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CONTACT in ABAQUS: HARD and SOFT contact techniques
I am using Abaqus for nonlinear FEA of contact between material1 (soft foam like) and material2 (hard plastic type) in vertebrae for my thesis. I have encountered many problems during this exercise, because I have to include the results of various contact conditions in this situation with friction. I am trying to get results for
- Hard contact (using penalty constraint enforcement)
- Soft contact (using linear and exponential Pressure-Overclosure relations)
The major problem is convergence during both analyses.
In hard contact the problem was the contact formulation - surface restrictions that limits some connectivity characteristics (e.g 3D solids/faces joined at nodes, or, T-intersections) , it was because of some solid elements (of material2- hard) which were connected via only one node (instead or being connected via edge or face). Mesh refinements were not so much helpful, because they only divide elements, refinement has no thing to do with smoothness of connection of already existing large-sized elements. I was able to get results only with selecting Finite Sliding option, although no sliding occurs in physical model because or interdigitation). This matter put a question mark on my results.
In soft contact, the problem was pressure-overclosure relation. It is really difficult to decide what should be C_o (clearance at zero pressure) and P_o (pressure at zero clearance), because the nodes and elements faces of material1 and material2 are nearly at zero distance (<10E -4) from each other. By selecting a linear pressure-overclosure relation and setting the value of contact surface stiffness "'K'" roughly equal to maximum value available in the siffness matrix of an element at the contact surface, I started my analysis. Soon I realized that the stresses at contact interface are higher even than stresses of Hard contact. so I reduced the value of "'K'" and re-run the analysis, repeating the analysis and reducing "'K'" till I got interface stresses equal in both cases finally. This value of "'K'" is now K_o for my analysis.
I have K_o , but the problem is still C_o and P_o, I can calculate P_o from P_o = (K_o) x (C_o), because C_o is very very small (less than 10E -4) , and if multiplied with (K_o = 350) gives some thing really funny ( i.e Pressure required to clost the clearance will be 0.0017). I have tried to run the analysis with some higher values of P_o but the analysis is not converging.
Anyone who has some idea on how to tackle such problems, please help.