Hello Mehdi,
The element direction is not the same as the material direction "a". All you need to do is make sure the material directions "a", "b", and "c" are correct for your material. Don't worry about the element direction; it does not matter.
By default, the element direction is from node 1 to node 2 of that element as defined on the *ELEMENT_SOLID card. So, the element direction has to do with the node numbering of the element. You can align the element direction to be in the same direction as material direction "a" using LSPP, but this step is not necessary.
The material models look at the material coordinate system (a, b, c) and not the element coordinate system.
Are the material axis a, b, and c in the correct orientation in your model? If so, then that's all you need. Your job is done as far as material direction goes.
Reno.
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