On the interaction between corrosion and fatigue which determines the remaining life of bridges

This paper studies the prior effect of corrosion on fatigue on the growth of cracks that arise from natural corrosion in steel bridges. It is shown that these 2 effects need to be simultaneously analysed. If not, then the resulting life is not conservative. This paper presents a simple methodology for performing this coupled analysis.     

THE EFFECTS OF CATHODIC POTENTIAL AND CALCAREOUS DEPOSITS ON CORROSION FATIGUE CRACK GROWTH RATE IN SEA WATER FOR TWO OFFSHORE STRUCTURAL STEELS

The effects of cathodic protection potential, corrosion products and stress ratio on corrosion fatigue crack growth rate have been studied on offshore structural steels. These materials were cathodically polarised in seawater and 3% sodium chloride solution at three potentials of -0.8, -1.0 and -1.1 V(SCE). The corrosion fatigue crack growth rate in seawater was greater than that in air and increased with more negative potentials. The maximum acceleration of crack growth rate in seawater was observed at the crack growth plateau which was independent of ΔK. Calcareous deposits precipitated within the cracks resulted in an increase of crack opening level and contributed to a reduction of the corrosion fatigue crack growth rate. Such a corrosion-product-wedging effect could be evaluated by using an effective stress intensity range, ΔK_eff. The estimation of corrosion fatigue crack growth rate in terms of ΔK_eff clarified the effect of hydrogen embrittlement under a cathodic potential. Thus the processes of cracking in seawater at cathodic potentials resulted from mechanical fatigue and hydrogen embrittlement with calcareous deposits reducing the crack growth rate. All these three mechanisms were mutually competitive.     

Corrosion-Mechanical Properties of Materials for Reactor Vessels

We established that, in the course of electrolytic hydrogenation, an aqueous medium of boron regulation with pH 8 significantly accelerates the growth of fatigue cracks in 15Kh2MFA and 15Kh2NMFA hull steels, especially in the metal of welded joints, as a result of hydrogen embrittlement at the tip of a crack. An increase in the strength of steels by means of heat treatment, a decrease in the frequency of alternating loading, and an increase in the stress ratio significantly accelerate the growth of fatigue cracks. We discovered the strongest influence of hydrogenation in the near-threshold region for low values of the stress intensity factor. The tendency toward passivation and the anticorrosive properties of 15Kh2MFA steel (i _cor = 5 mA/m²) are higher than those of 15Kh2NMFA steel (i _cor = 20 mA/m²), which can be explained by the higher inhomogeneity of the structure of the latter. The metal of welded joints has the lowest cyclic corrosion crack resistance and the highest tendency toward hydrogen embrittlement under conditions of stable or transient modes.     

A COMPARISON OF THE STRAIN INTENSITY AND CYCLIC J APPROACHES TO CRACK GROWTH

Two parameters describing the growth of fatigue cracks are compared. They are the cyclic J integral ΔJ and the strain intensity expressed as an equivalent stress intensity ΔK_eq-. By referring to cyclic stress-strain data obtained from hysteresis loops in high strength ferritic steels at room temperature and austenitic and ferritic steels at elevated temperature it is shown that: (i) for short cracks the parameters are simply related and (ii) both parameters adequately link fatigue crack growth rates observed in the separate high strain fatigue (HSF) and linear elastic fracture mechanics (LEFM) regimes. Correction factors for thumbnail cracks and the conditions under which the relations need further modification are discussed.     

THE GROWTH OF SMALL CORROSION FATIGUE CRACKS IN ALLOY 2024

Abstract— The corrosion fatigue crack growth characteristics of small surface and corner cracks in aluminium alloy 2024 is established. The damaging effect of salt water on the early stages of small crack growth is characterized by: (1) crack initiation at constituent particle pits, (2) intergranular microcracking for a≤100μm, and (3) transgranular small crack growth for a≥100μm. In aqueous 1% NaCl and at a constant anodic potential of −700 mV_SCE, small cracks exhibit a factor of three increase in fatigue crack growth rates compared to laboratory air. Small cracks exhibit accelerated corrosion fatigue crack growth rates at low levels of Δ K (< 1 MPa√m) below the long crack Δ K _th value. When exposed to Paris regime levels of crack tip stress intensity, small corrosion fatigue cracks exhibit growth rates similar to that observed for long cracks. Similar small and long crack growth behavior at various levels of R suggest that crack closure effects influence the corrosion fatigue crack growth rates of small cracks for a≥100 μm. Contrary to the corrosion fatigue characteristics of small cracks in high strength steels, no pronounced chemical crack length effect is observed for alloy 2024 exposed to salt water.     

A COMPARISON OF NEAR-THRESHOLD FATIGUE CRACK PROPAGATION IN TWO HIGH STRENGTH STEELS

Abstract— Fatigue crack growth rate data for long and short cracks were obtained from two high strength steels used in the aerospace industry. Differences in the behaviour of the two cracks were noted and explained in terms of the three-dimensional nature of short cracks compared to two-dimensional long cracks, the interaction between cracks, the applicability of Linear Elastic Fracture Mechanics (LEFM) and their effect of crack closure.     

Das Verhalten rostfreier Stahlgußlegierungen unter gleichzeitiger mechanischer und korrosiver Belastung

Behaviour of stainless cast alloys under combined mechanical and corrosive attack In media with 3000 mg B/1 (as boric acid, H₃BO₃) and varying chloride contents down to 1 mg Cl^?/1 13%-Cr-steels show pitting. Under constant load cracks appear simultaneously. As can be expected, 17%-Cr-steels are more resistant and duplex ferritic austenitic steels exhibit no corrosion cracking in 3% NaCl-solution even after 10 000 hrs. Generally, corrosion resistance improves with increasing contents of ferrite forming elements. Corrosion cracks are interpreted in termes of anodic stress corrosion cracking. Notch impact bending tests with hydrogen charged specimens show that deformability is reduced.     

Effect of acetic acids on the corrosion crack growth of 3.5NiCrMoV steels in high temperature water: synergistic interaction between stress corrosion and corrosion fatigue

Stress Corrosion Cracking (SCC) tests (pH: 3 ~ 5) and Corrosion Fatigue (CF) tests (R = 0.2, 0.1 Hz) were conducted to evaluate the effect of acetic acid on the corrosion crack growth behavior in high temperature water at 150°C. Acetic acid significantly influenced the corrosion fatigue cracking behavior of turbine disc steels in high temperature water. The CF crack growth rates of turbine disc steels increase until the organic acid concentration reaches a critical saturation value (between pH 4 and pH 3) because of the crack tip sharpening. Below the critical value of pH, the CF crack growth rates decreases because of the crack tip blunting. The corrosion fatigue crack growth rate is accelerated by the interaction of the fatigue and the stress corrosion in the test environment. The synergistic interaction should be accounted for in the realistic prediction of the corrosion fatigue life of turbine steel (3.5NiCrMoV steels) in high temperature water of acetic acid solution. With the high temperature corrosion fatigue data obtained in this study, it is possible to assess the life of turbine components in high temperature and high pressure.     

Fracture parameters for sintered steels

Measurements of the plane strain fracture toughness K _Ic of sintered steels have frequently been invalid because the requirement that P _max/P _Q<1.1 (where P _max = maximum load and P _Q=load used to calculate K _Ic) has not been met. We show that the reason for the criterion not being met is that sintered steels have a considerable crack growth resistance K _R. Values obtained in the past for K _Ic probably have been over-estimates of the initiation value of the crack growth resistance K _i and under-estimates of the maximum crack growth resistance K . The important point is that the assessment of the toughness of sintered steels by a single parameter is not appropriate. Test methods to determine the crack growth resistance of sintered steels are discussed. Crack growth, which is difficult to detect by visual observation, can be determined by compliance techniques. Because of the porous nature of sintered steel, fatigue cracks are unnecessary at the tip of the notch and indeed are undesirable as they can easily cause errors in toughness measurements through inadvertent overloading. The thickness requirement for plane strain measurements can also be relaxed.     

The mechanism of the nucleation of corrosion cracks

X-ray diffraction analysis was used to study changes in the distribution of stresses of the first and the second kind in surface layers of stressed Cr18Ni10Ti steel specimens during the nucleation and growth of corrosion cracks. Based on the results obtained, a mechanism of nucleation of corrosion cracks in 18-8-type steels was postulated.     

Crack growth from naturally occurring material discontinuities under constant amplitude and operational loads

This paper discusses how cracks that grow from small naturally occurring material discontinuities under operational load spectra behave. The growth of small cracks under a representative maritime aircraft flight load spectrum is discussed first. The results of this study, when taken in conjunction with the authors previous studies into cracks growing under combat aircraft load spectra, illustrate how for cracks that grow from small naturally occurring material discontinuities under operational load spectra crack growth can often be easily and accurately computed. It is also shown that the Hartman–Schijve variant of the NASGRO crack growth equation is able to accurately represent the growth of small cracks in two different rail steels. It is further shown that the growth of both small and long cracks can be described by a family of da/dN versus ΔK curves and that, for 7050-T7451, the experimental procedures commonly used to determine a closure free da/dN versus ΔK curve produce curves that are consistent with those obtained using the Hartman–Schijve equation and allowing for small variations in the term ΔK_thr.     

Improved fracture toughness of ultrahigh strength steel through control of non-metallic inclusions

In recent years, ultrahigh-strength steels, which can be employed successfully at yield strengths of 1400 MPa or higher, have been used increasingly for critical structural applications in aircraft and aerospace vehicles. Most recently, there has been increased demand, however, for ultrahigh-strength steel with superior plane-strain fracture toughness, K _IC, and for the steels suitable for large-sized structural applications; isotropy regarding the property has especially been required. One potential solution to this problem is to control nonmetallic inclusions of the steels. This review concentrates on recent topics concerning improved K _IC of ultrahigh-strength steels, i.e. low-alloy and highly alloyed secondary hardening steels, through control of non-metallic inclusions. The major factors controlling the property are discussed for each of the techniques.     

Cyclic fracture toughness of structural alloys with surface cracks at low temperatures

In the present paper, we study specific features of crack propagation from surface flaws in full-scale sheets used in manufacturing pressure vessels for cryogenic applications. The process of crack propagation consists of several stages and terminates in stages of surface through or central through cracks under conditions of low-frequency repeated tension. The effect of a decrease in temperature from 292 to 77°K on crack growth behavior was studied for sheets with a thickness of 2, 8, and 12mm. We describe a procedure for testing for crack-growth resistance at cryogenic temperatures and construct fatigue crack growth diagrams. It is shown that zones of influence of the front and rear faces of the specimen on the stress and strain fields near the crack front arise in the plane of a semielliptic crack. The shape of the interface of these zones can be approximated by a second-order curve. Variations in the thickness of the specimen and the test temperature affect the slope of the curve, i.e., the interface of the zones of influence. Specific features of the fracture process in the material of the plate with surface cracks manifest themselves most adequately at points of the crack front located on the indicated interface. We suggest a procedure for estimating the cyclic crack growth resistance of highly ductile stainless steels that is based on the use of the cyclic J-integral. We propose to regard the lengthl of the interface of the zones of influence of the front and rear faces of the specimen as a geometric parameter of the crack. It is used to construct kinetic fatigue crack growth diagrams for specimens with semielliptic surface cracks.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 9–19, January – February, 1995.     

FATIGUE CRACK PROPAGATION IN AUSTENITIC STAINLESS STEEL UNDER LOW FREQUENCY LOADING AND SALT WATER CONDITIONS

Fatigue crack growth rates (FCGR) for a steel similar to AISI 316LN have been evaluated in air and salt water. At free corrosion potential, a short crack effect appeared for crack lengths shorter than 4  mm. For longer cracks, a segment (called region I) of the corrosion FCGR curve can be described by an equation with the same coefficients as for steels of ordinary resistance to corrosion. Thus, the process controlling crack propagation is presumably the same for both steels. Corrosion FCGR in region II (range of longer crack lengths and higher Δ K values, where the curve cannot be described by the previously mentioned equation) are slower than for ordinary steels. The plateau in FCGR observed at a cathodic potential corresponds to the higher plateau at the free corrosion potential. An interpretation is presented for the shape of the corrosion FCGR curves at both potentials.     

Cyclic corrosion cracking in steel at some nuclear power stations at high water temperatures

Cyclic corrosion cracking is considered for low-carbon low-alloy nuclear power station steels as influenced by the high-parameter coolant. Numerous results are examined on the effects of temperature, frequency, asymmetry coefficient, and form of loading cycle on the fatigue crack growth rates in nuclear power station steels. The mechanics are defined and parameters are derived that define the effects of high-temperature water. The magnitude and character of the threshold range in the stress-intensity coefficient for cyclic corrosion cracking differ from those in static corrosion cracking. The threshold range in the stress-intensity coefficient for cyclic corrosion cracking can be used in engineering as an independent characteristic in the cracking of constructional materials. An autoclave test method has been proposed that enables one to determine the effects of the loading cycle amplitude, frequency, asymmetry coefficient, and form on the rate of fatigue crack growth in a single specimen. A mathematical technique has been developed for calculating the fatigue crack growth rates for constructional elements when the cyclic corrosion cracking mechanism applies for a wide range of cyclic loading parameters.Translated from Problemy Prochnosti, No. 10, pp. 34–41, October, 1995.     

Q345q桥梁钢和Q345qNH耐候钢在模拟工业大气+除冰盐混合介质中的腐蚀行为

采用干湿循环加速腐蚀实验,研究了Q345qNH耐候钢和Q345q桥梁钢在模拟西北地区工业大气环境的腐蚀介质(除冰盐+0.01 mol/L NaHSO₃)中的腐蚀行为。用失重法研究了两种钢的腐蚀动力学曲线,并使用XRD、SEM 和电化学工作站等手段分析了两种钢腐蚀不同时间后锈层的物相、形貌结构及其电化学特性。结果表明：在除冰盐+NaHSO₃的混合介质中,Q345qNH钢腐蚀100 h前的失重稍大于Q345q钢,腐蚀100 h后桥梁钢的失重量明显大于耐候钢;两种钢的腐蚀产物均由α-FeOOH,β-FeOOH,γ-FeOOH,Fe₂O₃,Fe₃O₄和FeOCl构成,但是Q345q钢中生成的不稳定β-FeOOH和FeOCl的含量明显高于Q345qNH钢,锈层的稳定性降低;随着腐蚀时间的延长两种钢锈层的自腐蚀电位均增大,自腐蚀电流密度均波动性减小,Q345qNH耐候钢的自腐蚀电位增大的速度高于Q345q钢,腐蚀后期其锈层的保护性优于普通桥梁钢。两种钢在混合介质中的腐蚀行为受多离子的耦合效应影响,锈层的致密性因β-FeOOH和FeOCl等不稳定腐蚀产物的生成而降低,但是仍有一定的保护性。Q345qNH耐候钢在除冰盐+0.01 mol/L NaHSO₃混合介质中的耐蚀性优于Q345q普通桥梁钢。     

The influence of aqueous environment of coolant of power plants on cyclical crack resistance of steels

The results of investigations of cyclic corrosion crack resistance of carbon steel of grade 20 and low-alloyed pipe steels of grade 15GS and 12Kh1MF are presented. The variants of water standard conditions of power plants are used as test environments. A significant activating impact of the aquatic environment on the kinetics of growth of fatigue cracks of the steels under study is shown. The most significant corrosive effect is observed when tested in an aqueous environment with addition of organic acid. In the low-frequency range of cyclical loading (0.04–0.0008 Hz), the frequency does not influence the characteristics of the cyclic corrosion crack resistance of steels. To eliminate the influence of stress cycle asymmetry on the diagram of corrosion-fatigue crack resistance of steels, it was proposed to use as a parameter of crack growth rate (CGR) the effective range of stress intensity factor (SIF), functionally associated with the factor of stress cycle asymmetry. When the temperature of the medium is increased from 80 to 150°C and to 280°C, the form of kinetic diagram of cyclic corrosion crack resistance is changed, resulting in a decrease in crack growth rate at the middle and upper sections of the diagram.     

Cathodic-Polarization Effects on the Long/Short Corrosion-Fatigue Crack Growth Rate of an Offshore Steel

We study the cyclic crack growth rate of an offshore steel. Compact specimens with long cracks and cylindrical specimens with short surface cracks were tested in air (at frequencies of 5–10 Hz) and in a 3.5% aqueous solution of NaCl (0.2 Hz). In the presence of cathodic polarization (–950 mV/SCE), the growth rate of long cracks is higher than under the conditions of free corrosion. The plateau appearing in this case in the da/dN–K diagrams indicates the presence of hydrogen embrittlement. For K > 10 MPa, the growth rates of short and long cracks coincide.     

A study of fatigue crack growth from artificial corrosion pits at welded joints under complex stress fields

The paper studies the effects of artificial corrosion pits and complex stress fields on the fatigue crack growth of full penetration load‐carrying fillet cruciform welded joints with 45° inclined angle. Parameters of fatigue crack growth rate of welded joints are obtained from S‐N curves under different levels of corrosion. A numerical method is used to simulate fatigue crack growth using different mixed mode fatigue crack growth criteria. Using polynomial regression, the crack shape correction factor of welded joints is fitted as a function of crack depth ratios. Because the maximum circumferential stress criterion is simple and easy to use in practice, fatigue crack growth rate is modified using this criterion. The relationship of effective stress intensity factor, crack growth angle and crack depth is studied under different corrosion levels. The simulated crack growth path obtained from the numerical method is compared with the actual crack growth path observed by fatigue tests. The results show that fatigue cracks do not initiate at the edge or bottom of pits but at the weld toes where the maximum stress occurs. The artificial corrosion pits have little effect on the effective stress intensity factor ranges and crack growth angle. The fatigue crack growth rates of welded joints with pits 1 and 2 are 1.15 times and 1.40 times larger than that of the welded joint with no pit, respectively. The simulated crack growth path agrees well with the actual one. The fatigue life prediction accuracy using the modified formulation is improved by about 18%. The crack shape correction factor obtained using the maximum circumferential stress criterion is recommended being used to calculate fatigue life.     

Corrosion Fracture of Structural Metallic Materials: Effect of Electrochemical Conditions in Crack

Abstract: The work is a compressed review based on the summarised results and the original approach for study of corrosion crack growth, taking into account local electrochemical conditions in the crack tip, which was developed at the Karpenko Physico‐Mechanical Institute of NASU. The model scheme of the pre‐fracture zone in the corrosion crack tip, which can be defined by the local values of pH of solution, electrode potential of metal E and stress intensity factor K_I is proposed. For its realisation, the special method and testing equipment for corrosion crack growth study and local electrochemical measurements in the crack were developed. The variation of the electrochemical conditions in corrosion cracks was studied, and it has been found that some stabilised levels of the pH and E values can be achieved in the tip of a non‐propagating and a propagating crack under static and cyclic loading during of exposure time. On this ground, the method for forecasting of the threshold stress intensity factor K_ISCC under stress corrosion cracking was proposed using these characteristic values of pH and E. This method was also adopted for the determination of the threshold stress intensity factor K_th under corrosion fatigue. The special method for determining corrosion fatigue crack growth rate diagrams based on consideration of extreme electrochemical conditions in the crack tip was developed. It has been proven that such diagrams reflect the extreme influence of the environmental factor on corrosion fracture of material, and they may be recommended as the base for the remaining lifetime calculation of the structural elements exploited under environmental conditions.