Significance of HAZ Softening on Strain Concentration and Crack Driving Force in Pipeline Girth Welds

2005 ◽  
Author(s):  
Ming Liu ◽  
Yong-Yi Wang ◽  
David Horsley
Keyword(s):  
Driving Force ◽  
Crack Tip ◽  
Functionally Graded ◽  
Weld Strength ◽  
Heat Affect Zone ◽  
Crack Driving Force ◽  
Strain Concentration ◽  
Symmetric Deformation ◽  
Girth Welds ◽  
The Impact

Modern micro-alloyed, control-rolled TMCP steels generally have good strength, high toughness, and good weldability. However, these valuable properties come along with certain undesirable features, such as low strain hardening (high yield to tensile ratio), low ductility as measured by uniform elongation (elongation at ultimate tensile strength), and possible heat-affect-zone (HAZ) softening due to reduced hardenability. These undesirable features are particularly detrimental in strain-based design of pipelines. Although the phenomenon of HAZ softening has been known for a long time, the impact of the HAZ softening on the integrity of pipeline girth welds was not well understood. The objective of this work was to understand the impact of HAZ softening on girth weld integrity. Finite element analysis was conducted to investigate the effects of HAZ softening on crack driving force and strain concentration in girth welds under longitudinal tensile loading. The material properties of WM and BM were obtained from an X100 girth weld. The HAZ was modeled as a functionally graded material based on its measured hardness. The models contained surface-breaking defects located at the fusion boundary simulating lack-of-sidewall fusion defects. The analysis results showed that increased CTOD driving force can be expected due to HAZ softening. The extent of increase is positively related to the width and degree of softening of the HAZ. On the other hand, weld strength overmatch reduces the total CTOD driving force. The strain concentration in the softened HAZ circumferentially remote from a surface-breaking defect was small. However, high strain concentration existed over the circumference covering the length of the defect. This concentration was primarily attributable to the existence of the defect and secondarily to the HAZ softening. One significant result from this work was that the relative increase in CTOD driving force and strain concentration due to HAZ softening was independent of defect size. In other words, on a relative basis, HAZ softening was no worse on large defects than on smaller defects. This result should be helpful in rationalizing the effects of HAZ softening for defects of various sizes that exist in field applications. Non-symmetrical crack-tip deformation occurred with softened HAZ. A large proportion of the crack-tip deformation was located in the HAZ. The magnitude of non-symmetric deformation increased with the increase of HAZ width and degree of softening. Even higher degree of non-symmetric deformation occurred with the increase of weld overmatching level. The structural significance of reduced total CTOD driving force and increased un-symmetric deformation at the crack tip due to weld strength overmatch warrants further study. The reduction in total CTOD driving force alone does not necessarily results in a higher level of weld integrity if the “intrinsic” toughness of the HAZ is substantially lower than the weld metal.

Effects of Weld Strength Heterogeneity on Crack Driving Force in Stress and Strain Based Design Scenarios

2014 ◽  
Author(s):  
Stijn Hertelé ◽  
Noel O’Dowd ◽  
Matthias Verstraete ◽  
Koen Van Minnebruggen ◽  
Wim De Waele
Keyword(s):  
Driving Force ◽  
Large Scale ◽  
Limit State ◽  
Weld Strength ◽  
Force Response ◽  
Crack Driving Force ◽  
Key Factor ◽  
Weld Properties ◽  
Girth Welds ◽  
Strain Hardening Behavior

Weld strength mismatch is a key factor with respect to the assessment of a flawed girth weld. However, it is challenging to assign a single strength mismatch value to girth welds, which are generally heterogeneous in terms of constitutive behavior. The authors have recently developed a method (‘homogenization’) to account for weld strength property variations in the estimation of crack driving force response and the corresponding tensile limit state. This paper separately validates the approach for stress based and strain based assessments. Whereas homogenization is reliably applicable for stress based assessments, the strain based crack driving force response is highly sensitive to effects of actual heterogeneous weld properties. The sensitivity increases with increased weld width and decreased strain hardening behavior. For strain based design, a more accurate methodology is desirable, and large scale testing and/or advanced numerical modeling remain essential.

Weld Strength Mismatch in Strain Based Flaw Assessment: Which Definition to Use?

2013 ◽  
Vol 135 (6) ◽  
Author(s):  
Stijn Hertelé ◽  
Wim De Waele ◽  
Rudi Denys ◽  
Matthias Verstraete ◽  
Koen Van Minnebruggen ◽  
...  
Keyword(s):  
Tensile Strength ◽  
Ultimate Tensile Strength ◽  
Driving Force ◽  
Experimental Validation ◽  
Weld Strength ◽  
Crack Driving Force ◽  
Strain Capacity ◽  
Strength Based ◽  
Key Factor ◽  
Girth Welds

Weld strength mismatch is a key factor in the strain based assessment of flawed girth welds under tension. A strength overmatching weld shields potential flaws within the weld itself from remotely applied deformations and consequently reduces crack driving force. Although this effect has been recognized for decades, different weld strength overmatch definitions exist, and it is not yet fully established which of those is most relevant to a strain based flaw assessment. In an effort to clarify this unsolved question, the authors have performed a large series of parametric finite element analyses of curved wide plate tests. This paper provides an experimental validation of the model and subsequently discusses representative results. It is found that crack driving force is influenced by the shape of the pipe metals' stress–strain curves, which influences the representativeness of two common mismatch definitions (based on yield strength and on ultimate tensile strength). Effects of strength mismatch on strain capacity of a flawed girth weld are best described on the basis of a flow stress, defined as the average of yield and ultimate tensile strength. Based on the observations, a framework for a new strain capacity equation is proposed.

Weld Strength Mismatch in Strain Based Flaw Assessment: Which Definition to Use?

2012 ◽  
Author(s):  
Stijn Hertelé ◽  
Wim De Waele ◽  
Rudi Denys ◽  
Matthias Verstraete ◽  
Koen Van Minnebruggen ◽  
...  
Keyword(s):  
Tensile Strength ◽  
Ultimate Tensile Strength ◽  
Driving Force ◽  
Experimental Validation ◽  
Weld Strength ◽  
Crack Driving Force ◽  
Strain Capacity ◽  
Strength Based ◽  
Key Factor ◽  
Girth Welds

Weld strength mismatch is a key factor in the strain based assessment of flawed girth welds under tension. A strength overmatching weld shields potential flaws within the weld itself from remotely applied deformations and consequently reduces crack driving force. Although this effect has been recognized for decades, different weld strength overmatch definitions exist and it is not yet fully established which of those is most relevant to a strain based flaw assessment. In an effort to clarify this unsolved question, the authors have performed a large series of parametric finite element analyses of curved wide plate tests. This paper provides an experimental validation of the model and subsequently discusses representative results. It is found that crack driving force is influenced by the shape of the pipe metals’ stress-strain curves, which influences the representativeness of two common mismatch definitions (based on yield strength and on ultimate tensile strength). It can be concluded from further observations that effects of strength mismatch on strain capacity of a flawed girth weld are best described on the basis of a flow stress, defined as the average of yield and ultimate tensile strength. Based on the observations, a framework for a new strain capacity equation is proposed.

Estimation of Tensile Strain Capacity of Vintage Girth Welds

2020 ◽  
Author(s):  
Bo Wang ◽  
Banglin Liu ◽  
Yong-Yi Wang ◽  
Otto Jan Huising
Keyword(s):  
Driving Force ◽  
Tensile Strain ◽  
Software Tool ◽  
Ground Movement ◽  
Small Scale ◽  
Crack Driving Force ◽  
Strain Capacity ◽  
Girth Welds ◽  
The Impact ◽  
Tensile Strain Capacity

Abstract Being able to estimate the tensile strain capacity (TSC) of vintage girth welds is sometimes necessary for the integrity management of vintage pipelines. Assessing girth weld integrity could be a top priority after a confirmed ground movement event. Decisions may also be needed about the disposition of a girth weld when weld anomalies are found. Typical fitness-for-service (FFS) procedures, such as API 1104 Annex A and API 579/ASME FFS-1, generally target materials under nominally elastic conditions and strain demands less than 0.2%. These procedures may produce overly conservative results when the strain demand exceeds 0.2%. This paper summarizes the development and validation of a TSC estimation tool for vintage girth welds under PRCI funding. The work consisted of three components: the development of a TSC model for vintage girth welds, the implementation of the model into a software tool, and the experimental validation of the performance of the tool using curved wide plate (CWP) tests. The TSC model was developed following the procedures established through a previous PRCI-PHMSA cofounded work. Finite element analyses (FEA) were performed to obtain a crack-driving force database while considering the salient features of vintage girth welds, such as larger weld caps and weld strength mismatch levels. The TSC model was then derived from the crack-driving force database using apparent toughness values representative of vintage girth welds. A graphical user interface (GUI) and a user manual were developed to facilitate the application of the TSC model. The software tool produces TSC estimates based on geometry, material, loading, and flaw characteristics of a girth weld. For inputs that might not have readily available values, recommended values are provided. The tool allows the evaluation of the impact of various input parameters on TSC. The performance of the TSC estimation tool was evaluated against eight purposely designed CWP tests. Accompanying small-scale material characterization tests, including chemical composition, round bar tensile, microhardness, and Charpy impact tests, were performed to provide additional inputs for the evaluation of the tool. The tool is shown to provide reasonably conservative estimates for TSC. An example problem is presented to demonstrate the application of the tool. Gaps and future work to improve the tool are highlighted at the end of the paper.

Multi-Tier Tensile Strain Models for Strain-Based Design: Part 2 — Development and Formulation of Tensile Strain Capacity Models

2012 ◽  
Author(s):  
Ming Liu ◽  
Yong-Yi Wang ◽  
Yaxin Song ◽  
David Horsley ◽  
Steve Nanney
Keyword(s):  
Driving Force ◽  
Tensile Strain ◽  
Weld Strength ◽  
Experiment Data ◽  
Pipe Wall Thickness ◽  
Crack Driving Force ◽  
Strain Capacity ◽  
Welding Processes ◽  
Tensile Strain Capacity

This is the second paper in a three-paper series related to the development of tensile strain models. The fundamental basis of the models [1] and evaluation of the models against experiment data [2] are presented in two companion papers. This paper presents the structure and formulation of the models. The philosophy and development of the multi-tier tensile strain models are described. The tensile strain models are applicable for linepipe grades from X65 to X100 and two welding processes, i.e., mechanized GMAW and FCAW/SMAW. The tensile strain capacity (TSC) is given as a function of key material properties and weld and flaw geometric parameters, including pipe wall thickness, girth weld high-low misalignment, pipe strain hardening (Y/T ratio), weld strength mismatch, girth weld flaw size, toughness, and internal pressure. Two essential parts of the tensile strain models are the crack driving force and material’s toughness. This paper covers principally the crack driving force. The significance and determination of material’s toughness are covered in the companion papers [1,2].

A Fatigue Crack Driving Force Parameter

2008 ◽  
Author(s):  
Ying Xiong ◽  
Zengliang Gao ◽  
Junichi Katsuta ◽  
Takeshi Sakiyama
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Driving Force ◽  
Crack Tip ◽  
Force Model ◽  
Crack Driving Force ◽  
R Ratio ◽  
Two Parameter ◽  
Better Than

Most of the previous parameters that utilized as a crack driving force were established in modifying the parameter Kop in Elber’s effective SIF range (ΔKeff = Kmax–Kop). This paper focuses on the physical meaning of compliance changes caused by plastic deformation at the crack tip, the test was carried out for structural steel under constant amplitude loading, and differences of several parameter ΔKeff in literature are analyzed quantificationally. The effect of actual stress amplitude at the crack tip on fatigue crack growth is investigated, and improved two-parameter driving force model ΔKdrive(=Kmax)n(ΔK^)1−n) has been proposed. Experimental data for several different types of materials taken from literature were used in the analyses. Presented results indicate that the parameter ΔKdrive is equally effective or better than ΔK(=Kmax-Kmin), ΔKeff(=Kmax-Kop) and ΔK*(=(Kmax)α(ΔK+)1−α) in correlating and predicting the R-ratio effects on fatigue crack growth rate.

Sensitivity of Plastic Response of Defective Pipeline Girth Welds to the Stress-Strain Behavior of Base and Weld Metal

2011 ◽  
Author(s):  
Stijn Hertele´ ◽  
Wim De Waele ◽  
Rudi Denys ◽  
Matthias Verstraete
Keyword(s):  
Tensile Stress ◽  
Yield Stress ◽  
Driving Force ◽  
Uniform Elongation ◽  
Element Model ◽  
Ultimate Tensile Stress ◽  
Weld Strength ◽  
Plastic Strains ◽  
Crack Driving Force ◽  
Strain Behavior

One of the key parameters influencing the acceptability of a pipeline girth weld defect subjected to remote plastic deformation is the strength mismatch between weld and base metal. However, no single definition exists for weld strength mismatch, as it can be defined either on the basis of yield stress, ultimate tensile stress or any intermediate flow stress. To investigate the relevance of such definitions, the authors have performed a series of analyses of curved wide plate tests, using a validated parametric finite element model. The results indicate that, whereas yield stress overmatch determines crack driving force for small plastic strains, ultimate tensile stress overmatch is the more important parameter for advanced plastic strains and determines the eventual failure mode. Further, the strain capacity and exact crack driving force curve are additionally determined by uniform elongation and crack growth resistance.

Limit Load Solution and Crack Driving Force Estimation Scheme for Embedded Flaws in Pipeline Girth Welds

2015 ◽  
Author(s):  
Aurélien Pépin ◽  
Tomasz Tkaczyk ◽  
Noel O’Dowd ◽  
Kamran Nikbin
Keyword(s):  
Driving Force ◽  
Research Work ◽  
Limit Load ◽  
Embedment Depth ◽  
Material Behaviour ◽  
Force Estimation ◽  
Crack Driving Force ◽  
Estimation Scheme ◽  
Girth Welds ◽  
Subsea Pipelines

The acceptability of flaws in a subsea rigid pipeline is usually sanctioned based on the results of an engineering criticality assessment (ECA), carried out considering all loads seen by the pipeline from fabrication to the end of service life. Reel-lay is an efficient installation method, frequently used for installing subsea pipelines. Unlike surface breaking flaws, embedded flaws are not directly assessed in a reeling ECA because the available assessment solutions are too conservative. A work around approach is often used, whereby acceptable surface breaking flaw sizes are deemed acceptable beneath the surface, provided that the embedment depth is equal to or greater than half of the flaw height. However, the results of more recent research work suggest that this approach could be non-conservative in some cases. In this work, a parametric finite-element (FE) study was carried out to assess the effect of the embedment depth, the crack length and the crack height on the load required to cause collapse of the shorter ligament of an embedded flaw. Subsequently, a closed form limit load solution was developed, and compared against available solutions for pipes subjected to tension. A J-based crack driving force (CDF) estimation scheme was developed for a selected material behaviour. Finally, recommendations were made for the direct reeling ECA of subsea pipelines with embedded flaws.

scholarly journals Evaluation of a finite element model for SENT testing of welded connections

2016 ◽  
Vol 7 (1) ◽  
pp. 7
Author(s):  
M. De Visschere ◽  
Sameera Naib ◽  
Wim De Waele ◽  
Stijn Hertelé
Keyword(s):  
Finite Element ◽  
Finite Element Model ◽  
Driving Force ◽  
Element Model ◽  
Potential Effect ◽  
Strength Properties ◽  
Weld Strength ◽  
Force Response ◽  
Crack Driving Force ◽  
Crack Tip Constraint

The SENT test has recently gained popularity for the characterization of the ductile tearing resistance of welded connections under low crack tip constraint. In addition to this practical purpose, Soete Laboratory adopts the SENT test as a tool to investigate effects of weld strength heterogeneity on the crack driving force response of weld defects. The numerical aspect of this investigation relies on a finite element model of a SENT specimen in which the heterogeneous strength properties of the weld region are defined on the basis of an imported hardness map. This paper evaluates the model in two respects. First, crack driving force response is validated on the basis of an experimental SENT test result of a non-welded specimen. Second, the potential effect of the transfer function between hardness and constitutive properties is illustrated. It is concluded that more work is required to improve the feasibility of weld hardness data as a means to characterize effects of weld strength heterogeneity.

Fitness-for-Service Fracture Assessments of Dissimilar Pipe Girth Welds Incorporating New Crack Driving Force Solutions

2014 ◽  
Author(s):  
Rodolfo F. de Souza ◽  
Claudio Ruggieri
Keyword(s):  
Driving Force ◽  
Large Scale ◽  
Structural Integrity ◽  
Numerical Solutions ◽  
Driving Forces ◽  
Limit Load ◽  
Assessment Procedure ◽  
Crack Driving Force ◽  
Alloy 625 ◽  
Girth Welds

The increasing demand for energy and natural resources has spurred a flurry of exploration and production activities of oil and natural gas in more hostile environments, including very deep water offshore production. Currently, structural integrity of submarine risers and flowlines conducting corrosive and aggressive hydrocarbons represents a key factor in operational safety of subsea pipelines. Advances in existing technologies favor the use of CMn steel pipelines (for example, API X65 grade steel) clad or mechanically lined with a corrosion resistant alloy (CRA), such as Alloy 625, for the transport of corrosive fluids. This work focuses on a fitness-for-service defect assessment procedure for strength mismatched welded components incorporating new crack driving force and limit load solutions. The study broadens the applicability of current evaluation procedures for J and CTOD which enter directly into structural integrity analyses and flaw tolerance criteria to provide a fairly comprehensive body of numerical solutions for crack driving forces in mismatched girth welds with circumferential surface cracks. This investigation also provides mismatch yield load solutions which are central to accurately predict failure load in strength mismatched structures subjected to large scale plasticity and ductile behavior. An approach is utilized to analyze the potential effects of the undermatching girth weld on critical flaw sizes for a typical lined pipe employed in subsea flowlines having a girth weld made of Alloy 625.

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