What is considered a "composite" is always changing. Just as there is no single definition, there is also no single analytical method that can safely predict their dynamic behavior. The same way you can’t obtain ideal performance by using a single material throughout an entire car, you can’t expect to use a single analytical method to predict the behavior of all composites.
Rule of Mixtures is probably the most known, and widespread method of estimating composite properties. Its notoriety in composite design circles is also its main problem: Rule of Mixtures has been over used, and applied to cases that do not even come close to respecting its original, simplifying assumptions. If you wish to trust your analysis, it is essential to find out when it is OK, and (more importantly) NOT OK to use Rule of Mixtures. This article will describe what this rule really says, and will show some consequences of abusing this “rule of thumb” for composite behavior.