Microplasticity initiation of aluminium matrix alloy elastically stressed in pure mode III

A new theoretical analysis of pre-cracked aluminium alloy tested in pure mode III deformation is presented. The expressions obtained define the stresses developed around the crack tip as a function of the pre-crack length α and give the possibility of determining the extent of the plastic zome with respect to the crack tip base line. For an applied stress not exceeding the yield point, the theoretically predicted dimensions of the plastic zone of 0.5α is well established experimentally by microstructure deformation measurements giving values ranging from 0.2α ± 0.005α to 0.6α ± 0.005α. The spreading of the plastic zone from the crack tip base line outwards into the surrounding material is due to the existence of high deformation paths comprising slipping within the grains and even entire blocks. The high stacking fault energy of the aluminium structure, together with precipitates of the second phase which act as perfect dislocation obstacles, give rise to the activation of numerous crystallographic slip systems 〈uvw〉{hhh}. The result is a region of deformation at very low applied stresses in a plastic zone of spherical shape around the crack tip. Favourable grain configuration and orientation of boundaries increases the amount of deformation, and associated with this is the existence of microcrack formation further enhancing the deformability.