Solid Mechanics: Stress 
Case 1: t_{xy} > 0 and s_{x} > s_{y} 
The principal axes are counterclockwise to the current axes (because
t_{xy} > 0) and no more
than 45º away (because s_{x}
> s_{y}).

Case 2: t_{xy} < 0 and s_{x} > s_{y} 
The principal axes are clockwise to the current axes (because t_{xy} < 0) and no more than
45º away (because s_{x} >
s_{y}).

Case 3: t_{xy} > 0 and s_{x} < s_{y} 
The principal axes are counterclockwise to the current axes (because
t_{xy} > 0) and between
45º and 90º away (because s_{x} < s_{y}).

Case 4: t_{xy} < 0 and s_{x} < s_{y} 
The principal axes are clockwise to the current axes (because t_{xy} < 0) and between 45º and
90º away (because s_{x} <
s_{y}).

Case 5: t_{xy} = 0 and s_{x} > s_{y} 
The principal axes are aligned with the current
axes (because s_{x} >
s_{y} and t_{xy} = 0).

Case 6: t_{xy} = 0 and s_{x} < s_{y} 
The principal axes are exactly 90° from the
current axes (because s_{x}
< s_{y} and t_{xy} = 0).

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