Removing tied surfaces

Dear all,
I'm working with tied surfaces (*TIE option) in a crack propagation analysis 
and I would like to remove this fastening between a pair of surfaces in the 
following step. Does anyone know how could I do it? Do I have to go to contact 
interactions necessarily?
Regards,
Diego González
Department of Materials
CEIT
P. Manuel Lardizabal 15
20018 San Sebastian, Spain
e-mail: dgonzalez@xxxxxxx