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• The residual stress characteristics of LSPed Ti6Al4V is described. • An extended N-R model incorporating closure stress effect is established for LSP. • Effect of closure stress on fatigue crack propagation behavior is deeply analyzed. • The fatigue life of LSPed Ti6Al4V is predicted. With the wide application of titanium alloy in various engineering fields, the service life of Ti alloy product has drawn much more attention. It's been proven that the laser shock peening (LSP) is an effective technology to improve the fatigue performance of titanium alloy by introducing a large and deep compressive residual stress. In this paper, a microstructural fracture mechanics model incorporating the crack closure stress effect is presented to investigate the reinforcement mechanism of laser shock peened (LSPed) titanium alloy. Firstly, a rigid-plastic simplified model is utilized to describe the residual stress characteristics of LSPed Ti6Al4V. Then the extended Navarro-Rios model incorporating crack closure stress effect is established to systematically analyze the crack propagation behavior. Finally, the effect of crack closure stress on fatigue life can be accurately predicted. Result shows that the crack propagation processing is a behavior reflecting the deceleration and acceleration of the crack growth rate. A higher threshold stress is required for LSPed Ti6Al4V to promote the propagation of fatigue crack. The closure stress effect reduces the barrier stress and critical crack length in each grain leading to the highly raised fatigue life of LSPed Ti6Al4V. Besides, the closure stress effect has a greater improvement in fatigue performance for a sample with a shorter crack length and thus can be neglected for a long crack. [ABSTRACT FROM AUTHOR]

Highlights • The residual stress characteristics of LSPed Ti6Al4V is described. • An extended N-R model incorporating closure stress effect is established for LSP. • Effect of closure stress on fatigue crack propagation behavior is deeply analyzed. • The fatigue life of LSPed Ti6Al4V is predicted. Abstract With the wide application of titanium alloy in various engineering fields, the service life of Ti alloy product has drawn much more attention. It’s been proven that the laser shock peening (LSP) is an effective technology to improve the fatigue performance of titanium alloy by introducing a large and deep compressive residual stress. In this paper, a microstructural fracture mechanics model incorporating the crack closure stress effect is presented to investigate the reinforcement mechanism of laser shock peened (LSPed) titanium alloy. Firstly, a rigid-plastic simplified model is utilized to describe the residual stress characteristics of LSPed Ti6Al4V. Then the extended Navarro-Rios model incorporating crack closure stress effect is established to systematically analyze the crack propagation behavior. Finally, the effect of crack closure stress on fatigue life can be accurately predicted. Result shows that the crack propagation processing is a behavior reflecting the deceleration and acceleration of the crack growth rate. A higher threshold stress is required for LSPed Ti6Al4V to promote the propagation of fatigue crack. The closure stress effect reduces the barrier stress and critical crack length in each grain leading to the highly raised fatigue life of LSPed Ti6Al4V. Besides, the closure stress effect has a greater improvement in fatigue performance for a sample with a shorter crack length and thus can be neglected for a long crack. [ABSTRACT FROM AUTHOR]

We elucidate that the cyclic deformation behavior of TWIP steels is governed by the cumulative effect of grain size and twin boundaries, by investigating fatigue properties of 20Mn-0.6C TWIP steels with two different grain size of 15 μm and 50 μm. At low strain amplitude (Δε /2 = 0.6%), the 15 μm 20Mn-0.6C TWIP steel exhibited a small initial cyclic hardening followed by a long stage of secondary hardening until saturation. In contrast, a short hardening stage followed by a long softening stage was observed in 50 μm 20Mn-0.6C TWIP steel. At high strain amplitude (Δε /2 = 1.6%), a rapid cyclic hardening followed by a long saturation stage up to fracture was observed in 15 μm 20Mn-0.6C TWIP steel. However, a rapid cyclic hardening followed by a long saturation stage up to fracture was noted in 50 μm 20Mn-0.6C TWIP steel. Fatigue life increased with decreasing grain size and strain amplitude and the ratio of plastic strain increased at the expense of elastic strain with increasing strain amplitude. The hardening ratio of the 15 μm 20Mn-0.6C TWIP steel was significantly higher as compared to 50 μm 20Mn-0.6C TWIP steel at the identical strain amplitude. This intriguing phenomenon is attributed to abundant boundaries leading to high work-hardening ability of 15 μm 20Mn-0.6C TWIP steel. Additionally, the spacing between striations at the crack initiation zone increased with reducing grain size and increasing strain amplitude. The increased striation spacing implied that the speed of fatigue crack propagation was enhanced with the number of fatigue cycles, which promoted maximum stress value involving cyclic hardening. [ABSTRACT FROM AUTHOR]