A predictive fatigue life model for anodized 7050 aluminium alloy

The objective of this study is to predict fatigue life of anodized 7050 aluminum alloy specimens. In the case of anodized 7050-T7451 alloy, fractographic observations of fatigue tested specimens showed that pickling pits were the predominant sites for crack nucleation and subsequent failure. It has been shown that fatigue failure was favored by the presence of multiple cracks. From these experimental results, a fatigue life predictive model has been developed including multi-site crack consideration, coalescence between neighboring cracks, a short crack growth stage and a long crack propagation stage. In this model, all pickling pits are considered as potential initial flaws from which short cracks could nucleate if stress conditions allow. This model is built from experimental topography measurements of pickled surfaces which allowed to detect the pits and to characterize their sizes (depth, length, width). From depth crack propagation point of view, the pickling pits are considered as stress concentrator during the only short crack growth stage. From surface crack propagation point of view, machining roughness is equally considered as stress concentrator and its influence is taken into account during the all propagation stage. The predictive model results have been compared to experimental fatigue data obtained for anodized 7050-T7451 specimens. Predictions and experimental results are in good agreement.     

A study of fatigue crack growth from artificial corrosion pits at welded joints under complex stress fields

The paper studies the effects of artificial corrosion pits and complex stress fields on the fatigue crack growth of full penetration load‐carrying fillet cruciform welded joints with 45° inclined angle. Parameters of fatigue crack growth rate of welded joints are obtained from S‐N curves under different levels of corrosion. A numerical method is used to simulate fatigue crack growth using different mixed mode fatigue crack growth criteria. Using polynomial regression, the crack shape correction factor of welded joints is fitted as a function of crack depth ratios. Because the maximum circumferential stress criterion is simple and easy to use in practice, fatigue crack growth rate is modified using this criterion. The relationship of effective stress intensity factor, crack growth angle and crack depth is studied under different corrosion levels. The simulated crack growth path obtained from the numerical method is compared with the actual crack growth path observed by fatigue tests. The results show that fatigue cracks do not initiate at the edge or bottom of pits but at the weld toes where the maximum stress occurs. The artificial corrosion pits have little effect on the effective stress intensity factor ranges and crack growth angle. The fatigue crack growth rates of welded joints with pits 1 and 2 are 1.15 times and 1.40 times larger than that of the welded joint with no pit, respectively. The simulated crack growth path agrees well with the actual one. The fatigue life prediction accuracy using the modified formulation is improved by about 18%. The crack shape correction factor obtained using the maximum circumferential stress criterion is recommended being used to calculate fatigue life.     

FATIGUE LIFE DISTRIBUTION AND GROWTH OF CORROSION PITS IN A MEDIUM CARBON STEEL IN 3%NaCl SOLUTION

Abstract— Statistical fatigue tests have been conducted on a structural medium carbon steel, S45C, in room air and in 3%NaCl solution, using five cantilever-type rotary bending fatigue testing machines which were specially manufactured for the purpose of the present study. Fatigue life distribution was examined at three and five stress levels in air and in 3%NaCl solution, respectively, and twenty specimens were allocated to each stress level. In room air, it was found that fatigue life distributions followed the three-parameter Weibull distribution, which were closely related to fracture morphology. In 3%NaCl solution, they also followed the Weibull distribution, but the scatter in fatigue life was smaller in comparison to that in air. It is suggested that the decrease in the scatter of fatigue life may be attributed to a smaller fraction of crack initiation life in 3%NaCl solution. The growth of corrosion pits was investigated using a laser microscope. The distribution of corrosion pit depths followed the log-normal distribution, and the corrosion pit depths increased with increasing time or the number of cycles. It was found that the growth of corrosion pits was accelerated by stress cycling and the depths increased with increasing stress level. Based on these results, a growth law of corrosion pits, including the effect of stress cycling, is proposed.     

Very high cycle fatigue of a high strength steel under sea water corrosion: A strong corrosion and mechanical damage coupling

This paper is focused on the effect of sea water corrosion on the gigacycle fatigue strength of a martensitic–bainitic hot rolled steel R5 used for manufacturing off-shore mooring chains for petroleum platforms in the North Sea. Crack initiation fatigue tests in the regime of 10⁶ to 10¹⁰ cycles were carried out on smooth specimens under three different environment conditions: (i) without any corrosion (virgin state) in air, (ii) in air after pre-corrosion, and (iii) in-situ corrosion-fatigue under artificial sea water flow. A drastic effect of sea water corrosion was found: the median fatigue strength beyond 10⁸ cycles is divided by 5 compared to virgin state specimens. The crack initiation sites were corrosion pits caused by pre-corrosion or created during corrosion-fatigue under sea water flow. Furthermore some sub-surface and internal crack initiations were observed on specimens without any corrosion (virgin state). Crack propagation curves were obtained in mode I in air and under sea water flow. Calculation of the stress intensity factor at the tip of cracks emanating from hemispherical surface pits combined with the Paris–Hertzberg–Mc Clintock crack growth rate model showed that fatigue crack initiation period represents most of the fatigue life in the VHCF regime. Additional original experiments have shown physical evidences that the fatigue strength in the gigacycle regime under sea water flow is mainly governed by the corrosion process with a strong coupling between cyclic loading and corrosion.     

Corrosion fatigue behavior of a heat-treated carbon steel and its statistical characteristics

The investigation of fatigue damage in corrosive environments is an important problem, because such environments reduce fatigue strength far below the typical fatigue strength determined in air. In this study, rotating bending fatigue tests of plain specimens in NaCl solution were carried out using a heat-treated 0.45% carbon steel, in order to clarify the physical background of corrosion fatigue damage. The emphasis is to perform the successive observations by the plastic replica method. The results show that corrosion pits are generated at the early stages of cycling, then most of them grow with further cycling until a crack is initiated from each corrosion pit. The initiation of corrosion pits from slip bands is only observed in the case when the stress range is relatively large, in the range of stress under which slip bands are produced in air. After initiation of a crack, a crack propogates by accompanying frequent interaction and coalescence with other cracks. The growth rate of an especially small crack in NaCl solution is larger than that in air. However, the growth rate of a comparatively large crack is smaller in NaCl solution than in air. Moreover, the statistical characteristics of corrosion fatigue behavior were investigated by exhibiting the distributions of crack initiation life and crack length.     

EIFS-based crack growth fatigue life prediction of pitting-corroded test specimens

Corrosive environment causes corrosion pits at material surface and reduces the fatigue strength significantly. Fatigue crack usually initiates at and propagates from these locations. In this paper, a general methodology for fatigue life prediction for corroded specimens is proposed. The proposed methodology combines an asymptotic stress intensity factor solution and a power law corrosion pit growth function for fatigue life prediction of corroded specimens. First, a previously developed asymptotic interpolation method is proposed to calculate the stress intensity factor (SIF) for the crack at notch roots. Next, a growing semi-circular notch is assumed to exist on the specimen’s surface under corrosive environments. The notch growth rate is different under different corrosion conditions and is assumed to be a power function. Fatigue life can be predicted using the crack growth analysis assuming a crack propagating from the notch root. Plasticity correction is included into the proposed methodology for medium-to-low cycle fatigue analysis. The proposed methodology is validated using experimental fatigue life testing data of aluminum alloys and steels. Very good agreement is observed between experimental observations and model predictions.     

CRACK NUCLEATION AND PROPAGATION IN BLADE STEEL MATERIAL

Stress corrosion cracking and corrosion fatigue of 12 Cr steel in sodium chloride solution has been investigated. Tests have been performed in air at room temperature and in aqueous solution with 22% NaCl at 80°C. The influence of corrosion pits on crack nucleation has been investigated. On fracture surfaces tested in environment (22% NaCl solution), crack initiation was observed in correspondence with corrosion pits; in this case fatigue life can be described using a fracture mechanics approach. The ΔK value for crack nucleation from a pit in rotating bending fatigue tests is very low in air (about 3 MPa√m). The results of slow strain rate tests on smooth specimens show that there is a threshold stress intensity, K_ISCC, of about 15 MPa√m and a plateau in stress corrosion crack growth rate of about 10^-5mm/s.     

当量加速腐蚀条件下7B04-T6高强度铝合金疲劳裂纹扩展规律研究

基于编制的机场环境加速试验谱,针对关键结构高强度铝合金件进行当量腐蚀试验,在实验室条件下成功地模拟和再现了服役环境条件的腐蚀损伤,借助复型法观测得到了腐蚀损伤的演化规律;通过预腐蚀疲劳试验和疲劳断口扫描电镜定量分析,得到了裂纹长度a与循环次数N数据集,分析了裂纹扩展速率da/d N与应力强度因子幅值ΔK的对应关系,定量表征了不同程度腐蚀损伤对疲劳裂纹扩展行为的影响规律.结果表明,在腐蚀初期,疲劳裂纹扩展过程中有经典的小裂纹扩展阶段;随着腐蚀损伤的加重,小裂纹行为不明显;腐蚀损伤越严重,疲劳裂纹扩展速率越快,结构抗疲劳性能显著退化.     

Corrosion fatigue life prediction of a steel shaft material in seawater

Corrosion fatigue behaviour of a medium strength structural material was studied in air and in 3.5% NaCl solution. Emphasis was placed on the study of corrosion pit formation and the development of cracks from pits. Pitting and crack propagation were quantified throughout the fatigue loading thereby allowing a model to be developed that included the stages of pitting and the pit-to-crack transition in order to predict the fatigue life. The results showed that a large number of corrosion pits with small size form at a very early stage in the fatigue lifetime. The number of pits and subsequent cracks was found to be higher at higher stress levels leading to multiple crack development and coalescence. When compared to air, fatigue life in a corrosive environment was significantly reduced at low stress levels due to pitting damage, indicating a dominant role of corrosion over that of mechanical effects. The corrosion fatigue model proposed shows good agreement with the experimental test data at lower stress levels but predicts more conservative lifetimes as the stress increases. Kitagawa–Takahashi diagram was produced for both test environments where it is indicated that the fatigue limit can be eliminated in a corrosive environment.     

The interaction between pitting corrosion,grain boundaries,and constituent particles during corrosion fatigue of 7075-T6 aluminum alloy

Concomitant corrosion fatigue research was performed on 7075-T6 aluminum alloy to gain an increased understanding of how microstructure influences pit growth, pit-to-crack transition, and critical crack propagation to fracture. Two thicknesses of rolled sheet and an extrusion of 7075-T6 aluminum alloy were etched and subjected to concomitant corrosion fatigue in a 3.5% sodium chloride solution. Testing was interrupted at various intervals to obtain information on pit generation, growth, and potential cracking. Results indicated that microstructure has a significant influence on pit-to-crack transition and fatigue crack propagation. Constituent particles competed with corrosion pits as critical crack nucleation sites, with some affecting the critical crack by either nucleation of additional cracking or linkage with the main crack. Post-fracture analysis confirmed the presence of noncritical cracks within the corroded region, related to pitting and constituent particles.     

Fatigue crack initiation and growth on a steel in the very high cycle regime with sea water corrosion

This paper is devoted to the effect of corrosion on the gigacycle fatigue strength of a martensitic-bainitic hot rolled steel used for manufacturing offshore mooring chains for petroleum platforms. Smooth specimens were tested under fully reversed tension between 10⁶ and 10¹⁰ cycles in three testing conditions and environments: (i) in air, (ii) in air after pre-corrosion and (iii) in air under real time artificial sea water flow. The fatigue strength at greater than 10⁸ cycles is reduced by a factor more than five compared with non-corroded specimens. Fatigue cracks initiate at corrosion pits due to pre-corrosion, if any, or pits resulting from corrosion in real time during the cyclic loading. It is shown that under sea water flow, the fatigue life in the gigacycle regime is mainly governed by the corrosion process. Furthermore, the calculation of the mode I stress intensity factor at hemispherical surface defects (pits) combined with the Paris-Hertzberg-Mc Clintock crack growth rate model shows that fatigue crack initiation regime represents most of the fatigue life.     

CORROSION FATIGUE OF AN HSLA STEEL

Abstract— Single-pitted specimens of an HSLA steel, were tested in laboratory air and in 1 M NaCl solution to study the influence of a corrosive environment on its fatigue life.
The growth of fatigue cracks and the partitioning of the fatigue life into fatigue crack initiation and fatigue crack propagation were studied by photographing the pit and the cracks developing on it periodically during testing. Non-propagating or dormant surface cracks were not observed in this study. Fractography using SEM showed the locations of fatigue crack initiation. The mechanisms of corrosion fatigue were studied by performing tests in 1 M NaCl at different test frequencies. Corrosion pits proved to be crack initiation sites. Hydrogen embrittlement was found to be unimportant in the corrosion fatigue of HSLA steel in this study. The 1 M NaCl corrosive environment appeared to reduce the fatigue life of this material by a dissolution mechanism. The effect of pit depth was studied by testing specimens having various pit depths. An effect of pit size was apparent. Fatigue life decreased with increasing pit depth. Pit depth, rather than the ratio of pit depth to pit diameter, influenced fatigue behaviour. A non-damaging pit depth was found.     

应力比对航空高强LD2CZ铝合金腐蚀疲劳裂纹扩展的影响

利用扫描电镜联合液压伺服试验机,并借助于Walker公式研究了应力比对预腐蚀不同时间航空高强LD2CZ铝合金疲劳裂纹扩展的影响,在应力比分别为0．05,0．5,0．7的条件下对预腐蚀0,15,30d的LD2CZ铝合金单边缺口板状试样进行了疲劳加载试验,得到了其疲劳裂纹扩展速率曲线,并拟合出了Walker公式中的材料常数。结果表明：裂纹扩展速率会随着应力比的增加以及腐蚀损伤的加深而增大,拟舍得到的Walker公式可用来定量化地表征应力比和腐蚀损伤对疲劳裂纹扩展速率的影响。     

Surface characterization and influence of anodizing process on fatigue life of Al 7050 alloy

The present study investigates the influence of anodizing process on fatigue life of aluminium alloy 7050-T7451 by performing axial fatigue tests at stress ratio ‘R’ of 0.1. Effects of pre-treatments like degreasing and pickling employed prior to anodizing on fatigue life were studied. The post-exposure surface observations were made by scanning electron microscope (SEM) to characterize the effect of each treatment before fatigue testing. The surface observations have revealed that degreasing did not change the surface topography while pickling solution resulted in the formation of pits at the surface. Energy dispersive spectroscopy (EDS) was used to identify those constituent particles which were responsible for the pits formation. These pits are of primary concern with respect to accelerated fatigue crack initiation and subsequent anodic coating formation. The fatigue test results have shown that pickling process was detrimental in reducing the fatigue life significantly while less decrease has been observed for anodized specimens. Analyses of fracture surfaces of pickled specimens have revealed that the process completely changed the crack initiation mechanisms as compared to non-treated specimens and the crack initiation started at the pits. For most of the anodized specimens, fatigue cracks still initiated at the pits with very few cracks initiated from anodic coating. The decrease in fatigue life for pickled and anodized specimens as compared to bare condition has been attributed to decrease in initiation period and multi-site crack initiations. Multi-site crack initiation has resulted in rougher fractured surfaces for the pickled and anodized specimens as compare to bare specimens tested at same stress levels.     

CRACK INITIATION AND PROPAGATION BEHAVIOUR OF A HEAT-TREATED CARBON STEEL IN CORROSION FATIGUE

Abstract— When estimating fatigue damage quantitatively it is important to clarify its physical basis. In this study, rotating bending fatigue tests of a heat-treated 0.45% carbon steel were carried out in 3% NaCl solution, in order to clarify the physical basis of corrosion fatigue damage from successive observations of plastic replicas. The results show that corrosion pits are generated during the early stages of cycling, then most of them grow with further cycling until a crack is initiated from each corrosion pit. The initiation of corrosion pits from slip bands is observed only in the case when the stress range is relatively large, and in the range of stress for which slip bands are produced in air. After initiation of a crack, the crack propagates by frequent interactions and coalescence with other cracks. The growth rate of an especially small crack in NaCl solution is larger than that in air. However, the growth rate of a comparatively large crack is smaller in NaCl solution than in air.     

Fatigue strength of gas turbine compressor blades

An experimental procedure has been developed for the investigation of fatigue and crack growth resistance of materials and real compressor blades. Methods for the determination of stress intensity factors in specimens and in blades with cracks have been justified. Investigations have been performed into the influence of manufacturing residual stresses and surface defects in the form of simulators of dents, corrosion pits, and nonmetallic inclusions on fatigue strength of steels and a titanium alloy. The characteristics of the material crack growth resistance have been studied considering the effect of the medium (sea water) and stress ratio in a cycle, as well as fatigue strength of newly-manufactured blades and those after being in operation. Specific features of fatigue crack propagation in blades have been considered and a method for predicting the life of blades with cracks has been justified.     

Mean stress effects on crack propagation rate and crack closure in 7050-T76 aluminum alloy

High strength aluminum alloys that are cyclically loaded in tension are known to show increased fatigue crack propagation rates when the mean stress is increased. It has been suggested that this increase in growth rate may be due to a lower crack closure stress. A smaller closure stress results in a higher effective fatigue crack propagation stress. Measurements of the closure stress were made using an extensometer placed across the crack in order to study the effects of crack closure on the fatigue crack growth rate. The closure stress was determined from the change in slope of load-displacement curves. The growth rate data at various mean stresses were plotted as a function of effective stress intensity factor. In all tests the cyclic stress amplitude was 80 MPa. For specimens run at mean stress levels of 44 MPa and 70 MPa the crack faces were found to close, but on increasing the mean stress to above 120 MPa no closure was observed. The growth rate data from specimens run at mean stresses from 44 MPa to 120 MPa coincided when plotted vs effective stress intensity factor but that crack growth rates for a mean stress of 226 MPa were higher than those obtained at lower mean stresses. We concluded that effects other than crack closure also influence the crack growth rate. Much greater scatter was found in the fatigue crack growth rates of our thicker specimens (5.6mm thick) than in our thinner specimens (2.5 mm thick). Indirect measurements of the length over which the crack faces were closed showed that the length closed decreased sharply with increasing mean stress. At a mean stress of 44 MPa the length closed at minimum stress was several millimeters, while at 70 MPa it had been reduced to less than 1mm.     

Fatigue crack formation and growth from localized corrosion in Al-Zn-Mg-Cu

The effect of precorrosion on the fatigue life of aluminum alloy 7075-T6511 was measured, physical characteristics of corrosion topography plus fatigue damage were established by microscopy, and a corrosion modified equivalent initial flaw size (CM-EIFS) was established using fracture mechanics modeling. Fatigue life is reduced by clustered corrosion pits on the L-S surface from laboratory-EXCO exposure. Cracks initiate from pits clustered as a semi-elliptical surface micronotch rather than the deepest pits, consistent with shape-dependent stress intensity. Marker band analysis establishes that the number of cycles to form a crack about a pit cluster can be a significant fraction of total fatigue life. The CM-EIFS, back-calculated from fracture mechanics analysis of measured fatigue life, equals measured initiating-pit cluster size provided that important inputs are provided; such favorable comparison validates this approach to corrosion-fatigue interaction. Calculated CM-EIFS provides a metric to characterize alloy corrosion damage, and can be used to forward-model the effects of stress and loading environment on fatigue life distribution, critical for efficient alloy development. Use in prognosis of the fatigue performance of a service-corroded surface is hindered by uncertain non-destructive characterization of corrosion topography.     

The influence of salt fog exposure on the fatigue performance of Alclad 6xxx aluminum alloys laser beam welded joints

Laser welding is increasingly used for the fabrication of lightweight and cost-effective integral stiffened panels in modern civil aircraft. As these structures age in service, the issue of the effect of corrosion on their damage tolerance requires attention. In this work, laboratory data on the influence of salt fog corrosion on the fatigue behavior of cladded 6156 T4 aluminum alloy laser welded specimens are presented. The experimental investigation was performed on 6156 T4 laser butt welded sheets. Prior to fatigue testing the welded joints were exposed to laboratory salt fog corrosion exposure for 720 h. The results showed that the clad layer offers sufficient corrosion protection both on base metal and the weld. Fatigue testing was followed by standard metallographic analysis in order to identify fatigue crack initiation sites. Crack initiation is located in all welded samples near the weld reinforcement which induces a significant stress concentration. Localized corrosion attack of the clad layer, in the form of pitting corrosion, creates an additional stress concentration which accelerates crack initiation leading to shorter fatigue life relative to the uncorroded samples. The potency of small corrosion pits to act as stress concentration sites has been assessed analytically. The above results indicate that despite the general corrosion protection offered by the clad layer, the localized attack described above leads to inferior fatigue performance, a fact that should be taken under consideration in the design and maintenance of these structures.     

Influence of frequency and exposure to a saline solution on the corrosion fatigue crack growth behavior of the aluminum alloy 2024

The aim of this study is to identify domains where the interactions between mechanical, environmental and microstructural parameters may occur during corrosion fatigue crack growth in the aluminum alloy 2024. The scope considered encompasses the influence of frequency and of alternate immersion in saline solution. Corrosion fatigue crack propagation tests have been carried out under sinusoidal and saw-tooth waveforms, and at different frequencies, load ratios, grain orientations and tempers, in air, distilled water and 3.5% NaCl in permanent and alternate immersion. The stress corrosion cracking behavior of the alloy 2024 has also been considered in order to evaluate the possible contribution of this type of damage during corrosion fatigue crack growth. In 3.5% NaCl, growth rates were found to decrease with decreasing frequency. In alternate immersion, growth rates were increased by up to an order of magnitude for the ΔK values considered compared to permanent immersion and air. The possible mechanisms that govern the corrosion fatigue behavior of the 2024 alloy are discussed in terms of a competition between passivation and anodic dissolution and/or hydrogen embrittlement. Finally, it is proposed that the fatigue crack growth enhancement observed during permanent immersion is related to a crack-tip hydrogen embrittlement mechanism. Hydrogen would be produced by anodic dissolution in relation with film rupture periodicity and then be dragged into the process zone. In alternate immersion, precipitate-free zone dissolution would govern crack advance, as during stress corrosion cracking.