Retarding Crack Growth by Static Pressure Hold for Pipeline Steel Exposed to a Near-Neutral pH Environment

2016 ◽  
Author(s):  
Mengshan Yu ◽  
Weixing Chen ◽  
Karina Chevil ◽  
Greg Van Boven ◽  
Jenny Been
Keyword(s):  
Growth Rate ◽  
Cyclic Loading ◽  
Crack Growth Rate ◽  
Crack Initiation ◽  
Crack Growth ◽  
Growth Curve ◽  
Pipeline Steel ◽  
Early Stage ◽  
Pipe Surface ◽  
Neutral Ph

From extensive investigations for over 30 years since the discovery of near-neutral pH stress corrosion cracking (NNpHSCC), the physical processes of crack initiation and growth have been determined, despite that some details in various aspects of crack initiation and growth are still to be understood. The growth curve is a function of crack growth by direct dissolution of steels at localized areas on pipe surface during initiation or at the tip of a crack during early stage of crack growth (the dissolution growth curve), and by a process involving the interaction of fatigue and hydrogen embrittlement (corrosion fatigue, the hydrogen enhanced fatigue growth curve) in Stage II after crack initiation and early stage of crack growth. For the latter case, recent research shows that crack growth rate can be substantially enhanced by variable amplitude cyclic loading. One of the most severe scenarios of cyclic loading in terms of crack growth rate is the underload type of pressure fluctuations that is often found within 30 km downstream of a compressor station. This investigation is aimed to evaluate pressure scenarios that could reduce or retard crack growth during pipeline operation. Specifically, the effect of pressure holds was investigated. Different periods of static hold were performed to an X65 pipeline steel exposed to a near-neutral pH solution. It was found that a static hold at the maximum load for one hour immediately after a large depressurization-repressurization cycle (underload cycle) yielded the lowest crack growth rate, which was about one third of that of constant amplitude fatigue without the static hold. Static holds for a period shorter or longer than one hour have yielded higher crack growth rates. This observation can be applied to field pipelines during operations to retard crack propagation.

Environmental Effect of Crack Growth Rate of Pipeline Steel in Near-Neutral pH Soil Environments

2004 ◽  
Author(s):  
Weixing Chen ◽  
Robert Sutherby
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Crack Growth ◽  
Soil Chemistry ◽  
Pipeline Steel ◽  
Crack Tip ◽  
Growth Behavior ◽  
Crack Growth Behavior ◽  
General Corrosion ◽  
Neutral Ph

The laboratory work reported here was initiated to determine whether different soils can be shown to give rise to different growth rate for a given pipeline steel. Two soil synthetic environments with different near neutral pH value were designed based on various soil chemistries collected near the pipeline in the field where near-neutral pH SCC was found. The crack growth behavior in both the environments were determined using compact tension specimen. The crack growth rate was in situ monitored by the potential drop system. It was found that soil chemistry has a profound effect on crack growth rate. Although it is insensitive to the soil chemistry and cyclic frequency, the crack growth rate in the high ΔK regime has been significantly enhanced in comparison with that in air. In the low ΔK regime, the growth rate is shown to have minor dependence on ΔK value but strong dependence on the testing environments. The observed crack growth behavior in different ΔK regimes and environments was related to the crack tip sharpness and crack crevice wideness as a result of corrosion and room temperature creep deformation. Soil solutions with low general corrosion rate are associated with a blunt crack tip and wide crack crevice, which would result in lower stress intensity at the crack tip and weaker crack closure effect, respectively. Similarly, a loading wave allowing shorter creep time on a given volume of material at the crack tip at high loading stress tends to produce a sharper crack tip and narrow crack crevice. These two factors have opposite effect on crack growth rate, and the observed crack growth rate reflects the combined effect of these two opposite factors.

Effect of Cyclic Loading on Crack Propagation of X-70 Pipeline Steel in Near-Neutral pH Solution

2005 ◽  
Vol 297-300 ◽  
pp. 2501-2507 ◽  
Author(s):  
Hao Guo ◽  
Guang Fu Li ◽  
Xun Cai ◽  
Jiasheng Bai ◽  
Wu Yang
Keyword(s):  
Growth Rate ◽  
Cyclic Loading ◽  
Crack Growth Rate ◽  
Tensile Stress ◽  
Crack Propagation ◽  
Crack Growth ◽  
Pipeline Steel ◽  
Stress Axis ◽  
Mode I ◽  
Propagation Period

Crack propagation of X-70 pipeline steel in near-neutral pH solution was studied under different modes of cyclic loading. A revised equation of crack growth rate vs. Δ K was obtained. Average crack growth rate increased with cycles under conditions of different R values. Linear shape notch specimen made cracks much easier to initiate and propagate than V-shaped notch specimen did. For different R values, the curves of crack growth rate with cycles were similar, but the platform propagation period and quick propagation period were different obviously. Crack growth rate at both periods increased and thus failure time decreased markedly with decrease of R value. The propagation directions of cracks were different under different cyclic loading conditions. Under mode I (single tensile stress) cyclic loading, cracks were straight and perpendicular to the tensile stress axis, while under mixed-mode I/III (tensile/shear stress) cyclic loading, cracks were sinuous and did not propagate in the direction perpendicular to the main tensile stress axis.

scholarly journals Fatigue crack growth rate and crack initiation and fracture life under mixed modes I and II.

1986 ◽  
Vol 35 (395) ◽  
pp. 930-935 ◽  
Author(s):  
Toshimitsu YOKOBORI ◽  
Takeo YOKOBORI ◽  
Kenji ISHII ◽  
Kiyoshi SATO ◽  
Kazuo SHYOJI
Keyword(s):  
Growth Rate ◽  
Fatigue Crack ◽  
Crack Growth Rate ◽  
Fatigue Crack Growth ◽  
Crack Initiation ◽  
Crack Growth ◽  
Fatigue Crack Growth Rate ◽  
Mixed Modes

scholarly journals The setup and application of the multi-scale in-situ test system for fatigue damage analysis

2018 ◽  
Vol 165 ◽  
pp. 04005
Author(s):  
Yi Shi ◽  
Xiaoguang Yang ◽  
Guolei Miao ◽  
Duoqi Shi
Keyword(s):  
Growth Rate ◽  
Fatigue Crack ◽  
Crack Growth Rate ◽  
Crack Initiation ◽  
Crack Growth ◽  
Test System ◽  
In Situ Test ◽  
Multi Scale ◽  
Macro Scale

This essays aims at introducing the setup for the multi-scale in-situ test system which is applied for the fatigue crack initiation test. The setup of the experiment system is first introduced, including the image capturing system, optical path system, image acquisition and storage system and the three-axis mobile platform. Then the preparation of micro speckle and the corresponding technique for spatial adjustment are improved to realize the DIC measurement in micro scale. Finally three experiments from macro-meso scale to macro scale: fatigue initiation test, the observation of micro crack and the fatigue crack growth rate in macro scale were conducted to verify the application of the system. The test result can indicate the location of crack initiation, the crack growth rate and the evolution of displacement/strain field, etc.

Application of Load Sequence to Control Crack Growth in Steel Pipelines Under Near Neutral pH SCC

2016 ◽  
Author(s):  
Olayinka Tehinse ◽  
Weixing Chen ◽  
Jenny Been ◽  
Karina Chevil ◽  
Sean Keane ◽  
...  
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Crack Growth ◽  
Oil And Gas ◽  
Pressure Fluctuations ◽  
Operating Pressure ◽  
Variable Amplitude ◽  
Neutral Ph ◽  
Novel Approach ◽  
Load Sequence

Pipelines are designed to operate below a maximum operating pressure in service. However, there are pressure fluctuations during operation. The presence of pressure fluctuations creates a drive for crack growth in steel pipes. In order to prevent catastrophic failure of pipelines, there is need for better understanding of the contribution of pressure fluctuations to crack growth rate in steel pipelines. Analysis of pressure fluctuation data in oil and gas pipelines shows that there are different types of fluctuations in a pipe due to friction loss with distance from the pump or compressor station. All these fluctuation types show a form of variable amplitude loading classified in this research as underload, mean load and overload. Studies of some structural systems shows that underload can cause acceleration of crack growth while retardation of crack growth is observed after an overload. This research aims to apply pressure fluctuations to manage integrity of steel pipelines through a novel approach of load sequence involving underload and overload in near neutral pH environment. Clear knowledge of the effect of load interaction involving load sequence of underload and overload is vital to control crack growth in steel pipelines under near neutral pH environment. The result of crack growth rate under different load sequence on X65 steel indicate that increase in overload ratio of 2, 3 and 4 caused an increase in crack growth rate of 1.68E−3, 1.89E−3 and 2.31E−3 mm/block respectively. These results are compared with results from other tests under variable amplitude without load sequence. Analyses were carried out on the morphology of the crack tip and the fracture surface after the test.

Fatigue Crack Initiation and Growth of Aluminum Alloy with Stress Ratio Effects

2013 ◽  
Vol 594-595 ◽  
pp. 1105-1111 ◽  
Author(s):  
Haftirman Idrus ◽  
M. Afendi ◽  
Wong Chun Hoe
Keyword(s):  
Growth Rate ◽  
Fatigue Crack ◽  
Crack Growth Rate ◽  
Crack Initiation ◽  
Crack Growth ◽  
Stress Ratio ◽  
Fatigue Crack Initiation ◽  
Stress Intensity Factor Range ◽  
Value Range ◽  
Compact Design

Fatigue crack initiation and growth of aluminum alloys with stress ratio were investigated due to it was widely used in aircraft production parts. Various types of aluminium alloy have been selected (6063-T6, 7075-T6, and 2024-T351). Compact design standard based on ASTM standard E647-11 was used for specimen. Cyclic loading experiment was conducted using Instron 8801 Hydraulic Server Machine with da/dN software for setup and parameter setting. Investigations on crack propagation and fracture surface were done by using Scanning Electron Microscope (SEM) to obtain the image of the specimen surface. Further analysis was done on the image to study on the crack initiation and propagation. Various stress ratio effects were set for the compact specimens having thickness 12.7 mm. Relationship between crack growth rate and the stress intensity factor range were further identified with the stress ratio effects. The gradients of crack growth rate increase while the stress ratio, R increase. Higher R-ratio results in higher value range of minimum load applied. Paris law and Modified Forman law were used as comparison with the experimental data for validation purposes and to provide the level of precision.

The Variation of Crack Energy Density (CED) on Creep Crack Growth

2007 ◽  
Vol 353-358 ◽  
pp. 106-109
Author(s):  
C.S. Jeong ◽  
Byeung Gun Nam ◽  
Katsuhiko Watanabe
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Energy Density ◽  
Crack Growth ◽  
Early Stage ◽  
Creep Crack Growth ◽  
Creep Crack ◽  
Crack Behavior ◽  
Creep Crack Growth Rate ◽  
Unified Description

Creep crack growth (CCG) rate has been organized frequently by C* or Ct parameter However, crack behavior of early stage under unsteady state condition has not been explained. Crack energy density (CED), which has been proposed as a parameter that can provide a unified description of crack behavior with no restriction on constitutive equation, can give the general expression about creep crack growth rate. By applying Ct and the concept of CED to the results, we showed that creep crack growth rate for all ranges of creep can be explained in a unified way by CED and its derivatives. Moreover, the physical meaning of the Ct is clarified in the discussion.

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