Reg-element deletion

Dear Friends.

i am working on machining simulation.

when the element reaches the ultimate stress it should remove(chip flow) from part.

i used in input file

*shear failiure,type=johnson cook,Element deletion=yes

but still the removal is not good 

so what can i do.

and if need to erite usersubroutine in VUMAT

please give me some information about that

if any body having usersoubroutin for element deletion case pease send me

please please

Mani

IIT delhi

India