On estimating axial high cycle fatigue behavior by rotating beam fatigue testing: Application to A356 aluminum alloy castings

doi:10.1016/j.msea.2017.05.008

Materials Science and Engineering: A

Volume 697, 14 June 2017, Pages 95-100

https://doi.org/10.1016/j.msea.2017.05.008 Get rights and content

Abstract

Three methods available in the literature to estimate the axial S-N curve from the rotating bending fatigue performance were summarized. Axial and rotating bending fatigue tests were conducted at various alternating stresses on specimens excised from A356 aluminum alloy castings in the high cycle fatigue range. These data were used to assess the effectiveness of the models. Among the three models, the one developed by Manson and Muralidharan did not provide good estimates of the axial fatigue performance from rotating bending data. The geometric correction factor proposed by Philipp yielded the best fit, however, did not alter the Basquin exponent whereas the one proposed by Halford and Manson provided a poor estimate. Only the model proposed by Esin provided accurate estimates of fatigue performance as well as the Basquin exponent.

Introduction

The prediction of fatigue performance has been of interest to engineers so that mechanical components can be design accordingly. Several attempts to estimate fatigue performance have been made [1], [2], [3], [4], [5], mostly relying on tensile properties either directly measured or estimated by hardness data. Although these methods provide good initial estimates, it has been recommended [6] that these methods should not be used for final design, and should always be accompanied by fatigue testing.

One of the most commonly used fatigue testing methods has been rotating bending, in which a bending moment is applied to a rotating specimen. As a result, maximum stresses are generated on the surface of the specimen, where fatigue crack is expected to be initiated. In contrast, axial fatigue tests generate uniform stresses on the cross-section of fatigue specimens, and consequently subsurface defects can also initiate a fatigue crack. Because rotating beam tests take significantly less time than the axial tests, there is an abundance of rotating beam fatigue data in the literature.

Although much progress has been made in the last several decades on how microstructural features and structural defects, such as pores and inclusions, affect fatigue performance, even a wider understanding can be gained by interpreting multiple datasets together. Currently, rotating beam fatigue data cannot be used in combination with those obtained in axial fatigue tests for joint analysis. Although there are some models presented in the literature for conversion of rotating beam fatigue data to axial fatigue data,there is no study in the literature in which those models were compared, to the authors’ knowledge. The present study is intended to fill this gap by using fatigue data obtained by rotating beam and axial fatigue tests on specimens excised from A356 aluminum alloy castings.

Section snippets

Background

When the applied stress is below the yield strength, σ_y, of the material, the relationship between the applied stress amplitude, σ_a, and resultant fatigue life follows the Basquin Law [7]: $σ_{a} = σ_{f}^{'} N_{f}^{b}$ where σ^ʹ_f is the fatigue strength coefficient (MPa) and b is the Basquin exponent. Both Basquin parameters are strongly affected by the material [8], specimen geometry [9] as well as the type of test conducted [10]. Most models in the literature are built on the strain-based fatigue life. Eq. (1) can

Experimental details

The specimens were excised from an A356 aluminum wheel hub casting. The chemical composition of the A356 alloy is given in Table 2.

Heat treatment of A356 involved a solution treatment of 8 h at 540 °C, quenching in cold water, natural aging at room temperature for 24 h, followed by artificial aging of 8 h at 170 °C.

Five tensile tests were run based on ASTM-E8 standard. Tensile specimens had an initial diameter of 8.9 mm and a gage length of 32 mm. Twelve uniaxial fatigue tests were conducted at R=−1

Results

The microstructure of the specimens is presented in Fig. 3, which shows a fine Si eutectic structure. The dendrite arm spacing (DAS) is 52 µm.

The results of the tensile tests are presented in Table 3, where S_T is tensile strength and e_F is elongation. The fatigue data for both tests are presented in Fig. 4. Note that at a given stress amplitude, rotating beam fatigue test yielded longer fatigue lives. Moreover, the slope of the two lines are different with the rotating beam results having a

Discussion

It is not surprising that all three factographs showed multiple casting defects which led to premature fracture. The maximum elongation that the alloy can be expected to exhibit at a yield strength of 200 MPa is ~23.2% [23], [24]. The average elongation of the specimens tested in this study, as shown in Table 3 is only 2.3%, or 10% of what the alloy is capable of. When elongation is this low, major structural defects including coarse, old oxide films, such as those on the surfaces of ingots, can

Conclusions

1.
Major structural defects were found on the fracture surfaces of tensile, axial and rotating beam fatigue test specimens, which led to premature fracture. These defects are old oxide bifilms and pores. In one of the specimens, the pore that initiated fatigue failure was interpreted as a bubble, entrained during mold filling.
2.
Multiple propogating cracks were found on the fracture surface of the rotating beam fatigue specimen, consistent with results reported previously in the literature.
3.
Four

Acknowledgements

HO would like to acknowledge a graduate assistantship provided by Alotech Corporation.

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¹: Currently at Auburn University.

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On estimating axial high cycle fatigue behavior by rotating beam fatigue testing: Application to A356 aluminum alloy castings

Abstract

Introduction

Section snippets

Background

Experimental details

Results

Discussion

Conclusions

Acknowledgements

Int. J. Fatigue

Mater. Sci. Eng.: A

Int. J. Fatigue

Int. J. Fatigue

Eng. Fract. Mech.

Mater. Sci. Eng.: A

Mater. Sci. Eng.: A

Exp. Mech.

J. Eng. Mater. Technol.

J. Mater. Eng. Perform.

Proc. ASTM

Phys. Rev. Lett.

Failure of Materials in Mechanical Design: Analysis, Prediction, Prevention

Exp. Mech.