Final answer:
The selection of materials for filling cracks is influenced by the thermal expansion coefficient, mechanical properties, and compatibility with the repaired material. This ensures minimal thermal stress and durability in varying temperatures and forces.
Step-by-step explanation:
The selection of materials used for filling cracks depends on various factors, including coefficient of thermal expansion, compatibility with the material being repaired, and the mechanical properties of the fill material. For instance, in the use of metal implants in the human body, researchers look for metal coatings with expansion coefficients matching that of the base metal to prevent thermal stress and resultant cracks at the interface. In dental applications, fillings that have similar coefficients of expansion to tooth enamel, such as porcelain in composite fillings, are preferred to reduce discomfort and the risk of cracks from eating something very hot or cold. Furthermore, the mechanical properties such as strength and flexibility play a role, since the materials need to withstand the forces and pressures they will encounter in service. For example, power lines require enough slack to accommodate expansion and contraction due to temperature changes without snapping.