Steel structures have many advantages, including excellent seismic performance, high strength, fast construction speed, stable quality, light weight, and green environmental protection [
1,
2,
3,
4]. However, due to the external environment as well as internal defects and human factors, steel structures are likely to accumulate damage over the course of their service life. Although traditional welding, bolted connections, and external steel plate reinforcement methods can improve the bearing capacity of the damaged structures, they may also cause further damage and additional weight to the structures [
5,
6,
7,
8,
9]. There are many advantages to using fiber-reinforced polymer sheets for reinforcement, such as corrosion resistance, light weight, simple construction, environmental friendliness, etc., which can be adopted to improve the bearing strength, corrosion resistance, and compressive buckling capacity of the steel structures [
10,
11,
12,
13,
14].
The advantages of fiber-reinforced polymer (FRP) include corrosion resistance, light weight, simplicity of construction, no pollution, and superior fatigue resistance. Steel structures can be strengthened using FRP to increase their ultimate strength and buckling resistance, as well as protect them from corrosion [
15,
16,
17,
18]. At present, the main fiber-reinforced polymers are carbon-fiber-reinforced polymer (CFRP), glass-fiber-reinforced polymer (GFRP), and basalt-fiber-reinforced polymer (BFRP). Basalt fiber is a pure, natural, inorganic material made from natural rocks. After use, it can be returned to nature without special treatment. In comparison with CFRP and GFRP, BFRP has the advantages of environmental protection, low cost, superior insulation, low hygroscopicity, and abundant raw materials [
19,
20].
Many researchers have carried out experiments on the bonding properties of FRP-to-steel interface, and corresponding theoretical formulae have been proposed [
21,
22,
23,
24,
25]. Doroudi et al. [
26] investigated the behavior of CFRP-to-steel bonded joints under quasi-static cyclic loading using nine single shear pull-off test specimens, in which three different bond thicknesses and both quasi-static monotonic and quasi-static cyclic loading methods were considered. The results showed that the stiffness of the load-displacement curves tends to decrease with an increasing number of loading cycles. Bocciarelli et al. [
27] conducted an experimental study on the fatigue behavior of steel structures retrofitted by FRP materials. The results showed that the fatigue resistance of the interface between the steel plate and the CFRP plate is remarkably superior to that of welded cover plates. Dehghani et al. [
28] proposed a new method for the analysis of bonded connections of CFRP and steel substrates. A comparison of results obtained from their proposed model and experimental data showed that the ultimate debonding load could be accurately estimated by their proposed model. In this model, unlike some previous bond-slip models, the ultimate debonding load is independent of the thickness of the adhesive. Biscaia et al. [
29] investigated the performance of CFRP-to-steel bonded joints under different temperatures (20 °C, 35 °C, 50 °C, 65 °C, 80 °C, and 95 °C). According to the local bond-slip behavior of the tested specimens, a temperature-dependent bond-slip model with a bi-linear shape was proposed and implemented into commercial software based on the finite element method. Yu et al. [
30] conducted an experimental study on the behavior of CFRP-to-steel bonded interfaces by testing a series of single-lap bonded joints. The test results showed that the bond strength of such bonded joints depends strongly on the interfacial fracture energy among other factors. Wu et al. [
31] conducted a series of static and fatigue tests on UHM (ultra-high modulus) CFRP plate and steel plate double strap joints to investigate the effect of residual bond strength and residual bond stiffness on bond behavior. Wu et al. [
32] studied the bond characteristics between ultra-high modulus (UHM) CFRP laminates with a modulus of 460 GPa and steel through a series of experiments with double strap steel joints bonded with UHM CFRP laminates. The effect of two types of adhesives (Araldite and Sikadur) on the failure modes, bond strength, effective bond length, CFRP strain distribution, adhesive layer shear stress distribution, and bond-slip relationship were discussed. Then, theoretical models were employed for the prediction of the specimen bond strength and effective bond length, and their applicability for UHM CFRP steel joints was verified by comparisons with experimental results. Golewski [
33] studied the tensile behavior of double- and triple-adhesive single-lap joints. It was found that the energy required to damage the joint when using double-sided adhesive tape is several times greater than that required to damage a joint made with epoxy. Jawdhari et al. [
34] investigated the bond-slip relationship between CFRP rod panels and concrete. They found that the debonding load of a CFRP rod panel is higher than that of an externally bonded conventional CFRP plate of similar cross-sectional area and mechanical properties. Mukhtar et al. [
35] tested the adhesive bond between FRP laminate and concrete in both double shear and mixed mode (shear/peeling) using a novel test apparatus. Test results showed that the bond capacity decreases as the peel angle increases. Altaee et al. [
36] studied the bond-slip relations of CFRP-steel joints using a new bilinear model. The proposed model resulted in excellent predictions in terms of the ultimate strength, failure mode, and all other interfacial properties. Jawdhari et al. [
37] carried out a three-dimensional nonlinear finite element analysis to study the strip panels’ interfacial and flexural properties. As a result, strip panels with finger joints were found to carry more load for a given CFRP thickness and splice plate length.
Most of the above studies focused on the bonding behavior of the interfaces regarding shear forces. However, the failure of the FRP-to-steel interface is related to the mixed-mode debonding failure, i.e., the combined actions of shear and peeling forces [
38,
39,
40]. In order to study the failure mode and debonding behavior of the interface between BFRP (basalt fiber reinforced polymer) sheet and structural steel under mixed-mode loading conditions, eighteen specimens with different initial angles were tested in this study. The specimens were designed with different initial angles to ensure that the interface was tested under shear and peeling mixed-mode conditions. Finally, a new calculation method for predicting the bond strength between BFRP sheets and steel under shear-peeling fracture is proposed.