Effect of Stress Components on Fatigue Crack Growth Rate under Multiaxial Stress Condition

Abstract

Multiaxial fatigue tests were carried out to investigate the effect of stress components (Δσ, Δτ, σ_st., τ_st.) on fatigue crack growth.
The test method was the combined tension and tortion one in phase loading by using thin-walled hollow cylindrical specimens with pre-cracks. The experiments were carried out under the condition of equal equivalent stress amplitude, Δσ_eq=√Δσ²+3Δτ²
Under the condition of mixed modes, the fatigue crack growth rate increases remarkably with increasing stress ratio. This characteristics is due to the effect of static tensile stress σ_st. on Δτ.
In this paper, the effect of static tensile stress was incorporated in the constitutive equation of fatigue crack growth rate under the mixed mode fatigue condition of Δσ and Δτ. Then, the following constitutive equation of fatigue crack growth rate including the effects of Δσ, Δτ, σ_st. and τ_st. was obtained.
db/dN=A(ΔK_Ib)^mA=(A_R=0)^1/r*
r^*={1+(σ_st/Δτ)^1/2/7 (Region IIa) 1 (Region IIb)
A_R=0={2.16×10^-5×0.230^m (Region IIa) 3.64×10^-5×0.147^m (Region IIb)
m={f(σ₂/σ₁) (Region IIa) g(σ₂/σ₁) (Region IIb)