Analysis of composite laminates with intra- and interlaminar damage

Failure process of composite laminate under quasi-static or fatigue loading involves sequential accumulation of intra- and interlaminar damage. Matrix cracking parallel to the fibres in the off-axis plies is the first intralaminar damage mode observed. These cracks are either arrested at the interface or cause interlaminar damage (delamination) due to high interlaminar stresses at the ply interface. This paper summarises recent theoretical modelling developed by the authors on stiffness property degradation and mechanical behaviour of general symmetric laminates with off-axis ply cracks and crack-induced delaminations. Closed-form analytical expressions are derived for Mode I, Mode II and the total strain energy release rates associated with these damage modes. Dependence of strain energy release rates on crack density, delamination area and ply orientation angle in balanced and unbalanced symmetric laminates is examined and discussed. Also, stiffness degradation due to various types of damage is predicted and analysed.