Initiation of free-edge delamination in composite laminates

Edge delamination onset on composite laminate was investigated for a carbon/epoxy G947/M18 composite material. On the edge of laminates, out-of-plane stresses arise, even up to material failure. Layer thickness is also known as influencing delamination onset stress. Making use of (i) a conventional model assuming plies homogeneity, damageable elastic behaviour, plane interlaminar surface also infinite interfacial stiffness and (ii) a local stress tensor correction near the edge, allows an efficient calculation of the full stress tensor, thanks to an asymptotic method. Then, a stress criterion was studied. The assessment of criterion parameters from test results was based on conjugate gradient method and experimental thickness effect. Edge delamination tests were performed on several specimens with various lay-ups and loaded under interlaminar shear, tension and compression. Acoustic emission was used to detect delamination initiation. As expected, these tests have exhibited layer thickness influence on onset stress. The assessment of shear and tension parameters shows good agreement between theory and experimental results. But the influence of interlaminar compressive stresses seems insufficiently taken into account in the criterion formulation.