解:(a)σ,=25MPa 40MPa 26 MPa 0)o/MPa =20 MPa ,=-40MPa /MPa (b)=-26MPa S0ME F/MPa O,=30 MPa G,=0 0, =-30 MPa (c)O=-50 MPa 50MPa omPa F/MPa 50MP (d o=40MPa I=10 MPa 20MPa t/MPa o,=41MPa C=4IMPa G,=0 OMPa ) 39°3 O/MPa G3=-61MH an=39°35 50MPa 7-9各单元体如图所示。试利用应力圆的儿何关系求 (1)主应力的数值 (2)在单元体上绘出主平面的位置及主应力的方向。 t/MPa 解:(a)a1=160.5MPa -30 0 /MPa 30.5MPa o=-23.56 70MPa (b)01=36.0MPa /MPa G,=0 丁3=-176MPa C1=65.6° 140MPa a/MPa ao gOMPa D 03D V D 03D D W 03D V V   03D V  E 03D V D 03D D W 03D V V   03D V  F 03D V D W D  V  03D V  V  G 03D V D 03D D W 03D V V   03D V     c $ D    D 03D V V   03D V  $  D  E 03D V V   03D V  $  D 03D 03D q V 03D W  03D q   VD  & R  03D q  03D q V V V  03D W  03D   q VD  03D q 03D V [ V \  W 03D R V 03D 03D q 03D 03D D qc 03D V   03D V   V V D q  VD W 03D V 03D 03D 03D 03D   V  V    q D V 03D W 03D V D    R V & 03D 03D 03D V [ V    V    q D V  03D V  D V W  03D &   R