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In this article, both the global and local instability of Variable Angle Tow (VAT) sandwich plates with variable stiffness skin layers under axial compression is studied, with emphasis on exploring the reasons for the occurence of the non-uniform wrinkling phenomena. The present sandwich plate model is according to the Extended High-order SAndwich Panel Theory (EHSAPT), in which the classical laminated plate theory is utilized for the two skin layers, whereas the high-order theory is applied to characterizing the core layer. The IsoGeometric Analysis (IGA) method combined with the principal of virtual work is employed to derive the eigenvalue equation that governs the critical  instability problem of VAT sandwich plates under axial compression. Both the membrane and non-membrane state is taken into account. The accuracy and effectiveness of the developed IGA model are validated by comparing against the results available in literature and FE results obtained using ABAQUS. Effects of the core thickness and fiber orientation angle on both the instability behaviours of VAT sandwich plates under the non-membrane state are discussed in numerical examples. It is found that for the case of variable stiffness skin layers, the non-uniform feature of both the pre-critical in-plane stress distribution and the bending stiffness in the skin layers are responsible for the occurence of the non-uniform instability of the sandwich plates under the non-membrane state. Moreover, the benign load redistribution mechanism offered by the VAT configurations remain in effect even in the case of local instability for the sandwich plates. The results presented herein may be beneficial for the design of VAT sandwich  structures under compression.

Isogeometric analysis, Periodic instability, Non-uniform instability, Wrinkling, Sandwich plate, Variable angle tow composite