Patent ID: 12258760

Similar reference characters denote corresponding features consistently throughout the attached drawings.

DETAILED DESCRIPTION

As shown inFIGS.1-3, the linkage for increasing the ductility of fiber reinforced polymer bars10includes a solid central shaft12and first and second hollow receiver portions18,20, respectively. The first hollow receiver portion18has opposed open and closed ends22,24, respectively, and, similarly, the second hollow receiver portion20has opposed open and closed ends26,28, respectively. The closed end24of the first hollow receiver portion18is secured to a first end14of the solid central shaft12and, similarly, the closed end28of the second hollow receiver portion20is secured to a second end16of the solid central shaft12. As indicated inFIG.3, each of the first and second hollow receiver portions18,20has an outer diameter d1associated therewith which is greater than an outer diameter d3of the solid central shaft12, and each of the first and second hollow receiver portions18,20has a central channel42,44, respectively, extending axially from, and in communication with, the corresponding one of the open ends22,26.

Each of the solid central shaft12, the first hollow receiver portion18and the second hollow receiver portion20may be cylindrical, as shown, and may further be axially aligned such that the open ends22,26of the first and second hollow receiver portions18,20, respectively, are opposed with respect to one another. A first transition portion34may be located between the closed end24of the first hollow receiver portion18and the first end14of the solid central shaft12. Similarly, a second transition portion36may be located between the closed end28of the second hollow receiver portion20and the second end16of the solid central shaft12. Each of the first and second transition portions34,36may be substantially frustoconical in shape, as shown.

In use, the central channels42,44of the first and second hollow receiver portions18,20, respectively, are adapted for respectively partially receiving first and second fiber reinforced polymer bars30,32. Each of the central channels42,44of the first and second hollow receiver portions18,20, respectively, may have a threaded surface38,40, respectively, for releasably engaging the first and second fiber reinforced polymer bars30,32. A strong adhesive, such as epoxy, may also be used to secure the first and second fiber reinforced polymer bars30,32within their corresponding central channels.

It should be understood that at least the solid central shaft12of the linkage for increasing the ductility of fiber reinforced polymer bars10may be made from any suitable material having a ductility which is greater than that associated with the first and second fiber reinforced polymer bars30,32. As a non-limiting example, at least the solid central shaft12may be made from steel. The greater ductility of the steel allows the linkage10to replace what is ordinarily the highly stressed part of an FRP bar under tension. The tensile failure force of linkage10is about 80-90% of the rupture force of the FRP bar. Additionally, linkage10may be coated with any suitable type of anticorrosive agent or the like. As a further alternative, the first hollow receiver portion and the second hollow receiver portion18,20may have textured or deformed outer surfaces for enhancing their bond strengths with concrete. It is contemplated that all elements of the linkage are made of the same material, for instance, all elements are made from steel.

The outer diameter d3of the solid central shaft12may be selected such that tensile force in the solid central shaft12does not exceed 80%-90% of the tensile force in the fiber reinforced polymer (FRP) bars30,32in order to avoid any uncertainty associated with material strength. Thus, as an example, the outer diameter d3may be selected to meet the following criterion:

π4⁢(d⁢3)2⁢fsu<π4⁢(df)2⁢ffr,
where fsuis the ultimate strength of the solid central shaft12, dfis the nominal diameter of each of the FRP bars30,32, and ffris the ultimate strength of each of the FRP bars30,32.

The diameter d2of each of the central channels42,44may be selected to be larger than the nominal diameter dfof each of the FRP bars30,32to allow for a gap sufficient for receiving enough epoxy to bond the FRP bars30,32within their respective central channels. The bonding between each of the central channels42,44and the FRP bars30,32can also be made mechanically by manufacturing FRP bars,30,32with threaded ends. The outer diameter d1of the first and second hollow receiver portions18,20may be selected to ensure that the first and second hollow receiver portions18,20do not yield when the FRP bars30,32reach their ultimate tensile strength at maximum load for the entire linkage. Thus, the following criterion may be used for selecting the outer diameter d1of the first and second hollow receiver portions18,20:

π4⁢(df)2⁢ffr<π4[(d⁢1)2-(d⁢2)2]⁢fsy,
where fsyis the yielding strength of the first and second hollow receiver portions18,20.

With reference toFIG.2, the length of each of the first and second hollow receiver portions18,20L1may be selected to ensure that the full tensile strength of the FRP bars30,32can develop without experiencing pull off failure between the FRP bars30,32and the surrounding epoxy/steel, such that

π4⁢(df)2⁢ffr≤π⁢df⁢τ⁢L⁢1⇒L⁢1=df⁢ffr4⁢τ,
where τ is the minimum bond strength between a) epoxy and steel; and b) epoxy and the FRP bar.

In experiment, a linkage for increasing the ductility of fiber reinforced polymer bars10was constructed from mild steel and used with glass fiber reinforced polymer (GFRP) bars. A displacement control uniaxial tension machine with a 100 kN capacity was used for testing. The GFRP bars had nominal diameters dfof 9.84 mm, with an ultimate strength ffrof 850-900 MPa and a rupture strain of 1.6%, giving a maximum axial force of 64.6-68.4 kN. The outer diameter d3of the solid central shaft12was selected to be larger than the nominal diameter of the GFRP bars, with a diameter of 12 mm, such that the achieved tensile force of the steel became close to, but less than, the GFRP bar tensile strength; i.e.,

π4⁢(d⁢3)2⁢fsu<π4⁢(df)2⁢ffr,
as discussed above.

The length L2of the transition portions34,36was selected to be as small as possible for the purpose of providing a smooth transition from the first and second hollow receiver portions18,20to smaller solid central shaft12. The L3of the solid central shaft12was 200 mm in the experiments, which provided good ductility overall, with the plastic deformation being much higher than the elastic deformation, thus not requiring any increases to the length. Additionally, in the experiments, the length L1of each of the first and second hollow receiver portions18,20was 250 mm, and the length L2of the transition portions34,36was 50 mm. The outer diameter d1of each of the first and second hollow receiver portions18,20was 25 mm, the diameter d2of each of the central channels22,26was 16 mm, and the diameter d3of the solid central shaft12was 12 mm. The yielding strength fsyof the steel was 379 MPa, and the ultimate strength of the steel fsuwas 538 MPa. The epoxy used in the experiments was Sikadur® Hex-300, manufactured by Sika®.

A direct uniaxial tension test was performed on the linkage for increasing the ductility of fiber reinforced polymer bars10with the geometrical and material properties described above. One end of the linkage10was attached to a GFRP bar and the other end was gripped directly to the jaw of the direct uniaxial tension testing machine. The experimental load-displacement relationship for the tested linkage10and the GFRP bar is shown inFIG.4. For comparison purposes, the load-displacement relationship for a GFRP bar of a total length of 900 mm (equivalent to the tested linkage and GFRP bar combination) is shown on the same graph.

The load-displacement relationship of the linkage-GFRP bar combination shown inFIG.4demonstrates a substantial increase in ductility, specifically about four times larger in the displacement when compared to the load-displacement relationship of the GFRP bar alone. Furthermore, the load-displacement relationship of the linkage-GFRP bar combination is almost identical to a typical tensile stress-strain response for a mild steel bar. This level of enhancement in the ductile response was achieved while reaching a maximum tensile force of 51.8 kN, which represents 80% of the rupture strength of the GFRP bar alone. This percentage can be further improved to 90% or 95% of the ultimate strength of the GFRP bar by increasing the diameter of the solid central shaft12. The final failure mode of the linkage-GFRP bar combination was characterized by a typical necking (i.e., cup-cone) ductile failure within the upper end of the middle third of the specimen (i.e., when arranged vertically in the testing machine, the upper end of the solid central shaft12). The large plastic deformation achieved at solid central shaft is very useful for enhancing the ductile performance of flexural controlled members (e.g., beams) if the center of the ductile link is made to coincide with the center of the anticipated plastic hinge region (i.e., the region of maximum bending moment).

It is to be understood that the linkage for increasing the ductility of fiber reinforced polymer bars is not limited to the specific embodiments described above, but encompasses any and all embodiments within the scope of the generic language of the following claims enabled by the embodiments described herein, or otherwise shown in the drawings or described above in terms sufficient to enable one of ordinary skill in the art to make and use the claimed subject matter.