I am writing to ask about a unidirectional composite material T700/M21, how can I understand why this material has the same modulus of elasticity in direction 2 and 3 which means (E2 = E3,E1) in an article and its constitutive law using chevalier book by adopting a quasi transversal isotropic material is different where I have E1 = E2, E3 and not E2 = E3 and E1. what is the method to choose the axis and to define the modulus of elasticity as E, G, V in the best way to not be confused and to contribute a good simulation. thank you
A unidirectional composite material cannot have the same modulus in all directions. Unidirectional means that all fibers run in the same direction. Obviously, this direction defines the axis for the first modulus of elasticity because here this parameter will have a maximum. T700 is a standard modulus carbon fiber with a tensile modulus of 230 GPa, now the ratio of matrix resin and fiber determines what the modulus of the composite is.
The other two moduli are in orthogonal direction and mostly determined by the characteristics and ratio of the matrix material, in your case M21. Below is a table for unidirectional T700/M21, but I do not know whether your material has the same fiber content and curing conditions. So don't copy them but use the numbers here to check the validity of your numbers.
List of physical properties of T700/M21, from this source.