## 1 Introduction  
Due to a variety of organic and inorganic compounds, progress and growth has been made possible when it comes to present-day materials, advanced polymers, engineering alloys, structural ceramics, etc. [1]. The use of new materials happens as a consequence of current technology trends. Material properties are expected to adapt to current changes in technology and to have a spectrum of functions and characteristics that have not been introduced yet. As a result, materials are merged,and their advantages are preserved. Functionally graded materials (FGM) meet the needs of all the mentioned requests in technology. Belonging to the family of engineering composite materials, FGM are modern materials that feature a continuous or discontinuous variation of the chemical composition through a defined direction. Detailed analysis and scientific experimentation have shown that FGM are able to constitute a gradient property, which is not the case with other homogeneous materials or composites. Present-day engineering faces a significant number of obstacles that could be overcome with these newly established materials with functionally graded composition. Mechanical characteristics like Poisson's ratio, density of material, modulus of elasticity, shear modulus, and thermal expansion coefficient change through a defined direction, where a property gradient can be stepwise or continuous (linear, exponential, or parabolic) (Figure 1). The existing materials and their qualities could be completely utilized by the FGM. The following factors are encompassed: reduction of transverse shear stress, the enhancement of mechanical and thermal characteristics as well as delamination prevention between the layers, which is one of the most crucial and the most commonly studied issues related to composite laminates [2] (Figure 2). Metal/ceramics are the most frequently used FGM, metal being superior to ceramics regarding strength, toughness, and high thermal conductivity, while ceramics features a good temperature resistance, low thermal conductivity, and good antioxidant characteristics. FGM that contain both metal and ceramic constituents enhance thermal-mechanical characteristics between layers. As a consequence of continuous change of properties at the interface, FGM avoids delamination. Functionally graded materials, being modern materials in the group composite materials, represent a popular topic discussed among numerous authors in recent years, as evidenced by a large number of publications in renowned journals in the field of composite materials. The actuality and the importance of the topic are addressed in numerous reviewed papers [3; 4; 5; 6], which undoubtedly indicates the intention of the authors to illustrate the current state of research in this area and point to further  
Figure 1: Gradient types. Figure 2: Material structure: (**a**) composite laminate; (**b**) functionally graded materials (FGM). research directions related to this very interesting area. Overall, FGM plates and shells under the impact of mechanical load or temperature can be studied using a 3D elastic theory or equivalent layer theories, which means classical plate theory (CPT), first-order shear deformation theory (FSDT), and higher-order shear deformation theory (HSDT). In order to eliminate the disadvantages of CPT in the analysis of moderately thick and thin plates [7], as well as to exclude the shear correction factors in FSDT [8], higher-order shear deformation theories (HSDT) were introduced. The most commonly used HSDT theory is third-order shear deformation theories (TSDT) developed by [PERSON] [9; 10] for laminate composite materials, taking into account the effects of shear deformation and satisfying condition that the laminates upper and lower surface stress-free. Later, the aforementioned theory was applied to the analysis of the FGM plates [11].