Environmentally assisted fatigue-crack growth in 7075 and 7050 aluminum alloys

Tests of 7050-T7451, 7050-T651 and 7075-T651 aluminum alloys show that these alloys exhibit similar fatigue-crack-growth kinetics in water vapor. The kinetics conform with the model for transport-controlled crack growth. The saturation water vapor pressure for the 7075-T651 alloy is lower than that for the 7050-T651 and 7050-T7451 alloys because tha latter alloys have rougher fracture surfaces. A slight increase in saturation pressure for 7050-T7451 compared to 7050-T651, on the other hand, is attributed to the difference in yield strengths. Good agreement between the experimentally determined saturation pressures and those predicted by the model provides further confirmation for the model for transport-controlled fatigue crack growth and its applicability to high-strength aluminum alloys.     

Determination of fatigue related discontinuity state of 7000 series of aerospace aluminum alloys

The current methods of fatigue life estimation do not model the interaction of stress with microstructural discontinuities. An experimental study of fatigue crack nucleation in 7075-T6 and 7079-T6 aluminum alloy sheet was performed to obtain more information on the role of discontinuities in the microstructure and to determine the relative contributions of each type of discontinuity. Some specimens were machined from recently manufactured material without any coating (cladding or anodizing). Some were fabricated from old clad-anodized material from an aircraft fuselage and old anodized material from a stock of unused wing panels. In the uncoated materials, large constituent particles and the surface roughness were found to be important factors in fatigue crack nucleation. In the coated materials, discontinuities related to the cladding and anodizing appeared to be the controlling factors in crack nucleation and particles did not play any role. The coated specimens showed a low scatter and a relatively low fatigue life. Multiple nucleation sites were observed in the coated materials.     

A MODEL FOR SMALL FATIGUE CRACK GROWTH

Abstract— Based on the assumption that normalized Kitagawa-Takahashi diagrams for different materials are the same, a unified model for microstructurally small fatigue crack and physically small fatigue crack growth rates was developed to describe their behaviour under different fatigue stress ranges. The stress-sensitive blocking effect of microstructural barriers to small fatigue crack growth is satisfactorily simulated by the model. Incorporated with the materials fatigue limit and microstructural barrier spacing, this model can be easily used in the prediction of small fatigue crack lifetime. Small fatigue crack growth rates of previous experimental studies in 7075-T6 Aluminium alloy and HT60 steel under different stress ranges are in an envelope between two boundary prediction curves corresponding to the largest and smallest stress ranges applied in the experiments. Problems concerning model accuracy and model application are also discussed in the present paper.     

Effect of low temperature on fatigue crack formation and microstructure-scale propagation in legacy and modern Al–Zn–Mg–Cu alloys

Temperature dependence of fatigue crack formation and microstructure-scale growth from constituent particles in 7075-T651 and 7050-T7451 is quantified via load induced fracture surface marker-bands. Larger and more abundant particles in 7075-T651 lead to increased crack formation frequency and decreased life. Crack growth rates are similar between alloys and decreased with decreasing temperature, paralleling crack formation behavior. The temperature dependence is attributed to hydrogen environment embrittlement, but is not sufficiently understood to fully model the observed behavior.     

Prediction of low-cycle fatigue-life by acoustic emission—2: 7075-T6 aluminum alloy

Low cycle high stress fatigue tests were conducted by tension-tension on an Alclad 7075-T6 aluminum sheet alloy, until rupture. Initial crack sizes and orientations in the fatigue specimens were randomly distributed. Acoustic emission was continuously monitored during the tests. Extremal peak-amplitudes, equivalent to extremal crack-propagation rates, are shown to be extremally Weibull distributed. The prediction of the number of cycles left until failure is made possible, using an ordered statistics treatment and an experimental equipment parameter obtained in previous experiments (Part 1). The predicted life-times are in good agreement with the actual fatigue lives. The amplitude distribution analysis of the acoustic signals emitted during cyclic stress has been proven to be a feasible nondestructive method of predicting fatigue life.     

Fatigue property of semisolid A357 aluminum alloy under different heat treatment conditions

Effect of heat treatment conditions on fatigue property of a semisolid A357 aluminum alloy under cyclic tensile loading was investigated. Comparison of the fatigue property of the semisolid A357 under T5 and T6 heat treatment conditions with other aluminum alloys including conventional casting A357-T6 alloy and four wrought aluminum alloys: 2024-T4, 7075-T6, 5052-T6 and 6061-T6 was made. It is found that the fatigue strength of the semisolid A357 under both heat treatment conditions is much higher than that of the casting A357-T6 alloy, comparable to that of the 6061-T6, but lower than that of the 2024-T4 and 7075-T6. Two-parameter Weibull distribution of fatigue data for the semisolid A357 under the two heat treatment conditions was constructed to show the statistical significance in fatigue lifetime. Fatigue fracture surface of the semisolid A357 under T5 and T6 heat treatment conditions was examined using scanning electron microscope (SEM). In the stable crack propagation region, the semisolid A357-T5 shows fatigue damage species of severely deformed grains, void coalescence, striations and ridgelines, while the A357-T6 displays less plastic deformation as revealed by the fatigue damage features of intergranular cracks, and transgranular cleavage patterns.     

Variation of the cyclic strain-hardening exponent in advanced aluminium alloys

The cyclic strain-hardening exponents for five fatigue-resistant aluminium alloys were determined throughout the fatigue life to study the degree of cyclic stability of these alloys. Data were compared with results for 2024-T4 aluminum and for two high-pressure steels. The strain-hardening exponent increased logarithmically in all cases except 2024-T4, although the increase was small and did not exceed 33% over the fatigue life. 7475-T351 aluminium alloy was found to be entirely stable, and 7075-T7351 almost so. These were followed in order of rising sensitivity by 2014-T6, 7050-T73651, and 2124-T851 aluminium alloys, and 28NiCrMo7.4 and 30CrNiMo8 steels. 2024-T4 aluminum alloy demonstrated a strong decrease in strain-hardening exponent with fatigue life.     

Elastic–plastic fatigue crack growth analysis under variable amplitude loading spectra

Most fatigue loaded components or structures experience a variety of stress histories under typical operating loading conditions. In the case of constant amplitude loading the fatigue crack growth depends only on the component geometry, applied loading and material properties. In the case of variable amplitude loading the fatigue crack growth depends also on the preceding cyclic loading history. Various load sequences may induce different load-interaction effects which can cause either acceleration or deceleration of fatigue crack growth. The recently modified two-parameter fatigue crack growth model based on the local stress–strain material behaviour at the crack tip [1,2] was used to account for the variable amplitude loading effects. The experimental verification of the proposed model was performed using 7075-T6 aluminum alloy, Ti-17 titanium alloy, and 350WT steel. The good agreement between theoretical and experimental data shows the ability of the model to predict the fatigue life under different types of variable amplitude loading spectra.     

Mean stress relaxation during cyclic straining of high strength aluminum alloys

Cycle-dependent relaxation may alter mean stress values and thus affect fatigue crack initiation life. This phenomenon is an issue both for accuracy of estimated fatigue lives and for the success of methods of intentionally introducing beneficial mean stresses. Although rigorous plasticity theories may describe cycle-dependent relaxation, it is not feasible to incorporate their complex mathematics into strain-based life estimates when loading histories contain millions of cycles. Empirical models are, on the other hand, more numerically efficient and, therefore, for the present purpose, constitute a better choice for modeling this transient phenomenon. Experiments have been performed to study mean stress relaxation in aluminum alloys 7075-T6511 and 7249-T76511. Preliminary tests were also conducted on alloy 7475-T651. Two empirical models are used to illustrate the experimentally observed cycle-dependent relaxation.     

Fatigue of 7075-T651 aluminum alloy under constant and variable amplitude loadings

The fatigue crack growth (FCG) behavior of 7075-T651 aluminum alloy was studied under constant and variable amplitude loadings in vacuum, air and 1% NaCl solution. In the study of constant amplitude loading fatigue, the stress ratios were 0.1 and 0.85 and the loading frequency was 10 Hz. In the study of variable amplitude loading fatigue, the load spectrums were tension type and tension–compression type, and the average loading frequency was about 5 Hz. The results of FCG tests, under constant and variable amplitude loadings, validated the unified two parameter driving force model, accounting for the residual stress and stress ratio effects on fatigue crack growth.     

The role of magnesium in CF and SCC of 7000 series aluminum alloys

Preliminary fatigue crack growth and surface reaction experiments were carried out on a 7075-T651 aluminum and an AZ31 magnesium alloy to examine the chemical role of magnesium in corrosion fatigue and stress corrosion cracking susceptibility of 7000 series (AlMgZn and AlMgZnCu) alloys. The evidence, although limited, strongly supports the further reactions of magnesium with water as being the cause for environmental cracking (CF and SCC) susceptibility of 7000 series aluminum alloys. This explanation is very different from the current hypotheses that are based on deformation (e.g., slip planarity), and opens up new avenues for improving the environmental cracking resistance of high strength aluminum alloys.     

Environmental effects on low cycle fatigue of 2024-T351 and 7075-T651 aluminum alloys

The environmental effects on the low cycle fatigue (LCF) behavior of 2024-T351 and 7075-T651 aluminum alloys were studied at room temperature. The specimens were subjected to identical LCF tests at strain ratio R of −1 and frequency of 5 Hz in three environments: vacuum, air and 1% NaCl solution of pH 2. A separate group of specimens was pre-corroded in 1% NaCl solution and then LCF-tested in air. Their strain–life relations and cyclic stress–strain responses were investigated and compared. Furthermore, the fracture surface morphology was evaluated to find the association of LCF behavior and fractographic features under different environmental conditions.     

A low cycle fatigue model of a short-fibre reinforced 6061 aluminium alloy metal matrix composite

A low cycle fatigue model has been developed to predict the fatigue life of both the unreinforced aluminium alloy and the short-fibre reinforced aluminium alloy metal-matrix composites based solely on crack propagation from microstructural features. In this approach a crack is assumed to initiate and grow from a microstructural feature on the first cycle. The model assumes that there is a fatigue-damaged zone ahead of the crack tip within which the actual degradation of the material takes place. The low-cycle fatigue crack growth and the condition for failure are controlled by the amount of cyclic plasticity generated within the fatigue-damaged zone ahead of the crack tip and by the ability of the short fibres to constrain this cyclic plasticity. The fatigue crack growth rate is directly correlated to the range of crack-tip opening displacement. The empirical Coffin–Manson and Basquin laws have been derived theoretically and applied to compare with total-strain controlled low-cycle fatigue life data obtained on the unreinforced 6061 aluminium alloy at 25 °C and on the aluminium alloy AA6061 matrix reinforced with Al₂O₃ Saffil short-fibres of a volume fraction of 20 vol.% and test temperatures from −100 to 150 °C. The proposed model can give predicted fatigue lives in good agreement with the experimental total-strain controlled fatigue data at both high strain low-cycle fatigue and low strain high-cycle fatigue regime. It is remarkable that the addition of high-strength Al₂O₃ fibres in the 6061 aluminium alloy matrix will not only strengthen the microstructure of the 6061 aluminium alloy, but also channel deformation at the tip of a crack into the matrix regions between the fibres and therefore constrain the plastic deformation in the matrix. The overall expected effect is therefore the reduction of the fatigue ductility.     

Growth characteristics for fatigue cracks embedded in fastener hole walls

A unique method for producing either single or double fatigue cracks embedded in fastener hole walls is described. The method repeatedly produces small semielliptical fatigue cracks and is valuable for basic experimental evaluations dealing with crack growth from fastener holes. Test specimens were prepared from 7075-T651 aluminum alloy plate for collecting constant amplitude embedded crack growth data for open and filled hoies without load transfer. Fractographic data are presented to show crack front shape and growth rates. Crack shape data correlated well with similar data derived from naturally induced embedded cracks. Analytical predictions were made for the same cases evaluated experimentally. Good correlations between the analytical and experimental results were obtained.     

Mixed-mode fatigue crack growth under biaxial loading

Studies on crack growth in a panel with an inclined crack subjected to biaxial tensile fatigue loading are presented. The strain energy density factor approach is used to characterize the fatigue crack growth. The crack growth trajectory as a function of the initial crack angle and the biaxiality ratio is also predicted. The analysis is applied to 7075-T6 aluminium alloy to predict the dependence of crack growth rate on the crack angle. The effect of crack angle on the cyclic life of the component and on the cyclic life ratio is presented and discussed.     

Microstructure-based fatigue modeling of cast A356-T6 alloy

High cycle fatigue (HCF) life in cast Al-Mg-Si alloys is particularly sensitive to the combination of microstructural inclusions and stress concentrations. Inclusions can range from large-scale shrinkage porosity with a tortuous surface profile to entrapped oxides introduced during the pour. When shrinkage porosity is controlled, the relevant microstructural initiation sites are often the larger Si particles within eutectic regions. In this paper, a HCF model is introduced which recognizes multiple inclusion severity scales for crack formation. The model addresses the role of constrained microplasticity around debonded particles or shrinkage pores in forming and growing microstructurally small fatigue cracks and is based on the cyclic crack tip displacement rather than linear elastic fracture mechanics stress intensity factor. Conditions for transitioning to long crack fatigue crack growth behavior are introduced. The model is applied to a cast A356-T6 Al alloy over a range of inclusion severities.     

Understanding fatigue crack growth in aluminum alloys by in situ X-ray synchrotron tomography

In situ 3D X-ray synchrotron tomography of fatigue crack growth was conducted in a 7075-T6 aluminum alloy. Local measurements of da/dN were possible with the 3D data sets obtained from tomography. In situ measurements of crack opening displacement (COD) were obtained, illustrating the possibilities for quantifying fatigue crack closure. Quantitative microstructural analysis enabled an assessment of the role of brittle inclusions on fatigue crack propagation. A significant increase in preferential crack growth through the inclusions was observed.     

Experimental approach to dimple fracture mechanisms under short pulse loading

Dimple fracture mechanisms are discussed for three kinds of aluminum alloys on the basis of an experimental approach and a finite element (FEM) analysis. The void growth and coalescence process was observed by an optical microscope and a scanning electron microscope. The fractographic observation for aluminum alloys 7075-T651 and 6061-T651 showed that several large voids called a dominant void are nucleated at inclusion sites or the second-phase particles ahead of the crack tip and followed by fine voids initiation leading coalescence of the dominant voids with the crack tip. On the other hand, in aluminum alloy 2017-T3, voids are nucleated very close to the crack tip and directly coalesce with the crack tip. FEM computation results suggested that the void nucleation and growth process is closely related to the triaxial stress state ahead of the crack tip.     

NOTCH SENSITIVITY OF ALUMINUM-LITHIUM ALLOYS IN FATIGUE

In order to evaluate the notch fatigue strength and notch sensitivity of aluminum-lithium, 2090 and 8090, alloys, rotary bending fatigue tests have been carried out using circumferentially notched specimens with different stress concentration factors. The results were compared with those of traditional aluminum, 2024T4 and 7075-T6511, alloys. It was found that 2090 and 8090 alloys showed superior notch fatigue strength in comparison to the conventional aluminum alloys. The notch sensitivities to the crack initiation limit of the aluminum-lithium alloys were lower than those of 7075-T6511, while they were nearly equal to those of 2024T4 for blunt notches. The notch sensitivities to the crack propagation limit were also lower in aluminum-lithium alloys, in particular the 8090 alloy, than in the conventional aluminum alloys. It was suggested that the decreased notch sensitivities of the aluminum-lithium alloys were attributed to both the crack propagation mode and the excellent propagation resistance related to their microstructures.