|Table of Contents| Section Contents| Previous Section| Next Section|
Figure 2-1a is a general three dimensional stress element, showing three normal stresses sigma_x, sigma_y, and sigma_z, all positive; and six shear stresses tau_xy, tau_yx, tau_yz, tau_zy, tau_zx, and tau_xz, also all positive. The element is in static equilibrium and hence
tau_xy = tau_yx, tau_yz = tau_zy and tau_zx = tau_xz
Outwardly directed normal stresses are considered as tension and positive if they act in the positive direction of a reference axis. The first subscript of a shear stress component is the coordinate normal to the element face. The shear stress component is parallel to the axis of the second subscript. The negative faces of the element will have shear stresses acting in the opposite direction; these are also considered as positive.
Figure 2-1b illustrates a state of biaxial or plane stress, which is the more usual case. For this state, only the normal stresses will be treated as positive or negative. The senses of the shear stress components will be specified by clockwise (cw) or counterclockwise (ccw) convention. Thus, in Figure 2-1b, tau_xy is ccw and tau_yx is cw.