## 5 Results and Discussion  
A356 is an aluminum-silicon (Al-Si) system casting alloy, which has a certain mechanical strength along with castability, weldability and corrosion resistance. Meanwhile, A356 aluminum alloy is a hypoeutectic alloy, whose mechanical characteristics are affected by the morphology of eutectic silicon particles. Casting defects, shrinkage, eutectic Si morphology and other factors affect the final service performance of the alloy [29]. According to previous research, the tensile strength of A356 has been reported to be between 150 and 320 MPa, the proof strength to be between 100 and 230 MPa, and the elongation at fracture to be typically in the range of 2 to 8% [30; 31; 32; 33]. The engineering curve and the transformed real stress-strain curve are shown in Figure 4. According to the experimental results in this paper, the proof strength of the experimental material A356 is approximately 224 MPa, the tensile strength is approximately 330 MPa, and the elongation at fracture is approximately 7%. In terms of the data, the mechanical experimental results of A356 used in this paper are more consistent with the results of previous studies. Figure 3: Specimens and FE models of different experiments. Figure 5 shows the dynamic tensile experimental results of A356. From the data curves, we obtained the proof strength and the ultimate strength at different strain rates. At the low strain rate of 0.001/s, the proof strength was 224 MPa. When the strain rate was increased to 500/s, the proof strength reached 262 MPa. Similarly to the proof strength, the flow stress also increased with the increase of the strain rate. In addition, the ultimate strength was 332 MPa at the low strain rate of 0.001/s and 418 MPa when the strain rate was increased to 500/s, showing a significant strain-rate strengthening effect. The results indicate that A356 alloy is strain-rate sensitive, which is consistent with previous studies on other aluminum alloys [24; 34]. In addition, similarly to the trend in proof strength, the true strain at fracture increased with increasing strain rate, as shown in Figure 5a. When the strain rate was 0.001/s, the fracture strain was approximately 0.067, and at the strain rate of 500/s, the true strain at fracture increased to 0.096. This was also reported in studies of aluminum alloys 3003, 5052, etc. [24; 35]. Based on previous studies of the dynamic properties of aluminum alloys, the effect of strain rate variation on the strength and flow behavior of the material is strongly related to the work hardening. Figure 4: Stress–strain curve of cast aluminum A356. Figure 5: Dynamic properties of A356 under different strain rates. (a) True stress–strain curves; (b) True stress–strain rate relationship. Parameters related to the stress state, such as the stress triaxiality and the Lode angle parameter, were calibrated by the DIC technique and FEM analysis. First, in order to clarify the change of stress triaxiality of the specimen during loading, we assigned a real strain hardening curve to the integral points in the FEM model. Then, the numerical analysis was performed to extract the stress triaxiality, Lode angle parameter and equivalent plastic strain data during the loading process by observing the elements at the crack initiation location in the specimen. With the DIC technique, we could obtain the equivalent plastic strains for different specimens at fracture. Thus, the equivalent fracture strains corresponding to the different stress-state-related parameters were obtained. As shown in Figure 6, we obtained the stress triaxiality curves of the elements at the location of the least cross-sectional area of each specimen during tensile loading through the FEM simulation results. During the steady loading stage of the specimen, the triaxiality was maintained near a constant value. Therefore, the average triaxiality value of each specimen in the stable loading stage was selected as the stress triaxiality of the specimen for the process of analyzing the FEM results.