I am doing a contact Debonding analysis in ansys. I have a matrix and a fiber that i am pulling it by appling a force.

The result are coming but the force is not increasing from zero value and it starts with a value of around 160 N then goes to 280N and then suddenly becomes zero.

The problem is initially it should follow a linear patter. I changed the force reaction also to displacement but it does not work. Any suggestions?