纤维增强高分子材料的拉伸试验

FRP materials are strong in the fiber direction because the high-strength fibers carry most of the load, while the polymeric resin matrix transfers stress between fibers. The material behaves elastically until fibers fracture, causing sudden, explosive failure. Strength increases as fiber volume percentage increases, with typical materials containing approximately 50% fibers by volume.

Perpendicular to the fibers, FRP materials depend entirely on the resin's strength, which is much lower than the fibers themselves. The resin's primary role is transferring stress and protecting fibers from mechanical and environmental damage, not providing structural strength in all directions. This directional weakness demonstrates the orthotropic nature of unidirectional FRP composites.

According to the rule of mixtures, composite properties result from the weighted mean of constituent parts. Fiber class—typically glass, aramid, or carbon—affects final product properties and cost. As fiber percentage increases, material strength increases proportionally. Individual fiber properties differ significantly from composite properties, requiring careful selection of compatible fibers and resin for specific applications.

Carbon FRP rebars exhibit significantly higher strength and modulus of elasticity compared to E-glass rebars, which are softer but more ductile. Both materials fail immediately after reaching maximum load with minimal ductility compared to metals. The stress strain characteristics of steels show far greater deformation capacity, highlighting FRP's brittle failure behavior and limited strain capacity.

Conventional Universal Testing Machine grips with serrations destroy the resin and cause premature end failures in FRP rebars. Embedding 12 inches of each rebar end in steel sections and filling gaps with high-strength epoxy provides proper load transfer without damaging the composite. This specialized end connection ensures valid test results and prevents grip-induced failures.

FRP materials are increasingly used in bridge decks, parking garages, and marine structures where deicing salts cause rapid deterioration of conventional steel reinforcement. FRP bars and plates are lighter and corrosion-resistant, making them ideal for naval structures, pipelines, and boating applications. They can be used as original reinforcement or epoxied to existing concrete structures for repair and strengthening.

FRP materials fail suddenly and explosively with little to no warning, producing a fibrous flower-like fracture structure and popping sounds as fibers break. Unlike ductile metals that exhibit extensive deformation and strain hardening before failure, FRP specimens fail at a fraction of the strain capacity of metals. This brittle behavior reflects the low strain capacity of individual fibers and the elastic behavior of the composite until fracture.