Appendix B.4 - Arguments #4-6: Material Stiffness

The stiffnesses of the cohesive material relate the separation of the cohesive layer to the tractions. You must supply a valid stiffness for each direction of separation in the cohesive element. The three stiffness values are Knn, Kss, and Ktt which are the stiffnesses in the local three, one, and two directions respectively. A subscript of n denotes the local 3-direction, s denotes the local 1-direction, and t denotes the local 2-direction. The traction-separation relationship is given as:

where tn, ts, and tt are the tractions in the three directions. The nominal strains in the corresponding directions are εn, εs, and εt. The nominal strains are related to the three separations (δ) using the nominal thickness of the cohesive element, To (also referred to as the initial thickness).

It is required to always use a nominal thickness of one. Note this value is explicitly defined in the cohesive layer properties. It is not necessarily the physical thickness of the cohesive element. The response of the material is linear elastic until damage is initiated. Damage initiation is predicted differently depending on the criterion selected in argument 1. See Appendix B.5 for more information on the available damage initiation criteria.