Application range of unidirectional carbon fiber sheet in structural reinforcement

1. Bending component repair

The repair of steel flexural members is usually to paste unidirectional carbon fiber sheet directly on the bottom of the tension flange. When the flexural member is stressed, the carbon fiber and the tension flange jointly bear the tensile force to improve the bearing capacity of the beam. Carbon fiber is suitable for various forms of steel beam reinforcement, including I-shaped section steel beams, rectangular section steel beams, steel plate beams, and steel-concrete composite beams. Tests have shown that after the use of carbon fiber reinforcement, the beam's flexural bearing capacity has been improved. The degree of improvement is related to factors such as the amount of fiber attached, the elastic modulus of the fiber, the elastic modulus of the steel, and the yield strength of the steel. Although the strength of carbon fiber is high, the elastic modulus is similar to steel. When the overall structure reaches the yield load, the strength of the carbon fiber material is only about 200 MPa, which is far from the available strength of carbon fiber, and its high-strength advantage fails to fully play its role in strengthening the steel structure.

For damaged steel beams, bonding carbon fiber reinforcement and repair can not only restore its lost stiffness, bearing capacity and improve its fatigue performance, but also protect the steel structure and play a dual effect of reinforcement and corrosion protection. When sticking unidirectional carbon fiber, stick the carbon fiber to the surface of the damaged part, so that a part of the load is transferred to the carbon fiber through the adhesive layer, which can reduce the nominal stress of the damaged part of the steel structure. Reduce the crack growth rate or stop the crack growth, thereby prolonging the service life of the structure. Studies have shown that after steel beams with damage defects are reinforced with carbon fiber boards with high elastic modulus, the rigidity can basically be restored to more than 90% of the rigidity of steel beams without damage. The increase in ultimate bearing capacity varies with the amount of reinforcement and damage.

2. Repair of tension (compression) components

Sticking carbon fiber reinforcement has a good effect on the improvement of the bearing capacity of the steel structure under tension (compression). Experimental research on carbon fiber reinforced hollow pipe columns shows that the reinforcement effect of pasting carbon fiber cloth along the circumferential direction of the square pipe is far better than the longitudinal direction, and the ultimate bearing capacity is increased by 18%. When reinforced with carbon fiber in the longitudinal direction, the failure form is the peeling failure of the unidirectional carbon fiber cloth and the steel structure. When the ring carbon fiber is used for reinforcement, the peeling between the unidirectional carbon fiber cloth and the steel structure and the carbon fiber cloth fracture will not occur, and the steel column will eventually undergo local buckling failure. The uniaxial tensile test of the carbon fiber cloth reinforced steel plate shows that the yield strength and ultimate load of the specimens with unidirectional carbon fiber cloth have been greatly improved, and the increase in the yield load increases with the increase of the bonding area. In addition, a limit state of carbon fiber cloth fracture is added to characterize the failure of the specimen.

3. Repair of steel pipe line under internal pressure

Because the steel pipe line is often exposed to high temperature, high pressure and harsh working conditions and filled with flammable, explosive, toxic, corrosive and other media, once the pipeline leaks and blasts will bring terrible consequences, so ensuring the safety of the pipeline has important practical significance . The steel pipe wire is wound and reinforced with carbon fiber. Under the action of internal pressure, the pipe wall mainly bears the hoop stress and the steel pipe wire expands. The radial deformation is constrained by the externally wound carbon fiber, so that the carbon fiber and the steel pipe line are stressed together, which reduces the circumferential stress of the pipe wall and improves the ability of the pipeline to withstand internal pressure. However, if a certain prestress is applied to the unidirectional carbon fiber so that the steel pipe line has already formed a compressive stress in the circumferential direction before it bears the internal pressure, and it is superimposed with the circumferential tensile stress generated by the internal pressure, the reinforcement effect will be better.

4. Fatigue repair of steel structure

The main feature of fatigue failure is that the failure stress is lower than the static stress intensity. It is one of the main failure modes of engineering structures such as crane beams and bridges under repeated loads. The results of survey statistics show that steel components in actual projects generally require a high number of cycles, and the number of stress cycles in 50 years is close to 10 million. After the component is reinforced with carbon fiber, under the action of alternating load, the bearing capacity is improved and the stress amplitude is reduced, which indirectly improves the fatigue strength and fatigue life of the component. Studies have shown that after FRP reinforcement, the remaining fatigue life of fatigue-damaged steel structures is doubled, and the reinforcement effect is very obvious. For welded components, carbon fiber bears part of the stress at the weld toe, and the steel stress at the weld toe is significantly reduced, which leads to a reduction in the stress concentration factor at the weld toe of the steel and improves the fatigue performance near the weld toe of the steel component. After carbon fiber reinforcement, the fatigue strength of the steel structure specimen with stress cycles exceeding 500,000 cycles is much higher than that of the original weld, and the fatigue strength of 2 million cycles with a 97% guarantee rate can reach 100.BMPa. Therefore, the use of carbon fiber paste technology to improve the fatigue performance of steel components has strong engineering application significance.

5. Prestressed carbon fiber reinforcement

The ordinary surface bonding method is used to reinforce the steel structure. Because the modulus of elasticity is close to that of steel, under normal use, the strength of carbon fiber is not fully utilized, and the reinforcement has no significant effect on the performance improvement during normal use. After prestressing the carbon fiber sheet, and then pasting it on the tension surface of the beam for reinforcement, the above problems can be effectively solved. In addition, the use of pre-stressed drawing tools for reliable anchoring at both ends can also prevent the carbon fiber sheet and steel beam from indicating premature peeling failure. Therefore, prestressed carbon fiber reinforcement has a good application prospect in steel structure reinforcement. However, there are few researches and applications of using prestressed carbon fiber sheets to reinforce steel structures at home and abroad.

Influencing factors of strengthening steel structure with unidirectional carbon fiber sheet

When the overall structure reaches the yield load, if the elastic modulus of carbon fiber is not high, the stress is far from the available strength of carbon fiber. Only when the steel yields, the unidirectional carbon fiber can transmit a larger load, and its high-strength characteristics cannot be exerted. In order to achieve the reinforcement effect, a large amount of carbon fiber is required. Therefore, in order to increase structural rigidity and improve structural fatigue performance, materials with high elastic modulus should be selected. In addition, the greater the thickness of the unidirectional carbon fiber sheet, the better the reinforcement effect, but the greater the interfacial shear stress, the more serious the stress concentration.

The glue layer serves as a load transfer medium between the steel structure and the unidirectional carbon fiber. The performance and thickness of the glue layer have a great influence on the reinforcement effect and the utilization rate of carbon fiber. For the reinforcement effect, the greater the shear modulus of the adhesive layer, the smaller the thickness of the adhesive layer, and the better the reinforcement effect. For the degree of stress concentration, the greater the thickness of the adhesive layer, the more serious the stress concentration of the reinforced component. The greater the shear modulus of the adhesive layer, the smaller the thickness of the adhesive layer, which will increase the degumming range of the adhesive layer and affect the reinforcement effect. Studies have shown that the thickness of the adhesive layer is preferably between 0.05 and 0.3 mm.

In order to reduce the shear stress and peeling stress of the interface at the end of the carbon fiber sheet,

To avoid premature damage to the glue layer and interface in this area, the following measures can be taken:

1. A certain amount of glue overflow is formed at the end of the carbon fiber sheet to reduce the peeling stress at the end;

2. When using carbon fiber plate, cut the plate end into a 45° bevel;

3. Wrap carbon fiber cloth strips to uranium or mechanically anchor the end area of the unidirectional carbon fiber sheet

Prospect of the technology of strengthening steel structure with unidirectional carbon fiber sheet

The unidirectional carbon fiber sheet reinforced steel structure can improve the rigidity and bearing capacity of the original structure on the basis of avoiding the defects of traditional reinforcement methods. In particular, it can significantly increase the fatigue life of fatigue-damaged steel structures and delay the growth of fatigue cracks. Therefore, the unidirectional carbon fiber sheet reinforced steel structure is a promising reinforcement method and will be widely used in practical engineering.