Decoupling fracture modes in non-standard test specimens: state of the art

We call here as non-standard an interfacial fracture specimen that features an asymmetry w.r.t. the crack plane (e.g., bimaterial joint). Such specimens generally undergo mixed-mode (I/II) fracture even if they are loaded in pure mode (I or II). Aiming, however, to characterize the pure-mode fracture of them, several attempts have been made to decouple mode I and mode II. Ouyang et al. [1] focused on the bimaterial case. They stated that mode decoupling is achieved when the differential equation of the mode I (mode II) fracture is only governed by the interfacial normal (shear) stress and relative transverse (axial) displacement. A similar statement was recently made by Bennati et al. [2], who provided a more general decoupling condition covering the case where both adherents feature bending-extension coupling. The same topic was also investigated by Maimí et al. [3], who provided a different decoupling condition from that by Bennati et al. Wang et al.’s [4] “strain-based” decoupling condition is the same with that by Ouyang et al. based on an assumption (i.e., matching the axial strains of the two adherents) simpler than solving the mathematical problem. In parallel, individual authors sometimes adopt different decoupling criteria (e.g., matching the bending rigidities of the two adherents), usually without justifying their choice, though. The present work brings together and reviews the scattered—and sometimes overlapping—contributions, aiming to elucidate the confusion observed.