High Temperature Creep Conditions Research Articles

Stress state kinetics near stress raisers under high-temperature creep conditions

Stress state kinetics near stress raisers under high-temperature creep conditions

Studies on crack growth rate under high temperature creep, fatigue and creep-fatigue interaction—II: On the role of fracture mechanics parameter [formula omitted

For high temperature creep, fatigue and creep-fatigue interaction, several authors have recently attempted to express crack growth rate in terms of stress intensity factor K I = α aσ g , where a is the equivalent crack length as the sum of the initial notch length a 0 and the actual crack length a ∗ , that is, a = a 0 + a ∗ . On the other hand, it has been shown by Yokobori and Konosu that under the large scale yielding condition, the local stress distribution near the notch tip is given by the fracture mechanics parameter of aσ gƒ(σ g ), where a is the cycloidal notch length, σ g is the gross section stress and ƒ(σ g ) is a function of σ g . Furthermore, when the crack growth from the initial notch is concerned, it is more reasonable to use the effective crack length a eff taking into account of the effect of the initial notch instead of the equivalent crack length a. Thus we believe mathematical formula for the crack growth rate under high temperature creep, fatigue and creep-fatigue interaction conditions may be expressed at least in principle as function of a effσ g , σ g and temperature. In the present paper, the geometrical change of notch shape from the instant of load application was continuously observed during the tests without interruption under high temperature creep, fatigue and creep-fatigue interaction conditions. Also, the effective crack length a eff was calculated by the finite element method for the accurate estimation of local stress distribution near the tip of the crack initiated from the initial notch root. Furthermore, experimental data on crack growth rates previously obtained are analysed in terms of the parameter of a eff σ g with gross section stresses and temperatures as parameters, respectively.

Studies on crack growth rate under high temperature creep, fatigue and creep-fatigue interaction—I: On the experimental studies on crack growth rate as affected by [formula omitted] and temperature

Many experimental studies have been reported on the measurements of crack growth rate and the observation of crack growth behaviour under high temperature creep, fatigue and creep-fatigue interaction in literatures. However, many of them have been done in air atmosphere. Furthermore, in many of them the measurements of the crack growth rate have been carried out by interrupting intermittently the running of the testing machine. In such experiments the complex effects due to the atmosphere, the interruption period and the corresponding unloading operation for the crack length measurement might have been involved. In the present paper in order to eliminate such effects, series of experimental studies on the crack growth behaviour under creep, fatigue and creep-fatigue interaction conditions on 304 stainless steel have been carried out by using high temperature microscope and observing the crack length continuously during running the test without interruption in vacuum of 10 −5mm Hg. Among the results, it was found that crack growth rates on a time basis, da/dt, under high temperature creep and creep-fatigue interaction conditions can not be described in terms of solely elastic stress intensity factor k i or only net section stress σ net, both independent of gross section stress σ g. The relation between crack growth rate and stress intensity factor under high temperature fatigue condition changes with some trend according to gross section stress at lower K I level and it can be approximately described in terms of stress intensity factor K I only, at higher K I level. The threshold stress intensity factor and the threshold net section stress under high temperature creep, fatigue and creep-fatigue interaction conditions appears to be almost independent of temperature.

A simple method of constructing an Ashby-type deformation mechanism map

An Ashby-type deformation mechanism map may be considerably simplified by plotting in the form of normalized stress versus the reciprocal of the homologous temperature. In this form, the boundaries separating the various fields appear as straight lines and the constant strain rate contours may be approximated as straight lines. Representative maps are presented for conditions of high temperature creep, and a simple procedure is outlined for constructing several maps for the same material at different grain sizes.

The friction-stress concept during high-temperature creep

AbstractUnder high-temperature creep conditions the dependence of the steady-state creep rate (es) on applied stress (σ) and temperature (T) can be described by an equationes = Aon exp (−Qc/RT)………(1)where A and n are constants and Qc the activation energy for creep. For most pure metals n is ∼4.0 and Qc = QSD (activation energy for self-diffusion). Creep theories can readily account for these stress- and temperature-dependences. However, with two-phase alloys n and Qc are generally substantially higher than those for pure metals and single-phase alloys. For example, with T.D. nickel n ≃ 40 and Q c = 795 kJ/lmol compared with n = 5.0 and Q c = 276 for pure nickel. Many precipitation-hardening alloys show the same trends. No creep theories can account for these high n and Q c values. In particular, recovery-creep theories suggest maximum values of n ≃ 4.0 and Q c = Q SD.

Effect of nonstationary loading cycles on the durability of thin-walled power components in high-temperature creep conditions

Effect of nonstationary loading cycles on the durability of thin-walled power components in high-temperature creep conditions

Stress Concentration Effects in Tertiary Creep

IN most cases, fracture under high-temperature creep conditions results from the formation and linking up of small voids on the grain boundaries within the metal. A period of accelerating creep precedes the fracture and it is generally assumed that this acceleration is caused by the reduction in stress-carrying area through the formation of these voids1,2.