Fracture analysis of notched composites

Abstract

Static fracture of notched composites has drawn the attention of numerous researchers in recent years and it has been studied both experimentally and theoretically. One of the proposed theories for predicting fracture of composites is the “average stress criterion” due to Whitney and Nuismer. A similar theory based on fracture mechanics has been used by Waddoups et al. Pipes et al have recently generalized these concepts by introducing a three-parameter fracture model and a radius superposed method. In the present paper the mechanisms behind the cut-off of the stress peak at the notch are investigated using the fictitious crack model (FCM). In the intense energy region close to the notch, a fictitious crack is assumed to form when the uniaxial tensile strength is exceeded. On the surfaces of this fictitious crack, cohesive forces act. These forces reduce with increased width of the fictitious crack and vanish at a certain crack width. Hereafter the crack is considered as a real crack. The reduction of the cohesive forces can be assumed to follow various curves, three of which are shown in Fig. 1(b). A common feature for all curves is however, that the area under the curve is equal to the fracture energy ℷ c. It should also be emphasized that the fictitious crack merely represents a damaged zone in the composite than a sharp crack. Some preliminary finite element calculations performed with the fictitious crack model show good correlation with experimental results and the redistribution of stresses at increased external load is illustratively demonstrated.