A recommended good practice is that the first row of elements around the crack tip have a radius of approximately a/8 or smaller where a is the crack length. This recommendation assumes that the first row of elements have a triangular or wedge shape but is still a reasonable recommendation for the edge length of rectangular or hexahedral elements. In addition to a refined mesh with well-shaped elements 3D models should have structured hexahedral meshes in the crack region. If that type of mesh is not possible you should invoke the unstructured mesh method (UMM). It is important the crack front as well as the whole crack region to have a refined mesh with well-shaped elements to obtain high quality fracture mechanics results. That's because fracture mechanics evaluations require higher quality elements than basic structural evaluations.
You need sufficient divisions to ensure a refined mesh with well-shaped elements along the entire crack front. The number of elements along the crack front is dependent on the size of the elements around the crack tip. It is recommended that aspect ratio is kept at a minimum, less than 4:1, in all directions but if possible lower aspect ratios should be used. For cracks with radius ratio greater than 5:1 will just require a more refined mesh than cracks with smaller radius ratios.
For more information, check the Fracture Analysis Guide on Ansys Help site following the link: