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

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 ΔKeff. In addition, the influence of the material on the contribution of the shear stress τθa, max, which arises at the plane where the maximum resultant stress amplitude σθa, max occurs, to the crack closure was investigated. For three typical industrial materials, the effective resultant stress range Δσθ, eff could be uniformly arranged regardless of nonproportional loading conditions by using σθa, max + k'τθa, max with k'=1/2. Moreover, expressing the stress components Δσθ, eff, σθa, max and τθa, max in terms of strain components Δεθ, eff, εθa, max and γθa, 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.