Effective Tensile Stiffness of Finite-Width Quasi-Isotropic Laminates

Finite width laminate tensile specimens cut out of a plate with a quasiisotropic [I 30'/90⁰]s-layup are considered in regard to their effective axial stiffness properties. Due to the existing free-edge effect, the effective coupon stiffness can be clearly reduced in comparison to the predictions of classical laminated plate theory. The respective stiffness reduction depends on both the width of the given laminate specimen and also the applied uniaxial tensile direction. This means that the quasi-isotropic laminate as a finite width specimen actually does not show an isotropic uniaxial tensile stiffness. For the description of this effect a closed-form analysis is presented and the respective stiffness reduction is derived in a consistent way. For comparison and validation purposes detailed finite element analyses have also been performed showing good agreement with the closed-form results.