Local tangent directions on surfaces in space

Local tangent directions are needed on surfaces in space; for example, to provide a convention for describing components of slip on an element-based contact surface or components of stress and strain in a shell. The convention used in Abaqus for such directions is as follows.

The default local 1-direction is the projection of the global x-axis onto the surface. If the global x-axis is within 0.1° of being normal to the surface, the local 1-direction is the projection of the global z-axis onto the surface. The local 2-direction is then at right angles to the local 1-direction, so that the local 1-direction, local 2-direction, and the positive normal to the surface form a right-handed set (see Figure 1.2.2–1). The positive normal direction is defined in an element by the right-hand rotation rule going around the nodes of the element. The local surface directions can be redefined; see “Orientations,” Section 2.2.5.

這句話 if the global x-axis is within 0.1° of being normal to the surface, the local 1-direction is the projection of the global z-axis onto the surface.是什么意思