Issue 31

R. Citarella et alii, Frattura ed Integrità Strutturale, 31 (2015) 138-147; DOI: 10.3221/IGF-ESIS.31.11 141 component. The method works by replacing one or more elements in the uncracked mesh by crack-blocks that contain sections of crack front. Two types of crack-blocks are available: A - Standard crack-blocks. - The standard crack-blocks reduce to a single element on their back faces and merge with the rest of the mesh via shared nodal numbers. - The crack-blocks are designed to replace elements in the mesh by updating element connectivities and node numbers (Fig. 3). - The crack-blocks consist of “through” and “quarter circular” crack blocks. B - Large crack-blocks. - The large crack-blocks contain multiple nodes on their back faces and are used with surface-based tying to connect them to the surrounding (dissimilar) mesh. - The crack-blocks consist of “through” and “quarter circular” crack blocks. (a) (b) Figure 3 : Standard crack-block mesh (a) and detail of the crack-block (b) . (a) (b) (c) Figure 4 : Large crack-block mesh (a) , detail of un-meshed large crack-block (b) and meshed crack-block (c) . The crack-blocks have a varying number of “rings” of elements around the crack front. The innermost ring contains “collapsed” elements to represent the singularity in the stress and strain field at the crack front. A full control of the nodes along the crack front and the radial nodes closest to the crack front allows to reproduce a singularity best suited to LEFM or EPFM. Although the crack-blocks are referenced as “quarter circular” or “through” crack blocks, the user has control of the initial crack front shape which may be defined by fitting a spline through a series of points for the greatest flexibility in definition. Loading (e.g. pressure load) and boundary conditions are updated as the crack blocks are incorporated into a mesh. By processing fracture mechanics parameters from a cracked mesh and adding a crack growth algorithm, it is possible to carry out automatic crack growth prediction. This introduces a number of additional challenges. In order to obtain best results from the mesh within the crack-blocks, their outer boundaries must be manipulated to reduce internal element distortion as much as possible. Further, to allow a crack to develop through a model, it must be possible to allow crack-blocks to transfer from one position to another. This requires manipulation of the elements outside the crack