Fracture Toughness for Transverse Loading and a New Model of Crack Initiation in Unidirectional Fiber Reinforced Composites

Abstract

Fracture toughness tests were carried out to clarify the effects of notch root radius, notch length and specimen thickness on the fracture toughness for transverse loading of unidirectional fiber reinforced plastics. As a result, it was found that an increase of notch root radius led to higher fracture toughness, while the fracture toughness was almost constant with the change of notch length or specimen thickness. In view of the observation that the fiber-matrix interfaces debonded first in the vicinity of the notch tip and then the macroscopic crack propagated partly through the debonding interfaces and partly through the resin, a model of crack initiation of the transverse cracking was proposed. By use of the model, the relation between fracture toughness and notch root radius was explained concisely. Furthermore, it was shown that the fracture toughness of the specimen with a crack, i.e. the minimum fracture toughness, was able to be estimated from that of the specimen with the notch.