Role of Crack Closure in Crack-Size-Dependent Corrosion-Fatigue Crack Growth of X65 Steel Exposed to Sour Brine Environment

Author(s):  
Baotong Lu ◽  
Stephen J. Hudak ◽  
Carl F. Popelar

Corrosion-fatigue in sour brine (SB) environments is a significant design consideration in deepwater floating production systems. Extensive testing over the past 20 years has shown that sour brine environments can reduce the fatigue life of line pipe steels by factors of 10× to 50× compared to fatigue lives measured in laboratory air; moreover, the extent of material degradation depends on a multitude of loading, environmental, and materials variables. Thus, in 2010 Southwest Research Institute (SwRI) embarked on an industry-supported Joint Industry Project (JIP) to develop a quantitative model to predict the effects of these variables on corrosion-fatigue crack growth rate (CFCGR) in offshore structure steels exposed to sour brine environments. Phase 1 of this JIP had successfully developed and validated such a model in the intermediate fatigue crack growth rate regime — i.e., with CFCGRs between 10−4 ∼ 10−2 mm/cycle. However, the Phase 1 model gave overly conservative CFCGRs at rates in the low growth rate regime below 1 × 10−4 mm/cycle, corresponding to S-N corrosion-fatigue lives in the high-cycle fatigue regime. It was hypothesized that these conservative predictions might result from the fact that the model did not consider effects of crack closure that could significantly reduce the effective crack-driving force in this low growth rate regime, a process that might also give rise to crack-size effects. Thus, the primary objective of the current study was to assess whether or not crack closure is responsible for the conservativism in the Phase 1 CFCGR model, as well as to explore related crack-size effects that in theory would not be predictable with conventional linear elastic fracture mechanics. Both of these possible effects are explored here using critical CFCGR experiments on X65 steel in sour brine under loading conditions for which the nominally applied mechanical driving force (ΔK), as well as the stress ratio (Rσ) and loading frequency were held constant, while crack closure measurements were made as the crack grew from 2 mm to about 10 mm. The crack closure measurements were made using elastic compliance measurements made with a specially designed, high-sensitivity clip gage. Results indicate that a crack-size dependence of CFCGR did occur and could be correlated using a crack-closure-corrected effective stress intensity factor (ΔKeff). These results have provided a foundation for extending the JIP’s Phase 1 CFCGR model into the low growth rate regime in the ongoing Phase 2 of the JIP.

1987 ◽  
Vol 36 (12) ◽  
pp. 774-780
Author(s):  
Toshio Terasaki ◽  
Tetsuya Akiyama ◽  
Masatoshi Eto ◽  
Yasuhumi Matsuo ◽  
Masaharu Kusuhara

Author(s):  
Raghu V. Prakash ◽  
Dhinakaran Sampath

Corrosion fatigue growth behavior of structural steels at low cyclic frequency is characterized by an increase in crack growth rate in the threshold and Paris regions, due to the simultaneous action of cyclic mechanical load (fatigue) and corrosive environment. Knowledge on the effect of load sequence on corrosion fatigue crack growth is important to set out the realistic design and prognosis criteria for components operating under corrosive environments. In this study, the corrosion fatigue crack growth rate under the effect of hold-time (1000s), at a maximum stress intensity factor (Kmax), interspersed during cyclic load on was studied experimentally on a Mn-Ni-Cr steel under 3.5% NaCl solution at a constant stress intensity factor range (ΔK) of 15 MPa √m; the corrosion crack growth rate was evaluated for three different frequencies of: 0.01, 0.1 and 1 Hz. As a result of hold time at the peak load, the exposure time for the crack-tip to interact with the environment increased, which could enhance the corrosion crack growth rates. To verify if this corrosion effect can be contained, electrode potential of (−) 850 mV and (−) 950 mV SCE was applied to the specimen to reduce the extent of corrosion contribution to crack growth rate. The fatigue crack growth rate (da/dN) increased when the frequency was decreased from 1 to 0.01 Hz at all electrode potentials. However, the crack growth rate at 0.01 Hz increased by an order of magnitude with a tensile hold at Kmax for 1000 s compared with the crack growth rate during continuous cyclic load for a given electrode potential. The crack growth rate reduced when the electrode potential was decreased to −950 mV SCE. The enhancement of corrosion fatigue crack growth rate with the introduction of a hold-time is explained using crack-tip strain rate assisted anodic dissolution.


Author(s):  
Baotong Lu ◽  
Brian P. Somerday ◽  
Stephen J. Hudak

Laboratory testing has shown that sour brine environments can reduce the fatigue life of offshore steels by factors of 10× to 50× compared to fatigue lives measured in laboratory air. Thus, in order to ensure safe, reliable, and environmentally-friendly deepwater development, the effect of these sour service environments must be properly accounted for in riser and flowline design. However, to ensure that the environmental effect is fully captured, tests need to be conducted at cyclic loading frequencies representative of those experienced in service (typically 0.1 Hz or less), which makes corrosion-fatigue testing very time-consuming and costly. Consequently, there has been a need for predictive models that can reduce the dependence on extensive long-term testing, while at the same time enable existing data to be interpolated and/or extrapolated over a broad domain of relevant mechanical, environmental, and material variables. In response to this need, a Joint Industry Project (JIP) was organized by Southwest Research Institute® (SwRI®) with the objective of developing and validating an analytical model to predict corrosion-fatigue performance of structural steels in sour brine environments. The resulting model is based on the kinetics of hydrogen generation and transport to a fracture process zone (FPZ), where embrittlement occurs in the hydrostatic stress field ahead of the growing crack. The advantage of this kinetic model is that details of the embrittlement process, which are not presently well defined, need not be included since corrosion fatigue crack growth (CFCG) is governed by the rate-controlling process (RCP) in the elemental kinetic steps that supply hydrogen to the FPZ. A general outline of this model is provided here and its validation against independently generated experimental data is demonstrated. The validated model has been implemented in spreadsheet format for convenience as an engineering tool. Due to the fundamental concepts underpinning the model, the engineering tool is shown to be adaptable to predicting CFCG rates in steels exposed to a variety of other environments — including hydrated and dehydrated sour crude oil, moist H2S gas, sweet brine, and seawater — with and without cathodic polarization. An extension of this Phase 1 model from intermediate to lower CFCG rates is currently underway in Phase 2 of the JIP but will not be discussed in detail in the present paper. The primary objective of this paper is to introduce the engineering tool based on the Phase 1 analytical model and demonstrate its functionality in quantifying CFCG rates over wide ranges of mechanical variables (stress-intensity factor range (ΔK), load ratio (Rσ), and cyclic loading frequency), environmental variables (H2S partial pressure, pH, temperature, applied potential), and material variables (yield strength).


1981 ◽  
Vol 103 (4) ◽  
pp. 298-304 ◽  
Author(s):  
T. Shoji ◽  
H. Takahashi ◽  
M. Suzuki ◽  
T. Kondo

The role of mechanical factors, such as ΔK, R, and K˙ (loading rate), and its significance on corrosion fatigue crack growth acceleration were discussed in terms of crack tip strain rate and/or nucleation rate of fresh metal surface. A new parameter for characterizing corrosion fatigue crack growth was proposed, paying attention to rates of crack tip mechanochemical reactions, i.e., oxide film rupture rate, passivation rate, and solution renewal rate, which are influenced by the crack tip mechanical condition, microstructure of material, and environment. Hence a new parameter da/dt]air, the time base pure fatigue crack growth rate which was related closely to crack tip deformation rate, was introduced as a measure of actual crack tip strain rate. In various combinations of materials and environments, it was shown that the value of da/dt]air determines a crack growth rate in the environment, irrespective of mechanical factors such as ΔK, Kmax, R, and K˙, or frequency.


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