Fatigue of aluminium alloy 2024-T351 in humid and dry air

Reversed bending fatigue tests conducted on specimens of aluminium alloy 2024-T351 in dry and humid air at stress levels of 248, 276, 290, 317 and 359 MPa showed that at low stress amplitude humid air reduces the fatigue life by as much as 21%. Mirco-hardness tests showed that the reduction in fatigue life is primarily attributed to localized hydrogen-induced overageing. SEM analysis and microhardness data were combined with past studies to propose a mechanism for environmentally induced fatigue in aluminium alloy 2024-T351 over a wide range of stress levels.     

Modelling fatigue crack growth in shot-peened components of Al 2024-T351

Microstructural fracture mechanics concepts are used to develop a model to incorporate shot-peening effects into crack propagation laws and fatigue life predictions. Shot peening produces a residual stress which resists opening of the crack and also produces a work-hardened layer within which the flow stress is raised. The model takes account of these effects to give an accurate prediction of the increase in fatigue life. The model was also used to derive the conditions for crack arrest, and the results are presented in the form of a fatigue damage map (FDM). The FDM can be used for the determination of safe loads in durability and maintainability analyses.     

Weld residual stress effects on fatigue crack growth behaviour of aluminium alloy 2024-T351

The interaction between residual stress and fatigue crack growth rate has been investigated in middle tension and compact tension specimens machined from a variable polarity plasma arc welded aluminium alloy 2024-T351 plate. The specimens were tested at three levels of applied constant stress intensity factor range. Crack closure was continuously monitored using an eddy current transducer and the residual stresses were measured with neutron diffraction. The effect of the residual stresses on the fatigue crack behaviour was modelled for both specimen geometries using two approaches: a crack closure approach where the effective stress intensity factor was computed; and a residual stress approach where the effect of the residual stresses on the stress ratio was considered. Good correlation between the experimental results and the predictions were found for the effective stress intensity factor approach at a high stress intensity factor range whereas the residual stress approach yielded good predictions at low and moderate stress intensity factor ranges. In particular, the residual stresses accelerated the fatigue crack growth rate in the middle tension specimen whereas they decelerated the growth rate in the compact tension sample, demonstrating the importance of accurately evaluating the residual stresses in welded specimens which will be used to produce damage tolerance design data.     

Fatigue of 2024-T351 aluminium alloy at different load ratios up to 1010 cycles

Fatigue properties of 2024-T351 aluminium alloy are investigated in the high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regime. Endurance tests are performed with ultrasonic equipment at 20 kHz cycling frequency at load ratios of R = −1, R = 0.1 and R = 0.5 up to 10¹⁰ cycles. Additional servo-hydraulic tests between 8 and 10 Hz at R = 0.1 show no frequency influence on fatigue lifetimes. Linear lines in double logarithmic S–N plots are used to approximate data. Slope exponents of approximation lines increase with increasing numbers of cycles for all load ratios. Failures above 5 × 10⁹ cycles (R = −1 and R = 0.1) or 10¹⁰ cycles (R = 0.5) occur, and no fatigue limit is found. Fatigue cracks leading to failures above 10⁹ cycles are initiated at the surface or slightly below at broken constituent particles or at agglomerations of fractured particles, which are probably Al₇Cu₂(Fe, Mn). Specimens stressed with more than 10¹⁰ cycles at R = −1 without failure show several cracks starting at constituent particles. Maximum crack lengths are 30 μm, which is considerably below grain size.     

Effect of cutting speed and tool rake angle on the fatigue life of 2024-T351 aluminium alloy

Ring-shaped specimens of 2054-T351 aluminium alloy were machined orthogonally on a lathe equipped with a quick-stop device at cutting speeds of 0.5–1.5 m s^?1 with tools having positive rake angles in the range 10–30°. The machined specimens were then fatigued at a selected stress and the resulting fatigue lives were compared with that of the virgin material. The surfaces of the specimens were examined using optical and scanning electron microscopy.The fatigue life of the machined specimens was found to increase with increasing cutting speed or tool rake angle. The fatigue life of the specimens machined at higher cutting speeds was higher than that of the virgin material, due to the presence of compressive residual stresses in the surface layers. At lower cutting speeds the surface damage was so severe that, in spite of the presence of compressive residual stresses in the surface layers, the fatigue life of the machined specimens was lower than that of the virgin material.     

Dependence of fatigue life on the surface integrity in the machining of 2024-T351 aluminium alloy — unlubricated conditions

An experimental investigation was conducted to study the dependence of fatigue life on the surface integrity in the machining of 2024-T351 aluminium alloy under dry unlubricated conditions. Cutting speeds ranging from 100 to 250 ft min^–1 (30.48 to 76.2 m min^–1 and tool rake angles ranging from 10 to 30° were used. The results of the investigation show that the damage in the surface due to machining consists of a wide variety of defects such as cracks, long straight grooves, cavities, microcracks and macrocracks, and severe plastic deformation, etc. The severity of the damage decreases with an increase in the cutting speed and tool rake angle. An increase in the cutting speed or tool rake angle resulted in an increase in the fatigue life of the specimen.     

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.     

Numerical simulation of fatigue crack propagation in friction stir welded joint made of Al 2024-T351 alloy

In this work, fatigue crack propagation in thin-walled aluminium alloy structure with two friction stir welded T joints has been simulated numerically. Crack propagation in stiffened part of the structure between two friction stir welded T joints is analysed by using the eXtended Finite Element Method (XFEM), including software ABAQUS, as well as MORFEO, for modelling and results display. Tensile fatigue loading is applied, with stress ratio R = 0, and maximum stress σ_max = 10 MPa. Material properties (Al 2024-T351, as used in aeronautical industry) in different welded joints zones are adopted from available literature data. Following results are obtained by numerical analysis: stress–strain and displacement state in the structure, position of the crack tip and value of stress intensity factor for every crack propagation step, as well as the structural life estimation, i.e. number of load cycles, N, also for each crack propagation step. Using these results the number of cycles at which the crack starts to propagate in an unstable manner is predicted.     

Influence of pulse sequence and edge material effect on fatigue life of Al2024-T351 specimens treated by laser shock processing

Laser shock processing (LSP) is being considered as a competitive alternative technology to classical treatments for improving fatigue, corrosion cracking and wear resistance of metallic materials. The purpose of this paper is to present a fully 3D finite element model for predicting the residual stresses that result from the LSP of aluminum alloy Al2024-T351 samples of interest for aeronautic industry in order to optimize the laser treatment to increase the fatigue life of the material. In order to correlate the simulation results with experimental data, three different laser shock processing strategies (pulse sequences) were performed on fatigue specimens and their fatigue life were compared. The starting points of cracks were identified by means of optical and scanning electron microscope examinations and a correlation with the maximum tensile stress regions predicted by the numerical model has been established.     

The effect of specimen thickness on the experimental characterization of critical crack-tip-opening angle in 2024-T351 aluminum alloy

Although the crack-tip-opening angle (CTOA) has been shown to be well suited for modeling stable crack growth and instability for thin sheet aluminum alloys, its behavior for increasing thickness has not been thoroughly evaluated. To investigate this behavior, fracture tests were performed on specimens made of 2024-T351 aluminum alloy with thicknesses of 2.3, 6.35, 12.7, and 25.4 mm. The surface CTOA exhibited an initially high value followed by a transition to a “constant” value after a short amount of crack extension, with this transition decreasing significantly with increasing specimen thickness. The critical CTOA was shown to decrease with increasing specimen thickness and appears to possibly be approaching a lower limiting value.     

Effect of cutting speed and tool rake angle on residual stress distribution in machining 2024-T351 aluminium alloy — unlubricated conditions

The residual stress distribution in the machining of 2024-T351 aluminium alloy was measured using an electrolytic etching technique. Ring-shape specimens were machined under unlubricated orthogonal conditions with high-speed steel tools having rake angles of 10, 15, 20 and 25° at cutting speeds ranging between 0.5 and 1.25 m sec^–1. The results of the investigation show that the residual stresses are compressive at the machined surface and decrease with depth beneath the machined surface. The maximum (near-surface) residual stress and the depth of the severely stressed region increase with an increase in the cutting speed. There seems to be little change in the residual stress distribution due to a change in the rake angle. The results are interpreted in terms of the variations in the amount of surface-region deformation produced by changes in cutting conditions.     

Comparison of retardation behaviour of 2024-T3 and 7075-T6 Al alloys

Retardation in fatigue crack growth rate following the application of single and periodic tensile overloads was studied for 2024‐T3 and 7075‐T6 aluminium alloys. Tests were performed at constant stress and at constant stress intensity factor ranges, at a load ratio of R= 0.1, at a baseline ΔK in the 10–20 MPa√m range which corresponds to the Paris regime. Overload ratios of 1.3–1.65 were studied with overload spacing, n, varying from 20 to 10 000 cycles. 2024‐T3 displayed an order of magnitude higher retardation, N_d, due to single tensile overloads compared to 7075‐T6. Periodic overloads induced maximum retardation when n/N_d≈ 0.5 for both alloys, the magnitude being only 15% higher for 2024‐T3.     

Characterization of inertia-friction welds between a rapidly-solidified Al-Fe-Mo-V alloy and IM 2024-T351

The structures, mechanical properties and fracture behaviour of inertia-friction welds produced between rapidly-solidified/powder metallurgy (RS/PM) Al-9Fe-3Mo-1V (wt %) and ingot metallurgy (IM) 2024-T351 aluminium were investigated. Visual examination showed the axial displacement experienced by the specimens during welding and the degree of metal expulsion from the weld interface (i.e. flash) to increase with an increase in axial force. The weld flash was observed to originate principally from the IM 2024-T351, which was consistent with the lower elevated-temperature strength of this precipitation-hardened alloy. Although the weld interface region remained nearly flat in welds produced using low axial force, this surface became increasingly curved (concave into the Al-9Fe-3Mo-1V alloy) with an increase in axial force. Microstructure analysis using both light and analytical electron microscopy characterized the heat- and deformation-affected zones (HDZs) in each of the base metals and the weld interface regions. The HDZ directly adjacent to the weld interface in the IM 2024-T351 exhibited fine, recrystallized alpha aluminium grains and an absence of S precipitates present in the base metal microstructure. The HDZ directly adjacent to the weld interface in the Al-9Fe-3Mo-1V exhibited fine alpha grains and fine, spherical and acicular dispersoids, which in part originated from the plastic deformation and fracture of coarse base metal dispersoid particles. The extent of this dispersoid-refined region was greatest at the centre of the weld as opposed to the outer periphery, and in the low rather than the high axial force weld. At the weld interface in the vicinity of the axial centre line, the occurrence of highly localized mechanical mixing between the two alloys was determined using both light and electron microscopy and electron-microprobe analysis techniques.Microhardness traverses showed relatively little variation in hardness across the weld interface and an absence of hardness degradation at any location relative to the unaffected base metals. Room-temperature transverse-weld tensile testing showed tensile strengths to range between 85 and 90% of the RS/PM base metal, with fracture occurring in the Al-9Fe-3Mo-1V HDZ remote from the weld interface. Three-point guided bend testing also revealed fracture to occur in the Al-9Fe-3Mo-1V HDZ. SEM fractographic analysis of the fracture surfaces found fracture in the Al-9Fe-3Mo-1V to involve microvoid formation at dispersoid/alpha aluminium interfaces and subsequent ductile rupture in the alpha aluminium matrix.     

Fatigue failure of adhesively patched 2024-T3 and 7075-T6 clad and bare aluminium alloys

The fatigue behaviour of adhesive patches used for repairing aircraft components was investigated. Adhesive patches were simulated using single‐lap shear specimens on clad and bare 7075‐T6 and 2024‐T3 aluminium alloy substrates. Stress–life curves were generated under constant amplitude loading at three stress ratios: R=?1, 0 and 0.5. In the bare materials, failure always occurred in the adhesive itself leaving the substrates intact. At fatigue lives below about 100 000 cycles, the clad alloy specimens also failed in this manner. However, at lower stress levels, the clad alloys failed by cracks initiating in the cladding layer along the end of the lap and subsequently propagating through the substrate. The fatigue strength of the substrate, due to the adhesive patch on the clad materials, was reduced by an order of magnitude compared to the Military Handbook values.     

Constant and variable amplitude fatigue testing of aluminum alloy 2024-T351 with ultrasonic and servo-hydraulic equipment

Constant amplitude (CA) and variable amplitude (VA) fatigue lifetimes of the aluminum alloy 2024-T351 were measured with servo-hydraulic (8–70 Hz) and ultrasonic testing equipment (20 kHz) at positive load ratios. Experiments in the high cycle fatigue regime served to identify influences of frequency and testing method on lifetimes. CA tests showed similar numbers of cycles to failure for both methods. Ultrasonic tests were performed in pulsed mode. In ultrasonic VA tests vibration amplitude of successive pulses of 2000 cycles length is varied. Servo-hydraulic VA tests are performed by varying the load of successive blocks. Servo-hydraulic VA tests with block length 2000 cycles delivered lifetimes similar to the ultrasonic tests. No frequency effect is found in CA and VA tests. Cracks are preferentially initiated at secondary phase particles at both frequencies. Lifetimes in servo-hydraulic VA tests are reduced when block length is decreased from 2000 to 200, 20 and single load cycles. Varying the load for each successive cycle at 50 Hz is realized with a feed-forward optimization of control parameters. Lifetimes differ by a factor 6 for different block lengths indicating a strong load sequence effect.     

Analysis of local microstructure and hardness of 13 mm gauge 2024-T351 AA friction stir welds

Abstract

The friction stir welding process has been used to join 13 mm gauge 2024-T351 aluminium alloy plates together. A detailed microstructural study of the resulting weld was carried out using differential scanning calorimetry, hardness testing, scanning electron microscopy and electron backscatter diffraction. Differential scanning calorimetry was used to explain the hardness results at a number of regions across the weld in terms of co-cluster dissolution and reformation and S phase formation, coarsening and dissolution. The 'onion rings' structure found in the nugget weld was shown to be the result of a combination of the slight grain size variations and a change in nature and size of the particles present, i.e. intragranular v. intergranular. The variation in corrosion properties and hardness of the rings is discussed in terms of the local microstructure and quench sensitivities.     

A study of corrosion fatigue behaviour of anodized and unanodized 2024-T3 aluminium alloy

The corrosion fatigue (CF) behaviour, under constant deflection bending conditions with a pulsating tension stress form, of 2024-T3 aluminium alloy, unanodized and anodized to form a thick porous film, in 3.5% NaCl solution has been investigated. It was found that E _corr varies very little until specimen fracture under low frequency CF conditions, whereas E _corr drops rapidly when approaching the later fracturing stage of the CF process under high-frequency conditions for unanodized specimens. However, a slow drop in E _corr was detected from the commencement of the CF process, and lasted up to a much more rapid drop at a later fracturing stage for the anodized specimen. This behaviour presumably can be explained by the cracking of the anodic film and the theory of imperfect recovery of the surface film. It is suggested that the E _corr monitoring technique may be useful for determining the remnant CF life for existing structural parts of this alloy or other aluminium alloys regardless of whether or not they are anodized. Furthermore, the T3 temper provides a microstructure which may retard main-crack formation and penetration in the CF process of the anodized alloy, thus mitigating partly the negative effect of the readily crackable anodic film.