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.     

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.     

CORROSION FATIGUE OF MILD STEELS IN A SODIUM NITRATE SOLUTION

Abstract— Cyclic loading of mild steels in 4 N NaNO₃ produces intergranular cracking similar to that associated with stress corrosion cracking at static loads, even when the cyclic loads are below the threshold for cracking under static conditions. However, under some circumstances cyclic loading promotes transgranular crack growth, the velocity of which may increase or decrease with increasing stress intensity factor. The transition from intergranular to transgranular cracking, or vice versa, is determined by the relative velocities of the two modes of failure, that observed having the higher velocity. The suppression of transgranular cracking as the stress intensity factor increases is only observed when the loading involves a compressive component and the effect is explained in terms of corrosion products interfering with the cracking process. Tension-tension loading in nitrates produces crack growth characteristics similar to those observed in oil.     

Crack growth prediction for underground high pressure gas lines exposed to concentrated carbonate–bicarbonate solution with high pH

A crack growth model for high pH stress corrosion cracking of pipeline steels is presented based on the assumption that the cracking is dominated by the repeated rupture of passive film at the crack tip. The model is validated by the experimental data available. It provides a reasonably good prediction to effects of various factors relating to materials, environment and loading conditions. Although the fatigue damage produced in operation of gas lines is negligible, the contribution due to cyclic load-promoted crack tip dissolution needs to be considered in remaining life prediction. Finally, the procedure for field application is discussed.     

Effect of loading cycle asymmetry,stress concentration,and fretting corrosion on the fatigue resistance of alloy D16T

Experimental data are presented for the fatigue strength of smooth specimens, specimens with a stress concentrator, and specimens under fretting corrosion conditions for alloy D16AT at three levels of loading cycle asymmetry coefficient R = –1, 0, and –0.5. It is established that fatigue resistance for alloy D16AT under fretting corrosion conditions is governed by the amplitude of operating stresses but it is practically independent of changes in loading cycle asymmetry coefficient. The possibility is demonstrated of calculating the fatigue limit for specimens with a stress concentrator with different loading cycle asymmetry from known values of the threshold stress intensity factor, the theoretical stress concentration factor, and the length of an nondeveloping crack at fatigue limit. Limiting stress amplitude diagrams for smooth specimens under fretting corrosion conditions have a different nature for the relationship between amplitude and average stresses of the loading cycle and therefore they cannot be described by a general analytical expression.Translated from Problemy Prochnosti, No. 5, pp. 47–51 May, 1990.     

Determining permissible stresses in simple cyclic loading

Cyclic loading stress is examined for a limiting-state diagram which characterizes the fatigue resistance, the permissible stresses, the cyclic cracking resistance, and the limiting fatigue crack size. It has been found for 09G2 steel that the limiting crack size is dependent on the stress cycle asymmetry if the permissible stresses are determined for a given strength margin coefficient. One can construct a permissible-stress diagram for a given crack size. In that case, the strength margin coefficient is dependent on the stress cycle asymmetry, which enables one to make better use of the material. Translated from Problemy Prochnosti, No. 1, pp. 97–102, January–February, 1997.     

Determining cyclic corrosion cracking resistance for titanium alloys with allowance for electrochemical conditions at the fatigue corrosion crack tip

Published data are examined on how various factors affect fatigue crack growth rates. Basic diagrams have been constructed for the cyclic cracking resistance in Ti-6AI-4V and Ti-6AI-6V-2Sn alloys in air, distilled water, and 3.5% NaCl for use in working-life calculations. Appropriate heat treatment can produce two microstructures in a titanium alloy, one of which has the largest cyclic cracking resistance, while in the second, the cracks grow at the lowest rate. The cyclic corrosion cracking resistance for a titanium alloy should be determined in relation to the state of stress and strain and to the electrochemical conditions at the corrosion fatigue crack tip, while the variations in fatigue crack growth rate for a given stress intensity factor in a corrosive medium are due to differing electrochemical conditions at the crack tip during the testing on different specimens. Basic diagrams can be derived for titanium alloys by using a physically sound methodology developed previously for steels, which is based on invariant diagrams for cyclic cracking resistance in air and in the corresponding medium, which can be constructed in relation to extremal working and electrochemical conditions at corrosion-fatigue crack tips.Translated from Problemy Prochnosti, No. 12, pp. 3–11, December, 1993.     

High-temperature cracking resistance in cast steel in reforming oven tubes

Summary Results are given from high-temperature tests (770, 870, 920, and 960°C) to estimate the short-time and cyclic cracking resistance for cast NK-40 steel tubes from reforming ovens in the initial state and after use for 75,400 h. The crack growth resistance in static loading has been determined from the crack resistance limit, and in cyclic loading from the kinetic diagrams for the fatigue failure in terms of the crack growth ratev against the scale of the nominal stress intensity coefficient K. Metallography shows that there are differences in structure in the various states, and material that has been used has pores and microcracks, whose numbers are largest in the inner wall layers. Cyclic tests with constant K show substantial increase in the crack growth rate in the defective material.The cracking stability limit falls as the test temperature rises, while the fatigue crack growth rate increases, particularly above 920°C. Increased asymmetry in the loading cycle (R=–1; –0.5; 0; 0.4) is accompanied by deterioration in the cyclic cracking resistance. There are two opposite trends in the effects of loading frequency (f=0.001–10 Hz) on the failure kinetics: in the high-amplitude loading range, reduction inf increases the crack growth rate, while in the low-amplitude range, there is a decrease. The results are explained in terms of creep and fatigue crack opening.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 26, No. 2, pp. 68–74, March–April, 1990.     

A REVIEW OF MODELLING SMALL-CRACK BEHAVIOR AND FATIGUE-LIFE PREDICTIONS FOR ALUMINUM ALLOYS

Abstract— The small-crack effect, where small fatigue cracks grow faster and at lower stress-intensity factors than large cracks, has been found to be significant for many materials and loading conditions. In this paper, plasticity effects and crack-closure modelling of small fatigue cracks are reviewed. A crack-closure model with a cyclic-plastic-zone-corrected effective stress-intensity factor range (related to the cyclic J -integral) and microstructural data on crack-initiation sites were used to calculate small-crack growth rates and fatigue lives for unnotched and notched specimens made of two aluminum alloys. The crack-closure transient from the plastic wake was shown to be the dominant cause of the small-crack effect and plasticity effects on the cyclic-plastic-zone-corrected stress-intensity factor range were negligible except at extremely high stress levels. Small-crack growth rates and fatigue lives under both constant-amplitude and spectrum loading from tests and analyses agreed well.     

Relation between the fatigue-failure diagram and fatigue curve

A method of converting the endurance curve to a fatigue-failure diagram is developed theoretically. The method consists in the solution of a system of three equations of the number of cycles of specimen failure for different levels of applied stress, as determined by the integration of the relationship between rate of crack growth and the stress-intensity factor, and also in the determination of three unknowns — the threshold stress-intensity factor and two parameters of the Paris equation. Confirmation of the method on the basis of published data on the endurance and crack resistance of steels subjected to cyclic loading indicated its applicability for practical calculations. Use of the method makes it possible to increase the informativeness of fatigue tests and to obtain a large amount of additional information on the results of numerous experimental investigations of the fatigue strength of materials.Translated from Problemy Prochnosti, No. 11, pp. 38–44, November, 1991.     

Fatigue fracture of AMg6N alloy under loading at sound frequencies

Cyclic crack growth resistance tests of AMg6N alloy under loading at frequencies of 20 to 10 kHz have demonstrated that the rate of fatigue crack propagation decreases with increasing frequency and the threshold stress intensity factors increase exponentially with the frequency of strain cycling. Fractographic observations of fracture surfaces of the specimens have revealed that an increase in the loading frequency is accompanied by a decrease in the fatigue striation spacing and in the size of the striation microzones by intnsifying the processes of secondary cracking and the formation of fretting products. This leads to a decrease in the rate of fatigue crack propagation and an increase in the threshold values of the stress intensity factors. An increase in the asymmetry of the loading cycles reduces the contribution of delamination and the formation of fretting products to the process of fracture of the alloy and results in a smaller fraction of the striation relief and in an earlier occurrence of the elements of quasistatic fracture by dimples, which is the cause of the reduction in characteristics of the cyclic crack growth resistance under asymmetrical loading. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 2, pp. 94–105, March–April, 1999.     

FATIGUE UNDER CYCLIC COMPRESSIVE LOAD

For ultra-high strength steels and aluminium alloys, a fatigue crack could initiate from a notch tip under cyclic compressive load. The threshold value for fatigue crack initiation under compressive load can be as great as four times that under tensile load. The crack grew at a decreasing rate until eventually it stopped growing altogether under cyclic compressive load with a maximum length of 0.2-0.5 mm. If the minimum compressive load was near zero, i.e. compression to zero load cycling, the threshold value was near that under tensile loading and the compressive fatigue crack could continue to grow; however, the crack growth rate under compression to zero load fatigue was 10–100 times less than that under the tensile fatigue loading.     

Influence of radiation damage on the cyclic crack resistance of austenitic steels and alloys (review)

The results of experimental investigations of the cyclic crack resistance of austenitic steels and nickel alloys in the annealed and cold-worked conditions at various temperatures with various dosages of radiation damage are analyzed and summarized. In the absence of rheological processes at a crack tip irradiation reduces crack growth in cyclic loading and for steels a correlation of this effect with the degree of radiation strengthening is observed. In weld joints after irradiation the cyclic crack resistance may drop. High-temperature aging of annealed steels promotes a decrease in crack growth rate and preliminary cold working does not play a significant positive role in preservation of cyclic crack resistance after irradiation in loading at reduced temperatures and has a negative influence at temperatures above 600°C. A decrease in loading frequency or the presence of holds in the cycle with a stress intensity factor equal to K_max significantly reduces fatigue crack growth resistance and radiation damage strengthens this effect.Translated from Problemy Prochnosti, No. 7, pp. 10–20, July, 1991.     

FATIGUE CRACK PROPAGATION OF SINTERED SILICON NITRIDE IN VACUUM AND AIR

Abstract— Static and cyclic fatigue in vacuum and air have been investigated for pressureless sintered silicon nitride. Cyclic crack propagation occurred even in vacuum where environmental effects are negligible. Hence such damage is attributed to pure mechanical fatigue unaffected by environment. Crack growth occurs in air even below K _ISCC and the growth rate is higher than in vacuum. Therefore, it is believed that cyclic crack growth in air is accelerated by stress corrosion cracking for a wide range of K _max values.     

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.     

几种生物陶瓷材料的裂纹扩展特性

采用破坏力学中的双扭矩实验法，研究了玻璃陶瓷、云母陶瓷、氧化铝陶瓷、氧化锆陶瓷材料分别在大气、水环境中的静负荷和循环负荷下的裂纹扩展特征，阐明了水环境和循环负荷对材料裂纹扩展特性（KI－V特征）的影响。所研究的材料在水环境下的裂纹扩展速度均加快，但玻璃陶瓷和氧化锆陶瓷材料更为明显。在静负荷下这几种材料的裂纹扩展阻力由小到大的次序为：玻璃陶瓷（N－0），玻璃陶瓷（N－11），云母陶瓷，氧化锆陶瓷和氧化铝陶瓷，对于氧化铝和氧化锆陶瓷材料在循环负荷下的裂纹扩展速度均明显加快。     

A study of crack growth retardation due to artificially induced crack surface contact

The contact of the cracked surfaces during a part of a loading cycle generally results in a reduced crack growth rate. A critical experiment was designed to evaluate the influence of the crack surface contact on crack growth. A round compact specimen made of 1070 steel with a round hole at the wake of the fatigue crack was designed. Two mating wedges were inserted into the hole of the specimen while the external load was kept at its maximum in a loading cycle. In this way, the wedges and the hole in the specimen were in firm contact during the entire loading cycle in the subsequent loading. Experiments showed that the addition of the wedges resulted in a reduction of crack growth rate in the subsequent constant amplitude loading. However, crack growth did not arrest. With the increase in the subsequent loading cycles, crack growth rate increased. The traditional crack closure concept cannot explain the experimental phenomenon because the effective stress intensity factor range was zero after the insertion of the wedges. The detailed stress–strain responses of the material near the crack tip were analyzed by using the finite element method with the implementation of a robust cyclic plasticity theory. A multiaxial fatigue criterion was used to determine the fatigue damage based upon the detailed stresses and strains. The crack growth was simulated and the predicted results were in good agreement with the experimental observations. It was confirmed that the stresses and strains near the crack tip governed cracking behavior. Crack surface contact reduced the crack tip cyclic plasticity and the result was the observed retardation in crack growth.     

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.     

THE EFFECT OF ENVIRONMENT ON FATIGUE CRACK GROWTH BEHAVIOR OF 2021 ALUMINUM ALLOY

Abstract— The effects of dry hydrogen, moist air, distilled water and hydrazine environments on the fatigue crack propagation behavior of 2021 aluminum alloy have been investigated over a wide range of growth rates spanning about six orders of magnitude. Environmental interactions in the intermediate and near-threshold crack growth regions are shown to be associated with different fracture characteristics and mechanisms. Scanning Auger and X-ray photoelectron spectroscopic analysis of fracture surface corrosion deposits revealed that oxide induced crack closure phenomena, which considerably influence the near threshold corrosion fatigue behavior of low strength steels and some aluminum alloys, are not of importance for the present 2021-T6 aluminum alloy. The mechanistic aspects of environmentally influenced fatigue behavior of the alloy are discussed in the light of hydrogen embrittlement, chemical reactions and crack closure concepts.     

Crack initiation and growth path under multiaxial fatigue loading in structural steels

In real engineering components and structures many accidental failures occur due to unexpected or additional loadings, such as additional bending or torsion. There are many factors influencing the fatigue crack paths, such as the material type (microstructure), structural geometry and loading path. It is widely believed that fatigue crack nucleation and early crack growth are caused by cyclic plasticity. This paper studies the effects of multiaxial loading paths on the cyclic deformation behaviour, crack initiation and crack path. Three types of structural steels are studied: Ck45, medium carbon steel, 42CrMo4, low alloy steel and the AISI 303 stainless steel. Four biaxial loading paths were applied in the tests to observe the effects of multiaxial loading paths on the additional hardening, fatigue crack initiation and crack propagation orientation. Fractographic analyses of the plane orientations of crack initiation and propagation were carried out by optical microscope and SEM approaches. It is shown that these materials have different crack orientations under the same loading path, due to their different cyclic plasticity behaviour and different sensitivity to non-proportional loading. Theoretical predictions of the damage plane were conducted using the critical plane approaches, either based on stress analysis or strain analysis (Findley, Smith–Watson–Topper, Fatemi–Socie, Wang–Brown–Miller, etc). Comparisons of the predicted crack orientation based on the critical plane approaches with the experimental observations for the wide range of loading paths and the three structural materials are shown and discussed. Results show the applicability of the critical plane approaches to predict the fatigue life and crack initial orientation in structural steels.