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E. Maggiolini et alii, Frattura ed Integrità Strutturale, 25 (2013) 117-123 ; DOI: 10.3221/IGF-ESIS.25.17 118 hand, it is difficult and awkward when the maximum stress or the crack initiation and propagation is outside the bisector or generally when the component is three dimensional or geometrically complex. The implicit gradient method reinforces the idea that the damage should be related to the average of the stress components occurring on the body, where the values near to the critical point are more important than the far away field (by a weight function). The computed effective stress is representative of the overall damage in the process zone. The influence zone dimension is simply regulated by material properties and is indicated by the length c . In a uniaxial fatigue case, the authors proposed to only average the first principal stress; for multi-axial fatigue it is necessary to use a multiaxial criterion, for instance, by using stress invariants or critical plane approaches, it is possible to define the ratio ρ between the hydrostatic component and the deviatoric component in the tension field averaging [5]. M ATHEMATICS OF THE PROBLEM n a body of volume V, it is possible to define a non-local effective tension σ eff in a generic point X as an integral average of an equivalent local tension σ eq , weighted by a Gaussian function ψ (x,y) depending on the distance between points x and y of the body:         eq V eff ,int eq v V ψ x,y σ (y)dV 1 (x) ψ x,y σ y dy  Vr(x) ψ x,y dV       in V (1) where ψ = 2 2 x y 2L 2 e 2L   [mm -2 ] L c 2  [mm] (2) By approximating Eq. (1), it is possible to define an effective stress σ eff by the Helmholtz equation [1, 2, 6] using Neuman boundary conditions ( eff n 0     ) [6]. 2 2 eff ,IG eff ,IG eq c       in V (3) here σ eq is the first principal stress and c is a material coefficient (for instance, it is 0.2 mm for weldable construction steel). Figure 1: 2D geometry with a variable angle, size in mm. A NALYSIS BY SOFTWARE WITH A BUILT - IN PDE SOLVER he initial investigated geometry is a simple 2D geometry (Fig. 1), with the under linear elastic plane stress condition and remote tensile stress equal to one. The geometry has a notch-opening angle ranging from 0° to 180° and a notch tip radius varying from 0 to 1 mm, the initially investigated parameter being the null radius. I T