Ship structures are subject to complex and variable loading conditions during long-term service, including wave-induced forces, wind loads, and mechanical vibrations. These cyclic and multiaxial stresses make critical components, such as decks, hatch coamings, longitudinal stiffeners, and welded joints, highly susceptible to fatigue damage. Fatigue crack initiation and propagation are among the most critical factors affecting structural integrity and service life. With the ongoing trends of ship enlargement and structural lightweighting, conventional material design approaches are increasingly unable to meet the stringent requirements for fatigue resistance. Therefore, surface strengthening techniques, especially shot peening (SP), have emerged as effective methods for improving fatigue performance. Focusing on Q355, a widely used marine structural steel, the work aims to systematically investigate the effects of SP parameters on fatigue strengthening and establish a framework for optimized surface treatment. A combination of finite element analysis (FEA) and high-cycle fatigue testing was employed. Finite element models were developed to simulate both single-particle and multi-particle shot impacts, allowing for a detailed assessment of the evolution of residual compressive stresses (Acrs), equivalent plastic strain (Apeeq), and stress concentration factors (Ks) under varying shot diameters, velocities, and coverage levels. To translate simulation results into actionable guidance, comprehensive quantitative indicators were established, including the residual compressive stress integral (Acrs), which reflected the depth and magnitude of the stress field, and the equivalent plastic strain used to estimate the degree of cold work (CW%). A multi-response optimization framework based on the Desirability Function Approach (DFA) was implemented. By assigning specific weights to Acrs, Acrs,and Ks, the work balanced the beneficial effects of stress strengthening and hardening against the detrimental effects of surface roughness. Experimental validation was conducted on Q355 specimens through three optimized processes (OP-1, OP-2, and OP-3). Fatigue tests were performed at stress amplitudes of 321.0 MPa and 288.9 MPa (R = ‒1) to evaluate the enhancement in service life. The results demonstrated that optimization of peening parameters significantly enhanced surface properties and fatigue resistance. SP treatment increased the surface microhardness of Q355 steel to a maximum of 251HV and created a deep compressive residual stress layer with an integral value reaching ‒245 MPa·mm. In fatigue testing, all optimized processes substantially extended the fatigue life of the specimens. Under low stress amplitude of 288.9 MPa, the OP-2 process, identified as the optimal configuration, achieved a remarkable fatigue life increase of 173.21% compared to unpeened specimens. Fractographic analysis revealed three failure modes: subsurface single crack source, subsurface multiple crack sources, and surface crack source. The optimized parameters successfully shifted the crack initiation sites from the surface to the subsurface. Specifically, for the OP-2 group, the probability of subsurface-initiated failure exceeded 75%, which corresponded to the longest fatigue life, while the surface failure probability was reduced to only 25%. Furthermore, the stability of the residual stress field under cyclic loading was found to be a key factor. OP-2 specimens exhibited the minimum stress relaxation and the strongest anti-relaxation capability, leading to a mean life improvement of 9% to 80% over other peened groups at 288.9 MPa. A multi-response optimization method for the SP of Q355 steel was established that simultaneously considered residual stress stability, surface hardening, and stress concentration. It is concluded that optimizing SP parameters effectively suppresses surface crack initiation by promoting a transition of the crack source to the subsurface, thereby achieving a synergistic improvement in fatigue performance. The proposed framework proves that relying solely on initial residual stress levels is insufficient and the stability and distribution of the stress field are critical for accurate fatigue life prediction. These findings provide both theoretical insights and practical engineering guidance for the optimization of surface strengthening processes and the fatigue life design of critical ship components.
Key words
shot peening /
Q355 steel /
fatigue performance /
residual stress /
surface hardening /
stress concentration
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Funding
National Natural Science Foundation of China (202502JC0055, 52371335); Hubei Provincial Natural Science Foundation for Young Scientists (20231J0222); National Natural Science Foundation of China Young Scientists Fund (52501398)
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