## 4 Mechanical Properties
### Longitudinal and Hoop Stress Distributions of Sleeve  
4.1 Longitudinal and Hoop Strain Curves for the Middle Section of the Sleeve near Steel Bar under Uniaxial Tension and Repeated Tension and Compression (Only Final Tension Process) Loading  
Figure 18 shows the comparison of the longitudinal strain curves near the steel bar side of the sleeve, and Figure 19 shows the comparison of the hoop strain curves of the sleeve under uniaxial tension and repeated tension and compression (only the last tension process) loading. The variation trends of the longitudinal and hoop strains of the sleeve were fairly consistent between the cases of uniaxial tension and repeated tension and compression (only the last tension process) loading. The longitudinal strain was mostly forced via compression and the hoop strain was mostly forced via tension near the steel bar side at the middle cross-section of the sleeve (the reasons are presented in Section 6). The difference was that the load was constant but the longitudinal and hoop strains of the sleeve had increased (the curve was in the horizontal section) at the beginning of the load of the last tensile part after the high-stress tension and compression. This was because all the measurement data were zeroed, and then the connection was changed to uniaxial tension after high-stress repeated tension and compression loading. After the zeroing process of the data acquisition instrument, the strain of each strain gauge was zero while the residual strain (from \(-111\) to \(161\)\(\mu\epsilon\)) of the sleeve existed. The residual strain of the sleeve was eliminated with the increase in the uniaxial tension load, so a horizontal section of the curve existed. Figure 18: Load–longitudinal strain curves for the middle section of the sleeve near the steel bar under uniaxial tension and repeated high-stress tension and compression loading (the final tension process) (SZ3). (**a**) \(L=6d\), (**b**) \(L=8d\), (**c**) \(L=10d\), (**d**) \(L=12.5d\). The influence of the lapping length on the average longitudinal and hoop strains of the middle section of the sleeve is shown in Figure 20. The longitudinal compressive strain and the hoop tensile strain of the middle section of the sleeve were decreased under uniaxial tension and repeated tension and compression (only the final tension process) loading with the increase in the lapping length of steel bars. Figure 19: Load–hoop strain curves for the middle section of the sleeve near the steel bar under uniaxial tension and repeated high-stress tension and compression loading (the final tension process) (SH3). (**a**) \(L=6d\), (**b**) \(L=8d\), (**c**) \(L=10d\), (**d**) \(L=12.5d\). Figure 20: Effect of lapping length on the longitudinal and hoop strains of the middle section of the sleeve under the ultimate load during uniaxial tension and repeated tension and compression (only the final tension process) loading. 4.2 Longitudinal and Hoop Strain Curves of the Middle Section of the Sleeve near Steel Bar Side under High-Stress Repeated Tension and Compression Loading  
Figure 21 shows the longitudinal strain curves of the middle section of the sleeve near the steel bar side under high-stress repeated tension and compression loading. When the lapping length was short, the curve was inclined toward the upper left and lower right sides. With the increase in the lapping length, the curve was inclined toward the lower left and upper right sides, indicating that the sleeve strain behaviour changed with the change of the lapping length. For the specimen with a lapping length of \(6d\) (Figure 21a), when it was under tension, the sleeve was mostly compressed longitudinally (the ordinate in Figure 21 was greater than 0). Under compression (ordinate in Figure 21 was less than 0), the sleeve was compressed when the load was small and stretched as the load increased. The reasons for these observations are discussed in Section 6.