Convergence problem of cohesive element in abaqus

Hello, 

Would there will be anyone like to share the experiences of using cohesive element in Abaqus. Recently I come across some trick proplems while using the cohesive element with bilinear cohesive law to model the slide of interface.

First,I don't know if there is a exact meaning of the stiffness, but it seems that it is related to the convergence. After many times of trial, I find I can only get a convergent result by using a couple of values. Nevertheless, I don't know the meaning of the stiffness.

Secondly, convergence sames also related to the increament of time step, I can get a result by using a large increment by cannot get the result by a smaller time increment.

 Third, about the tie constraint in abaqus. We know there are two ways of connecting cohesive element with other element in the model. One is by sharing node, one is by tie constraint.   But I cannot get a same result by using these two method respectively. For example,the results of strain distribution is a bit higher by using tie constraint than by sharing nodes. Would there be anybody tell me the possible reason.