Size effect in fracture of unidirectional composite plates

Fracture of notched, unidirectionally reinforced composite plates with well -bonded ductile matrices is typically preceded by the formation of long, di screte plastic shear zones aligned in the fiber direction. Onset of fractur e is associated with a critical tension stress in a certain small process z one ahead of the notch tip; the critical stress is often equal to the tensi le strength of the unnotched composite plate. Length of the shear zones can be estimated by plastic limit-analysis, and the local tension stress ahead of the notch found by superposition of the stress caused by remotely appli ed loads with the stress induced by the shear tractions in the plastic zone , which is shown to be dominated by a logarithmic singularity. In as-fabric ated boron-aluminum composites, this fracture mechanism was analyzed and co nfirmed by numerous experiments (G.J. Dvorak, J. Zarzour and Y. Benveniste, Engineering Fracture Mechanics 42, 501-517, 1992). Since the notch tip fie ld is not described by a stress intensity factor, experimental notched stre ngth data cannot be interpreted in terms of a single material property, suc h as toughness. An alternative scaling procedure is outlined for prediction of notched strength of wide plates on the basis of data obtained from smal l size specimens.