Influence of Heat Treatment Conditions in Fatigue Crack Propagation Behaviour of 8090 Alloy (original) (raw)
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Near-threshold fatigue crack growth in 8090 Al-Li alloy
Metallurgical and Materials Transactions A, 1995
Near-threshold fatigue crack growth was studied in 8090-T8771 A1-Li alloy tested in moist laboratory air. The testing was conducted using (1) the ASTM E-647 load-shedding procedure, (2) a powerlaw load-shedding procedure, and (3) a constant-amplitude (CA) loading procedure. Crack closure in the three procedures was analyzed. In reconciling fatigue crack growth rates (FCGRs) with different crack closure levels under identical testing parameters, the conventional AKefr (=Kr~-Kop) fails to correlate the test data and the modified Ms (=Kma x-XKop, where X is the shielding factor, defined by an energy approach) is proven to be the true crack driving force. A parallel slip-rupture model is proposed to describe the mechanism of near-threshold fatigue crack growth in this alloy. The model explains the mode transition from crystallographic slip band cracking (SBC) to subgrain boundary cracking (SGC)/brittle fracture (BF) in terms of a microstructure-environment synergy. The transition is related to the material's short-transverse grain size.
Analysis of the Fatigue Damage Behavior of AW2099-T83 Al-Li Alloy under Strain-Controlled Fatigue
Frattura ed Integrità Strutturale
Microstructural characteristics, monotonic and strain-controlled cyclic axial behaviors of AW2099-T83 Aluminum-Lithium alloy were investigated. Grain sizes and structures are not uniform in the different orientations studied. High strength and low ductility characterize the tensile behavior of the alloy under static loading. Strain-controlled fatigue testing was conducted at strain amplitudes ranging from 0.3% to 0.7%. Over this range, macro plastic deformation was only observed at 0.7%. Cyclic stress evolution was found to be dependent on both the applied strain amplitude and the number of cycles. Limited strain hardening was observed at low number of cycles, followed by softening, due probably to damage initiation. With low plastic strain, analytical approach was adopted to profile the damaging mechanism for the different applied strain amplitude. Because of the absence of fatigue ductility parameters due to low plasticity, a three-parameter equation was used to correlate fatigue life. Fractured specimens were studied under SEM to characterize the fracture surface and determine the controlling fracture mechanisms. The fractography analysis revealed that fracture at low strain amplitudes was shear controlled while multiple secondary cracks were observed at high strain amplitude. Intergranular failure was found to be the dominant crack propagation mode.
Fatigue-crack propagation in aluminum-lithium alloys processed by power and ingot metallurgy
A study has been made of the mechanics and mechanisms of fatigue crack propagation in a commercial plate of aluminum-lithium alloy 2090-T8E41. In Part I, the crack growth and crack shielding behavior of long (~5 mm) through-thickness cracks is examined as a function of plate orientation and load ratio, and results compared to traditional high strength aluminum alloys. It is shown that rates of fatigue crack extension in 2090 are, in general, significantly slower (at a given stress intensity range) than in traditional alloys, although behavior is strongly anisotropic. Differences in growth rates of up to 4 orders of magnitude are observed between the L-T, T-L, and T-S orientations, which show the best crack growth resistance, and the S-L, S-T, and L + 45, which show the worst. Such behavior is attributed to the development of significant crack tip shielding (i.e., a reduction in local crack driving force), primarily resulting from the role of the crack path morphology in inducing crack deflection and crack closure from the consequent asperity wedging. Whereas crack advance perpendicular to the rolling plane (e.g., L-T, etc. ) involves marked crack path deflection and branching, thereby promoting very high levels of shielding to cause the slowest growth rates, fatigue fractures parallel to the rolling plane (e.g., S-L, etc. ) occur by an intergranular, delamination-type separation, with much lower shielding levels to give the fastest growth rates. The implications of such "extrinsic toughening" effects on the fracture and fatigue properties of aluminum-lithium alloys are discussed in detail.
Fatigue of aluminium—lithium alloys
International Materials Reviews, 1992
alloys are a class of low density, high strength, high stiffness monolithic metallic materials that have been identified as prime candidates for replacing 2000 and 7000 series aluminium alloys currently used in commercial and military aircraft. In this review, the cyclic fatigue strength and fatigue crack propagation characteristics of aluminium-lithium alloys are reviewed in detail with emphasis on the underlying micromechanisms associated with crack advance and their implications to damage tolerant design and lifetime computations. Compared with traditional aerospace aluminium alloys, results on the fatigue of binary AI-Li, experimental AI-Li-Cu, and near commercial AI-Li-Cu-Zr and AI-Li-Cu-Mg-Zr systems indicate that alloying with Li degrades the lowcycle fatigue resistance, though high-cycle fatigue behaviour remains comparable. The alloys, however, display superior (long crack) fatigue crack growth properties, resulting from a prominent role of crack tip shielding, principally due to deflected and tortuous crack path morphologies, induced by the shearable nature of coherent b' precipitates, crystallographic texture, and anisotropic grain structures. Environmental fatigue resistance is comparable with 2000 series alloys and better than 7075-type alloys. The accelerated growth of small fatigue cracks, strong anisotropy, poor short-transverse properties, and a sensitivity to compression overloads are the principal disadvantages of AI-Li alloys. IMR/237 * Although this class of alloys contain alloying elements such as copper, magnesium, and zirconium in addition to aluminium and lithium, the term aluminium-lithium (or Al-Li) alloys is generally used to encompass all aluminium alloys containing greater than 0·5 wt-% lithium. International Materials Reviews 1992 Vol. 37 NO.4 153 157 220 Elongation, % 26 33 2·6 *SHT solution heat treatment; CWO cold water quench. to"y and O"u are the yield and ultimate tensile strengths, respectively. ordered (L1 2 structure), metastable, and spherical J' particles, in rx (fcc) AI-Li solid solution matrix. Owing to the low particle/matrix misfit strains ('" -0'120/0), the precipitates tend to remain coherent with the matrix and retain their spherical morphology, even for particle diameters as large as 300 nm. 54 In the naturally aged condition (or underaged tempers), microstructures show fine homogeneous distributions of J' particles (2-5 nm in diameter) in the matrix and at grain boundaries. With increased aging time and/ or temperature, the matrix J' precipitates coarsen, and equilibrium J particles nucleate heterogeneously along grain boundaries, thereby resulting in the formation of Li-depleted
Analysis of Low-Cycle Fatigue Behavior of AW2099-T83 Al-Li Alloy
Frattura ed Integrità Strutturale
Microstructural characteristics, monotonic and strain-controlled cyclic axial behaviors of AW2099-T83 Aluminum-Lithium alloy were investigated. Grain sizes and structures are not uniform in the different orientations studied. High strength and low ductility characterize the tensile behavior of the alloy under static loading. Strain-controlled fatigue testing was conducted at strain amplitudes ranging from 0.3% to 0.7%. Over this range, macro plastic deformation was only observed at 0.7%. Cyclic stress evolution was found to be dependent on both the applied strain amplitude and the number of cycles. Limited strain hardening was observed at low number of cycles, followed by softening, due probably to damage initiation. With low plastic strain, analytical approach was adopted to profile the damaging mechanism for the different applied strain amplitude. Because of the absence of fatigue ductility parameters due to low plasticity, a three-parameter equation was used to correlate fatigue life. Fractured specimens were studied under SEM to characterize the fracture surface and determine the controlling fracture mechanisms. The fractography analysis revealed that fracture at low strain amplitudes was shear controlled while multiple secondary cracks were observed at high strain amplitude. Intergranular failure was found to be the dominant crack propagation mode.
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2000
Tensile properties and fatigue crack propagation behavior of a 2195-T8 Al-Li alloy were investigated at different stress ratios, with particular emphasis on their dependence on specimen orientation. Specimens with orientations of 0, 15, 30, 45, and 90 deg to the rolling direction were tested. The alloy contained a strong brass-type texture and a profuse distribution of platelike precipitates of T 1 (Al2CuLi) phase on {111} matrix planes. Both tensile strength and fatigue thresholds were found to be strongly dependent on the specimen orientation, with the lowest values observed along the direction at 45 deg to the rolling direction. The effect of stress ratio on fatigue threshold could generally be explained by a modified crack closure concept. The growth of fatigue crack in this alloy was found to exhibit a significant crystallographic cracking and especially macroscopic crack deflection. The specimens oriented in the L-T + 45 deg had the smallest deflection angle, while the specimens in the L-T and T-L orientations exhibited a large deflection angle. The dependence of the fatigue threshold on the specimen orientation could be rationalized by considering an equivalent fatigue threshold calculated from both mode I and mode II values due to the crack deflection. A four-step approach on the basis of Schmid’s law combined with specific crystallographic textures is proposed to predict the fatigue crack deflection angle. Good agreement between the theoretical prediction and experimental results was observed.
On the growth of small fatigue cracks in aluminum-lithium alloy 2090
Scripta Metallurgica, 1986
It is the objective of this article to examine the behavior of small (2 to 1000 ..mu..m) fatigue cracks in a commercial Al-Li-Cu-Zr alloy, and to compare results with those determined on conventional long (greater than or equal to 20 mm) crack samples. The development of ultra-lightweight aluminum-lithium alloys has aroused much interest in the aerospace industry with the prospect
Fatigue crack propagation in aluminum- lithium alloy 2090: Part I. long crack behavior
Metallurgical Transactions A, 1988
A study has been made of the mechanics and mechanisms of fatigue crack propagation in a commercial plate of aluminum-lithium alloy 2090-T8E41. In Part II, the crack growth behavior of naturallyoccurring, microstructurally-smaU (2 to 1000/xm) surface cracks is examined as a function of plate orientation, and results compared with those determined in Part I on conventional long (~>5 mm) crack samples. It is found that the near-threshold growth rates of small cracks are between 1 to 3 orders of magnitude faster than those for long cracks, subjected to the same nominal stress intensity ranges (at a load ratio of 0.1). Moreover, the small cracks show no evidence of an intrinsic threshold and propagate at AK levels as low as 0.7 MPa~mm, far below the long crack threshold AKrH. Their behavior is also relatively independent of orientation. Such accelerated small crack behavior is attributed primarily to restrictions in the development of crack tip shielding (principally from roughness-induced crack closure) with cracks of limited wake. This notion is supported by the close correspondence of small crack results with long crack growth rates plotted in terms of AKcff (i.e., after allowing for closure above the effective long crack threshold). Additional factors, including the different statistical sampling effect of large and small cracks with microstructural features, are briefly discussed.
Materials Research, 2005
Aluminum-lithium alloys are candidate materials for many aerospace applications because of their high specific strength and elastic modulus. These alloys have several unique characteristics such as excellent fatigue crack growth resistance when compared with that of the conventional 2000 and 7000 series alloys. In this study, fatigue crack propagation behavior has been examined in a commercial thin plate of Al-Li-Cu-Mg alloy (8090), with specific emphasis at the fatigue threshold. The results are compared with those of the traditional Al-Cu-Mg alloy (2024). Fatigue crack closure is used to explain the different behavior of the compared alloys.
Fatigue crack propagation in aluminum-lithium alloy 2090: Part II. small crack behavior
Metallurgical transactions, 1988
A study has been made of the mechanics and mechanisms of fati9ue crack propagation in a commercial plate of aluminum-lithium alloy 2090-TBE41. In Part II, the crack growth behavior of naturallyoccurring, microstructurally-small (2 to 1000 um) surface cracks is examined as a function of plate orientation, and results compared with those determined in Part I on conventiona 1 long (~5 nm) crack samples. It is found that the near-threshold growth rates of small cracks are between 1 to 3 orders of magnitude faster than those for long cracks, subjected to the same nominal stress intensity ranges (at a load ratio of 0.1). Moreover, the small cracks show no evidence of an intrinsic threshold and propagate at 6K levels as low as 0.7 MP arm, far be 1 ow the long crack thresho 1 d 6KTH. The i r behavior is also relatively independent of orientation. Such accelerated small crack behavior is attributed primarily to restrictions in the development of crack tip shielding (principally from roughness-induced crack closure) with cracks of limited wake. This notion is supported by the close correspondence of small crack resu 1 ts wi th long crack growth rates plotted in terms of 6K e ff (i .e., after allowing for closure above the effective long crack threshold). Additional factors, including the different statistical sampling effect of 1 arge and sma 11 cracks wi th mi.crostructura 1 features, are briefly discussed.