0 =ocos a osin 20 /2 a=0 a max C=90 0 amin a=45° a max 0 0 amin 正负号规定: 正应力一拉应力为正,压应力为负 切应力一自外法线n顺时针转向它,为正;逆时针为负6     2 = cos   =  sin2 / 2  = 0   max =   = 90 0   min =   = 45 2 max    =  = 0 0   min= 正负号规定： 正应力—拉应力为正，压应力为负 切应力—自外法线 n 顺时针转向它，为正；逆时针为负