The normal contact force is not allowed to become larger then cohesion in tension (positive values of ). If it would become larger, then the contact is broken, a gap is assumed and the contact force is put to 0. To have really a positive for extension of the contact spring, the order of the two nodes as specified in the element record for the contact spring should be correct.
Notice that when you use control_mesh_generate_contactspring to obtain the contact spring elements, you are not sure what the first and what the second node of an element will be, and thus you should not use this group_contactspring_cohesion record. Otherwise, it is not important what you use as first and second node, so that control_mesh_generate_contactspring can be used safely.
If this group_contactspring_cohesion is not specified, infinite cohesion is assumed.
The index specifies the element_group, see element_group.