# Numerical Simulation Analysis of Concrete-Filled Circular CFRP-Steel Middle Long Columns
## 2 Axial Compression Test
### Experimental Process  
A 500 t microcomputer-controlled electro-hydraulic servo pressure testing machine was used for the axial compression experiment. As shown in Figure 1, five displacement gauges were, respectively, set at the 1/4 equidistant point of the component and the bottom plate and were used to measure lateral displacement and longitudinal displacement. Four pairs of longitudinal and circumferential resistance strain gauges were set at the middle section of the outer wall of the steel pipe along the circumference and were used to measure the strain at the middle section of the outer wall of the steel pipe [17; 18; 19]. After pasting CFRP, another four strain gauges were set at the middle section of the column in the horizontal direction. The layout of the strain gauges is based on the relevant provisions of the national standards \"Metallic Materials--Tensile Testing at Ambient Temperature\" (GB/T 228-2002) [20] and \"Test Method for Tensile Properties of Oriented Fiber Reinforced Plastics\" (GB/T 3354) [21]. Dynamic and static strain collector DF3817F was used for data acquisition. The data collection was carried out from the beginning of loading the samples. At this time, the pressure gauge pointer of the testing machine started to rotate, and the data collection was uninterrupted during the test until the samples were damaged. Compared with the static strain collector, the dynamic and static collector has obvious advantages, which can be real-time online monitoring, alarming, and printing. It can draw graphics in the software, saving labor costs and reducing errors caused by manual recording so as to improve the overall work efficiency in the test process.  
Before the test begins, the upper and lower surfaces of samples should be calibrated and leveled to prevent eccentric compression. When the surface is paved with fine sand, it is leveled horizontally. In order to avoid uneven edges, steel sheets were inserted into the four corners to realize the basic leveling of the surface. In the elastic range, the load of each stage was about 1/10 of the estimated ultimate load. When the steel pipe yielded, the load of each stage was about 1/15 of the estimated ultimate load. When the sample was close to failure, the load of each stage was about 1/20 of the estimated ultimate load, and the load reached a maximum when the pressure gauge pointer of the testing machine began to return to zero. In this paper, the ultimate bearing capacity is defined as the maximum  
\begin{table}
\begin{tabular}{c c c c} \hline
**Concrete Strength Grades** & **Compressive Strength** & **Elastic Modulus** & **Poisson’s Ratio** \\ \hline C40 & 41.2 MPa & 3.25 \(\times\) 10\({}^{4}\) & 0.2 \\ C50 & 51.3 MPa & 3.45 \(\times\) 10\({}^{4}\) & 0.2 \\ C60 & 60.8 MPa & 3.60 \(\times\) 10\({}^{4}\) & 0.2 \\ \hline \end{tabular}
\end{table}
Table 3: Concrete parameters.  
\begin{table}
\begin{tabular}{c c c c c} \hline
**Material Model** & **Tensile Strength** & **Elastic Modulus** & **Wall Thickness** & **Poisson’s Ratio** \\ \hline SKO & 3500 MPa & 2.51 \(\times\) 10\({}^{5}\) & 0.167 mm & 0.367 \\ \hline \end{tabular}
\end{table}
Table 5: CFRP parameters.  
load that the specimen can bear. At this time, the brittle failure of CFRP will occur, and a large cracking sound can be heard. If the loading continues, the elastoplastic bending instability of the sample will occur. In order to ensure the safety of the test, the test is terminated [22; 23; 24]. The failure mode of all specimens is shown in Figure 2.