I am doing hydraulic fracturing using cohesive element and use MAXS for damage initiation criterion. The manual says MAXS is maximum nominal stress. I am not sure how to set this value. For example, if my initial stress is -6e6 (6MPa)and rock tensile strength is zero. And if I set positive 2e6 Pa in the first value(normal direction) in below line, there are things I'd like to have clarification: Thanks for your help.

1. The positive value means tension?

2. MAXS means maximum tensile stress at cohesive element?

3. If my rock has zero tensile strength, do I set the first value to be zero ( rock initially under compression with compression stress of -6MPa, once it is in tension, it is damaged?)  If I set it to zero, the error says The maximum normal traction at failure must be greater than zero

*Damage Initiation, criterion=MAXS
2e+06, 1e+07, 1e+07

Data lines to specify damage initiation for CRITERION=QUADS or CRITERION=MAXS without the RATE DEPENDENT parameter 

First line

1. Maximum nominal stress in the normal-only mode.

2. Maximum nominal stress in the first shear direction (for a mode that involves separation only in this direction).

3. Maximum nominal stress in the second shear direction (for a mode that involves separation only in this direction).

4. Temperature, if temperature dependent.

5. First field variable.

6. Second field variable.

7. Etc., up to four field variables.