Nbi min = Ci

(1)

where N_b is the number of cycles to fracture, _{i min} is the minimum creep rate intensity and C_i is the parameter which in the general case depends upon the material deformability properties but is independent of the cycle stress level and is a constant at developed the given principal stress ratio.

On the basis of the elastic viscoplastic model of the material deformation developed at the Institute for Problems of Strength and a large body of experimental data on viscoplastic behaviour of structural materials of different classes (heat-resistant steels 15Kh2MFA, 15Kh2IMFA, 106I2MFA, metastable steels 08Kh18N10T-VD, VNZ-25, titanium alloys VT-6S and VT-20) the above approach was further developed to the form applicable for the theoretical engineering calculations.

Here, the following experimentally justified prerequisites are used:

• when the material is being deformed, a condition is met of the existence of the stress-strain diagram in reduced stresses and strains which is invariant to the stress type;

• the shape of the materials stress-strain diagram under static loading, as well as of the cyclic creep curve is determined by the intensity of the simultaneous development of competing processes of the material strain hardening and the reduction in the load carrying capacity of a structural element (specimen) being loaded due to a change in its geometry and shape during plastic deformation;

• creep is considered as a process associated with the relaxation of local nonequilibrium stresses due to the rate effect of the preceding deformation;

• the magnitude of the parameter C_i is determined by the evolution rate of the above competing processes of the material hardening and the reduction in the load carrying capacity of the structural element under study.

On the basis of eqn. (1) considering the above hypotheses we get the following equation:

(2)

where a and b are the material constants at the given temperature _imax; is the maximum stress intensity in a cycle; is the intensity of stress corresponding to the transition from a quasistatic to fatigue fracture mechanism calculated on the basis of the material reference data.

A complex experimental verification of the developed criterion performed on tubular specimens under repeating loading at a complex stress state

confirmed the validity of the proposed computational and experimental method for all the materials studied.