A new shear strain function-based plate model in the framework of refined couple-stress theory for size-dependent hygro-thermomechanical analysis of microlaminated composite plates

This paper aims to develop an analytical framework for the hygro-thermomechanical analysis of laminated microplates by integrating the refined couple-stress theory with a new shear strain function-based deformation model. The formulation captures anisotropic material behaviour and incorporates size dependency through an intrinsic length-scale parameter aligned with the laminate’s fibre direction. The governing equations are derived with the principle of virtual work and solved using Navier’s closed-form solution for simply supported plates. A detailed parametric study highlights the influence of microstructural size effects, shear deformation, and hygro-thermal fields on the bending response of laminated microplates. The inclusion of couple-stress effects leads to significantly reduced deflections relative to classical models, underscoring the need for scale-dependent formulations at the microscale. The proposed theory also provides higher accuracy than conventional higher-order shear deformation theories (HSDTs) without relying on shear correction factors. Overall, the formulation demonstrates better accuracy than other higher-order theories, making it a reliable analytical tool for evaluating numerical methods and designing advanced microscale-laminated composite structures operating in hygro-thermal environments.