Issue34

G Meneghetti et alii, Frattura ed Integrità Strutturale, 34 (2015) 109-115; DOI: 10.3221/IGF-ESIS.34.11 110 [3,4]. In plane problems, the mode I and mode II NSIFs for sharp V-notches, which quantify the intensity of the asymptotic stress distributions in the close neighbourhood of the notch tip, can be expressed by means of the Gross and Mendelson’s definitions [5]:   1 1 1 0 0 2 lim r K r                (1)   2 1 2 0 0 2 lim r r K r                (2)    rr  r     notch bisector r A=R 0 2   R 0 Figure 1 : (a) Polar coordinate system centred at the notch tip. (b) Control volume (area) of radius R 0 surrounding the V-notch tip. where (r,θ) is a polar coordinate system centred at the notch tip (Fig. 1a), σ θθ and τ rθ are the stress components according to the coordinate system and λ 1 and λ 2 are the mode I and mode II first eigenvalues in William’s equations [6], respectively. The condition θ = 0 characterizes all points of the notch bisector line. When the V-notch angle 2α is equal to zero, λ 1 and λ 2 equal 0.5 and K 1 and K 2 match the conventional stress intensity factors of a crack, K I and K II, according to the Linear Elastic Fracture Mechanics (LEFM). The main practical disadvantage in the application of the NSIF-based approach is that very refined meshes are needed to calculate the NSIFs by means of definitions (1) and (2). The modelling procedure becomes particularly time-consuming for components that cannot be analysed by means of two-dimensional models. Recently, Nisitani and Teranishi [7,8] presented a new numerical procedure suitable for estimating K I for a crack emanating from an ellipsoidal cavity. Such a procedure is based on the usefulness of the linear elastic stress σ peak calculated at the crack tip by means of FE analyses characterized by a mesh pattern having a constant element size. In particular Nisitani and Teranishi [7,8] were able to show that the ratio K I /σ peak depends only on the element size, so that the σ peak value can be used to rapidly estimate K I , provided that the adopted mesh pattern has been previously calibrated on geometries for which the exact value of K I is known. This approach has been theoretically justified and extended also to sharp V-shaped notches subject to mode I loading [9] giving rise to the so-called Peak Stress Method (PSM), which can be regarded as an approximate FE-based method to estimate the NSIFs. Later on, the PSM has been extended to cracks subjected to mode I as well as mode II stresses [10]. The element size required to evaluate K 1 and K 2 from σ peak and τ peak , respectively, is several orders of magnitude greater than that required to directly evaluate the local stress field. The second advantage of using σ peak and τ peak is that only a single stress value is sufficient to estimate K 1 and K 2 , respectively, instead of a number of stress-distance FE data, as usually made by applying definitions (1) and (2). Since the units of the mode I and mode II NSIFs, K 1 and K 2 , depend on the notch opening angle, a direct comparison of the NSIF values cannot be performed. This problem was overcome by Lazzarin and Zambardi [11], who proposed to use the total elastic strain energy density (SED) averaged over a sector of radius R 0 (Fig. 1b) for static [11-14] and fatigue [11,15,16] strength assessments. With reference to plane strain conditions, the SED value can be evaluated as follows: 1 2 2 2 1 1 2 2 1 1 0 0 e K e K W E R E R                   (3) where e 1 and e 2 [11] are two parameters which depend on the notch opening angle 2α and the Poisson’s ratio ν. In principle, Eq. (3) is valid when the influence of higher order, non-singular terms can be neglected inside the control volume. In the case of short cracks or thin welded lap joints, for example, the T-stress must be included in the local SED evaluation [17].