Search

Ni–Ti–Co is a new shape memory alloy (SMA) composition that has a higher strength and a lower superelastic temperature range than the traditional binary Ni–Ti SMA composition. In this paper, the superelastic properties of Ni–Ti–Co bars that are important for seismic applications were studied and compared with those of Ni–Ti bars. The superelastic behavior, strength, strain recovery, low-cycle fatigue characteristics, and fracture strains of Ni–Ti–Co and Ni–Ti SMA with different heat treatment strategies and testing temperatures from −40 °C to 50 °C were investigated with seismic applications in mind. The effect of low-cycle fatigue loading and temperature variations on the superelasticity degradation of Ni–Ti–Co SMA were evaluated. The results showed that the yield stress of Ni–Ti–Co SMA at room temperature was 1.41–1.74 times that of the Ni–Ti SMA. Ni–Ti–Co SMA exhibited 100% strain recovery when unloaded from 5% strain in a temperature range from −40 °C to room temperature; while at 50 °C, a residual strain of 0.6% was observed when unloaded from 5% strain. For comparison, the Ni–Ti SMA lost superelasticity when the temperature was reduced to 0 °C. When subjected to low-cycle fatigue loading, the stability of the superelastic behavior (maintaining yield stress, energy dissipation and strain recovery) of Ni–Ti–Co SMA at −40 °C was better than that at room temperature and 50 °C. In particular, the superelasticity of Ni–Ti–Co SMA at −40 °C was superior to that of the Ni–Ti SMA at 0 °C. At −40 °C, the Ni–Ti–Co SMA showed almost no loss in yield stress, damping ratio and recovery strain during the first 100 cycles of fatigue loading. Moreover, from 100th to 471st cycle, at which the fracture occurred, the strain recovery of Ni–Ti–Co SMA showed almost no degradation. [ABSTRACT FROM AUTHOR]

Iron-based shape memory alloys (FeSMAs) are emerging as a promising material for use in post-tensioning concrete structures to provide self-centering capabilities during a seismic event. Past experimental studies on FeSMA focused on strengthening or repairing existing structural components under gravity loading. In addition to the structural rehabilitation, FeSMA also have potential for use in self-centering columns subjected to seismic loads. However, the basic material properties, such as strength, ductility, recovery strain, actuation stress (i.e. prestress) stability, low-cycle fatigue resistance, and temperature dependence of FeSMA related to self-centering column applications have not been studied extensively to-date. To fill this knowledge gap and determine the feasibility of using FeSMA in self-centering columns, this study performed a comprehensive characterization and analysis of FeSMA both before and after actuation (i.e. thermal stimulation). The strength, ductility, energy dissipation, and recovery strain of FeSMA before actuation were tested at different temperatures from −40 °C to 50 °C. After actuation, the actuation stress, low-cycle fatigue resistance, and strain capacity of FeSMA were tested at different temperatures from −40 °C to 50 °C and prestrain levels from 4% to 30%, and under low-cycle fatigue loading with strain amplitudes from 0.5% to 1.0%. The results from this study demonstrated that FeSMA exhibit high ductility, cyclic actuation stress stability, and low-cycle fatigue resistance at temperatures from −40 °C to 50 °C. Furthermore, it was found that increasing the prestrain level can effectively increase the post-actuation strain amplitude at which the actuation stress reduces to zero. A prestrain level between 15% and 20% is recommended for application of FeSMA in self-centering columns. The research findings from this study demonstrated the feasibility of using FeSMA in self-centering columns subject to seismic loading. [ABSTRACT FROM AUTHOR]

Cu-Al-Mn (CAM) shape memory alloys (SMAs) are cost effective, have a high low-cycle fatigue life and superelastic limit, and a wide temperature application range compared with other types of SMAs. These characteristics of CAM SMAs have resulted in increased interest in their use in civil engineering applications, particularly as reinforcement in concrete structures and as dampers in steel structures. However, these applications require machining of the CAM SMA bars for connection with other structural elements. This study presents the methods and results of the first systematic research on the machinability of CAM SMAs. The key machinability characteristics of CAM SMAs: chip formation, cutting temperature, tool wear, workpiece surface roughness, and diameter deviation, were studied and compared with those of conventional NiTi SMAs and commonly used steels: mild steel (MS) and 304 stainless steel (SS). The effects of a wide range of cutting parameters: cutting speed ranging from 15 to 120 m/min , feed rate ranging from 0.1 to 0.2 mm/revolution , and depth of cut ranging from 0.5 to 1.5 mm, were investigated. The results from this study showed that the tool wear from machining CAM SMAs was close to that from machining SS and slightly higher than that from machining MS, but much lower than that from machining NiTi SMAs. In all the cases considered here, the tool wear from machining CAM SMAs was found to be 0.6–1.8 times that from machining SS, 0.8–2.4 times that from machining MS, and 1/7–1/21 that from machining NiTi SMAs. After a continuous machining test with a total cutting length of 4.5 m, the nose wear from machining CAM SMAs was found to be 1.6 times that from machining MS, the average flank wear from machining CAM SMAs was found to be three times that from machining MS, and the diameter deviation (relative diameter difference from that of the first sample) of CAM SMAs was only 10 μm larger than that of MS. [ABSTRACT FROM AUTHOR]

Characteristics of near-fault ground motions warrant special consideration due to their severe and impulsive effects on structures. These characteristics are unique compared to far-field ground motions, upon which nearly all seismic design criteria are based. The objectives of this study were to explore the shake table response of reinforced concrete, to investigate near-fault ground motion effects on reinforced concrete bridge columns subjected to near-fault ground motions, and to provide a framework for the evaluation of bridge columns near active faults. Two large-scale columns were designed and tested under a near-fault ground motion on a shake table at the University of Nevada, Reno. One column represented the current California Department of Transportation far-field design, and the other was based on the American Association of Highway and Transportation Officials provisions. The most unique measured response characteristic in both columns was the large residual displacements even under moderate motions. A new hysteresis model was developed to capture this effect and was incorporated in an analytical model. Based on this finding, a framework for the evaluation of reinforced concrete bridge columns with respect to the control of residual displacement was proposed. DOI: 10.1061/(ASCE)0733-9445(2007) 133:7(982) CE Database subject headings: Impulsive loads; Seismic design; Bridges, concrete; Concrete, reinforced; Hysteresis; Strain rate; Plastic hinges; Columns; Ground motion.

This paper discusses the superelastic behavior of shape memory alloy, Nitinol (NiTi), when used as reinforcement in concrete beams. The ability of NiTi to recover and reduce permanent deformations of concrete beams was investigated. Small-scale concrete beams, with NiTi reinforcement were tested under half-cycle loads. The reinforcement ratio varied from 0.1 to about 0.9%. The experimental results showed that the average residual displacement in the NiTi reinforced beams was less than one-fifth of that of the steel reinforced test beams. The stiffness of NiTi reinforced test beams however was lower than those of the steel reinforced test beams. An analytical study of load-deflection relationship for a series of hybrid beams was undertaken. It was found that a hybrid system that incorporated NiTi combined with high strength steel or carbon fiber reinforced plastic bars was a better choice for design because of their relatively high stiffness. DOI: 10.1061/(ASCE)0899-1561(2007)19:6(454) CE Database subject headings: Deflection; Concrete beams; Cracking; Shape memory effect; Ductility; Flexural strength; Deformation.

An analytical and experimental study of reinforced concrete flared bridge columns was conducted to determine an appropriate retrofit method for columns with inadequate shear capacity under earthquake loads. Four 0.3-scale models were tested on a shake table to failure, one representing as-built columns and the other three retrofitted with jackets, one with steel, one with a glass fiber reinforced plastic (FRP), and the third with a carbon FRP jacket. The jackets were designed to keep the plastic hinge away from the column end. A new lay up method was developed for FRP fabrics used on nonprismatic members. The results showed that all jackets improved the seismic performance. The effectiveness of the jackets was nearly the same for all materials. The method to estimate shear capacity based on the guidelines of the California Department of Transportation and a modified version of that method were found to closely estimate the shear capacity. However, the method in the Federal Highway Administration seismic retrofit manual overestimated the shear capacity and was unconservative. CE Database subject headings: Bridges; Retrofitting; Concrete columns; Earthquake loads; Concrete, reinforced; Shear strength.