Calculation of stress intensity factors for interlaminar cracks in composite laminates

Owing to the oscillatory characteristics of stresses and displacements near the crack tip, individual strain-energy release rates for bimaterial interface cracks no longer exist. Strain-energy release rates for finite crack extension, Delta a, are introduced. For interface cracks lying between two monoclinic media, the explicit quadratic relationships between stress intensity factors and these finite-extension strain-energy release rates are established. Very accurate stress intensity factors can be obtained by calculating these finite-extension strain-energy release rates in the finite element analysis and by using these quadratic relationships. An alternative and efficient method based on near-tip crack surface displacement ratio is also introduced to obtain these stress intensity factors. An infinite 'two-ply' graphite/epoxy laminate under remote uniform tension and an edge-delaminated symmetric [theta/ - theta/ - theta/theta] graphite/epoxy laminate subjected to uniform axial extension are selected for numerical investigation. (C) 1997 Elsevier Science Limited.