1. Data processing of tensile experiment.
Figure 1 shows the stress-strain curve of static tensile test. As can be seen from the figure, the maximum real strain is 1.46%, and the corresponding maximum real stress is 471MPa. There is no obvious yield phenomenon of GJV450 in the tensile process, and the real strain reaches 0.2% plastic strain when the real strain is about 0.4%, and the real stress 389MPa is taken as the yield strength. The elastic modulus measured by tangent method is 203GPa.
2. Experimental data processing of Hopkinson pressure bar.
The strain rate that can be achieved in the experimental process can not be controlled by the air pump, but the strain rates obtained are all near the expected strain rate, which is calculated by the actual strain rate in the actual calculation.
Figure 2 shows the stress-strain curves at various temperatures when the expected strain rates are 3000s-1, 6000s-1, 8000s-1, 10000s-1, and figure 3 shows the stress-strain curves at different strain rates at 20 °C, 200 °C, 400 °C and 600 °C. Fig. 4 is a bar diagram of the maximum stress of the material at different temperatures and strain rates.
As can be seen from figures 2 and 3, the stress-strain curves under all parameters show a trend similar to the static tensile curve, and the stress increases sharply with the increase of strain in the stage of elastic deformation, which is due to the increase of strain. the increase of dislocation density in the material hinders the movement of dislocations and the flow stress increases sharply. At the end of elastic deformation, the plastic deformation occurs and the increasing trend of stress slows down. after increasing to a certain value, the sample is unloaded, this is because when the strain continues to increase, under the combined action of force and heat, the thermal softening resistance to strain strengthening is obvious, and the increasing trend of stress slows down.
As can be seen from figure 2, at each expected strain rate, the flow stress of GJV450 is obviously affected by temperature, showing a trend that the temperature increases and the material flow stress decreases, and at each strain rate, the flow stress does not plummet with the increase of temperature, this is because the thermal softening effect of the material increases with the increase of temperature, the material is more likely to slip and the flow stress decreases. At each expected strain rate, the flow stress of the material varies with temperature, which indicates that the thermal softening effect of GJV450 is independent and is not affected by the strain rate.
As can be seen from figures 3 and 4, the flow stress of the material increases at first and then decreases with the increase of strain rate when the temperature is 20 °C and 200 °C, and when the temperature is 400 °C and 600 °C. the flow stress of the material increases at first and then decreases and then increases again with the increase of strain rate. This shows that the variation of material flow stress with strain rate is affected by temperature, and there are both strain rate strengthening effect and strain rate weakening at different temperatures. The reason for this phenomenon is that the adiabatic temperature rises seriously at high strain rate, which leads to thermal softening effect; at the same time, the material is easy to produce micro-cracks at high strain rate, and the internal damage form of the material changes. It is worth noting that even if the flow stress decreases with the increase of strain rate at a certain temperature, on the whole, the flow of materials should increase with the increase of strain rate, showing the strain rate strengthening effect as a whole.