of this fracture behavior in terms of an R‐curve representation (fracture resistance as a function of apparent crack length). The specific problems of R‐curve evaluation that exist in AZT due to nonlinear deformation behavior are addressed and the influence of the observed crack growth mechanisms is discussed.     

压痕-急冷法测定增韧氧化铝基陶瓷复合材料的抗热震性能

采用压痕-急冷法测试了Al2O3-SiCw，Al2O3-ZrO2和Al2O3-TiCp三种陶瓷基复合材料的抗热震性能，并与急冷强度法测试结果进行了对比分析。实验结果表明，两种方法之间存在一致性。三种陶瓷基复合材料与基体Al2O3相比抗热震性均有较大幅度的提高，其中Al2O3-SiCw复合材料显示出最为优越的抗裂纹扩展能力与抗循环热震性能。材料的增韧效果是产生这一现象的主要原因。压痕-急冷法与急冷强度法相比免去了热震后的强度测试，具有使用试样数目少，数据具有统计效应，可直接观测裂纹扩展等优点。     

Features of fatigue crack growth due to repeated thermal shock

Abstract Thermal shock loading, such as that which occurs when a hot material is sprayed with cold water, produces a very high stress level near the exposed surface that eventually may lead to the development of cracks. Further growth of the cracks under repeated thermal shock is a very complex phenomenon due to the transient nature of the highly non-linear thermal stresses and the strong influence of the environment. There are cases in industry where cracks created by thermal shocks have arrested and stopped, and others where the cracks have progressed. Understanding this difference in behaviour is very important to the operators of pressure plant. This paper describes an experimental examination of crack growth in pressure vessel steel specimens exposed to repeated thermal shock. A test-rig that achieves large-scale thermal shocks through the repeated water quenching of heated flat plate specimens is used. The effect of steady-state loads on the growth is also analysed. Environmental effects due to the aqueous nature of the testing environment are found to be a major contributor to the crack growth kinetics.     

A simple methodology to visualize crack propagation for ceramic materials

Analysis of the crack growth behavior is critical for fracture and thermal shock assessment. The current work presents a simple methodology to visualize cracks in ceramic materials. The procedure is exemplified for refractory materials on the basis of images obtained during mechanical loading using a wedge splitting test. Complementary in situ crack growth observation verify that for one of the materials growth and opening displacement of the main crack is accompanied by pronounced micro-cracking, branching, and bridging processes. Apparent fracture resistance and thermal shock resistance parameters are discussed.     

钨铜梯度材料热震过程中显微组织及热性能

针对目前W-Cu功能梯度材料(FGM)在长期热震循环过程中的稳定性缺乏相应研究的问题,以化学镀W-10wt%Cu复合粉体和Cu粉为原料,通过叠层压制和常压气氛烧结的工艺制备了W-10wt%Cu/W-20wt%Cu/W-30wt%Cu层状梯度材料。在600℃、800℃、1000℃温度下进行热震试验,对试样在不同热震温度、热震次数下的显微组织和热学性能变化进行了研究。试验结果表明,随着热震温度升高,渗出至试样各梯度层表面的Cu逐渐增加。当热震温度达到1000℃时,试样各梯度层表面出现大量Cu聚集成片的现象,同时在W-20wt%Cu/W-30wt%Cu界面处发现了界面裂纹。随着热震次数的提高,在W-10Cu层中,Cu逐渐渗出表面并在内部留下微孔。此外,W-Cu FGM的热导率随热震次数的增加而减小,在1000℃经过200次热震后,室温热导率由200.54 W·(m·K)?1降至159.23 W·(m·K)?1,降低了20.60%。该结果揭示了热震循环中裂纹形成与显微组织变化的耦合失效机制,明确了W-Cu FGM安全服役的范围。      

Ultimate Bearing Capacity Analysis and Sizing Optimization of a Cracked Reactor Pressure Vessel in the Coupled Thermo-Mechanical Field

Pressurized thermal shock (PTS) can subject a crack surface to a very high tensile stress. Also the material toughness is obviously decreased in the cooling process, so it is necessary to study the influence of PTS on the ultimate bearing capacity of a reactor pressure vessel with defects. A 3-D finite element model is established for the beltline region around an inner crack. The FEM is used to reveal the transient temperature field and stress field, and the XFEM is adopted to simulate the ductile crack propagation. To ensure that the strength requirement is satisfied, the ultimate internal pressures of vessels with different crack sizes and different wall thicknesses are obtained. The result shows that the ultimate bearing capacity of the base wall with shallow surface cracks at high temperature is mainly controlled by tensile strength, while it is also affected by the fracture toughness of the material under the severe PTS. The stress in the early stage of the PTS is mainly the thermal stress, and later is the thermo-mechanical coupling stress. The impact of the crack depth on the bearing capacity of the structure is much greater than that of the crack length.     

Preparation and thermomechanical characterisation of aluminum titanate flexible ceramics

Flexible ceramics may be useful, for example to process refractory materials with an improved resistance to thermal shocks. A natural flexible sandstone, itacolumite, is mainly constituted of interlocked quartz grains and contains microcracks. Its microstructure allows some free motion between grains that induces its flexibility. The aim of this study is to prepare and characterize flexible aluminum titanate ceramics by mimicking the microstructure of itacolumite. Aluminum titanate (AT) has a high thermal expansion anisotropy that induces grain boundary microcracking leading to flexibility. Here, the flexibility is the capacity of the material to endorse large strain-to-rupture level. This concept is also closely related to a low value of the stiffness induced by damage mechanisms. In this study, the flexibility has been estimated by the measurement of the deflection at fracture on three-point bending test. By preparing AT samples sintered according to different heating cycles, the correlations between the sintering cycle, the microstructure and the flexibility have been studied. Grain size and microcrack width have been observed by scanning electron microscopy. A major parameter for flexibility is the microcrack volume fraction within the sample. Three types of AT materials have been processed: non flexible (NF), flexible (F), and very flexible (VF). Their thermal and mechanical behaviors have been investigated and showed that NF has a brittle behavior while F and VF have a nonlinear ductile one. This was found to be due to grain boundary microcracks network and to the interlocking of grains. VF is more flexible than F because its microcracks are wider. Flexibility improves the thermal shock resistance: F and VF have a higher thermal shock resistance than NF. Moreover, thermal expansion measurements during thermal cycles showed anomalous effects induced by crack closure when heating and crack opening when cooling.     

先进近零膨胀陶瓷研究进展

陶瓷材料易受外界温度剧变的影响,在材料中产生热引力,导致陶瓷材料失效.因此近零膨胀陶瓷在耐热冲击性能上具有显著优势,应用十分广泛,一直是材料学界研究的热点.介绍了磷酸盐、钛酸铝、微晶玻璃、钨酸锆几类典型的耐热冲击陶瓷材料,重点介绍了其热膨胀性能、现阶段的应用情况和国内外的发展状况.并提出了研究中存在的问题及其今后的发展方向.     

The Fractography and Crack Patterns of Broken Glass

The topographical features which appear on the fracture surfaces of broken glass objects and the resulting crack patterns which develop are Nature’s documentation of the fracture event. They are considered after a brief discussion of glass strength. Strength is central to the fracture surface features for it determines the strain energy release rate and the dynamics of crack extension. The surface features known as the mirror, the mist, and the hackle are illustrated and addressed through the principles of fracture mechanics and associated energy criteria. Quantitative aspects of the fracture process such as the stress level at fracture for a glass object are directly related to the size of the fracture mirror. The concept of a fracture mirror constant is related to the strength. Formation of the mist and hackle surface regions are also fundamentally addressed, as is crack branching. Distinctive crack patterns that evolve during fracture, that is the traces of the cracks intersecting the glass free surfaces, are described. Dicing fragmentation of high-strength tempered glass and the long sword-like shards of low-strength annealed glass fracture are contrasted through their strain energies. Characteristic cracking patterns are reviewed for several common glass fractures including those for pressure breaks, both bottle explosions and flat glass window failures from wind pressure whose basic similarities are described. The patterns of crack branching or forking, the branching angles and the crack length prior to forking, are also discussed. Other glass crack patterns such as those from impact and thermal stress are also considered.     

Simulation of disruption loads on first-wall high-temperature materials

The behaviour of carbon materials under thermal load in fusion reactors has been simulated by laser-pulse irradiation in a scanning electron microscope (SEM). In this way material damage, such as thermal shock crack formation and propagation, and erosion behaviour, can be studied in situ in the SEM with high lateral resolution. The dependence of damage initiation and propagation on the laser-beam parameters (pulse number, energy, spot size, spot duration and energy density), is of special interest. The damage behaviour is strongly determined by special materials structures. Because of its fibre reinforcement, the investigated CFC composite materials proved to be more stable to erosion and crack formation than homogeneous finegrained graphites. High-temperature damage may be diminished by the use of carbon materials with creep-resistant components.