On invariant combinations of Qij coefficients and a novel invariant IQ

Invariants of the plane-stress elasticity stiffness matrix [Q] play a key role in composite design, as they simplify design processes. Multiple invariants have been proposed independently in the past, such as the 'trace' Tr=trace([Q]) by Tsai and Melo or the invariants U1, U4 and [U5] which are often used in context of lamination parameters. The present paper presents a parametric description of rotation-independent (invariant) linear combinations of the coefficients Qij of [Q]. It is shown that the aforementioned invariants are special cases of the developed parametric description, in which a novel invariant IQ=Q11+Q22+Q12+Q66 plays a key-rol