Study on The Mechanical Properties of Steel - Basalt Fiber Composite Reinforcement (SBFCBs)

. Steel bar and Basalt Fiber are combined to obtain a new structural material with high strength, high elastic modulus, high toughness, corrosion resistance, low cost and other excellent comprehensive performance: Steel Basalt Fiber Composite Bars (SBFCBs). In this paper, three different types of composite bars were tested by monotonic tensile tests, and the failure patterns of steel bars were introduced in the process of stretching, and the yield strength, ultimate strength, elastic modulus and stress-strain curves of steel bars were obtained. Test results showed that the stress-strain curve of SBFCBs was obviously double-folded, and SBFCBs exhibited stable post-yielding stiffness after the reinforcement yielded. The stress-strain curve model of SBFCBs under uniaxial tension was derived according to the material's compounding rule. By sorting the experimental data and comparing it with theoretical values, we could prove the accuracy of the model.


Introduction
Wu Zhi-shen and Wu Gang proposed a new composite material. Its inner core is ordinary steel bars and the oute r cover is steel fiber composite bars (SFCBs). SFCBs co mbine the advantages of FRP and steel bars: the role of s teel bars in the initial stage SFCBs will be guaranteed to have a higher elastic modulus; FRP is linear elastic, and t he steel bar is elastoplastic. FRP continues to function aft er the steel bar yields, and the stress-strain relationship o f SFCBs at this stage has obvious secondary stiffness [1]. Gu Xing-yu and Shen Xin studied the mechanical prope rties of high modulus basalt fiber-steel wire composite b ars, analyzed the mechanical mechanism of basalt fiber-s teel wire composite bars, and concluded that the stress-st rain relationship curve of basalt fiber-steel wire composit e bars has double Linear feature [2]. Xiao Tong-liang and Qiu Hong-xing proposed a steel-basalt fiber composite t endon curve and hysteresis rule for the asymmetry of ten sion and compression of steel-basalt fiber composite ten dons, and gave a suggested value for the modulus of unl oading modulus [3].
BFRP bars have the advantages of high strength, goo d durability, light weight, etc. The disadvantages are mai nly reflected in low elastic modulus and poor ductility; t he main advantages of steel bars are high elastic modulu s and good ductility, so we can combine the two to get a new materials with good comprehensive properties such as high strength, high elastic modulus, and corrosion resi stance [4][5][6][7]. In order to study the mechanical properties of SBFCBs, this paper conducted unidirectional tensile t ests on 3 different types of SBFCBs, and derived the stre ss-strain constitutive model of SBFCBs.

Theoretical analysis of mechanical properties of SBFCBs
In this paper, the composite rule is used to analyze the str ess-strain relationship of steel-basalt fiber composite bar s (SBFCBs). This rule holds that the interface between th e inner core of the steel bar and the outer layer of the co mposite bar fiber can be well bonded, and deformation c oordination under load [8][9]. The structural diagram of S BFCBs is shown in figure 1. The stress  -strain  of st eel-basalt fiber composite bars is shown by the following formulas (1) and (2) In the formula: b  is the fiber stress; b  is the fiber A is the area of fiber cladding in the cross section of the composite rib; s  is the reinforcement stress; s  is the steel strain; s A is the area of the inner core of the composite reinforcement. The stress-strain curve of the inner core steel bar in S BFCBs was simulated by the double-fold ideal elastic-pl astic model, and the stress-strain curve of BFRP was sim ulated by the complete linear elastic model, as shown inf igure 2. During the tensile test, since the yield strength of the reinforcement is 400MPa, when the strain value of the co mposite reinforcement is around 0.002, the reinforcemen t begins to yield, which specifically shows that the stress growth rate slows down [10]. It is considered that the rein forcement after yield is not considered, and the core rein forcement cannot continue to bear higher loads, so the su bsequent increased loads are all borne by the composite r einforcement and basalt fiber, which is called secondary stiffness. During the entire tensile test, it is assumed that the core steel bar is a completely elastic plastic material, the steel bar is completely elastic at the initial stage, and completely plastic after yielding. The basalt fiber is com pletely elastic throughout the process. ( ) ( )

Specimen size
The models and materials of SBFCBs are shown in

Test methods and equipment
To ensure the normal conduct of the tensile test, the SBF CBs must be anchored at both ends before the test. BFR P is a typical anisotropic material. The transverse and lon gitudinal strength ratios of basalt fiber are small. If tradit ional clip anchors are used, BFRP will fail early in the an choring area. So the straight barrel bonded anchorage (St eel pipe specification: 50mm×5mm, length is 350 mm, a s shown in figure 4) was used in this test. The SBFCBs s pecimens in this test were shown in figure 5.
This test used jacks for loading (as shown in figure 6, 7). Using a pressure ring to measure the tensile force of SBFCBs. Using Y-B-15 handheld strain gauge display(a s shown in figure 8)and Strain gauge to measure the tensi le strain of SBFCBs. This test uses DH3817 dynamic an d static strain test system to collect data.  Installing SBFCBs, pressure rings, jacks, reaction fra mes, etc. before starting the test. This test used the meth od of tension control, the loading rate was 0.05 t/s.

The failure process and failure pattern of the specimen
The entire test process is based on the yield of the steel b ars in SBFCBs. It can be divided into two stages: at the b eginning of loading, the steel bar is in an elastic working state and has not yet yielded, and the core of the steel bar and the outer basalt fiber share the load; The core steel b ar yields first and cannot continue to bear higher loads. T he additional load is borne by basalt fiber, which manifes ts as the strain growth of the reinforcement material acce lerates. With the continuous increase of the load, the bas alt fiber eventually breaks down and its bearing capacity. When it reaches the maximum value, its bearing capacit y drops rapidly and the test ends. It can be found that SB FCBs have the following main characteristics during the stretching process.
(1) The failure part of the test piece is concentrated i n the middle area of the reinforcement, and there is almo st no fiber failure in other places.
(2) Before the test, the anchorage was blocked to pre vent the reinforcement from being pulled out before it w as damaged. After the test, through observation, there wa s no obvious deformation and crack at the orifice of the a nchorage, indicating that the anchorage effect of the anc horage on the composite reinforcement was very good.
(3) The damage of SBFCBs started from the breakag e of the outer basalt fiber. During the loading process, th SBFCBs specimen e harsh sound of the gradual break of the fiber will be he ard. When the ultimate load is reached, the outer basalt fi ber is suddenly broken and accompanied by a loud noise. The morphology of SBFCBs failure is filamentary blasti ng, as shown in Figure 9.

Test results and analysis
According to the formula (3) derived from the law of co  mposite materials, combined with the physical properties  of materials in table 2 and table 3 figure 10 below.
It can be clearly seen from Fig. 10 that the stress-strai n curves of SBFCBs all show obvious double-folded line s. After the steel bar is yielded during the stretching proc ess, the composite bars have obvious secondary stiffness. It agrees well with the theoretical curve.Through the co mparison and analysis,wo can find some facts.
(a) The measured yield strain of composite bars is ver y close to that of ordinary steel bars.
(b) The measured ultimate strength of the composite bar is less than the theoretical value. It can be judged fro m the ultimate strength of SBFCBs that the core reinforc ement and basalt fiber are effectively bonded, and they a re jointly stressed, but it cannot guarantee that all basalt f ibers and the core reinforcement deform synchronously when bearing the load, which results in the composite rei nforcement ultimate strength below theoretical value.
(c) The ultimate strain of composite reinforcement is slightly smaller than the elongation of basalt fiber

Conclusion
In this paper, three different types of SBFCBs were subje cted to tensile tests,and the test values were compared wi th theoretical values to draw the following conclusions: (1) During the tensile test, the failure of SBFCBs began with the fracture of the outer basalt fiber. During the loading of the specimen, the core steel bar yielded first. As the load increased, the outer basalt fiber assumed a larger load. When the ultimate strength was (2) The stress-strain curve of SBFCBs was obviously double-folded. Taking the steel bar yield as the dividing point, the test process was divided into the initial stage a nd the post-yield stage: when the steel bar was yielded, it could withstand larger loads and exhibit a clear "post-yi eld stiffness", which was called secondary stiffness.
(3) The stress-strain curve model of SBFCBs under uniaxial tension was derived according to the material's compounding rule. By sorting the experimental data and comparing it with theoretical values, we could prove the accuracy of the model.