Dynamic fracture of graphite/epoxy composites stiffened by buffer strips: An experimental study

Fracture responses of unidirectional graphite/epoxy composite coupons enhanced by buffer strips are investigated under impact loading conditions using digital image correlation technique and high-speed photography. Composite coupons made of phenylethynyl terminated imide oligomer (PETI-5) as matrix and IM7 graphite fiber as reinforcement are studied. Buffer strips are made of the same material but with a different stacking sequence to attain quasi-isotropy. Edge-notched coupons are subjected to impact loading along the axis of symmetry. The effectiveness of methods used for attaching the buffer strip, namely, co-curing at elevated temperatures and adhesive bonding at room temperature, are also examined. The optically measured stress intensity factor histories reveal that both methods provide nearly identical fracture responses. However, the crack initiates much later in coupons stiffened using adhesive bonding method than its co-cured counterpart and thus shows higher stress intensity factor at initiation. The residual stresses are shown to be responsible for the difference in the fracture responses.