Fracture in Laminated Materials

The effects of weak interfaces in laminated materials are examined in terms of the triaxial tensions operative at a crack tip. Depending on the orientation of the interface relative to the crack, the triaxial tensions may cause distinct types of delamination during crack propagation. The crack may be arrested by delamination and may have to be re-initiated under conditions of uniaxial tension. Alternatively the crack may be divided by delamination into a series of parallel cracks travelling independently in several layers. If these layers are sufficiently thin, a triaxial stress system cannot be supported, and the cleavage mode of fracture may be suppressed. These two basic laminate geometries are discussed with reference to the mode of crack propagation in structural steels and high-strength composites with a variety of bond strengths. The principles of laminar composites can be applied also to materials that are inherently brittle, for example, those which do not possess a sufficient number of independent slip systems to exhibit ductile behavior. Results of work on zinc aluminum laminates are used to illustrate this effect where potential stress concentrations in the hexagonal-close-packed layers can be relieved in the face-centered-cubic layers because of the abundance of slip systems. The authors describe techniques that have been used to examine the fracture properties of laminates, and finally examples of natural laminates are reviewed where fracture resistance is increased due to decohesion of the interphase interfaces in the fracture path.