PSI - Issue 39

Dmitry O. Reznikov / Procedia Structural Integrity 39 (2022) 236–246 Author name / Structural Integrity Procedia 00 (2019) 000–000

239

4

Fig. 1. Influence of overload on the rate of fatigue crack growth CA is a constant amplitude loading mode; OL is a constant amplitude loading with a single overload, UL is a constant amplitude loading with a single underload The traditional models that were developed to describe fatigue crack growth under constant amplitude loading modes are commonly used to describe the kinetics of fatigue crack under variable loading conditions ( , ) da f K R dN   , (1) with the initial condition: for N = 0: a=a 0 (2) where a is the characteristic size of the crack (for example, its depth), a 0 is the initial size of the crack; N is the number of loading cycles; R = S min / S max is the stress ratio; S min and S max are the minimum and maximum values of the first principal stress in the loading cycle, K = YS √π a is the stress intensity factor , Y is the correction function for the geometry and loading pattern, Δ K = K max - K min is the range of the stress intensity factor in the loading cycle. In particular, the modified Paris equation, that accounts for the stress ratio, can be used:

m

da dN

1 R          C K

(3)

where C and m are constants depending on the material and loading conditions.

c)

b)

a)

Fig. 2. Model of plastic zones at the crack tip during its retardation after overload 1 - plastic zone induce by the overload, 2 - plastic zone induced by the current CA loading cycle a) the beginning of the retardation effect, b) crack propagation in the retardation zone, c) the end of the retardation effect