# Prof. Yu's Research Group in the Cloud

## Structures vs Materials

Structures and materials often appear together and sometimes even used interchangeably. The main reason is that the boundary between structures and materials is not clear. Even our first solid mechanics course usually called either strength of materials or mechanics of materials  is mainly talking about structures: rods, shafts, columns, beams, pressure vessels, etc. With the coming of composites, this difference between these two becomes even more elusive.

In my opinion, a material, specifically a structural material, is a substance structures can be made of. At least one of the dimensions of materials are not related with material properties. For example, the properties of material provided in bulky form  are not related with any of its dimension. The properties of composites tapes are not related with its longitudinal length. Materials are shaped to a specific geometry to form structures.The  behavior of structures depends not only on the material it is made of, but also depend on the specific geometry, and the support conditions and loading conditions.

I will appreciate any of your comments. Any suggestions for more concise and precise definition of materials are welcome. This differentiation is important is for our taxonomy as we have tools developed for modeling composite structures and tools for modeling composite materials.

1. In the book  Micromechanics of Composite Materials by Jacob Aboudi et al, it has a good description regarding materials and structures, which are quoted here:

The difference between a structure and a material comes down to the presence of boundary. A material is a substance of which a body is composed. The material itself has no boundaries, but rather may be thought of as what is present at a point in the body. Scientist and engineers have developed ways to represent materials through properties that describe how the material behaves at a point in a body, such as Young's modulus, thermal conductivity, density, yield stress, Poisson's ratio, and coefficient of thermal expansion. The body itself, on the other hand, is a structure. It has boundaries and its behavior is dependent on the conditions at these boundaries. For example, a given a steel beam, the beam itself is a structure, while the material is steel. This distinction between materials and structures is natural and extremely convenient for structural engineers and materials scientists.