Fatigue of 7075-T651 aluminum alloy

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Abstract

Extensive fatigue experiments were conducted using 7075-T651 aluminum alloy under uniaxial, torsion, and axial-torsion loading. Detailed fatigue results were reported. Different mean stresses were applied in the experiments and the mean stress was found to have a significant influence on the fatigue strength of the material. A tensile mean stress decreased the fatigue strength dramatically. Fatigue damage was found to occur under compression–compression loading. In addition, axial-torsion experiments using tubular specimens were conducted under different loading paths to study the multiaxial fatigue behavior. Fatigue cracking behavior was found to be dependent on the loading path as well as the loading magnitude. When the loading magnitude was high, the material displayed shear cracking. When the loading stress was below a certain level, the material exhibited tensile cracking. For most loading cases under investigation, the material displayed a mixed cracking behavior. A kink was found in the shear strain versus fatigue life curve from the pure torsion experiments, and it was associated with a distinctive transition of cracking behavior. The Smith–Watson–Topper (SWT) parameter with a critical plane interpretation was found to correlate well with most of the experiments conducted in terms of fatigue lives. However, the SWT parameter cannot deal with the uniaxial fatigue conditions where the maximum stress is low or negative. More importantly, the model fails to correctly predict the cracking behavior observed experimentally on the material. A critical plane criterion based on a combination of the normal and shear components of the stresses and strains on material planes was found to better correlate the fatigue experiments in terms of both fatigue life and cracking behavior. The characteristics of the multiaxial fatigue criterion were discussed based on the experimental observations on 7075-T651 aluminum alloy.

Introduction

Aluminum alloys are widely used in the aircraft industry due to the high strength-to-density ratio. Extensive studies have been conducted to understand the fatigue behavior of aluminum alloys over the years. The fatigue process consists of crack initiation and crack propagation to failure. Smooth specimens are usually used to study the crack initiation behavior. Fatigue lives are related to the stress and strain quantities. Cracked specimens are used to investigate the crack growth behavior and often the fracture mechanics approaches are used to characterize crack propagation. Different approaches have been developed emphasizing either crack initiation or crack growth for the design and evaluation of a load-bearing structure. For example, there are two basic approaches currently used in the fatigue design in the US Navy and Air Force: “safe-life” and “damage tolerance” [1]. The first approach considers the fatigue crack nucleation only while the second approach predicts crack growth. The “safe-life” approach may be appropriate for the fatigue design of a structure while the “damage tolerance” approach may be more suitable for the evaluation of an existing component.

Although disputes exist with respect to a proper definition of crack initiation, it is an accepted conclusion that a majority of the life is spent on the crack initiation phase in the high cycle fatigue regime. New high strength alloys often have small critical flaw sizes and as a result, most of the lifetime of the structures made from these alloys is spent in initiating fatigue cracks. Knowledge and predictions of the crack initiation life are important for the assessment of the fatigue life of a structural component. Crack initiation behavior is the base for the crack growth predictions in a unified approach for fatigue life predictions [2].

Most experimental studies on aluminum alloys were concentrated on uniaxial tension–compression loading with the mean stress effects [3], [4], [5], [6]. Stress–life and strain–life methods are often used based on the stabilized stress–strain hysteresis loops. It is often found that in the lower plastic strain region, the Coffin–Manson relationship does not obey the single slope behavior for the aluminum alloys. Endo and Morrow [3] observed that the usual linear log–log relationships between the fatigue life and the elastic and plastic strains did not provide an adequate correlation of the experimental results for 2024-T4 and 7075-T6 aluminum alloys. Sanders et al. [7] showed that the plots of the plastic strain amplitude versus the fatigue life for the aluminum alloys reflected the linearity of the Coffin–Manson relationship down to a critical level of plastic strain. The deviation of the fatigue results of the aluminum alloys from the single slope behavior of a Coffin–Manson plot was related to the relative inability of the microstructure to develop homogeneous slip during low plastic strain cycling. Fatemi et al. [8] applied a bi-linear relationship to the stress amplitude versus fatigue life curve of 14 aluminum alloys including 7075 aluminum alloy. It was shown that the bi-linear S–N model provided a better representation of the data than the commonly used single slope linear model.

Fracture mechanics approaches were also used to study the fatigue initiation life of aluminum alloys [9], [10], [11]. Short-crack analyses were used to simulate the total fatigue life (S–N) of unnotched specimens made of 7075-T6 and tested under constant amplitude loading [9]. Gill and Pao [10] detected fatigue crack initiation, determined the local notch tip stresses and strains, and compared the fatigue crack initiation of 7075 aluminum alloy in the as-polished condition and in the polished-and-pitted conditions. Döring et al. [11] developed a short crack model based on the integration of Paris type crack growth equation. The initial starter crack length is determined by using a backward integration of the Paris type law based upon the experimentally determined data from uniaxial loading. It was observed that cracked constituent particles constituted the majority of the observed fatigue crack nucleation sites in several aircraft aluminum alloys [12]. These constituent particles, which are inherent in the material, are formed during the cooling process when some of the alloying elements solidify more rapidly than the aluminum. The distribution of these particle sizes were treated as the initial crack sizes in predicting fatigue life [12]. A microstructure based multiscale fatigue model was developed to estimate the fatigue behavior for 7075 aluminum alloy with the consideration of damage incubation, microstructurally small crack growth, and long crack growth [13].

Engineering components often experience multiaxial stresses. A fatigue criterion is needed to assess the fatigue behavior under multiaxial stress loading. A major development in fatigue life predictions is the confirmation of the critical plane approaches for multiaxial fatigue. A critical plane approach concerns a critical plane in a given material for a known stress state on which cracks nucleate. The notion is that cracking behavior is material and loading magnitude dependent.

Smith, Watson, and Topper (SWT) [14] developed a fatigue model to consider the mean stress effect for uniaxial loading by using the cyclic strain amplitude and the maximum stressFP=σmaxΔε2where Δε/2 is the strain amplitude and σmax is the maximum stress in a loading cycle. FP denotes “fatigue parameter”. Socie [15] extended the SWT parameter to multiaxial fatigue with a critical plane interpretation. The critical plane was defined as the material plane where the normal strain amplitude was a maximum. The normal strain amplitude, Δε/2, and the maximum normal stress, σmax, in Eq. (1) are taken on the critical plane. For tension–compression loading, the criterion predicts a cracking plane with its normal being parallel to the axial stress direction. The criterion predicts ±45° cracking planes under pure torsion loading. It has been commonly recognized that the SWT parameter is particularly suitable for aluminum alloys.

Despite comprehensive work, many questions remain to be answered. Limited efforts have been made on the multiaxial fatigue study of the aluminum alloys. Particularly, the cracking behavior of the aluminum alloys under different stress states has not been well investigated. In the current study, extensive fatigue experiments were conducted using 7075-T651 aluminum alloy under uniaxial, torsion, and axial-torsion loading. Different mean stresses were applied in the experiments to study the mean stress effects on fatigue. Cracking behavior under different stress states was studied. Fatigue under compression–compression loading was experimentally investigated. The SWT fatigue criterion with the critical plane interpretation and a modified SWT parameter were evaluated based on the experimental results.

Section snippets

Material and specimens

The material used in the current experimental investigation was 7075-T651aluminum alloy. As shown in Fig. 1, four types of testing specimens were used in the testing program. They were uniaxial dog-bone shaped plate specimens (Fig. 1a), uniaxial solid cylindrical specimens (Fig. 1b), solid cylindrical specimen for torsion (Fig. 1c), and tubular specimens for axial-torsion loading (Fig. 1d). The dog-bone plate specimens were machined from a large plate. Both the uniaxial cylindrical solid

Smith, Watson, and Topper (SWT) criterion

The Smith, Watson, and Topper (SWT) [14] criterion has been known to correlate well with the fatigue experiments of aluminum alloys. The critical plane extension of the SWT criterion is considered in the current investigation. Fig. 11 shows the SWT parameter versus the fatigue life for the uniaxial specimens experimentally tested in the current investigation. The solid line in Fig. 11 was obtained by fitting the fully reversed uniaxial fatigue data using the three-parameter equation

Modified SWT parameter

Jiang and Sehitoglu [19] extended the SWT parameter to consider the general stress state and cracking behavior. With a slightly different form, the modified SWT criterion can be expressed byFP=2bΔεσmax+1-b2ΔτΔγwhere σ and τ are the normal stress and shear stress, respectively, on a material plane. ε and γ are the normal strain and shear strain corresponding to the normal stress, σ, and shear stress, τ, respectively. The symbol Δ denotes range in a loading cycle and the subscript “max”

Discussion

The experiments conducted in the current investigation confirm that significant fatigue damage can be produced under compression–compression loading for the 7075-T651 aluminum alloy. Usually, a material is dominated by one type of cracking behavior. In contrast, 7075-T651 aluminum alloy displays shear cracking, mixed cracking, and tensile cracking dependent on the loading magnitude. Earlier investigations indicate that many materials are dominated by mixed cracking behavior [19], [27], [28],

Conclusions

A systematic experimental investigation was conducted on the fatigue behavior of 7075-T651 aluminum alloy under the uniaxial, torsion, and axial-torsion loading conditions. The mean stress has a significant effect on fatigue life. Fatigue damage was found to occur under compression-compression loading. The material displays shear cracking, mixed cracking, and tensile crack behavior dependent on the loading magnitude. The Smith, Watson, and Topper (SWT) fatigue criterion can predict the fatigue

Acknowledgements

The Missile Defense Agency (F49620-03-1-342) and the Office of Naval Research (N000140510777) sponsor this work. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Missile Defense Agency, the Air Force Office of Scientific Research, the Office of Naval Research, or the US government.

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