Defining principal strain + direction

[taminator]

Homework Statement

Question: The strain state in a point is defined by the tensor:
0.9 0 0
0 -0.4 0.2
0 0.2 0.6
- Define principal strains and directions in the point
- Explain how deforms the element shown in figure 2b (they just drew a cube on 3d axis)
- Draw the principal directions

Homework Equations

So i know that ε^3 - I1ε^2 - I2ε - I3 = 0

The Attempt at a Solution

and for the I values, i subbed the tensor values in and got I1 = 0.7, I2 = 0.02, I3 = -0.252
I have a feeling I'm subbing the wrong values though. is εx = εxx from the tensor above?
Becuase the end equation i get, does not give me any roots.

 

[radou]

I think you have a good feeling. For example, I1 = 0.9 - 0.4 + 0.6 = 1.1 .

 

[taminator]

ahh indeed I've got 2 values wrong.
So now i got I1 = 1.1, I2 = 0.1 and I3 = -0.252, but i still can't find my roots?