DIC-based strain evolution of HRB500E reinforcing-bar upset straight-threaded sleeve splices under monotonic and cyclic loading

To evaluate the strength, deformation recovery, and local load-transfer behavior of upset straight-threaded sleeve couplers for HRB500E reinforcing bars under monotonic and cyclic loading, 54 specimens with nominal bar diameters of 14, 16, 18, 20, 22, and 25 mm were tested. The program included monotonic tension, high-stress cyclic tension–compression, and large-deformation cyclic tension–compression tests. Extensometers were used to measure residual deformation and total elongation at maximum force, while digital image correlation was employed to characterize sleeve surface strain fields. Under monotonic tension, all specimens failed by fracture of the parent bar outside the sleeve. The mean tensile strengths ranged from 720.46 to 771.72 MPa, residual deformations were below 0.075 mm, and total elongations at maximum force ranged from 11.57 to 17.56%. After high-stress cyclic loading, specimens with bar diameters of 16–25 mm exhibited mean tensile strengths of 735.80–754.27 MPa and residual deformations of 0.0030–0.0877 mm, whereas buckling occurred in the unsupported bar segment of some 14 mm specimens. Under large-deformation cyclic loading, the 16–25 mm specimens retained high tensile strength after the prescribed cycles, while the 14 mm specimens showed greater buckling susceptibility and some 25 mm specimens exhibited pinching near zero load. Digital image correlation revealed relatively continuous sleeve strain distributions in bar-fracture specimens, but pronounced compressive strain concentration on the buckling side. These results provide experimental evidence for evaluating the splice-level performance of high-strength reinforcing-bar mechanical couplers and may provide useful guidance for connection detailing and their potential application in seismic structural systems.