A novel coupled hygro-thermo-mechanical multiscale-APFEA (HTM-multiscale-APFEA) algorithm is developed to precisely predict the mechanical and failure behavior of thick fiber-reinforced laminated composites under hygrothermal conditions. The Fourier and Fick’s equations are simultaneously solved by the proposed fully- coupled temperature-moisture concentration (FCTM) algorithm to account for their mutual effects on each other. Two subroutines, one responsible for the homogenization, failure analysis, and also linking the macro and micro models of the composite material, and another for establishing the constitutive equations and failure analysis of the cohesive/adhesive material, are integrated into a user-defined material code extended for also defining the materials failure behavior. The effects of temperature and moisture conditions on the stiffness, strength, coefficients of hygrothermal expansions (CHEs), and fracture toughness of matrix and cohesive/adhesive materials are considered. The results of the HTM-multiscale-APFEA algorithm are verified against the results of experimental data, analytical high-order theories, and reliable numerical analysis. Furthermore, the failure analysis of composite laminate under hygrothermal conditions coupled with mechanical loading (bending) is carried out by the HTM-multiscale-APFEA algorithm, and compared with experimental data. The results have shown that the transverse tensile strength, shear strength, and transverse Young’s modulus of the composite material degrade respectively about 72%, 13%, and 40% in the most severe hygrothermal conditions at the material boundaries; while the normal stiffness and normal strength of the cohesive/adhesive material decline by about 73% and 47%, respectively, at outer layers. In addition, the mixed-mode fracture toughness of the adhesive material elevates from 0.55 × 103 N/m at inner layers to 2.14 × 10 3 N/m at outer layers of adhesive material. The proposed algorithm is highly flexible and can easily be adapted to model different types of laminated composites.

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