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

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

The fatigue behaviour of an ultra-high strength steel as influenced by the water vapour content and temperature of an air environment

Total fatigue lives, fatigue crack initiation and propagation lives and fatigue crack propagation rates of notched (K_t = 3.15) SAE 4340 steel specimens tested under program loading have been determined in air environments having 45 different water vapour content/temperature combinations. Forty three of these involved unsaturated (less than 100% RH) conditions. Maximum fatigue lives and minimum crack propagation rate did not occur in very dry air, but in an environmental region between 3 and 50% RH and 40–100°C. The bearing of the results on the common concepts of “corrosion fatigue” and “atmospheric corrosion fatigue” is discussed.     

A relative crack opening time correlation for corrosion fatigue crack growth in offshore structures

A considerable amount of research has been carried out on the prediction of mean stress effects on fatigue crack growth in structures. Newer types of structure are now being developed for use in highly dynamic, harsh marine environments, particularly for renewable energy applications. Therefore, the extent to which mean stresses can enhance corrosion‐assisted fatigue damage in these structures needs to be better understood. A new theoretical model that accounts for mean stress effects on corrosion fatigue crack growth is proposed. The model is developed based on the relative crack opening period per fatigue cycle and by considering only the damaging portion of the stress cycle. The baseline data for the modelling exercise are the data obtained at a stress ratio of 0.1 in air and seawater tests conducted on compact tension specimens. The model is validated by comparison with experimental data and with other fatigue crack propagation models. The proposed model correlates fairly well with experimental data and the other models examined.     

Evaluation of stress corrosion resistance and corrosion fatigue fracture behavior of ultra-high-strength P/M Al–Zn–Mg alloy

Quasi-static tensile tests in air and slow strain rate tests (SSRTs) in a 3.5% NaCl solution were conducted in an ultra-high-strength P/M Al–Zn–Mg alloy fabricated through powder metallurgy. Attention is also paid to fatigue strength and fatigue crack growth behavior in laboratory air and in a 3.5% NaCl solution. The alloy has extremely high strength of about 800 MPa. However, elongation at break remains small, at about 1.3%. The final fracture occurs by a macroscopically flat crack normal to the tensile axis, with little reduction in area and little shear lip on the periphery of a smooth sample. However, it fails microscopically in a ductile manner, with dimples. Dimple size is less than 1 μm, because the grain size of the alloy is extremely small. Strengthening mechanisms operating in the alloy are: small grains, sufficient metastable η′ phase in a matrix, and intermetallic compound acting as a fiber reinforcement. The SSRT strength in a 3.5% NaCl solution decreases slightly at a very low strain rate, that is smaller than those observed in aluminum alloys sensitive to stress corrosion. This means that the crack initiation resistance to stress corrosion is superior. However, under cyclic loading, the corrosion fatigue strength becomes lower than that conducted in air, because pitting corrosion on a sample surface acts as a stress concentrator. Crack initiation site of quasi-static and fatigue failure of the alloy is at inclusions, and hence, it is essential to decrease inclusions in the alloy for the improvement of the mechanical properties. Fatigue crack resistance of the alloy is inferior to conventional Al–Zn–Mg alloys fabricated by ingot metallurgy, because the fatigue fracture toughness, or ductility, of the alloy is inferior to other Al alloys, and intergranular cracking promotes crack growth. However, no influence of 3.5% NaCl solution on corrosion fatigue crack growth is observed, although an investigation is required into whether stress corrosion crack growth occurs or not, and at the same time, and of corrosion fatigue crack growth behavior at lower stress intensity. The fracture surface and crack initiation sites are closely examined using a high-resolution field emission type scanning electron microscope, and the fracture mechanisms of the alloy are discussed.     

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

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

Growth behaviour of small surface fatigue cracks in AISI 304 stainless steel

A series of axial tensile fatigue tests (R = 0.1) was carried out to investigate the initiation and the growth behaviours of very small surface fatigue cracks under two different surface conditions (viz. smooth and pitted surfaces) of AISI 304 stainless steel at room temperature. This paper deals with both of the two approaches regarding the analysis of fatigue: the approach based on the concept of fracture mechanics and low cycle fatigue. In particular, both the initiation and growth of cracks and the coalescence of small cracks by fatigue in the specimen have been investigated by the methods of surface replicas and photomicrographs. Quantitative information such as the initiation period, growth and coalescence behaviours of small cracks, and crack growth properties were systematically obtained. The results show that the accurate determination of these parameters is critical for the application of fracture mechanics to fatigue life assessment.     

Fatigue crack initiation and propagation in several nickel-base superalloys at 650°C

The modes of crack initiation and propagation of several nickel-base superalloys have been examined after fatigue and creep-fatigue testing at 650°C. In fatigue, crack initiation was transgranular and frequently associated with porosity or inclusions in the higher strength alloys. These defects were usually located at the surface, except for tests at low strain ranges where larger, internal defects often initiated failure. Although fatigue crack initiation was transgranular, in those alloys with grain sizes of less than 15 μm, fatigue crack growth quickly became intergranular. This transition was environmentally assisted and did not occur for subsurface cracks until the crack broke through to the atmosphere. In the creep-fatigue cycle, which included a 900 s tensile dwell, crack initiation and propagation wer e both intergranular in all alloys.     

Effect of thick DLC coating on fatigue behaviour of magnesium alloy in laboratory air and demineralised water

Rotating bending fatigue tests have been performed using Diamond‐like carbon (DLC) coated specimens of a wrought magnesium alloy, AZ80A, in laboratory air and demineralised water and the effect of DLC coating on fatigue and corrosion fatigue behaviour was studied. Three film thicknesses of 3.5 μm, 13 μm, and 25 μm (two‐layer film) were evaluated and particular attention was paid to the role of thick DLC coating. In laboratory air, the fatigue strengths of the DLC‐coated specimens were higher than that of the substrate specimen and increased with increasing film thickness. This was because hard DLC coating with good adhesion suppressed the crack initiation due to cracking of inclusions or cyclic slip deformation on the substrate surface. In demineralised water, the fatigue strength of the 3.5‐μm DLC‐coated specimen was the same as that of the substrate specimen due to the penetration of the water through pre‐existing film defects, while the 13‐μm and 25‐μm DLC‐coated specimens showed increased corrosion fatigue strength with increasing film thickness and also exhibited nearly the same fatigue strength as in laboratory air except for a few premature failed specimens, indicating a potential of thick DLC coating or two‐layer coating for complete improvement of corrosion fatigue strength in aqueous environments.     

Investigation of small fatigue crack initiation and growth behaviour of nickel base superalloy GH4169

Surface replication method was utilized to monitor the small fatigue crack initiation and growth process of single‐edge‐notch tension specimens fabricated by nickel base superalloy GH4169. Three different stress levels were selected. Results showed that small fatigue cracks of nickel base superalloy GH4169 initiated from grain boundaries or surface inclusions. The small fatigue crack initiation and growth stages took up about 80–90% of the total fatigue life. Multiple major cracks were observed in the notch root, and specimen with more major cracks seemed to have smaller fatigue life under the same test conditions. At the early growth stage, small crack behaviour might be strongly influenced by microstructures; thus, the crack growth rates had high fluctuations. However, the stress level effect on the small fatigue crack growth rates was not distinguishable for the three different stress levels. And no clear differences were found among the crack initiation lives by using replication technique.     

AF1410与300M钢的腐蚀冲击疲劳行为

根据舰载飞机起落的服务条件提出了腐蚀冲击概念和试验方法，考察了两种起落架材料在盐水中的腐蚀冲击疲劳行为，包括冲击疲劳寿命，裂纹萌生与扩展速率。尽管两种材料在空气中的冲击疲劳寿命几乎相等。但300M钢在盐水中的冲击疲劳寿命下降幅度较大。在盐水介质中，氢脆加速300M钢冲击疲劳裂纹的萌生和扩展。局部塑性变形区优先腐蚀促使AF1410钢的裂纹萌生，盐水对AF1410钢的裂纹扩展速率没有影响。     

CRACK NUCLEATION AND PROPAGATION IN BLADE STEEL MATERIAL

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

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

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

Effect of inclusion on subsurface crack initiation and gigacycle fatigue strength

The effect of inclusions on crack initiation and propagation in gigacycle fatigue was investigated experimentally and analytically in six high strength low alloy steels. Fatigue testing was performed at very high numbers of cycles through ultrasonic fatigue tests at 20 kHz. Inclusions at subsurface are common sites for fatigue crack nucleation in these alloys when cycles to failure was >10⁷ cycles. A significant change in the slope of the S–N curve was observed accompanying the transition from surface to subsurface crack initiation. A deterministic model has been developed to predict the total fatigue life, i.e. crack initiation life and crack propagation life, from the measured inclusion sizes. The predicted fatigue strength agreed reasonably well with the experimental results. It is a tendency that smaller inclusions are associated with longer fatigue life. The results demonstrated that the portions of life attributed to subsurface crack initiation between 10⁷ and 10⁹ cycles are >99%.     

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

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

Initiation and early growth mechanisms of corrosion fatigue cracks in stainless steels

The mechanism of corrosion fatigue crack initiation in stainless steels was examined in both air and chloride solutions. For a tempered martensitic, a duplex and a soft martensitic steel it is shown that the decrease of the fatigue strength from the value measured in air to that measured in corrosive environments depends primarily upon the stability of the protective film. If the passivity is stable, cracks are found to originate almost exclusively at oxide inclusions. Cracking or debonding were found to occur. For the duplex steel in the vicinity of the inclusion there were pronounced emerging persistent slip bands. They cause localized corrosion attack, thus allowing cracks to be formed more easily. If pitting is superposed, crack nucleation always occurs at the base of the pit. Pit formation and growth rate are accelerated by cyclic loading.     

An engineering approach to fatigue analysis based on elastic‐plastic fracture mechanics

Nowadays cast iron components are widely used in highly stressed structures. Component lifetime is strongly influenced by inhomogeneities caused by the material's microstructure and the manufacturing process (graphite particles, (micro‐)shrinkage pores, inclusions). Inhomogeneities often act as a fatigue crack starter. Lifetime until failure may be divided into stages for crack initiation, short and long crack growth. Initiation of a crack of technical size (a ≈ 1mm) is often dominated by the growth of short cracks. The paper presents an approach to analyse the mechanically short fatigue crack growth based on elastic‐plastic fracture mechanics considering the closure behaviour of short cracks. The effective J‐integral range is used as a crack driving force. Finite element analysis results as well as analytical solutions to approximate the crack driving force are presented. The application of the approach is successfully demonstrated for cast iron material EN‐GJS‐400‐18‐LT using data from fatigue tests, microstructure and fracture surface analyses to assess the fatigue life.     

STATISTICAL INVESTIGATION OF THE EFFECT OF LABORATORY AIR ON THE FATIGUE BEHAVIOUR OF A CARBON STEEL

In order to clarify the effect of the atmospheric conditions on fatigue damage, rotary bending fatigue tests were carried out on smooth specimens of a normalized 0.37% carbon steel in controlled laboratory air. The air conditions used in the tests were moist air at 20 °C, moist air at 35 °C and dry air at 35 °C. The influence of atmosphere on crack initiation and propagation behaviour was investigated in detail based on successive observations of the surface. Experimental results showed that the fatigue life was superior at 20 °C compared to 35 °C by a factor of 2, while the effect of moisture was small compared to that of temperature. The statistical investigation of crack initiation and propagation behaviour indicated that the temperature strongly affects the crack initiation process; conversely, moisture plays an important role in the propagation process of cracks smaller than 0.3  mm. Moreover, the distribution characteristics of crack initiation life, crack propagation life, fatigue life and crack growth rate were analysed by assuming either a Weibull distribution or a log-normal distribution.     

Crack propagation of pipeline steel exposed to a near-neutral pH environment under variable pressure fluctuations

This investigation attempts to understand the corrosion fatigue crack propagation behavior of pipeline steels exposed to near-neutral pH environments. The fatigue loading was designed to simulate the underload-type variable amplitude pressure fluctuations found during pipeline operation. The effects of amplitudes (R ratios) of underload and minor cycles were investigated. It has been found from this investigation that the crack growth rate is enhanced significantly through load interaction of the variable amplitude fatigue. The acceleration factor is found to be up to 2.7 and 5.3 for tests in air and in the near-neutral pH solution, respectively. The crack growth rate decreases with R ratios of underload and minor cycles for tests both in air and in near-neutral pH environments. The latter could enhance crack propagation by a factor of up to 11, as compared with the crack growth rate in air. The critical R ratio of minor cycles at which the minor cycles do not contribute to crack propagation through load interaction was determined to be as high as 0.982, which is much lower than the threshold determined by constant amplitude fatigue. This critical R ratio could be utilized to demarcate stress corrosion cracking and corrosion fatigue, and should be incorporated as one of the design principles for components/structures subjected to variable amplitude cyclic loading.     

6061-T6 铝合金激光焊接接头腐蚀疲劳裂纹扩展

目的研究6061-T6铝合金激光焊接接头的腐蚀疲劳裂纹扩展特性,并分析裂纹扩展的影响因素。方法利用光纤激光器,焊接尺寸为150 mm×100 mm×4 mm(焊接方向、横向、熔深方向)的6061-T6铝合金,采用SE(B)三点弯曲疲劳裂纹扩展试验并利用连续降K法,分别在空气和人工海水中进行疲劳裂纹扩展试验,通过使用金相显微镜(OE)和扫描电子显微镜(SEM),对金相结构进行观测分析。结果同样工艺参数的焊接接头,在海水中疲劳裂纹门槛值(4.063 016 MPa·m~(0.5))大于空气中的门槛值(3.479 166 MPa·m~(0.5));在疲劳裂纹扩展中速区(da/dN10~(-5) mm/cycle)时,海水焊接接头疲劳裂纹扩展速率大于空气中的,低速区(da/d N10~(-5) mm/cycle)则小于在空气中的。结论成形良好的焊缝、晶粒细小的焊缝组织有助于接头疲劳裂纹扩展性能的提高;中速区,海水中疲劳裂纹扩展速率偏大,主要是由腐蚀条件下焊缝裂纹尖端阳极溶解和交变载荷共同作用导致;低速区,海水中疲劳裂纹扩展速率偏小,主要原因是腐蚀产物堆积于疲劳裂纹扩展尖端,产生较强裂纹闭合效应。