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Claudio Ruggieri - One of the best experts on this subject based on the ideXlab platform.
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Fracture resistance testing of dissimilar nickel–chromium girth welds for clad line pipes
International Journal of Fracture, 2017Co-Authors: Diego F B Sarzosa, Vitor S. Barbosa, Caio C. P. Santos, Eduardo Hippert, Claudio RuggieriAbstract: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 ( $$J{-}\Delta a$$ J - Δ a and $$\mathrm {CTOD}{-}\Delta a$$ CTOD - Δ 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.
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J-dominance and size requirements in Strength-Mismatched fracture specimens with weld centerline cracks
Journal of the Brazilian Society of Mechanical Sciences and Engineering, 2015Co-Authors: Rodolfo F. Souza, Claudio RuggieriAbstract: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 $$J$$ J -integral. Previous research efforts have enabled establishing size requirements for valid $$J$$ 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 $$J$$ 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 $$J$$ 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 $$M=(b\sigma _{0}^{WM})/J$$ M = ( b σ 0 W M ) / J , where $$b$$ b is the remaining crack ligament and $$\sigma _{0}^{WM}$$ σ 0 W M denotes a reference (yield) stress for the weld material, at which a one-parameter characterization of crack-tip stress fields still remains valid.
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engineering approach for circumferential flaws in girth weld pipes subjected to bending load
International Journal of Pressure Vessels and Piping, 2015Co-Authors: Marcelo Paredes, Claudio RuggieriAbstract:Abstract 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.
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J and CTOD estimation formulas for C(T) fracture specimens including effects of weld Strength overmatch
International Journal of Fracture, 2013Co-Authors: Rafael G. Savioli, Claudio RuggieriAbstract:This work focuses on an evaluation procedure to determine the elastic–plastic J -integral and Crack Tip Opening Displacement (CTOD) fracture toughness based upon the η -method for C(T) fracture specimens made of homogeneous and welded steels. The primary objective of this investigation is to enlarge on previous developments of J and CTOD estimation procedures for this crack configuration while, at the same time, addressing effects of Strength Mismatch on the plastic η -factors. The present analyses enable the introduction of a larger set of factors η for a wide range of crack sizes (as measured by the a / W -ratio) and material properties, including different levels of weld Strength Mismatch, applicable to pipeline and pressure vessel steels. Very detailed non-linear finite element analyses for plane-strain and 3-D models of C(T) fracture specimens with centerline-cracked welds provide the evolution of load with increased load-line and crack mouth opening displacement required for the estimation procedure. Overall, the present study, when taken together with previous investigations, provides a fairly extensive body of results to determine parameters J and CTOD for different materials using C(T) specimens with varying overmatch conditions.
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Further results in J and CTOD estimation procedures for SE(T) fracture specimens – Part II: Weld centerline cracks
Engineering Fracture Mechanics, 2012Co-Authors: Marcelo Paredes, Claudio RuggieriAbstract:Abstract This work examines the effect of weld Strength Mismatch on J and CTOD fracture parameters using single edge notch tension (SE(T)) specimens. A central objective of the study is to enlarge on previous J and CTOD estimation procedures based upon plastic eta factors ( η ) described in Part I for centerline-cracked welds. The present analyses provide a large set of η -factors for a wide range of crack sizes ( a / W -ratio) and material properties, including different levels of weld Strength Mismatch, which are applicable in improved estimation equations for J and CTOD for weld centerline notched SE(T) specimens.
I.a. Khan - One of the best experts on this subject based on the ideXlab platform.
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Characterization of near tip stresses for a plane-strain off-center crack in a Strength Mismatched weld
International Journal of Mechanical Sciences, 2015Co-Authors: Suranjit Kumar, I.a. Khan, Vivek Bhasin, P.k. Singh, R. K. SinghAbstract:Abstract The past two decades have seen an increasing emphasis on the development of flaw assessment procedures accounting for the state of stress (constraint) near the crack tip. 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 recent work, the authors have proposed a novel three-parameter scheme to characterize the crack tip opening stress, for small-scale yielding, for a crack lying at the center of a Strength Mismatched weld. The proposed scheme was validated with detailed finite element analyses using the modified boundary layer formulation. In the present work, the previously proposed scheme is extended to an off-center crack lying in the weld region. Apart from analyzing an off-center crack, studies are performed on an interface crack in a weld having finite width. To validate the proposed scheme, modified boundary layer analyses are carried out for a wide range of offset ratio values (ratio of the crack tip-to-weld center distance to the weld half-width), ranging from 0.33 to 0.9, and Strength Mismatch ratio values (ratio of yield Strength of weld to yield Strength of base material), ranging from 0.6 to 1.6. Our studies revealed that the proposed analytical scheme works well for all offset ratios and weld Mismatch ratios. For an interface crack, crack tip stresses depend mainly on the Strength Mismatch ratio between the base and weld material. Practically no significant effect is observed of the weld width on the crack tip opening stress of an interfacial crack.
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Characterization of crack tip stresses in plane-strain fracture specimens having weld center crack
International Journal of Solids and Structures, 2014Co-Authors: Suranjit Kumar, I.a. Khan, Vivek Bhasin, R. K. SinghAbstract:Abstract 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 ( I p ) 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.
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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
International Journal of Mechanical Sciences, 2014Co-Authors: I.a. Khan, Vivek Bhasin, Jayanta Chattopadhyay, R. K. Singh, K. K. Vaze, A.k. GhoshAbstract:Abstract 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.
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Influence of Weld Strength Mismatch on Crack Length Measurement Using Compliance Technique
Procedia Engineering, 2014Co-Authors: Suranjit Kumar, I.a. Khan, Vivek Bhasin, K. K. VazeAbstract:Abstract 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.
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An insight of the structure of stress fields for stationary crack in Strength Mismatch weld under plane strain mode-I loading—Part I: Pure bending specimen
International Journal of Mechanical Sciences, 2012Co-Authors: I.a. Khan, Vivek Bhasin, Jayanta Chattopadhyay, A.k. GhoshAbstract:Abstract In-service inspections of many nuclear power plants have revealed that cracks are most likely to occur in or the regions near the weld. Interfacial cracks under elastic as well as in elastic–plastic conditions have already been extensively discussed in literature. However, the problem of crack lying at the centre of weld is less understood. Though several detailed numerical studies have been performed to investigate the influence of weld Strength Mismatch on crack-tip stress fields till date, however, the detailed insight of the structure of stress fields under large scale plasticity is still lacking. The present article is intended to bridge that gap. In this work, detailed structure of the global plastic fields which occur in a deeply cracked (a/W>0.3) Mismatch welded pure bending specimen, under fully plastic condition, is presented. Aspects related to the state of stress at the interface of two materials are discussed. It is shown that a family of five fields proposed in this work is adequate to cover all practical cases of weld Mismatch. Proposed fields were confirmed by detailed full-field finite element analyses. Excellent agreement is observed between the proposed theoretical solutions and the numerical results.
Fumiyoshi Minami - One of the best experts on this subject based on the ideXlab platform.
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numerical analysis of Strength Mismatch effect on stress field in charpy specimen
Welding in The World, 2015Co-Authors: Yasuhito Takashima, Yugo S Yamada, Tsunehisa Handa, Kenji Oi, Fumiyoshi MinamiAbstract: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 K V. It is found that K V 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 K V.
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Evaluation method for Charpy impact toughness of laser welds based on lateral contraction analysis
Welding in the World, 2014Co-Authors: Yasuhito Takashima, Mitsuru Ohata, Tatsuya Nishi, Hiroto Shoji, Fumiyoshi MinamiAbstract:An evaluation of Charpy impact toughness of the weld metal for laser beam welds is often impossible due to occurrence of fracture path deviation (FPD) from the weld metal into base steel. Therefore, the improved toughness evaluation method using side-grooved specimen instead of normal specimen has been suggested for preventing FPD. However, for a pre-qualification of the toughness of laser welds, the toughness value obtained with side-grooved specimen should be corrected to the toughness value of normal Charpy specimen. In this study, the method for correction of side-grooved Charpy energy for laser welds to normal Charpy impact toughness suggested by Hagihara et al. (Q J Jpn Weld Soc 25: 165–172, 2007) is advanced by means of FE analysis with focus on the deformation behaviors associated with Strength Mismatch. Furthermore, the distinctive Strength Mismatch of laser welds on lateral contraction of Charpy specimen has been analyzed by means of the Weibull stress approach. The applicability of the advanced engineering method is demonstrated by comparing experimental result, whereas the further verification should be accumulated.
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Constraint-based assessment of fracture in welded components
Welding in the World, 2013Co-Authors: Fumiyoshi Minami, Yasuhito Takashima, Mitsuru Ohata, Yoichi YamashitaAbstract:A toughness correction method, developed as ISO 27306, specifies an equivalent crack tip opening displacement (CTOD) ratio, β , for the correction of CTOD toughness for constraint loss in structural components. Using β , the standard fracture toughness specimen and structural components are linked at the same level of the Weibull stress. This paper applies the IST method to the fracture assessment of welded components. Effects of the Strength Mismatch and residual stress in welds on β are implemented into the toughness correction. The Strength Mismatch does not have an influence on β , provided that the Strength Mismatch between the weld and base metals is in the range from 0.9 to 1.2. On the other hand, residual stress exerts a large influence at a low CTOD level below full yielding: β is elevated by the tensile residual stress. It is shown on the failure assessment diagram that the CTOD toughness correction with β eliminates the excessive conservatism often found in the conventional approach and leads to accurate fracture assessments of welded components.
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Strength-Mismatch Effect on Steel Weld HAZ-Toughness in CTOD and Charpy Tests
Welding in the World, 2010Co-Authors: Yasutake Chiba, Mitsuru Ohata, Keiji Murayama, Susumu Satoh, Kyohsuke Miyashiro, Fumiyoshi MinamiAbstract:The effects of Strength-Mismatch between the base and weld metal in welded joints on CTOD and Charpy impact toughness of heat-affected zone (HAZ) are investigated in this study. Both toughness properties are compared, in consideration of the effect of the local brittle zone (LBZ) in HAZ, and Strength-Mismatch effects are discussed analytically. Two types of welded joints for 490 MPa Strength class structural steel with the same thickness of 25 mm, which were Strength-matched and overmatched joints, are prepared under the same welded conditions (heat input = 4 KJ/mm) with different welding consumables. The critical CTOD of the HAZ for the overmatched joint exhibits the lower values in wide temperature range of ductile-to-brittle fracture transition than that for the matched joint, whereas microstructures and total LBZ size along crack-tip are consistent with each other. On the contrary, significant difference between the matched and overmatched joints in the lower temperature range cannot be observed in the Charpy impact toughness. The reason why the Strength-Mismatch effects on the fracture toughness are different between the CTOD and Charpy impact tests is discussed.
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Significance of Strength Mismatch in Fracture Performance Evaluation of Welded Joints — Characterization of Mismatch Effect by the Weibull Stress Criterion
Welding in the World, 2009Co-Authors: Fumiyoshi Minami, Mitsuru Ohata, Yasutake Chiba, Takamasa Manabe, Keiji MurayamaAbstract:The effect of Strength Mismatch in welds on the fracture driving force of the heat-affected zone (HAZ) is investigated by 3-dimensional FE-analysis. The Weibull stress is used for the evaluation of the fracture driving force. The CTOD ratio, γ = δ_Mismatch /δ_Match, is proposed to quantify the Mismatch effect, where δ_Mismatch and δ_Match are CTODs of the Mismatched joint and the matched joint, respectively, at the same level of the Weibull stress. The CTOD ratio γ is decreased by Strength overmatching, which is due to the constraint effect of the overmatch WM on the stress fields in the HAZ. The equivalent CTOD ratio, β = δ/δ_WP, is employed to correct the constraint loss in the structural component, where δ and δ_WP are CTODs of the fracture toughness specimen and the wide plate component, respectively, at the same level of the Weibull stress. The analysis using β and γ indicates that Strength overmatching leads to a severe requirement of the HAZ toughness, which cannot be directly derived from the CTOD — remote strain relationship for the wide plate joint. By contrast, the toughness requirement of the WM is relaxed by Strength overmatching, as a result of the shielding effect of the overmatch WM.
R. K. Singh - One of the best experts on this subject based on the ideXlab platform.
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Characterization of near tip stresses for a plane-strain off-center crack in a Strength Mismatched weld
International Journal of Mechanical Sciences, 2015Co-Authors: Suranjit Kumar, I.a. Khan, Vivek Bhasin, P.k. Singh, R. K. SinghAbstract:Abstract The past two decades have seen an increasing emphasis on the development of flaw assessment procedures accounting for the state of stress (constraint) near the crack tip. 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 recent work, the authors have proposed a novel three-parameter scheme to characterize the crack tip opening stress, for small-scale yielding, for a crack lying at the center of a Strength Mismatched weld. The proposed scheme was validated with detailed finite element analyses using the modified boundary layer formulation. In the present work, the previously proposed scheme is extended to an off-center crack lying in the weld region. Apart from analyzing an off-center crack, studies are performed on an interface crack in a weld having finite width. To validate the proposed scheme, modified boundary layer analyses are carried out for a wide range of offset ratio values (ratio of the crack tip-to-weld center distance to the weld half-width), ranging from 0.33 to 0.9, and Strength Mismatch ratio values (ratio of yield Strength of weld to yield Strength of base material), ranging from 0.6 to 1.6. Our studies revealed that the proposed analytical scheme works well for all offset ratios and weld Mismatch ratios. For an interface crack, crack tip stresses depend mainly on the Strength Mismatch ratio between the base and weld material. Practically no significant effect is observed of the weld width on the crack tip opening stress of an interfacial crack.
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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
International Journal of Mechanical Sciences, 2014Co-Authors: I.a. Khan, Vivek Bhasin, Jayanta Chattopadhyay, R. K. Singh, K. K. Vaze, A.k. GhoshAbstract:Abstract 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.
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Characterization of crack tip stresses in plane-strain fracture specimens having weld center crack
International Journal of Solids and Structures, 2014Co-Authors: Suranjit Kumar, I.a. Khan, Vivek Bhasin, R. K. SinghAbstract:Abstract 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 ( I p ) 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.
Vivek Bhasin - One of the best experts on this subject based on the ideXlab platform.
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Characterization of near tip stresses for a plane-strain off-center crack in a Strength Mismatched weld
International Journal of Mechanical Sciences, 2015Co-Authors: Suranjit Kumar, I.a. Khan, Vivek Bhasin, P.k. Singh, R. K. SinghAbstract:Abstract The past two decades have seen an increasing emphasis on the development of flaw assessment procedures accounting for the state of stress (constraint) near the crack tip. 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 recent work, the authors have proposed a novel three-parameter scheme to characterize the crack tip opening stress, for small-scale yielding, for a crack lying at the center of a Strength Mismatched weld. The proposed scheme was validated with detailed finite element analyses using the modified boundary layer formulation. In the present work, the previously proposed scheme is extended to an off-center crack lying in the weld region. Apart from analyzing an off-center crack, studies are performed on an interface crack in a weld having finite width. To validate the proposed scheme, modified boundary layer analyses are carried out for a wide range of offset ratio values (ratio of the crack tip-to-weld center distance to the weld half-width), ranging from 0.33 to 0.9, and Strength Mismatch ratio values (ratio of yield Strength of weld to yield Strength of base material), ranging from 0.6 to 1.6. Our studies revealed that the proposed analytical scheme works well for all offset ratios and weld Mismatch ratios. For an interface crack, crack tip stresses depend mainly on the Strength Mismatch ratio between the base and weld material. Practically no significant effect is observed of the weld width on the crack tip opening stress of an interfacial crack.
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Characterization of crack tip stresses in plane-strain fracture specimens having weld center crack
International Journal of Solids and Structures, 2014Co-Authors: Suranjit Kumar, I.a. Khan, Vivek Bhasin, R. K. SinghAbstract:Abstract 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 ( I p ) 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.
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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
International Journal of Mechanical Sciences, 2014Co-Authors: I.a. Khan, Vivek Bhasin, Jayanta Chattopadhyay, R. K. Singh, K. K. Vaze, A.k. GhoshAbstract:Abstract 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.
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Influence of Weld Strength Mismatch on Crack Length Measurement Using Compliance Technique
Procedia Engineering, 2014Co-Authors: Suranjit Kumar, I.a. Khan, Vivek Bhasin, K. K. VazeAbstract:Abstract 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.
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An insight of the structure of stress fields for stationary crack in Strength Mismatch weld under plane strain mode-I loading—Part I: Pure bending specimen
International Journal of Mechanical Sciences, 2012Co-Authors: I.a. Khan, Vivek Bhasin, Jayanta Chattopadhyay, A.k. GhoshAbstract:Abstract In-service inspections of many nuclear power plants have revealed that cracks are most likely to occur in or the regions near the weld. Interfacial cracks under elastic as well as in elastic–plastic conditions have already been extensively discussed in literature. However, the problem of crack lying at the centre of weld is less understood. Though several detailed numerical studies have been performed to investigate the influence of weld Strength Mismatch on crack-tip stress fields till date, however, the detailed insight of the structure of stress fields under large scale plasticity is still lacking. The present article is intended to bridge that gap. In this work, detailed structure of the global plastic fields which occur in a deeply cracked (a/W>0.3) Mismatch welded pure bending specimen, under fully plastic condition, is presented. Aspects related to the state of stress at the interface of two materials are discussed. It is shown that a family of five fields proposed in this work is adequate to cover all practical cases of weld Mismatch. Proposed fields were confirmed by detailed full-field finite element analyses. Excellent agreement is observed between the proposed theoretical solutions and the numerical results.