Corrosion Fatigue Crack Initiation Research Articles

Evaluation of Corrosion Fatigue Crack Initiation Life of 13Cr Steel

In this research, corrosion fatigue crack initiation tests using 13 Cr stainless steel were conducted in synthetic seawater to investigate the corrosion fatigue crack initiation life (Nc). In addition, a corrosion fatigue crack initiation and propagation model was suggested also. Until now, corrosion fatigue crack initiation life was treated ambiguously. The important point of this model is the suggestion of the moment of the corrosion fatigue crack initiation life at which the critical size of the corrosion pit changes to the crack. As the crack which emanates from the pit is usually small, accordingly it is treated as a small crack. In addition, the observation of the corrosion fatigue fracture surfaces using SEM were conducted. And the fracture mechanics analysis using an intrinsic crack model was conducted for the treatment of the small crack. Finally, the following were obtained. In the case without a clear stress concentration point which seems to fall into a corrosion fatigue crack initiation, as the meaning to define the number of cycles, at which the fatigue crack propagation rate becomes faster than the corrosion pit growth rate so that the fatigue crack initiates from the pit and its propagation starts in earnest, as the reasonable and universal corrosion fatigue crack initiation life (Nc) has been clarified.

Corrosion fatigue phenomena learned from failure analysis

The purpose of this paper is to present corrosion fatigue phenomena learned from failure analysis. The emphasis is focused upon the importance of the role of corrosion pit in the corrosion fatigue crack initiation process and intergranular fracture surface in the corrosion fatigue crack propagation process of steam turbine blade. The hydrogen effect is also stressed in the crack propagation process in the corrosion fatigue crack propagation of the cargo oil tank ship structural members for very large crude oil carrier. Finally the future problems should be solved in corrosion fatigue failure are touched in brief.

Effect of precipitates on the corrosion–fatigue crack initiation of ISO 5832-9 stainless steel biomaterial

The correlation between the microstructure and fatigue crack initiation behavior (in air and 0.9% NaCl solution) of ISO 5832-9 austenitic stainless steel biomaterial was investigated. A combination of techniques such as extraction of precipitates, SEM, EDS and X-ray diffraction was found to be very useful to characterize Z-phase precipitates, which are abundant in this steel. An analysis of fatigue crack initiation, using SEM, revealed that fatigue and corrosion–fatigue cracking initiate preferentially in the elongated Z-phase precipitates, which are parallel to the tensile direction, and in coarse precipitates usually associated with Al-rich nonmetallic inclusions. Crack initiation is caused by particle rupture rather than by separation of the particle/matrix interface. The findings indicate that coarse Z-phase precipitates and nonmetallic inclusions are detrimental to the fatigue properties of the steel. These microstructural constituents accelerate the initiation of fatigue and especially corrosion–fatigue cracks.

Effect of microstructure on fatigue properties of steels in Sea Water

In this study, fatigue crack propagation properties in sea water are investigated, when considering the usage for hull structural steels. The rate of the optimum ferrite-bainite (F/B) steels shows superior resistance even in a sea water.Corrosion fatigue properties in sea water are investigated for both conventional ferrite-pearlite (F/P) steels and the F/B steels. It is clarified that fatigue strength of the F/B steels is higher than that of the F/P steels. As to corrosion fatigue crack propagation rates, the F/B steel originally has much superior resistance in normal conditions, and acceleration factors of corrosion crack propagation rate in sea water to that of air is almost constant regardless of microstructure in steels. These two characters brings superior propagation resistance in sea water. As to corrosion fatigue strength, the F/B steel has the advantage of not only a long fatigue crack propagation life, but also long fatigue crack initiation life. This extended initiation life in the F/B steel comes from the small number of corrosion pits, which might be corrosion fatigue crack initiation sites. This difference of pit number may depend on homogeneity to general corrosion. In case of the F/P steels, the microstructure is not so homogeneous, therefore, general corrosion is not uniform. In case of the F/B steels, the general corrosion is uniform because of fine and homogeneous microstructures.

Corrosion Fatigue Crack Initiation and Early Growth Behavior in Stainless Steels. 3rd Report. Influence of Phase Fraction.

Corrosion Fatigue Crack Initiation and Early Growth Behavior in Stainless Steels. 3rd Report. Influence of Phase Fraction.

On Corrosion Fatigue Emanating from Notches: Stress Field and Electrochemistry

We analyze the processes of initiation and growth of corrosion fatigue cracks on the surface of a semicircular notch depending on the stress field and local electrochemical processes running inside the notch. For this purpose, we consider a typical corrosion-active system formed by low-strength carbon steel placed in a 3% NaCl solution. In this system, the electrochemical dissolution of the metal is predominant. A semicircular notch is regarded both as a stress concentrator and as a local corrosion electrolyzer. The field of elastoplastic stresses near the notch is computed by the finite-element method with the help of the CASTEM-2000 software package. The early stages of corrosion fatigue are investigated and the existence of a critical density of cracks q * on the surface of the notch is demonstrated (above this density, the cracks undergo intense coalescence leading to the formation of the macrocrack). We propose a model of this process and an analytic relation for the prediction of the number of cycles N * corresponding to the critical density q *. This relation takes into account the constants of electrochemical dissolution of the metal, the density of corrosion current i cor, and the effective stress σeff, which can be regarded as the mean level of stresses in a segment 0 ≤ x ≤ x eff from the notch tip.

Evaluation of Fatigue Damage and Fatigue Crack Initiation Process by Means of Atomic-Force Microscopy

Researches on fatigue crack-initiation processes in metals by using atomic-force microscopy (AFM) were reviewed. For fatigue of a low-carbon steel and α-brass in air, morphologies and sizes of slip-bands and crack initiation processes were discussed, and it was found that the depth of an intrusion drastically increased with its outgrowth to a crack. With coalescence of cracks, the width of cracks increased rapidly. Transgranular cracks were initiated when their accumulated sliding distances, those were evaluated from measured intrusion depth as a function of intrusion angle relative to the stress-axis, reached a critical value. The critical value was independent of stress amplitude, mean stress, and grain-size. Change of surface roughness during fatigue process was also presented.In corrosion fatigue crack initiation study of a stainless steel and a high-strength aluminum alloy, even for a crack whose optical micrographs showed that it was initiated from corrosion pit, a micro-crack was sometimes found from AFM images before pit formation. On the other hand, for a crack whose optical micrographs showed that it was initiated from grain-boundary without corrosion pit, AFM image sometimes indicated that there was a small pit at the crack initiation site.These results showed that AFM is a very useful and powerful tool for elucidating micro-mechanisms of fatigue crack initiation process.

Corrosion Fatigue Crack Initiation in Aluminum Alloys 7075 and 7050

Mechanisms of corrosion fatigue crack initiation by pitting in high-strength, 7000-series aluminum alloys were studied. The alloys used were Al 7075-T7351 (UNSA97075) and Al 7050-T7451 (UNS A7050). Corrosion pits were formed in a 0.6-M sodium chloride (NaCl) solution by the electrochemical reaction between the constituent particles and the surrounding matrix. Depending on whether these particles were cathodic (C-type) or anodic (A-type) relative to the alloy matrix, pits formed as a consequence of matrix or particle dissolution. In the Al 7075 alloy, C-type and A-type particles were present. In the Al 7050 alloy, only C-type particles were present. The C-type particles were identified, using transmission electron microscopic techniques, as Al23CuFe4 in the Al 7075 alloy and Al7Cu2Fe in the Al 7050 alloy. Because these particles tend to cluster parallel to the rolling plane in the rolling direction, significant pit growth from pit coalescence was observed following prolonged exposure to an aerated 0.6 M NaCl solution. The effect of preexisting corrosion pits on fatigue crack initiation was investigated in an ambient air environment (20°C and 42% relative humidity) using blunt-notch fracture mechanics specimens oriented in the short-transverse direction to obtain cracks in the same orientation as those that grow from rivet holes. Results to date indicate that the presence of corrosion pits can significantly shorten the fatigue crack initiation life and decrease the threshold (ΔK/√ρ)th of the alloy by as much as 50%. Post initiation analyses further confirmed that, when corrosion pits were present, fatigue cracks always initiated from these pits. In the absence of pits, fatigue cracks initiated from large inclusions. Identification of constituent particles, mechanism of pit formation and growth, and analyses of fatigue crack initiation kinetics were discussed.

Corrosion Fatigue Crack Initiation Life of a Ship Structural Steel in Dilute Sulfuric Acids

This paper describes corrosion fatigue for notched specimens in dilute sulfuric acids. The acids are typical corrosion environments of condensate from coal vapor in a coal cargo hold. Fatigue data were consolidated as the relationships between the equivalent stress at a notch root and a fatigue crack initiation life. Life in dilute sulfuric acids was shorter than that in seawater because of the severe dissolution of iron. Long term corrosion fatigue was also estimated from the relationship between the stress intensity factor range to the fatigue crack initiation and the life. Corrosion wastage was considered as an estimate of the fatigue crack initiation life.

113 高強度アルミニウム合金における腐食疲労き裂発生機構のAFMによる観察(GS-3 疲労き裂の発生と伝ぱ)

Corrosion fatigue crack initiation process of a high-strength aluminum alloy, 7075-T651 was observed by means of a scanning atomic force microscopy (AFM). Plane bending fatigue tests were carried out either ill air or in 3% NaCl solution. The fatigue strength in aqueous environment was much lower than that in air. Grain boundaries around specific grains were preferential site of anodic dissolution, and cracks were initiated from these grain boundaries.

Overload effects on corrosion fatigue crack initiation life and life prediction of aluminum notched elements under variable amplitude loading

In this study, corrosion fatigue tests under both constant and variable amplitude loading are carried out to investigate the overload effect on the corrosion fatigue crack initiation (CFCI) life and life prediction model of the notched specimens of LY12CZ aluminum alloy sheets, which has the continuous strain hardening characteristics. Analysis of test results of CFCI life in 3.5% NaCl solution environment shows that overload increases the CFCI threshold but has no appreciable effect on the CFCI resistance coefficient, and thus increases the CFCI life. Further analysis gives the overload effect factor for CFCI life, z, which can be used to characterize the load interaction effect in load spectrum. Furthermore, three load spectra are designed and used for testing and predicting the CFCI lives under variable amplitude loading (VAL). Test results of CFCI life under above mentioned load spectra show that the load pattern has great effect on CFCI life of LY12CZ alloy notched elements even the difference among the load spectra is small. It means that the overload effect should be taken into account in CFCI life prediction under VAL. Finally, the equivalent stress amplitude approach is put forward to predict the CFCI life under VAL by using Miner’s rule and the CFCI life expression considering the overload effect. The predicted CFCI lives of LY12CZ alloy notched elements under VAL agree well with test results. Based on the above test results and analysis, it may be thought that Miner’s rule can be used to predict the CFCI life of notched elements of metals with continuous strain hardening characteristics if the overload effect on CFCI life is taken into account in life prediction under VAL.

Corrosion Fatigue Crack Initiation and Early Growth Behavior in Stainless Steels. 2nd Report, Influence of Microstructure on Early Crack Growth.

Fatigue tests have been conducted on a duplex stainless steel, SUS 329 J 4 L, and an austenitic stainless steel, SUS 304, in order to clarify the influence of microstructure on crack initiation and early growth. Crack initiation of SUS 329 J 4 L occurred principally at austenite phase in room air and in 3% NaCl solution. In SUS 304, cracks were generated within austenite phase and at austenite grain boundary in room air and in 3% NaCl solution, respectively. The early crack growth behavior in the microstructurally small crack region was influenced by the phase change at crack front for SUS 329 J 4 L, and the crack hesitated at the phase boundary. Also in SUS 304 the fluctuations in crack growth rate were found at austenite grain boundaries, which were less remarkable in 3%NaCl solution than in room air. Early crack growth immediately after the initiation was enhanced in 3% NaCl solution for both stainless steels.

The Initiation of Corrosion Fatigue Cracking from Pitting in 3.5Ni-Cr-Mo-V Low-Pressure Rotor Steel.

Corrosion and Corrosion fatigue properties of 3.5 Ni-Cr-Mo-V steam turbine low-pressure rotor steel were investigated. Immersion tests were performed on 3.5Ni-Cr-Mo-V steel by dipping the specimen into deionzed water and 10 ppm NaCl solution. Observation of SEM and X-ray cartography showed that MnS inclusions were partly dissolved and corrosion pits were initiated from the edge of the dissolved MnS. Long-term corrosion fatigue tests were done in aqueous environment up to 5×108 stress cycles. The S-N curve was consisted of two different curves and there appeared substantial reduction in fatigue strength over 5×107 stress cycles. The fatigue crack initiation and growth from pits can be considered as the main factor of reduction of fatigue strength. Assuming the corrosion pit as a semi-elliptic surface crack, the critical stress intensity factor range ΔΚth for corrosion fatigue crack initiation was calculated. The average value of ΔΚth was around 1.50 MPam1 / 2, about one half of the value obtained from conventional macroscopic fatigue crack growth rate tests.

Corrosion fatigue crack initiation of Al–Zn–Mg–Mn alloy in 0.5 M Na 2SO 4 solution

The fatigue crack initiation behavior of Al alloys with Mn, especially the pit-to-crack transition stage following the pit growth stage, was investigated by measuring the propagation rate of the crack emanating from the pit in 0.5 M Na 2SO 4 solution. Experimental results were discussed in terms of the role of Mn in pit-to-crack transition.

On the corrosion fatigue crack initiation model and expression of metallic notched elements

In the present study, attempts are made to extend the application of the mechanical model for the fatigue crack initiation (FCI) and the FCI life formula of metallic notched elements in laboratory air to those in the corrosive environment. The test results and analysis of the corrosion FCI (CFCI) life of aluminum alloys and Ti6A14V show that the expression of the CFCI life obtained by modifying the FCI life formula in laboratory air can give a good fit to the test results of the CFCI life. The salt water (3.5% NaCl) environment has no effects on the CFCI resistant coefficient compared with the FCI resistant coefficient in laboratory air. However, 3.5% NaCl environment greatly decreases the CFCI threshold of aluminum alloy, but has little effect on the CFCI threshold of Ti6A14V. The loading frequency ranging from 1 Hz to 10 Hz has no appreciable effect on the CFCI life, and thus, the CFCI threshold of aluminum alloys investigated. Hence, the expression for the CFCI life of metallic notched elements proposed in this study is a better one, which reveals a correlation between the CFCI life and the governing parameters, such as, the geometry of the notched elements, the nominal stress range, the stress ratio, the tensile properties and the CFCI threshold. However, this new expression of the CFCI life needs to be verified by more test results.

Corrosion Fatigue Crack Initiation and Early Growth Behavior in Stainless Steels.

Fatigue tests have been performed on a dual-phase stainless steel, SUS 329 J 4L, and an austenitic stainless steel, SUS 304, in order to clarify the crack initiation and early crack growth behavior in 3% NaCl. Crack initiation in room air occurred due to ductile transgranular slip in both stainless steels. In contrast, cracks were generated at the austenite-ferrite phase boundary of SUS 329 J 4L and at the corrosion pit of SUS 304 in 3% NaCl. Early crack growth in the microstructurally small crack region seemed to be enhanced in 3% NaCl for both stainless steels. In larger regions, the crack growth behavior for SUS 329 J 4L was coincident with the da/dN-ΔKeff relationships for long cracks, while a breakdown in ΔK similitude was found for SUS 304. Environmental effects were less evident for long cracks, except for a slight increase in da/dN at high ΔK region in both stainless steels.

Corrosion-fatigue life prediction for notched components based on the local strain and linear elastic fracture mechanics concepts

An initiation-propagation model based on the local strain and linear elastic fracture mechanics concepts has been investigated for application to predict the fatigue life of notched components exposed to a corrosive environment. Estimates of the corrosion-fatigue crack initiation lives were obtained using strain-life relationships. The Paris power law was used to obtain the estimates of corrosion-fatigue crack propagation lives. Estimated corrosion-fatigue lives were compared with the experimentally obtained corrosion-fatigue life data using centre-notched specimens of three types of modified Al-2.5Mg alloy exposed to an Arabian Gulf seawater environment. Good fatigue life estimates were obtained both in air and in Arabian Gulf seawater environments for all three types of alloy. It is shown that good corrosion-fatigue life predictions can be made by determining the relevant fatigue parameters via a few simple constant-amplitude fatigue tests on smooth specimens and a few crack growth rate tests in the environment at the frequency of interest.

A fatigue-life prediction methodology for notched aluminum-magnesium alloy in gulf seawater environment

Local-strain and linear-elastic fracture-mechanics (LEFM) methodologies have been investigated for prediction of the corrosion-fatigue life of notched components of specially developed Al-2.5Mg alloys exposed to Arabian Gulf seawater environment. Corrosion-fatigue crack initiation life estimates were obtained using strain-life relationships; corrosion-fatigue crack propagation life estimates were obtained using LEFM relationships. The total corrosion-fatigue life was considered to be the sum of the crack initiation and crack propagation lives. Estimated corrosion-fatigue lives were compared with experimentally obtained corrosion-fatigue life data using center-notched specimens of three types of Al-2.5Mg alloys (containing different amounts of chromium) exposed to Arabian Gulf seawater environment. Two notch geometries, a circular notch (Kt= 2.42) and an elliptical notch (Kt= 4.2), were investigated. Good corrosion-fatigue life predictions can be obtained using local-strain and LEFM methodologies by determining the relevant material constants via a few simple fatigue tests on smooth specimens and a few crack-growth-rate tests in the environment at the frequency of interest.

Recent Advances for Corrosion Fatigue Mechanisms.

A critical but non exhaustive review of the aqueous corrosion fatigue mechanisms is proposed with a particular emphasis on the corrosion-deformation interactions, The limits of the classical approaches of corrosion fatigue crack initiation and propagation predictions from the electrochemical point of view are pointed out. It is shown how the cyclic plastic deformation can locally modify the electrochemical reactions and how corrosion (anodic dissolution and/or hydrogen effects) can change the local plasticity and microfracture. The experimental methods to detect the localized corrosion fatigue damage are detailed. The crack propagation mechanisms near the fatigue threshold are underlined, with a particular attention for the anodic dissolution and hydrogen effects and the interaction between corrosion fatigue and stress corrosion cracking. Finally recent numerical simulations of corrosion fatigue damage and lifetime at mesoscopic scales are proposed and trends for future researches are given.

Cracking process of Fe-26Cr-1Mo during low cycle corrosion fatigue

The corrosion fatigue (CF) life has been divided classically into the initiation'' and propagation'' periods. Usually, the crack initiation process dominates the component lifetime under the low cycle CF condition because the crack propagates rapidly one initiated. Despite much work done on the research of the CF crack initiation mechanisms, however, a full understanding of crack initiation is still lacking. There are some limitations in explaining the CF crack initiation in an aqueous solution using the above four mechanisms individually. And, it is difficult to conduct experiments in which one mechanism along can be examined. Although CF is complicated, it is possible to reproduce a specific experiment condition which will have the dominant factor affecting the CF crack initiation. Once the cracks initiate on the smooth metal surface, their coalescence, micropropagation and macropropagation will take place successively. The initiated cracks propagate first in the range of several grains, and the behavior of the microcrack propagation is different from that of macrocrack propagation. For Fe-26Cr-1Mo ferritic stainless steel, the fundamental research work of straining electrode has been done by many investigators, but the observation of the material surface at different deformation processes has not been reported. In the present study, themore » detailed observation of the cracking process of the material has been carried out in low cycle CF.« less