American Composites Manufacturer’s Association’s Guidelines and Recommended Practices for Fiber Reinforced Polymer (FRP) Architectural Products, Section 3.7 “Servicability, Durability, and Long Term Effects”:
The durability of FRP composites products is widely considered superior to mild steel, aluminum alloys and stainless steels, or typical steel reinforced concrete. FRP materials have been in wide-spread use since the late 1940’s under a variety of conditions and numerous applications. Because of inherent corrosion resistance, FRP products have found broad use in harsh environments such as chemical processing plants, coal fired power plants, semi-conductor facilities and sewage treatment plants. FRP composites are used in extreme climates such as the tropics and Arctic regions due to the inherent ability to withstand temperature extremes without significant change in mechanical properties coupled with imperviousness to moisture absorption.
Unlike thermoplastics, thermoset composites are less susceptible to performance degradation as a result of long-term loads. Large scale structural applications such as pressure vessels and radomes demonstrate that FRP composites are capable of safely withstanding sustained loads over prolonged periods. Creep studies of FRP composites indicate that these properties are controlled largely by the matrix material.
Creep-related strains and deformations can occur in long term, axial loaded columns and beams at significantly high applied loads (Zureick and Scott, 1998, Sa et.al 2011, respectively). Some linear high elongation cores will also demonstrate creep behavior, so if high continuous loading, specifically in compression, is envisioned, these types of cores should be avoided, or the density increased to suit.
Proven in service as automotive springs, helicopter rotors, pressure vessels, boat hulls, and aircraft structures, properly designed FRP has excellent life under cyclic loading conditions. Given tensile fatigue loading conditions, FRP material shows very little change in tensile strength but does change in modulus of elasticity, proportional to the load. Flexural fatigue test results have also proven sandwich core laminates to be well adapted to fatigue loadings transverse to the plane of the laminate, but as with all materials loaded in fatigue, the design load should be well below the designed ultimate load.
The moisture absorption of FRP composites is negligible. Dimensional change and stresses due to moisture absorption are an insignificant design factor for design loads below levels which cause matrix cracking. Due to the inherent moisture impermeability and relative high strain at failure of the polymer material, the freeze-thaw performance of FRP composites is excellent when compared to conventional materials such as concrete. As is typical with all moisture mitigative detailing, exterior components should be provided with drainage paths to eliminate standing water and thus prevent ice damage. Sandwich core laminates must be fabricated without voids, and all openings cut through the finished laminate must be isolated from the core to ensure no water transmission or absorption by the core.
Ultraviolet (UV) Exposure
Weathering of FRP composites is related to degradation of the polymeric portion of the matrix by ultraviolet (UV) exposure. In thermoset composite, this attack is only on the surface of the material and typically progresses only 2 to 5 mils into the laminate surface. This is only a cosmetic affect and does not usually affect the structural performance. As with other building materials, the early stages of UV attack can cause color shift or yellowing and gloss changes. FRP composites should be protected from UV by an opaque gel coat surface or by painting the exposed surfaces. Incorporating UV screens into the matrix is also useful. Of these techniques, gel coating is the most common because it provides good surface finish and a deep 10 to 20 mil (mm) thick protective surface. Gel coating is used by the marine industry to provide a durable long life finish on boat hulls. Properly prepared FRP composites can also accept a wide variety of surface coatings, including oil- and water-based paint, as well as plural component systems such as urethanes. Factors influencing the weatherability of a gel coated surface include the type of gel coat resin, amount and type of fillers and colorants in the gel coat.