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8.1.3 Boundary Conditions
Boundary conditions are necessary to uniquely define the solution of the problem under study. Improperly defined boundary conditions can lead to underconstrained or overconstrained problems.
If the problem is underconstrained the solution is not uniquely defined and FEBio will not be able to find a solution. The most common example of an undercontrained problem is one where the rigid body modes are not constrained. A rigid body mode is a mode of deformation that does alter the stress in the body. Affine translation and rotation of the entire mesh are two ways to introduce a rigid body mode. In a (quasi-)static finite element analysis all rigid body modes must be properly constrained in order to find a unique solution. This can often very easily be achieved by constraining and edge or face of the model from moving at all.
If the problem is over-constrained, FEBio will usually find an answer but it is probably not the solution that was sought. The most common cause of an over-constrained model is one that has conflicting boundary conditions. For example, a force is applied to a node that is fixed.