Effect Of Strength Mismatch Research Articles

Effect of strength mismatch on limit load of dissimilar metal welded joint for connecting pipe-nozzle of nuclear pressure vessel

The interface region cracks in dissimilar metal welded joint (DMWJ) for connecting pipe-nozzle of a reactor pressure vessel (RPV) to safe end are important concerned cracks in fracture assessment. In this work, based on a large amount of finite element analyses, the effect of strength mismatch on the limit loads of two interface cracks in an actual DMWJ has been investigated for wide ranges of strength mismatch factors and crack sizes. The results indicate that for shallower cracks, the limit load is dominated by fully yielding of the safe end pipe and are independent of strength mismatch and crack size. For deeper and longer cracks, the limit load is dominated by fully yielding of the ligament area materials. As the mismatch factor M increases from undermatch (M < 1) to overmatch (M > 1), the limit loads increase. To improve the load-bearing capacity of the DMWJ and prevent plastic collapse failure, a high degree of strength overmatch (increasing strength of buttering and weld metals) should be designed and adopted. The normalized limit load solutions as functions of mismatch factor and crack size have been obtained for the two interface cracks.

Effects of material properties and mismatch on unified constraint parameter

In order to develop unified constraint-based fracture assessment method, it needs to investigate the effects of material properties and mismatches on the unified constraint parameter and develop the parameter solutions for cracked structures. In this work, the effects of material properties (yield strength and hardening exponent) and strength mismatch on the in-plane and out-of-plane unified constraint parameter Ad* of semi-elliptical surface cracks in homogeneous and welded plates have been investigated. The results show that the parameter Ad* does not essentially change with yield strength and hardening exponent of materials for cracks in homogeneous plates under bending and tension loads. The parameter Ad* of weld centerline cracks in welded plates under bending and tension loads is essentially independent on the strength mismatch. These advantages of Ad* are conducive to the development of its solutions for homogeneous and welded structures containing cracks. The effect of weld width on Ad* is related to loading mode, mismatch (undermatch or overmatch) and crack size. The mechanism of the influence of material properties, strength mismatch and weld width on Ad* has been explained by its definition and the crack-tip plastic strain distribution. The development and application of Ad* solutions for homogeneous and welded structures also have been discussed.

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%.

Fatigue failure transition analysis of load carrying cruciform welded joints by extended local approach

This study was mainly concerned with the fatigue crack initiation points transition relationship between the weld toe and weld root in Load-carrying Cruciform Weld Joint (LCWJ) considering the mismatched filler material yield strength and geometrical sizes under cyclic loading conditions. The critical failure locations were investigated by the experimental tests and numerical simulations. To analyze the fatigue characteristic indicators of the potential fatigue initiations, different configurations of mismatch ratios and local geometries (plate thickness, weld size and penetration ratio) were designed to conduct the fatigue tests and cyclic calculations of CLWJ. Subsequently, New uniform effective notch energy indicator was proposed to characterize the low and high cycle fatigue life by extending the SED method. The related analytical formulations for weld toe and weld root were established to predict the low and high cycle fatigue assessment indicator of LCWJ considering the effects of plasticity and yield strength mismatch and geometry configurations. The effective notch energy relationship between the weld toe and weld root in LCWJ was estimated by the established analytical solutions.

Strength mismatch effect on residual stress of 10CrNi3MoV steel considering the back-chipping process

The paper investigates the Welding Residual Stresses (WRS) of 10CrNi3MoV high strength steel multipass butt welded joints by experimental measurement and numerical calculation considering the effects of weld metal strength mismatch and back-chipping processing. Firstly, a thermo-metallurgical-mechanical simulation model was developed incorporating the Effect of Solid-State Phase Transformation (SSPT) to obtain multipass welding residual stress distributions after the back-chipping processing for evenmatched filler material. Phase transformation modeling of diffusionless transformation kinetics based on the Koistinen-Marburger formula was performed by a user subroutine implemented in ABAQUS. Additionally, strain transformation calculation due to volumetric and heating change during the SSPT was completed by the user-defined subroutine. Then, the effects of weldment mismatch and back-chipping processing on WRS for the multipass welds were analyzed. Comparing the calculated residual stress results (with and without consideration of SSPT) with experimental data shows that more accurate WRS predictions are obtained when the SSPT effect is incorporated in the simulation procedure. For evenmatched welded joints, the compressive WRS were distributed in the top and back weld layer due to the involvement of SSPT behavior. While the considerable tensile WRS in undermatched weldments was obtained without considering the SSPT, the magnitudes were lower than those in Heat Affected Zone (HAZ). The proposed finite element model can predict the WRS of high-strength steel multipass welded joints and provide a theoretical basis for conducting the structural integrity assessment.

Effect of Mechanical Heterogeneity on Strain and Stress Fields at Crack Tips of SCC in Dissimilar Metal Welded Joints.

The crack tip strain and stress condition are one of the main factors affecting stress corrosion cracking (SCC) behaviors in the dissimilar metal welded joint of the primary circuit in the pressurized water reactor. The mechanical property mismatch of base metal and weld metal can significantly affect the stress and strain condition around the crack tip. To understand the effect of different weld metals on strain and stress fields at SCC crack tips, the effects of strength mismatch, work hardening mismatch, and their synergy on the strain and stress field of SCC in the bi-material interface, including plastic zone, stress state, and corresponding J-integral, are investigated in small-scale yielding using the finite element method. The results show a significant effect of the strength mismatch and work hardening mismatch on the plastic zone and stress state in the weld metal and a negligible effect in the base metal. J-integral decreases with the single increase in either strength mismatch or work hardening mismatch. Either the increase in strength mismatch or work hardening mismatch will inhibit the other’s effect on the J-integral, and a synthetic mismatch factor can express this synergistic effect.

Fatigue crack growth behavior of Ni-Cr-Mo-V steel welded joints considering strength mismatch effect

High strength steel welded joints are usually fabricated by heterogeneous weld metal for reducing the negative impacts of microstructural characteristics and defects on mechanical properties, such as hydrogen embrittlement cracking, joint toughness decreasing, etc. To reflect material heterogeneity in the mechanical characteristics, two kinds of weld filler material are selected to obtain Evenmatched Welded Material (E-WM) and Undermatched Welded Material (U-WM) marine Ni-Cr-Mo-V steel welded joints. The Fatigue Crack Growth (FCG) behaviors of Base Metal (BM) and related welded joints are investigated considering load ratio (0.1, 0.4, 0.7) and specimen state (as-welded and heat-treated) effects. The experimental FCG trends for BM and weldments have been compared with the trends available in standards. The FCG rate (da/dN) results show the U-WM demonstrates higher FCGR curves than BM and E-WM. Additionally, it demonstrates no significant difference about FCGR for E-WM and U-WM under high R-ratios (0.4 and 0.7). Moreover, both E-WM and U-WM in as-welded state presents higher fatigue crack propagation resistance than in the Post Welded Heat Treatment (PWHT) state. The fracture mechanism of FCG was analyzed according to the fatigue crack trajectory in microstructures and fractography. Transgranular fracture behavior were observed with some secondary particles in E-WM, while the intergranular fracture was exhibited with few tiny secondary microcracks in U-WM.

Strength mismatch effect on residual stress of high strength steel butt-welded joints

This study was mainly concerned with the residual stresses of 10CrNi3MoV high strength steel multipass butt welded joints by experimental measurement and numerical calculation considering the weld metal strength mismatch effect. Firstly, the Solid-State Phase Transformation (SSPT) processing of weldment was described by the thermo-metallurgical-mechanical simulation model. The residual stress distribution on the basis of the established procedure. Phase transformation modeling of diffusionless transformation kinetics based on the Koistinen-Marburger formula was performed by a user subroutine implemented in ABAQUS. Additionally, strain transformation calculation due to volumetric and heating change during the SSPT was completed by the user-defined subroutine. The results show that more accurate predictions of welding residual stress are obtained when the SSPT effect is incorporated in the simulation procedure. For evenmatched welded joints, the compressive residual stresses were distributed in the top and back weld layer due to the involving of SSPT behavior. The proposed finite element model can predict residual stress of high strength steel multipass welded joints and provide a theoretical basis to conduct the structural integrity assessment.

Strain-Based Failure Assessment Based on a Reference Strain Method for Welded Pipelines

Abstract Engineering critical assessment (ECA) is an evaluation procedure for structures with flaws and has been widely applied for assessing pipeline integrity. The standards for structural integrity assessment, including BS 7910, involve stress-based ECA, and they are known to produce overly conservative results. Therefore, strain-based ECA has been recently developed as an alternative approach. One of the effective methods for improving the accuracy of strain-based ECA is the reference strain method. However, only a limited number of studies have applied this method to welded pipelines. Therefore, a numerical analysis based on strain-based ECA was performed for girth-welded joints with a circumferentially oriented internal surface crack. Particular attention was given to the strength mismatch effects. The equivalent stress–strain curve in BS7910 was used to reflect the strength mismatch effects in the reference strain. The results of the proposed method were validated with the results of a finite element analysis (FEA) in terms of the J-integral. Previous methods and the proposed method exhibit a reasonable correlation of the J-integral in the case of over-matching (OM). In the under-matching (UM) cases, while the previous procedures tended to underestimate or excessively overestimate the elastic-plastic energy release rate in comparison with the FEA, the proposed method evaluated the J-integral of pipelines with sufficient accuracy.

Numerical Investigation of Strength Mismatch Effect on Ductile Crack Growth Resistance in Welding Pipe

The effect of strength mismatch (ratio between the yield stress of weld metal and base metal, My) on the ductile crack growth resistance of welding pipe was numerically analyzed. The ductile fracture behavior of welding pipe was determined while using the single edge notched bending (SENB) and single edge notched tension (SENT) specimens, as well as axisymmetric models of circumferentially cracked pipes for comparison. Crack growth resistance curves (as denoted by crack tip opening displacement-resistance (CTOD-R curve) have been computed using the complete Gurson model. A so-called CTOD-Q-M formulation was proposed to calculate the weld mismatch constraint M. It has been shown that the fracture resistance curves significantly increase with the increase of the mismatch ratio. As for SENT and pipe, the larger My causes the lower mismatch constraint M, which leads to the higher fracture toughness and crack growth resistance curves. When compared with the standard SENB, the SENT specimen and the cracked pipe have a more similar fracture resistance behavior. The results present grounds for justification of usage of SENT specimens in fracture assessment of welding cracked pipes as an alternative to the traditional conservative SENB specimens.

Combined effect of prestrain history and strength mismatch on crack tip constraints in welded specimen

Combined effect of prestrain history and strength mismatch on crack tip constraints in welded specimen

Application of fracture mechanics to welds with crack origin at the weld toe: a review Part 1: Consequences of inhomogeneous microstructure for materials testing and failure assessment

This two-part paper provides an overview on the state-of-the-art in the application of engineering fracture mechanics to weldments. This, of course, cannot be exhaustive but is limited to butt and fillet welds with crack initiation at weld toes. In the present first part, the authors briefly focus on the susceptibility of welds to cracks and other defects. Following this, they discuss in more detail the consequences of material inhomogeneity across the weld for fracture mechanics. Inhomogeneity causes scatter in fracture toughness and strength mismatch effects both of which have to be considered in fracture toughness testing, crack driving force determination, and fracture assessment of welded components. Part 2 of the paper series will add a discussion on welding residual stresses and questions of applying fracture mechanics to residual as well as total lifetime estimation of welds under cyclic loading.

Finite Element Plastic Limit Loads of Complex Cracks in Pipes With Two-Layered Materials.

Based on the detailed three-dimensional (3D) finite element (FE) limit analyses, the present study investigates the plastic limit loads of complex-cracked pipes with two-layered materials for determining maximum load-carrying capacity or critical crack length of pipes with two-layered materials. The complex cracks in pipes with two-layered materials consist of a partial through-wall crack and 360-deg circumferential surface crack in the inner side of pipe in the same plane in pipe, which could be developed in the preemptive weld overlay region on the dissimilar metal weld (DMW) of nuclear pipe. In terms of FE limit analyses for complex-cracked pipes with two-layered materials, total thickness of pipe, depth of 360-deg internal surface crack, length of partial through-wall crack and the effect of strength mismatch between two materials are systematically considered in the present study. As for loading conditions, axial tension, global bending moment, and internal pressure are employed in the present FE analyses, and then, the confidence of the present FE procedure is confirmed by comparing the FE results with the existing solutions for complex cracks in single material. The results of the present FE plastic limit loads are compared with the existing solutions for complex-cracked pipes with two-layered materials. Also, a simple approach using equivalent single material based on the weighted average concept instead of using the properties of two materials is suggested for predicting plastic limit loads of two-layered materials. The present results can be applied to leak-before-break (LBB) analyses of nuclear piping with weld overlay.

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.

Strength Mismatch Effects on Charpy Absorbed Energy and CTOD Fracture Toughness

Strength Mismatch Effects on Charpy Absorbed Energy and CTOD Fracture Toughness

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.

Failure assessment on offshore girth welded pipelines due to corrosion defects

AbstractCorrosion is an electrochemical process in offshore pipelines where the material strength begins to decrease as corrosion advances. Numerous studies have been performed to determine the remaining strengths (failure pressure) of corroded pipelines. Currently the axial corrosions of the girth welded pipelines still leave much to be understood. This study attempted to simulate girth welded pipeline with various corroded depths and lengths in order to compare with offshore pipeline design manuals. Based on the numerical results, the influence of corrosion defects parameters on remaining strengths were investigated for girth welded pipelines. The investigation on the effect of strength mismatch revealed that in the cases of under‐matched, higher failure pressures are obtained. Comparisons of current results with B31G‐2012 and DNV‐RP‐F101 demonstrated that both codes may produce somewhat conservative predictions on the failure pressure. Furthermore, an equation was proposed to evaluate the corrosion progress across girth welded pipelines.

Numerical Analysis of Constraint and Strength Mismatch Effects on Local Fracture Resistance of Bimetallic Joints

In this paper, the finite element method (FEM) based on GTN model was used to investigate the in-plane/out-of-plane constraint and strength mismatch effects on local fracture resistance of A508/Alloy52Mb bimetallic joint. TheJ-resistance curves, crack growth paths and local stress-stain distributions in front of crack tips were calculated for cracks with different constraints and strength mismatches. The results show that the local fracture resistance of the interface crack in this joint is sensitive to constraint and strength mismatch effects. With increasing in-plane constraint (crack deptha/W), out-of-plane constraint (specimen thicknessB) and strength mismatch degree, the plastic strain and stress triaxiality around crack tip increase, and the corresponding crack growth resistance decreases. The crack with strength mismatch factorM=1 displays a markedly higher crack growth resistance than the other cracks withM&gt;1 andM&lt;1. It also has been found that there is an interaction between in-plane/out-of-plane constraint and strength mismatch for the bimetallic joint. With increasing in-plane/out-of-plane constraint, strength mismatch effect on fracture toughness becomes weaken. For accurate and reliable safety design and failure assessment of the bimetallic joint structures, the effects of constraint and strength mismatch on local fracture resistance need to be considered.

Numerical analysis of strength mismatch effect on stress field in Charpy specimen

The influence of strength mismatch adjacent to the V-notch in Charpy specimen on stress field has been numerically analyzed with a 3D finite element method. It is found that the softer material adjacent to the V-notch provides a “shielding effect” from high-speed straining introduced by impact loading. As this leads to reduction in flow stress ahead of the V-notch, lower opening stress compared with homogeneous specimen is found in the case of the V-notch within the harder material in heterogeneous specimen. By contrast, in the case of the V-notch within the softer material, plastic strain is concentrated near the V-notch and the opening stress is elevated by constraint due to hard material adjacent to the V-notch. The Charpy impact test was conducted with the Charpy specimen extracted from the clad steel consisted of low- and high-strength steels in order to investigate the strength mismatch effect Charpy absorbed energy KV. It is found that KV value is affected by strength mismatch adjacent to the V-notch and the Weibull stress criterion can be applied to the quantitative evaluation of the strength mismatch effect on KV.

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.