As shown in above figure, a small bar is embedded in a large one. Assume a thin layer glue is attached between two bars. I try to add cohesive elements between them in order to simulate what circumstance would cause debonding behavior.

There's a function to insert cohesive seams in mesh module. But it fail if I select entire external surface (6 faces) of the inside bar. Error message states that Abaqus/CAE could not determine crack boundaries. However, other type of selection is available. For example, select any 4 or 5 surfaces of inner bar can create cohesive elements successfully.

Is there any guideline to use this function because there's no much information in documentation. Thanks a lot.