Damage initiation and growth in laminates with ply drop-offs under quasi-static and fatigue loading

The thickness of a laminated composite structure can be varied by reducing the number of plies through the thickness of the composite material (i.e. using ply drop-offs), where one or more plies are terminated prematurely at some position within the composite, or at the surface. One of the disadvantages of using ply drop-offs is the likelihood of stress concentrations at the terminations of the dropped plies, as these stress concentrations frequently lead to the premature development of damage. In this work, composite coupons with a double ply drop-off have been fabricated using out-of-autoclave CFRP. Coupons cut from the panels have been subjected to quasi-static and fatigue loading and the development of damage has been monitored using microscopy, and in some coupons, by using a chirped fibre Bragg grating (CFBG) sensor. The first damage appeared as resin cracks within the resin pocket associated with the ply drop-off and this damage developed subsequently into delaminations. Under quasi-static loading, the delaminations grew stably within the ply drop-off region with increasing load; under fatigue loading, the delaminations grew with a constant growth rate. The paper will present observations on the initiation and growth of the damage.