Corrosion Fatigue Crack Research Articles

The effect of long-term operation on fatigue and corrosion fatigue crack growth in structural steels

Operational degradation of steels under a combined action of mechanical loading and aggressive hydrogenating environments is a crucial problem for structural integrity. Among the mechanical properties, the characteristics of brittle fracture resistance, namely, impact strength and fracture toughness, as well as the plasticity characteristics, are highly sensitive to the operational degradation of structural steels. However, in the case of exposing of structures beside environmental influences to cyclic mechanical loading scenarios, their reliability depends on the degradation degree due to fatigue and corrosion fatigue. Tendency of in-service changes in the fatigue characteristics (fatigue crack growth rate and fatigue strength) can be unambiguous. In-service lifetime of structure due to the non-monotonic character of operational degradation of certain mechanical characteristics is divided into two stages: deformation strengthening (stage I) and in-bulk dissipated damaging (stage II). The operational degradation of steels is accelerated under influence of corrosive hydrogenating environments as a result of intensification of dissipated damaging. The effect of environments on acceleration of fatigue crack growth rate is mainly revealed in the mid-region of stress intensities, more significantly for in-service degraded metal, indicating its susceptibility to stress corrosion fatigue.

Mechanistic understanding on the fatigue cracking in fretting corrosion of alloy 690TT under partial slip regime in 100 ℃ and 290 ℃ water

This study comparatively investigated the fretting corrosion fatigue cracking of alloy 690TT under partial slip regime in 100 ℃ and 290 ℃ water. The results revealed that the occurrence of continuous dynamic recrystallization. The process at 290 ℃ was considerably faster resulting in smaller size of ultra-fine grained structure. The increase in the water temperature resulted in the transformation of the crack initiation mechanism from intrusion/extrusion theory to the low-angle dislocations boundary attack mode. Furthermore, an increase in water temperature resulted in corrosion overcoming the influence of mechanics in crack propagation. In addition, the relevant mechanisms were discussed.

Experimental investigation on corrosion fatigue crack initiation and growth of heat-treated U75V rail steel

Understanding the corrosion fatigue mechanism of rail materials is of great importance to guide the assessment of the structural integrity of modern railway components. To this purpose, the corrosion fatigue crack (CFC) initiation and growth behaviors of heat-treated U75V rail steel are investigated under 3.5 wt% NaCl solution in terms of high-cycle fatigue, fatigue crack growth (FCG) rate, together with surface morphology and microstructure. Experimental results reveal that the U75V rail steel is susceptible to corrosion fatigue failure. Examinations on fractured samples clearly show that the electric potential difference between ferrite and cementite triggers the preferential anodic dissolution of ferrite, leading to corrosion fatigue crack initiation. The crack initiation is not only solely determined by the size of the corrosion pit, but also strongly influenced by its local shape. In comparison with the fatigue crack growth in air, the corrosion FCG tends to grow in a relatively straight path, because of hydrogen embrittlement. Additionally, the CFC behaviors indicate a tendency to transverse through soft-oriented grains and deflect upon encountering hard-oriented grains.

The effect of current and lambda on white-etch-crack failures

White etching cracks (WECs) have been associated with premature failure of wind turbine roller bearings. Various drivers for the generation of WECs have been identified such as loading conditions, slip, steel quality, lubrication, hydrogen embrittlement, corrosion fatigue cracking, and stray electrical currents passing through the surface. In this work, a benchtop test rig utilizing a three-ring-on-roller test configuration was used to investigate the effect of electrical current and operation in different lubricating regimes, defined by lambda (λ), on high-quality bearing steel samples tested in a commercially available power transmission EP gear lubricant. It was observed that there is an inverse correlation between the magnitude of electric current applied to the ring/roller system and time-to-failure. Higher current magnitudes lead to shorter time-to-failure than lower current magnitudes, with macropitting as the main failure mode. Sub-surface investigation revealed the presence of WECs in all cases. For the same current magnitude, tests conducted in boundary and mixed lubrication regimes showed that time-to-failure increased as λ increased, and the tests resulted in WEC related macropits, whereas tests conducted in near-hydrodynamic regime resulted in surface damage with no macropit. It was also noted that a shift toward near-hydrodynamic lubrication resulted in a distinct surface distress on the roller surface. Furthermore, there seems to be a transition in the mixed regime during which the surface distress occurred. The damage on the surface of the test samples resembled non-spatially, periodic, groove-like corrugations and, in some cases, crater-like depressions. Sub-surface imaging, performed by sequential sectioning, revealed the presence of WECs in all cases, and broad, branching cracks that were more prevalent under the more severe boundary conditions.

Effect of corrosion on the fatigue crack propagation properties of butt weld with G20Mn5QT cast steel and Q355D steel in 3.5‐wt% NaCl solution

AbstractThe corrosion fatigue crack propagation (FCP) of the butt weld with G20Mn5QT cast steel and Q355D steel in 3.5‐wt% NaCl solution was investigated experimentally. Considering the influences of stress ratio and loading frequency, the corrosion FCP rate curve was obtained. The results indicated that the corrosion FCP rate in the corrosion environment is three times that in air. Increasing the stress ratio or decreasing the loading frequency will increase the corrosion FCP rate. The fracture morphology of the specimens in the corrosion environment had more corrosion pits and intergranular characteristics due to anodic dissolution than that in air. Anodic dissolution and hydrogen embrittlement jointly promoted the corrosion FCP of butt weld in simulated seawater. The corrosion FCP rate model was established considering the effects of stress ratio and loading frequency.

Fatigue crack growth of EH36 steel in air and corrosive marine environments

EH36 steel used in offshore platforms is susceptible to corrosion fatigue in marine environment. The corrosion fatigue performance in EH36 steel is influenced by various factors, such as maximum applied load, loading frequency and stress ratio. To investigate such effects, fatigue crack growth (FCG) tests on compact tensile (CT) specimens of EH36 steel were carried out in three different environments, including air, immersed seawater and wet-dry cyclic environment, by considering different maximum loads, loading frequencies and stress ratios. From the experimental tests, the relation of crack development and loading cycles and the relation of fatigue crack growth rate (FCGR) and stress intensity factor (SIF) range were obtained. Constants (C and m) in Paris law were obtained through fitting technique based on experimental data. Due to corrosion of marine environment, the fatigue life of EH36 steel decreases while the FCGR increases. The corrosion fatigue crack growth (CFCG) experimental results were compared with the predictions from Paris-law recommended in BS7910. A new CFCG calculation model consisting of environmental acceleration factors was proposed and evaluated for accuracy, and such model can be used for predicting the remaining fatigue life of EH36 steel in marine corrosive environment under the specified conditions. The fracture morphology of the specimens in different environments was observed by scanning electron microscope (SEM) to analyze the fracture mechanism. The fracture morphology of the CT specimens in two corrosive environments is almost same and appears as cleavage fracture.

A peridynamic model for corrosion-fatigue failure behaviour

Corrosion-fatigue is the main factor affecting the service life of aero-engine parts in marine or coastal environments. A numerical simulation method for modelling corrosion-fatigue failure based on peridynamic (PD) theory is presented. We established a coupling relationship between the mechanical and chemical fields according to the influences of displacement on damage, stress on corrosion behaviour, and corrosion behaviour on fatigue behaviour. Subsequently, we verified the model using the corrosion-fatigue crack growth prediction for S355 steel. The prediction and test results show the influence of corrosion behaviour on the fatigue crack growth rate. In addition, we verified the model through the corrosion-fatigue life prediction of a Ti-6Al-4V alloy sample with a corrosion pit. Compared with the test results, the prediction accuracy was limited to a scatter band of factor 2. These results indicate the validity of the proposed model in simulating the corrosion-fatigue behaviour.

Failure analysis of overheated coil leakage in ethylene cracking furnace

The ethylene cracking furnace is the core equipment of ethylene production plants. The safe and efficient operation of the furnace is essential for the production of ethylene and other raw materials. This article analyses the finned tube cracking failure on the convection portion of the ethylene cracking furnace. In order to find out the radical causes of the failure, Metallography, microcracking, chemical composition, mechanical property testing, hardness testing, and finite element modeling were adopted. The original fracture shows a characteristic of columnar crystal cracks with the presence of discontinuous fatigue striations, which is consistent with corrosion fatigue cracks. The fracture area at the tip of the crack shows a dimple fracture pattern, which is a characteristic of plastic cracking. At the same time, a large amount of chromium carbide precipitates at grain boundaries, indicating the presence of intergranular corrosion. Moreover, it worked under conditions of thermal stress and corrosive media. These indicate that corrosion fatigue and intergranular corrosion lead to the cracking of the finned tubes.

Effect of different loading conditions on corrosion fatigue crack growth rate of a nickel-based alloy in supercritical water

Advanced ultra-supercritical (A-USC) technology improves the initial steam temperature to 650 °C and more. In such an extremely corrosive medium environment, metallic materials are susceptible to corrosion fatigue. In the present study, the aim is to evaluate the effect of maximum stress intensity factor (Kmax), stress ratio (R), waveform, frequency, and holding time (th) on the corrosion fatigue cracking behavior of C-HRA-2 (a candidate nickel-based alloy for A-USC units) in supercritical water (650 °C/25 MPa). The results show that with the increase of Kmax, the corrosion fatigue crack growth rate (CFCGR) increases and satisfies the exponential relationship. The decrease in R and frequency both cause the rise of CFCGR, which satisfies the logarithmic linear relationship. Sinusoidal and triangular wave loading have no significant effect on the CFCGR. Trapezoidal wave with th has a larger CFCGR while with the rise in th, the CFCGR increases. The crack propagation path is straight, perpendicular to the loading direction, and secondary cracks are also observed. Transgranular and intergranular mixed fractures occurred during continuous loading. The proportion of intergranular fracture increases significantly as holding time is applied. The corrosion fatigue crack growth of C-HRA-2 in supercritical water can be explained by dynamic embrittlement mechanism.

Assessment of the residual life of turbine runners with operational defectiveness

The residual life of the runners of hydraulic turbines in the presence of operational defects is estimated. The main problems of the operation of hydraulic turbines associated with technological defects and exhaustion of the standard resource are described. The main requirements for initial data to be used in estimation of the residual resource and the requirements for predicting the residual resource of runners based on the results of surveys and analysis of their technical condition are specified. We have classified and briefly described the applied approaches and techniques used in estimation of the residual resource. The main damaging factors affecting the residual life of the runners are revealed: deformation aging of the metal, cavitation, corrosion and fatigue damage to the elements of runners. The most characteristic defects are divided into three groups: zones of cavitation erosion; corrosion-fatigue cracks; and weld defects. Particular attention is paid to corrosion-fatigue cracks identified using flaw detection. The mechanism of crack formation and the most probable location of the cracks in the runner are shown. Statistical data on the number of cracks at the onset of the runner operation and at the time of shutdown maintenance are presented. The main statistical parameters of the sample and the parameters of crack size distributions including the distribution law are determined. The distribution law is exponential for the crack length parameter; whereas for the crack opening width it is log-normal. The revealed multidirectional cracks are located at the surface, subsurface or inner layer of the metal. They arise from operational defects (ulcers, craters, undercuts or delamination) and grow during operation of the turbine units. We also present the design schemes of elements with cracks used for quantification of resources according to the criteria of fracture mechanics. The results of calculations for static and dynamic crack resistance are presented as the dependence of stress intensity factors on the crack size. The levels of the total accumulated damage to the runners, the values of the residual life at the stage of crack nucleation and development were determined for 11 hydraulic units in the «start-stop» and «working» cycles. The main conclusion is that the total operating time of the hydraulic turbine runners significantly exceeds the standard operating life, while the residual resource is insufficient for a further period of long-term operation.

Life prediction of corrosion-fatigue based on a new crack growth rate model with damage and the extended finite element method

A fatigue crack growth rate model introducing corrosion damage is proposed in this paper, where the fracture parameter is modified by the damage variable and the exponential influence factor. The corrosion damage mechanism is coupled to the crack growth rate model, which quantifies the influence of the environment and reflects the interaction between corrosion and fatigue. To accurately predict the corrosion-fatigue crack growth life by the extended finite element method (XFEM), a simple crack surface updating method is developed. This method does not depend on the complex level set marching algorithm which often brings serious computational convergence problem, so it has good robustness and high efficiency. Moreover, the stress intensity factors at arbitrary crack tip can be calculated by combining the interaction integral method and the piecewise interpolation method, so it is suitable for the double crack-tip problem. The corrosion-fatigue crack growth life is calculated based on the cycle-by-cycle method and the corresponding algorithm is developed. The corrosion-fatigue crack growth life can be accurately predicted by calculating the corrosion damage and crack propagation length under each load cycle. In this paper, sufficient examples are given to verify the accuracy of the proposed method in simulating crack propagation path and predicting corrosion-fatigue life. Finally, this method is well applied to a practical engineering structure.

Corrosion fatigue crack growth in stainless steels: A peridynamic study

Fatigue crack growth (FCG) behaviors are accelerated by corrosive environments, resulting in higher corrosion FCG (CFCG) rates and more rapid structural failure. Current local-theory-based numerical methods do not effectively capture the CFCG behaviors of metallic materials. Recently, peridynamic (PD) models, which are nonlocal-theory-based, have demonstrated good descriptive ability for discontinuous problems, making it promising to simulate CFCG via PD methods. In this paper, a peridynamics-based CFCG simulation framework is proposed and applied for the CFCG simulation of the stainless steel, Type-634. This framework provides explanations of corrosion-induced faster crack growth from the view of degradations of material properties. And the simulated results agree well with the experimental testing results, demonstrating the effectiveness of the proposed model. Moreover, the effects of corrosion-related degradation function parameters and loading frequencies on CFCG results are explored, and the results demonstrate the generalizability of the simulation method. With further development and embedding more accurate physical models, this simulation framework will effectively assist real CFCG tests.

Study on Stress Intensity Factor of the Pit-Crack Model for Portal Crane Girders

In shipbuilding gantry cranes in long-term service in the coastal humid salt-spray environment, the main beam is affected in many places by corrosion fatigue-formed corrosion pits, affecting the structural strength of the main beam. This paper focuses on the impact of corrosion-generated pits under the conditions of crack nucleation. Corrosion fatigue crack nucleation is a local damage evolution process and the stress intensity factor criterion is one of the critical conditions to discern whether a crack sprouts at the corrosion pit. This paper establishes a corrosion pit-crack model based on the overall finite element model of the portal crane, and uses ANSYS software to simulate the stress intensity factor under complex boundary conditions. The results show that the different sizes and depth-to-diameter ratio of the pits greatly affect the value of the stress intensity factor and the stress concentration phenomenon may be the main factor causing the emergence of cracks; the change in the size of the pits on the stress intensity factor is not obvious, but with the increase in the depth-to-diameter ratio of the pits, the stress intensity factor of the pit-crack model is significantly increased. According to the hypothesis of semi-ellipsoidal pitting, the relationship between the stress intensity factor and the stress concentration factor at the pit is proposed, and its calculation results are within 5% error compared with the finite element method, and it is found that the effect of the etch pit on the stress intensity factor is positively correlated with the stress concentration factor; with the increase in the etch pit depth–diameter ratio, the crack budding location changes with the change in the stress concentration location, and the crack is more likely to be generated from the shoulder of the etch pit when the depth–diameter ratio of etch pit exceeds 1.

Study on crack propagation life of corrosion fatigue in orthotropic steel deck in steel bridges

The study of the corrosion fatigue life of steel bridges is of much significance. The remaining life of corrosion fatigue crack propagation is found to be lesser than fatigue crack propagation. The finite element model is established, the key points are meshed in small size, and the fatigue crack damage of local components is analyzed.The crack propagation life evaluation method of corrosion fatigue in orthotropic steel deck structure bridges is obtained based on the multi-scale finite element method. Stress intensity and corrosion are the main factors that affect the crack propagation life of corrosion fatigue. The stress intensity factor was analyzed by diaphragm spacings, diaphragm thickness, and roof thickness,It is understood that the thickness of the diaphragm has a great influence on the stress strength factor. Temperature, humidity and morphological characteristics of corrosion pits affect the rate of corrosion when exploring the influence of corrosion factors.The analysis of corrosion fatigue crack growth process shows that corrosion has a great influence on the remaining life of fatigue crack propagation.

Effect of 0.1 wt% Nb on the microstructure and corrosion fatigue performance of high strength steels

In this manuscript, the effect of 0.1% Nb addition on the corrosion fatigue behavior of the marine engineering steel has been studied through corrosion fatigue experiments, electrochemical hydrogen charging, and a series of microscopic characterization methods. The results show that BM steel has a high corrosion fatigue sensitivity in the marine environment. Compared to BM steel, the corrosion fatigue sensitivity of 0.1 Nb steel is reduced by 8% in the marine environment, and it reduced by 40% in the hydrogen charging environment. 0.1% Nb adding inhibits hydrogen concentration in local areas by forming irreversible hydrogen traps NbC and by improving the microstructure, and it lowers the diffusible hydrogen content and total hydrogen content inside the steel by inhibiting electrochemical hydrogen evolution reaction to hold back the corrosion fatigue crack initiation and propagation, thereby improving the ability of resisting corrosion fatigue cracking for the marine engineering steel.

Corrosion fatigue behavior of AA 7020 alloy in seawater

Abstract The effect of two-step aging treatment on the fatigue behavior of welded AA 7020 Al. alloy was investigated. For fatigue tests to be carried out in atmospheric and seawater environments, a plane bending tester at a stress ratio of R = 0 was used. Experiments in the control group were carried out with unwelded specimens at a strain rate of R = −1 in air. The gradual heat treatment, two-step aging, significantly improved the fatigue and corrosion fatigue behavior of welded AA 7020 alloys. Unwelded specimens showed better fatigue properties than the welded ones. In the experiments in the seawater environment, pitting, and exfoliation corrosion occurred on the surfaces. It was determined that the corrosion pits reduce the fatigue life of welded and unwelded specimens. It was also observed that the corrosion sensitivity of the welded specimens increased in the heat affected zone and resulted in corrosion fatigue cracking.

Microstructure-dependent corrosion fatigue crack growth behavior of Ti-6Al-4V alloy in simulated body fluid

The fatigue crack growth rate (FCGR) of Ti-6Al-4V alloys in simulated body fluid (SBF) was first determined by direct current potential drop (DCPD) method. The results show that the acceleration effect of SBF corrosion environment on FCGR of lamellar structure is far greater than that of equiaxed and bimodal structures, especially in Paris region, which is mainly due to the high corrosion sensitivity of the lamellar α/β phase boundary to SBF under fatigue loading.

Study on Corrosion Fatigue Properties of the Welded Joints of SMA490BW Weathering Steel

The polarization curve test and corrosion fatigue test of SMA490BW weathering steel welded joints for high-speed train bogies were carried out by electrochemical workstation and fatigue testing machine, respectively, and the fatigue fractures were observed and analyzed by scanning electron microscope (SEM). The results show that: The mechanical properties of the SMA490BW weathering steel welded joints welded by MAG welding are good, the tensile strength is 519MPa, and the elongation after fracture is 26.67%. In 3.5wt% sodium chloride solution, the self-corrosion potential of the heat-affected zone is the lowest, the self-corrosion current density is the largest, and the corrosion resistance is the worst, the weld is in the middle, and the base metal is the best. The corrosion fatigue limit of SMA490BW weathering steel welded joints in 3.5wt% sodium chloride (NaCl) solution is 236.55MPa, which is much lower than the fatigue limit of 335.22MPa in air. SEM fracture analysis showed that the fatigue cracks of the joints in the air originated from the weld pores, while the corrosion fatigue cracks originated from the pitting pits on the surface of the specimens.

Corrosion fatigue crack growth behavior of a structurally gradient steel for high-speed railway axles

Considering the complex service environments that high speed railway axles are subjected to, the fatigue crack growth (FCG) behavior of a structurally gradient axle steel with different pre-crack depths both in air and a corrosive medium was investigated at a frequency of 5 Hz. The results indicated that in the high ΔK region, the FCG rate was dramatically accelerated by corrosion but the gap narrows as ΔK decreased. The accelerated corrosion FCG rate was a comprehensive result of the acceleration effect of the anodic dissolution, hydrogen-enhanced localized plasticity and the retardation effect of corrosion-induced crack-tip blunting. Despite the fact that the corrosion resistance gradually decreased as the pre-crack depth increased, the FCG rate in the corrosive medium gradually decreased. This was because fatigue loading played a more important role than corrosion in accelerating the corrosion FCG rate.

Effect of micro-galvanic corrosion on corrosion fatigue cracking of the weld joint of high strength bridge steel

The local electrochemical and corrosion fatigue cracking behavior of the weld joint of Q690qE high-strength bridge steel was investigated in simulated marine environment. It reveals the weld joint was vulnerable to corrosion fatigue failure in polluted marine atmosphere with the corrosion fatigue life considerably lower than that of parent metal. The micro-galvanic corrosion effect between CGHAZ and WM played a dominant role in the corrosion fatigue failure of the weld joint, leading to the final fracture mostly at CGHAZ. The micro-galvanic corrosion was caused by Volta potential variation among various subzones, which was evidenced by local electrochemical techniques.