Innovative Use of Shape Memory Alloys in Seismic Resilient Structures

Background

Conventional seismic design philosophy of steel structures, such as special moment resisting frames (SMRFs), special concentrically braced frames (SCBFs), eccentrically braced frames (EBFs) and buckling-restrained braced frames (BRBFs), relies on the yielding of structural elements to dissipate seismic energy. However, the resultant residual deformations can pose significant difficulties to post-earthquake rehabilitation of buildings and infrastructure, leading to long-period suspension of functionality. In extreme cases, earthquake-damaged structures can be unrecoverable and eventually demolished due to excessive residual deformations and damages, which will cause enormous wastes of resource and energy. In this light, the concept of seismic resilience has been introduced and attracted considerable attentions in the development of novel steel structural systems. The seismic resilient design of steel structures requires the mitigation of earthquake-induced damages and post-earthquake residual deformations to promote the structural restorability and reduce the losses and environmental impacts associated with repairing/demolition of damaged structures.

With the development of metallurgical technologies, the value of SMAs in seismic resilient structural systems has been increasing recognised. SMA is a family of smart metallic materials that exhibits the unique shape memory effect, i.e., the ability to restore residual deformations upon heating. Within a certain temperature range, some categories of SMA (e.g. Ni-Ti SMA) are able to fully recover inelastic deformations spontaneously upon unloading (known as superelasticity). These extraordinary characteristics (see Fig. 1) make SMA an optimum material for applications in seismic resilient steel structures, especially to reduce post-earthquake residual deformations and damages.

Fig. 1 Characteristics of Ni-Ti shape memory alloy: superelasticity and shape memory effect.

Impacts of Research

Over the past few years, the Laboratory for Resilient Steel and Smart Structures has endeavoured to push the state-of-the-art of research on seismic resilient steel structures and smart SMA materials. Research activities have been undertaken in the following themes:
(1) Material properties and cyclic behaviour of SMAs.
(2) Innovative SMA structural components and devices for seismic resilience.
(3) Applications of SMA in seismic resilient steel structures.

The major objectives of the research include the following:
(1) To perform experimental studies to improve the scientific understanding of the properties and cyclic behaviour of smart SMA materials.
(2) To develop and validate the applicability of SMA components/devices for applications in seismic resilient steel structures.
(3) To develop reliable material models and finite element (FE) modelling techniques for accurate numerical simulations of the behaviour of SMA components, as well as the structural response of structural systems equipped with SMA.
(4) To develop practical design guidelines for the proposed SMA components/devices and the corresponding structural systems.

In the short term, the research activities have provided the much-needed test data of the properties and cyclic behaviour of SMA materials, components, devices, and structural systems equipped with SMA. Moreover, practical modelling and design techniques for different SMA components have been developed. The knowledge gained from the research has advanced the scientific understanding and evoked further research activities on SMA components and their applications in earthquake engineering. The research activities have also contributed to the training/education of research personnel and engineering students.

In the medium term, the knowledge gained from the research will benefit the industry by potentially advancing the manufacturing technique of SMA products (especially those designated for seismic or structural applications), as well as the seismic design methodology of steel structures with innovative applications of SMA. These can contribute to fostering the economic competitiveness of China in the sector of advanced manufacturing and construction technology.

In the long term, the broader application of SMA in seismic resilient steel structures can improve the safety and sustainability of buildings and infrastructure in earthquakes. This will benefit the welfare of the society by reducing life and economic losses during extreme earthquake events, as well as the costs associated with repair or demolition of earthquake-damaged structures. Specifically, seismic resilient steel structures equipped with SMA can considerably reduce costly and dangerous post-earthquake repair work. This will enhance the post-earthquake restorability of buildings and infrastructure, and consequently the seismic resilience of the society itself, to moderate earthquakes. After a strong earthquake that is beyond the design limit, seismic resilient steel structures equipped with SMA will be more repairable than conventional steel structures: with the latter likely requiring complete demolition due to the large residual deformations. The enhanced repairability of steel structures will reduce the environmental impact and carbon footprint of the construction industry through the reduction of post-earthquake demolition and the associated waste of resources. In addition, it is expected that the broader exposure of the research to the general public will enhance public awareness of earthquake hazards and enhance the seismic resilience at society level.

Representative Research Activities and Outputs

Theme 1 Material properties and cyclic behaviour of SMAs

Experimental studies have been undertaken to examine the properties of Ni-Ti SMA fabricated in different forms, including bolts (bars) [1, 2], washers [3, 4] and plates [5, 6]. Fig. 2 shows a typical setup and recorded stress-strain curve of such tests. A particular focus has been placed on evaluating the self-centring behaviour (superelasticity) of Ni-Ti SMA following cyclic loading. In addition, various material characterisation techniques (see Fig. 3), including differential scanning calorimetry (DSC), X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) have been employed to ascertain the microscopic structure (phase distribution) and phase transition temperatures of Ni-Ti SMA materials, which are directly related to their superelastic behaviour at different temperatures. In addition, a simplified “hybrid” modelling technique (Fig. 4) [6] has been developed for numerically simulating the cyclic behaviour of Ni-Ti SMA, in order to accurately capture the superelastic and energy-dissipation behaviour of SMA in finite element analysis.

Recently, research has been extended to iron based SMA (Fe-SMA) to investigate its shape memory effect and low cycle fatigue performance (Fig. 5) for potential applications in earthquake engineering.

Fig. 2 A typical test setup and result of SMA plate under cyclic loading.

Fig. 3 Material characterisation techniques: (a) Employed DSC testing platform; (b) a representative EBSD result of Ni-Ti SMA showing the phase distribution.

Fig. 4 Hybrid modelling of Ni-Ti SMA: (a) modelling framework; (b) example of simulation.

Fig. 5 Low cycle fatigue test of Fe-SMA: (a) test setup; (b) cyclic test results.

Theme 2 Innovative SMA structural components and devices for seismic resilience

Various types of SMA structural components and devices have been developed for practical applications in seismic resilient steel structures.

To overcome the risk of premature buckling of conventional SMA components (e.g. wires, bars and plates), Ni-Ti SMA angles (Fig. 6) have been developed via advanced hot forming and heat treatment processes. With a much higher buckling resistance of SMA angles than that of SMA plates, the self-centring and energy dissipation capacities of SMA can be effectively utilised under both tensile and compressive loadings. SMA angles also enable increased design flexibility of connection with other structural members. A preliminary experimental test (Fig. 6) has demonstrated the satisfactory self-centring performance of the developed SMA angles.

Fig. 6 Development and testing of Ni-Ti SMA angles: (a) prototype SMA angle; (b) test setup; (c) cyclic test results.

Fig. 7 Developed self-centring damper: (a) constitutive components; (b) working mechanism.


The research group also developed a friction-based self-centring damper equipped with SMA bars (Fig. 7) for applications in braced frame systems. The damper relies on SMA bars and spring washers to provide the self-centring capacity, and steel-brass friction pairs to provide the required energy-dissipation. Specifically, the damper is featured with a multi-stage energy-dissipation mechanism that is favourable for performance-based seismic design. A series of cyclic loading tests (Fig. 8) have demonstrated an excellent self-centring and energy-dissipation performance of the developed damper.
Fig. 8 Cyclic tests of self-centring damper: (a) test setup and observation; (b) typical cyclic test results.

Theme 3 Applications of SMA in seismic resilient steel structures

The applicability and performance of various SMA components/devices in seismic resilient steel structures have been evaluated with structural analyses [7, 8]. Typical prototype structures and analytical results are shown in Fig. 9. In addition, large-scale validation tests of steel braced frames equipped with SMA self-centring dampers are currently underway (Fig. 10).
Fig. 9 Analyses of prototype structures equipped with SMA components/devices: (a) braced frame structures with SMA self-centring dampers; (b) moment resisting frames with SMA bars and washers.

Fig. 10 Test setup of large-scale braced-frame with SMA self-centring damper.


References
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