섬유 강화 고분자 재료의 인장 시험
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Overview
출처: 로베르토 레온, 버지니아 공대, 블랙스버그, 버지니아 토목 및 환경 공학부
섬유 강화 중합체 재료(FRP)는 중합체 수지에내장된 세로 섬유에 의해 형성되는 복합 재료로, 따라서 하나 이상의 방향을 따라 정렬된 섬유로 폴리머 매트릭스를 생성한다. 가장 간단한 형태로 FRP 재료의 섬유는 질서 있고 병렬 방식으로 정렬되므로 정형 외과 재료 특성을 부여하므로 재료가 두 방향으로 다르게 행동한다는 것을 의미합니다. 섬유와 평행하게, 재료는 매우 강하고 뻣뻣한 반면, 섬유에 수직은 매우 약할 것입니다, 강도는 단지 전체 매트릭스 대신 수지에 기인 할 수 있기 때문에.
이러한 단방향 구성의 예는 철근 콘크리트 구조에 사용되는 기존의 강철 막대를 모방하는 상용 FRP 보강 막대입니다. FRP 재료는 보행자 교량 및 계단과 같은 독립형 구조물로 사용되며 기존 구조물을 강화하고 수리하는 재료로도 사용됩니다. 얇고 긴 플레이트는 종종 강도를 추가하기 위해 기존 콘크리트 구조에 epoxied됩니다. 이 경우 FRP 막대는 외부 보강 역할을 합니다. FRP 바와 플레이트는 더 가볍고 부식성이 뛰어나므로 교량 데크와 주차장에서 디빙 슬랫이 기존 막대의 급속한 악화로 이어지는 응용 프로그램을 찾고 있습니다.
이 실험실 운동에서는 단방향 표본의 인장 거동이 궁극적인 강도와 변형 능력에 중점을 두고 연구될 것입니다. 시편의 동작은 고장까지 탄력이 있을 것으로 예상되며, 이는 갑작스럽고 폭발적인 방식으로 발생할 것으로 예상된다. 이 동작은 경질 변형 용량과 고장 전에 경화변형을 나타내는 연성 강철과 대조되어야 합니다.
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!