Qauantitative Expression of Creep Damage Elements in Ductile Materials and its Application to creep Life Assessment

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The constitutive equations for constant load creep were determined for as-received, prestrained, artificially aged and service-exposed specimens under accelerated creep conditions . The logarithm of creep rate varics lincarly with true strain as expressed by cg.(1)given below. The stress and temperature dependence of the imaginary initial strain rate and the acceleration factor S were determined. The imaginary initial strain rate corresponds to the steady state creep rate under a constant stress. The rate controlling mechanism is independent of prestrain and structural deterioration. The difference in the imaginary initial strain rate arises from the defference in mechanically defined structure factor. On the other hand, the effect of structural deterioration on the magnitude of S comes chiefly from the decrease in n_S in the state equation for Svalue expressed by eq.(2)In epsilon^^・=In epsilon^^・_0+Sepsilon , (1)where epsilon^^・ is the creep rate, epsilon is the true strain, and epsil … More on^^・_0 is the imaginary initial strain rate at zero strain. The stress and temperature dependence of epsilon^^・_0 and S are expressed as below :epsilon^^・_0=A sigma^n exp(Q_0/RT)(2)and S=A_S exp(m_Sepsilon_p)・sigma_0^<ns> exo(Q_s・RT) (3)where Q_0 is the apparent activation energy for creep, Q_s is the parameter describing the temperature dependence of s, sigma is the true stress at the onset of reloading creep andn is the stress exponent, The magnitude of A is given by an equation, A=A_0 exp[(m-n)epsilon_p], where A_0and m are constants and epsilon_p is the prestrain before reloading creep. The symbols A_s, ms and ns are constants. The values of Q_0 and n are quite insensitive to the structural degradation and prestraining. This means that the rate controlling mechanismof degraded specimens is identical with that of unused specimens. On the other hand, the magnitude of the imaginary initial strain rate increases while that of S decreases with aging. Integrating eq. (1) with time, one obtains the rupture life, tr as below :t_r=1/(Sepsilon^^・_0) (4)A numerical calculation showed that the effect of oxidation during creep makes creep life shorter at lower stresses than that extrapolated from higher stress. Actually, the effect of oxidation on creep life becomes much stronger than that of structural deterioration, when the stress is as low as anoperating conditions. Less