Nowadays, there is a high demand for materials with a low weight and high strength, for many engineering applications. Among them, FG-GPL porous materials have attracted the interest of many researchers due to their mechanical potentials in aerospace and marine industries. A large variety of works from the scientific literature have focused on the static and/or dynamic behavior of different structural members, such as beams, plates, shells, with arbitrary shapes and made of composite materials [
1,
2,
3,
4]. For example, Zhang et al. [
5] applied the DSC-regularized Dirac-delta method using the Timoshenko theory to explore the dynamics of FG-GPL porous beams resting on elastic foundations and subjected to a moving load. Based on a shear and normal deformation theory and by employing the Ritz approach, Priyanka et al. [
6] investigated the stability and dynamic responses of porous beams made of FG-GPLs. Moreover, the free vibrations of rotating, FG-GPL, porous Timoshenko beams were studied by Binh et al. [
7], using the generalized differential quadrature method (GDQM). Xu et al. [
8] adopted the differential transformation method to investigate the free vibration behavior of FG-GPL porous beams based on the Euler–Bernoulli beam theory under a spinning movement. Ganapathi et al. [
9] proposed a trigonometric shear deformation theory, including a thickness stretching effect, to study the dynamic problem of curved beams made of FG-GPL porous nanocomposites, and proposed a closed-form solution as valid tool for further computational investigations. Yas and Rahimi [
10] applied the GDQM to study the thermal vibration of FG-GPL, porous Timoshenko beams. Safarpour et al. [
11] applied the 3D elasticity theory in conjunction with the GDQM to study the bending and free vibration behavior of porous annular and circular plates made of FG-GPLs under different boundary conditions. A novel computational method was proposed by Nguyen et al. [
12] to evaluate the static bending and free vibration response of FG-GPL porous plates based on a first-order shear deformation theory (FSDT), while using a polygonal mesh with parabolic shape functions. Furthermore, the nonlinear free vibrations of porous plates made of FG-GPL nanocomposites, resting on an elastic foundation, were investigated using the GDQ approach by Gao et al. [
13], using classical plate theory (CPT) and von Kármán-type nonlinearities. The same FSDT basics were applied by Saidi et al. [
14] to study analytically the stability and vibrations of FG-GPL porous plates under an aerodynamical loading. The classical finite element approach and Rayleigh-Ritz procedure for a comprehensive investigation of the free and forced vibration behavior, and the static response of FG-GPL porous annular sector plates, were considered by Asemi et al. [
15] using an FSDT approach. In addition, Phan [
16] applied a refined plate theory to analyze the free and forced vibrations of porous plates made of FG-GPL nanocomposites, while using the (NURBS) non-uniform rational B-spline approximations. An analytical solution to the wave-propagation problem of FG-GPL porous plates was presented by Gao et al. [
17], based on different plate theories, such as CPT, FSDT, or higher order theories (HSDTs). Zhou et al. [
18] combined the 3D elasticity theory and GDQM to assess the free vibrations of FG-GPL porous plates, whereas in Ref. [
19], the authors proposed a multiple scale approach and Galerkin method in order to define the nonlinear, forced vibration response of porous, thin, rectangular plates made of FG-GPL nanocomposites, including the von Kármán-type nonlinearities. Furthermore, a deep review on FG-GPL porous structures was performed by Kiarasi et al. [
20]. The fabrication issues of these structures represent a challenging aspect for many practical applications. A novel quadrilateral element was proposed by Ton-That et al. [
21], in line with the FSDT and Chebyshev polynomials, to analyze FG-GPL porous plates/shells. In addition, a variational differential quadrature (VDQ) was proposed by Ansari et al. [
22] for solving the free-vibration response of post-buckled, arbitrarily shaped porous plates made of FG-GPL nanocomposites, based upon a third-order shear deformation theory (TSDT). The static and free-vibration analysis of FG-GPL annular plates, cylindrical shells and truncated conical shells, with various boundary conditions, within a three-dimensional elasticity theory, were also investigated by Safarpour et al. [
23]. Bahaadini [
24] defined a further analytical solution to the free vibration problem of FG-GPL, porous, truncated conical shells, according to a Love’s first approximation theory, while examining the influences of porosity coefficients, weight fractions and geometries of GPLs, on the free vibration of the structure. Babaei and his coauthors analyzed the stress-wave propagation and natural frequencies of porous joined conical-cylindrical shells made of FG-GPLs [
25] and joined conical-cylindrical-conical shells [
26] by using the classical finite element method (FEM). Based on the Donnell’s theory and the Galerkin approach, the internal resonance of metal foam cylindrical shells made of FG- GPLs was studied by Ye and Wang [
27]. In the further work [
28], the authors employed the Galerkin method and an improved version of Donnell nonlinear shell theory to investigate the nonlinear vibration of metal foam cylinders reinforced with GPLs. Moradi et al. [
29] applied the moving least squares (MLSs) interpolations using an axisymmetric model to analyze stress waves’ propagation in FG-GPL, porous, thick cylinders in a thermal gradient environment. Based on the FSDT, Salehi et al. [
30] solved analytically the nonlinear vibration of imperfect, FG-GPL, porous nanocomposite cylindrical shells, whereas in Ref. [
31] the authors applied the GDQM to investigate the free vibration of sandwich pipes, considering the effects of porosity and a GPL reinforcement on the conveying fluid flow. Among the recent literature, Zhou et al. [
32] examined the flutter and vibration properties of FG-GPL, porous cylindrical panels under a supersonic flow. At the same time, the vibration of FG-GPL porous shells was analytically investigated by Ebrahimi et al. [
33]. Pourjabari et al. [
34] analytically investigated the effect of porosity on the free and forced-vibration characteristics of GPL-reinforcement composite cylindrical shells in a nonlocal sense, based on a modified strain gradient theory (MSGT). In line with the previous works, a limited attention has been paid to the buckling response of FG-GPL porous materials and structures. Among the available literature, Zhou et al. [
35] presented an accurate nonlinear buckling study of FG-GPL, porous, composite cylindrical shells based on Donnell’s theory and HSDT. Shahgholian-Ghahfarokhi et al. [
36,
37] investigated the torsional buckling behavior of FG-GPL, porous cylindrical shells, according to a FSDT and Rayleigh-Ritz method. Similarly, Yang [
38] applied the Chebyshev polynomials-based Ritz method to study the natural frequencies and buckling response of FG-GPL porous rectangular plates, using the FSDT approach. Dong [
39] investigated the buckling behavior of spinning cylindrical shells made of FG-GPL porous nanocomposites, while applying a FSDT and Galerkin approach. A novel numerical DQ-FEM solution to investigating the buckling and post-buckling of FG-GPL porous plates with different shapes and boundary conditions was applied by Ansari et al. [
40]. Kitipornchai [
41] analyzed the natural frequencies and elastic buckling of FG-GPL porous beams using the Timoshenko beam approach and the Ritz method. Twinkle et al. [
42] focused on the impacts of grading, porosity and edge loads on the natural frequency and buckling problems of porous cylindrical panels made of FG-GPLs. Nguyen [
43] investigated the buckling, instability and natural-frequency response of FG porous plates reinforced by GPLs using three-variable higher order isogeometric analysis (IGA). Rafiei Anamagh and Bediz [
44], instead, applied the FSDT to study the free vibration and buckling behavior of porous plates made of FG-GPLs using a spectral Chebyshev approach.
In the available literature, it seems that the static, buckling and dynamic behavior of porous spherical shells made of FG-GPLs has not been surveyed so far, despite their geometry being of great interest in various engineering applications, such as heat exchangers or energy absorbers, among other applications in the areas of aerospace, mechanical engineering and marine engineering. Among the different shell geometries, a spherical shell structures, indeed, features a high strength with a simple geometry, even compared to a cylindrical structure. The design of such structural members considering only static loading conditions may fail in dynamic situations. In such context, we focus on the buckling capacities of spherical shells made of porous FG nanocomposites reinforced by graphene, due to their exceptional flexibility and enhanced physical features. It is well known from the literature, indeed, that porous ceramic nanocomposites can ensure different beneficial effects, such as a reduced electrical and thermal conductivity; low weight; reasonable hardness; and resistance to wear, corrosion and high-temperature applications [
45]. Among the few works on spherical shell dynamics available in the literature, we cite Refs. [
46,
47], where a Ritz-Galerking procedure was proposed to solve a dynamic buckling problem for clumped spherical members. A finite difference method was applied, instead, in [
48,
49,
50], to check for the sensitivity of the dynamic buckling response of spherical caps to some initial manufacturing imperfections. Novel theoretical shear deformation theories were applied in Refs. [
51,
52] to treat the buckling response of isotropic and orthotropic shallow spherical caps, whose problem was solved analytically by means of Chebychev series [
51], or numerically according to classical finite elements [
52]. At the present state, however, there is a general lack of works from the literature focusing on the dynamic buckling of GPL-reinforced porous nanocomposite spherical shells, whose aspects are explored here according to the 3D elasticity basics and Green deformation nonlinearities, rather than common shell theories and Von-Karman nonlinearities, as proposed in [
53].
The equilibrium equations of a pre-buckling state are determined from the principle of virtual work, whose solution is found according to classical finite elements. The buckling loads are obtained according to the nonlinear Green strain field and the generalized geometric stiffness concept, for spherical caps featuring a uniform and non-uniform pattern of GPLs in the metallic matrix, including open-cell internal pores and for various porosity distributions along the shell’s thickness with uniform and symmetric FG patterns. More specifically, five different patterns of GPL dispersion pattern are assumed throughout the shell’s thickness, namely, a FG GPL-X, A, V, UD and O patterns. A systematic investigation checks for the effects of various porosity distributions and GPL patterns, along with the weight fractions and porosity coefficients of nano-fillers and different polar angles, on the buckling behavior of FG-GPL, porous spherical caps.