Weld Strength Mismatch Research Articles

Evaluation of strain based ECA method in DNVGL-RP-F108

Pipelines may be subject to strain-controlled external loads exceeding yield strength of materials due to many causes, e.g., soil settlements and ground movement. There are several strain based fracture mechanics assessment methods to determine the maximum tolerable defect sizes in girth welds in pipelines. One of these methods is the DNVGL-RP-F108 procedure which was developed from a joint industry project for assessing reeled pipes and has been used for both offshore and on-shore pipelines. However, as with other methods, it requires the weld strength either even- or over-matching the parent steel which may be difficult to achieve for high strength steels. There are also uncertainties about the adequacy of the DNVGL-RP-F108 guidance addressing the effects of internal pressure and welding-induced residual stresses. In this study, the maximum tolerable defect sizes in the girth welds of an X70 spiral welded pipeline were determined using the DNVGL-RP-F108 approach. The effects of several variables on the assessment results, including weld strength mismatch, stress-strain curve magnitude, internal pressure level, fracture toughness, residual stresses and misalignment, were also investigated. A comprehensive finite element analyses were performed to verify the crack driving force determined by the DNVGL-RP-F108 approach. It was found that the DNVGL-RP-F108 approach can be conservatively applied to girth welds with a weld strength under-match by 5% in all cases, and even by 10% for most cases, for the conditions investigated. It was also revealed that use of this guidance could be unduly conservative for strength over-matching welds.

Tensile Strain Capacity Prediction Model of an X80 Pipeline with Improper Transitioning and Undermatched Girth Weld.

As an important component of strain-based design, the tensile strain capacity (TSC) concept has been extensively used for pipelines that experience expectable plastic strain for both installation and service. However, some stress-based designed pipelines have experienced unforeseen plastic strain in the past decade that resulted in failure. It seems that the tensile strain capacity has gradually become an important requirement for geohazard risk management and pipeline maintenance of stress-based design pipelines. The tensile strain capacity of an X80 pipeline is investigated. The assessment in this work was based on the fracture initiation–control-based limit state. This limit state corresponds to the onset of stable tearing and generally provides a reasonably conservative estimate. Besides that, factors such as wall thickness, material’s strain hardening capacity, toughness, weld strength mismatch, HAZ (heat-affected zone) softening, pipe wall thickness, high–low misalignment, and internal pressure were also investigated to construct a prediction model of the X80 vintage pipeline.

Investigation of CMOD based η factors for clamped SET specimens with mismatched weld based on finite element analyses

This work studies the J-integral of weld centerline crack using clamped single-edge notched tension (SETc) specimens. Extensive finite element analyses (FEA), covering both plane-strain and three-dimensional SETc specimens with mismatched weld, are carried out to evaluate plastic η factors for estimating the J-integral based on load–displacement curve. A wide range of specimen configurations and material properties, including different crack size, specimen width, weld width, strain hardening exponent, weld strength mismatch and the ratio of elastic modulus to yield stress, is considered in present FEA. Based on the results obtained from FEA, the effects of specimen configurations and material properties on plastic η factors are examined and a set of fitting formulae for the plastic η factor is proposed. The proposed η factors are applicable in improved estimation of J-integral for welded SETc specimens with weld centerline crack.

Influence of weld joint geometry and strength mismatch on load bearing capacity of API pipeline

The load bearing capacity of pipeline weld joints with varying wall thicknesses and strength mismatch was assessed using finite element analyses. The structural integrity of API X65-API X80, API X42-API X65, and API X42-API X80 weld joints was investigated under tensile, internal pressure and bending load.Pipe joint combinations with an outer diameter of 762 mm (30 inches) had wall thickness ratios of 1.22, 1.54, and 1.89. The pipe joint's end face with different wall thicknesses was machined with a taper angle of 4°∼45° along the longitudinal direction of the low strength and thick pipe.According to parametric studies, the tensile strength and moment of a pipe joint with a wall thickness ratio less than 1.5 were unaffected by the wall thickness ratio and taper angle; however, those of a pipe joint with a wall thickness ratio greater than 1.5 were significantly reduced at low taper angles. The wall thickness ratio and taper angle had no effect on the failure pressure of the pipe joint with different wall thicknesses.

Revealing the fatigue crack propagation mechanism of a Ni-based superalloy electron beam welded joint through in-situ SEM observation

In this study, in-situ scanning electron microscope (SEM) fatigue crack propagation experiments for electron beam welded joints of GH4169 Ni-based superalloy are carried out. Based on material characterization and analysis, the corresponding fatigue crack propagation mechanism of the welded joint (WJ) is investigated. Compared to the base material (BM), the WJ has comparable static strength, but worse plasticity. Significantly, the WJ has longer fatigue crack growth life than that of the BM. The fatigue crack growth rate (FCGR) curves of the WJ and the BM intersected with each other. In particular, under low stress intensity factors (ΔK) level, the FCGR of the WJ is lower than that of the BM. Under high ΔK level, the FCGR of the WJ is higher than that of the BM. The steep FCGR curve of the WJ can be attributed to the unique weld texture and a shielding effect caused by the weld strength mismatch.

Effects of Fixture Configurations and Weld Strength Mismatch on J-Integral Calculation Procedure for SE(B) Specimens.

This work presents the development of a J-integral estimation procedure for deep and shallow cracked bend specimens based upon plastic ηpl factors for a butt weld made in an S690 QL high strength low alloyed steel. Experimental procedures include the characterization of average material properties by tensile testing and evaluation of base and weld metal resistance to stable tearing by fracture testing of square SE(B) specimens containing a weld centerline notch. J-integral has been estimated from plastic work using a single specimen approach and the normalization data reduction technique. A comprehensive parametric finite element study has been conducted to calibrate plastic factor ηpl and geometry factor λ for various fixture and weld configurations, while a corresponding plastic factor γpl was computed on the basis of the former two. The modified ηpl and γpl factors were then incorporated in the J computation procedure given by the ASTM E1820 standard, for evaluation of the plastic component of J and its corresponding correction due to crack growth, respectively. Two kinds of J-R curves were computed on the basis of modified and standard ηpl and γpl factors, where the latter are given by ASTM E1820. A comparison of produced J-R curves for the base material revealed that variations in specimen fixtures can lead to ≈10% overestimation of computed fracture toughness JIc. Furthermore, a comparison of J-R curves for overmatched single-material idealized welds revealed that the application of standard ηpl and γpl factors can lead to the overestimation of computed fracture toughness JIc by more than 10%. Similar observations are made for undermatched single material idealized welds, where fracture toughness JIc is overestimated by ≈5%.

Prediction of fully plastic J-integral for weld centerline surface crack considering strength mismatch based on 3D finite element analyses and artificial neural network

This work mainly focuses on determination of the fully plastic J-integral solutions for welded center cracked plates subjected to remote tension loading. Detailed three-dimensional elastic–plastic Finite Element Analyses (FEA) were implemented to compute the fully plastic J-integral along the crack front for a wide range of crack geometries, material properties and weld strength mismatch ratios for 900 cases. According to the database generated from FEA, Back-propagation Neural Network (BPNN) model was proposed to predict the values and distributions of fully plastic J-integral along crack front based on the variables used in FEA. The determination coefficient R2 is greater than 0.99, indicating the robustness and goodness of fit of the developed BPNN model. The network model can accurately and efficiently predict the elastic-plastic J-integral for weld centerline crack, which can be used to perform fracture analyses and safety assessment for welded center cracked plates with varying strength mismatch conditions under uniaxial loading.

Fracture resistance testing of dissimilar nickel–chromium girth welds for clad line pipes

This work presents an experimental investigation of the ductile tearing properties for the girth weld of a typical C–Mn pipe internally clad with ASTM UNS N06625 Alloy 625 using measured crack growth resistance curves (\(J{-}\Delta a\) and \(\mathrm {CTOD}{-}\Delta a\) curves). Here, the material of the external pipe is a typical API 5L Grade X65 pipeline steel whereas the inner clad layer is made of a nickel–chromium corrosion resistant alloy steel. Testing of the girth weld employed side-grooved, clamped SE(T) specimens with a weld centerline notch to determine the crack growth resistance curves based upon the unloading compliance method using a single specimen technique. This experimental characterization provides additional toughness data which serve to evaluate the effectiveness of current procedures in determining accurate experimentally measured R-curves for this class of material, including the effects of weld strength mismatch.

A framework for fracture assessments of dissimilar girth welds in offshore pipelines under bending

This work describes the development of a framework for structural integrity assessments of dissimilar girth welds in clad and lined pipes subjected to high levels of bending load for a wide range of pipe geometries, crack configurations, weld grooves and weld strength mismatch levels based upon an EPRI estimation scheme for crack driving forces coupled with a weld groove simplification procedure and the equivalent stress–strain relationship. Systematic validation of the method is provided and the crack driving force estimation accuracy is demonstrated through comparisons with numerical tools and 3-D finite element analyses for reeled pipelines.

The study of weld strength mismatch effect on limit loads of part surface and embedded flaws in plate

Strength mismatching between weld metal and parent material is common in pressure vessel and piping industry. To carry out the fracture assessment of weld defects, it is required to accurately estimate the reference stress or limit load for the flawed section. Weld strength mismatch introduces complexity in determination of the limit load. Although it is acceptable to use the weaker material properties in the assessment, it can be overly conservative. Industrial standards R6 and BS7910 provide the methods for weld strength mismatch, but no limit load ratio solutions are available for semi-elliptical surface and embedded flaws which are the most common cracks in pressure vessels and pipework. It is useful to understand whether the existing limit load ratio solutions in R6 and BS7910 can be used for part surface and embedded flaws. This paper reviews the assumptions used in the existing reference stress solutions and discusses its suitability for mismatch conditions. The effects of weld strength mismatch on limit loads of the part surface and embedded flaws are studied using 3D elastic-plastic finite element analyses. The results are compared with the R6 and BS7910 solutions. The validity of the handbook solutions of through-wall flaws for part surface and embedded flaws in plate and the effects of weld strength mismatch and weld strip width on limit loads are discussed.

CTOD estimation procedure for clad pipe girth welds subjected to bending load

This work describes a crack driving force estimation procedure applicable for structural integrity assessments of dissimilar girth welds in clad pipes subjected to high levels of bending load. The purpose of this study is to explore the application of the equivalent stress-strain relationship approach coupled with a weld bevel simplification scheme and the EPRI procedure to evaluate the CTOD for defects located at the weld centerline, including the effects of the weld strength mismatch, weld bevel geometry and the clad layer thickness. Parametric 3D finite element analyses of pipes with circumferential part-through cracks are performed to validate the weld joint simplification and to assess the accuracy of the crack driving forces predictions. Finally, the results show that the estimations using the proposed methodology are in close agreement to those obtained directly from finite element analyses

Mechanical Properties Variations and Comparative Analysis of Dissimilar Metal Pipe Welds in Pressure Vessel System of Nuclear Plants

The experimental investigations of two dissimilar metal weld (DMW) joints between SA508Gr.3Cl.1 ferritic steel and SS304LN austenitic stainless steel using Inconel 82/182 (ERNiCr-3/ENiCrFe-3) and Inconel 52/152 (ERNiCrFe-7/ENiCrFe-7) filler metals have been conducted in the present work. The integrity assessment of DMW joints and the mechanical properties variations has made pertaining to ASME Section-III and Section-IX. Mechanical tests comprising bend test, transverse tensile test (TTT), tensile test, Charpy impact test, microhardness measurement have been carried out along with microstructural evolution using the standard test specimens according to respective ASTM standards. Bend tests have shown that interfaces of the SA508–Inconel, Inconel–Inconel, and Inconel–SS304LN are free from any lack of fusion or cracks. TTTs have shown that failures of the specimens are from the SS304LN indicating integrity of the weld joint. Tensile tests confirm that tensile strength of the different regions agreed the required strength as per ASME Section-II. The weld strength mismatch and plastic instability strength (PIS) are found to be important factors during integrity assessment of joints. Based on the comparative investigations, owing to better mechanical properties, Inconel 82/182 filler metals could be an optimum choice over Inconel 52/152 filler metals for present DMW joints required in pressure vessel system of nuclear plants.

Framework for Key Influences on Tensile Strain Capacity of Flawed Girth Welds

A key influence factor in the strain-based assessment of pipeline girth weld flaws is weld strength mismatch. Recent research has led to a framework for tensile strain capacity as a function of weld flow stress (FS) overmatch. This framework is built around three parameters: the strain capacity of an evenmatching weldment, the sensitivity of strain capacity to weld FS overmatch, and the strain capacity at gross section collapse (GSC). A parametric finite element study of curved wide plate (CWP) tests has been performed to identify the influence of various characteristics on each of these three parameters. This paper focuses on flaw depth, tearing resistance of the weld, stress–strain behavior of the base metal, and weld geometry. Influences of these characteristics are mostly found to be limited to one or two of the three framework parameters. A preliminary structure is proposed for equations that further develop the strain capacity framework.

Limit Loads of Surface and Embedded Flaws in Plate with Weld Strength Mismatch

Strength mismatching between weld metal and parent material is common in pressure vessel industry. To carry out the fracture assessment of weld defects, it is required to accurately estimate the reference stress or limit load for the flawed section. Weld strength mismatch introduces complexity in determination of the limit load. Although it is acceptable to use the weaker material properties in the assessment, it can be overly conservative. Industrial standards R6 and BS7910:2013 provide the methods for weld strength mismatch, but no limit load ratio solutions are available for semi-elliptical surface and embedded flaws which are the most common cracks in pressure vessels and pipework. It is useful to understand whether the existing limit load ratio solutions in R6 and BS7910 can be used for surface and embedded flaws. This paper reviews the assumptions used in the existing reference stress solutions and discusses its suitability for mismatch conditions. The effects of weld strength mismatch on limit loads of the surface and embedded flaws are studied using 3D elastic-plastic finite element analyses. The results are compared with the R6 and BS7910 solutions. The validity of the handbook solutions of through-wall flaws for surface and embedded flaws in plate and the mismatch effect on limit loads are discussed.

J-dominance and size requirements in strength-mismatched fracture specimens with weld centerline cracks

The strong dependence of crack-tip fields on specimen geometry and remote loading, particularly for moderate-to-low hardening materials under LSY conditions underlies the constraint loss phenomenon and invalidates the one-parameter characterization of unstable cracking behavior in terms of the \(J\)-integral. Previous research efforts have enabled establishing size requirements for valid \(J\) fracture toughness testing based on the dominance of the HRR singularity which are essentially applicable to homogeneous materials. Only little effort has been expended in the past years to extend the one-parameter characterization of crack-tip conditions to welded structural components. The present work addresses the coupling effect of specimen geometry and weld strength mismatch on the crack-tip deformation limits for which \(J\)-dominance is valid in weld centerline fracture specimens. Very detailed nonlinear finite element analyses of plane-strain models for common specimen geometries, including deep and shallow crack bend SE(B) and tension SE(T) fracture specimens, with weld centerline cracks enable resolving the required crack-tip stress fields from which the variation of crack-tip constraint with increased values of \(J\) can be assessed. The present study provides further insight into the coupled effects of specimen geometry and weld strength mismatch on the levels of deformation limit defined by \(M=(b\sigma _{0}^{WM})/J\), where \(b\) is the remaining crack ligament and \(\sigma _{0}^{WM}\) denotes a reference (yield) stress for the weld material, at which a one-parameter characterization of crack-tip stress fields still remains valid.

Engineering approach for circumferential flaws in girth weld pipes subjected to bending load

The current investigation pursues extending the applicability of fully plastic solutions for J-integral and crack-tip opening displacement (CTOD), originally cataloged in a series of handbooks (also known as EPRI handbooks), in defective girth weld pipes subjected to bending load for a wide range of surface crack dimensions and weld strength mismatch levels. The suitability of the given set of solutions in this work is constrained by a certain number of parameters that are derived from the coupled effect of weld strength mismatch and configurational effects upon near-tip stress-strain fields in heterogeneous media. These results reveal a weak dependence of coupled effect of weld strength mismatch and weld groove size upon crack driving force, when the ratio of the mismatch between weld and base normalizing stresses is considered moderate (≤1.3), whereby the proposed estimation method becomes essentially valid for failure assessment procedures and fitness-for service (FFS) evaluations.

An insight of the structure of stress fields for stationary crack in strength mismatch weld under plane strain mode-I loading – Part II: Compact tension and middle tension specimens

In-service inspections of many nuclear power plants have revealed that cracks are most likely to occur in a weld or in the regions near the weld. In part-I, the problem of a stationary crack lying at the centre of a weld in a pure bending specimen SE(PB) was analysed. The detailed structure of the global plastic fields for a deep crack, under fully plastic condition, was presented. Aspects related to the state of stress at the base-weld interface were discussed. To enhance our understanding of the weld strength mismatch effects, other commonly used fracture specimens, that is, compact tension C(T) and middle tension M(T) specimens having a weld centre crack were analysed in the present investigation. The influence of weld mismatch on the structure of global stress fields (leading to plastic yielding of the ligament) as well as on the crack tip constraint was studied. It is demonstrated that, when a crack is postulated at the centre of a weld, a family of stress fields proposed in part-I for a SE(PB) specimen is applicable to a C(T) specimen also. The studies performed in this article, along with part-I, have established that in comparison to slip line field analysis, the modified upper bound theorem is simple and more general. It is applicable to macroscopically homogeneous materials and can also account for weld mismatch effects.

Characterization of crack tip stresses in plane-strain fracture specimens having weld center crack

Fracture toughness of metals depends strongly on the state of stress near the crack tip. The existing standards (like R-6, SINTAP) are being modified to account for the influence of stress triaxiality in the flaw assessment procedures. These modifications are based on the ability of so-called ‘constraint parameters’ to describe near tip stresses. Crack tip stresses in homogeneous fracture specimens are successfully described in terms of two parameters like J–Q or J–T. For fracture specimens having a weld center crack, strength mismatch ratio between base and weld material and weld width are the additional variables, along with the magnitude of applied loading, type of loading, and geometry of specimen that affect the crack tip stresses. In this work, a novel three-parameter scheme was proposed to estimate the crack tip opening stress accounting for the above-mentioned variables. The first and second parameters represent the crack tip opening stress in a homogeneous fracture specimen under small-scale yielding and are well known. The third parameter accounts for the effect of constraint developed due to weld strength mismatch. It comprises of weld strength mismatch ratio (M, i.e. ratio of yield strength of weld material to that of base material), and a plastic interaction factor (Ip) that scales the size of the plastic zone with the width of the weld material. The plastic interaction factor represents the degree of influence of weld strength mismatch on crack tip constraint for a given mismatch ratio. The proposed scheme was validated with detailed FE analysis using the Modified Boundary Layer formulation.

Influence of Weld Strength Mismatch on Crack Length Measurement Using Compliance Technique

Evaluation of crack growth during fracture and fatigue-crack-growth test using unloading compliance technique is well established for standard homogeneous fracture specimens. Three Point Bend specimens are widely used to measure fracture toughness and fatigue-crack-growth rates in metallic materials. A compliance functions is given in ASTM E1820-09 [5] to find out the in-situ crack length based on crack mouth opening displacement (CMOD). One of the inpute required in compliance function is the young's modulus of elasticity of the specimens material. To quantify the fracture toughness of weld joints, specimens are machined from weldments. Such specimens comprises of both weld and base materials. In general, base and weld materials have different young's modulus of elasticity (E). ASTM (E-1820) method doesn’t account for the influence of mismatch in “E” values of base and weld material on the compliance function.In this work, an effective modulus of elasticity is proposed for TPB specimens having weld center crack to account for the influence of mismatch in “E” values of base and weld material and geometry of weld on the measurement of crack length using ASTM compliance function. Numerical studies were carried out on TPB specimens. Compliances were calculated by linear elastic 3D Finite Element analysis. Wide range of relative crack length (a/w) varying from 0.3 to 0.7 and practical range of weld width were considered. Crack lengths calculated based on proposed effective modulus of elasticity were compared with the results obtained from ASTM (E-1820) technique (applicable to homogeneous specimens). Our studies revealed that, in comparison to ASTM technique, the proposed scheme for the TPB specimens having weld centre crack allows a very accurate evaluation of actual crack size.

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

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.