Corrosion Fatigue Life Research Articles

Structural reliability assessment of offshore wind turbine support structures subjected to pitting corrosion‐fatigue: A damage tolerance modelling approach

AbstractThe structural integrity of offshore wind turbine (OWT) support structures is affected by one of the most severe damage mechanisms known as pitting corrosion‐fatigue. In this study, the structural reliability of such structures subjected to pitting corrosion‐fatigue is assessed using a damage tolerance modelling approach. A probabilistic model that ascertains the reliability of the structure is presented, incorporating the randomness in cyclic load and corrosive environment as well as uncertainties in shape factor, pit size and aspect ratio. A non‐intrusive formulation is proposed consisting of a sequence of steps. First, a stochastic parametric Finite Element Analysis (FEA) is performed using SMART© crack growth and Design Xplorer© facilities within the software package ANSYS. Secondly, the results obtained from the FEA are processed using an Artificial Neural Network (ANN) response surface modelling technique. Finally, the First Order Reliability Method (FORM) is used to calculate the reliability indices of components. The results reveal that for the inherent stochastic conditions, the structure becomes unsafe after the 18th year, before the attainment of the design life of 20 years. The FEA results are in very good agreement with results obtained from analysis steps outlined in design standard BS 7910 and other references designated as ‘theoretical analysis methods’ in this study. The results predict, for the case study, that the pit growth life is approximately 56% of the total pitting corrosion fatigue life. Sensitivity analysis results show that the aspect ratio of pits at critical size plays a significant role on the reliability of the structure.

Reliability Prediction of Variable Amplitude Corrosion Fatigue Life of TP140 Casing Steel

Based on the constant amplitude CF test of oil well tube material TP140small specimen, the procedures of predicting the variable amplitude corrosion fatigue (VACF) life of given reliability of TP140 were developed. Through the multi-sample sinusoidal loading constant amplitude CF life texts, the reliability distribution of CF life was analyzed. The CF life prediction of the specified reliability was obtained under 4 different stress levels, and the corresponding P-S-N curve expressions were obtained. Using VACF loading block spectrum and P-S-N curve expressions, the VACF life was calculated without considering the loading sequence effect. Furthermore, 5 VACF life tests were carried out using the same loading block spectrum. The VACF life with reliability was predicted by the principle of reliability and statistics. VACF life was calculated and compared with the predicted life. Test results and analysis show that the predicted results agree well with the experimental results, and CF life of TP140 casing steel follows a lognormal distribution at the given equivalent stress level, which has been substantiated.

Effects of grain structure related precipitation on corrosion behavior and corrosion fatigue property of Al–Mg–Si alloy

An Al–Mg–Si alloy with three types of grain structure (fully, partly, and none recrystallized grains) were prepared to determine the effects of grain structure related precipitation on corrosion resistance and corrosion fatigue property in this paper. Corrosion immersion tests, corrosion fatigue tests were conducted for the purpose. Through the investigations, a mechanism different from the existent mechanisms for expounding the corrosion behavior of Al–Mg–Si alloys is proposed in this paper. It was found that low angel grain boundaries (LAGBs) with continuous Mg-Si segregation play a dominant role on the penetration depth of corrosion; while the intragranular dislocations induced precipitates determine the corrosion morphology on the surface of the sample; in addition, the high angel grain boundaries (HAGBs) and precipitation free zones (PFZs) in the periphery do not decrease the corrosion resistance of the experimental alloy in the conditions of this paper. Furthermore, a deeper penetration of corrosion rather than the severer attack of corrosion on the surface results in a shorter corrosion fatigue life, as well as a corrosion dominated fracture failure during corrosion fatigue tests.These results may supplement the existing corrosion mechanisms of Al–Mg–Si alloy.

The effect of near-surface plastic deformation on the hot corrosion and high temperature corrosion-fatigue response of a nickel-based superalloy

Abstract Surface treatments such as shot peening to inhibit fatigue crack initiation are essential processes when designing gas turbine components for aerospace applications. It is therefore crucial to understand the effects of shot peening in representative service environments. Here, the influence of surface treatment on the high temperature corrosion fatigue response of a polycrystalline nickel-based superalloy is considered, an area that has not previously been explored. Two shot peening conditions; 110H 7A 200% and 330H 7A 200%, along with a polished surface were chosen. Specimens were salted and exposed to SO2 gas during fatigue testing at 700 °C. A range of novel techniques including SEM, EBSD and axial chromatism profilometry were used to analyse the near surface cold work and surface condition before and after testing. EBSD local misorientation maps, paired with an increase in corrosion-fatigue life, suggest that a greater depth of cold work produced by the smaller shot size (110H), is providing a significant benefit in terms of hot corrosion and corrosion-fatigue performance. This paper concludes that the presence of a substantial layer of cold work is required to account for any metal loss due to the effects of hot corrosion. It is also evident that cold work hinders fatigue crack initiation and delays the onset of pit to crack transition.

Analysis on damage evolution and corrosion fatigue performance of high-strength steel wire for bridge cable: Experiments and numerical simulation

Corrosion fatigue (CF) failure is one of the typical failure modes of high-strength steel wires for bridge cables because the cables are subjected to the long-term cyclic loads and exposed to the heavy polluted environment at the same time. In this paper, the damage evolution process and CF performance were investigated, and the service life was predicted for steel wire with machine-cut notch via experiments and numerical simulation. Firstly, a novel experimental device was constructed to apply fatigue load and corrosive medium on multiple specimens simultaneously. With the constructed device, two groups of experiments were conducted to analysis the fatigue performance and CF performance of surface notched wire specimens, and the fatigue life and CF life of steel wire were obtained under fatigue load and the coupling effect of corrosive medium and fatigue load (CCF) respectively. Additionally, in order to study the fatigue damage evolution process and CF damage evolution process of surface notched steel wire under fatigue load and CCF respectively, a fatigue damage constitutive equation and a CF damage constitutive equation were proposed respectively according to continuum damage mechanics, and experimental data were used to determine the value of each parameter in these two equations. With the proposed model, damage evolution process in steel wire was simulated under fatigue loads and CCF respectively. The effect of corrosive medium on wire life was discussed. The results show that the fatigue life and CF life of surface notched wire obtained from simulation are consistent well with the experimental results, which shows that the simulation method can achieve the expected goal, and has high reliability in predicting the fatigue life and CF life, and simulating the fatigue damage evolution and CF damage evolution of surface notched wire. For the fatigue damage evolution process and CF damage evolution process, damage evolves gradually, and the damage evolution rate gradually increases. There are more and more damage elements at the bottom of machine-cut notch, and some elements inside the steel wire are also damaged. Comparing the damage distribution of steel wire subjected to fatigue load only and CCF, it can be found that corrosive medium will cause the damaged elements to appear earlier. Under the same stress range, the fatigue life and CF life of steel wire with machine-cut notch are significantly different, but the fatigue damage evolution rate and CF damage evolution rate are similar. It can be concluded that the difference in service life is mainly due to the corrosive medium that causes the damaged elements near the surface notch appear earlier, especially under higher stress ranges.

Research on surface residual stresses generated by milling Ti6Al4V alloy under different pre-stresses

Machining-induced residual stress plays a significant role in the corrosion resistance and fatigue life of the manufacturing end product. In the current study, milling experiments were performed to investigate the influence of pre-stress on the surface residual stresses. For this purpose, finite element (FE) simulations of the orthogonal cutting based on arbitrary Lagrangian-Eulerian approach were performed. To validate the simulation results, a device was designed to stretch the workpiece with pre-stresses before the machining process. The residual stresses obtained under different pre-stresses were measured based on the varied bending deflection of the workpiece after each material layer removal. As a result, the general variation trend of residual stress profiles that obtained under different pre-stresses was found well consistent with the finite element simulation results, indicating that the proposed residual stress measuring principle is reliable. Furthermore, the pre-stress loaded on the Ti6Al4V part can significantly affect the distribution of machining-induced residual stresses, implying that surface residual stresses can be adjusted by employing appropriate values of pre-stresses during the machining process.

An improved continuum damage mechanics model for evaluating corrosion–fatigue life of high-strength steel wires in the real service environment

In this paper, an improved continuum damage mechanics (CDM) model for predicting the corrosion-fatigue life of high-strength steel wires is proposed considering the effects of both the corrosive environment and external force. A numerical simulation program is implemented in ANSYS based on this model; subsequently, a typical case study is performed and the effects of the corrosion environment, pit shape, and tensile stress are examined. Results compared to real engineering projects indicate that the developed model is reasonable and can be used to describe the corrosion-fatigue evolution process of high-strength steel wires in the real service environment.

Corrosion fatigue analysis of stay cables under combined loads of random traffic and wind

After the failure of their protection system, stay cables may subject to environmental corrosion and fatigue loading simultaneously. To study the cable safety during the bridge service life, a general framework was proposed in this paper to estimate the corrosion fatigue life of stay cables under the combined action of random traffic and wind. A simplified approach integrated with the wind-vehicle-bridge system was first introduced to calculate the stress time histories of the stay cables. Based on the traffic data and wind records at the bridge site, the procedure to predict the stress ranges of bridges in service was applied to the North Channel Bridge (NCB) of Hangzhou Bay Bridge as an example and the most critical stay cable was determined. The time-variant corrosion fatigue model of the stay cable was then established considering the deterioration of the steel wires and the corrosion distribution inside the cable. Three levels of corrosion rates (low, medium, and high) were also defined in the model for different environmental conditions, and the corrosion fatigue analysis of the stay cable was finally conducted. The results show that the coupled effects of corrosion and fatigue greatly reduce the lifetime of the wire, and the fatigue resistance of the stay cable in the bridge service life is insufficient under certain corrosion rates.

Effect of stored energy on corrosion fatigue properties of ultrafine grained Fe-20%Cr steel by equal channel angular pressing

Corrosion fatigue properties of ultrafine grained (UFG) Fe-20%Cr steel by equal channel angular pressing was investigated in term of grain refinement and stored energy. The corrosion fatigue life of the ECAP processed and the post-ECAP annealed sample was analysed. Under annealing temperature of 773 K, due to the recrystallization stage, there was a little difference in microstructure from the four passes ECAP. After the annealing process, the strength of the samples pressed four passes decreases, while the elongation showed more ductile. Two mechanisms for crack propagation have been well recognized for stainless steel. One is slipping step dissolution mode. In this model, the slip step formed by active dislocation at a crack tip is anodic dissolution reaction, so that crack tip advance to further distance. The other is stress adsorption mechanism. The corrosion fatigue cracks initiation and cracks propagation process of iron-chromium alloy can be characterized by corrosion pits and intergranular fracture, respectively. The corrosion fatigue behaviour of non-equilibrium and equilibrium grain boundary of iron-chromium alloy was occupied by crack initiation.

Influence of geothermal environment on the corrosion fatigue behaviour of standard duplex stainless steel X2CrNiMoN22-5-3

In the geothermal environment corrosion fatigue may severely lower the lifetime expectancy of high alloyed steels. Therefore, the corrosion fatigue (CF) of duplex stainless steel X2CrNiMoN22-5-3 was investigated in the Northern German Basin electrolyte at 369 K using a specifically designed corrosion chamber. Although the failure mechanism is independent of surface roughness (low scatter ranges technical surface: TN=1:1.35, polished surface: TN=1.1.95), the life expectancy in purely alternating axial cyclic load to failure was clearly related to surface finish and applied stress amplitude. Specimen with technical surfaces tested at high stress amplitudes (>275 MPa) lasted longer (cycles to failure: P50% at Sa 300 MPa=5x105) than specimen with polished surfaces (cycles to failure: P50% at Sa 300 MPa=1.5x105). When applying a protective potential a significant increase of CF life range from 4.7 x 105 (free corrosion potential) to 107 cycles (potential range from USHE = –450 to – 900 mV) was observed. Microstructural analysis of the CF damage revealed horizontal grain attack within corrosion pit cavities, multiple fatigue cracks and preferable deterioration of austenitic phase. At low stress pitting is the initiating crack growth process whereas at high stress amplitudes the formation of micro cracks is reason for crack propagation and failure.

Damage evolution simulation and life prediction of high-strength steel wire under the coupling of corrosion and fatigue

A simulation method is proposed to study corrosion fatigue (CF) damage evolution and predict the CF life of steel wire. The initial finite element model of corroded steel wire is developed by the cellular automaton and the damage evolution simulation is realized by a user material subroutine based on CF damage model. The proposed method can achieve the expected goals compared with experimental results. The mutual promotion of corrosion and fatigue results to a large performance degradation in steel wire. The damage evolution of random pit model is more complex and the CF life is shorter than semi-ellipsoid pit models.

A reliability assessment model for an outdoor 88 kV XLPE cable termination

A metropolitan power utility has been experiencing systematic and premature failures of a particular type of XLPE cable terminations operating at 88kV (rms). The utility therefore desired to understand the modes of failure in order to evaluate the risk with the terminations still in service. Failed terminations were forensically analysed. The dominant modes of degradation were identified as electro-thermal, galvanic corrosion and thermomechanic fatigue. Based on the weakest link principle, a reliability model was developed using the life factor models corresponding to the identified modes of degradation. Galvanic corrosion and thermomechanic fatigue life factors were found to have the predominant influence on the overall reliability of the termination as they reduced the cable termination life by more than 10 years. The model can be used to track the probability of the remaining life of the terminations as well as evaluating the effectiveness of alternative designs and/or in-situ repairs.

Impact of High-Velocity Oxy-Fuel ZrO2 Coating on Corrosion Resistance and Fatigue Life of AISI 316L Austenitic Stainless Steel

Purpose. The purpose of this research paper is to investigate the corrosion and fatigue life of AISI 316L austenitic stainless steel in the absence and presence of high-velocity oxy-fuel ZrO2 coating. Design/Methodology/Approach. AISI 316L austenitic stainless steel is chosen for the investigation, keeping in mind, its widespread usage in naval and marine applications where the members are exposed to corrosive sea water environment. ZrO2 coating is a popular surface treatment provided to mechanical members to improve their corrosion resistance. Being a refractory material, ZrO2 inhibits the corrosion of the AISI 316L austenitic stainless steel in marine applications. But, the study of the effect of ZrO2 coating on the corrosion and fatigue life of the material hitherto is scarce and hence the present investigation is undertaken. The corrosion and fatigue analysis of the coated specimens are carried out by taking two control parameters, namely, rotational speed of job and axial speed of torch, into consideration and applying L4 Taguchi orthogonal array. Findings. The corrosion resistance of the material has increased but the fatigue strength has decreased upon coating of ZrO2 on AISI 316L austenitic stainless steel. The failure has occurred because of the formation of oxide layers on the steel during coating.

Modelling guidelines for corrosion-fatigue life prediction of concrete bridges: Considering corrosion pit as a notch or crack

Corrosion accelerates the fatigue damage accumulation and decreases the fatigue life of reinforced concrete bridges. A critical issue in the corrosion-fatigue life prediction is to deal with the relationship between fatigue crack and corrosion propagation. This paper proposes two modelling methods for fatigue life prediction of reinforced concrete bridges subjected to coupled corrosion-fatigue effect: model I considers the corrosion pit as a notch and model II treats the corrosion pit as a surface crack. The chloride ingress, cyclic load, corrosion pit growth, fatigue crack growth and concrete severe cracking are systematically considered. Following that, a probabilistic analysis is also performed to include various uncertainties in the two models, and the effects of environment aggressiveness level and traffic frequency on the prediction results are investigated. The results show that corrosion pit-induced stress concentration has a significant effect on the fatigue behavior, and modelling the corrosion pit as a notch is the preferred method for theoretical prediction. For the extreme aggressiveness environment and low traffic frequency conditions, considering the corrosion pit as a crack is an alternative modelling method for the fast assessment of aging reinforced concrete bridges due to its simplicity.

Effect of Shot Peening Time on fatigue and corrosion fatigue life for A27 cast carbon Steel

A 27 cast carbon steel, which was used in induced draft fan blades of Nasiriyah thermal electricity station was chose to actualize the aim of this paper by studying the effect of shot peening time on dry and corrosion fatigue life. .Many specimens from the metal choice for tensile and fatigue tests were equipped according to ASTM (E8/E8M-09) some of preparing specimens were subjected to shot peening by steel ball in diameter 2.25 mm for different time (10, 20, 30) minute. Hardness, residual stress, surface roughness , dry fatigue and corrosion fatigue were tested. The result showed an improvement in the tensile strength for shot peening comparing with the base metal. The tensile strength of the shot peening metal at 20 minute was higher than the tensile strength of the shot peening at 10,30 minute. Also shot peening contributed in improvement fatigue life but the best time was 20 minute and this improvement witness decay in fatigue life by corrosion fatigue test due the effect of environment in addition of cyclic loads.

Influence of combined shot peening and PEO treatment on corrosion fatigue behavior of 7A85 aluminum alloy

The plasma electrolytic oxidation (PEO) with coating thickness of ~10 μm and ~35 μm on 7A85 aluminum alloy was performed using the alkaline silicate based electrolyte, and the corresponding influence on fatigue behavior in air and 3.5% NaCl solution was studied. Results show that PEO treatments reduced the corrosion fatigue life of 7A85 aluminum alloy. The corrosion fatigue life of samples with thin PEO coatings (10 μm-thick) is decreased for the occluded battery promoted localized corrosion, while that of samples with thicker PEO coatings (35 μm-thick) is reduced for the low ductility of the oxide coating. The shot peening (SP) treatment with ceramic shots was introduced as pretreatment of PEO, The surface residual stresses as well as the residual stress of the substrate beneath the coating were analyzed by X-ray stress analyzer. The corrosion fatigue performance of shot peening combined PEO treated samples is improved for the residual compressive stress field induced by shot peening. Whereas, enhanced notch effect was induced by shot peeing surface roughening and oxide coating local overgrowth, so the corrosion fatigue life of samples with SP, surface polishing, PEO, respectively is further improved for the reduction of notch effect.

A critical appraisal of laser peening and its impact on hydrogen embrittlement of titanium alloys

Many efforts have been made to understand the effects of hydrogen on titanium alloys, resulting in an abundance of theoretical models and papers. Titanium alloys are crucial advanced materials that provide an excellent combination of a high strength-to-weight ratio and good corrosion behaviour even though they are reasonable to corrosion attack. Titanium alloys are susceptible to hydrogen embrittlement when comes into contact with hydrogen, and galvanic pair with an active metal current, or the pH is greater than 12 or less than 3 or an impressed current. In view of the fact that hydrogen behaves differently with α and β phases, hydrogen degradation may vary markedly in titanium alloys. Hydrogen diminishes the corrosion and erosion resistance and fatigue life of in-service titanium components. A laser peening or laser shock peening is a novel technique for making the metal surfaces and sub-layers densify. It evokes that laser shock peening adoption results in yielding and plastic deformation, thereby creating high compressive residual stresses extending below the surface of the material which is desirable for hydrogen embrittlement resistance and reduction of crack initiation and growth of the component. This article is a review of information relating hydrogen embrittlement of titanium alloys and surface modification technique which influence the strength potential of titanium alloys.

Corrosion Fatigue of Duplex Stainless Steel X2crnimon22-5-3 Exposed to the Geothermal Environment of the Northern German Basin Under Different Applied Potentials and Surface Finishes

The corrosion fatigue (CF) of duplex stainless steel X2CrNiMoN22-5-3 was investigated by means of applying purely alternating axial cyclic load to failure. Hour-glass shaped specimens with different surface finishing were tested at different stress amplitudes in a corrosion chamber allowing for the circulation of Northern German Basin electrolyte heated to 369 K. The influence of a protective potential on the CF was investigated by implementing a potentiostat. The significant increase of CF life range from 4,7 x 105 (free corrosion potential) to 107 cycles (potential range from USHE = –450 to –900 mV) was observed. The influence of surface condition was significantly demonstrated with polished surface finish showing a higher CF life expectancy than precisely turned surface finish. Microstructural analysis of the CF damage revealed horizontal grain attack within corrosion pit cavities, multiple fatigue cracks and preferable deterioration of austenitic phase.

Influence of ultrasonic rolling on surface integrity and corrosion fatigue behavior of 7B50-T7751 aluminum alloy

The surface integrity and corrosion fatigue (CF) performance of the 7B50-T7751 aluminum alloy treated by one (UR1), three (UR3), and six passes (UR6) of ultrasonic surface rolling process (USRP) were investigated in this paper. The results revealed that a modified surface layer with refined microstructure and increased microhardness was formed after the USRP, and a compressive residual stress (CRS) was induced. The CF life of all USRP-treated samples was improved, especially the life of the UR1 sample with an optimal surface integrity. The synergistic effect of surface layer microstructure refinement and CRS effectively have inhibited the occurrence and development of pitting and intergranular corrosion, prevented the initiation of CF cracks at the surface, thus led to the movement of the fatigue crack nucleation site toward the subsurface. Overall, the CRS has played a major critical role. The severe surface damage in the modified layer of the UR6 sample resulted in a partial relaxation of the CRS, in favor of secondary fatigue crack initiation, therefore when using the USRP treatment to improve the CF life of the 7B50 aluminum alloy, the effect of surface integrity should be fully considered.

Durability method on corrosion fatigue performance of AH 32 steel

A durability method in view of cathodic protection is proposed to improve the corrosion fatigue resistance of AH 32 steel in seawater. By aid of corrosion fatigue tests, the effects of thermal spraying Zn (zinc) and Cr (chromium) coating corrosion fatigue lives are quantitatively determined, respectively, and electrochemical measurement and fracture analysis are used to analyze the life-prolonging mechanism of these two coatings on corrosion fatigue. The results show that both Zn and Cr coating improve the corrosion fatigue resistance of AH 32 steel, and the effect enhances with the decrease of stress. The effect of Cr coating on corrosion fatigue of AH 32 steel mainly reflects in extending the crack initiation life for its better corrosion resistance. While the effect of Zn coating on corrosion fatigue lies in not only inhibiting the initiation of corrosion fatigue but also restraining crack propagation as cathodic protection materials. To sun up, Cr coating has a better durability effect than Zn coating at higher stress level, while Zn exceeds Cr at low stress level.