Monofilament SMA wire is one of the most popular elements because of its mature manufacturing technology, good availability in market, high load resistance efficiency, and simple way of working. Monofilament SMA wires are tension-only elements, and are often considered in a damping device. Monofilament SMA wires have already been used in historic buildings such as the S. Giorgio Church, the St. Francis Basilica and the San Feliciano Cathedral (Indirli and Castellano, 2008). Preload can be applied to the SMA wires to enhance the initial stiffness, “yield” strength, and self-centering capability. The fundamental mechanical behavior of individual SMA wires has been widely studied since the 1990s (Piedboeuf et al., 1998; Tobushi et al., 1998; Wolons et al., 1998; Dolce and Cardone, 2001; Tamai and Kitagawa, 2002). One consensus from the existing studies is that an SMA wire, if produced through an appropriate manufacturing process, can exhibit outstanding ductility, hysteretic repeatability, recoverability, and fatigue performance. However, the behavior of an SMA wire is indeed sensitive to the temperature and loading rate.

However, great challenges still exist in practical implementation of SMA wires. One concerning issue is the difficulty of anchorage, especially for SMA wires with a diameter exceeding 1 mm. This problem results from the smooth surface of the wires and high hardness of the SMA material, making conventional grippers such as U-connectors ineffective. Another concern is the relatively small load resistance of individual SMA wires, rendering hundreds or even thousands of SMA wires needed in a “full-scale” device. This not only complicates the arrangement of the wires but also makes anchorage more challenging. Therefore, SMA wires are more suited to proof-of-concept studies, whereas the opportunity for application in modern construction is remote.

Randomly distributed superelastic SMA fibers have recently been proposed to promote crack closure capability of concrete members. Failure to control crack of a concrete structure is often responsible for corrosion of the reinforcement and hence deterioration of the functionality and serviceability over its service life. SMA fibers are expected to be a viable alternative to conventional steel fibers, where the latter undergoes permanent residual deformation after yielding and the cracks of the member would not close if no external treatment (e.g., external tensioning) is applied. The SMA fibers are also very competitive when compared with fiber reinforced polymer (FRP) products which have limited ultimate tensile strain (and hence tends to fail in a brittle manner) and behave linearly with no energy dissipation capability.

The development of superelastic SMA fibers is still in its early stage, where strands and wires (Daghash and Ozbulut, 2016; Sherif et al., 2017, 2018) were mostly considered. Bonding is critical issue that cannot be easily resolved because form-setting of superelastic SMA fibers is difficult and expensive and often requires special treatment such as annealing. Some researchers have studied the use of thermally activated martensitic SMAs as fibers in cementitious matrix to provide prestressing, and these fibers are more easily produced into dog-bone shaped, paddled, chipped, and crimped shapes (Lee et al., 2018, 2021).

Larger scale SMA bars may be more preferred for civil engineering application because of their large load resistance and ease of construction. One of the most typical application scenarios for SMA bars is beam-to-column connections, where SMA bars are further machined into the bolt form (Speicher et al., 2011; Fang et al., 2014; Wang et al., 2017a; Fang et al., 2017, 2018a). These bolts, which are often preloaded, provide moment resistance, energy dissipation, and self-centering capabilities for the connections. Alternatively, SMA bolts can be buckling-restrained such that they are subjected to both tension and compression, providing stable two-directional flag-shaped hysteresis. Some new damping devices have been developed based on this concept (Wang and Zhu, 2018a; Qiu et al., 2020).

Caution should be exercised that SMA bolts are vulnerable to premature fracture at the threaded section. According to the existing experimental study (Fang et al., 2014; 2015a), the net threaded-to-shank diameter ratio of SMA bolts are recommended to range between 1.25 and 1.4. A smaller ratio would cause unexpected fracture at a large strain, whereas an overly large ratio would cause more significant material waste during the machining process. In addition, the performance of SMA bars/bolts is generally inferior to SMA wires, and special annealing is needed to trigger superelasticity (Wang et al., 2016).

SMA cable is a viable solution that exhibits the benefits of both SMA wires and bars. An SMA cable is produced by helically wrapping a series of SMA strands, and each strand consists of a series of helically wrapped monofilament SMA wires. Existing experimental studies on SMA cables (Reedlunn et al., 2013; Mas et al., 2017; Sherif and Ozbulut, 2017; Fang et al., 2019a) showed that their hysteretic behavior is comparable to monofilament SMA wires; compared with SMA bars, SMA cables are more cost effective and requires no annealing. However, the performance tends to deteriorate when an increased number of SMA wires are involved in a cable. This phenomenon could be due to the more pronounced nonlinear stress distribution for larger scale SMA cables. Moreover, as SMA cables are harder than steel ones, anchorage is a major challenge. Failure of SMA cables is often found at the anchorage due to slippage. Shi et al. (2021) recently proposed two mechanical anchorage solutions, namely, spike-type and end stop-type anchorages, and their reliability was examined through a series of tests. End stop type was found to be most suited to SMA cables. SMA cables have now been considered in many structural members such as braces (Chen et al., 2020) and isolation bearings (Liang et al., 2020). In 2021, SMA cable-based bearings was used for the first time in the Datianba #2 highway bridge in China (Fang et al., 2022b).

The available tests on superelastic SMA flat plates are limited. Murasawa et al. (2006) first carried out tensile tests on SMA plates, and confirmed the strong link between inhomogeneous deformation behavior and macroscopic deformation behavior when they are subjected to cyclic loading. Zhou et al. (2021) examined the tension-release behavior of SMA flat plates, with particular focus of their potential applications in beam-to-column connections. A similar study was conducted by Wu et al. (2022) who focused on timber connections. The SMA plates were found to display similar hysteretic behavior to SMA bars. For practical application, SMA plates may be bolt-connected to the adjacent members, rendering potential weak sections and hence premature fracture near the bolt holes. This should receive sufficient attention, but no study is currently available.

SMA U-shaped plate is a special type of damping element that can provide large deformability and relatively high flexibility, making it particularly suitable for isolation bearings. A unique bending behavior is displayed by a U-shaped plate which consists of a flexible circular region. The geometry allows either in-plane or out-of-plane paralleled movements between two straight legs. So far, the only experimental results on SMA U-shaped plates were reported by Wang and Zhu (2018b), who confirmed that the element could exhibit stable flag-shaped hysteretic behavior when subjected to multiple loading cycles, with residual deformation being less than 2% of the maximum deformation. The main concern is the low-cycle fatigue facture which tends to occur at the junction between the circular region and the straight legs, where a high stress demand exists. The occurrence of fracture may also be related to the manufacturing process where stress concentration and cold hardening occur during the bending process. Fe-SMA U-shaped plates were also tested (Zhang J. et al., 2022), where superior low-cycle fatigue resistance was observed.

Angles are a common type of connecting elements used in connections. They are expected to transfer axial force, shear force, and bending moments, and provide energy dissipation for the connections when earthquake happens. When angles are endowed with superelasticity, extra self-centering capability is enabled. Wang et al. (2020) conducted an experimental study on SMA angles. The results are similar to those from the SMA U-shaped plate tests. Again, low-cycle fatigue fracture at stress concentration location occurred which should receive sufficient attention.

SMA rings enable flexible deformation at any direction of loading because of their bisymmetric configuration. This is an essential advantage compared with other SMA elements which are expected to act only in one or two directions. The load carrying mechanisms of an SMA ring subjected to pushing and pulling actions are not the same, leading to slightly different hysteretic behaviors. Under idealized situations, e.g., no fracture happens, the SMA rings would first experience bending and finally be stretched when under pulling; and would under significant bending until fully compressed when under pushing. Dong et al. (2006) first conducted a proof-of-concept study on 1 mm-thick SMA rings. Gao et al. (2016) tested a full scale SMA ring, and employed it in a cross-braced system with the ring serving as central damping elements linking four tension cables. The manufacturing steps of an SMA ring include disk cutting, heating, hammer forging, ring rolling, and annealing. Alternatively, Choi et al. (2018) applied wire cutting to flat SMA plates to produce SMA rings.

SMA helical springs can have a very large stroke and could recover large deformations accompanied by moderate energy dissipation. The high flexibility and large deformability make them ideal candidates for structural isolation and other scenarios where large deformation demand is expected (Liu et al., 2019). The typical manufacturing of a SMA helical spring involves: 1) heating SMA wires or bars to low-red heat temperatures, 2) bending the softened SMA around a mandrel to produce a helix, and 3) applying annealing to the finished coils to enable form-setting and to achieve uniform properties and good superelasticity. Small-scale tests have been done by Aguiar et al. (2010) and Liu et al. (2019), who reported excellent self-centering capability of the SMA springs with an equivalent damping ratio of more than 2%. Larger scale tests were conducted by Speicher et al. (2009), based on which a detailed analytical model proposed by Mirzaeifar et al. (2011) was verified. The influence of varying material properties and temperatures on the hysteretic behavior of SMA helical springs was revealed by Savi et al. (2015).

SMA disc springs, also known as SMA Belleville washer springs, are a relatively new family member of SMA elements. The key benefits include small size for narrow-space installation, small risk of premature fracture (because of the upper limit of deformation when flattened), and flexible load resistance and deformability realized via different stacking patterns (i.e., in series, in parallel, or in combination). An SMA disc spring could be produced by either form-setting or waterjet cutting (Speicher et al., 2009; Maletta et al., 2013). SMA disc springs deform via the development of hoop stress, where the maximum stress is induced at the two edge corners of the cross-section, and the remaining cross-sectional area remains at low-stress levels (Fang et al., 2016). Because of this stress distribution pattern as well as the special conical geometry, the load resistance of SMA disc springs is generally not large. Fang et al. (2020b) and Zheng et al. (2021) have considered using SMA disc springs for rocking bridge piers, where these springs serves as energy dissipative holding-down devices to enable a controlled rocking behavior.

SMA friction springs exhibit the largest load carrying capacity among all the available SMA element types, so they are especially suited to structures with heavy load resistance and energy dissipation demands (Fang et al., 2015b). The concept of SMA friction springs stemmed from conventional high-strength steel friction springs which consist of outer and inner steel rings placed alternately via wedged contact surfaces (Wang et al., 2019a; Zhang R. et al., 2022). The compressive deformation of conventional friction springs is provided by expansion of the outer rings and contraction of the inner rings. Similarly, an SMA ring spring set consists of a series of SMA outer rings and strong steel inner rings placed alternately. The main difference is that the nonlinear deformation demand is only provided by the expansion of the SMA outer rings, whereas the steel inner rings stay elastic.

Due to the large recoverable strain provided by the SMA, SMA friction springs can have much larger deformability than the conventional steel friction springs; the energy dissipation is also much increased because of the combined effect of superelasticity and friction. As the SMA outer rings have a uniform hoop stress distribution over the ring thickness, the load resistance efficiency is high, and therefore the SMA ring springs normally achieve larger load resistance than SMA tensile elements, e.g., cables and bars, given the same consumption of the SMA material. SMA friction springs have been applied to high-performance braces and beam-to-column connections which have been verified via a series of experimental programs (Wang et al., 2017b, 2019b; Fang et al., 2018b, 2019b).