A continuous, unidirectional fiber reinforced composite lamina is an orthotropic material. As discussed in Stress-Strain Relations of Materials, there are 9 independent material constants for an orthotropic material. For a thin plate such as a lamina, the plane stress assumption holds and the number of independent constants can be further reduced from 9 to 4 (see this section for details).
The stress-strain relations can be written as
only four of , , , , and are independent material constants.
Due to the large number of possible fiber-matrix combinations and their volume fraction ratios, these constants are usually not available without conducting a series of experiments. Nontheless, estimated values can be obtained, assuming that the properties of both the matrix and the fibers are known.