# Investigation of Impact Behavior of STS304L Steel Plate Under Cryogenic Temperature
## 3 Results and Discussion  
### Mechanical Behavior  
In this study, the impactor was driven by gravity to exert a load on the specimen, and the resulting force was plotted against the displacement between the impactor and specimen. The displacement of the specimen was defined as the distance traveled by the impactor after contacting the upper surface of the specimen. The force-displacement relationship was measured throughout the impact energy absorption process to evaluate the mechanical behavior of the specimens.  
When the impact load was first applied, a fluctuation region appeared. As the strain-hardening effect occurred, both the displacement and force increased linearly until they reached their peak values. Subsequently, necking was observed, where the displacement increased without a corresponding increase in force. The mechanical behavior of STS304L at all test temperatures exhibited similar trends, as depicted in Figure 6a-c.  
The force-displacement history characterizes the dynamic response, including the peak force, absorption energy, and stiffness, as summarized in Table 3. The slope of the force-displacement curve, which represents the section where the displacement and force increase linearly, is steeper at lower temperatures. This indicates that a greater force was required to induce displacement, suggesting that stiffness increased with a decreasing temperature, enhancing resistance to deformation. Across all test conditions, increasing the impact energy led to a greater displacement and peak force. However, beyond a certain impact energy threshold, the force curve decreases sharply without a corresponding  
\begin{table}
\begin{tabular}{c c c c} \hline
**Impact Weight [kg]** & **Temperature [\({}^{\circ}\)C]** & **Impact Energy [J]** & **Impact Velocity [m/s]** \\ \hline \multirow{8}{*}{32.546} & & 50 & 1.824 \\  & & 70 & 2.159 \\  & & \(\cdots\) & \(\cdots\) \\  & & Crack initiation & \(\sqrt{2 gh}\) \\  & & energy & \(\sqrt{2 gh}\) \\ \hline \multirow{8}{*}{32.546} & & 50 & 1.824 \\  & & 70 & 2.159 \\  & \(-100\) & \(\cdots\) & \(\cdots\) \\  & & Crack initiation & \(\sqrt{2 gh}\) \\  & & energy & \(\sqrt{2 gh}\) \\ \hline \end{tabular}
\end{table}
Table 2: Summary of test scenarios.  
increase in displacement, accompanied by fluctuations due to the formation of a hole at the impact site. This indicates that the specimen lost its resistance to impact loading owing to crack initiation [20; 21].  
Figure 6: Displacement–force history for different temperatures: (**a**) 20 \({}^{\circ}\)C; (**b**) \(-\)100 \({}^{\circ}\)C; and (**c**) \(-\)193 \({}^{\circ}\)C.  
Figure 7 shows a schematic of the force-displacement history at cracking, illustrating the influence of temperature on the mechanical behavior. At 20 \({}^{\circ}\)C, no crack initiation was observed until 109 J, while at 110 J, cracks began to form. Similarly, the impact energy absorption capacity was 174 J at \(-\)100 \({}^{\circ}\)C and 184 J at \(-\)193 \({}^{\circ}\)C. This corresponds to a 59.63% increase at \(-\)100 \({}^{\circ}\)C and 68.80% increase at \(-\)193 \({}^{\circ}\)C compared with that at 20 \({}^{\circ}\)C, indicating that STS304L exhibited a negative temperature sensitivity in impact energy absorption.  
As temperature decreases, dislocation mobility becomes limited, resulting in a higher dislocation density than that at higher temperatures [22; 23]. Because greater free energy is required to move dislocations, more impact energy is necessary to achieve the same deformation owing to an increasing stiffness. Therefore, temperature plays a crucial role in determining the impact of energy absorption capacity.