## 3 Test Results with Discussions  
### Recovery Capacity of NiTi Foam Subjected to Static Load with Discussions  
The states of the specimens after 10% strain compressive loading and after recovery are shown in Figure 3.  
The maximum strain of NiTi bulk material under static loading is 10% [1], beyond which the shape memory effect will be damaged. Therefore, to illustrate the advantage of NiTi foam over its solid counterpart in terms of deformability with shape memory effect maintained, 10% strain  
Figure 3: Illustration of a specimen (**a**) after quasi-static loading and (**b**) after recovery by heating (view angle are slightly different). Figure 2: Illustration of the split Hopkinson pressure bar (SHPB) at Construction Lab, Nanyang Technological University, Singapore. is chosen, and 20% strain is chosen as twice the maximum strain of NiTi bulk with shape memory effect unaffected. The test results Figure 4 suggest that the strength of the shape memory foam increases initially, and then decreases with the increasing cycling number. The initial strengthening is caused by the compression and incomplete recovery: for each loading cycle, although only 10% overall compressive strain is applied, the NiTi foam cannot fully recover to the unloaded configuration, as seen from Tables 1 and 2. Take the first cycle of compression and recovery of the 10% strain case as an example. Before test, the diameter is 40.00 mm, and the thickness is 12.20 mm, while after compression and recovery, the diameter becomes 40.18 mm, and the thickness becomes 11.92 mm. First, it is clear that the specimen diameter increases and the thickness decreases. Furthermore, a simple calculation suggests that the volume of the specimen decreases, leading to increased relative foam density, and hence increased strength. It is the competing mechanism between consolidation due to densification and damage accumulation due to multiple loading that determines the foam strength in each cycle. With this underlying mechanism, both observations for 10% and 20% strain can be analyzed in a unified criterion. On one hand, loading causes the densification of the foam, whose density increases as the volume of foam specimen decreases, even after recovery by heating. A relatively denser foam after loading provide a higher stress-strain curve compared with those with less dense foam, which has a strengthening effect. On the other hand, multiple loading and recovery induces damage accumulation, which has a weakening effect of the foam. Therefore, initial loading cycles enhance the foam, leading to a higher stress-strain curve in the next cycle, and further loading exceeding a cycle number induces damage accumulation. In addition, the response and recovery of bulk NiTi are examined [19], which shows that among the five loading cycles: in the initial two loading cycles, the strength is relative low while in the third and fourth loading cycles, the strength goes high. When it comes to the fifth cycle, the stress-strain curve lowers. Despite the difference between the stress-strain curves of the NiTi foam and its counterpart bulk solid, the two material perform similarly with increasing cycle numbers in terms of strength. A failure test was carried out to investigate the failure pattern and ultimate strength of the NiTi foam under quasi-static compressive loading. Figure 5 shows generally that the stress-strain relationship is a straight line, indicating that the stress increases with strain almost linearly, which is different from that of low-density metal foam, which has a stress plateau. At 25% strain, a short and sharp drop occurs, corresponding to the detachment of the foam part at the specimen fringe, as shown in Figure 5. With the progress of the 0.4 mm/min compression, more foam parts gradually break apart from the fringe. From 37% strain corresponding to around 160 MPa stress, the NiTi foam parts continue to fall off the specimen, which results in a disrupted pattern in the stress-strain relationship.