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[FAQ]

If one looks at the bilinear law, it is a triangle, and hence three points are needed to define this triangle (traction vs. separation law): 1. Peak stress at which damage initiates (sigma^max or T^max) 2. Displacement (separation) at which this occurs 3. Displacement (separation) where damage fully occurs (complete separation) In terms of input, one will notice that ANSYS has bilinear laws for INTER20x elements as well as CONTA17x elements. There are different types of inputs for each. Usually, item #1 is input directly for any of the bilinear models. This is directly related to the peak stress (one would usually measure force in test, so convert that to stress by dividing by area) obtained by test. For item #2, this is either defined by (a) contact stiffness KN (or KT) for contact elements or (b) ratio of #2 vs. #3 for INTER20x elements. The idea with this value is that it defines the ‘elastic’ slope – i.e., at what separation (displacement) value is the behavior still elastic. For item #3, input the separation distance or fracture energy density. The fracture energy density is the area under the triangle. This is harder to estimate, but if the test data provides the value of G, then convert that for input here. This is done for both normal direction and tangential direction. ANSYS usually assume that the elastic stiffness (item #2 above) is similar for normal and tangential directions, so one end up with 5-6 coefficients to input for the CZM model.