May 4, 2022 at 10:09 pm

Sheldon Imaoka

Ansys Employee

COMBIN39 is just a node-to-node element. Thus, the node is not directly associated with an area or volume - that is why we use force vs. deflection in its definition. It is just basically a spring, like K*x=F, similar to the FE matrix equation we solve [K]{x}={F}.

When you have solid elements, for example, the integration point for the constitutive relationship is associated with a 'volume', so that is why you end up with stress vs. strain definition instead. From the constitutive relationship (stress vs. strain) and element properties, we go to a form of [K]{x}={F}. So your comment is reversed - we do not go from COMBIN39 force-deflection curve to stress vs. strain but do the opposite; everything is transformed to stiffness [K], deformation {x}, and forces {F} to solve as a matrix equation.

If you have traction vs. separation (bond-slip), did you intend to use something like a cohesive zone model? If your reinforcement is modeled as a beam element (single node describing cross-section), then the COMBIN39 is suitable, and you need to estimate the area associated with a node to convert the bond-slip relationship to force-displacement. If, however, your reinforcement is a solid, then you may want to look at the cohesive zone model instead (see

*Fracture Analysis Guide*, Sections 3.5 and 3.6 on modeling interface delamination). The cohesive zone model is defined as a traction-separation law (units of stress vs. displacement, such as MPa - mm).