Dynamic fragmentation of expanding ductile structures: Defect opening, stress release fronts and cohesive zone interactions

Abstract

The fragmentation of a one-dimensional expanding structure under dynamic loading conditions is analyzed, since the early work of Mott, as a competition between the activation of local defects and the inhibition of other ones by release fronts emitted when defect failure occurs. The celerity of these so-called Mott waves is computed under the assumption of immediate defect stress relaxation at failure time and it controls inhibition. However, in ductile metals, defects consist in progressively forming localized necks breaking by a process coupling plasticity and ductile failure, i.e., pore growth and coalescence. Failure is thus a non-instantaneous and dissipative process and the defect interaction problem is more complex. On the one hand, the propagation of release fronts is delayed and their celerity is a function of the effective cohesive model governing defect evolution. On the other hand, when the release fronts emitted by two neighboring defects meet as they are still opening, their subsequent evolution is driven by the interplay between the inertia of the block separating them and the evolution of the two cohesive stresses: defect unloading before complete opening occurs in some cases. The whole structure fragmentation process then involves broken, partially open (arrested necks) and inhibited defects and lead to fragment sets different from the ones predicted under the Mott assumption.