## 3 Test Results with Discussions
### Recovery Capacity of NiTi Foam Subjected to Dynamic Load with Discussions
Subsequently, the compressive loading speeds onto the specimen are different, resulting in different loading rates. From the figures, it is observed that the higher the strain rate, the higher the dynamic strength of the NiTi foam, which implies that the NiTi foam is a rate-sensitive material. Figure 7: Nominal stress vs. nominal strain-1.0 MPa: (**a**) first test; (**b**) second test; (**c**) third test; (**d**) fourth test. When specimen 1 after the SHPB tests is examined, it is found that under the first loading, some major cracks initiate in the center and propagate through the whole specimen, but the cracked specimen remains as an entire piece, as shown in Figure 9a. However, in the second test, the cracks develop and the specimen falls into pieces, as shown in Figure 9b. The failure prevents the specimen to be tested in the subsequent loading cycles, which accounts for why different specimens undergo different numbers of loadings. Again, it is observed that the dynamic strengths of the second test of both specimens under 1.5 MPa loading are higher than those of the first test. When zoomed-in, the foam failure is clear: some joints fracture and disconnect with adjacent parts, as illustrated in Figure 10.  
Figure 8: Nominal stress vs. nominal strain-1.5 MPa: (**a**) first test (specimen 1); (**b**) second test (specimen 1); (**c**) first test (specimen 2); (**d**) second test (specimen 2). Figure 9: Specim 1 after: (**a**) first test; (**b**) second test. For the same reason, the test stops for both specimens after the first loading cycle when subjected to 2.0 MPa of gas gun pressure, because the specimen surface becomes significantly smaller and uneven with broken joints. Thus, further loading does not make sense. As expected, the strain rate effect of this NiTi foam is significant when compared with the first test for each specimen under lower loading intensities, as shown in Figures 7a, 8a and 11. The higher the strain rate, the higher the dynamic strength. The failure pattern of NiTi foam subjected to dynamic load is significantly different from that subjected to quasi-static load: major cracks initiate and develop in the specimen center under dynamic loading. The mechanism is that dynamic compression in the out-of-plane direction leads to a quick expansion in the in-plane direction (the plane refer to the specimen round surface), resulting in quick tension in the in-plane direction. While material tends to become brittle when loaded with a relatively higher strain rate, and recall that the largest tensile stress occurs in the disc center, cracks initiate and develop in the center. On the contrary, while the foam disc is compressed quasi-statically, there is sufficient time for the stress to distribute. Then, the joint failure does not initiate at the center; instead, small foam parts fall off the fringe. The different failure patterns suggest that the shape memory NiTi foam deforms differently when subjected to different loading rates. In fact, the three major factors governing the mechanical properties (such as stiffness and strength) of the porous/cellular solids are porosity (or relative density, which is defined as the ratio of foam density to density of the bulk materials from which the foam is made), pore size, and pore distribution. However, almost all of the current theories on cellular solids, including metal foam (ductile) and foam  
Figure 11: Stress vs. strain-2.0 MPa, first test (specimen 1). Figure 10: Joint failure of shape memory NiTi foam in SHPB test (1.5 MPa). concrete (brittle), focus on the dependency of foam property on only the relative density, and neglect the effect of the pore size and pore distribution. It is believed that the influence of pore size and pore distribution on foam properties is also important and worth investigating, for which a considerable amount of test data are required. We will examine it in the future.