Significance of Biaxial Stress on the Strain Concentration and Crack Driving Force in Pipeline Girth Welds With Softened HAZ

2007 ◽  
Author(s):  
Ming Liu ◽  
Yong-Yi Wang
Keyword(s):  
Driving Force ◽  
Hoop Stress ◽  
Surface Strain ◽  
Biaxial Stress ◽  
Material Anisotropy ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Crack Driving Force ◽  
Loading Sequence ◽  
Longitudinal Strains

The effect of the biaxial stress and HAZ softening on the crack driving force of girth weld defects was investigated using finite element analyses (FEA). The defects were of elliptic shape and located on the inner surface of the pipe. The crack driving force is represented by the crack tip opening displacement (CTOD) normal to the cracked plane (Mode I). The effect of hoop stress on a homogeneous pipe was revisited at first. It was found that the application of hoop stress tends to increase the crack driving force. However, in the practical range of longitudinal strains (≤4.0%), the effects of hoop stress is not monotonic. For example, at a constant longitudinal strain, as the pre-existing hoop stress increases, the driving force may firstly increase then decrease. The combined effect of HAZ softening and biaxial stress was then studied. With the application of hoop stress, the increase of the crack drive force due to HAZ softening was amplified. It was found that the crack driving force can be closely correlated with the surface strain measured over a structurally significant scale right above the defect. In addition, the effects of loading sequence and material anisotropy on the crack driving force were also briefly examined. The increase of the crack driving force from the hoop stress is more pronounced when it is applied prior to the application of longitudinal strains than the reverse loading sequence. The material anisotropy was found to further increase the crack driving force and therefore representative material models are necessary to analyze the anisotropy effects.

Modeling and CMOD Mapping of Surface-Cracked Wide Plates

2008 ◽  
Author(s):  
Da-Ming Duan ◽  
Yong-Yi Wang ◽  
Yaoshan Chen ◽  
Joe Zhou
Keyword(s):  
Test Data ◽  
Driving Force ◽  
Mouth Opening ◽  
Quality Data ◽  
Crack Mouth Opening Displacement ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Crack Driving Force ◽  
Strain Capacity ◽  
Test Conditions

Curved wide plate (CWP) tests are frequently used to measure the tensile stress and strain capacity of pipeline girth welds. The parameters affecting the CWP measurement include specimen geometry and cooling setups. High-quality data is obtained when valid test conditions are confirmed. Crack mouth opening displacement (CMOD) is often measured in CWP tests. CMOD is a direct indicator of the amount of deformation at the cracked plane. It is an indirect indicator of the crack driving force (CDF) imparted on the crack. For a given test geometry and material, certain relationships can be derived between the measured CMOD and the more conventional representation of crack driving force, such as CTOD (crack tip opening displacement) and J-integral. Such relationships are a key element in fracture toughness testing standards. This kind of relationship is also particularly useful in strain-based design where CWP specimens are used for strain capacity and flaw growth prediction. In this paper finite element (FE) analysis is first used in modeling CWP testing conditions for X100 specimens with girth weld flaws to validate the test conditions. A novel approach called CMOD mapping is then developed to characterize the flaw behavior which, by making a direct use of CMOD test data from the CWP tests, is used to estimate the crack growth in the CWP. Finally analysis of strain limits using crack driving force (CDF) for the CWP specimens is also given by comparing experimental test data and FE estimation.

Numerical Analysis of the Crack Driving Force of Mismatched Girth Welded Pipes Subject to Large Plastic Deformations

2021 ◽  
Author(s):  
Kai Wu ◽  
Hong Zhang ◽  
Yue Yang ◽  
Xiaoben Liu
Keyword(s):  
Crack Length ◽  
Driving Force ◽  
Crack Depth ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Plastic Deformations ◽  
Crack Driving Force ◽  
Ground Deformations ◽  
Global Strain ◽  
Large Plastic Deformations

Abstract Strength mismatched pipes with part-through cracks can suffer large plastic deformation from permanent ground deformations caused by geohazards. Thus, the crack driving force involved in engineering critical assessments plays an important role in guaranteeing pipeline integrity when pipes are subjected to complex loads induced by a hostile environment. In this paper, Python scripts are developed to generate up to 200 finite element models of strength mismatched pipes with various crack sizes under large plastic deformations based on the commercial software ABAQUS. The effects of crack length, crack depth, and strength mismatch factors on the evolution of crack tip opening displacement (CTOD) and global strain were investigated. An approximately linear relationship was observed in all cases tested with global strain values varying from 0.5% to 3%. Meanwhile, the value of the CTOD increased with the increase of crack length and crack depth, and decreased with increasing mismatch factor from the undermatch to the overmatch conditions. The effect of the crack depth on the CTOD is comparatively larger than the crack length, which presented an obvious change of the CTOD for deep cracks coupled with undermatched conditions. Overmatched welding only affected the value of CTOD slightly, while a drastic increase of CTOD value was observed for the undermatched welding conditions, especially for deep and long cracks.

Fracture Behaviour of Thin Sheet Stainless Steel

2012 ◽  
Vol 525-526 ◽  
pp. 549-552
Author(s):  
Nenad Gubeljak ◽  
Darko Jagarinec ◽  
Jožef Predan ◽  
John Landes
Keyword(s):  
Fracture Toughness ◽  
Stainless Steel ◽  
Crack Initiation ◽  
Driving Force ◽  
Crack Tip ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Crack Driving Force ◽  
Integrity Assessment ◽  
Crack Tip Opening

The differences in fracture behavior between the compact tension C(T) and the middle tensile M(T) specimens make structure integrity assessment uncertain. Two different types of specimens C(T) and M(T) specimens made from stainless steel have been used for fracture toughness testing at the room temperature by the principles of the ASTM 1820-05 standard procedure. Stable crack initiation and crack propagation occurred for the C(T) specimens at lower values of crack driving force than for the M(T) specimens. Crack tip opening displacement-CTOD has been directly measured on the surface of specimens by using a stereo-optical grading method. The critical crack tip opening displacement at crack initiation CTODi has been measured as a plastic Stretch Zone Width (SZW) during a post test fractographic inspection. Comparison between the CTOD-R curves of both types of specimens shows some difference between the C(T) and the M(T) specimens, but a more significant difference appeared in the crack driving force, as consequence of different constraint (triaxiality) of the C(T) versus the M(T) specimens. Therefore, the result obtained by test on laboratory C(T) specimens cannot be directly used as fracture toughness material properties in a structure integrity assessment, except as a conservative lower bound estimate.

Comparison of Flaw Driving Force Estimates in Simulated Weld Residual Stress Fields

2013 ◽  
Author(s):  
David J. Dewees ◽  
Robert H. Dodds
Keyword(s):  
Residual Stress ◽  
Driving Force ◽  
Three Dimensional ◽  
Crack Size ◽  
J Integral ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Weld Residual Stress ◽  
Future Work ◽  
Crack Tip Opening

Previous work has focused on the methods and results for calculating flaw driving force in simulated three-dimensional (3D) weld residual stress (WRS) fields using contour (J) integral techniques. This paper extends that work to look at explicit modeling of the crack tip opening displacement (CTOD) in these same WRS fields, and for the same range of semi-elliptical flaws. Comparison is made between the predicted trends of driving force with crack size for the calculated driving force (J-integral) versus the “measured” value (CTOD). Implications for fracture assessments are given, and recommendations for future work are made.

Tensile Strain Limits of Buried Defects in Pipeline Girth Welds

2004 ◽  
Author(s):  
Yong-Yi Wang ◽  
Wentao Cheng ◽  
David Horsley
Keyword(s):  
Driving Force ◽  
Tensile Strain ◽  
Slope Instability ◽  
Crack Driving Force ◽  
Factors Affecting ◽  
Soil Movement ◽  
Strain Design ◽  
Girth Welds ◽  
Grade Defect ◽  
Longitudinal Strains

Buried defects, such as lack-of-sidewall fusion defects, are some of the most commonly occurring defects in mechanized girth welds. Although some of the existing ECA (Engineering Critical Assessment) procedures permit the assessment of the significance of buried defects, their application is limited to the nominally elastic applied stress range. The assessment of buried defects is more complex than that of surface-breaking defects. There is much more experimental data on the behavior of surface-breaking defects than buried defects. One simplistic approach is to treat buried defects as surface-breaking defects under a generally accepted assumption that buried defects are less detrimental than surface-breaking defects of the same size. This paper focuses on the behavior of girth welds containing buried defects subjected to high longitudinal strains. The high longitudinal strains in onshore pipelines may be caused by soil movement such as seismic activity, slope instability, frost heave, mine subsidence, etc. For offshore pipelines, the highest longitudinal strains typically occur during pipe laying operations. The paper describes a strain design methodology based on a crack driving force method that has been previously applied to obtain tensile strain limits of surface-breaking defects. The focus of this paper is the application of the crack driving force methodology to examine the factors affecting the strain limits of girth welds containing buried defects. By using crack driving force relations in conjunction with a constraint-sensitive fracture mechanics approach, tensile strain limits are derived as a function of material grade, defect size, toughness, and pipe wall thickness. The paper concludes with the comparison of strain limits between buried and surface-breaking defects.

The Effect of Pressure Induced Hoop Stress on Bi-Axially Loaded through Wall Cracked Cylindrical Structures – A Strain Based Method

2011 ◽  
Vol 110-116 ◽  
pp. 1525-1530
Author(s):  
M.V.N. Sivakumar ◽  
B. N. Rao ◽  
S. R. Satishkumar
Keyword(s):  
Finite Element ◽  
Internal Pressure ◽  
Hoop Stress ◽  
Pipeline Steel ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Ductile Tearing ◽  
Effect Of Pressure ◽  
Bending Load ◽  
Axially Loaded

This paper presents a simplified strain-based fracture mechanics approach to study the effect of pressure induced hoop stress on bi-axially loaded through walled cracked (TWC) pipes subjected to an external bending load in combination with internal pressure. Elastic-plastic finite element analyses are conducted to establish the relation between global strain and Crack tip opening displacement (CTOD). In the finite element model X65 pipeline steel is considered using power-law idealization of stress-strain, and the inelastic deformations, including ductile tearing effects, are accounted for by use of the Gurson–Tvergaard–Needleman model. Several parameters are taken into account, such as crack length, internal pressure and material hardening. Strain based crack driving force equation is used and maximum load criterion is adopted to determine the critical strain from ductile tearing in the cracked pipeline. The results suggest that presence of pressure-induced hoop stresses increases the fracture response in high-hardening materials and their effects are significant due to large plastic-zone size.

Low-Constraint Back-Bend Test of High Strength Steels: Crack Driving Force Calibration

2006 ◽  
Author(s):  
Ming Liu ◽  
Yong-Yi Wang
Keyword(s):  
Laboratory Test ◽  
Driving Force ◽  
Laboratory Tests ◽  
Finite Thickness ◽  
Crack Tip ◽  
Correlation Equation ◽  
Crack Tip Opening Displacement ◽  
Crack Driving Force ◽  
Wide Range ◽  
Low Constraint

It has been well-established that the experimentally measured toughness of materials depends on the crack-tip constraint levels. Accurate assessment of the integrity of real structures requires that the laboratory tests be conducted at similar constraint levels as those experienced by the structures. Conventional laboratory tests are usually designed at high constraint levels to obtain “conservative” toughness values. However, pipelines usually experience low-constraint loads; therefore the assessment results using the conventional laboratory test data can be overly conservative. Back-bend specimen is designed as a low-constraint laboratory test. To obtain the fracture toughness from the test, it is necessary to develop a correlation between the crack driving force, i.e. the crack tip opening displacement (CTOD), and the overall load and displacement. A semi-analytical correlation equation for back-bend tests is presented in this paper. The equation is based on the slip-line theory which was originally developed for rigid-perfectly plastic materials under plane strain conditions. The equation has been extended to take account of the elasticity, yield strength, and strain hardening of the materials. The geometry factors such as the ligament thinning and finite thickness are also investigated. The predicted CTOD driving force by the correlation equation shows a good match with the finite element calculations for a wide range of material properties and specimen dimensions.

Crack Driving Force in Anisotropic Media due to Non-Elastic Deformation

2004 ◽  
Vol 261-263 ◽  
pp. 75-80
Author(s):  
G.H. Nie ◽  
H. Xu
Keyword(s):  
Elastic Deformation ◽  
Driving Force ◽  
Strain Energy ◽  
Elastic Stress ◽  
Complex Function ◽  
Crack Driving Force ◽  
Special Cases ◽  
Degree Of Anisotropy ◽  
Minimum Strain Energy ◽  
Orthotropic Media

In this paper elastic stress field in an elliptic inhomogeneity embedded in orthotropic media due to non-elastic deformation is determined by the complex function method and the principle of minimum strain energy. Two complex parameters are expressed in a general form, which covers all characterizations of the degree of anisotropy for any ideal orthotropic elastic body. The stress acting on the long side of ellipse can be considered as a crack driving force and applied in failure and fatigue analysis of composites. For some special cases, the resulting solutions will reduce to the known results.

Experimental Study on Fracture Toughness of Q460C the High-Strength Construction Steel at Low Temperature

2011 ◽  
Vol 71-78 ◽  
pp. 890-897 ◽  
Author(s):  
Yuan Qing Wang ◽  
Yun Lin ◽  
Yan Nian Zhang ◽  
Yong Jiu Shi
Keyword(s):  
Low Temperature ◽  
High Strength ◽  
Crack Tip Opening Displacement ◽  
Opening Displacement ◽  
Three Point Bending ◽  
Function Fitting ◽  
Ductile Brittle Transition Temperature ◽  
Boltzmann Function ◽  
Fracture Appearance ◽  
Crack Tip Opening

Three point bending tests were carried out on 14mm-thick Q460C the high-strength structural steel at low temperature, and scanning electronic microscope of the fracture appearance was analyzed. The results showed that the obvious feature of brittle mechanism was shown on the three point bending specimen fracture whose testing took place at -40°C. And the crack tip opening displacement value of Q460C steel, which was less than that of Q235 steel, Q345 steel and Q390 steel at low temperature, tended to decrease with respect to the temperature reduction. Moreover, a Boltzmann function fitting analysis was applied to the experimental data, and the ductile-brittle transition temperature and the changing regularity were obtained.

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].

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