Initiation

In this section, we also remain in a 2d setting, but will explicitly mention those results that do not generalize to dimension 3.

The mechanics community is of two minds when it comes to crack initiation, or the lack thereof. It claims loud and clear that crack initiation is not within the purview of fracture, because the onset of the cracking process is impurity or imperfection related, yet it relentlessly seeks to predict crack initiation, appealing to extraneous ingredients. Such is one of the motivations of the theory of damage (Lemaître and Chaboche, 1985) which, in essence, substitutes damaged areas for cracks. The thickness of the damaged area – the “process zone” – is controlled by a damage parameter which is in turn assumed to follow an a priori postulated evolution law. A careful tailoring of the damage parameters permits one to control the thickness of the process zone and to collapse it, in the limit, into cracks. The associated phenomenology is however troublesome: What do the damage variables represent and why do they care to follow the postulated evolution laws?