Prova di trazione di materiali polimerici fibrorinforzati
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Overview
Fonte: Roberto Leon, Dipartimento di Ingegneria Civile e Ambientale, Virginia Tech, Blacksburg, VA
I materiali polimerici rinforzati con fibre (FRP) sono materiali compositi formati da fibre longitudinali incorporate in una resinapolimerica, creando così una matrice polimerica con fibre allineate lungo una o più direzioni. Nella sua forma più semplice, le fibre nei materiali FRP sono allineate in modo ordinato e parallelo, conferendo così caratteristiche del materiale ortotropico, il che significa che il materiale si comporterà in modo diverso nelle due direzioni. Parallelamente alle fibre, il materiale sarà molto forte e/o rigido, mentre la perpendicolare alle fibre sarà molto debole, in quanto la resistenza può essere attribuita solo alla resina anziché all’intera matrice.
Un esempio di questa configurazione unidirezionale sono le barre di rinforzo in FRP disponibili in commercio, che imitano le barre di acciaio convenzionali utilizzate nella costruzione in cemento armato. I materiali FRP sono utilizzati sia come strutture autonome come ponti pedonali e scale, sia come materiali per rafforzare e riparare le strutture esistenti. Le piastre sottili e lunghe sono spesso epossidate alle strutture in calcestruzzo esistenti per aggiungere resistenza. In questo caso, le barre FRP fungono da rinforzo esterno. Le barre e le piastre in FRP sono più leggere e resistenti alla corrosione, quindi stanno trovando applicazioni in ponti e parcheggi, dove le lamelle antighiaccio portano a un rapido deterioramento delle barre convenzionali.
In questa esercitazione di laboratorio, verrà studiato il comportamento di trazione di un campione unidirezionale, con particolare attenzione alla sua resistenza e capacità di deformazione finali. Ci si aspetta che il comportamento del campione sia elastico fino al fallimento, che dovrebbe verificarsi in modo improvviso ed esplosivo. Questo comportamento dovrebbe essere contrastato con quelli degli acciai duttili, che presentano un’ampia capacità di deformazione e indurimento della deformazione prima del fallimento.
Principles
Procedure
Results
Typical stress-strain curves for the E-glass FRP plate specimens are shown for the plate with the two uniaxial layers aligned longitudinally (Fig. 1) and respectively perpendicularly (Fig. 2) to the direction of loading. For the case of the load applied parallel to the fibers (Fig. 1), the maximum force was 12.32 kips, corresponding to a tensile strength of 98.6 ksi. The failure occurred at a strain of 2.98% and the modulus of elasticity, calculated from a line tangent at 30% of the ultimate load, was 5686 ksi. Since an extensometer was not used, this value should be taken only as indicative of the Young's modulus. The behavior is essentially linear to failure. The results are reasonable for a material specified at 50% E-glass fiber volume.
Figure 1: Stress-strain curves for the E-glass FRP plate: load applied parallel with the fibers.
For the case of the load applied perpendicular to the fibers (Fig. 2), the maximum force was 2.72 kips, corresponding to a tensile strength of 10.9 ksi. The failure occurred at a strain of 2.24 and the modulus of elasticity, calculated from a line tangent at 30% of the ultimate load, was 640 ksi.
Figure 2: Stress-strain curves for the E-glass FRP plate: load applied perpendicular to the fibers.
As expected, there was a very large difference between the two directions as shown in the comparison graph (Fig. 3). This emphasizes the tailorability of the material properties; in this case we have a material that is strong in one direction and weak in the other.
Figure 3: Stress-strain curves for the E-glass FRP plate: load applied parallel (blue) and respectively, perpendicular (orange) to the fibers.
The failure surfaces bear witness to this, with the one for the fibers aligned longitudinally showing numerous broken fibers and the one with the fibers aligned perpendicularly showing the typical surface for a resin failure at an interface.
The plot in Fig. 4 shows a comparison of the behavior of the FRP rebars. There is a very significant drop in strength (a factor of about 2) and modulus of elasticity (about a factor of 4) decrease when we compare the carbon FRP and the E-glass FRP curves. All of these FRP materials can be seen to have very little or no ductility, failing immediately after carrying their maximum load.
Figure 4: Linearized stress-strain curves for E-glass (orange) and respectively, carbon (blue) FRP rebars.
Applications and Summary
FRP materials are light, strong composites used extensively in both civil, mechanical, and aerospace applications. They are made up of strong fibers embedded in a resin or similar matrix, and they are manufactured in many forms, including prepeg strips and laminates. Their strength and stiffness can be tailored by varying the amounts, types, and directionality of the fibers. FRP materials have a much smaller deformation capacity than metals or polymers and give little warning of failure, thus are important to study the manner and mechanics of failure.
FRP materials are used in a myriad of civil engineering applications from transportation to construction materials, marine to electronic applications, and even consumer products to business equipment. There are GFRP poles and towers for hanging power and telephone lines, FRP stairwells and parking garages, FRP roofing, seawall reinforcement, FRP marine fenders, and ground anchorage to name a few. They are also extensively used to strengthen and repair structures.
Many highway structures, such as the Prodeck Bridge System and Auto Skyway, employ FRP materials to help reinforce and support the loads that traverse the bridge in the road systems. Even the guardrails that one sees on the sides of the highways can be built using FRP materials. FRP materials are also used to transport people over pedestrian bridges, such as the Aber Feldy Golf Club Bridge in Scotland and the Shank Castle footbridge in Cumbria, U.K.
Many marine applications use FRP materials for their resistance to corrosion and salt. FRP is used extensively in the boating industry, as well as for naval structures and pipelines. FRP materials are not only seen in practical construction applications, but also in fun applications, such as in artistic architectural forms and roller coasters. The shooting arrow sculpture in San Francisco, named “Cupid’s Span”, is made from FRP materials, as are the pedestals in many roller coasters in Six Flags nationwide.
Transcript
Fiber-reinforced polymeric materials, FRP, are composite materials that are formed by embedding fibers in a polymeric resin, creating a matrix that is very strong in the direction of the fibers.
In their simplest form, fibers in FRP materials are aligned orderly in one direction, and encased in resin, causing the material to behave orthotropically. Mechanical properties of these materials are very different in the direction of the fibers compared to the other two principle directions.
An FPR material is very strong in the direction of the fibers because of the high strength of the fiber, behaving elastically until the fibers fracture, and the material fails in an explosive manner. The material is, however, very weak in the perpendicular direction because of the very much lower strength of the resin.
In this video, the tensile behavior of a unidirectional specimen will be studied, with emphasis on its ultimate strength and deformation capacity.
The strength of FRP materials is directly related to the strength of the individual fibers. As the percentage of fibers in a material increase, the strength of the material increases. Typical materials have approximately 50% fibers by volume.
The unidirectional strength of FRP is often used in reinforcing bars, or rebars, but can be realized in more than one direction of the material by controlling the direction of the fibers.
Fibers can be places in random directions, or single plys of uniaxial layers can be place in alternating directions, resulting in two strong directions and one weak direction. The fiber and resin used to make an FRP must be chosen to be compatible with each other and meet application requirements.
The class of the fiber used, typically glass, aramid, or carbon, affects the properties and cost of the final product. In general, the fibers have very low strain capacity, resulting in sudden failures without any evidence of ductility.
The resin primary acts to transfer stress and protect fibers from mechanical and environmental damage. During manufacturing, pressure is applied to squeeze out as much resin as possible to increase the strength of the material. It is important to note that the individual fiber properties are not the properties of the composite. Instead, according to the rule of mixtures, properties of the composite are a result of the weight and mean of the constituent parts.
In the next section, we will conduct simple tension tests on a Universal Testing Machine to compare the stress/strain behavior of glass and carbon FRP, while taking care to properly prepare the samples to obtain valid results.
Obtain four FRP specimens. Two will be from a unidirectional 0.5 inch E-glass FRP plate cut into one-by-eight inch specimens: one along the direction of the fibers, and one perpendicular to the fibers. The third specimen will be a 0.25 inch carbon FRP rebar, and the fourth will be a 0.25 glass FRP rebar. The rebar specimens should be about 24 inches long.
Prepare the FRP rebar specimens in advance by embedding 12 inches of the ends in slightly larger steel round and rectangular sections and filling the empty spaces with high strength epoxy. Allow several days for curing, according to the epoxy specifications.
This type of end connection is needed because the serrations in conventional UTM grips will destroy the resin and lead to premature end failures. Proceed in the same manner as the other tension tests, by turning on the UTM and initializing its software. Then, insert a specimen into the grips, and lock it in place.
Load the specimen in displacement control at a rate of about 0.2 inches per minute. As the specimen begins to fail, popping sounds will be heard and small shards will begin to fall off the specimen. Followed by an explosive failure of the material, which separates into a fibrous flower-like structure.
Here is the stress/strain curve for the E-glass FRP plate specimen being loaded in the direction of the fibers. From this graph, we can determine the maximum force, tensile strength, and strain and calculate the modulus of elasticity. These results are reasonable for a material specified at 50% E-glass fiber volume showing essentially liner behavior.
This graph shows the same material loaded perpendicular to the direction of the fibers. We can see a decrease in the maximum force, tensile strength, strain, and the modulus of elasticity. Note that a significant amount of the strength measured in this particular specimen comes from the fibers in the outside protective layers, in which the fibers are randomly oriented. The very large difference between the two directions emphasizes the tailorability of the material properties. In this case, we have a material that is strong in one direction, and weak in the other.
The failure surfaces bear witness to this, with the one for the fibers aligned longitudinally showing numerous broken fibers, and the one with the fibers aligned perpendicularly showing the typical surface for a resin failure at an interface. Comparing the behavior of the FRP rebars, there is a very significant difference in strength and modulus of elasticity. Both materials fail immediately after carrying their maximum load.
The difference between the strong carbon FRP bar, and the softer, but far more ductile E-glass one, is obvious in this linearized graph. However, there is little ductility, as they fail at a fraction of the strain of metals such as a36 steel.
FRP materials are used in a myriad of civil engineering applications, including original construction and repair applications. Let’s look at a couple of common uses of FRPs.
FRP sheets, laminates, and bars can be impregnated with resin and precured for use in field applications. The FRP bars and plates are light and corrosion resistant, so they’re finding applications in bridge decks and parking garages, where deicing leads to rapid deterioration of conventional bars.
Many marine applications also use FRP materials for their resistance to corrosion and salt. FRP is used extensively in the boating industry, as well as for naval structures and pipelines.
You’ve just watched JoVE’s introduction to tension testing of fiber-reinforced polymeric materials, or FRPs. You should now understand the components of FRPs and standard laboratory testing for determining their strength.
Thanks for watching!