Issue 49

B. El-Hadi et alii, Frattura ed Integrità Strutturale, 49 (2019) 547-556; DOI: 10.3221/IGF-ESIS.49.51 550 where C and m are properties constant for a material regrouped in tab.3, ΔK=K max -K min is the SIF range, N is number of cycles, a is crack length and da is change in crack length. Figure 3 Finite element model mesh of the repaired plate. (a) t=2.29 mm, (b) t=6.35 mm and (c) near crack mesh refinement. Plate thickness 2.29 (mm) Plate thickness 6.35 (mm) m=3.2828 m=4.224 C=3.63e-13 C=1.51e-15 Table 3 : Material constants in Paris law for aluminum plates [21]. The adopted method was to evaluate SIFs with ABAQUS using J-integral. The energy approach states that fracture will occur when the energy release rate, reaches a critical value G c , and is consistent with the idea that failure occurs when the SIF in mode I, K I , reaches a critical value, called the fracture toughness (K IC ). For linear elastic material, J (equivalent to G) is related to the SIF by [29]:   2 2 2 * * 1 1 2 J G G G K K K E G             (2) where I, II and III denotes the modes of fracture, and E*=E / (1-  2 ) for plane strain case (3D configuration). The J- integral in pure mode I loading condition, can then be correlated to KI using the relation: 2 1 I EJ K v   (3) where E is Young’s modulus and  is the Poisson’s ratio. If an arbitrary contour is considered as illustrated in Fig.4, the J-integral is given by: i i u J wdy T ds x       (4) where w is the strain energy density, T i are components of the traction vector, u i are the displacement vector components, and ds is a length increment along the contour Γ . The strain energy is defined as: