## 3 Results and Discussions
### The Results of the Stress-Strain Curves  
#### 3.2.1 The Influence of Different Strain Rates on Dynamic Impact Shear Properties  
The dynamic shear stress-strain curves of aluminum alloy 7050-T7451 in three forming directions at different strain rates were shown in Figure 8. The analysis of Figure 8 showed that the material had experienced roughly three stages of deformation evolution processes:  
Figure 6: Diagram of the shear surfaces in TD. Figure 7: Scanning electron microscope (SEM) diagram of the shear surface in TD. In the first stage, the shear stress increased with the increase of strain, the first stage belonged to the early stage of elastic deformation, in which the grain structure will produce rotation and deformation in shear zone along the shear direction under the combined action of shear stress and extrusion stress. At this stage of deformation, the shear modulus of material obtained by Figure 8 at different strain rates were shown in Table 3. From Table 3, it can be seen that the shear modulus increased with the increase of strain rate in the same direction, the aluminum alloy 7050-T7451 had obvious strain rate strengthening effect in the stage of elastic deformation. At A point of Figure 8, compared with quasistatic tests, the shear stress in dynamic shear tests was a bit unstable near the yield point, and the curve tended to be gentle after the end of plastic deformation. The above indicated that the force of material internal structure was balanced after plastic deformation [19]. At point A, when strain rate exceeded \(2.5\times 10^{4}\) s\({}^{-1}\), the shear stress decreased in a fixed strain region (10-20%), which indicated that the strain rate within the strain peak range had an enhancement to material [20]. In the second stage, there was no apparent yield platform of material. The shear stress had a wavy change around a fixed value with the increase of strain, which belonged to serrated yielding. The above had shown that the strain strengthening effect of three forming directions in the strain rate range of \(2.5\times 10^{4}\) s\({}^{-1}-4.5\times 10^{4}\) s\({}^{-1}\) was not obvious, which was caused by Dynamic Strain Aging  
Figure 8: Dynamic stress-strain curves of different forming directions: (**a**) ND; (**b**) TD; (**c**) RD. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Shear Modulus/(GPa)** & **ND** & **TD** & **RD** \\ \hline
**2.5 \(\times\) 10\({}^{4}\) s\({}^{-1}\)** & **20** & **15** & **15** \\
**3.5 \(\times\) 10\({}^{4}\) s\({}^{-1}\)** & **21** & **19** & **20** \\
**4.5 \(\times\) 10\({}^{4}\) s\({}^{-1}\)** & **22** & **24** & **22** \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The shear modulus of aluminum alloy 7050-T7451 at different strain rates. (DSA) characteristics of specimens [21], and the result of the interaction between working hardening effect and thermal softening effect caused by adiabatic temperature rise, both as the increase of strain and strain rate. When strain increased to 40%, the shear stress of different strain rates under the same direction tended to be about 300 MPa, which was because the interaction of thermal softening effect and strain rate strengthening effect canceled each other out at the strain of 40% [2], and the macroscopic performance was that the flow stress was insensitive to strain rate. In the third stage, the thermal softening effect of material exceeded the sum of strain hardening [22] and strain rate hardening effect, which led to thermal viscoelastic instability and adiabatic shear, then the carrying capacity of specimens was reduced, and the shear failure occurred finally. It was consistent with the previous analysis of the generation of adiabatic shear and the shear failure analysis of material. In addition, as shown in Figure 8, with the increase of strain rate, the shear stress and yield strength of three forming directions were all rising, which was shown a certain positive strain rate strengthening effect and a certain strain rate sensitivity.