Hysteresis behavior of t-stub connections with superelastic shape memory fasteners

Abstract

This study compares the energy dissipative characteristics of bolted t-stub connections using steel and shape memory alloy (SMA) fasteners. The initial phase of the study focused on the optimization of the SMA superelastic effect using two different heat treatment temperatures. The samples were then subjected to tensile testing to determine transformation stress, tensile strength, and fracture strain. In addition, low cycle fatigue tests were conducted to examine the energy dissipative characteristics of these superelastic SMAs heat treated at the two temperatures. The cycled samples were then tensile tested to determine the effect of fatigue cycling on transformation stress, tensile strength, and fracture strain. Results from the mechanical tests were analyzed to determine the preferred heat treatment temperature that resulted in the least residual strain and the largest energy dissipative characteristics. The second phase of the study involved t-stub connection testing using steel and SMA double-ended threaded rod fasteners. The optimum heat treatment determined in the first phase was used to develop the SMA fasteners for the t-stub connection tests. Experimental hysteresis results from the t-stub connection tests were used to compare the energy dissipation capacity of the connections with SMA and steel fasteners.

Introduction

In the Northridge earthquake of January 17, 1994, more than 150 structures experienced brittle fractures in their welded moment connections, which demonstrated in no uncertain terms the vulnerability of welded connections to seismic loading. This premature brittle failure of welded connections was also noticed in the 1995 Hyogo-ken Nanbu (Kobe) earthquake. Ductility in the components of structural systems has been demonstrated to be effective in dissipating vibrations induced due to earthquake and/or blast. Researchers have identified that the ductility of structural components and systems would genuinely enhance structural performance subjected to dynamic loads in general, and earthquake/blast loads in particular. Recently, shape memory alloys (SMAs) that have two unique characteristics known as shape memory and superelastic effects, have been used for seismic applications in structural systems. This is due to SMAs’ high energy dissipating capabilities and their ability to withstand large strains (up to 10%) without undergoing permanent deformation.

Studies related to the discovery and behavior of the SMAs by Otsuka and Wayman [1], Greninger and Mooradian [2], Chang and Read [3], Buehler and Wang [4], and Castleman and Motzkin [5] from 1932 to 1968 contributed immensely to the understanding of SMAs.

Shape memory alloys are a special class of smart materials that can recover from large strains (up to 10%) through two distinct phenomena of shape memory effect (SME) and superelasticity (SE) as shown in Fig. 1(a) and (b), respectively. The SME describes the ability of the material to restore the original shape of a plastically deformed sample through the application of a thermal process. This phenomenon results from a crystalline phase change known as the thermoelastic martensitic transformation. Below the martensitic finish temperature, $M_f$, the alloy is in a martensitic phase and the microstructure is characterized by self accommodating twins. If a stress is applied, deformation proceeds by twin boundary movement resulting in a detwinned martensitic microstructure. Upon heating the material to above its austenite finish temperature, $A_f$, the alloy returns to its parent austenitic phase and deformation is recovered.

The SE refers to the ability of a material to return to its original shape upon unloading after deformation. Above the austenite finish temperature, $A_f$, the material is in its parent austenitic phase and the atomic structure is a simple cubic lattice. As load is applied to the material, a stress-induced phase change results and the material undergoes a solid-state phase transformation from austenite to detwinned martensite. Since the detwinned martensitic phase transformation is stress induced, removal of the applied load causes the material to return to its parent austenitic phase. From an atomic point of view, the cubic austenite lattice undergoes a shear-like strain due to the applied stress, resulting in an unstable detwinned martensitic structure. Upon removal of the applied stress, the crystalline lattice returns to its original cubic structure.

A number of researchers have investigated the feasibility of incorporating superelastic shape memory alloys into smart structures for seismic resistance or damage tolerant structural and mechanical systems. Strnadel et al. [6] reported the effects of cycling conditions on three types of thin metal coupons of superelastic Ni–Ti and Ni–Ti Cu shape memory alloys. This study showed that both transformation stress and energy dissipation diminish and the residual deformation increases as the material is cycled. Tobushi et al. [7] reported the influence of strain rate on 0.75 mm diameter superelastic NiTi wires. Their results indicated that strain rates higher than 10% per minute increased the forward transformation stress and the dissipated work but decreased the reverse transformation stress and energy. Wilde et al. [8] studied a variable isolation system for elevated highway bridges consisting of laminated rubber bearings and SMA bars for small, medium, and large sizes of earthquake loading. Saadat et al. [9] investigated the potential of using Ni–Ti alloys in auto-adaptive energy dissipation mechanisms for coastal and inland structures subjected to hurricane loading. Tamai and Kitagawa [10] proposed a new type of seismic resisting member made of superelastic wire for exposed type column base plates with SMA anchorage and a braced frame with SMA damper. The damping properties and damping functional behavior of SMA was discussed in detail by Humbeeck [11].

Ocel et al. [12] proposed a steel connection consisting of four large diameter SMA bars connecting the beam flange to the column flange, which served as the primary moment transfer mechanism. The experimental investigation included cyclic dynamic loads, which showed that connection test specimens exhibited a high level of energy dissipation, large ductility capacity, and no strength degradation after being subjected to cycles up to 4% drift. DesRoches et al. [13] compared cyclic behavior of shape memory alloy bars to those of more common SMA wires and showed that the residual strain was not dependent on the diameter of the material. The tests involved cyclic straining of test specimens up to 6% in increments of 1%. This study also evaluated the cyclical characteristics of SMA wire and bars to determine the re-centering and damping properties as a function of bar diameter, cyclical strain, and loading frequency. The results showed that with proper heat treatment, nearly ideal superelastic properties can be obtained in both wire and bar form of the superelastic Ni–Ti shape memory alloys. Finally, DesRoches and Smith [14] presented a critical review of shape memory alloys detailing their potential and limitations in seismic resistant design and retrofit.