plasticity induced fatigue crack closure

hi friends am doin my my project on" FEA of plasticity induced crack closure". my specimen size is 80*80 mm. by using symmetry condition i modelled 1/4th of model in ANSYS. therfore i created 40*40 mm plate. with initial crack length of 4mm.means its MT specimen of 80*80mm with 8 mm crack length, by symmetry condition i modelled 40*40 with 4mm crack lenth. for  this condition i have to apply cyclic loading of maximum stress and minimum stress and i have to find stress value at which the crack opens means i have to apply cyclic loading between maximum and minimum stress and stress at which crack opens have to be find out and crack opens means then the node have to be released at that particular node i.e meaning that crack propagates. and then at next node again cyclic loading have to be applied between maximum and minimum stress and again cyclic loadin have to be applied .cyclic loading have to be modeled by using ANSYS (APDL). am havin no idea in APDL am just familiar in ANSYS GUI. please just give me sugggestions.  my mail id isjohnsmart1987@gmail.com.  please send me some APDL coding related to fatigue crack propagation for some model . or send some coding model for fatigue crack propagation based on APDL........................plzzzzzzzzzzz i need ur help .am just helpless in doin my project