ABSTRACT
A general theory of constitutive relations has to be able to describe an infinite number of classes of materials, distinguished by properties of symmetry and invariance. The question is now to know if the constitutive laws and the associated variables are indifferent to the choice of the coordinate system during motion of the body. The response of such a characteristic Representative Elementary Volume (REV) submitted to a given loading depends on the loading direction. The question which is now raised is whether, when the REV is loaded in different directions, some of them give the same response. The constitutive law has to be written to respect the material symmetry and then the number of independent elastic coefficients can be reduced. A monoclinic material is a material with only one plane of symmetry. The stiffness matrix or compliance matrix do not have to be modified by transformation with respect to a symmetry of this plane.
