Corrosion Fatigue Crack Initiation Life Research Articles

A continuum damage model for prediction of crack initiation life of pitting corrosion and fatigue

Corrosion fatigue damage can lead to degradation of engineering structures and components. As a major stage of the total corrosion fatigue life, the crack initiation of pitting corrosion and fatigue is studied. A continuum damage mechanics model for predicting the crack initiation life of pitting corrosion fatigue is presented based on a pure fatigue damage model and a fracture-mechanics-based pit-to-crack transition criterion. The synergistic effect of pitting corrosion and fatigue are quantified as a time-varying amplification of the fatigue stress field. The proposed model is validated by corrosion fatigue test data of two different metallic materials. The predicted curves of corrosion fatigue crack initiation life are in good agreement with the test data. The model demonstrates a good prediction of corrosion fatigue crack initiation life, i.e., corrosion fatigue life is always lower than fatigue life, and the lower the stress, the more significant the life reduction.

Modeling for corrosion fatigue crack initiation life based on corrosion kinetics and equivalent initial flaw size theory

In present paper, a novel, effective and economical model based on corrosion kinetics and equivalent initial flaw size theory was proposed to predict the corrosion fatigue crack initiation (CFCI) life of E690 steel in simulated seawater. Comparison between the predicted S-N curves and the measured data showed that this model was of nice accuracy under high peak stresses, while non-negligible deviation existed under low peak stresses due to lack of enough measured data or consideration for the stress effect on corrosion rate. Despite of that, thought of this model still has a good practicability in predicting CFCI life of steels.

Modelling of corrosion fatigue crack initiation on martensitic stainless steel in high cycle fatigue regime

This paper presents an analytical model for assessing the corrosion fatigue crack initiation life on a martensitic stainless steel X12CrNiMoV12-3 in high cycle fatigue regime (between 105 and 107 cycles). Based on in-situ electrochemical measurements during corrosion fatigue tests in NaCl aqueous solution, the corrosion fatigue crack initiation mechanism was identified. Two main stages were investigated: (i) the fracture of the passive film by slip bands and (ii) the free dissolution of the metal developing fatigue crack initiation from a critical corrosion defect. The depassivation stress threshold corresponds to the median fatigue strength at 107 cycles for fatigue corrosion tests. For an applied stress range less than this threshold, the depassivation phenomenon was not observed at 107 cycles and no crack initiation occurred. The proposed model takes into account the depassivation process induced by the slip bands emergence at the specimen surface and the corrosion rate under cyclic loading. The experimental results are compared to the proposed model taking into account mechanical and electrochemical material parameters.

Prediction of corrosion fatigue crack initiation behavior of A7N01P-T4 aluminum alloy welded joints

Through investigating the corrosion fatigue crack initiation behavior of A7N01P-T4 aluminum alloy welded joints in 3.5 wt.% NaCl solution, corrosion fatigue crack initiation life is formulated as [Formula: see text] and the mechanism of corrosion fatigue crack initiation is proposed. SEM and TEM tests revealed that several corrosion fatigue cracks formed asynchronously and the first crack does not necessarily develop into the leading crack. The uneven reticular dislocations produced by fatigue loading are prone to piling up and tangling near the grain boundaries or the second phases and form the “high dislocation-density region” (HDDR), which acts as an anode in microbatteries and dissolved to form small crack. Thus the etching pits, HDDR near the grain boundaries and second phases are confirmed as the main causes inducing the initiation of fatigue crack.

Dorf Ketal acquires Indian catalyst firm Filtra

This paper presents an analytical model for assessing the corrosion fatigue crack initiation life on a martensitic stainless steel X12CrNiMoV12-3 in high cycle fatigue regime (between 105 and 107 cycles). Based on in-situ electrochemical measurements during corrosion fatigue tests in NaCl aqueous solution, the corrosion fatigue crack initiation mechanism was identified. Two main stages were investigated: (i) the fracture of the passive film by slip bands and (ii) the free dissolution of the metal developing fatigue crack initiation from a critical corrosion defect. The depassivation stress threshold corresponds to the median fatigue strength at 107 cycles for fatigue corrosion tests. For an applied stress range less than this threshold, the depassivation phenomenon was not observed at 107 cycles and no crack initiation occurred. The proposed model takes into account the depassivation process induced by the slip bands emergence at the specimen surface and the corrosion rate under cyclic loading. The experimental results are compared to the proposed model taking into account mechanical and electrochemical material parameters.

Corrosion–Fatigue Life Prediction for 2024-T62 Aluminum Alloy Using Damage Mechanics-Based Approach

An approach based on the continuum damage mechanics was applied to predict corrosion–fatigue crack initiation life of 2024-T62 aluminum alloy. A fatigue test in air, a pre-corrosion–fatigue test, and a corrosion–fatigue test on smooth standard specimens were performed. The fatigue lives are strongly reduced by the corrosive environment of 5 wt.% NaCl continuous salt spray compared with non-corroded specimens. Damage evolution models for fatigue in air and pre-corrosion–fatigue of smooth specimens were established, which forms the basis for solving the corrosion–fatigue problem. Finally, the method of corrosion–fatigue life prediction was presented. The predictions comply with the experimental data.

해수환경중 부식피로균열 발생수명 평가에 관한 연구

With regard to corrosion fatigue crack initiation life (Nc), it has been treated ambiguously for the member which doesn't have stress concentration area. In this research, in order to clarify the corrosion fatigue crack initiation life (Nc), corrosion fatigue tests were carried out. Reasonable and universal corrosion fatigue crack initiation life (Nc) was defined and corrosion fatigue crack initiation/propagation model was suggested also. As the fatigue 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 was conducted. And the fracture mechanics analysis using an intrinsic crack model was conducted for the treatment of the small crack. Finally, the followings were obtained. When there is no clear stress concentration point which seems to fall into a corrosion fatigue crack initiation life, the significance of the definition and suggestion of the moment of the reasonable and universal corrosion fatigue crack initiation life (Nc), 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 propagates in earnest, has been clarified.

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 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.

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.

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.

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.

Prediction method of corrosion fatigue crack initiation life based on corrosion pit growth mechanism.

Corrosion fatigue crack initiation behavior of low alloy steel has been investigated in water. It was observed that corrosion fatigue process was composed of three stages, namely, corrosion pit growth, transition into crack from the corrosion pit and corrosion fatigue crack propagation. The corrosion pit size was found to increase with time t following the relation : corrosion pit size ∝ t1/3. The condition of transition into crack from the corrosion pit was determined by a stress intensity factor, by assuming the corrosion pit as a sharp crack. The critical pit condition (ΔK)p was 1.2 MPa √(m) for 2.5 NiCrMoV and 3.5 NiCrMoV in 90°C water with 100 ppb dissolved oxygen. Based on the abovementioned results, residual life prediction method for corrosion fatigue crack initiation was proposed.

The stress rising and holding time characteristics of corrosion fatigue crack initiation, propagation and fracture life.

The corrosion fatigue crack initiation life is not affected independently by each stress rising time tR and stress holding time tH, but by the sum of the tR and tH, that is, by stress frequency, f=1/(2tR+tH), On the other hand, the corrosion fatigue crack propagation life is influenced independently by tR and tH. The tH characteristics shows the transition behaviour from the cyclic dependent mechanism into the time dependent mechanism. The crack propagation life occupies major part of fracture life. The fracture life law diagram plotted by the fracture time t and fracture cycles N, as normalized stress corrosion life tscc, and corrosion fatigue life (cycles) NCF(tH=0), respectively, shows approximately the same curve with repect to the parameter tR.

On the Corrosion Fatigue Crack Initiation Life for Welded Joints of Mild Steel in Sea Water

In this study, the authors investigated the fatigue crack initiation life Nc at the toe of butt welded joint and at the toe of cruciform butt welded joint subjected to pulsating tensile load with the cycling rate of 10 cpm both in sea water and in air. The results of the tests were compared with those estimated according to the concept proposed in the previous paper, in which a method of estimation of the fatigue crack initiation life Nc of a notched mild steel plate both in sea water and in air was shown on the basis of the stress-strain function D.As a result of this study, the following conclusions were obtained :(1) The corrosion effect factor Kc at the toe of manual arc welded joints in sea water is nealy equal to that in, the notched mild steel base plates in sea water.(2) The fatigue crack initiation life of the toe of manual arc welded joint of a mild steel under the corrosive environment of sea water in ambient temperatures can be estimated by using the results of completely reversed strain controlled fatigue tests of smooth round bar specimen of base metal in air.