What does environmental resistance mean when spoken about in a fiber reinforced polymer composites context?
First, let us understand the basic failure modes of composites under tensile, compression and shearing forces. The three fundamental failure modes of fiber reinforced polymer matrix composites are (1) fiber fracture, (2) polymer resin fracture or degradation, and (3) fiber/matrix interfacial separation.
1. Fiber Fracture
Under loading conditions, the polymer matrix fractures either perpendicular to the fiber direction (tension), or parallel to the fibers (shear), or some off-set angle (again shear). Matrix crushing can also occur under compressive loading (axial or transverse). Polymer resins will also break down chemically, typically becoming chalky or softening through a time-based degradation.
3. Fiber/Matrix Interfacial Separation
The interface between the polymer resin and the fiber is a form of adhesively bonded joint. When the adhesion of the polymer to the fiber is poor, then failure can be seen as a clean separation of the polymer resin from the fiber (i.e., there is no residual resin on the fiber surface).
Examples of fiber, polymer resin and interfacial failure are depicted in the following illustrations.
Second, what is the environment of exposure? The principal environmental exposures that polymer materials are 
susceptible to are temperature (high and low), fluids, gases and radiation (solar and nuclear). How these environments impact a composite material is fundamentally a polymer resin issue. With the ability of forming thermoplastic polymers during cure with higher molecular weights, a range of chemical additives and a molecular enhanced structure, they tend to perform better in adverse environments. However, the specific tailoring of the thermoplastic polymer for a specific environment requires precision in the fabrication and often comes with a higher price per weight.