PSI - Issue 17

D. Camas et al. / Procedia Structural Integrity 17 (2019) 894–899 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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certain number of loads to obtain a stable and continuous enough plastic wake. In three-dimensional analysis, one load cycle is usually applied between node releases because of the numerical cost (Roychowdhury and Dodds (2003)). In this work, the crack growth scheme is analyzed, but only the influence of the number of load cycles after releasing the last set of nodes on the plasticity induced crack closure. The numerical accuracy is analyzed in terms of crack closure and opening values which are analyzed all along the thickness. For this scope, a CT specimen of aluminum has been modelled three-dimensionally and some calculations were made.

2. Finite element model

A compact tension (CT) specimen has been studied in this work. A scheme can be seen in Fig. 1. Main dimensions of the specimen for all the cases analyzed are W =50mm, a =20mm and b =3mm. The commercial engineering simulation software ANSYS has been used to develop the three-dimensional model. The geometry and the way loads are applied in this problem allows for considering symmetry boundary conditions and for modelling only a quarter of the whole specimen.

Fig. 1. CT specimen scheme.

The mesh around the crack front and where the crack grows usually is regular, having the elements the same size during the whole process. However, in this analysis, in order to save computational cost, the mesh becomes smaller as it approaches to the final crack length. Besides, the element size in this area must be small enough to determine properly the stress and strain fields which present deep gradients by the crack front. The specimen has been meshed considering two different areas. Near the crack front, a homogeneous and structured mesh with hexahedral elements have been used. The most remote areas have been meshed with an unstructured mesh with tetrahedral elements that allow an important size transition. A parametric meshed has been considered so that the size of the elements is related to the applied load. The parameter that has been considered as reference is the Dugdale’s plastic size (equation 1). = 8 ( ) 2 (1) Apart from the size along the crack growth direction, the mesh size along the thickness must be good enough to determine the stress and strain behavior along the thickness with accuracy as Camas et al. (2011), Camas et al. (2012), García-Manrique et al. (2017) and Lopez-Crespo et al. (2008) showed, specially near the surface, where the gradients are deeper.