A Criterion for Splitting Crack Initiation in Unidirectional Fiber-Reinforced Composites

Abstract

Unidirectional fiber-remforced composites exhibit a high degree of anisot ropy in strength. Crack initiation from a pre-existing crack almost always occurs along the fiber direction; it is known as the splitting crack. One criterion for the splitting crack initi ation under mixed mode condition is proposed. In this criterion, the splitting crack initia tion is characterized by a unique interaction curve on the K f σ -K f τ plane, where K f σ and K f τ are the tensile and shear stress intensity factors along the fiber direction. Fracture toughness tests under mixed mode loading are systematically carried out on two graphite-epoxy systems: AS4/3501-6 and IM7/85517. For both composites, the condi tion of the splitting crack initiation can be characterized by a vertical line of K f σ = con stant on the K f σ -K f τ plane. This means that the tensile stress intensity factor along the fiber direction is responsible for the splitting crack initiation. Furthermore, it is found that under the conditions of K f σ = 0 or K f σ < 0, damage occurs at locations away from the tip of the pre-existing crack before crack initiation and that the fracture toughness under such conditions is high.