Page 316 - Introduction to Continuum Mechanics

300 The Elastic Solid

so that

Indeed, one can easily see that all Q^/ with an odd number of the subscript 1 are all zero.
That is, among the 21 independent coefficients, the following eight (8) are zero

and the constitutive equations involve 13 nonzero independent coefficients. Thus, the stress
strain laws for a monoclinic elastic solid having the^JCs plane as the plane of symmetry, are:

or

Or, in contracted notation, the stiffness matrix is given by