Mechanical splicing of superelastic Cu–Al–Mn alloy bars with headed ends

Abstract

This paper examines the feasibility of mechanical splicing using a steel coupler to connect headed ends of superelastic Cu–Al–Mn alloy (Camalloy) bars and steel reinforcing bars to be used in concrete structures. Although threading of Camalloy is as easy as that of steel, mechanical splicing using threaded ends requires machining of Camalloy bars into dog-bone shape to avoid brittle fracture at the threaded ends. The machining process requires significant time and cost and wastes substantial amount of the material. This paper attempts to resolve this issue by applying mechanical splicing using steel couplers to connect headed ends of Camalloy and steel reinforcing bars. To study its feasibility, we prepare 3 specimens wherein both ends of each Camalloy bar (13 mm diameter and 300 mm length) are connected to steel reinforcing bars. The specimens are tested under monotonic, single-cycle, and full-cycle tension loading conditions. From these tests, we observed (1) excellent superelasticity with recoverable strain of around 6% and (2) large ductility with fracture strain of over 19%. It should be emphasized here that, in all the specimens, ductile fracture occurred at the locations apart from the headed ends. This is in sharp contrast with brittle fracture of headed superelastic Ni–Ti SMA bars, most of which took place around the headed ends. From the results of the microstructural analysis, we identified the following reasons for avoiding brittle fracture at the headed ends: (1) Precipitation hardening increases the strength around the boundary between the straight and headed (tapered) portions, where stress concentration takes place. (2) The strength of the straight portion does not increase significantly up to the ductile fracture if its grain orientation is close to 〈0 0 1〉.

Orientation Dependence of Plasticity and Fracture in Single-Crystal Superelastic Cu-Al-Mn SMA Bars

“…The superior ductility without strong hardening observed in Groups G2 to G4 are noteworthy from the viewpoint of seismic applications, where ductility is critical. The small or moderate hardening is important in avoiding brittle fracture at connections (Kise et al 2018). To the authors' knowledge, such superior ductility of single-crystal SMAs-especially in the form of bars of 8-mm diameter and 40-mm gauge length-has not been reported in the literature; however, the orientation dependence of superelasticity (Horikawa et al 1988) and multistage superelasticity (Otsuka et al 1979) were reported for single-crystal superelastic Cu-Al-Ni SMAs.…”

Feasibility of Roll-Threading Superelastic Cu-Al-Mn SMA Rods

“…One may argue that it is better to perform roll-threading at the α þ β phase before the solution-heat treatment because Cu-Al-Mn alloy has excellent cold workability (Kainuma et al 1996). However, this was intentionally avoided here because the rolled threads can deform significantly during the solution-heat treatment, and the high strain at the rolled threads can prevent crystal growth to obtain single crystals (Kise et al 2018).…”

“…To avoid brittle fracture at the cut threads, however, machining of SMA rods into a dogbone shape is necessary, as demonstrated in superelastic Ni-Ti SMA rods (DesRoches et al 2004;McCormick et al 2007); this process requires significant time and cost and wastes substantial amounts of the material. To address these issues, the feasibility of mechanical splicing was studied between steel rods and Cu-Al-Mn SMA rods with headed ends (Kise et al 2018). In the feasibility study, it was demonstrated that ductile fracture took place apart from the heat-treated headed ends by controlling the microstructure.…”

“…This method aims to roll-thread single-crystal Cu-Al-Mn SMA rods without losing their excellent superelasticity obtained by aging. Because no additional heat treatment from the ordinary manufacturing process is necessary, roll-threading is cost effective and suitable for mass production-this is the key difference from the heading process proposed previously (Kise et al 2018). It has been long recognized that both the material cost itself and the processing cost for connection are the critical issues in commercializing SMAs in structural applications (Ozbulut et al 2011;Chang and Araki 2016;Lecce and Concilio 2014).…”

SMA bending bars as self-centering and damping devices

“…Based on the application, the SMAs can be used in the forms: (1) fibers and wires used for retrofitting cementitious composites, (2) wires and rebars used for reinforcing concrete structures, and (3) bars used for damping and self-centering devices [19][20][21][22][23][24][25]. Recently, confinement, self-centering devices, energy dissipation devices, and passive control devices have been studied and developed using SMA wires such as works done by Choi et al, Araki et al and Qui et al and Dolce et al [26][27][28][29][30].…”