It is well known that for very short cracks the stress intensity factor K is not a suitable parameter to estimate the stress level over the small but finite Stage II process zone activation region of size r(s) near the crack tip, within which crack growth events take place. A critical appreciation,of the reasons for the limitations on the applicability of AK as a fatigue crack propagation (FCP) parameter, when the crack length a is of the same order of magnitude or smaller than the size of the 'fatigue-fracture activation region'. As is presented. As an alternative to Delta K the range Delta sigma s of the cyclic normal stress at a point situated at the fixed distance s = r(s)/2, ahead of the crack tip, inside the fatigue-fracture activation region, is proposed. It is observed that the limitation on the use of Delta K when the crack is short, is mathematical (and not physical) but this inconvenience is easily circumvented if the stress Delta sigma s at the prescribed distance is used instead of Delta K since nowadays Delta sigma s can be obtained numerically by using finite element methods (FEM). It follows that the parameter Delta sigma s is not restricted by the mathematical limitations on AK and so it would seem that there is, a priori, no reason why the validity of the parameter Delta sigma s cannot be extended to short cracks. It is shown that if the Paris law is expressed in terms of Delta sigma s(pi r(s))(1/2) instead of Delta K the validity of the modified Paris law can be extended to short cracks. A coherent estimate of the value of the fatigue-fracture activation region r(s) is derived in terms of the fatigue limit Delta sigma(FL) obtained from S-N tests and of the threshold value Delta K-th obtained from tests on long cracks where both relate to Stage II crack growth that ends in failure, namely, r(s) = (Delta K-th/Delta sigma(FL))(2)/pi. An overall, threshold diagram is presented based on the simple criterion that, for sustained Stage II FCP, Delta sigma(s) must be greater than Delta sigma(FL). The study is based on a simple continuum mechanics approach and its purpose is the investigation of the suitability of both Delta K and Delta sigma s to characterise the crack driving force that activates complex fracture processes at the microstructure's scale. The investigation pertains to conditions that lead to the ultimate failure of the component at values of Delta sigma(s) > Delta sigma(FL).
