Short-term creep,stress-rupture strength,and failure of molybdenum-tungsten alloys at high temperatures

Results are given for determining the characteristics of short-term creep and stress-rupture strength of molybdenum-tungsten alloys with solid-solution and combined strengthening prepared by powder metallurgy at temperatures of 1500, 1750, and 2000°C. Empirical equations are obtained describing the dependence of creep rate and time to failure on applied stress. It is established that introduction into Mo-30% W alloy of finely dispersed niobium carbide and carbon particles markedly improves its high-temperature strength characteristics.Translated from Problemy Prochnosti, No. 5, pp. 41–47, May, 1990.     

Dehydration creep of concrete at high temperatures

In this paper, a new approach for modeling the transient component of the load induced thermal deformation is proposed in order to predict the concrete behavior when subjected to high temperatures with a concomitant applied load. This component is conventionally referred to as transient creep strain. In this approach, the transient creep strain is split into a drying creep component and a newly introduced dehydration creep strain. The former is related to the evolution of the hygrometric state of the material, while the latter is related to the material dehydration which results from the heating induced chemical transformations. Therefore, a dehydration variable is defined and then introduced as a driving variable of the transient creep for temperatures exceeding 105°C. This thermo-hydro-damage model is implemented using a finite element code and␣numerical simulations are performed and compared to experimental findings in order to assess the predictive character of the proposed model.

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Résumé Dans cet article, une nouvelle approche pour la modélisation de la composante transitoire de la déformation thermique induite sous charge est proposée afin de prédire le comportement du béton à hautes températures. Cette composante est conventionnellement connue sous le nom du fluage thermique transitoire. Dans cette approche, le fluage thermique transitoire est décomposé en fluage de dessiccation et en une composante, nouvellement introduite, de fluage de déshydratation. La première composante est due à l’évolution hygrométrique du matériau tandis que la deuxième est due à la déshydratation du matériau qui résulte des transformations chimiques induites par l’augmentation de la température. Par conséquent, une variable de déshydratation est définie et est introduite comme une variable régissant le fluage thermique transitoire lorsque la température dépasse 105°C. Ce modèle thermo-hydro-endommageable est implémenté dans un code aux éléments finis. Des simulations numériques sont effectuées et comparées à des résultats expérimentaux pour analyser les capacités prédictives du modèle proposé.

     

Correlations between short-term,long-term static,and low-cycle strength characteristics of low-alloy molybdenum alloys at high temperatures

As a result of joint analysis of previously obtained experimental data on the mechanical properties of low-alloy molybdenum alloys of the systems Mo-Al-B, Mo-Zr-B, Mo-Zr-Hf-B and Mo-Re for different types of uniaxial tension, we have established that for the considered class of materials in the high-temperature region there exist close correlations between short-term, long-term static, and low-cycle strength characteristics, which are described by functional relationships common to all the materials of the indicated class. Translated from Problemy Prochnosti, No. 12, pp. 43–51, December, 1994.     

Deformation of Cu–P alloys at high temperatures

Abstract

The deformation behaviour of Cu–P alloys has been investigated by torsion and tensile testing over a range of strain rates and temperatures. The torsion flow curves are interpreted in terms of dynamic softening processes, and the curves obtained during interrupted testing are used to examine static-restoration behaviour. Constitutive equations relating flow strength to strain rate and temperature are deduced, with allowance made for the effect of deformation heating, and implications of the equation constants are discussed. It is shown from tensile results that a state of superplasticity can be achieved in alloys containing 3·8 and 6·8 wt-%P. Superplasticity can occur only if the small α grain size is stable and if the temperature and strain rate fall within certain limits. The activation energy associated with superplastic flow has been determined.

MST/52     

Cyclic fracture toughness of structural alloys with surface cracks at low temperatures

In the present paper, we study specific features of crack propagation from surface flaws in full-scale sheets used in manufacturing pressure vessels for cryogenic applications. The process of crack propagation consists of several stages and terminates in stages of surface through or central through cracks under conditions of low-frequency repeated tension. The effect of a decrease in temperature from 292 to 77°K on crack growth behavior was studied for sheets with a thickness of 2, 8, and 12mm. We describe a procedure for testing for crack-growth resistance at cryogenic temperatures and construct fatigue crack growth diagrams. It is shown that zones of influence of the front and rear faces of the specimen on the stress and strain fields near the crack front arise in the plane of a semielliptic crack. The shape of the interface of these zones can be approximated by a second-order curve. Variations in the thickness of the specimen and the test temperature affect the slope of the curve, i.e., the interface of the zones of influence. Specific features of the fracture process in the material of the plate with surface cracks manifest themselves most adequately at points of the crack front located on the indicated interface. We suggest a procedure for estimating the cyclic crack growth resistance of highly ductile stainless steels that is based on the use of the cyclic J-integral. We propose to regard the lengthl of the interface of the zones of influence of the front and rear faces of the specimen as a geometric parameter of the crack. It is used to construct kinetic fatigue crack growth diagrams for specimens with semielliptic surface cracks.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 9–19, January – February, 1995.     

Effects of diffusion and primary creep on intergranular cavitation at high temperatures

International Journal of Fracture - Next-generation reactors are expected to play a crucial role in power production in the foreseeable future. Due to the extreme anticipated operating temperatures...     

Tensile and creep performance of a novel mullite fiber at high temperatures

The novel fiber CeraFib75 with a composition near to pure mullite was analyzed with respect to its potential for high-temperature applications. This mullite fiber free of glass phase was aimed to overcome the strength of commercial oxide fibers at high-temperatures. Tensile tests at room and high temperatures ranging from 900 to 1400 °C and creep tests were performed. Nextel™720, another crystalline mullite-alumina fiber, was tested as a reference. Microstructure and crystal phase analysis of the new fiber revealed mullite grains with traces of γ- and α-alumina in-between; it contains occasionally defects causing a reduced strength at room-temperature. Remarkably, at temperatures beyond 1200 °C, CeraFib75 presented a higher tensile strength than Nextel™720. During tensile tests at 1400 °C, an extended region of inelastic deformation was observed for CeraFib fibers only, which was related to a grain boundary sliding mechanism. Creep rates were of the same order of magnitude for both fibers.