## principal axes for orthotropic material??

i am very much confused about the term 'Principal Axis'..my situation is im not able to understand the following lines from a book

'..............Since it is difficult to determine the three principal axis of a specific
element to define the orthotropic material tensor, only the isotropic material model is used for the 3D FE models......'

my question is whether this 'principal axis' is assosiated with moment of inertia or whether it is principal planes in strength of materials?....i referred many websites but still not clear...a simple and understandable explanation is needed.
thanks.
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 Quote by mikewinifred '..............Since it is difficult to determine the three principal axis of a specific element to define the orthotropic material tensor, only the isotropic material model is used for the 3D FE models......' my question is whether this 'principal axis' is assosiated with moment of inertia or whether it is principal planes in strength of materials?....i referred many websites but still not clear...a simple and understandable explanation is needed. thanks.
Hi mikewinifred!

From wikipedia:
 A familiar example of an orthotropic material with three mutually perpendicular axes is wood, in which the properties (such as strength and stiffness) along its grain and in each of the two perpendicular directions are different
So these are local properties of the material, and have nothing to do with the shape.

Moment of inertia, as you know, depends on the shape (and density etc).

The reason why they both have principal axes is that they are both tensors … and that's how tensors are

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