The Straight Edge Constraint controls the addition of an additional constraint, forcing the edge to be straight. The default is to add this constraint to all edges unless linear shape functions have been chosen in the
Discretization section of the interface. Using this additional constraint removes the need to use a mesh with only one element per edge. Internal nodes will make the model singular because the truss element only has stiffness in the axial direction in a geometrically linear problem. The same problem is present also when using higher-order shape functions, even if there is only one element along an edge since there are internal nodes.
The additional constraints increase the solution time, especially for large 3D and transient problems. The default mesh, when using Physics-controlled mesh in the
Mesh node, is to use one element per edge only, so that the extra constraints are not needed unless the shape functions are of higher order.
Select the Disable for linear discretization order check box to suppress the addition of the straight edge constraints when linear shape functions are used. This is the default state.
