Local Coordinate Systems
Boundaries
Many quantities for a membrane can best be interpreted in a local coordinate system aligned to the membrane surface. Material data, initial stresses-strains, and constitutive laws are always represented in the local coordinate system.
This local membrane surface coordinate system is defined by the boundary coordinate system (tl, t2, n).
The quantities like stresses and strains are also available as results in the global coordinate system after a transformation from a local (boundary) system.
Local Edge System
Many features, such as an edge load, allow input in an edge local coordinate system. The orthogonal local edge coordinate system directions xl, yl, and zl are defined so that:
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The first direction (xl) is along the edge. This direction can be visualized by selecting the Show edge directions arrows checkbox in the View settings.
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The third direction (zl) is the same as the membrane normal direction of the adjacent boundary.
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The second direction (yl) is in the plane of the shell and orthogonal to the edge. It is formed by the cross product of zl and xl; yl = zl × xl.
The local edge system can be visualized by plotting the components of the local edge transformation matrix with an Arrow Line plot. The matrix components are defined per feature. For instance, the variable name for the xx-component is <interface>.<feature_tag>.TleXX.
When an edge is shared between two or more boundaries, the directions may not always be unique. It is then possible to use the control Face Defining the Local Orientations to select from which boundary the normal direction zl should be picked. The default is Use face with lowest number.
If the geometry selection contains several edges, the only available option is Use face with lowest number, since the list of adjacent boundaries would then be different for each edge. For each edge in the selection, the face with the lowest number attached to that edge is then used for the definition of the normal orientation.