The microstructure and environment influence on fatigue crack growth in 7049 aluminum alloy at different load ratios

Abstract

The role of microstructure and environmental influence on fatigue crack has been investigated on a high strength 7049 aluminium alloy, artificially aged to underaged (UA) and overaged (OA) microstructures with approximately the same yield strength properties, but differing in the mode of slip deformation. The UA alloy deforms by planar slip while the OA alloy by wavy slip. Crack growth measurements were performed at constant load ratios, between 0.8 and −1, in ambient air and vacuum. The influence of load ratio is discussed in terms of slip deformation mechanisms, microstructure, and the influence of environment using the two intrinsic parameters, ΔK and K_max.

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, ΔK_app=K_max−K_min, and when closure is induced, the driving force is decreased to a smaller effective stress amplitude ΔK_eff=K_max−K_cl.

Based on the crack closure concept, Schmidt and Paris [18] plotted ΔK_th and K_max, in terms of an R-ratio and identified the two thresholds. Later, Döker and Marci [19] plotted the ΔK_th vs K_max 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 K_max, as the relevant crack tip driving forces. They state that for crack propagation to occur, two driving forces, ΔK and K_max ΔK and K_max 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 K_max 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: ΔK_th, K_max.