EFFECT OF STATIC-MODE ON FATIGUE-CRACK GROWTH BY A UNIFIED MICROMECHANIC MODEL

A micromechanic model which considers the influence of damage in the f orm of microvoids, cavities and microcracks on the Fatigue Crack Growt h Rate (FCGR) is considered. Two modes of crack growth are discussed: (i) the slip mode (Neumann type), influenced by the stress intensity f actor range (Delta K) and (ii) the ''static mode'', in which the forma tion of new surfaces is attributed to K-max and which causes damage in itiation and growth in the process zone. Initiation of damage results from a statistical strength distribution of material elements whereas the damage growth is described as a probabilistic process in which the local stress concentration causes further breakage of the neighboring elements. The FCGR curve in the near threshold region is modelled usi ng an averaging technique that includes canceling of incomplete slip s teps. It is assumed that these steps are of a microstructural characte ristic length and obey the normal distribution. In the Paris regime, a n increase in the static mode influence causes an acceleration in the FCGR and leads to a continuous increase in the Paris exponent (m) from 2, in the case of pure slip mode, to m approximate to 4. The instabil ity at K = K-max ensues from the accumulation of a critical amount of damage ahead of the tip. Using the proposed model, where the material is represented by a field of unidirectional elements distributed in th e crack plane in a beehive shape, a complete da/dN curve, including ne ar-threshold behavior, a power law dependence and an instability point (K-C), was obtained without an artificial combination of partial mode ls. The model uses six micromechanic material constants with each cons tant having a definite physical meaning. Examples for two alloys demon strate a good fit between the simulated and experimental curves.