Introduction: Shear buckling is considered one of the primary instability modes in sandwich panels with flexible cores. The negligible shear stiffness of the core significantly reduces the panel’s stability against shear loads, while the occurrence of thickness-direction nonlinear deformations and core compressibility complicates the buckling mechanism, thereby challenging the simplifying assumptions of conventional theories.

Methods: In this study, an improved higher-order sandwich panel theory is developed for the stability analysis of thick panels featuring a flexible core, wherein the displacement fields of the face sheets and the core are defined independently as higher-order functions along the thickness direction. Furthermore, the stress compatibility conditions and displacement continuity at the face sheet–core interfaces are rigorously enforced, thereby capturing the intricate bending–shear–extension couplings with a constant number of unknown kinematic variables. The governing equations are derived via the principle of minimum potential energy and subsequently solved using the Galerkin method.

Findings: Numerical results revealed that the proposed theory estimates the shear buckling load lower than the HSAPT by 2.3% for symmetric square panels, 1.9% for unsymmetric square panels, 2.1% for symmetric rectangular panels, and 2.0% for unsymmetric rectangular panels. Furthermore, this theory exhibits an enhanced capability to distinguish and predict localized wrinkling modes. The findings also indicated that face sheet thickness asymmetry reduces the critical buckling load due to the degradation of effective panel stiffness.

Conclusion: The reduced critical loads yielded by the present formulation are attributed to the rigorous satisfaction of inter-component continuity conditions alongside core shear compliance, thereby correcting the unconservative estimations inherent in prior models. Hence, the proposed model provides a reliable, high-precision framework for the safe design and structural optimization of sandwich panels under in-plane shear conditions.