Recall from above that a functionally graded material is one that varies in one or more properties throughout its dimensions. These constitute a relatively young class of materials with promising applications in a variety of areas. For example, the living tissues in your body, including your bones, are classified as (natural) functionally graded materials, so if scientists want to develop materials capable of replacing these, they are looking to develop a functionally graded material. They are also useful in aerospace applications, where materials that can withstand a large thermal (temperature) gradient are needed. Functionally graded materials are commonly found in energy conversion devices and have also been used in gas turbine engines. They can also be good at preventing the propagation of cracks through the volume of a material, which makes them promising candidates for defense applications like developing bullet-resistant materials to create armors for humans and vehicles.