Influence of Mean Load Pressure Fluctuations on Crack Growth Behavior in Steel Pipelines

2018 ◽  
Author(s):  
Olayinka Tehinse ◽  
Weixing Chen ◽  
Karina Chevil ◽  
Erwin Gamboa ◽  
Lyndon Lamborn
Keyword(s):  
Crack Growth ◽  
Pressure Fluctuations ◽  
Stress Sensitivity ◽  
Crack Growth Behavior ◽  
Mean Stress ◽  
Load Pressure ◽  
R Ratio ◽  
Stress Maximum ◽  
Minimum Stress ◽  
Pump Compressor

Internal pressure fluctuations during pipeline operations could contribute to crack growth in steel pipelines. These pressure fluctuations create a variable amplitude loading condition with large amplitude cycles at near-zero stress ratio, R (minimum stress / maximum stress) and small amplitude cycles (minor cycles) at near +1 R ratio which can both affect crack propagation. Mean stresses fluctuate with pressure due to fluid friction losses proportional to the distance from the pump/compressor station. A deeper understanding of mean stress sensitivity on crack growth rate in steel pipelines is sought. The aim of this research is to retard crack growth in pipelines by prescribing pressure fluctuations, thus controlling mean stress effects on imperfection growth in steel pipelines under a near neutral pH environment. This study shows that prescriptive mean load pressure fluctuations can be used to reduce crack growth rates in steel pipelines, thus expanding pipeline integrity management methods.

scholarly journals Fatigue life and crack growth behavior of post welded Aluminum 5183 alloy

2018 ◽  
Vol 165 ◽  
pp. 21013 ◽  
Author(s):  
Vidit Gaur ◽  
Manabu Enoki ◽  
Toshiya Okada ◽  
Syohei Yomogida
Keyword(s):  
Grain Size ◽  
Fatigue Life ◽  
Crack Growth ◽  
Growth Behavior ◽  
Crack Growth Behavior ◽  
Mean Stress ◽  
Threshold Values ◽  
R Ratio ◽  
Fatigue Failures ◽  
Mean Stresses

In this study we investigated the fatigue life and crack growth behavior of Al-5183 alloy. Microscopic analysis revealed nearly equi-axed grains and no texture in longitudinal or cross-sectional plane of the welded plates. Gas porosities with an average size of 45 μm, comparable to grain size (55 μm), were present and often initiate fatigue failures. Load-controlled cyclic tests at different stress-ratios (R = -1, -0.5, 0.1, 0.5. 0.7 and 0.8) revealed decrease in fatigue lives with increase in R-ratio. At R > 0.7, no fatigue failure could be observed, suggesting a probable mean-stress saturation effect on fatigue. Mean stress also tends to control the crack initiation sites: surface initiated failures at low mean stresses while sub-surface pores induced failures at higher mean stresses. Fatigue-crack growth tests on CT specimens at different R-ratios (0.1, 0.5 and 0.8) revealed reduction in crack growth rates (and in threshold values) with increasing R-ratio. The ΔK applied for pores responsible for fatigue failures were often lower than or near to the threshold values and also, the size of such pores was of order of magnitude of grain size, thus crack initiated from pores are short cracks and further tests are progress.

High pH Crack Growth Sensitivity to Underload-Type of Pressure Fluctuations

2018 ◽  
Author(s):  
Hamid Niazi ◽  
Hao Zhang ◽  
Kaitlyn Korol ◽  
Weixing Chen
Keyword(s):  
Growth Rate ◽  
Crack Growth ◽  
Pressure Fluctuations ◽  
Loading Conditions ◽  
Constant Amplitude ◽  
Variable Amplitude ◽  
High Ph ◽  
R Ratio ◽  
Pump Compressor ◽  
Crack Growth Rates

High pH Stress Corrosion Cracking (HpHSCC) is a significant threat to the buried pipelines, which are protected through simultaneous coating and cathodic protection strategies. In the past decades, extensive research has been devoted to assessing the influence of environmental and metallurgical factors on the susceptibility to HpHSCC. With reference to mechanical factors, previous studies employed either slow strain rate or constant amplitude testing methods. However, the pressure fluctuation data extracted from pipeline operations has indicated that pipelines experience highly variable amplitude loading conditions during their service. Accordingly, an important consideration in managing HpHSCC is load interaction. Statistics show a higher probability of HpHSCC failures within the 30 km downstream from pump/compressor stations where the pipeline steels experience elevated service temperatures, with incipient higher susceptibility to HpHSCC. However, the pipeline sections within the 30 km downstream from pump/compressor stations also experience the underload-type of pressure fluctuations that feature a maximum pressure close to the design limit, frequent and large amplitudes of depressurization, resulting in low stress ratio, R (minimum stress/maximum stress), and many smaller pressure fluctuations (minor cycles) with R ratio closer to +1.0. It has been well characterized that the underload-minor-cycle-type of pressure fluctuations has the significant acceleration effect on crack growth rates in near-neutral pH (NNpH) environments. However, the effect of the underload-type of pressure schemes on HpHSCC crack growth has not been well developed. In this research work, a cathodically protected X65 steel specimen in the developed high pH solution, composed of 1N Na2CO3 and 1N NaHCO3, was subjected to different loading conditions. These loading waveforms simulate underload cycles (R = 0.5), minor cycles (R = 0.9) and variable amplitudes consisting of both underload and minor cycles, respectively. The HpHSCC test results showed that the highest and lowest crack growth rates were obtained in high and low R ratio constant amplitude loading conditions, respectively. Furthermore, an intermediate crack growth rate was obtained under variable amplitude loading condition. These results indicate that the underload cycles retard crack growth rate in high pH environments.

An Experimental Investigation of the Fatigue Crack Growth Behavior of a Pressure Vessel Material

2006 ◽  
Author(s):  
Xiaogui Wang ◽  
Zengliang Gao ◽  
Tianwen Zhao ◽  
Fei Ding ◽  
Yanyao Jiang
Keyword(s):  
Experimental Investigation ◽  
Crack Growth ◽  
Pressure Vessel ◽  
Great Influence ◽  
Maximum Load ◽  
Crack Arrest ◽  
Crack Growth Behavior ◽  
Constant Amplitude ◽  
R Ratio ◽  
Vessel Material

An experimental investigation was conducted on 16MnR, a pressure vessel material widely used in China. Standard compact specimens were subjected to several different R-ratios (the ratio of the minimum load over the maximum load in a loading cycle) and loading amplitudes. Three different notch sizes ranging from very sharp notch to blunt notch were used. In addition to constant amplitude load, experiments were conducted with overloads being applied at different crack lengths. Two-step high-low sequence loading experiments were conducted to study the influence of the loading sequence. The experimental results show that the R-ratio has an insignificant influence on crack growth of the material. A single overload at the start of crack growth from a notch results in a crack arrest. The application of an overload during a constant amplitude loading experiment retards the crack growth significantly. A higher constant amplitude loading history has a great influence on the crack growth in the subsequent lower constant amplitude loading. The size of the notch together with the R-ratio has a great influence on the early crack growth from the notch.

The Impact of Pressure Fluctuations on the Early Onset of Stage II Growth of High pH Stress Corrosion Crack

2020 ◽  
Author(s):  
Hamid Niazi ◽  
Hao Zhang ◽  
Lyndon Lamborn ◽  
Weixing Chen
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Stress Corrosion ◽  
Crack Growth ◽  
Pressure Fluctuation ◽  
Pressure Fluctuations ◽  
Crack Size ◽  
Stage I ◽  
Stage Ii ◽  
R Ratio

Abstract Steel pipelines undergo the following sequential stages prior to high pH stress corrosion cracking (HpHSCC) failure, viz., formation of environmental condition, initiation of the intergranular cracks followed by cracks coalescence to form critical crack size (Stage I), mechanically dictated crack growth with higher rate (Stage II) compared to Stage I, rapid crack propagation to failure (Stage III). From fracture mechanics perspective, the crack size reaches the critical value at the onset of stage II; consequently, stress intensity factor (K) ahead of the crack tip exceed the critical value (KISCC). Although many researches have been devoted to understanding HpHSCC behavior, the mechanical conditions that accelerate the onset of stage II remains unknown. This study investigates the mechanical loading conditions that yield to early onset of stage II with respect to the most severe loading condition in operating pipeline, underload-minor-cycle type of pressure fluctuation. In this study, several loading scenarios were applied to pre-cracked CT specimens exposed to 1 N NaHCO3-1N Na2CO3 at 40° C and −590 mVSCE. The first series of tests were conducted through applying variable amplitude loading waveforms to determine the K value below the KISCC. It was observed the crack growth rate decreases from 1.5 × 10−7 mm/s to 2.5 × 10−8 mm/s when Kmax decreases from 36 to 15 MPa·m0.5. Then, both constant amplitude and variable amplitude loading scenarios with the Kmax = 15 MPa·m0.5 were applied to pre-cracked CT specimens. It was observed that low R-ratio constant amplitude cycles yield to highest crack growth rate (3.6 × 10−7 mm/s), which was one order of magnitude higher than other waveforms. However, comparing the intergranular crack advancement per block resulted in similar crack growth rates for those waveforms containing low R-ratio cycles. These results imply that stage I of crack growth is assisted by fatigue due to low R-ratio cycles. It was observed that loading/unloading frequency of low R-ratio cycles has a direct relation with crack growth rate at stage I, i.e., high frequency cycles accelerate onset of stage II. The implication of these results for pipeline operator is that pressure fluctuation, particularly large and rapid pressure fluctuation at the sites susceptible to HpHSCC, threatens the pipeline integrity. Avoiding such pressure fluctuations, if possible, increase pipeline lifespan and prevents catastrophic damages by intergranular stress corrosion crack growth through delaying the onset of stage II of HpHSCC crack growth.

Energy response of S355 and 41Cr4 steel during fatigue crack growth process

2018 ◽  
Vol 53 (8) ◽  
pp. 663-675 ◽  
Author(s):  
Grzegorz Lesiuk ◽  
Mieczysław Szata ◽  
Dariusz Rozumek ◽  
Zbigniew Marciniak ◽  
José Correia ◽  
...  
Keyword(s):  
Experimental Data ◽  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Propagation ◽  
Crack Growth ◽  
Stress Effect ◽  
Mean Stress ◽  
Mean Stress Effect ◽  
R Ratio ◽  
The Mean

In this research, a novel approach of the fatigue crack growth rate description has been proposed. Based on theoretical and experimental approach, the mean stress effect expressed by R-ratio is present in classical da/dN–Δ K diagram. According to energy approach – based on the irrevocably dissipated energy accumulated in material (hysteresis loop) during fatigue process – the mean stress effect can be minimalized. Experimental validation of the proposed model was performed using results of fatigue crack propagation data for S355 and 41Cr4 steels in terms of strain energy density parameter Δ S or cyclic J-integral range –Δ J. In contrast to the force approach based on Kmax (or Δ K), the energy parameters Δ S or Δ J represent unambiguously the fatigue crack propagation rate, without influence of mean stress effect – R-ratio. However, in near threshold range of kinetic fatigue fracture diagram, the energy parameter displays a slight dispersion of the experimental data. According to the crack closure theory and its U-Elber parameter, the dispersion of experimental data is decreased. Therefore, the crack closure effects have a high significance in energy model – similar to the ‘force approach’ based on Δ K concept.

Paris law relations for an epoxy-based adhesive

2019 ◽  
Vol 234 (2) ◽  
pp. 291-299 ◽  
Author(s):  
AVM Rocha ◽  
A Akhavan-Safar ◽  
R Carbas ◽  
EAS Marques ◽  
R Goyal ◽  
...  
Keyword(s):  
Crack Growth ◽  
Damage Parameter ◽  
Growth Behavior ◽  
Crack Growth Behavior ◽  
Mode I ◽  
Mode I Crack ◽  
Loading Conditions ◽  
Fatigue Crack Growth Behavior ◽  
Paris Law ◽  
R Ratio

Depending on the expression used in Paris law relation, crack growth rate can be a function of loading conditions. However, for design purposes, it is costly and time consuming to obtain a complete map for fatigue crack growth behavior of adhesives. Accordingly, using a damage parameter where the Paris law constants are similar for different loading conditions is very helpful. In this paper, mode I crack growth (FCG) tests were performed at different loading conditions. Results show that performing one experiment would be enough to obtain Paris law constants if the effect of R ratio is considered in the relations.

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