Creep damage mechanisms

Of particular interest in the present chapter is the effect of test atmosphere on creep and creep damage mechanisms. While there are undoubtedly several factors that can promote creep cavitation and contribute to the observed changes in stress exponent and activation energy, the fact remains that the strain rates are substantially higher in air than in inert atmospheres, as shown in Fig. 8.12. This phenomenon is a direct consequence of the topotactic oxidation reaction of SiC whiskers exposed at the surface. As described by Porter and Chokshi,38 and subsequently by others,21,22 at high temperatures in air, a carbon-condensed oxidation displacement reaction occurs in which graphitic carbon and silica are formed at the whisker interface via... 

M. F. Ashby and B. F. Dyson, Creep damage mechanisms and mechanics, in Advances in Fracture Research, edited by S.R. Valluri et al (Pergamon Press. Oxford, 1984), 3 30. 

Interestingly, even if the microstructural evolutions are different, advanced austenitic stainless steels and the Incolloy 800 alloy, which is close to nickel-based alloys, are subjected to the same creep damage mechanisms as martensitic steels and conventional austenitic stainless steels. The same modelings may be applied and lead to creep lifetime predictions in agreement with experimental data up to the longest experimental lifetimes published in the literamre. 

Murakami S. 1983. Notion of continuum damage mechanics and its application to anisotropic creep damage theory. Engng. Mater.Technol (105) 99-105. 

The probability analysis is performed to estimate the probability of a specific adverse consequence resulting from a loss of containment that occurs due to an aging mechanism. The PoF analysis should address all damage mechanisms to which the equipment being studied is or can be susceptible. Further, it should address the situation where equipment is or can be susceptible to multiple damage mechanisms (e.g., thinning and creep) [10]. 

The deformation and damage mechanisms in creep of ceramics and hard materials are similar to those in metals [150,151]. Under normal loading conditions (in the absence of severe elastic constraint) ceramics fracture at room temperature before any significant plastic flow. Dislocation glide in ionically bonded ceramics is complicated by the presence of both anions and cations, which create electrostatic (Coulombic) barriers to shear. As in metals, three creep regimes have been identified. The initial high strain-rate, observed on applying the load, decreases rapidly... 

Both damage mechanisms cause a decrease of the effective cross section of the specimen and stress concentrations by notch effects. This is the reason for the rapid increase in the strain rate observed in tertiary creep. 

Krajcinovic, Dusan, Creep of structures - A continuous damage mechanics approach, J. Structural Mechanics, Vol. 11, no. 1, pp. 1-11. 1983. Krempl, E., Models of Viscoplasticity-Some Comments on Equilibrium (Back) Stress and Drag Stress , Acta Mech. vol. 69, p. 25-42,1987. van Krevelen, D.W., et. al.. Prop, of Polymers, Correlation w. Chem. Struc. Elsevier, London, 1972. 

Zhang, M.J. and Brinson, H.F., Cumulative Creep Damage of Polycarbonate and Polysulfone , Experimental Mechanics, June, 1986, p. 155-162. 

Zhu, S., et al. 2012. A generalized energy-based fatigue-creep damage parameter for life prediction of turbine disk alloys. Engineering Fracture Mechanics. 

At the macroscopic scale, the effect of the creep hold time is characterized by stronger damage while holding in compression than in tension during tests in air (Fig. 6.18(a)) [69,70]. Intergranular creep damage is rather rarely observed and is only present under tensile stress and therefore cannot explain hold time effects. These effects are explained by oxidation mechanisms active in steels with 9% Cr loaded under air at... 

One of the main challenges for some reactor components in austenitic stainless steels at high-temperature in-service conditions is the demonstration of their behavior up to 60 years. The evaluation of creep lifetime of these stainless steels requires on the one hand to carry out very long-term creep tests and on the other hand to understand and model the damage mechanisms in order to propose physically based predictions toward 60 years of service. 

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