CLASSIFICATION AND PROBABILISTIC MODEL DEVELOPMENT FOR CREEP FAILURES OF STRUCTURES: STUDY OF X-70 CARBON STEEL AND 7075-T6 ALUMINUM ALLOYS
Nuhi Faridani, Mohammad
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Creep and creep-corrosion, which are the most important degradation mechanisms in structures such as piping used in the nuclear, chemical and petroleum industries, have been studied. Sixty two creep equations have been identified, and further classified into two simple groups of power law and exponential models. Then, a probabilistic model has been developed and compared with the mostly used and acceptable models from phenomenological and statistical points of view. This model is based on a power law approach for the primary creep part and a combination of power law and exponential approach for the secondary and tertiary part of the creep curve. This model captures the whole creep curve appropriately, with only two major parameters, represented by probability density functions. Moreover, the stress and temperature dependencies of the model have been calculated. Based on the Bayesian inference, the uncertainties of its parameters have been estimated by WinBUGS program. Linear temperature and stress dependency of exponent parameters are presented for the first time. The probabilistic model has been validated by experimental data taken from Al-7075-T6 and X-70 carbon steel samples. Experimental chambers for corrosion, creep-corrosion, corrosion-fatigue, stress-corrosion cracking (SCC) together with a high temperature (1200 0C) furnace for creep and creep-corrosion furnace have been designed, and fabricated. Practical applications of the empirical model used to estimate the activation energy of creep process, the remaining life of a super-heater tube, as well as the probability of exceedance of failures at 0.04% strain level for X-70 carbon steel.