A mechanical model of fatigue crack propagation. Report 2

Abstract

The proposed model of fatigue crack propagation based on the solution of the cyclic elastoplastic problem of the stress-strain state [1] makes it possible to take into account the effect of the triaxial stress state on the deformation of the material at the crack tip. The proposed algorithm of calculations of the state of damage on the basis of the principle of linear damage summation and also the agreement between the calculated and experimental data confirm the assumption on the controlling role of low-cycle damage in the mechanics of crack propagation in cyclic loading described from phenomenological positions. The main advantages of the proposed model are: The possibilities of calculating endurance in crack propagation or calculating the crack propagation rate for cases in which the variation of the range of the stress intensity factor along the crack length in structural members takes place at a variable loading asymmetry; the possibilities of describing the effect of loading asymmetry on the fatigue crack-propagation rate using only the strain criterion (Coffin's equation) since the range of the plastic strain intensity at the crack tip is, as shown in [1], a function of not only the range of the stress intensity factor ‡K but also of its maximum value Kmax; the possibilities of describing the dependence of ‡Kth on loading asymmetry based on the assumption on the constancy of the size of the structural element for the given material; the possibilities of describing the crack propagation rate in all the three sections of the dL/dN=f(‡K) diagram, starting with the values ‡K similar to ‡Kth and ending with the value of K at which monotonic quasistatic fracture becomes the controlling process.