Study on Growth Life Prediction of Surface Crack in Metallic Materials under Nonproportional Loading

Abstract

Fatigue crack propagation tests were conducted under various nonproportional loading conditions on stainless steel SUS304 and titanium alloy Ti-6Al-4V. The crack opening displacement was also measured using digital image correlation to determine the effective resultant stress intensity factor range ΔK_eff. In addition, the influence of the material on the contribution of the shear stress τ_{θa, max}, which arises at the plane where the maximum resultant stress amplitude σ_{θa, max} occurs, to the crack closure was investigated. For three typical industrial materials, the effective resultant stress range Δσ_{θ, eff} could be uniformly arranged regardless of nonproportional loading conditions by using σ_{θa, max} + k'τ_{θa, max} with k'=1/2. Moreover, expressing the stress components Δσ_{θ, eff}, σ_{θa, max} and τ_{θa, max} in terms of strain components Δε_{θ, eff}, ε_{θa, max} and γ_{θa, max} established a unique relationship regardless of the material. Finally, the crack growth lifetime could be predicted with an error of approximately ±30% by using the proposed empirical formula.