Creep of ceramics

This review analyses a wide range of experimental data on the creep of ceramic materials and reveals many similarities with the creep of metals. It is demonstrated that there are two important differences in the creep behaviour of ceramics: (1) there is an enhanced role of diffusion creep, and (2) in the power-law regime, ceramics divide into two categories with stress exponents of 5 and 3, respectively. It is concluded that the behaviour with an exponent of 5 represents fully ductile behaviour as in f cc metals, whereas the behaviour with an exponent of 3 is due to dislocation climb from Bardeen-Herring sources under conditions where there is either a lack of five independent slip systems or, if five independent slip systems are available, a lack of interpenetration of these systems.     

Creep in chipboard

3- and 4-element spring and dashpot models have frequently been applied to the behaviour of some visco-elastic materials, although somewhat less frequently to wood and wood-based sheet materials which are themselves visco-elastic. However there exists a need to develop a good analytical procedure for fitting the non-linear response curves corresponding to these models to experimental data. This paper describes such a method and applies it to the data from a creep experiment on UF chipboard bending specimens under sustained three-point loading. The computer program written for the solution process illustrates the response curves on a graph plot.     

Creep in polycrystalline aluminium

Abstract

The present paper reports a study of creep in large grained polycrystalline aluminium. Two techniques were employed to understand this process in more detail. The first was stereoimaging to measure the local strain in the specimen. This measurement was made by photographing the specimen at various times during the creep test, which was carried out in a scanning electron microscope, and using these images to calculate the strain. The second technique was analysis of electron backscattering diffraction patterns, as generated in a scanning electron microscope. This technique allowed examination of changes in crystallography that accompanied the creep process. The results of the experiments showed that strain built up in different grains at different rates. There was never a discontinuity in strain across the grain boundary, and strain relaxation was observed in different grains at different times during the test. Recrystallisation was also observed to occur. In some cases, an existing grain migrated into another grain, presumably through strain induced grain boundary migration. In other cases, there appeared to be nucleation of a new grain with a different orientation.     

Ti65合金的初级蠕变和稳态蠕变

使用透射电镜(TEM)研究了Ti65合金在600~650℃、120~160 MPa条件下的蠕变变形行为及其微观变形机制。结果表明：初级蠕变变形机制主要由受攀移控制的位错越过α₂相的过程主导;稳态蠕变阶段蠕变机制主要由受界面处扩散控制的位错攀移的过程主导,且应力指数为5~7。在初级蠕变阶段α₂相与位错的相互作用是α₂相对合金高温强化的主要方式,在稳态蠕变阶段沿α/β相界分布的硅化物阻碍位错运动与限制晶界滑移是硅化物对合金强化的主要方式。     

Investigating Creep in Corrugated Packaging

In this study, the inverse relationship between lifetime and secondary creep rate was verified suggesting that shorter duration tests can be used to evaluate lifetimes. Also, the variability in creep rate was significantly lower than previous studies suggesting that using enhanced testing facilities could reduce required number of repetitions. A novel finding was an observed transition point in applied loading level where cyclic humidity conditions had the highest creep rates at low loads and constant high humidity conditions had the highest creep rate at high loads. Finally, it was determined that a standard moisture cycling history for a standard test protocol will be difficult to determine and the moisture sorption behaviour of each box type must be accounted for. Copyright © 2017 John Wiley & Sons, Ltd.     

Creep in transforming polycrystalline materials

An extension, taking recovery into account, has been made of Poirier's (1982) theory of transformation plasticity, that is, the acceleration of creep due to the introduction of accommodation dislocations in the plastic relaxation of the misfit of domains of new phase formed during phase transformation. This enables more realisticc distinctionsto be made between situations where transformation plasticity may be a significant effect in the Earth and where it may not.     

Creep in an electrodeposited nickel

This paper reports the experimental results on the creep behavior of electrodeposited ultrafine-grained nickel and its particle-reinforced nanocomposite. The objective of this research was to further improve the knowledge of the creep behavior of monolithic nickel and to explore the role of nano-sized SiO₂ particles in the potential creep strengthening of electrodeposited Ni nanocomposite. The creep behavior and microstructure of the pure ultrafine-grained nickel and its nanocomposite reinforced by 2 vol% nano-sized SiO₂ particles were studied at temperatures in the range from 293 to 573 K and at the applied tensile stresses between 100 and 800 MPa. The results indicate that the creep resistance of the nanocomposite may be noticeably improved compared to the monolithic nickel due to the interaction of the particles with dislocation motion. It was found that the applied stress interval can be divided into lower and higher stress intervals corresponding to dislocation (power-law) and exponential creep regions, respectively. Analysis of the creep data leads to the suggestion that the creep behavior of both electrodeposited nickel and its nanocomposite in power-law region may be grain boundary controlled. However, the mechanism responsible for the observed creep behavior at lower temperatures and the highest stresses is still not well established.     

Creep of germanium

Isothermal creep, as well as the response to incremental stress and temperature changes, were studied in germanium single crystals oriented for double slip, in the range 470 to 700° C. The stress-sensitivity of the compressive creep rate In / In is numerically close to 3 at low strains, but increases appreciably with deformation. This effect, and a similar strain dependence of the activation energy as determined by thermal cycling, are explained in terms of the curvature of the creep curves on the basis of Boltzmann's superposition principle. The Peierls barrier seems to be an important obstacle to dislocation movement at relatively low temperatures, when S-shaped creep curves are observed. Other barriers, with greater heights, seem to become increasingly effective above about 550° C. Although dislocation loops, and the formation and break-up of dipoles were observed by TEM, recovery mechanisms involving self-diffusion did not appear to make a substantial contribution to the creep within the range of temperatures used.     

Dislocations in ceramics

Abstract

The type and nature of dislocations in oxide ceramics are reviewed and compared. Slip systems in these materials tend to be limited and difficult to activate, and Burgers vectors are often large. Thus, dislocation dissociation has been an important theme of research and climb dissociation has been frequently observed. Elasticity theory suggests that climb dissociation should be expected whenever diffusion can occur, and may be widespread in other materials deformed at elevated temperatures. Low temperature behaviour remains an area where much work needs to be done, particularly to assess the role of the Peierls barrier on the dislocation-core configuration.

MST/270     

Fatigue in ceramics

The fatigue process in ceramic materials is reviewed. It is shown that there are few cases where fatigue effects have been unequivocally demonstrated. Mechanisms pertinent to the substantiated fatigue conditions are presented and discussed. The need for further definitive experimentation to deduce the realm of fatigue, and its dependence on microstructure, vis-a-vis the fatigue models, is emphasized.

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Résumé On passe en revue le processus de fatigue dans les matériaux céramiques. On montre qu'il existe quelques cas où les effets de fatigue ont été démontrés de manière non équivoque. On présente des mécanismes susceptibles de convenir à des conditions de fatigue dûment identifiées, et on les discute. On met en avant la nécessité d'expérimentation plus avancée en vue de déduire les bases de la fatigue et sa dépendance de la microstructure et ce par rapport aux modèles de fatigue existants.

     

Superplasticity in ceramics

It is now recognized that superplasticity is a potential deformation process in ceramics. This review summarizes the major characteristics of superplasticity and examines the reports of both transformation and structural superplasticity in ceramic and other non-metallic materials. It is shown that there are both similarities to and differences from metals. Similarities include the variation of strain rate with stress and grain size, but an important difference is the necessity to consider the role of intergranular glassy phases in ceramics. Superplasticity is also important in intermetallic compounds, and in geological materials where there is evidence for superplastic deformation both in laboratory experiments and in natural deformation.     

低蠕变砖压蠕变率测试结果的不确定度评定

根据GB／T5073--2005对一批低蠕变砖进行高温压蠕变率的测试,并对影响高温压蠕变率的不确定度分量进行分析和评定,给出了不确定度报告。结果表明该测试的不确定度主要来自重复性试验。     

Creep fracture in Al-SiC metal-matrix composites

Creep fracture behaviour of pure aluminium-matrix composites with 10–30 vol% SiC particulates at 623 K is reported. A comparison of tensile and compression creep data shows the existence of a transition stress. Above this transition stress no steady state creep is observed in tension. This transition stress is related to a transition from intergranular to transgranular fracture. The origin of transition stress is perhaps associated with the diffusional relaxation of stress concentration at the matrix/particle interface by lattice diffusion. The intergranular creep fracture of composites appears to be similar to that of unreinforced aluminium and it is power-law creep controlled. The transgranular creep fracture occurs by void nucleation and growth. The nucleation strain for voids is quite small and hence the tertiary stage starts before the end of the primary stage. The ductile fracture models overestimate the strain to fracture and do not predict the observed stress dependence of strain to fracture.     

Creep crack growth in brittle materials

During steady state crack growth by diffusive cavitation at grain boundaries, crack tip fields are relaxed due to the presence of a cavitation zone. In the present analysis, analytic solutions for the actual crack tip stress fields and the crack velocity in the presence of cavitation zone consisting of continuously distributed cavities ahead of the crack tip are derived using the smeared volume concept. Results indicate that the r ^–1/2 singularity is now attenuated to r ^{–1/2 +} (0<<1/2) singularity. The singularity attenuation parameter is a function of the crack velocity and material parameters. The crack growth rate is related to the mode I stress intensity factor K by K ² at relatively high load, K ⁿ at intermediate load, and approaches zero at small load near K _th. Meanwhile, the cavitation zone extends further into the material due to the stress relaxation at the crack tip and the subsequent stress redistribution. Such relaxation effects become very distinct at low crack velocity and low applied load. Key words: Creep crack growth, brittle material, diffusive cavity growth, sintering stress, crack tip stress field.     

Inverse Flux Creep in Ring-Shaped Superconductors

Anomalous flux creep is depicted in ring-shaped superconductors, where the vortex movement goes to the inner edge of the sample. The current dissipation phenomenon is inverted and an induction of the current is achieved. Thus, an increase in the current circulating by the superconductor is obtained. The experimental technique consists in the utilization of a closed magnetic circuit for the current induction and a magnet ring with the field opposite to the vortex trapped in the superconducting wall.