Composite materials are increasingly used in structural applications because they provide significant weight savings. The weight savings result from the combination of a lightweight, weak, and flexible matrix material with a strong and stiff reinforcing material in the form of fibers. Fiber-reinforced composites are used in a wide variety of applications, from laminated aircraft wings to sports equipment. One of the important properties of composites is that they are often characterized by different moduli of stiffness under tension and under compression. The difference between the moduli varies considerably depending on the material considered, but it is generally greater than 10%, therefore a bimodular approach seems to be a necessary development for a more accurate prediction of the mechanical behavior of this class of materials.