## 5 The Influence of Surface Residual Stress on Hydrogen Permeation Resistance
### Analysis of Mechanical Properties of Materials after Hydrogen Charging  
Figure 14 compares the specimens' tensile stress-strain curves with different surface residual compress stresses before and after 5 h of hydrogen charging. Based on the analysis of Figure 14, it can be inferred that after 5 h of hydrogen charging of all specimens, the tensile mechanical properties of the specimens with different surface residual compress stresses show different degrees of degradation. Among all different surface residual stress specimens, the untreated specimen with a surface residual stress of 120 MPa has a tensile strength of 542 MPa after hydrogen charging, which is decreased by 11.01%. The elongation is 16.94%, which is 6.16% less than that before hydrogen charging. The mechanical properties of this specimen change significantly due to the effect of hydrogen permeation. The specimen preprocessed with 0.2 MPa shot peening strength, leading to a surface residual compress stress of 120 MPa, exhibits a tensile strength of 554 MPa after hydrogen charging, which decreases by 9.77% before hydrogen charging, and the  
Figure 13: Fracture morphologies of samples with different surface residual stress after hydrogen charging for 5 h. (**a**) M = \(-\)120 MPa; (**b**) M = \(-\)215 MPa; (**c**) M = \(-\)293 MPa; (**d**) M = \(-\)336 MPa. elongation decreases by 4.91%. The specimen with a surface residual compress stress of 293 MPa has a tensile strength of 572 MPa after hydrogen charging, which decreases by 7.44%. The elongation is 18.11%, which decreases by 3.71%. Figure 15 shows the material properties' change curves of different surface residual stress specimens after 5 h of hydrogen charging. Figure 15a illustrates the measured tensile strengths and elongation ratios of different surface residual compress stress specimens after hydrogen charging. The analysis of Figure 15a indicates that the tensile strength of each hydrogen-charged specimen positively correlates with the residual compress stress on the specimen surface, which means the tensile strength of each specimen increases monotonically with the increase in the specimen's surface residual stress. However, the elongation of each hydrogen-charged specimen does not change monotonically with the variation of each specimen's surface residual compress stress. The specimens' elongation ratio tends to increase first and then decrease as the surface residual compress stress increases. The specimen with a surface residual compress stress of 293 MPa has the largest elongation after hydrogen charging. Figure 15b shows the curve of the specimens' hydrogen embrittlement sensitivity indexes. As illustrated in Figure 15b, it is evident that as the surface residual compress stress of the specimen increases, the specimens' hydrogen embrittlement sensitivity index shows a decreasing trend. When the surface residual compress stress of the specimen falls within the range of 293-336 MPa, the decreasing trend of the specimens' hydrogen embrittlement sensitivity indexes slows down. By comparison, it can be seen that the measured result of specimens with a surface residual compress stress of 336 MPa on tensile strength is 575 MPa after 5 h of hydrogen charging, which is the largest among the four  
Figure 14: Stress–strain curves of samples with different surface residual stress before and after hydrogen charging for 5 h. (**a**) M = \(-\)120 MPa; (**b**) M = \(-\)215 MPa; (**c**) M = \(-\)293 MPa; (**d**) M= \(-\)336 MPa. groups of specimens. The tensile strength is about 8.49% higher than that before hydrogen charging, and the elongation is 17.7%, which is 3.49% lower than that before hydrogen charging. The hydrogen embrittlement susceptibility index is 16.45%, which is the lowest among the four groups of specimens.