Miniature Creep Research Articles

J0310201 CrMoV鍛鋼環状切欠き材のクリープ損傷評価

Creep damage preferentially proceeds at stress concentrated portion such as notch roots under multiaxial stress states in high temperature components. It is necessary to establish creep damage assessment methods under these conditions for maintaining safety operation. So far, influence of notch on creep rupture strength, and damage condition at notch root after testing were reported. However, damage extension process at notch root is not clarified and argument of damage assessment methods is still on going. In this study, creep tests were carried out using the circumferential notch specimens on a CrMoV forging steel. Damage conditions at the notch root were observed with interrupted test specimens. Damage assessment by using the miniature creep specimens taken from damaged notch specimens was conducted. It was found that the location where the void number density is the largest is corresponding to the location where both the axial stress and the triaxiality factor yield the maximum values. The creep damage given to the notch specimen was underestimated by the subsequent miniature creep test, when creep voids initiated in the creep damaged specimen do not give any influence on ductility reduction for the miniature specimen.

Micromechanical investigations and modelling of a Copper–Antimony-Alloy under creep conditions

In many practical applications, creep damage is the limiting factor of a component’s lifetime. A micromechanical model of creep induced grain boundary damage is proposed, which allows for the simulation of creep damage in a polycrystal within the framework of finite element analysis. The model considers grain boundary cavitation and sliding according to a micromechanically motivated cohesive zone model while creep deformation of the grains is described following the slip system theory. The model can be applied to idealised polycrystalline structures, such as a Voronoi tessellation or, like demonstrated here, to real grain structures of miniature creep specimens. Creep tests with pure Cu single crystals and with a coarse-grained polycrystalline Cu-1wt.% Sb alloy at 823K have been performed and used to calibrate the polycrystal model. The grain structure of the polycrystalline Cu–Sb specimens has been revealed by the EBSD method. Extensive grain boundary sliding and cavitation has been observed in the crept specimens. Grain boundary sliding has been found to promote wedge-type damage at grain boundary triple junctions and to contribute significantly to the total creep strain. Furthermore, the assumed stress sensitivity of the models grain boundary cavity nucleation rate strongly influences the development of wedge-type damage.

High-temperature and low-stress creep anisotropy of single-crystal superalloys

The high-temperature and low-stress creep (1293K, 160MPa) of the single-crystal Ni-based superalloy LEK 94 is investigated, comparing the tensile creep behavior of miniature creep specimens in [001] and [110] directions. In the early stages of creep, the [001]-direction loading shows higher minimum creep rates, because a greater number of microscopic crystallographic slip systems are activated, the dislocation networks at γ/γ′ interfaces accommodate lattice misfit better, and γ channels are wider. After the creep rate minimum, creep rates increase more strongly as a function of strain for [110] tensile loading. This may be related to the nature of rafting during [110] tensile creep, which results in a more open topology of the γ channels. It may also be related to more frequent γ′ cutting events compared with [100] tensile creep.

High-temperature strength and damage evolution in short fiber reinforced aluminum alloys studied by miniature creep testing and synchrotron microtomography

The creep behavior of a squeeze-cast, short fiber reinforced Al metal matrix composite (MMC), consisting of an Al–11 wt.% Zn–0.2 wt.% Mg alloy reinforced with 15 vol.% Al 2O 3 Saffil® short fibers is investigated using miniature creep specimens. The small dimensions of the miniature creep specimens permit them to be machined from regions of an MMC block with different microstructures, thus allowing the effect of grain size and fiber texture on creep to be investigated on a more local level than is possible using conventional specimen geometries. The miniature creep specimens are subjected to uniaxial tensile stresses ranging from 3 to 40 MPa at temperatures between 573 and 623 K. It is shown that tests performed using the miniature creep specimen geometry are in good agreement with results previously obtained with standard creep specimens. Through interrupted creep experiments, it is observed that the creep back flow that occurs after unloading increases with increasing accumulated plastic strain. In the as-cast MMC, synchrotron microtomography reveals a fine distribution of pores whose spatial density increases with the spatial density of the fibers. The presence of fractured fibers in the crept MMC is also revealed. Some of the regions between fractured fiber fragments appear to be filled with matrix material, while others are voided.

In-situ observation of creep damage evolution in Al–Al2O3 MMCs by synchrotron X-ray microtomography

Abstract Creep damage by pore nucleation and growth limits the lifetime of components subjected to loading at high temperature. In this work, the creep damage evolution in the aluminium matrix composite AA6061 + 22 % (Al2O3)P is observed in-situ by the fast-tomography technique using a miniature creep device. Reconstructions of the gauge volumes of the creep samples show particle delamination, pore formation and growth at 300 °C and additional particle fracture at 200 °C during the creep process. The results reveal that both the number of pores and the total pore volume show exponential growth with creep time, whereas the average volume of individual pores remains almost constant. The total pore volume increases linearly with the creep strain with a stress-dependent factor of proportionality.

Deformation and Fracture Mode during Small Punch Creep Tests

The creep damage condition of components under elevated temperature is a requirement to guarantee safe life extension and continued operation. Destructive and/or non-destructive assessments are regularly applied to assess the remaining life of components during service. Uniaxial creep specimens have been traditionally employed for conventional tests to examine a series of high temperature creep properties. However the ability to remove these relatively large uniaxial specimens is limited due to the required size of the specimens with respect to the component dimensions. To overcome this shortcoming, small element testing techniques such as miniature creep (MC) and small punch creep (SPC) tests have recently been proposed to investigate creep properties. However their applications are limited as there is no established standard for the testing procedures and subsequent data evaluation. In order to aid the standardization of the SPC test method, this paper investigated the deformation and fracture of interrupted SPC tests. Results showed that the creep deformation in the SPC test could be classified into three conventional stages. Firstly, the crack of about 1mm in the diameter developed on the extended surface of the disc specimen at the end of the primary creep stage. Secondly, during secondary creep, circumferential cracking progressed in the through-thickness direction by about 0.1mm. Lastly, the tertiary creep region was extremely short and only appeared just before final fracture and failure. The result showed that the ratio of load in the SPC test to stress in the uniaxial creep proposed past was smaller than the experiment value. This result was due to the early crack formation in the disc specimen and the shear type crack development, and the difference of loading ball's diameter.

Novel experimentation techniques to study damage accumulation and microstructural instability during creep of super alloy single crystals at high temperatures (>1000°C)

The objective of the present work is to demonstrate that it is well worth the effort to develop novel experimentation techniques to study creep in Ni base super alloy single crystals above 1000°C. A miniature tensile creep specimen and a double shear creep specimen have been developed. These are useful in high temperature technology because they can be directly taken out from cast turbine blades (miniature tensile creep specimens) and provide multiaxial creep data (double shear creep specimens). Most importantly, these new techniques can be used to increase the level of microstructural understanding of creep. Thus miniature tensile creep experiments can be used to demonstrate that <100> directions parallel to, and perpendicular to, the solidification direction differ in creep rupture behaviour. This is related to the presence of cast micropores which align along primary dendrites. The double shear creep specimens, on the other hand, help in understanding the influence of stress multiaxiality on rafting, the directional coarsening of the γ′ phase. In a macroscopic crystallographic shear system of type <110> {110}, rafting occurs at 45° to the shear direction and is governed by the maximum principal stress.

A guide to condition assessment of gas turbine hot gas path components

Maintenance and/or replacement of hot gas path (HGP) components in gas turbines represent a major fraction of the cost of installation and operation. Condition assessment of HGP components can increase reliability and minimise costs, based on sound metallurgical and mechanical analysis. In this paper relevant improvements to condition assessment procedures and analyses are described, including γ’ precipitate sizing and lifing techniques used on precipitation-hardened Ni-based superalloys. Non-destructive testing, optical metallography and miniature creep testing are also covered. Understanding the operation and possible failure mechanisms of the component in service is the key to selecting an appropriate assessment procedure to facilitate optimisation of component lives and repair cycles

A guide to condition assessment of gas turbine hot gas path components

Maintenance and/or replacement of hot gas path (HGP) components in gas turbines represent a major fraction of the cost of installation and operation. Condition assessment of HGP components can increase reliability and minimise costs, based on sound metallurgical and mechanical analysis. In this paper relevant improvements to condition assessment procedures and analyses are described, including γ’ precipitate sizing and lifing techniques used on precipitation-hardened Ni-based superalloys. Non-destructive testing, optical metallography and miniature creep testing are also covered. Understanding the operation and possible failure mechanisms of the component in service is the key to selecting an appropriate assessment procedure to facilitate optimisation of component lives and repair cycles

Development of Miniature Creep Testing for Solders

This paper describes a new miniature creep tester and the size effect of creep and creep rupture times for solders. The new miniature creep tester was developed and creep rupture tests were carried out using five types of Sn-37 Pb specimens with different gage diameters. Creep rupture times of miniature specimens did not differ from those of bulk specimens at a higher stress but they were scattered around the bulk data at lower stresses. Creep rates of miniature specimens were slower than those of bulk specimens and creep ductility of miniature specimens was smaller than that of bulk specimens.

844 はんだのミニチュアクリープ試験法の開発

This paper describes a new miniature creep tester and the size effect of creep and creep rupture lives for solders. The new miniature creep tester was developed and creep rupture tests were carried out using five types of Sn-37Pb specimens with different diameters. Creep rupture times of miniature specimens were longer than those of bulk specimens. Creep rates of miniature specimens were slower than those of bulk specimens but the unique relationship held between the strain rate and rupture time in all types of specimens.

918 SUS304ステンレス鋼の3軸引張クリープ試験(GS-3 熱変形・高温疲労・熱疲労)

This paper describes the new triaxial creep testing and creep damage evaluation in a triaxial stress state. Equi-triaxial tension creep tests were carried out using SUS304 stainless steel at 923K. Triaxial creep damage was evaluated by utilizing a miniature creep testing for triaxially damaged specimens. Maximum principal, Mises and Huddleston stresses were applied to the test results and the suitability of the stresses was discussed for multiaxial creep damage evaluation.

"Direkte Bewertung der Kriechfestigkeit einer gerichtet erstarrten Turbinenschaufel mittels Minikriechproben

The present paper presents a novel approach which allows a direct assessment of the creep strength of thin walled DS turbine blades at temperatures up to 1100 °C using a grain morphology sensitive miniature creep specimen test technique. The work builds up on earlier investigations by DeMestral and co-workers ([i] B. DeMestral et al., Met. and Mat. Transactions A, 27A (1996) pp. 879–890 and [ii] B. DeMestral and G. Eggeler, Prakt. Metallogr., 29 (1992) pp. 174–191) on the influence of grain morphology on creep deformation and damage accumulation in superalloys with elongated macrograins. The paper reports the important features of this new procedure which include (i) the creep test technique (specimen geometry and grip design) and (ii) the removal of creep specimens from a DS CM 247 LC cast blade with respect to the morphology of the macrograins. The paper also reports creep results obtained with this new technique at temperature of 1030 °C and a tensile stress of 180 MPa and discusses the results on the basis of the associated microstructural creep deformation and damage mechanisms.

Modelling the multiaxial creep behaviour of alloy 800H

A creep constitutive model, based upon the θ-projection concept, is derived using constant-stress creep curves obtained from miniature creep specimens cut from the walls of Alloy 800H tubing. Initial verification of the model predictions under changing uniaxial stress conditions is made by comparison with a series of constant-load creep results. Two multiaxial constitutive models have been developed based on the θ description of uniaxial behaviour, the first based on a von Mises function of equivalent stress and the second based on the maximum principal stress criterion. These models are used to predict the creep deformation and rupture behaviour of tubular specimens under combined internal pressure and axial load. It is shown that both the multiaxial creep deformation and rupture behaviour of Alloy 800H at 900°C are well predicted by the model based on the maximum principal stress criterion.

Assessment of remaining life of fossil power plant parts by means of a miniature creep rupture test.

In order to validate a procedure for creep rupture tests using miniature specimens, a sampling technique and a method of making the miniature specimens were investigated. Finally, the use of the iso-stress creep rupture test on the miniature specimens to predict the remaining creep life of Cr-Mo-V, 21/4Cr-Mo and 11/4-1/2Mo steel was examined.The creep curve, rupture life and ductility of the miniature specimens tested in argon were similar to those of conventional sized specimens in air. The fracture mode of the miniature specimens was also consistent with that of conventional sized specimens.On a graph showing temperature T vs. log time to rupture tr, the creep rupture data were represented with an iso-stress line, respectively. Each iso-stress line shifted parallel with the stress level.Moreover, the sampling device that was developed was applied to the operating plant components. Samples have been successfully removed from it and were obtained without affecting the material properties around the core hole.

Anomalous primary creep behaviour of dilute aluminium-silicon alloys

Low temperature (750 °C) and high stress (800 MPa) creep curves of single crystal superalloy ERBO/1 tensile specimens loaded in the 〈001〉 direction show two creep rate minima. Strain rates decrease towards a first sharp local creep rate minimum at 0.1% strain (reached after 30 min). Then deformation rates increase and reach an intermediate maximum at 1% (reached after 1.5 h). Subsequently, strain rates decrease towards a global minimum at 5% (260 h), before tertiary creep (not considered in the present work) leads to final rupture. We combine high resolution miniature creep testing with diffraction contrast transmission electron microscopy and identify elementary processes which govern this double-minimum type of creep behavior. We provide new quantitative information on the evolution of microstructure during low temperature and high stress creep, focusing on γ-channel dislocation activity and stacking fault shear of the γ′-phase. We discuss our results in the light of previous work published in the literature and highlight areas in need of further work.

Characterization of the elevated temperature static load crack extension behavior of type 304 stainless steel

Creep crack extension rates in Type 304 stainless steel, obtained as a function of temperature over the range 650–800°C and as a function of specimen geometry at 750°C, are empirically correlated with both the net section stress and the apparent stress intensity factor. The results indicate that the stress intensity correlation is strongly dependent on specimen geometry, whereas the net section stress correlation appears to be generally valid. A direct correspondence between crack extension and local (crack tip) displacement is noted when creep crack extension rates at 750°C are compared with COD obtained from actual castings of the crack tip. By introducing the concept of a miniature creep specimen at the crack tip, a physical model for creep crack growth is developed, based on local stress relaxation and strain accumulation, that is consistent with both experimental observation and existing theories of steady state creep.