# Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites
## 5 Healing Assessment of Shape Memory Alloy Reinforced Self-Healing Metal Matrix Composites  
### Healing Assessment Techniques  
The healing efficiency of a composite is assessed by comparing the mechanical properties of the virgin composite with the post-heal mechanical properties of the same composite, using the following relationship:  
\[\frac{MP_{healed}}{MP_{virgin}}\ \times\ 100\%=\%\ \text{Healed}\]  
Sample testing:  
Various mechanical tests, including tensile, bending, and fatigue tests, were performed to study the healing behavior of SHMMCs, as detailed below.  
Tensile testing:  
Several earlier studies [52; 71] on NiTi-reinforced SHMMCs did not adhere to standard methods for fabricating tensile test specimens to assess healing efficiency. [PERSON] [39], [PERSON] et al. [34], [PERSON] et al. [68], and [PERSON] et al. [70] fabricated sub-size tensile test specimens to demonstrate healing efficiency, whose dimensions are specified in ASTM E8M: Standard Test Methods for Tension Testing of Metallic Materials [110]. The specifications of sub-size tensile test specimens are provided below (Figures 8 and 9). Table 3 presents the dimensions of the rectangular tension test specimen as shown in Figure 8, while Table 4 provides the dimensions of the round tension test specimen as shown in Figure 9.  
Bending test:  
[PERSON] [85], [PERSON] et al. [50; 69], [PERSON] et al. [51], and [PERSON] et al. [72] fabricated bending test [111; 112; 113] samples to assess the healing of SHMMCs. The bending test is advantageous as it provides an easily visible crack on the surface of the test sample and allows for partial damage without complete fracture. It also allows the performing of shape recovery tests to demonstrate healing in the samples [50; 69; 85]. Figure 10 shows the schematic of a three-point bend test setup.  
\begin{table}
\begin{tabular}{c c} \hline \hline  & **Dimensions (mm)** \\ \hline G—Gauge length & 20.0 \(\pm\) 0.1 \\ D—Diameter & 4.0 \(\pm\) 0.1 \\ R—Radius of fillet, min & 4 \\ A—Length of reduced section, min & 24 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Different dimensions of the round tension test specimen as illustrated in Figure 9.  
Figure 8: Rectangular Sub-size Tension Test Specimen. Reprinted, with permission, from ASTM E8/ESM-15 [110] Standard Test Methods for Tension Testing of Metallic Materials, copyright ASTM International. A copy of the complete standard may be obtained from www.astm.org (accessed on 6 January 2023).  
Figure 10: The three-point bend test setup used by [PERSON] and [PERSON]. The grey bar represents the test specimen, the blue arrows represent the force direction during the test, and the red arrows represent the relevant measurements. Reprinted/adapted with permission from Refs. [50; 85]. 2009, [PERSON], and 2013, [PERSON].  
Figure 9: Round Tension Test Specimen. Reprinted, with permission, from ASTM E8/ESM-15 [110] Standard Test Methods for Tension Testing of Metallic Materials, copyright ASTM International. A copy of the complete standard may be obtained from www.astm.org (accessed on 6 January 2023).  
Fatigue testing:  
[PERSON] et al. [68] fabricated middle-tension (M(T)) fatigue-crack growth rate samples using geometry from ASTM E647: Standard Test Method for Measurement of Fatigue Crack Growth Rates [114]. The fatigue tests were performed on an MTS 321.12 hydraulic load frame with a strain-controlled setup and were terminated after 350,000 cycles. Healing cycles were carried out at 592 \({}^{\circ}\)C for 24 h before retesting to determine the percentage of healing, with crack size quantification performed optically at the midpoint of each sample.