To answer this question, we need to plot trajectories of principal stresses . 为了回答这个问题，我们尚须画出主应力迹线图。
Thus the directions that emerge for extremizing the normal stress are the directions corresponding to the principal stresses . 因此，产生最大正应力的方向就是相应于主应力的方向。
The orientation of the principal stresses relative to the directions of the polarizer and first quarter-wave plate is of course immaterial . 主应力相对于起偏镜和第一个四分之波片方向的方位，当然是无关紧要的。
Major intermediate , minor principal stress 最大中最小主应力
Strength criterion of rock and effect of medium principal stress 岩石的强度准则及中间主应力的影响
