The microstructure and environment influence on fatigue crack growth in 7049 aluminum alloy at different load ratios
Introduction
Microstructure and environment play an important role in the fatigue crack growth resistance of aluminium alloys and have been investigated during the last two decades [1], [2], [3], [4], [5], [6], [7], [8], [9]. Synergetic effects of environment and loading make the understanding of the underlying mechanisms between microstructure and environment difficult.
Since the mid-1970s, increasing demands for fail-safe designs and damage-tolerant constraints have given importance to the fracture toughness and fatigue crack growth resistance properties [9]. In addition, the effects from stress corrosion cracking and corrosion fatigue have to be included as these involve time-dependent interactions between alloy microstructure, mechanical deformation, and local environmental conditions [1], [2], [3], [4], [5], [6], [7], [8], [9].
The mechanical behaviour of materials depend strongly on their microstructure [1], [2], [7], [10], [11], [12]. It is known that an aluminium alloy exhibits very different properties depending on whether it is cold rolled or heat treated under different temper conditions. Although the local microstructural features and the applied stress intensity range (ΔK) primarily govern slip characteristics and growth mechanisms, the resulting cyclic crack advance can be substantially influenced by the environment [1], [2], [3], [4], [5], [6].
Kirby and Beevers [13], demonstrated that even the seemingly innocuous environment such as laboratory air can lead to a marked increase in crack propagation rates when compared to a vacuum, in the near-threshold fatigue regime of 7XXX series aluminum alloys. Lin and Starke [14] showed that microstructure–environment interactions at low stress intensities could be completely different from those at higher growth rate levels. It is now recognised [15] that environmental effects on slow fatigue crack growth in aluminium alloys strongly depend on alloy composition, heat treatment, moisture content of the surrounding air, and the presence of certain embrittling species.
In the past, the near-threshold fatigue properties have been mainly discussed in light of crack closure mechanisms. The crack closure concept has served, since Elber [16], to explain most of the related phenomena with decreasing crack growth rates, due to plastic deformation at the crack tip, roughness of fatigue surfaces, oxide, etc. Crack branching may likewise lead to reduced crack growth rates [8], [17]. In the absence of closure, the crack tip driving force is related to the applied stress intensity, ΔKapp=Kmax−Kmin, and when closure is induced, the driving force is decreased to a smaller effective stress amplitude ΔKeff=Kmax−Kcl.
Based on the crack closure concept, Schmidt and Paris [18] plotted ΔKth and Kmax, in terms of an R-ratio and identified the two thresholds. Later, Döker and Marci [19] plotted the ΔKth vs Kmax to identify the two critical thresholds (ΔK*th and K*max) as minimum condition for crack growth. Vasudevan and Sadananda [20], [21] extended these results in terms of intrinsic behaviour in contrast to the crack closure concepts. They concluded that crack closure has little effect on the fatigue crack growth behaviour, when compared with microstructural and environmental effects.
Since Vasudevan and Sadananda [20] presented a new theory considering closure as a minor factor for crack advance, they have modelled fatigue crack propagation for a wide variety of materials by assuming two stress intensity parameters, ΔK and Kmax, as the relevant crack tip driving forces. They state that for crack propagation to occur, two driving forces, ΔK and Kmax ΔK and Kmax are needed simultaneously, instead of one. These two driving forces are intrinsic parameters of a material and they are valid for short as well as long cracks. The apparent differences in the behavior of short or long cracks is related to the residual stresses. Crack closure exists only behind the crack tip and is induced by roughness, oxides, plastic deformation etc, whereas crack tip plasticity produces compressive residual stresses in front of the crack tip.
Recently, Lang and Marci [22] have done an important analysis of the role of plasticity which is in agreement with the unified approach of Vasudevan and Sadananda. Lang and Marci's independent analysis provides convincing experimental proof that there is a Kmax threshold that must be met in addition to a ΔK threshold and provide a strong independent support for the unified approach. They also observed that the plasticity induced closure was very small.
The aim of this study is to examine the mechanisms governing the fatigue behaviour of commercial Al 7075 alloy under controlled microstructural and environmental conditions, specifically involving an underaged (UA) and overaged (OA) alloy, having the same crystallographic texture and yield stress, but with different precipitate microstructural features. Near-threshold fatigue behaviour at room temperature environments is compared with that of a vacuum for several load ratio values. Micromechanisms of fatigue crack growth are discussed in terms of the specific role of several concurrent processes involving environmentally assisted crack growth and intrinsic microstructural effects. Results are discussed on the basis of the main deformation mechanisms and microstructure, the embrittling influence of the environment (humid air and vacuum) and the two intrinsic parameters of crack growth: ΔKth, Kmax.
Section snippets
Material and experimental procedure
The 7049 Al alloy (for chemical composition see Table 1) was underaged (UA) and overaged (OA) (Table 2) such that approximately the same yield strength resulted. The overaged heat treatment was the standard temper designation (T7351). The UA treatment was carefully done to match the OA yield strength.
The mechanical properties of the two materials are listed in Table 3. The yield strengths of both microstructures are approximately the same (≈440 MPa); the tensile strength of the UA material is
Experimental results
The variation of fatigue crack propagation rates (da/dN) with the stress intensity factor range ΔK is shown in Fig. 1a for the underaged (UA) and in Fig. 1b for the overaged (OA) 7075 alloy, respectively. These alloys were fatigued in ambient air (20°C, 50% HR) and in vacuum (∼2.6×10−3 Pa) at R values of −1.0 and 0.8.
Fig. 2 shows the dependence of ΔKth on Kmax for the UA and OA 7049 Al alloy in ambient air and in vacuum. An L-shape curve is obtained for the OA alloy over the entire R range for
Discussion
Fig. 2 shows the expected L-shape curve for both microstructures at positive R-values. The UA alloy loses its fatigue resistance at negative R-ratios, but the OA alloy continues on the same (ΔKth−Kmax) curve. This could be caused by the compressive parts of loading, causing shear loads that induce tensile stresses, which result in secondary cracks parallel to the compression axis. SEM studies support this observation.
Fig. 3 (ΔK vs. R) shows that da/dN is mainly controlled by Kmax at R-values up
Summary
It is observed that the UA and OA microstructures of the 7049 alloy show distinct fatigue behaviour in the two environments of humid air and vacuum. The overall behavior is due to the complex relationship between the effect of the environment with microstructure and loading. We are in the process of identifying the mechnisms through the characterization of the detail fatigue results over the entire crack growth range and fracture surface topography. The key to the understanding such complex
Acknowledgments
Financial support by the ONR Arlington, USA is gratefully acknowledged.
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