Effect Of Strength Mismatch Research Articles

Significance of Strength Mismatch in Fracture Performance Evaluation of Welded Joints — Characterization of Mismatch Effect by the Weibull Stress Criterion

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 = δMismatch /δMatch, is proposed to quantify the mismatch effect, where δMismatch and δMatch 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/δWP, is employed to correct the constraint loss in the structural component, where δ and δWP 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.

Strength mis-match effect on limit loads for circumferential surface cracked pipes

The present work provides mis-match limit loads for welded pipes with constant-depth, circumferential part-through surface cracks in the centre of the weld metal. As limiting cases, circumferential through-wall and fully circumferential cracks are also considered. Based on systematic finite element limit analyses, mis-match limit loads are quantified by the strength mis-match ratio and only one geometry-related slenderness parameter, not requiring additional geometry-related parameters. This implies that mis-match limit loads for surface cracked pipes can be found from those for idealized axi-symmetric problems, which offers significant benefit in assessment of surface cracks in welds of pipes.

용접부 중앙에 표면균열이 존재하는 인장 평판에 대한 강도 불일치 한계하중 해석 및 간략 J-적분 예측

In this work, the effect of strength mismatch on plastic limit loads is quantified for similar metal weld plates with cracks under tension load, via three-dimensional, small strain elastic-perfectly plastic finite element analyses. Relevant variables related to plate geometry and crack length are systematically varied, in addition to the weld width. An important finding is that mis-match limit loads can be uniquely quantified through strength mis-match ratio and one geometry-related parameter. Based on the proposed limit load solutions, reference stress based J-integral estimates is also investigated. When the reference stress is defined by the mis-match limit load, predicted J-integral values agree overall well with FE results.

Mismatch limit loads and approximate J estimates for tensile plates with constant-depth surface cracks in the center of welds

The present work provides mismatch limit loads and approximate J estimates for tensile plates with constant-depth, part-through surface cracks in the center of the weld metal. Based on systematic three-dimensional FE limit analyses, effects of strength mismatch related variables on limit loads are firstly quantified by the strength mismatch ratio and one geometry-related parameter. Mismatch limit loads for part-through surface cracks are then correlated to those for two-dimensional, through-wall crack problems. Based on the proposed limit load solutions, the applicability of the reference stress based J estimates is also investigated. When the reference stress is defined by the mismatch limit load, predicted J values agree overall well with FE results.

The effect of undermatching on crack tip constraint in a welded structure of nodular irons

Abstract The main aim of this paper is to investigate the effect of yield strength mismatch of parent and weld metal on the fracture behavior of plain and heat-treated welded nodular cast iron. For this, experimental and theoretical studies were performed. Nickel electrode, which has lower yield strength value than that of the parent material, was used to join nodular cast iron by cold arc welding method. In experimental study, fracture toughness (J-integral) values were determined for various crack locations including base, weld metal and heat affected zone. In theoretical study, finite element analyses were performed for the same crack locations with different crack lengths. Stress triaxiality and equivalent plastic strain values were determined for all crack locations and various crack lengths. Finally, fracture toughness values of the base metal were determined as lower than those of the other regions including the weld metal and the heat affected zones. Stress triaxiality and equivalent plastic strain values used to clarify the effect of yield strength mismatch on the constraint around crack tip increased with growing crack length in annealed and under-matched models. In annealed models, stress triaxiality values became slightly lower but equivalent plastic strain values appeared higher than those of under-matched models.

Modelling of ductile fracture initiation in strength mismatched welded joint

In this paper, the effect of strength mismatch and width of the welded joints on the stress–strain distribution in the crack tip region has been discussed. The single-edge notched bend (SENB) specimens (precrack length a 0/ W = 0.32) were experimentally and numerically analysed. The model of local approach to fracture, proposed by Gurson, Tvergaard and Needleman, was used. High-strength low-alloyed (HSLA) steel was used as a base metal in quenched and tempered condition. The flux-cored arc-welding process in shielding gas was used. Two different fillers were selected to make over- and undermatched weld metal. The experimental analysis of fracture behaviour of the over- and undermatched welded joints was followed by numerical computations of void volume fraction in front of the crack tip. The critical void volume fraction, f c, used in prediction of the crack growth initiation on the SENB specimen had been previously determined on a round smooth specimen. Three widths of weld metal were considered: 6, 12 and 18 mm. A comparison of the crack tip opening displacement (CTOD) values corresponding to crack initiation in the SENB specimens is given, as determined both experimentally and using the GTN model.

Effect of Strength Mismatch on Ductile Crack Initiation Behavior from Notch Root under Static Loading

It has been well known that ductile fractures of steels are accelerated by triaxial stresses. The characteristics of ductile crack initiation in steels are evaluated quantitatively using two-parameter criterion based on equivalent plastic strain and stress triaxiality. It has been demonstrated by authors using round-bar specimens with circumferential notch in single tension that the critical strain to initiate ductile crack from specimen center depends considerably on stress triaxiality, but surface cracking of notch root is in accordance with constant strain condition. This study fundamentally clarifies the effect of strength mismatch, which can elevate plastic constraint due to heterogeneous plastic straining under static loading, on critical conditions for ductile cracking from the pre-notch root. In order to evaluate the stress/strain state in the pre-notch root of specimens, a thermal elastic-plastic finite element (FE) analysis has been carried out.

Effect of Strength Mismatch on Ductile Crack Initiation Behaviour from Notch Root

This study fundamentally clarifies the effect of strength mismatch, which can elevate plastic constraint due to heterogeneous plastic straining under static loading, on critical conditions for ductile cracking from the notch root. The critical condition for initiating ductile crack from the notch root for strength mismatched bend specimens under static loading is almost the same as that for homogeneous tensile specimens with a circumferential sharp notch under static loading.

The effect of yield strength mismatch on the fracture behavior of welded nodular cast iron

Abstract In this study, fracture toughness ( J -integral) values in cracked welded nodular iron were determined experimentally, and stress–strain analyses were done by the finite element method, for different crack sizes and locations with various strength levels. As a result, the effect of yield strength mismatch on the constraint around crack tip was discussed by means of stress triaxiality (ST) and plastic strain. It was found that ST values increase with crack length. Different regions in the weldment affect the extent of plastic deformation in the material.

Fracture toughness of welds—effect of brittle zones and strength mismatch

The aim of this study is to examine the effect of the welding procedure, the heat input in particular, on the changes induced in the mechanical behavior of welds on an high strength quenched and tempered steel.Welds were performed by submerged arc welding, in a high quenched and tempered steel RQT 601 with 25mm thick, using heat inputs of 2, 4.2 and 6.6kJ/mm. COD/J specimens were extracted transversely to the welding direction and tested according to BS 7448—Part 1 and Part 2, at a temperature of −20°C.The increase of heat input caused softening of the weld metal and heat-affected zone, due to microstructural changes and microstructural coarsening. A decrease in toughness was observed in the weld metal and heat-affected zone with the increase of heat input. The experimentally obtained value of the dimensionless factor m, relating J with COD, is similar to that predicted by the equations established in the literature. A general trend for the decrease of J and COD toughness with the increase in the size of the brittle zone sampled by the fatigue crack front is observed, though some scatter is always omnipresent.

Numerical investigation on J‐integral testing of heterogeneous fracture toughness testing specimens: Part I – weld metal cracks

ABSTRACTBased on extensive two‐dimensional (2D) finite element (FE) analyses, the present work provides the plastic η factor solutions for fracture toughness J‐integral testing of heterogeneous specimens with weldments. Solutions cover practically interesting ranges of strength mismatch and relative weld width, and are given for three typical geometries for toughness testing: a middle cracked tension (M(T)) specimen, single edge cracked bend (SE(B)) specimen and (C(T)) specimen. For mismatched M(T) specimens, both plane strain and plane stress conditions are considered, whereas for SE(B) and C(T) specimens, only the plane strain condition is considered. For all cases, only deep cracks are considered, and an idealized butt weld configuration is considered, where the weld metal strip has a rectangular cross section. Based on the present solutions for the strength mismatch effect on plastic η factors, a window is provided, within which the homogeneous J estimation procedure can be used for weldment toughness testing. The effect of the weld groove configuration on the plastic η factor is briefly discussed, concluding the need for further systematic analysis to provide guidance to practical toughness testing.

Numerical analyses of strength mis-match effect on local stresses for ideally plastic materials

The effect of strength mis-matched welded joints on the local stresses in the neighbourhood of a crack has been investigated, in the context of ideally plastic materials, using the finite element method and slip line field analysis. Several key parameters affecting the local stresses have been identified: degree of strength mis-match, M, slenderness of the weld, ( W− a)/ H, and crack location within the weld. Loading geometry effects have been considered by investigating tension and bending specimens. The analyses were performed for plane strain and plane stress conditions. It is found that, for the case when the crack locates within the weld metal, the crack tip stress triaxiality depends not only on M but also on ( W− a)/ H. The effect of ( W− a)/ H is worth noting, as its effect on the crack tip stress triaxiality is restricted to certain ranges of ( W− a)/ H, which in turn depend on M, geometry and loading. For an interface crack, the stress triaxiality at the softer side is higher than that for a crack in a specimen wholly made of the softer material; the reverse is true for the harder side. This explains the ductile failure behaviour of cracks observed in experiments: the crack path tends to deviate into the softer material in spite of its higher toughness. Another notable point is that, under sufficiently high under-matching conditions, a second peak stress ahead of and remote from the crack tip may occur, the magnitude of which depends on M and ( W− a)/ H. Present findings are believed to be vital for design and defect assessment of welded joints with conventional welding as well as advanced welding techniques.

Effect of strength mis-match and dynamic loading on ductile fracture initiation

It has been well known that ductile fracture of steels is accelerated by triaxial stresses. The characteristics of ductile crack initiation in steels are evaluated quantitatively using a two-parameter criterion based on equivalent plastic strain and stress triaxiality. The present study focuses on the effects of geometrical discontinuity, strength mis-match, which can elevate plastic constraint due to heterogeneous plastic straining, and loading rate on the critical condition for ductile fracture initiation using a two-parameter criterion. Fracture initiation testing has been conducted under static and dynamic loading using circumferentially notched round-bar specimens. In order to evaluate the stress/strain state in the specimens, especially under dynamic loading, a thermal elastic–plastic dynamic finite element (FE) analysis considering the temperature rise due to plastic deformation has been carried out. The tensile tests on specimens with an undermatching interlayer showed that the relationship between the critical equivalent plastic strain to initiate ductile fracture and stress triaxiality was equivalent to that obtained on homogeneous specimens under static loading. Moreover, the two-parameter criterion for ductile fracture initiation is shown to be independent of the loading rate. It was demonstrated that the critical global strain to initiate ductile fracture in specimens with strength mis-match under various loading rate can be estimated based on the local criterion, that is two-parameter criterion obtained on homogeneous specimens under static tension, by mean of FE-analysis taken into account accurately both strength mis-match and dynamic loading effects on stress/strain behaviors.

Fracture behaviour of diffusion bonded titanium alloys with strength mismatch

The present study considers the effect of strength mismatch on the fracture behaviour of diffusion bonded joints between commercially pure (CP) Ti and Ti-6Al-4V (Ti64), including dissimilar joints and sandwich structures with strength undermatching and overmatching. The aim of the investigation is to determine the influence of the interlayer thickness (for both higher and lower strength interlayers) and the bond quality on the deformation behaviour and fracture toughness of the joints. The influence of mechanical heterogeneity (strength mismatch) on the fracture behaviour of the interface in dissimilar joints was also investigated. Round bars of CP Ti and Ti64 having a diameter of 40 mm were diffusion bonded as dissimilar butt joints and sandwich structures containing lower strength (undermatching) and higher strength (overmatching) interlayers of different thicknesses. Round transverse tensile specimens and standard four point bend (single edge notch bend) specimens were extracted from the joints via spark erosion cutting. The four point bend specimens were fatigue precracked to introduce a sharp crack after introducing machine notches at the centre of the interlayers in the sandwich structures and at the interface in the dissimilar joints, and tested at room temperature. Some specimens were also prepared with the crack positioned away from the interface to determine the effect of notch position on fracture behaviour. The effect of strength mismatch on the crack tip opening displacement fracture toughness parameter of the joints has been evaluated. Crack initiation, crack growth, and crack deviation processes have been examined and fracture resistance curves (R curves) constructed for the joints. These results were used to explain the influence of mechanical heterogeneity of the joints and interlayer thickness on fracture behaviour.

Welded plate and T-stub tests and implications on structural behavior of moment frame connections

A series of tests on simple-welded plate specimens (SWPS) and T-stub tension specimens simulating some of the joint details in moment frame connections were conducted in this investigation. The effects of weld strength mismatch and weld metal toughness on structural behavior of these specimens were considered under both static and dynamic loading conditions. Finite element analyses were performed by taking into account typical weld residual stress distributions and weld metal strength mismatch conditions to facilitate the interpretation of the test results. The major findings are as follows: (a) Sufficient specimen size requirements are essential in simulating both load transfer and constraint conditions that are relevant to moment frame connections, (b) Weld residual stresses can significantly elevate stress triaxiality in addition to structural constraint effects, both of which can significantly reduce the plastic deformation capacity in moment frame connections, (c) Based on the test results, dynamic loading within a loading rate of 0.02 in/in/sec, as used in this study, premature brittle fractures were not seen, although a significant elevation of the yield strength can be clearly observed. However, brittle fracture features can be clearly identified in T-stub specimens in which severe constraint effects (stress triaxiality) are considered as the primary cause, (d) Based on both the test and FEA results, T-stub specimens provide a reasonable representation of the joint conditions in moment frame connections in simulating both complex load transfer mode and constraint conditions.

J and CTOD estimation for homogeneous and Bi-material fracture toughness testing specimens

This paper proposesJ and CTOD estimation schemes applied to fracture toughness testing, covering typical homogeneous and bi-material specimens. Recommendations are based on the plastic limit analysis (either slip line field or finite element limit analyses), assuming the rigid plastic material behavior. The main outcome of the present study is that theJ and CTOD estimation schemes (both codified and non-codified), recommended for homogeneous specimens, can be equally used for bi-material specimens with interface cracks. The effect of yield strength mismatch in bi-material specimens on the theJ-integral and CTOD is discussed.

Effect of Strength Mismatch and Dynamic Loading on Criterion for Ductile Crack Initiation

It has been well known that the ductile cracking of steel would be accelerated by triaxial stress state. Recently, the characteristics of critical crack initiation of steels are quantitatively estimated using the two-parameters, that is, equivalent plastic strain and stress triaxiality, criterion.This study is paid to the fundamental clarification of the effect of geometrical heterogeneity and strength mis-matching, which can elevate plastic constraint due to heterogeneous plastic straining, and loading rate on critical condition to initiate ductile crack using two-parameters. Then, the crack initiation testing were conducted under static and dynamic loading using round bar specimens with circumferential notch and strength mis-matching. To evaluate the stress/strain state in the specimens especially under dynamic loading, thermal elastic-plastic dynamic FE-analysis considering the temperature rise was used.The result showed that equivalent plastic strain to initiate ductile crack expressed as a function of stress triaxiality obtained from the homogeneous specimens with circumferential notch under static loading could be a material constant independent of strength mis-matching and loading rate. And it was demonstrated that by using this two-parameters criterion, the critical crack initiation of specimens with strength mis-matching under static or dynamic loading, such as critical load or critical displacement, could be predicted from thermal elastic-plastic FE-analyses.

Strength Mismatch Effect on Ductile Crack Initiation from Notch Root of Structural Steel

The characteristics of the condition to initiate ductile crack of steels has been quantitatively estimated using the “two-parameters, that is, equivalent plastic strain and stress triaxiality, criterion”. It has been demonstrated by authors using round-bar specimens with circumferential notch in single tension that the critical strain to initiate ductile crack from specimen center depends considerably on stress triaxiality, but surface cracking of notch root is in accordance with constant strain condition.This study provides the fundamental clarification of the effect of strength mis-matching, which can elevate plastic constraint due to heterogeneous plastic straining, loading mode and loading rate on critical condition to initiate ductile crack from notch root using two-parameters. The crack initiation testing using charpy type bend specimens with strength mis-matching near the notch were conducted under static and dynamic loading. To analyze the stress/strain state in the specimens especially under dynamic loading, thermal elastic-plastic dynamic FE-analysis considering the temperature rise was used. It has been shown that the strength mis-matching would not enhance the stress triaxiality at the notch root of bending specimen. The critical condition to initiate ductile crack from notch root for strength mis-matched bend specimens under both static and dynamic loading would be almost the same as that for homogeneous tensile specimens with circumferential sharp notch under static loading. It follows from these results that the ductile cracking from notch root surface under static and dynamic loading would be estimated by “critical plastic strain criteria” even if the strength mis-matching exists near the notch root

On the interrelationship between fracture toughness and material mismatch for cracks located at the fusion line of weldments

The effect of strength mismatch on brittle fracture initiated from cracks located at the fusion line of weldments are investigated. The scope of this research has been to derive expressions for the crack tip constraint and thereby open for transferability schemes from one geometry/mismatch condition to another. The procedure has been termed the J–Q–M approach, where Q quantifies the geometry effect and M the material mismatch effects. In the paper the approach is applied to specific weldments and the influence of fracture toughness on the effect of mismatch is discussed. The fracture toughness testing included series with up to 50 parallel test specimens and the theoretical calculations agreed well with the test results. The weld metal yield strength overmatch with respect to HAZ was detrimental for the fracture toughness of the HAZ at CTOD levels lower than about 0.25 mm, at higher toughness levels evenmatch resulted in the lowest toughness.

Residual stresses in strength-mismatched welds and implications on fracture behavior

The effects of weld strength mismatch on welding residual stresses were discussed based on a series of recent comprehensive studies. Two typical joint configurations were analyzed in detail: a butt-welded plate and a multi-pass girth weld. The butt-welded plate represents a severely under-matched weld, in which repair welding effects were also analyzed. For the girth weld, three weld strength levels were considered to investigate the weld strength mismatch effects on weld residual stress distributions. Fracture mechanics analysis was then performed for the multi-pass girth weld for a surface crack situated along the weld centerline. Welding-induced residual stress fields were assumed to be the dominant loading mode in all cases. With the dominance of welding-induced residual stress field, such as in some of the stress corrosion cracking cases experienced in the utility industry, stress intensity factors are shown to be an appropriate and convenient fracture mechanics parameter. The implications on ductile fracture behavior were also discussed in light of the results from this study.