Fatigue Crack Growth Behaviour in Friction Stir Welded Aluminium–Lithium Alloy Subjected to Biaxial Loads
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Abstract The effects of specimen thickness and positive stress ratio on the fatigue crack growth behavior of Ti-6Al-4V are discussed in this paper. Fatigue crack growth rates are shown to increase with increasing stress ratio. Stress ratio is shown to have a strong effect on fatigue crack growth behavior. However, for the range of specimen thicknesses examined in this study, the fatigue crack growth rates are shown to not be strongly affected by specimen thickness. A single intrinsic fatigue crack growth rate curve is obtained when the fatigue crack growth rates are plotted against the effective (closure-corrected) stress intensity factor range. Multiparameter extensions to the Paris law are also presented for the combined assessment of the effects of specimen thickness, stress ratio, and crack closure on the fatigue crack growth behavior of Ti-6Al-4V.
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Abstract Behaviour of fatigue crack growth and closure through a compressive residual stress field is investigated by performing fatigue crack growth tests on welded SEN specimens of a structural steel (JIS SM50A). Depending on the type of the initial residual stress in the region of crack growth, the growth and closure of the crack show different behaviour. In particular, in the transition region from a compressive residual stress field to a tensile residual stress field, the fatigue crack growth rates cannot be described by the effective stress intensity factor range Δ K eff , based on the measured crack opening stress intensity factor K op . Also it is found that the R'‐method using the data of d a /d N vs Δ K for residual stress‐free specimens, with the effective stress ratio R'[=( K max + K r )/( K min + K r )], gives non‐conservative predictions of the growth rates in the transition region. Observations of crack closure behaviour in this study indicates that partial opening of the crack occurs and this plays an important role in crack growth through a compressive residual stress field. Based on the concept of a partial opening point (defined and measured in this work), fatigue crack growth behaviour can be better explained.
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The prediction of service lifetime and the selection of alloys for maximum fatigue crack growth resistance are made complicated by the effect of prior loading histories on the rate of crack growth. Some research indicates that crack closure can be an important factor in accounting for this history effect. This paper presents a review of the lifetime on load-interaction effects on fatigue crack growth and indicates some of the future research needed to better understand the factors giving rise to history effects in the growth of long and short cracks.
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Aluminum-lithium alloys exhibit similar environmental fatigue crack growth characteristics compared to conventional 2000 series alloys and are more resistant to environmental fatigue compared to 7000 series alloys. The superior fatigue crack growth behavior of Al-Li alloys 2090, 2091, 8090, and 8091 is due to crack closure caused by tortuous crack path morphology and crack surface corrosion products. At high R and reduced closure, chemical environment effects are pronounced resulting in accelerated near threshold da/dN. The beneficial effects of crack closure are minimized for small cracks resulting in rapid growth rates. Limited data suggest that the 'chemically small crack' effect, observed in other alloy system, is not pronounced in Al-Li alloys. Modeling of environmental fatigue in Al-Li-Cu alloys related accelerated fatigue crack growth in moist air and salt water to hydrogen embrittlement.
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As well known, the friction stir welding is a novel welding process which is a solid state welding process for sheet or plate using the friction stir phenomenon. This paper describes the effect of welding condition such as the rotation speed and the travelling speed during the friction stir welding process on the micro Virkers hardness and the microstructure of friction stir welded joints in AI-7075-T651 plate. From those investigations, the highest hardness of stir zone was observed at the welding condition of SO-3. The microstructures of the friction stir welded joints was not dependent on the welding conditions, but in the SO-4 specimen, the friction stir welding defect like tunnel shape was found in stir zone.
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Fatigue crack growth mechanisms of long and small cracks were investigated in cast and wrought aluminum and titanium alloys with various microstructures (as-cast A535, 6061-T61, and mill-and beta annealed Ti-6Al-4V). In addition, friction stir welded and cold spray processed 6061-T61 were also investigated. The effects of microstructure on the fatigue crack growth response of each material were evaluated. Long crack growth data were generated on compact tension specimens at low and high stress ratios R=0.1 and 0.7, respectively. Small crack growth testing was performed on corner and surface flaw tension specimens at low stress ratio, R=0.1. Fatigue crack growth mechanisms at the microstructural scale of the materials were identified and will be discussed. Closure corrections were applied to long crack growth data, and the results were compared to experimental small crack growth data. Models for small crack growth predictions from long crack growth data will also be presented and discussed.
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