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

A micromechanic model which considers the influence of damage in the form of microvoids, cavities and microcracks on the Fatigue Crack Growth Rate (FCGR) is considered. Two modes of crack growth are discussed: (i) the slip mode (Neumann type), influenced by the stress intensity factor range (Delta K) and (ii) the ''static mode'', in which the formation of new surfaces is attributed to K-max and which causes damage initiation 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 using an averaging technique that includes canceling of incomplete slip steps. It is assumed that these steps are of a microstructural characteristic length and obey the normal distribution. In the Paris regime, an 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 instability 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 the crack plane in a beehive shape, a complete da/dN curve, including near-threshold behavior, a power law dependence and an instability point (K-C), was obtained without an artificial combination of partial models. The model uses six micromechanic material constants with each constant having a definite physical meaning. Examples for two alloys demonstrate a good fit between the simulated and experimental curves.