Issue 31

R. Citarella et alii, Frattura ed Integrità Strutturale, 31 (2015) 138-147; DOI: 10.3221/IGF-ESIS.31.11 139 In [1], experimental and numerical results of fatigue crack growth for a crack starting from a straight-fronted edge notch in an elastic bar under axial loading with or without superimposed cyclic torsion are given and the influence of different loading conditions on fatigue life of cylindrical specimens is discussed. The relations between crack opening displacement and the crack length measured on the free specimen surface are obtained, and it is shown that the growth of the crack fronts is dependent on the initial notch depth. Using the aforementioned relations, the crack front shape and crack growth rate in the depth direction can be predicted. The numerical simulations in [4] are based on the Dual Boundary Element Method [2-3] whereas, in this paper, the same calculations are performed based on the Finite Element Method (FEM). In the past a comparison between FEM and DBEM results on this kind of problems was already attempted but considering separately the two loading conditions [4-5]; now the comparison is extended in case of simultaneous application of the torsion and traction fatigue loads. The computational 3D fracture analyses deliver variable mixed mode conditions along the crack front. F RACTURE MECHANICS SIMULATIONS BY ZENCRACK Introduction he numerical studies are based on finite element (FE) analyses using the adaptive remeshing approach. This study employs ZENCRACK [6-8] for automated 3D remeshing and crack propagation calculations along with ABAQUS [9] as the finite element solver. ZENCRACK is a 3D crack analysis tool able to read in an uncracked finite element model and to produce a cracked finite element model. Stress intensity factors are calculated automatically from the results of the cracked finite element analysis. Furthermore, crack growth can be undertaken by extending the crack position. An updated finite element model is then created and run to simulate crack growth (Fig. 1). USER INPUT (FE mesh of uncracked comp.) USER INPUT (crack location, size) ZENCRACK (Creates mesh of crack comp.) FE CODE (Analysis) ZENCRACK (Evaluation of crack growth and update FE mesh) ZENCRACK (Next FE analyses) STOP No Yes Figure 1 : Flow chart for crack growth prediction analysis. Crack growth criteria In order to predict linear elastic fracture mechanics (LEFM) crack growth using FE method, three basic parameters are required: stress intensity factors (SIF), crack propagation direction (CPD) and crack growth material models. There are several approaches to calculating Stress Intensity Factors (SIF’s) such as: the crack tip opening displacement (CTOD) approach [5], the crack tip stress field approach [10] and the SIF extraction method from J-integral [5]. Using the crack opening displacement approach the SIF values can be obtained as follows:     2 2 4 1 p p I b b E K u u r              (1a)     2 2 4 1 p p II n n E K u u r              (1b)     2 2 4 1 p p III n n E K u u r              (1c) T