Default local material directions

The local material 1- and 2-directions lie in the plane of the shell. The default local 1-direction is the projection of the global 1-axis onto the shell surface. If the global 1-axis is normal to the shell surface, the local 1-direction is the projection of the global 3-axis onto the shell surface. The local 2-direction is perpendicular to the local 1-direction in the surface of the shell, so that the local 1-direction, local 2-direction, and the positive normal to the surface form a right-handed set (see Figure 1).

Figure 1. Default local shell material directions.

The default set of local material directions can sometimes cause problems; a case in point is the cylinder shown in Figure 2.

Figure 2. Default local material 1-direction in a cylinder.

For most of the elements in the figure the local 1-direction is circumferential. However, there is a line of elements that are normal to the global 1-axis. For these elements the local 1-direction is the projection of the global 3-axis onto the shell, making the local 1-direction axial instead of circumferential. A contour plot of the direct stress in the local 1-direction, σ11, looks very strange, since for most elements σ11 is the circumferential stress, whereas for some elements it is the axial stress. In such cases it is necessary to define more appropriate local directions for the model, as discussed in the next section.