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

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

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

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

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

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

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