## 4 Results and Discussion
### Hardness and Strength of the Jets  
Figure 11 shows the fragments and slugs of the SCJ formed from the two materials that are obtained from the underwater recovery experiment. In total, 18.7 g (46.8%) of the OFHC-Cu jet was recovered, of which the slug accounted for 15.68 g. In comparison, 27.9 g (61.2%) of the Mo jet was recovered in total, of which the slug accounted for 24.2 g.  
Seven recovered fragments of each of the OFHC-Cu and Mo jets were selected and analyzed using a hardness tester to determine their hardness. The results were averaged to yield the average hardness for each jet. As shown in Table 5, the recovered OFHC-Cu and Mo jet samples exhibited the average hardness of 571.61 and 2119.99 MPa, respectively. The hardness of the Mo jet fragments was approximately 3.7 times that of the OFHC-Cu jet fragments.  
Research [31] has shown that the hardness of most metallic materials is approximately three times their yield strength. On this basis, the yield strengths of the fragments of the two jets can be estimated to be 190.53 and 706.66 MPa, respectively. However, these values were only the yield strengths of the jet fragments at the completion of the experiment. Determining the yield strength of each material during the jet-formation process requires taking into consideration the stress softening caused by the high temperature of the material during this process.  
Based on the specific form of the J-C constitutive equation, the amount of stress softening due to the material temperature rise, \(\Delta\sigma_{T}\), can be expressed as follows:  
\[\Delta\sigma_{T}=A\bigg{(}\frac{T-T_{r}}{T_{m}-T_{r}}\bigg{)}^{m}. \tag{17}\]  
Because the cooling process of jet particles is unaffected by the strain-rate effect, the relationship between the yield strength of the material and the ambient temperature under quasi-static deformation conditions can be used to fit the correlation coefficient in Equation (17) to obtain a mathematical relationship between the amount of stress softening of the material and the ambient temperature. Furthermore, the strength of each jet can be estimated from the strength data for its fragments. Table 6 summarizes the results and the relevant parameters. As shown in Table 6, the OFHC-Cu and Mo jets exhibited average  
Figure 11: Recovered jet fragments and slugs. (**a**) OFHC-Cu; (**b**) Mo.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multicolumn{4}{c}{**Hardness of Different Jet Particles (MPa)**} & \multirow{2}{*}{
\begin{tabular}{c} **Average** \\ **(MPa)** \\ \end{tabular} } \\ \cline{1-1}  & & & & & & & & **(MPa)** \\ \hline OFHC-Cu & 514.86 & 546.25 & 658.05 & 557.03 & 494.27 & 701.20 & 529.57 & 571.60 \\ Mo & 1992.78 & 2107.52 & 2174.21 & 2085.94 & 1948.65 & 2279.14 & 2251.68 & 2119.99 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Hardness of the jets.  
strengths of 144.32 and 286.66 MPa after a fracture (that is, after the end of PD), respectively. The average strength of the Mo jet was approximately twice that of the OFHC-Cu jet.  
Figure 12 shows the theoretically calculated flow-stress distribution of each of the OFHC-Cu and Mo jets after a fracture. It is evident that the flow stress varied considerably between the elements in different parts of each jet due to the deformation environment. Overall, the flow stress of the Mo jet was significantly higher than that of the OFHC-Cu jet. From the head to the tail of each jet, the flow stress of the elements first decreased and thereafter increased, with the minimum value occurring at \(\frac{1}{3}\) of the distance from the head of the jet. The average flow stresses of the elements in the low-velocity (2000-4500 m/s) parts of the OFHC-Cu and Mo jets were 193.1 and 344.3 MPa, respectively. These values differ by 34% and 20% from the jet strength estimates in Table 6, respectively.