A CRITERION FOR SPLITTING CRACK INITIATION IN UNIDIRECTIONAL FIBER-REINFORCED COMPOSITES

Unidirectional fiber-reinforced composites exhibit a high degree of an isotropy in strength. Crack initiation from a pre-existing crack almos t always occurs along the fiber direction; it is known as the splittin g crack. One criterion for the splitting crack initiation under mixed mode condition is proposed. In this criterion, the splitting crack ini tiation is characterized by a unique interaction curve on the K(sigma) f-K(tau)f plane, where K(sigma)f and K(tau)f are the tensile and shear stress intensity factors along the fiber direction. Fracture toughnes s tests under mixed mode loading are systematically carried out on two graphite-epoxy systems: AS4/3501-6 and IM7/8551-7. For both composite s, the condition of the splitting crack initiation can be characterize d by a vertical line of K(sigma)f = constant on the K(sigma)f-K(tau)f plane. This means that the tensile stress intensity factor along the f iber direction is responsible for the splitting crack initiation. Furt hermore, it is found that under the conditions of K(sigma)f = 0 or K(s igma)f < 0, damage occurs at locations away from the tip of the pre-ex isting crack before crack initiation and that the fracture toughness u nder such conditions is high.