# 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
In addition, 92% of the flexural strength for Sn-20 wt.%Bi-NiTi SHMMC and 88% for Bi-10 wt.%Bi-NiTi SHMMC were recovered within an hour of healing. Figure 12 shows the snapshot of the healing L-shape restoration and crack closures of a bent Sn-20 wt.%Bi-NiTi bar at equal intervals recorded by [PERSON]. The image shows the progressive restoration of the bar's shape and the closure of cracks. The restoration of the bar's geometry took approximately 36 s and was fully accomplished within 74 s after placing the sample on a hot plate. As the temperature increased, the eutectic material within the matrix underwent melting, enabling its flow into the cracks and subsequent welding of the structure. The angular rate of recovery during this process was measured to be approximately 1 degree per second. [PERSON] et al. [70] used the Taguchi method to study design factors (volume fraction of SMA strip, pre-strain of SMA NiTi strip, and healing temperature) on NiTi SMA-reinforced Sn-13 wt.%Bi alloy SHMMCs. They aimed to optimize mechanical properties with minimal experiments. This study yielded the following four conclusions: (1) optimal conditions for ultimate tensile strength healing efficiency were 2.33% SMA volume fraction, 6% pre-strain, and 190 \({}^{\circ}\)C healing temperature; (2) ductility healing efficiency was optimized with 0.78% SMA reinforcement, 0% pre-strain, and 190 \({}^{\circ}\)C healing temperature; (3) higher SMA volume fraction improved ultimate tensile strength healing efficiency but decreased ductility healing efficiency; increased healing temperature improved both properties; pre-strain affected ultimate tensile strength only; and (4) >2.33% SMA volume fraction caused manufacturing difficulties and internal structural damage. Higher pre-strains and healing temperatures led to sample instability. [PERSON] et al. [71] synthesized Zinc based SHMMCs by embedding NiTi SMA wires into Zinc ZA-8 die-casting alloy (Zn-0.8 Al-0.015 Cu). The objective of the study was to assess the efficacy of two load transfer methods in the healing mechanism: direct load transfer through interfacial bonding between the metal matrix and SMA reinforcement, and indirect transfer via mechanical load transfer to a bolt embedded in the matrix. The findings revealed an increase in the volume fraction of SMA reinforcement resulted in a rise in ultimate tensile strength, but only mechanical load transfer samples with higher strain hardening showed significant regained strength after heat treatment for healing. Notably, the investigation identified two critical flaws in the healing system. Firstly, it was necessary  
Figure 11: Bending Test Specimens tested by [PERSON] and [PERSON]: (**a**) crack closure with applied heat and (**b**) straightening of specimen with applied heat. Figure 12: Snapshots of healing L-shape restoration and crack closures of bent Sn-20 wt.%Bi-NiTi bar at equal intervals taken by [PERSON]. Reprinted/adapted with permission from Ref. [69]. 2022, [PERSON] et al. to encase the sample in sand to maintain structural stability during the healing process. Secondly, the healing process resulted in only up to a 30% recovery of the original ultimate tensile strength and ductility in the fabricated SHMMCs, indicating limitations in achieving full restorative potential. [PERSON] et al. [68] synthesized SHMMCs by incorporating 2 vol% NiTi SMA wires reinforcement into Al-3 at.% Si matrix. Al-3 at.%Si was chosen as the matrix material due to its composition, providing 20% liquid and 80% solid at the healing temperature, ensuring structural stability. The authors provided compelling proof for the healing mechanism of the SHMMCs under both tensile and fatigue conditions, through a combination of experimental and computational approaches. The SHMMCs samples were carefully heat-treated for 24 hours at 592 \({}^{\circ}\)C to achieve a post-heal microstructure. Their research revealed that the SHMMC could recover over 90% of its strength after a healing cycle, with an average healing efficiency of 91.6% under tensile conditions.