## 1 Introduction  
Recently, graphene platelets (GPLs) have been spotlighted as a next-generation nanofiller for various composite materials because they provide excellent physical, chemical, and electrical properties [1; 2]. Composite structures reinforced with graphene platelets are called graphene platelet-reinforced composites (GPLRCs), and their properties are dramatically increased when only a tiny amount of GPLs are included [3]. GPLRC structures are typically fabricated in the form of beams, plates, and shells, and basic thermo-mechanical behaviors, such as static bending, free vibration, and thermal and mechanical buckling, have been intensively examined for the successful application of GPLRC structures in engineering applications [4; 5; 6]. Besides the GPL amount, the thermo-mechanical responses of GPLRC structures are also remarkably affected by the thickness-wise GPL dispersion distribution. Several purposeful functional distributions of GPLs have been suggested, and functionally graded GPLRC (hereafter FG-GPLRC) structures are being widely investigated with respect to these primitive functional distribution patterns [7; 8; 9; 10; 11; 12; 13]. The term _functionally graded_ was inspired by the concept of FG material (FGM), in which the material composition varies functionally and continuously in a specific direction [14]. The base material particles in FGMs vary continuously and functionally in a specific direction to maximize the target performance by overcoming the inherent demerits of lamination-type composites [15]. Maximizing the target performance of FG-GPLRC structures can be achieved by selecting the best primitive distribution pattern or by creatingan optimal distribution pattern, using the optimization technique [16]. This maximization can be achieved once the thermo-mechanical behaviors of these nanocomposite structures have been sufficiently investigated. Meanwhile, functionally graded GPLRC beams, plates, and shells are geometrically characterized by their small thickness compared to their characteristic length. Therefore, these structures can easily buckle when in-plane compressive loads are applied [17], and the structures abruptly lose their load-bearing strength when buckling occurs. In-plane compressive loads are frequently produced when the structures subject to the displacement constraint are exposed to thermal environments. Therefore, understanding thermal buckling and postbuckling in FG-GPLRC structures is important for their successful application in the target engineering field. Regarding thermal buckling, [PERSON] et al. [18] conducted modeling and analysis of the thermal buckling of FG-GPLRC plates resting on an elastic foundation by applying a two-step perturbation technique to a higher-order SDT. [PERSON] et al. [19] numerically investigated the effects of the GPL distribution pattern, concentration, and geometry on the thermal buckling and postbuckling behaviors of FG multilayer nanocomposite plates reinforced with GPLs. [PERSON] and [PERSON] [20] studied the thermal buckling of composite laminated plates reinforced by graphene sheets using a non-uniform rational B-spline (NURBS) based isogeometric finite element method. [PERSON] et al. [21] introduced a mathematical model to study the thermal postbuckling of multiscale fiber-reinforced graphene composites based on the Euler-Bernoulli beam theory and the von Karman geometric nonlinearity. [PERSON] et al. [22] carried out a size-dependent thermal buckling analysis of a micro-plate composed of epoxy, which is reinforced with FG-GPLs based on modified couple stress theory and sinusoidal SDT. [PERSON] et al. [23] numerically investigated the thermal buckling and postbuckling of edge-cracked FG-GPLRC beams resting on an elastic foundation with respect to the parameters of the GPL, crack, and elastic foundation. [PERSON] et al.