First you need to understand how fatigue cracks in real materials advance through the microstructure, and that is complex. See http://youtu.be/yiUrhwTsbd4.
i am working on fatigue crack propagtion too. i use ansys and i have generated o code but in montonic loading by using X-FEM so i trie do to cyclic loading.
i can send to you the code. and i am pleased to work together.