What is a Composite Material, Exactly?

We are so familiiar with composites that we almost forget what it is. Some say a composite material is a material which is anisotropic and heterogeneous.

First, I would like to argue that anisotropy cannot be used to define composites although composites are usually anisotropic. We do have isotropic composites, such as particle reinforced composites. And many times we idealize composites to be isotropic solids, aka black aluminum. Hence anisotropy cannot be a descriminator.

Second, How about heterogeneous? In the literature, heterogeneous material is almost used as a synonym for composite material (e.g. the celebrated micromechanics book by Nemat-Nasser used the term heterogeneous material directly). However we should quantify heterogeneity a little bit more. As we all know at a sufficient small scale, such as atomic scale, all materials are heterogeneous. For a material to be called a composite, the heterogeneities must be distinguishable at the continum scale. In other words, the behavior of different phases within a composite can be described in th framework of continuum mechanics.

Somebody also describe a composite as a material  composed of two or more materials with a recognizable interface between them. Again, the recognizable interface should be quantified. Are they recognizable by normal human eyes, or by Scan Electronic Microscopy? 

Hence from modeling perspective, I prefer to define composites as heterogeneous materials with heterogeneities which can be idealized as continuum. I know this definition is debatable. Any of your opinions are welcome.

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