Creep Contribution Research Articles

Fatigue and creep crack growth in oxide dispersion strengthened INCONEL MA-754

The influence of temperature, orientation, and environment on fatigue and creep crack growth behavior in oxide dispersion strengthened INCONEL MA-754 was examined. With an increase in temperature, crack growth rates increase due largely to an increasing creep contribution. Environment also may influence crack growth behavior, its effect depending on orientation. Orientation has a marked effect on crack growth because of the propensity for creep void formation along particle stringers in the microstructure, which form in the processing. The rate of crack growth can be enhanced if the aligned voids are parallel to the main crack or retarded if these voids are normal to the direction of the crack. In the transverse-longitudinal (T-L) orimation crack growth is faster on a time basis in creep than in fatigue; the reverse of this is true in the longitudinal-transverse (L-T) orientation. Predicted fatigue crack growth rates based on a cumulative damage model agree with experimentally determined growth rates.

Effects of temperature, ordering and composition on high cycle fatigue of polycrystalline Fe 3Al

The high cycle fatigue (HCF) behavior of several Fe 3Al-type alloys was determined in the temperature range 25–600°C. Two types of order were created by heat treatment: D0 3-type by slow cooling, and B 2-type (off-stoichiometric) by water quenching. D0 3-type order was effective in prolonging high cycle fatigue lives in Fe-28.7%Al, but not in Fe-23.7%Al at 25°C. This difference was attributed to the presence of superlattice dislocations in the former. The fatigue resistance of D0 3 material was not as severely degraded at elevated temperatures as was that of B2 material. A substantial creep contribution to fatigue behavior was noted in tests on B2 material at 560 and 600°C. Fracture surfaces consisted primarily of transgranular cleavage facets in all alloys and conditions of order, but crack initiation was intergranular in unnotched Fe 3Al tested in HCF. Fatigue striations were observed only at 500°C. The results are discussed in terms of dislocation configurations and two-phase microstructures observed by transmission electron microscopy in certain alloys and test conditions.

The thermomagnetic pressure difference in CO

The thermomagnetic pressure difference, which occurs in a heat conducting polyatomic gas under the influence of a magnetic field, has been measured for CO in a parallel plate arrangement. Data as a function of field orientation are used to identify the different contributions to this effect. In contrast to earlier data obtained in a less ideal geometry, the new results can be described in terms of Waldmann's phenomenological approach. This theory is found to somewhat overestimate the thermal creep contribution.

The influence of temperature cycling on the creep properties of Nickel 201 and Inconel 600 in combustion gas

The effect of temperature cycling on the creep behaviour of Nickel 201 and Inconel 600 in combustion gas has been studied. Specimens were tested both at constant temperature, 900° C, and at 900° C interrupted by temperarature drops down to ∼510° C. The creep straining has been analysed with respect to a weighted time parameter which includes the creep contribution during the lower temperatures of each cycle. With respect to this compensated time parameter, the temperature variations were generally observed to result in a strong acceleration in creep. The effect seemed to increase with increasing frequency of temperature drops, increasing grain size and decreasing stress. Thus, at low stress levels, large-grained specimens of both alloys experienced an acceleration even inabsolute creep rate upon cycling. The grain size dependency indicates that the destructive effect of the cycles is caused by crack formation. Surface cracking associated with grain boundary oxidation seemed to be the dominant cracking mode. It is suggested that, during creep in oxidizing environments, repeated periods of cooling might strongly accelerate the growth of surface creep cracks due to the difference in thermal expansion between metals and oxides. This difference causes high tensile stresses to arise in the metal in front of the grain boundary oxides, and the stresses are assumed to be high enough to nucleate microcracks along the boundary.

Recent insights on the swelling and creep of irradiated austenitic alloys

It appears that all austenitic Fe-Ni-Cr alloys eventually swell during neutron irradiation at a rate of ~1%/dpa over a surprisingly large range of temperature. In addition, there is no evidence that saturation of swelling will occur in commercial alloys at engineering-relevant levels. The sensitivities of swelling to variations in composition, fabrication and environment lie almost exclusively in the duration of the transient regime of swelling. Irradiation creep rates at high exposures are also expected to exhibit a similar parametric insensitivity, largely because the major creep contribution appears to be proportional to the swelling rate.

Helical anisotropy induced by annealing in metglas 2826

The magnetic anistropy induced by constant torsional strain annealing of an amorphous alloy with nominal composition Fe40Ni40P14B6 has been studied. The influence of annealing conditions is reported and discussed. The experimental results indicate that magnetoelastic coupling is the main source of the induced anisotropy, the anelastic and plastic creep contributions being almost negligible.

Creep properties of oxide and carbide fuels under irradiation

The creep behaviour of uranium dioxide and uranium carbide has been examined in both bend and compression experiments in DIDO Materials Test Reactor. In UO 2 no significant variation in creep rate with dose and temperature occured above ~10 25 fissions m −3 between 450°C and 1230°C, the high strain rates measured in compression at low doses being largely attributable to pore sintering. Both a linear rating and stress dependence were observed up to 40 MNm −2 and creep rates were found to be independent of grain size. At higher doses ( >6 × 10 26 fissions m −3 ) transient strains were incurred on varying stress and temperature due to the development of grain boundary gas bubbles. This also resulted in a six fold increase in the radiation creep constant between 6 × 10 26 and 1.2 × 10 27 fissions m −3. A similar pattern of behaviour with respect to rating and stress was observed in hyperstoichiometric UC between 450 and 800°C up to 1 × 10 27 fissions m −3. However the nominally steady state creep rate was a factor 8 lower than in UO 2 irradiated under the same conditions. The experimental results also suggest that the primary creep contribution to the initial strain in compression is much higher than in UO 2. There was no evidence of either transient strain on changing stress or of an increasing creep rate at high doses. The experimental observations are reported and discussed in relation to models for irradiation induced low temperature creep in ceramic fuels.

Thermo-mechanical fatigue behaviour and life prediction of C-1023 nickel based superalloy

Nickel based superalloys are used for manufacturing turbine blades and vanes components due to their ability to withstand high stress levels at high temperatures. The complex thermo-mechanical fatigue loadings that those components suffer (as a result of start ups and shutdowns) make life assessment a complex task. Working towards this end, the thermo-mechanical fatigue behaviour and life prediction of C-1023 nickel based superalloy was studied in this work. Several tests were conducted at different phase-shifts between mechanical and thermal cycles. It was found out that in-phase tests (where peak temperature coincides with maximum mechanical strain) were prone to intergranular fracture due to creep contribution. On the other hand, out of phase tests showed clear transgranular paths ascribed to fatigue damage. Finally, some simple damage parameters were found to be suitable tools for life prediction.

Long-range intermolecular coupling in concentrated poly- n-butyl methacrylate solutions and its dependence on temperature and concentration

The real ( J′) and imaginary ( J″) components of the complex compliance have been measured between 0.05 and 1.3 cycles/sec. for four solutions of a fraction of poly- n-butyl methacrylate with molecular weight 3.05 × 10 6 in diethyl phthalate. The concentrations ranged from 30 % to 60 % polymer by weight. The creep compliance, J( t), was measured over periods of time up to 10 5 sec. The dynamic measurements could be reduced to single composite functions by a modified method of reduced variables which includes a temperature-dependent spacing between long-range coupling points, as shown by earlier measurements at higher frequencies. The parameters for the WLF equation and the apparent heat of dissociation of coupling points, determined from the earlier measurements, were confirmed. The creep data ould not be so reduced because the dissociation of coupling points affects the plateau and terminal zones differently. The creep compliance at long times J( t) could be expressed as the sum of three terms: a network compliance J A , an Andrade creep contribution βt 1 3 , and a flow contribution t/ η. The dependence of β and η on temperature agreed with predictions made by combining the temperature dependences of the monomeric friction coefficient and the average spacing between coupling points. Similar agreement was obtained for the concentration dependence of β as well as qualitatively for the concentration dependence of η, which is approximately proportional to the 14.4 power of polymer concentration. The spacing between coupling points calculated from J A was in reasonable agreement with calculations from the maximum in J″ by the theory of Marvin. There are evidently several types of long-range intermolecular coupling in polymer systems which differ from the entanglement coupling described by Bueche.