Modelling stiffness degradation due to matrix cracking in angleply composite laminates

When a multidirectional composite laminate is subjected to in plane static or fatigue tensile loading, matrix cracks parallel to the fibres appear in the off axis plies long before catastrophic failure. Matrix cracking signif icantly reduces the laminate stiffness properties and triggers development of other harmful resin dominated damage modes such as delaminations. Concur rent matrix cracking in the adjacent off axis plies is an extremely complex problem to model analytically and has been analysed mostly using finite el ement methods. The present paper is concerned with the theoretical modelling of stiffness reduction in cracked angleply [theta (1)/theta (2)](s) laminates subjected to multiaxial in plane loading. A new approach based on the equivalent cons traint model of the damaged ply and an improved two-dimensional shear lag m ethod has been applied to model matrix cracking in angleply [theta (1)/thet a (2)](s) laminates. Stresses in the cracked ply are determined from a syst em of two coupled ordinary differential equations and used to calculate the in situ damage effective functions, which describe the stiffness loss. For angleply [+/- theta](s) laminates with a cracked midlayer, it is found tha t the reduction due to matrix cracking of the laminate axial and transverse moduli is more significant than in crossply laminates, while for the shear modulus, the opposite is true. Matrix cracking in such laminates can resul t in an increase in the Poisson's ratio - a phenomenon not observed in cros sply [0/90](s) laminates. In addition, matrix cracking in angleply [+/- the ta](s) laminates introduces coupling between extension and shear.