Ultra-high-strength steel (UHSS) structures are exposed to corrosive environments during service, and hydrogen-assisted cracking (HAC) may occur owing to stress corrosion cracking and hydrogen embrittlement. In this study, the HAC threshold stress intensity factor and fatigue life of UHSS steel were evaluated by applying stress in a corrosive environment to prevent structural fracture. For specimen with semicircular slits by electric discharge machining, fatigue limit was obtained by static fatigue test under corrosive environment. The fatigue limit of the crack specimen was evaluated by the fatigue limit of the experiment and HAC threshold stress intensity factor, and comparative evaluation was performed. On the surface of cracks, grain boundaries were embrittled by corrosion, and grains were clearly observed. Meanwhile, cracks in the surface direction propagated slightly, unlike cracks in the depth direction. The static fatigue limit of UHSS (SKD11:HV670) was determined to be 400 MPa, and the fatigue limit of the crack specimen could be evaluated. The experimental results agreed well with the evaluation results.

In this study, elastic waves were detected when different bending stresses were applied to cracked specimens of high-strength steel (SKD11: HV550) immersed in a 0.057 M solution of acetic acid (CH3COOH), and frequency characteristics were analyzed using time-frequency analysis. The dominant frequency obtained using the tensile test was approximately 103 kHz, and those in the acetic-acid solution without stress were approximately 32 and 101 kHz. The dominant frequencies of the crack specimens in which cracks propagated were approximately 30–40 (F1), 60–85 (F2), and 100–110 (F3) kHz. An elastic wave was obtained by corrosion, pitting, crack initiation, and propagation but not during the hydrogen aggregation time. The dominant frequencies of the crack specimens without crack propagation were approximately 28–33 (F1) and 94–109 (F3) kHz. These were the same as the dominant frequency in the acetic-acid solution under nonstress conditions. The fractured surface showed many traces of pitting and corrosion regardless of the applied stress, resulting in microcracks in the Cr carbide. -----------------------------------------------------------------------------------------------------------

Springs acting as suspension systems can damaged by external collisions, such as those with stones. Corrosion pitting occurs in the damaged areas, and cracks will initiate and fracture. However, the allowable corrosion pitting size can be increased by inducing residual stresses via shot peening. In this study, residual stresses are induced through shot peening (SP) and stress shot peening (SSP). Then, the harmless crack depth (ahml), the crack depths (a25, a50) that reduce the fatigue limit of non-SP smooth materials by 25% and 50%, and the crack depths (ahml) that can be detected using non-destructive inspection (NDI) are evaluated. The residual stress affects ahml: the larger the residual stress, the larger ahml will be. The aspect ratio also affects ahml. Because both SP and SSP exhibit ahml> (a25, a50), (a25, a50) can be rendered harmless. Hence, (a25, a50) can secure the safety and reliability of the steel via SP and SSP. Because both SP and SSP exhibit (aNDI1, aNDI2) > (a25, a50), the crack depths of and cannot be detected using NDI.