1.1 Finite element model dimensions

The size of the steel plate test piece is 500mm×20mm×5mm. The pasting length of carbon fiber cloth is 400mm, and the pasting width is 20mm, see Figure 1. In order to better simulate the actual test loading process, the loading method with surface load applied at the end of the steel plate.

1.2 Model establishment and handling of anchoring methods

Since the structure is symmetrical, when building the model, half of the structure is used to build the model. According to the needs of the analysis, the finite element models under the three anchoring methods of CFRP end unanchoring, u-shaped anchoring and permanent anchoring (ideal anchoring) were established respectively (see Figure 2).

Among them, the three-dimensional solid unit-three-dimensional solid unit-shell unit is adopted, that is, the solid45 unit is used for the steel plate and the middle adhesive layer, the shell41 unit is used for the carbon fiber cloth, and the u-shaped hoop is anchored by the CFRP winding anchorage shell41 unit simulation.

For the simulation of the anchoring end of the permanent anchoring method (ideal anchoring method), the spring unit is used to connect the CFRP cloth and the steel plate to achieve the permanent anchoring form of the CFRP end. The permanent anchoring is an ideal complete anchoring form.

1.3 Calculation scheme of U-shaped hoop anchorage

The model reinforcement design scheme is as follows:

(1) No anchoring at the end of CFRP. Paste 1 layer, 2 layers, 3 layers CFRP respectively;

(2) U-shaped anchor at the end of CFRP. The length of u-hoop is 2mm, but the number of layers of u-hoop is 1, 2, and 3 respectively;

(3) U-shaped anchor at the end of CFRP. The number of U-shaped layer is 1 layer, but the anchor length is 1mm, 2mm, 4mm;

(4) Permanent anchoring of CFRP ends. Paste 1 layer of CFRP, and the anchor length is 2mm.

2 Analysis of relevant parameters of CFRP end unanchored

2.1 Interfacial stress without CFRP end anchoring

It shows the interface shear stress curve of CFRP at different ends when CFRP is pasted with different layers.

It can be seen from Figure 3 that when the CFRP end is not anchored, the bonding shear stress at the CFRP end gradually increases as the number of CFRP layers attached to the structure increases. It can be seen that when the CFRP ends are not anchored in actual engineering structure reinforcement, the more CFRP layers are pasted, the earlier the CFRP ends will be peeled and damaged, resulting in a low CFRP utilization rate of multiple layers, resulting in waste of materials. Therefore, when pasting multiple layers, the end of CFRP must be anchored.

2.2 Interface bonding stress of different anchoring methods at the ends

When the end of CFRP is not anchored, the shear stress without anchor is the largest. The shear stress anchored by the U-shaped hoop is centered, mainly because the U-shaped hoop shares part of the CFRP end shear stress value. In the ideal state, the shear stress of the permanent anchoring method is the smallest, mainly because the permanent anchoring method assumes almost all the shear stress values in the adhesive layer. It can be seen that the stronger the anchoring method, the smaller the shear stress in the adhesive layer at the end, and the less likely it is to break. Therefore, in actual engineering, a stronger anchoring method should be adopted as much as possible.

2.3 Interfacial bonding stress with different U-shaped hoop anchor layers

The interface shear stress curve with different number of u-hoop layers when pasting 1 layer and 2 layers of CFRP can be concluded that as the number of U-hoop winding layers increases, the bonding stress between the interface of the end anchoring area decreases significantly. And as the number of u-hoop anchor layers increases, the smaller the shear stress at the CFRP end, the lower the probability of premature failure of the CFRP end. Within a certain range, the more u-shaped hoop layers, the stronger the anchoring degree of the CFRP end, and the better the effect of anchoring, but the u-shaped hoop should not exceed 3 layers at most.

2.4 Interfacial bonding shear stress with different u-shaped hoop anchor width

When one layer of CFRP is pasted, the u-shaped anchor widths at the ends of CFRP are 0cm, 1cm, 2cm, and 4cm respectively. Observation shows that within the effective bonding range, with the increase of the anchoring length, the shear stress at the end gradually decreases. Beyond the effective bonding length, the bonding shear stress at the end of the anchor length increases basically unchanged. Therefore, in actual structural reinforcement, the length of the end anchoring should be paid attention to to prevent the anchoring length of the end of the structure from being too short, resulting in the premature stripping failure of the CFRP end due to the shear stress concentration.

In conclusion 

In this paper, the finite element model of the u-hoop applied to the CFRP end under different parameter conditions and the u-anchor end to the interface bonding stress change law under different conditions can be analyzed, and the following conclusions can be drawn:

(1) When the end is not anchored, the stress on the bonding surface will be reduced to different degrees after the end is anchored, which alleviates the stress concentration at the end. The CFRP and the reinforced structure coordinate the force better and improve the CFRP reinforcement. The effect of the structure can effectively avoid the peeling and destruction of the end of the fiber cloth;

(2) Under the same external conditions, increasing the number of CFRP layers will increase the interface bonding shear stress. In actual structural reinforcement, the number of CFRP layers should not exceed 3 layers;

(3) Increasing the number of u-shaped hoop layers can reduce the bonding stress at the end and ensure that the ability of CFRP to work in coordination with the original structure is stronger;

(4) Within the effective bonding length range, as the u-shaped anchor width increases, the shear stress at the end bonding surface decreases continuously; beyond the effective bonding length, the interface bonding stress remains basically unchanged