# Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites
## 5 Healing Assessment of Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites
### Healing Assessment Results  
[PERSON] [39] used Sn-13 at.%Bi and Mg-5.7 at.%Zn-2.7 at.%Al as matrix materials, along with 1 vol% continuous, uniaxially oriented NiTi SMA wires as reinforcement, to synthesize SHMMCs. The NiTi SMA wires were sputter coated with 5 nm of gold to enhance matrix and fiber wettability. In the case of Sn-13 at.%Bi-NiTi, the crack formed during tensile testing was successfully closed, and the healed specimen exhibited a recovery of over 94% of its original tensile strength. However, for Mg-5.7 at.%Zn-2.7 at.%Al-NiTi, the composite showed only partial crack closure after healing due to rough crack walls preventing complete closure. The limited fraction of SMA reinforcement wires in the Mg-based alloy was unable to generate sufficient force to overcome the strength of the matrix material. [PERSON] et al. [34] conducted research aimed at demonstrating a proof-of-concept shape-memory alloy self-healing (SMASH) technology for aeronautical applications, specifically targeting fatigue cracks propagating through the matrix. The team designed thermodynamic compositions for three different alloy systems, Al-Si, Al-Cu, and Al-Cu-Si, to produce aluminum alloy matrices with predetermined eutectic phases. The local microstructure near the crack-initiation site was found to have a significant impact on the fatigue crack growth rate, with the presence of a discontinuous eutectic phase favoring longer fatigue life. Interestingly, the behavior of the crack as it grew shifted from an intergranular to a transgranular failure mechanism after repeated healing. However, Al-Cu or Al-Cu-Si alloys showed minimal signs of healing, indicating the complexity of the self-healing process in these systems. By conducting finite element simulations, [PERSON] et al. [115; 116; 117] investigated the effect of SMA reinforcement, the matrix's softening property, and the SMA's pre-strain on the healing of SHMMCs. The study found that prestraining of the SMA reinforcement and softening property of the matrix material at healing temperature is beneficial for crack closure and self-healing. SMA provides a more vital shape-recovery ability than non-transforming materials. [PERSON] et al. [118] conducted a study using 3D FEM/XFEM modeling to examine the impact of NiTi SMA wires on crack closure and healing performance in a self-healing mechanism involving microcapsules with a glass shell and healing agent. They observed that a higher volume fraction of SMA wires improved the healing process and increased ultimate fracture stress. Applying a 1% pre-strain to the SMA wires resulted in complete crack closure and improved adhesion of the healing agent. Increasing the wire radius or volume fraction enhanced fracture stress and the ability to close the crack. The thickness of the microcapsule affected its likelihood of breaking, while the microcapsule radius did not have a significant impact. Moreover, a stronger interface increased the probability of microcapsule breakage. [PERSON] [50] fabricated an SHMMCS composed of Sn-20 wt.%Bi and Bi-10 wt.%Sn matrices reinforced with a 20% volume fraction of NiTi wires. The Bi-10 wt.%Sn-NiTi composite exhibited self-healing properties, with the ability to heal at 145 \({}^{\circ}\)C while providing a 23% eutectic healing liquid to close cracks. All bent samples fully recovered from flexural strain, as shown in Figure 11. Figure 11a illustrates crack closure upon the application of heat, while Figure 11b demonstrates the straightening of the specimen with the application of heat. As heat permeated through the samples and elevated their temperatures, the shape-memory effect of the NiTi was activated, resulting in the straightening of the specimens and the closure of the cracks. It is worth noting that the complete recovery of flexural strain occurred well before the temperature approached the healing temperature (T\({}_{\rm H}\) = 145 \({}^{\circ}\)C) for the alloy.