Fatigue Crack Growth Behavior of Embedded Flaws in Sour Pipelines

2016 ◽  
Author(s):  
Feng Gui ◽  
Colum Holtam ◽  
Brandon Gerst ◽  
Ramgopal Thodla
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Fracture Mechanics ◽  
Crack Growth ◽  
Hydrogen Concentration ◽  
Crack Tip ◽  
Corrosion Process ◽  
Fatigue Performance ◽  
Loading Conditions ◽  
Hydrogen Charging

DNV-OS-F101 Appendix A provides procedures for the assessment of circumferential flaws located in subsea pipe girth welds using fracture mechanics methods, commonly referred to as engineering critical assessment (ECA). The purpose of the ECA approach is to provide critical flaw dimensions for given material properties and loading conditions in a conservative way. The results of the assessment are used to derive weld flaw acceptance (or weld repair) criteria to be used during pipeline installation. An ECA will typically consider flaws under installation and operation loading conditions, including fracture and fatigue crack growth (FCG) calculations. Internal and external surface-breaking flaws are assessed, along with embedded flaws. DNV-OS-F101 provides guidance on the appropriate FCG law to be used for the assessment of each flaw type under operational loading. For internal surface flaws exposed to sour production fluids (i.e. containing H2S) FCG rates (FCGRs) are known to be significantly higher than in air and, in the absence of relevant published data, project-specific testing is commonly performed to quantify fatigue performance. The recommendation for the assessment of embedded flaws is to use an air curve, as long as it can be substantiated that the fatigue performance is not reduced due to the environment. It has been demonstrated that the FCG behavior of C-Mn pipeline steels exposed to sour environments is dominated by bulk hydrogen charging effects, i.e. hydrogen charging by absorption from the exposed surfaces rather than the corrosion process at the crack tip. Therefore, it is expected that an embedded (or external) flaw in a sour pipeline will be located in steel containing absorbed hydrogen. This paper describes the results of an investigation aimed at understanding and quantifying the FCG behavior of embedded flaws in sour pipelines. For the purposes of this work, an embedded flaw refers to a crack propagating in hydrogen charged material but whose crack tip is not directly exposed to the sour environment. Hydrogen diffusion modelling and simulation studies were performed to predict the through wall hydrogen concentration in standard fracture mechanics specimens based on sour environmental conditions. Two novel test methods were developed to accurately measure FCGRs in hydrogen charged steel, one for single edge notched bend (SENB) specimens and one for compact tension (CT) specimens. FCGR tests were carried out using both methods. The FCGRs measured in hydrogen charged API 5L grade X65 pipeline steel were significantly higher than in air. In some cases, the observed FCGRs in hydrogen charged steel were higher than for specimens fully immersed in the sour environment. This is believed to be due to reduced environmental crack closure/blunting effects; the steel is charged with hydrogen, but there is no active corrosion process occurring inside the crack. The results of the present study indicate that the use of an air FCG curve for embedded (or external) flaws located in hydrogen charged steel may be non-conservative. Further work is required to establish the relationship between FCGR and hydrogen concentration in steel and to evaluate the implications for pipeline ECA calculations.

Applicability of Precracked Charpy Specimens for Determining Fatigue Crack Growth and J-R Properties: Experiments and Assessment of K and J Dominance

2014 ◽  
Author(s):  
Gustavo Henrique B. Donato ◽  
Rodrygo Figueiredo Moço ◽  
Tatiane Rossi Merlo
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Fracture Mechanics ◽  
Crack Growth ◽  
Driving Forces ◽  
Crack Tip ◽  
Structural Steels ◽  
Stress Fields ◽  
Small Scale ◽  
Main Challenge

Structural integrity assessments regarding Fatigue Crack Growth (FCG) and fracture phenomena are based on fracture mechanics theoretical background and rely upon the notion that a single parameter (usually K or J, respectively for linear elastic and elastic-plastic fracture mechanics) characterizes the crack-tip stress fields and controls local damage. However, the validity of K/J as crack-tip driving forces representative of local stress fields is only achieved if SSY (Small Scale Yielding) conditions prevail. It means that plasticity ahead of the crack must be small. Current standards (e.g.: ASTM E399, E1820, E647, ISO 12135) impose severe geometrical restrictions for the specimens (minimum thicknesses and crack depths) looking for plane strain (high constraint) conditions and therefore K and J-dominance. The main challenge is that thicknesses and/or planar dimensions of current real structures made of high toughness structural steels are in several cases not enough for the extraction of “valid” C(T), SE(B) or SE(T) specimens. In this context, subsized specimens are of great interest. As an example, Charpy geometries have been investigated during the last decades. This work is concerned about testing high structural steels and investigates the applicability of fatigue-precracked Charpy specimens for determining FCG (da/dN vs. ΔK) and J-R curves. The main issues are: i) verify the feasibility of the experiments in a servohydraulic machine in terms of scatter, control and repeatability; ii) quantify the validity limits of K and J for such reduced geometries. Samples had notches machined by EDM and were precracked reaching a/W=0.25 and a/W=0.45. FCG and J-R tests were successfully conducted with repeatability and refined 3D non-linear FE models were developed to provide compliance solutions and verify K and J dominance. Consequently, mechanical properties from subsized samples could be obtained and compared to data obtained from standardized C(T) specimens made of the same steel. The applicability of precracked Charpy geometry could be investigated, motivating further investigations in the field.

Cyclic Plasticity of a Cracked Structure Subjected to Mixed Mode Loading

2007 ◽  
Vol 348-349 ◽  
pp. 105-108 ◽  
Author(s):  
Sylvie Pommier
Keyword(s):  
Finite Element ◽  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Mixed Mode ◽  
Crack Tip ◽  
Cyclic Plasticity ◽  
Loading Conditions ◽  
Mixed Mode Loading ◽  
Crack Tip Plasticity

Cyclic plasticity in the crack tip region is at the origin of various history effects in fatigue. For instance, fatigue crack growth in mode I is delayed after the application of an overload because of the existence of compressive residual stresses in the overload’s plastic zone. Moreover, if the overload’s ratio is large enough, the crack may grow under mixed mode condition until it has gone round the overload’s plastic zone. Thus, crack tip plasticity modifies both the kinetics and the crack’s plane. Therefore modeling the growth of a fatigue crack under complex loading conditions requires considering the effects of crack tip plasticity. Finite element analyses are useful for analyzing crack tip plasticity under various loading conditions. However, the simulation of mixed mode fatigue crack growth by elastic-plastic finite element computations leads to huge computation costs, in particular if the crack doesn’t remain planer. Therefore, in this paper, the finite element method is employed only to build a global constitutive model for crack tip plasticity under mixed mode loading conditions. Then this model can be employed, independently of any FE computation, in a mixed mode fatigue crack growth criterion including memory effects inherited from crack tip plasticity. This model is developed within the framework of the thermodynamics of dissipative processes and includes internal variables that allow modeling the effect of internal stresses and to account for memory effects. The model was developed initially for pure mode I conditions. It was identified and validated for a 0.48%C carbon steel. It was shown that the model allows modeling fatigue crack growth under various variable amplitude loading conditions [1]. The present paper aims at showing that a similar approach can be applied for mixed mode loading conditions so as to model, finally, mixed mode fatigue crack growth.

Experiments and Interpretations of some Load Interaction Phenomena in Fatigue Crack Growth Related to Compressive Loading

2014 ◽  
Vol 891-892 ◽  
pp. 1353-1359 ◽  
Author(s):  
Christopher Benz ◽  
Manuela Sander
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Numerical Simulations ◽  
Crack Growth ◽  
Compressive Loading ◽  
Crack Tip ◽  
Interaction Effects ◽  
Loading Conditions ◽  
Compression Loading ◽  
Interaction Phenomena

The present paper addresses two issues regarding the influence of compressive loadings for fatigue crack growth. The first issue is the crack tip loading in compression. It will be verified that Kmaxand R are not suitable to account for compressive loading conditions at the crack tip. The second issue is the investigation of some basic load interaction effects in tension-compression loading. Especially loading conditions that were reported leading to an acceleration of fatigue crack growth were revisited. Numerical simulations of the experiments are used to interpret the results.

scholarly journals Effect of Bond-Line Thickness on Fatigue Crack Growth of Structural Acrylic Adhesive Joints

Materials ◽  
2021 ◽  
Vol 14 (7) ◽  
pp. 1723
Author(s):  
Yu Sekiguchi ◽  
Chiaki Sato
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Fatigue Behavior ◽  
Adhesive Bond ◽  
Bond Line ◽  
Loading Conditions ◽  
Fatigue Crack Growth Resistance ◽  
Line Thickness ◽  
Bond Line Thickness

With an increasing demand for adhesives, the durability of joints has become highly important. The fatigue resistance of adhesives has been investigated mainly for epoxies, but in recent years many other resins have been adopted for structural adhesives. Therefore, understanding the fatigue characteristics of these resins is also important. In this study, the cyclic fatigue behavior of a two-part acrylic-based adhesive used for structural bonding was investigated using a fracture-mechanics approach. Fatigue tests for mode I loading were conducted under displacement control using double cantilever beam specimens with varying bond-line thicknesses. When the fatigue crack growth rate per cycle, da/dN, reached 10−5 mm/cycle, the fatigue toughness reduced to 1/10 of the critical fracture energy. In addition, significant changes in the characteristics of fatigue crack growth were observed varying the bond-line thickness and loading conditions. However, the predominance of the adhesive thickness on the fatigue crack growth resistance was confirmed regardless of the initial loading conditions. The thicker the adhesive bond line, the greater the fatigue toughness.

scholarly journals Study on the Influence of the Gurson–Tvergaard–Needleman Damage Model on the Fatigue Crack Growth Rate

Metals ◽  
2021 ◽  
Vol 11 (8) ◽  
pp. 1183
Author(s):  
Edmundo R. Sérgio ◽  
Fernando V. Antunes ◽  
Diogo M. Neto ◽  
Micael F. Borges
Keyword(s):  
Growth Rate ◽  
Fatigue Crack ◽  
Crack Growth Rate ◽  
Fatigue Crack Growth ◽  
Plastic Strain ◽  
Crack Growth ◽  
Damage Model ◽  
Crack Tip ◽  
Strength Parameter ◽  
Stress Intensity Factor Range

The fatigue crack growth (FCG) process is usually accessed through the stress intensity factor range, ΔK, which has some limitations. The cumulative plastic strain at the crack tip has provided results in good agreement with the experimental observations. Also, it allows understanding the crack tip phenomena leading to FCG. Plastic deformation inevitably leads to micro-porosity occurrence and damage accumulation, which can be evaluated with a damage model, such as Gurson–Tvergaard–Needleman (GTN). This study aims to access the influence of the GTN parameters, related to growth and nucleation of micro-voids, on the predicted crack growth rate. The results show the connection between the porosity values and the crack closure level. Although the effect of the porosity on the plastic strain, the predicted effect of the initial porosity on the predicted crack growth rate is small. The sensitivity analysis identified the nucleation amplitude and Tvergaard’s loss of strength parameter as the main factors, whose variation leads to larger changes in the crack growth rate.

Effective Modeling of Fatigue Crack Growth in Pipelines

2012 ◽  
Author(s):  
Steven J. Polasik ◽  
Carl E. Jaske
Keyword(s):  
Fatigue Crack ◽  
Stress Intensity ◽  
Fatigue Crack Growth ◽  
Fracture Mechanics ◽  
Crack Growth ◽  
Growth Models ◽  
Critical Parameters ◽  
Operating Pressure ◽  
Mechanics Model ◽  
Key Variables

Pipeline operators must rely on fatigue crack growth models to evaluate the effects of operating pressure acting on flaws within the longitudinal seam to set re-assessment intervals. In most cases, many of the critical parameters in these models are unknown and must be assumed. As such, estimated remaining lives can be overly conservative, potentially leading to unrealistic and short reassessment intervals. This paper describes the fatigue crack growth methodology utilized by Det Norske Veritas (USA), Inc. (DNV), which is based on established fracture mechanics principles. DNV uses the fracture mechanics model in CorLAS™ to calculate stress intensity factors using the elastic portion of the J-integral for either an elliptically or rectangularly shaped surface crack profile. Various correction factors are used to account for key variables, such as strain hardening rate and bulging. The validity of the stress intensity factor calculations utilized and the effect of modifying some key parameters are discussed and demonstrated against available data from the published literature.

Finite Element Simulation of Stable Fatigue Crack Growth Using Critical CTOD Determined by Preliminary Finite Element Analysis

2014 ◽  
Vol 891-892 ◽  
pp. 1675-1680
Author(s):  
Seok Jae Chu ◽  
Cong Hao Liu
Keyword(s):  
Finite Element ◽  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Finite Element Simulation ◽  
Crack Growth ◽  
Crack Tip ◽  
Stress Intensity Factor Range ◽  
Crack Tip Opening Displacement ◽  
Element Analysis ◽  
Element Simulation

Finite element simulation of stable fatigue crack growth using critical crack tip opening displacement (CTOD) was done. In the preliminary finite element simulation without crack growth, the critical CTOD was determined by monitoring the ratio between the displacement increments at the nodes above the crack tip and behind the crack tip in the neighborhood of the crack tip. The critical CTOD was determined as the vertical displacement at the node on the crack surface just behind the crack tip at the maximum ratio. In the main finite element simulation with crack growth, the crack growth rate with respect to the effective stress intensity factor range considering crack closure yielded more consistent result. The exponents m in the Paris law were determined.

Characterization of Crack Tip Damage Zone Formation on Alloy 625 During Fatigue Crack Growth at 750°C by Transmission EBSD Method

2017 ◽  
Author(s):  
Yuji Ozawa ◽  
Tatsuya Ishikawa ◽  
Yoichi Takeda
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Power Plants ◽  
Crack Path ◽  
Damage Zone ◽  
Crack Tip ◽  
Loading Frequency ◽  
Zone Formation ◽  
Alloy 625

In order to clarify the mechanism of fatigue crack growth in alloy 625, which is a candidate material for use in advanced ultra supercritical power plants, the crack tip damage zone formation after a crack growth test conducted in high temperature steam was investigated. It was observed that the oxide thickness at the crack tip tended to increase with decreasing cyclic loading frequency. The crack path was a mix of transgranular and intergranular fractures. According to the grain reference orientation deviation (GROD) maps, it was revealed that the density of geometrically necessary dislocations (GNDs) in the matrix along the crack path and ahead of crack tip increased with an increase in the fatigue crack growth rate (FCGR) due to environmental effects. It was observed that (1) mobile dislocations at the crack surface were blocked due to the thick oxide layer, resulting in an increase in the density of GNDs, and (2) an increase in the density of GNDs might induce stress concentration at the crack tip, deformation twinning, and the acceleration of FCGRs.

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