# Superelastic Shape Memory Alloy Honeycomb Damper
## 3 Experimental Test
### Experimental Results  
Figure 7 illustrates the force-displacement relationship of the specimen, revealing responses in the form of a stable flag-shape. The red lines represent new cycle while the grey lines represent finished cycles. This suggests that the SHD maintains a steady hysteretic performance. The static residual displacement is determined to be 1.5 mm in tension and 1.9 mm in compression, respectively, when the specimen is subjected to a 10 mm displacement loading amplitude. This indicates that the SHD possesses an exceptional capacity for self-centering. The specimen demonstrates a macroscopic fracture strain of 6.9%, as determined by dividing the extended length by the net length of the honeycomb structure, which measures 145 mm. Nevertheless, this fracture strain slightly surpasses that which is typically observed in superelastic SMA plates or bars (less than 6% [24]). It should be noted that a discrepancy of at least 10% exists between the fracture strain observed in the experiment and the strain of the design objective.  
Figure 8 illustrates the different mechanical characteristics of the specimen as displacement-loading increases. The maximum forces are adopted at the point of maximum displacement during each loading cycle. The secant stiffness is defined as the ratio of the maximum forces to the maximum displacement. The dissipated energy, \(E_{D}\), is defined as the integral of the area contained by the hysteretic curve throughout each cycle. The equivalent viscous damping ratio is computed as:  
\[\xi_{eq}=\frac{E_{D}}{4\pi E_{s0}} \tag{7}\]  
where \(E_{s0}\) represents the energy dissipated by a linear system when subjected to the same maximum force and displacement throughout each half-cycle of the loading procedure.  
Figure 6: The test setup: (**a**) the frame and (**b**) the clamping of SHD.  
The maximal forces and secant stiffness of the specimen under tension are found to be smaller compared to the values obtained in compression. The origin of this phenomenon is attributed to the increased compressive stress exhibited by SMA materials compared to their tensile stress. As the displacement amplitude increases, the amount of dissipated energy also increases. However, the equivalent viscous damping ratio initially decreases and then marginally increases after a displacement of 6 mm. This suggests that the specimen has a steady ability to dissipate energy.  
The fracture parts are situated at the outermost part of the honeycomb cell's sharp edge and one of its interior connecting nodes (see Figure 9). Based on the observed fracture morphology of the specimen, it can be inferred that the failure occurred due to a notable stress concentration at the sharp edge during the tensioning or compressing processes. Shape memory alloys (SMA) exhibit a pattern of metal fatigue and the formation of microcracks following cyclic tension and compression. Moreover, microcracks tend to develop fast during the loading process, which ultimately results in the fracture of the specimen.  
Figure 7: Experimental response of the specimen at different loading amplitudes.