Fatigue delamination growth - Is UD testing enough?

Full scale composite structures typically consist of multi-directional lay-ups. However, fatigue delamination growth experiments are usually conducted on test coupons with a unidirectional lay-up. This raises the question how to transfer results gathered from such coupons to understand and predict fatigue delamination growth in full scale structures. Is testing unidirectional coupons sufficient, or is further data needed to take the fibre orientation change across a delaminating interface into account? The present work investigated the effect of fibre orientation on fatigue driven mode I delamination growth in a carbon fibre reinforced polymer composite. Specimens with a 0//0, a 0//45, and a 0//90 interface were tested. It was found that the fibre orientation affected the delamination growth rate, as it changed the nature and strength of toughening mechanisms such as fibre bridging and crack migration. Of the tested interfaces, the 0//0 interface exhibited the fastest delamination growth rates for a given fatigue load and pre-crack length, but further investigation is necessary to confirm that a 0//0 interface will indeed reliably provide 'worst case' delamination growth data.