Crack growth behavior of 7075-T6 under biaxial tension–tension fatigue☆
Introduction
It has been common practice to characterize the crack growth behavior in metallic materials under uniaxial fatigue. However, the majority of aerospace structural components experience combination of axial, bending, in-plane shear and torsional stresses resulting in complex stress states. It is thus appropriate to extend the fatigue crack growth studies under non-uniaxial loading conditions. There are many possible scenarios of these non-uniaxial loading conditions. One of these is the in-plane biaxial tension–tension fatigue which is the focus of the present study. The test material in this study was the aluminum alloy 7075-T6 which is widely used as the structural material in military and civilian aircraft fleet.
Cruciform type specimens are generally used with biaxial fatigue test machines to characterize crack growth behavior under in-plane biaxial loading condition. This arrangement provides a means to apply different types of cyclic biaxial load (perpendicular and parallel to crack), e.g. different biaxial stress ratios, in-phase, out-of-phase or with any other phase, same or different frequencies, etc. The biaxial stress ratio, is the ratio of the horizontal applied load to the vertical applied load. Liu and Dittmer investigated the fatigue crack growth behavior under different biaxial loading conditions (i.e. ratio) in aluminum alloys [1]. Their results showed that the direction of crack growth and crack growth rate were controlled by the larger biaxial stress component, and the effect of stress parallel to the crack had from small to negligible effects on crack growth rates. Yuuki et al. observed that biaxiality had negligible effect on crack growth rates at low stress levels, but noticeable effect at high stress levels [2]. Hopper and Miller found that the stress parallel to the crack causes a decrease in crack growth rates [3]. Anderson and Garrett observed that biaxial stress field causes an instantaneous or a gradual change in crack growth rate [4]. Shanyavskiy’s investigation showed that the crack growth rate increases with a larger biaxial stress ratio [5]. Sunder and Ilchenko concluded that the fatigue crack growth rates are sensitive to load biaxiality [6]. Lee and Taylor have reported that the fatigue life was shortened with increase of biaxiality stress ratio, however the crack growth rate versus crack driving force data was not provided [7]. Joshi and Shewchuk investigated the fatigue crack propagation in biaxial stress fields using plates with crack in bending mode, and their investigation also showed that crack growth rates are affected by stress parallel to the crack [8].
Overall, there are limited numbers of fatigue crack growth studies under in-plane biaxial loading condition unlike uniaxial loading case. Previous studies have demonstrated that a low biaxiality stress ratio has little or negligible effect on the crack growth rate, while a noticeable difference is seen at higher ratio. There is still, however a need for the investigation into the crack growth mechanisms, especially what are the differences in the microstructural fatigue mechanisms between uniaxial and biaxial situations and how they change with biaxiality stress ratio. This phenomenon was investigated in the present study. Furthermore, in-plane biaxial load causes the non-coplanar crack growth in several cases, which develops fracture mode II along with mode I. This situation is similar to fatigue crack propagation in composite materials and adhesive joints when subjected to uniaxial fatigue, where it is common practice to relate the crack growth rate with the strain energy release rate (G) instead of stress intensity factor (K) [9]. So, a G approach may be more appropriate to characterize the fatigue crack growth behavior under biaxial situation. This is also of importance due to the anisotropic properties present at the crack tip in cold-rolled metals including the aluminum alloy 7075-T6, which was the test material of this study. The direction and rate of crack growth could be affected by anisotropic properties that can create different damage mechanisms. A crack growth rate versus energy release rate approach was, therefore, investigated in the present biaxial fatigue crack growth study. This paper presents the details and results of these investigations.
Section snippets
Materials and methods
The material of this study was a 3.18 mm thick sheet of 7075-T6 aluminum alloy. Cruciform specimens (Fig. 1, Fig. 2) were machined from these sheets. Length and width of each arm was 120 mm and 45 mm, respectively, and the radius of curvature at the junction of arms was 45 mm. First, a hole of 6 mm diameter was drilled at the center of the specimen, and then a notch of 1 mm length and 0.25 mm width, at 45° to horizontal and vertical arms, was machined by electro-discharge method. Thereafter, a
Crack propagation
As mentioned earlier, the cold-rolling direction of the sheet was at 45° to the x- and y-axes in biaxial and 90° to the x-axis in uniaxial tests. In uniaxial and biaxial with λ = 1 tests, the crack initiated from precrack and propagated at 90° to the resultant force, 90° to the cold-rolling direction or coplanar to the initial direction of the precrack. In biaxial test with λ = 1, the resultant force was at 45° to the x- and y-axes. Fig. 4a shows the typical experimental crack initiation and
Region I
The threshold driving force (ΔKI ∼ 2.70 MPa*m1/2 or ΔG ∼ 3.16E-4 MJ/m2) for biaxial fatigue with λ = 1 was slightly lower than its counterpart for the uniaxial test (ΔKI ∼ 2.80 MPa*m1/2 or ΔG ∼ 3.26E-4 MJ/m2). As mentioned earlier, this study did not determine explicitly threshold strain energy release rate but these could be reasonably estimated from the measured crack growth data (i.e. from Fig. 8). Furthermore, these values are presented with intention of being qualitative values more than quantitative.
Conclusions
Effects of in-plane biaxiality fatigue loading condition on the crack growth behavior of aluminum alloy 7075-T6 were characterized. Cruciform specimens with a center hole having a notch at 45° to the rolling direction were tested in a biaxial fatigue testing machine. Two biaxiality ratios, λ (1 and 1.5) were investigated. A precrack was generated first from a notch which was in L–T orientation before conducting the actual tests. Uniaxial fatigue crack growth rates in L–T or T–L orientation were
Acknowledgements
The authors gratefully acknowledge the support of Office of Corrosion Policy and Oversight, OSD, Washington, DC (Mr. Richard A. Hays) and Aerospace Systems Directorate, Air Force Research Laboratory (Mr. Robert A. Reuter).
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The views expressed in this article are those of the authors and do not reflect the official policy or position of the United States Air Force, Department of Defense, or the U.S. Government.