Understanding Fatigue Crack Growth Behavior at Low Frequencies for a Mn–Ni–Cr Steel in 3.5 % NaCl Solution Under Controlled Cathodic Potential
28 Feb 2015-Materials Performance and Characterization (ASTM International)-Vol. 4, Iss: 2, pp 157-167
TL;DR: In this paper, a frequency shedding method is used to estimate fatigue crack growth rate over a range of frequencies between 0.01 and 5.1 Hz at a constant stress intensity factor range of 15MPa √m.
Abstract: Fatigue crack growth behavior at low loading frequencies for a Mn–Ni–Cr steel immersed in 3.5 % NaCl solution, with and without cathodic polarization, is investigated and presented in this paper. Frequency shedding method is used to estimate fatigue crack growth rate over a range of frequencies between 0.01 and 5 Hz at a constant stress intensity factor range of 15 MPa √m. The effectiveness of cathodic potential of −900 mV SCE in containing corrosion contribution to crack growth is estimated by comparing with published data on fatigue crack growth rate in lab air and 3.5 % NaCl solution without cathodic polarization. It is noted that there are three regions of crack growth rate as a function of applied frequencies when the data is plotted in log–log scale: linear crack growth rate, plateau crack growth rate, and high crack growth rate. The crack growth rate of steel in 3.5 % NaCl solution at 15 MPa √m is scanned for different cathodic potentials between −760 mV SCE and 1150 mV SCE at 0.01 and 0.1 Hz. The potentials at which the crack growth rates are the maximum and the minimum are found to be −760 and −950 mV, respectively. The optimum cathodic protection potential for the minimum corrosion fatigue crack growth rate ranges between −900 and −950 mV. This optimum potential for the minimum corrosion crack growth rate is more negative than the cathodic potential required for restraining the corrosion effects on fatigue strength of steel to normal ambient air behavior.
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01 Jan 2019
TL;DR: The developments to estimate the fatigue properties of materials using small volume of sample material—similar to scooped samples are presented and the optimum electrode potential for a fatigue crack growth study has been identified.
Abstract: Accurate and reliable life prediction is one of the challenges faced by engineers working in safety-critical domains such as power plants, transportation, and offshore structures. This paper presents the developments to estimate the fatigue properties of materials using small volume of sample material—similar to scooped samples. Cyclic ball indentation and cyclic small punch testing methods have been developed over a period of nearly two decades and have been demonstrated to predict the fatigue properties of in-service materials. Some salient results are discussed in this paper. In the case of offshore structures, the synergistic effect of mechanical loading at a low frequency combined with the corrosive environment accelerates the damage. Life prediction for such structures requires data on corrosion-fatigue crack growth at low frequencies. This is a time-consuming effort and hence there is a need to estimate the properties through novel test methods. Frequency shedding method is proposed to estimate the fatigue crack growth rate behavior in corrosive environments. Further, designers and operators of offshore equipment resort to avoidance of free corrosion through the use of electrode potentials. However, the choice of electrode potential is dependent on the stress state. Through a systematic study, the optimum electrode potential for a fatigue crack growth study has been identified and the same is discussed here. It is hoped that the results of this study and the directions shown to carry out the data generation under more realistic conditions would help the life prediction and life extension community at large.
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TL;DR: In this article, the authors analyzed the growth rate of small fatigue cracks in a high strength steel tested in 3.5% NaCl solution with cathodic protection in the Paris regime through the comparison with the corresponding results obtained in air or in high vacuum.
27 citations
TL;DR: In this article, the effect of cathodic polarization on crack growth was investigated by taking replicas from initially smooth specimens of a high strength steel under fully reversed push-pull loading while: (1) exposed to laboratory air, (2) immersed in a 0.6 M sodium chloride (NaCl) solution at open circuit potential (OCP) and (3) with an applied cathodic potential of 1250 mV (SCE).
Abstract: — Corrosion fatigue crack growth rates in high strength steel are often increased when a large cathodic polarization is applied. The corrosion fatigue mechanism in this case is generally considered to be due to hydrogen embrittlement. In the present study the crack growth process was carefully monitored by taking replicas from initially smooth specimens of a high strength steel under fully reversed push-pull loading while: (1) exposed to laboratory air, (2) immersed in a 0.6 M sodium chloride (NaCl) solution at open circuit potential (OCP) and (3) with an applied cathodic potential of —1250 mV (SCE). It is shown that the effect of cathodic polarization is dependent on the applied stress level and the nature of the cracking process, which in turn, is related to the sue of the crack. For stress levels at or below the in-air fatigue limit, failure did not occur for cathodically polarised specimens despite the number of loading cycles being 10 times that of the lifetime of identical tests in solution at the open circuit potential. At stress levels above the in-air fatigue limit the reduction in fatigue endurance caused by the presence of the corrosive environment can be partially recovered through cathodic polarization. The role of non-metallic inclusions in the cracking process under various exposure conditions is discussed, and a cracking mechanism is proposed.
16 citations
TL;DR: In this paper, the effects of cathodic protection potential, corrosion products and stress ratio on corrosion fatigue crack growth rate have been studied on offshore structural steels and the maximum acceleration of crack growth was observed at the crack growth plateau which was independent of ΔK.
Abstract: 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, ΔKeff. The estimation of corrosion fatigue crack growth rate in terms of ΔKeff 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.
12 citations
TL;DR: In this article, a frequency shedding method was proposed to estimate corrosion fatigue-crack growth characteristics, where the frequency is shed with crack advance using an exponential law, and crack closure estimates were obtained at periodic intervals of crack length and the effective stress intensity graphs suggest acceleration in crack growth rate due to corrosion as the frequency was reduced.
Abstract: Corrosion fatigue-crack growth characteristics are important for the design of marine and off-shore structures Design of critical components requires data on fatigue-crack growth rate at very low frequencies of the order of 10−2 to 10−3 Hz Experiments at low frequencies pose practical difficulties of enormous test duration To address this, it is proposed to estimate corrosion crack growth characteristics using a frequency shedding method where the frequency is shed with crack advance using an exponential law Fatigue-crack growth rate tests have been conducted on Ni–Mn–Cr steel at a constant ΔK range of 18 MPa√m (lower Paris regime) under lab air conditions as well as 35 % NaCl solution Crack growth rate data plotted as a function of test frequency presents a straight-line trend in log–log scale for a frequency range of 1–01 Hz; however, there is a change in trend when the frequencies are dropped further, which could be due to domination of corrosion mechanism To understand the role of crack closure, crack closure estimates were obtained at periodic intervals of crack length and the effective stress intensity graphs suggest acceleration in crack growth rate due to corrosion as the frequency is reduced
6 citations