Fracture and creep of an Al2O3-SiC (whisker)-TiC (particle) composite

High-temperature fracture strength and compressive creep of an electrodischarge-machinable composite, Al₂O₃-30.9 vol.% SiC whiskers-23 vol.% TiC particles have been studied to 1200 °C and 1450 °C, respectively, in inert atmosphere. Microstructures of fractured and deformed specimens were examined using scanning and transmission electron microscopy. Fast fracture occurred at T ≤ 1200 °C. Steady-state creep was achieved for T > 1350 °C at stresses < 80 MPa, with the rate-controlling mechanism being partially unaccommodated grain-boundary sliding, with a stress exponent of ≈ 1 and an activation energy of≈ 470 kJ/mol.     

An experimental approach for the analysis of early-age behaviour of high-performance concrete structures under restrained shrinkage

This paper presents a systematic approach for the experimental testing and analysis of the early-age thermo-mechanical behaviour of large prismatic high-performance concrete specimens under restrained autogenous shrinkage and realistic temperature conditions. The loading system can apply a partial degree of restraint in order to enable the characterisation of high-performance concrete specimens without premature termination of the test in case of significant restrained shrinkage. The instrumentation system can measure the strains and other parameters from the setting time with high accuracy and reliability. The analysis method takes into account the temperature effects on the measured properties and provides equations to determine the time-evolution of shrinkage, thermal expansion, stiffness and creep of the concrete. Results from the testing of 200 × 200 × 1000 mm specimens made with a 0.34 water-cement ratio concrete are presented, analysed and discussed in the paper to demonstrate the application of the proposed approach.     

A review of the development of creep age forming: Experimentation, modelling and applications

About twenty years ago, the process of Creep Age Forming (CAF) was invented, and since then has been developed for the manufacture of heat treatable aluminium alloy panel components particularly, it has been successfully used for aircraft wing panels. Significant research work has been carried out in recent years and process applications have been expanded. This paper contains a review of recent research and development of this novel forming process. It covers process applications in the aircraft industry and scientific research, including the development of forming tools, experimental studies, materials and process modelling and springback prediction. Some potential future applications and challenges for deeper understanding of this novel process are also discussed.     

基于等效等时应力应变曲线的燃气涡轮叶片多工况蠕变计算

基于等时应力应变曲线理论,提出了一种可用于计算多工况的等效等时应力应变曲线蠕变计算方法。运用该方法计算了某航空发动机燃气涡轮工作叶片,在先后经历4个状态(共150 h)持久试车工作状态后的残余变形,并将变形结果与其他蠕变计算方法以及真实发动机叶片经过150 h持久试车后的计量结果进行了对比分析。分析结果表明,与其他蠕变方法相比,本文作者提出的基于等效等时应力应变曲线蠕变计算方法在保持计算精度的同时,进一步提高了计算效率,更具有工程实用价值。     

Testing and modelling total suction effects on compressibility and creep of crushable granular material

Recent works have shown that delayed events of particle crushing are partially responsible of creep deformation in granular materials, and that Stress Corrosion Cracking promoted by high humidity within particles is the source of this mechanism. A number of experimental studies have focused on creep behaviour of water saturated samples and wetting-deformation after soaking dry material. However, there are few evidences of the effect of varying total suction in time-dependent deformation of partially saturated crushable material, and this mechanism have been rarely considered in constitutive models. The aims of this paper are to present experimental evidence of the effect of total suction on compressibility and creep of sandy sized samples from crushed rock, and to propose a simple one-dimensional elasto-plastic modelling approach based on the enhancement of an existing model. Oedometric compression tests at different total suctions are presented. The results show that compressibility and creep strains increase with both stress and humidity. The model proposed uses a time-dependent hardening law coupling suction with the amount of particle breakage. Based on preliminary calibrations, the model captures the effect of suction and time-dependent behaviour over a large range of total suction.     

Analysis of nickel cylindrical bump insertion into aluminium thin film for flip chip applications

This paper investigates the mechanical deformation and the electrical contact resistance of an electroplated Ni micro-cylinder called micro-insert inserted in an Al thin film. A modified nanoindentation apparatus is used to perform the experiments with 6 μm, 8.5 μm and 12.5 μm diameters micro-inserts having the same 5 μm height. Mechanical deformation of Ni micro-insert and Al film is described at different maximum loads corresponding to an equivalent stress of 0.8 GPa, 1.6 GPa and 3.2 GPa. At equivalent stress less than 1.6 GPa, Ni micro-insert exhibits an elastic deformation while at 3.2 GPa it presents an elastic-plastic deformation with a large amount of compression and penetration into micro-insert foundation. Visco-plastic deformation of Al film is noticed during hold at all maximum loads. Beside, Al creep parameters are extracted using a combined Maxwell/Kelvin-Voigt phenomenological model. Mechanical results are coupled to electrical contact resistance measurement.     

Effect of Creep of Ferritic Interconnect on Long‐Term Performance of Solid Oxide Fuel Cell Stacks

High‐temperature ferritic alloys are potential candidates as interconnect (IC) materials and spacers due to their low cost and coefficient of thermal expansion (CTE) compatibility with other components for most of the solid oxide fuel cells (SOFCs). However, creep deformation becomes relevant for a material when the operating temperature exceeds or even is less than half of its melting temperature (in degrees of Kelvin). The operating temperatures for most of the SOFCs under development are around 1,073 K. With around 1,800 K of the melting temperature for most stainless steel (SS), possible creep deformation of ferritic IC under the typical cell operating temperature should not be neglected. In this paper, the effects of IC creep behaviour on stack geometry change and the stress redistribution of different cell components are predicted and summarised. The goal of the study is to investigate the performance of the fuel cell stack by obtaining the changes in fuel‐ and air‐channel geometry due to creep of the ferritic SS IC, therefore indicating possible changes in SOFC performance under long‐term operations. The ferritic IC creep model was incorporated into software SOFC‐MP and Mentat‐FC, and finite element analyses (FEAs) were performed to quantify the deformed configuration of the SOFC stack under the long‐term steady‐state operating temperature. It was found that the creep behaviour of the ferritic SS IC contributes to narrowing of both the fuel‐ and the air‐flow channels. In addition, stress re‐distribution of the cell components suggests the need for a compliant sealing material that also relaxes at operating temperature.     

Effect of holding time in brazing cooling process on creep life of the solid oxide fuel cell with bonded compliant seal

In present paper, effect of holding time at 600 °C during the brazing cooling process on creep life of solid oxide fuel cell (SOFC) with bonded compliant seal (BCS) is investigated by the finite element method. The research indicates that creep crack initiation time in BCS structure increases significantly with the holding time increasing. Compared with that the traditional cooling method during the brazing process, the creep crack initiation time can be prolonged more than twice by the holding time of 150 h with the operating temperature of 600 °C, it increases from 14,949 h to 31,911 h. When the operating temperature is 800 °C, the creep crack initiation time of SOFC can hardly be affected if the holding time exceeds 10 h. Based on the creep damage analysis and considering the cost of the SOFC manufacturing process, it is recommended that the holding time should not be exceeded 300 h if the operating temperature is below 750 °C. And when the operating temperature is 800 °C, the recommended holding time should not be longer than 10 h. The research of the present paper can provide theoretical guidance for the long life manufacturing and reliability operation of SOFC.     

Using a strain based failure assessment diagram for creep–brittle materials

Recently, there has been considerable interest in studying creep crack growth in creep–brittle materials. For example, the methodologies for assessing creep ductile materials, using fracture mechanics parameters like C^* and C_t, have been extended to include creep–brittle materials [1]. This paper begins by examining these recent developments and outlines the difficulties in adopting these developments. An alternative approach is then proposed. This new approach leads from recent work proposed by the authors [2], where a strain based failure assessment diagram (SB-FAD) is used. Experimental results from a series of tests on a simulated heat affected zone of a low alloy steel are explored. The application of the methodology for assessing the initiation and growth of a defect in a creep–brittle material is also demonstrated.