Interaction between a major crack and small crack damage in aircraft sheet material

A study is made of the effect of small crack damage on the fracture tolerance of an elastic-plastic sheet material to a major crack. The study is motivated by concern for the influence of multiple-site fatigue damage in lap joints on the tolerance of aging aircraft fuselages to major cracks. Flat sheet geometries are analysed, both unreinforced and reinforced. Several analysis approaches are explored and assessed, including linear elastic fracture mechanics (LEFM), a modification of LEFM employing a damage-reduced fracture toughness, and a modification of the Dugdale model which makes use of a damage-reduced yield stress of the sheet material. An important feature of the interaction between a major crack and small crack damage is the fact that the plastic zone of the major crack engulfs at least several damage sites in geometries typical of most lap joint designs. The damage-reduced yield strength of the lap joint emerges as being central to understanding the role of damage, and simple formulas are given which indicate how damage erodes tolerance in the presence of a major crack.