For notches, the energy release rate is a function of crack size increment and it vanishes when the crack size increment approaches to zero. Further, the value of the critical notch stress intensity factor becomes dependent on the notch geometry. So does its unit. General criteria that link the macroscopic toughness for different notch angles with intrinsic microscopic parameters have been discussed in the literature [2,3], and no solution was provided. In this paper, we postulate that a small size cohesive zone exists in front of the notch tip. Based on a simple Barenblatt’s cohesive zone model for brittle fracture, we obtained a relation between the macroscopic notch stress intensity factor (notch toughness) and materials microscopic surface separation work. With this relation, a unified failure criterion for both cracks and notches is obtained. Figure 1: a) Notch fracture problem, b) cohesive zone driving force, c) cohesive zone resistance and d) a cohesive law. COHESIVE ZONE MODEL SOLUTION For isotropic elastic materials with traction free notch edges and mode I loading, the stress and displacement fields close to the notch tip can be written [6]: ( ) ( ) 1 θ θ σ λ λ Ni N i Nij N ij u K r h K r g (1) l ( ) N ij T K α β b) r ( )N ij T K x y β a) α θ c) CZ σ 0 σ u2 Separation CZ σ d)
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