**3. Summary**

Mechanical properties of materials represent the material's responses to different loading conditions and are macroscopic representations of the atomic bonding between the atoms from which they are constructed. It was suggested that the cohesive energy (*E*C) between two particles can be linked to the elastic constants of the materials and other various physical properties (such as melting temperature, atomic volume, lattice constants, and Debye temperature). Such a correlation opens the possibility of interlinking the material's electronic transport properties with the mechanical ones on the atomic level.

Measuring or evaluating correctly the mechanical properties of TE materials has the potential to bridge between the atomic (mechanical) and physical (electronic/transport) understanding of these materials to the fully developed working modules that will be optimal from both ends standpoint. That way, the material selection for the proper use will be much easier and efficient.

As it was shown in this review, knowing and controlling the mechanical properties of TE materials are paramount necessities for approaching practical TEGs and moving the entire TE technology onward in the Technological Readiness Level (TRL) scale. The material's elastic constants (e.g., Young's modulus and Poisson's ratio), strength, and fracture toughness are the most crucial for the designing practical devices (using finite element analysis). In such an approach, adequate modeling of TEGs could be prepared with lower experimental intervals while saving both money, time, materials, and man power. The elastic constants can provide the understanding about the material's stiffness, while the strength provides the loading conditions in which the material will maintain its original shape. Knowing the fracture toughness will provide the stress envelope in which the material could operate and its susceptibility to inherent fabrication faults.

Characterizing these mechanical properties (elastic constants, strengths, and fracture toughness) is handful enough and will provide both scientists and manufacturers sufficient data to improve and further proceed to practical TEGs. Characterization methods of these properties are varied with pros and cons to each. It is the authors' opinion to prefer the mechanical methods over the physical ones (such as in the case of Young's modulus measurement by sonic waves), so the results obtained will more accurately describe the material's response to mechanical loading. In evaluating the material's strength, it will be best to choose the type of testing method in which the loading conditions are as close as possible to the expected service conditions of the material. In order to establish a coherent database for all of the developed materials, it will be adequate to test all of these materials under compression and flexural conditions. This is due to the fact that most of the currently available published were obtained following compression conditions, and for the reason, flexure conditions are more susceptible to defect in the material. For measuring fracture toughness, it seems to be wise to choose other testing methods than the Vickers Indentation Fracture, which is prone to errors and uncertainties. It may be applied for a qualitative evaluation of the property as required for distinguishing between fabrication parameters, but for quantitative modeling and calculations, it will be better to use the testing method reported in the standards such as ASTM C 1421 [36] and ASTM B 771 [36].

The abovementioned mechanical properties are not the only mechanical properties at question but also the major ones that should to be evaluated. A more detailed mechanical design will require also the characterization of the fatigue limit and thermal fatigue resistance depending on the expected service conditions of the developed practical TE devices.
