Damage mechanics based probabilistic high‐cycle fatigue life prediction for Al 2024‐T3 using non‐intrusive polynomial chaos

This paper proposes a low‐cost method for predicting probabilistic high‐cycle fatigue life for Al 2024‐T3 based on continuum damage mechanics and non‐intrusive polynomial chaos (NIPC). To randomize Lemaitre's two scale fatigue damage model, parameters S and s are regarded as random variables. Based on small sample of test life, inverse analysis is performed to obtain samples of the two parameters. Statistic characteristics of the two parameters are calculated analytically through coefficients of NIPC. Fatigue test of aluminum alloy 2024‐T3 standard coupon and plate with hole under different spectrum loading shows that the proposed method is effective.     

Monotonic and low cycle fatigue behaviour of 2024‐T3 aluminium alloy between room temperature and 300 °C for designing VAWT components

In the present study, the results of the monotonic tension tests and low cycle fatigue tests performed on aluminium alloy EN AW‐2024‐T3 under various operating temperatures are presented in order to assess the fatigue behaviour of the aluminium alloy under evaluated temperatures. Monotonic tests were performed to determine the influence of temperature on mechanical properties of the material. The aim of cyclic tests was to acquire the parameters required for Manson–Coffin equation in order to plot strain–fatigue life curves. Moreover, stress–strain behaviour of the alloy and the cyclic hardening behaviour were evaluated using Ramberg–Osgood equation. Finally, P_SWT‐damage parameters for each temperature have been calculated for further investigation of the effects of the temperature on fatigue life using acquired data while taking the account of mean stress effect into calculations. Variations in the experimental data due to various test temperatures are presented for both monotonic and cyclic tests.     

Interactive creep–fatigue crack growth of 2024‐T3 Al sheets: selective transitional functions

Interactive creep–fatigue on aeronautical materials has been of a great concern to engineers and researchers. There are still more fundamental issues to be addressed. In this regard, 2024‐T3 Al sheets are considered to study the transitional behaviours that incorporatematerial, loading and geometry effects in the interactive creep–fatigue process. A dual‐scale fatigue crack growth da/dN‐ΔS model that includes microscale, macroscale and large scales is proposed based on the volume energy density criterion. The model generates the transitionalised crack length (TCL) compared with the fictitious crack length (FCL) yielded from the da/dN‐ΔK model that is only restricted to monoscaling. Crossover points are observed across TCL and FCL curves. With stress amplitude being fixed, three levels of mean stress are employed to discuss the variational effects of transitional functions (TFs). Applied loading conditions have an appreciable effect on the fatigue life cycles of 2024‐T3 Al sheets. Variation of R ratios leads to the interactive creep–fatigue behaviour and further enforces the selective TFs in the inherent multiscaling process. TCLs and FCLs are compared with test data of the 2024‐T3 Al sheet. Rationality and superiority of the dual‐scale crack growth model are validated.     

Plastic CTOD as fatigue crack growth characterising parameter in 2024‐T3 and 7050‐T6 aluminium alloys using DIC

The plastic range of crack tip opening displacement (CTOD) has been used for the experimental characterisation of fatigue crack growth for 2024‐T3 and 7050‐T6 aluminium alloys using digital image correlation (DIC). Analysis of a complete loading cycle allowed resolving the CTOD into elastic and plastic components. Fatigue tests were conducted on compact tension specimens with a thickness of 1 mm and a width of 20 mm at stress ratios of 0.1, 0.3 and 0.5. The range of plastic CTOD could be related linearly to da/dN independent of stress ratio for both alloys. To facilitate accurate measurements of CTOD, a method was developed for correctly locating the crack tip and a sensitivity analysis was performed to explore the effect of measurement position behind the crack tip on the CTOD. The plastic range of CTOD was demonstrated to be a suitable alternate parameter to the stress intensity factor range for characterising fatigue crack propagation. A particularly innovative aspect of the work is that the paper describes a DIC‐based technique that the authors believe gives a reliable way to determine the appropriate position to measure CTOD.     

Effect of cyclic hardening on fatigue performance of slide burnished components made of low‐alloy medium carbon steel

The slide burnishing process causes cyclic loading of the surface being treated, which provokes cyclic hardening. Using a forced‐controlled indentation test, the sixth “loading‐unloading” cycle was stabilised. The effect of the number of passes and the cyclic loading coefficient (CLC) on the fatigue performance of slide burnished specimens was investigated. Rotating bending fatigue tests were conducted using nine groups of hourglass shaped specimens, which were slide burnished through a different number of passes and CLC values. A stabilised cycle of the surface layer achieved with six passes, lead to largest fatigue limit, whereas the CLC exerted negligible influence on the fatigue performance. The observed phenomenon was explained through different residual stress relaxation rates, due to the rotating bending load, as well as with the obtained surface layer microstructure. The residual stress relaxation was investigated through rotating bending fatigue tests, using cylindrical fatigue specimens, followed by X‐ray stress analysis.     

The influence of low‐plasticity burnishing process on the fatigue life of friction‐stir‐processed Al 7075‐T6 samples

The effects of low‐plasticity burnishing (LPB) on the fatigue life of friction‐stir‐processed (FSP) Al 7075‐T6 plates were examined experimentally and numerically. Aluminum samples were taken from plates to test fatigue response in the presence of heat‐affected zone (HAZ) at different loading magnitudes. Finite element method was employed to numerically evaluate fatigue life of FSPed samples by means of the Smith–Watson–Topper (SWT) model. Through numerical analysis, the FSP and its cooling procedure were modelled on the basis of the arbitrary Lagrangian–Eulerian technique, and then, the effect of the LPB to assess fatigue response of samples was examined. Aluminum samples undergoing friction‐stir process presented lower‐fatigue life as stresses were highly concentrated within FSP regions. Involvement of LPB regained fatigue durability through compressive residual stress induced on the aluminum samples. The higher applied force over the LPB promoted compressive residual stress on the sample surface and improved fatigue life of samples. The predicted life results were found twice more in magnitude than those of experimentally obtained.     

The fatigue crack growth behaviour of 2524‐T3 aluminium alloy in an Al2O3 particle environment

The effect of the Al₂O₃ dust environment on the crack propagation behaviour of 2524‐T3 Al alloy was investigated. The results show that the Al₂O₃ dust environment reduces the fatigue crack growth rate (FCGR) of alloy especially at low ΔK. Many Al₂O₃ particles are deposited and stuck in the crack during fatigue loading which promotes crack closure, while this effect is gradually weakened with the increase of ΔK. The deposited Al₂O₃ particles induce the disorderly arranged slip bands (SBs) ahead of the crack tip which deflects the crack path making it more tortuous in the Al₂O₃ dust.     

Fatigue assessment of refill friction stir spot weld in AA 2024‐T3 similar joints

Refill friction stir spot welding is a solid‐state process technology that is suitable for welding lightweight materials in similar or dissimilar overlapped configuration. In this study, the fatigue behaviour of single overlapped spot joints of AA2024‐T3 was studied. To statistically analyse the fatigue data, a 2‐parameter Weibull distribution was deployed, considering several reliabilities (R_e = 0.99, R_e = 0.90, R_e = 0.5, R_e = 0.10). To obtain an optimized weld parameter according to the fatigue behaviour, 2 different weld conditions were studied, taking into account the effect of the hook formation. The microstructure analyses and microhardness profiles showed great similarity in both weld conditions. However, these conditions presented distinct interfacial hook profiles, in which the interfacial hook downward represented better fatigue life and infinite fatigue life at 15% of the maximum strength load. The fracture surfaces obtained from 3 different fracture modes were investigated by using scanning electron microscopy; the crack was tracked and described according to its fracture mechanisms from its initiation until the final failures. It was observed that the crack is initiated at hook profile.     

Multiaxial fatigue behaviour of A356‐T6

Aluminum alloy A356‐T6 was subjected to fully reversed cyclic loading under tension, torsion and combined loading. Results indicate that endurance limits are governed by maximum principal stress. Fractography demonstrates long shear mode III propagation with multiple initiation sites under torsion. Under other loadings, fracture surfaces show unique initiation sites coincidental to defects and mode I crack propagation. Using the replica technique, it has been shown that the initiation life is negligible for fatigue lives close to 10⁶ cycles for combined loading. The natural crack growth rate has also been shown to be comparable to long cracks in similar materials.     

Fretting fatigue behaviour of hard anodizing coated 2014‐T6 aluminium alloy with dissimilar mating materials under plane bending loading

The effect of hard anodizing coated 2014‐T6 aluminium alloy test samples with dissimilar mating materials on fretting fatigue was investigated. Fretting fatigue configuration involved bridge‐type pads on the flat specimen. Bridge‐type pads were made of AISI 4140 steel. All the fretting fatigue tests were conducted under plane bending loading with a stress ratio of R=?1. Coated and uncoated specimens were compared for microhardness, surface roughness, tangential force. The specimens were tested under both plain fatigue and fretting fatigue loading at ambient temperature. Micrographs obtained from scanning electron microscope showed that hard anodizing coating had tiny cracks through the thickness of the anodized layer. The hardness of hard anodized coating was higher than that of uncoated specimens and they also exhibited lower tangential force. However, the fretted region of the hard anodizing coated specimens was rougher than that of uncoated samples and despite lower tangential forces, fatigue lives of hard anodizing coated samples were inferior to those of uncoated samples. As the hard anodizing coating had pre‐existing tiny cracks and tension residual stress, cracks propagated from the hard anodizing coating through the interface into the substrate. We conclude that these may be the main reasons for inferior fretting fatigue lives compared with uncoated samples.     

The effects of load sequencing on the fatigue life of 2024-T3 aluminum alloy

Current fatigue life analysis of metallic rotorcraft dynamic components are based on a linear cumulative damage concept known as Miner's rule. This type of analysis assumes that there is no load sequence effect that occurs during the fatigue loading history. Past studies have shown that linear cumulative damage analysis of fatigue tests has produced life predictions that have been conservative as well as unconservative. Some of this uncertainty has been attributed to the fact that load sequence effects do exist in most fatigue load spectra. As a first phase the study reported herein was done to evaluate the load sequencing effects that could exist in commercial fixed-wing fatigue load spectra. To evaluate these effects a typical commercial wing spectra was reordered using a scheme that had previously been shown in fatigue block loading to produce the shortest fatigue lives. This scheme starts with the smallest load range in a load sequence and proceeds in ascending order until the largest load range is reached. Tests on open hole test specimens made of 2024-T3 aluminum alloy were conducted on the normal sequence of loads as well as the reordered scheme called lo–hi. Test results showed no significant differences between the fatigue lives of the normal load sequence and the reordered load sequence. A computer program called FASTRAN which calculates total fatigue life using only crack growth data was shown to predict the fatigue life of the spectrum tests with acceptable accuracy.     

The effect of pitting corrosion on fatigue life

Fatigue testing of pre‐pitted 2024‐T3 aluminium alloy specimens is performed in laboratory air at 22 °C and 40% RH to characterize the effect of pitting corrosion on fatigue life. Specimens, pre‐corroded in a 0.5 M NaCl solution from 48 to 384 h, have fatigue lives that are reduced by more than one order of magnitude after 384 h pre‐corrosion as compared to those of uncorroded specimens. The reduction in fatigue life is interpreted in terms of the influence of the time of exposure to the corrosive environment or pit size. The crack‐nucleating pit sizes, ranging from 20 to 70 μm, are determined from post‐fracture examinations by scanning electron microscopy. Fatigue lives are estimated using a fracture mechanics approach and are shown to be in good agreement with the actual data. A probabilistic analysis shows that the distribution of fatigue life is strongly correlated to the distribution in nucleating pit size.     

Fatigue crack growth analysis of 2024 T3 aluminium specimens under aircraft service spectra

The fatigue crack growth behaviour of 2024 T3 aluminium was investigated experimentally. The fatigue experiments were performed under constant stress amplitude, constant amplitude with single and multiple overloads and aircraft service spectra. The fatigue spectra used correspond to the air-to-air, air-to-ground and instrumentation and navigation flight phases. They were applied for different stress levels. In total 11 different random flight service spectra were examined. The retardation effects caused by the overloads on the fatigue crack growth behaviour and the fatigue crack growth under aircraft service spectra were predicted using an in-house-developed code. The code makes use of the strip plastic zone approximation to account for material hardening effects along the path of prospective crack growth. Crack growth is treated incrementally and corresponds to failure of material elements ahead of an existing crack after a certain critical number of fatigue cycles. For the simulation of irregular service spectra by equivalent sequences of distinguished stress cycles a modified rainflow counting method is utilized. Spectrum simulation accounts also for non-linearity in fatigue damage accumulation and load sequence effects. The computed fatigue curves fit well with the experimental results.     

Magnesium alloy defectology AZ91D high‐pressure die cast and influence on the fatigue behaviour

The fatigue results of a high‐pressure die cast of AZ91D magnesium alloy revealed the presence of different types of casting defects, which account for the large scattering in the number of cycles until failure. In this paper, this magnesium alloy has been analysed, and in an effort to reproduce the same surface and material conditions exhibited in automotive service components, the fatigue test samples were manufactured using a die that employs the same casting process and equipment. To examine the fracture surface of all the fatigue tests, a scanning electron microscope was used, and the source of the failure, so as to relate fatigue life with casting defect type, was identified. Five casting defect types that influence the fatigue behaviour were observed and classified: (a) isolated pores (blowholes), (b) micro‐porosity areas, (c) circular shrinkage cavities associated with the contraction and geometry of the casted specimen, (d) surface burrs associated with the die‐casting mould and (e) the presence of oxides or inclusions.     

Damage of Al 2024 alloy due to sequential exposure to fatigue, corrosion and fatigue

Fatigue and corrosion are two major concerns of aging aircraft research. Metal corrosion and fatigue act synergistically in various ways. They may be present separately or simultaneously. In this study Al 2024-T3 alloy was intermittently subjected to sequential corrosion and fatigue processes. Specimens were first subjected to various degrees of fatigue damage in air, then immersed in a corrosive solution for a fixed amount of time, and subsequently further fatigued in air to failure. The results are compared with those subjected to fatigue in air, and to corrosion then fatigue in air. We find that the total fatigue life (sum of fatigue cycles before and after corrosion) exceeds that of the virgin samples, provided that the initial fatigue cycle has consumed more than half of life solely fatigued in air. It is believed that the blunting of microcracks and the removal of other surface damage, such as intrusions/extrusions created at the initial stage of fatigue, are the major reasons for this increased fatigue life.     

Fatigue behaviour of die‐casting aluminum alloy suspension arms for scooters

The aim of the paper is at qualifying a methodology for the fatigue life assessment of structural components obtained by die‐casting for vehicle applications. Full‐scale fatigue tests were conducted on the back suspension arms of mid‐size scooters. Two loading conditions, i.e. bending and combined bending plus torsion, were considered as representative of typical operating conditions. Fatigue tests showed that the locations of failure initiation is different for the two loading configurations. Material was characterized by means of small size standard specimens extracted from the components in order to be representative of the effective material conditions, particularly of the surface. Both static (tensile) and fatigue tests were conducted, making use of three different R‐ratios. A finite element (FEM) model of the suspension arm, representative of full‐scale test conditions was set‐up in order to interpret the tests. The sub‐modelling approach was adopted in order to get accurate evaluations of the stress–strain fields with reasonable computing resources and elaboration time. Combined elastic and elastic–plastic analyses were necessary to estimate the stress cycles in the regions critical for fatigue damage. Employing the material properties given by small‐size specimens and the proposed FE analysis technique, predictions of either the locations of failure initiation and the fatigue strength of the component were obtained, in quite good agreement with full‐scale tests.     

Fatigue behaviour of friction stir welded AA2024‐T3 alloy: longitudinal and transverse crack growth

The fatigue crack growth properties of friction stir welded joints of 2024‐T3 aluminium alloy have been studied under constant load amplitude (increasing‐ΔK), with special emphasis on the residual stress (inverse weight function) effects on longitudinal and transverse crack growth rate predictions (Glinka's method). In general, welded joints were more resistant to longitudinally growing fatigue cracks than the parent material at threshold ΔK values, when beneficial thermal residual stresses decelerated crack growth rate, while the opposite behaviour was observed next to K_C instability, basically due to monotonic fracture modes intercepting fatigue crack growth in weld microstructures. As a result, fatigue crack growth rate (FCGR) predictions were conservative at lower propagation rates and non‐conservative for faster cracks. Regarding transverse cracks, intense compressive residual stresses rendered welded plates more fatigue resistant than neat parent plate. However, once the crack tip entered the more brittle weld region substantial acceleration of FCGR occurred due to operative monotonic tensile modes of fracture, leading to non‐conservative crack growth rate predictions next to K_C instability. At threshold ΔK values non‐conservative predictions values resulted from residual stress relaxation. Improvements on predicted FCGR values were strongly dependent on how the progressive plastic relaxation of the residual stress field was considered.     

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.     

Effects of TiN,CrN and TiAlN coatings using reactive sputtering on the fatigue behaviour of AA2024 and medium carbon steel specimens

The effects of ceramic coatings (TiN, CrN and TiAlN films) on the fatigue behaviour of 2024 aluminium alloy (AA2024) and medium carbon steel (MCS) specimens under cyclic loading have been investigated in this study. These ceramic thin films of about 2 μm thickness were deposited on the substrates using radio frequency reactive magnetron sputtering, with metallic Ti, Cr and Al targets in an Ar/N₂ gas environment. The (111), (200) and (220) and (311) peaks of a face centre cubic (fcc) structure in CrN films, and the (111), (200) and (220) peaks in TiN/TiAlN films were observed using X-ray diffraction method, respectively. Fatigue tests were performed using rotational bending conditions at a speed of 3000 rpm with five stress levels (115, 173, 231, 288 and 346 MPa for AA2024; 288, 346, 404, 462 and 519 MPa for MCS) in air. Scanning electron microscopy was used for the analysis of surface morphologies and initial failure stage. The results show that the fatigue life of coated specimens is significantly increased under lower cyclic loading, for example, from 42% at 288 MPa, to 98% at 230 MPa and up to 434% at 173 MPa, for AA2024 with TiN coatings. The improvements of mean fatigue life for MCS coated with TiN films are 88% at 519 MPa, 197% at 462 MPa and up to 597% at 404 MPa, respectively. The experimental results also depict that the coated specimens with higher surface hardness (TiAlN) tended to increase in fatigue strength and fatigue life over uncoated and TiN/CrN specimens.     

Plastically elastically dominant fatigue interaction in 316L stainless steel and 6061‐T6 aluminium alloy

Abstract Extensive studies involving multilevel loading have been performed to study the interaction effects of High–Low and Low–High loading sequences on various metals. ^{1 - 10} High–Low sequences generally yield ‘damage’ sums less than unity while ‘damage’ sums for Low–High sequences are typically > 1. It can be appreciated that the mechanisms governing fatigue behaviour under elastically dominant conditions differ from those observed under predominantly plastic conditions. This paper presents results on the interaction between plastically dominant fatigue (PDF) and elastically dominant fatigue (EDF) in 316L stainless steel and 6061‐T6 aluminium alloy. In addition, overstraining effects coupled with PDF and EDF interaction in 316L stainless steel are also reported.