Plane stress analysis of FRP laminated composites based on homogenization considering the viscoelastic behavior of the resin

To efficiently evaluate the anisotropic viscoelastic behavior of thin laminated composite plates, a theoretical framework is developed by reducing a three dimensional anisotropic viscoelastic constitutive law to plane stress conditions and integrating it with classical lamination theory. Based on this framework, numerical analyses using laminated shell elements are conducted to investigate time-dependent relaxation responses under tensile and bending loads for various stacking sequences. The validity of the proposed formulation is verified through comparison with results obtained under the assumption of three dimensional stress states reported in existing studies. As unique characteristics of laminated composites arising from the viscoelastic properties of the resin, it is shown that when plies exhibiting tension–shear coupling are present, local stress concentrations occur near interlaminar free edges, and their magnitudes increase as relaxation of the resin matrix progresses. Furthermore, for asymmetric stacking sequences, the macroscopic anisotropic elastic properties are found to change significantly over time due to viscoelastic relaxation of the resin. These results indicate that, in the design of thin-walled composite structures requiring long-term reliability, it is necessary to consider not only the initial elastic properties but also the time-dependent viscoelastic behavior of the resin and the resulting evolution of anisotropy in the laminate.