Multiaxial fatigue behavior and life prediction of 7075-T651 aluminum alloy under two-step loading

doi:10.1016/j.engfracmech.2020.107007

Engineering Fracture Mechanics

Volume 230, 1 May 2020, 107007

https://doi.org/10.1016/j.engfracmech.2020.107007 Get rights and content

Highlights

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Constant amplitude & two-step loading spectra multiaxial fatigue tests on AA7075-T651.
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Fatigue damage accumulation rate is decreased by the two-step loading interaction.
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Loading step corresponding to lower fatigue life dominates in two-step loading spectra.
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A non-linear accumulative damage model based on modified Basquin’s criterion.

Abstract

Focusing on the loading sequential effects on the multiaxial fatigue damage accumulation, tests including two series of constant amplitude loadings and 12 groups of two-step loading spectra were conducted on 7075-T651 aluminum alloy. Under the same equivalent stress amplitude, the multiaxial fatigue life raises with the increase of stress amplitude ratio. Fatigue damage accumulation rate is decreased by the two-step loading interaction. Cyclic loading curves and fracture morphology analysis reveal that the loading step corresponding to lower fatigue life is dominant. Considering the stress amplitude ratio effects and loading interaction, a non-linear accumulative damage model is proposed based on the modified Basquin’s criterion. The proposed model provides a more accurate calculation of accumulative fatigue damage compared with the other four commonly used models. Fine life prediction results are obtained by the proposed model with all predicted data within a factor of three.

Section snippets

Material and specimens

7075-T651 aluminum alloy was used in this work with the chemical composition shown in Table 1. The mechanical properties of 7075-T651 aluminum alloy are listed in Table 2, and the specimen geometry is shown in Fig. 1.

Multiaxial fatigue tests

Multiaxial fatigue tests are stress-controlled by SDN100/1000 electro-hydraulic servo fatigue testing machine with the sinusoidal loading wave of 5 Hz. The tests are carried out under completely reversed and in-phase tension-torsion loading. The axial and shear stress amplitudes

Review of constant amplitude loading tests

This part of the experiments was reported in reference [23]. A review is presented to make the research complete.

With stress amplitude ratio remaining $\sqrt{3}$ , multiaxial fatigue life decreases gradually with the increase of equivalent stress amplitude. As presented in Fig. 2, average specimen life is 83,123 cycles under 250 MPa of $σ_{eq}$ while it reduces to 11,423 cycles when $σ_{eq}$ equals to 350 MPa. Fig. 3 shows the relation between stress amplitude ratio and multiaxial fatigue life under 300 MPa of

Criterion for constant amplitude loading tests

Basquin [38] proposed a criterion which described the relation between stress amplitude and fatigue life and was applied to stress-controlled uniaxial fatigue life prediction. Which is, $\frac{Δ σ_{n}}{2} = σ_{a} = σ_{f}^{'} {(2 N_{f})}^{b}$

In which, $Δ σ_{n} / 2$ is half of the variation of normal stress, which equals to axial stress amplitude $σ_{a}$ . $σ_{f}^{'}$ is the tensile fatigue strength, b is the tensile fatigue strength coefficient. Based on Basquin’s criterion, a modified criterion is developed to predict stress-controlled multiaxial fatigue

Conclusions

(1)
Under stress amplitude ratio of $\sqrt{3}$ , multiaxial fatigue life of 7075-T651 aluminum alloy decreases with the increase of equivalent stress amplitude. While under fixed equivalent stress amplitude, the multiaxial fatigue life raises with the increase of stress amplitude ratio. Considering stress amplitude ratio effects, a modified Basquin’s criterion is proposed, which could predict the constant amplitude multiaxial fatigue life.
(2)
Fatigue damage accumulation rate is decreased by the two-step loading

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The authors gratefully acknowledge the financial support of the Fundamental Research Funds for the Central Universities (Grant No. 3122019194).

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View full text

Multiaxial fatigue behavior and life prediction of 7075-T651 aluminum alloy under two-step loading

Highlights

Abstract

Section snippets

Material and specimens

Multiaxial fatigue tests

Review of constant amplitude loading tests

Criterion for constant amplitude loading tests

Conclusions

Declaration of Competing Interest

Acknowledgments

Mater Des

Proc Eng

Eng Fract Mech

Int J Fatigue

Mater Sci Eng, A

Int J Fatigue

Int J Fatigue

Int J Fatigue

Int J Fatigue

Int J NonLin Mech

Int J Fatigue

Eng Fract Mech

Int J Fatigue

Int J Fatigue

Int J Fatigue

Eng Fract Mech

Mech Mater

Int J Fatigue

Probabilist Eng Mech