确定应力边界条件的两种方法

弹性力学问题中，确定应力边界条件有两种方法——比较法和公式法。比较法就是当边界面与坐标轴平行或垂直时，将微元体“移到”边界附近，把应力与边界面上的面力进行比较，从而求出应力边界条件。公式法就是当边界面与坐标轴成一般角度时，先求出边界面的外法线与坐标轴的方向余弦，再代公式。两种方法各有优点，它们是相通的，是特殊与一般、个性与共性的关系。

Two Methods for Determine Stress boundary Condition

There are two methods for determine stress boundary conditions in elasticity----comparative method and formula method. The former is, when the bounding surface parallel or vertical to the coordinate axis, ＂moving＂ the tiny body vicinity to the bounding surface, comparing the stress with the surface force on bounding surface, then, can obtain the stress boundary condition. The latter is, when the bounding surface and coordinate axis were put into a general angle, calculating the direction cosine between coordinate axis and outer normal on bounding surface, then applying the correction formula mechanically. These two methods have their own advantages, they are the special and general, individuality and commonness.