An experimental investigation of in-plane constraint effect on local fracture resistance of a dissimilar metal welded joint

In this paper, an experimental investigation on effect and mechanism of in-plane constraint induced by crack depth on local fracture resistance of two cracks (A508 heat-affected-zone (HAZ) crack and A508/Alloy52Mb interface crack) located at the weakest region in an Alloy52M dissimilar metal welded joint (DMWJ) between A508 ferritic steel and 316L stainless steel in nuclear power plants has been carried out. The results show that the local fracture resistance of the two cracks is sensitive to in-plane constraint. With increasing in-plane constraint (crack depth a/W), the fracture mechanism of the two cracks changes from ductile fracture through mixed ductile and brittle fracture to brittle fracture, and the corresponding crack growth resistance decreases. The crack growth path in the specimens with different in-plane constraints deviates to low-strength material side, and is mainly controlled by local strength mismatch, rather than toughness mismatch. For accurate and reliable safety design and failure assessment of the DMWJ structures, it needs to consider the effects of in-plane constraint on fracture mechanism and local fracture resistance. The new safety design and failure assessment methods incorporating constraint effect need to be developed for the DMWJ structures.     

An experimental investigation of local fracture resistance and crack growth paths in a dissimilar metal welded joint

An experimental investigation on the local fracture resistance and crack growth behavior in a Alloy52M dissimilar metal welded joint (DMWJ) between A508 ferritic steel and 316L stainless steel has been carried out by using the single-edge notched bend (SENB) specimens. The local J-resistance curves and crack growth paths of 13 cracks located at various positions in the DMWJ were determined, and the effects of the local strength mismatch on local fracture resistance, crack growth paths and integrity assessment for the DMWJ were analyzed. The results show that the cracks always deviate to the materials with lower strength, and the crack path deviations are mainly controlled by the strength mismatch, rather than toughness mismatch. The J-resistance curve with larger crack path deviation only reflect the apparent fracture resistance along the crack growth region, rather than the intrinsic fracture resistance of the material at the initial crack-tip region. Without considering the local fracture resistance properties of heat affected zone (HAZ), interface and near interface zone, the use of the J-resistance curves of base metals or weld metals following present codes will unavoidably produce non-conservative (unsafe) or excessive conservative assessment results. In most cases, the assessment results will be potentially unsafe. Therefore, it is recommended to obtain and use local mechanical and fracture resistance properties of all regions of the DMWJ if the complex local mismatch situation is a concern. And new integrity assessment methods based on local damage and fracture models also need to be developed for the DMWJs.     

Ductile-to-brittle transition in tensile failure due to shear-affected zone with a stress-concentration source: a comparative study on punched-plate tensile-failure characteristics of precipitation-hardened and dual-phase steels

The effect of shear-affected zone (SAZ), with a stress-concentration source induced by the punching process, on tensile properties was investigated. Tests using honed specimens (which have the same shapes and stress-concentration without any SAZ) and smooth specimens were conducted to compare the effect with that of the punched specimens. Dual-phase steel, which has a high work-hardening ability and low yield strength, and precipitation-hardened steel, which has a low work-hardening ability and high yield strength, were used in the tests. Materials with two tensile strength grades were prepared from both types of steel. Only the precipitation-hardened steel with higher strength grade punched specimen showed a brittle fracture with extremely short fracture-elongation, whereas the other specimens showed a ductile fracture. The fracture surface analysis revealed that cracks initiated in the maximum shear stress plane of the SAZ under tensile loading at first. We call the crack “shear crack.” The steel which showed brittle fracture used in this study easily exhibited plastic-strain localization compared with the other steels. If the shear crack is sharp, then the transition from ductile to brittle failure tends to occur. Furthermore, the strength characteristics of the punched specimen depend on the crack length dependency of the strength resistance and the failure phenomenon of the original material.     

Constraint-corrected fracture failure criterion based on CTOD/CTOA

In engineering design, a difficulty has always existed in those standard laboratory tests that cannot accurately predict the behavior of large structures like pipelines due to the different constraint levels. At present, extensive work has been done to characterize the constraint effects on fracture toughness by introducing a second parameter, while the systematic research on constrained transformation is inadequate. To address this issue, the ductile fracture process of X65 SENB specimen is simulated through the finite-element method coupled with the Gurson–Tvergaard–Needelman model. The parameters crack tip opening displacement (CTOD) and crack tip opening angle (CTOA) are chosen to characterize the fracture behaviors. The effects of specimen thickness on fracture toughness based on CTOD/CTOA and constraints ahead of crack tips in SENB specimen are studied. The results indicate that the critical values of CTOD/CTOA decrease with the increase of specimen thickness, but the constraint parameters are opposite. Furthermore, it finds that there is a near linear relationship between critical values of CTOD/CTOA and the stress constraint ahead of the crack tip. Thus, a constraint-corrected fracture failure criterion based on CTOD/CTOA is proposed, which can be used for the prediction and simulation of stable tearing crack growth in specimens and structures, made of this steel with any thickness value.     

The effect of specimen dimensions on mixed mode ductile fracture

The results from a series of experiments are presented to determine the effect of specimen dimensions on the ductile tearing resistance of A508 Class 3 forged steel at ambient temperature. Single edge notch tension specimens were subjected to Mode I, Mode II and combination of Modes I and II. Mode I tests on various specimen sizes reveal characteristic features found in earlier work, such as decreasing slope of the tearing resistance with increasing constraint (or specimen size). In contrast, for Mode II the tearing resistance is shown to be independent of specimen size, although dependent on initial crack length. The tests show that there is a competition between void growth and shear localisation as mechanisms for ductile crack extension. The dominance of one mechanism over the other is shown to be related to the local Mode I and Mode II components of the J-integral.     

A numerical analysis of failure characteristics of ductile layers in laminated composites

In this study, the failure of the ductile layers from collinear, multiple and delaminating cracks that occur in laminated composite systems was studied using a constitutive relationship that accounts for strength degradation resulting from the nucleation and growth of voids. The results indicate that, in laminated composites, void nucleation and growth ahead of the cracks occur at a much faster rate because of evolution of much higher stress values in the interface region. Except for short crack extensions, collinear and multiple cracks develop crack resistance curves similar to that seen for a crack in the ductile layer material as a homogenous isotropic cases. For delaminating crack cases, the fracture behaviour is strongly influenced by the delamination length. The resistance of the ductile layers to crack extension can be significantly reduced by short delamination lengths; however, for large delamination lengths the resistance to crack extension becomes greater than that seen for the ductile material. The results also show that, if the crack tip is at the interface, similar maximum stress values develop in the ductile layers as in the fracture test of the same ductile material, suggesting that ductile–brittle fracture transition behaviour of the ductile layers is dependent upon the extent of the cracks in the brittle layers and fracture characteristics of the brittle layers.     

Microstructure characterization of fractured steel beam-to-column connections

The failure of steel beam-to-column connections during the Northridge earthquake of January 1994 was both surprising and alarming to the structural engineering community. These steel moment resisting frames (SMRFs) are intended to behave in a ductile manner. However, in many steel buildings inspected in the Los Angeles area after the Northridge earthquake the connections exhibited brittle fracture. Despite recent laboratory testing of numerous large and small scale structural connections reasons for brittle fracture are still not clearly understood. Therefore, rational design methods to prevent or control brittle fracture in building structures have not been established. This paper investigates the contribution of the microstructure of welded connections to brittle failure. The various microstructures present in a fractured welded connection were characterized and their influence on crack initiation examined. The predominantly brittle failure is preceded in this case by a ductile crack which initiates in a brittle microstructure in the vicinity of the weld root, adjacent to the unfused backing bar.     

Ductile to brittle transition of an A508 steel characterized by Charpy impact test: Part I: experimental results

This study is devoted to the ductile-brittle transition behavior of a French A508 Cl3 (16MND5) steel. Due to its importance for the safety assessment of PWR vessels, a full characterization of this steel with Charpy V-notch test in this range of temperature was undertaken. The aim of this study is to provide a wide experimental database and microstructural observations to supply, calibrate and validate models used in a local approach methodology. Mechanical and fracture properties of the steel have been investigated over a wide range of temperatures and strain-rates. Effects of impact velocity on ductile-brittle transition curve, on ductile tearing and on notch temperature rise are presented and discussed. A detailed study of ductile crack initiation and growth in Charpy specimens is also carried out. From fractographic investigations of the microvoids nucleation around carbide second phase particles, a plastic strain threshold for nucleation is determined for this material. A508 Cl3 steels undergo a transition in fracture toughness properties with temperature, due to a change in fracture mode from microvoids coalescence to cleavage fracture. A systematic investigation on the nature and the position of cleavage triggering sites and on any change in the ductile to brittle transition (DBT) range has been carried out. This leads to the conclusion that manganese sulfide inclusions do not play an increasing role with increasing test temperature as recently mentioned in other studies on A508 Cl3 steel with a higher sulfur content. In a companion paper [Tanguy et al., Engng. Fract. Mech., in press], the numerical simulation of the Charpy test in the ductile-brittle transition range using fully coupled local approach to fracture is presented.     

An ultimate crack resistance concept

A two-parameter fracture criterion is discussed in terms of an ultimate crack resistance concept. The difference is found between the failure assessment diagram and the ultimate crack resistance. The concept of an ultimate crack resistance provides one with the ability to estimate crack resistance behaviour in cases of brittle, ductile and intermediate fracture states with the customary use of stress intensity factors.     

Probabilistic prediction of the crack resistance of nuclear pressure-vessel steels on the basis of a local approach. Part 2

On the basis of a new local probabilistic criterion of brittle fracture, a local criterion of ductile fracture proposed by the authors earlier, and the obtained approximate solution of the problem of stress-strain state near the crack tip, we develop a probabilistic model for the prediction of the crack resistance of pressure-vessel steels. The model enables one to predict the dependence of K _Ic on temperature for any given probability of brittle fracture and the influence of the thickness of the specimen on K _Ic. Bu using this model, we can also describe the temperature range of the brittle-ductile transition. The results of numerical calculations are compared with the experimental data for 15Kh2MFA pressure-vessel steel. It is shown that the proposed model fairly well describes the spread in the experimental data on the crack resistance of this type of steel. TsNII KM “Prometei,” St. Petersburg, Russia. Translated from Problemy Prochnosti, No. 2, pp. 5–22, March–April, 1999.     

A probabilistic model for cleavage fracture with a length scale--parameter estimation and predictions of stationary crack experiments

This study presents a large experimental investigation in the transition temperature region on a modified A508 steel. Tests were carried out on single-edge-notch-bend specimens with three different crack depth over specimen width ratios to capture the strong constraint effect on fracture toughness. Three test temperatures were considered, covering a range of 85 °C. All specimens failed by cleavage fracture prior to ductile tearing. A recently proposed probabilistic model for the cumulative failure by cleavage was applied to the comprehensive sets of experimental data. This modified weakest link model incorporates a length scale, which together with a threshold stress reduce the scatter in predicted toughness distributions as well as introduces a fracture toughness threshold value. Model parameters were estimated by a robust procedure, which is crucial in applications of probabilistic models to real structures. The conformity between predicted and experimental toughness distributions, respectively, were notable at all the test temperatures.     

面外约束对韧性材料的断裂韧度的影响

与脆性断裂的断裂韧度随厚度的增加而逐渐趋近于一个常数的变化趋势不同,韧性较好材料的弹塑性断裂韧度的特征为:在一定范围内随着厚度的增加,弹塑性断裂韧度逐渐增加。主要研究在保持试件的面内约束(a/w)保持不变的条件下,面外约束(厚度的变化)对同种材料的试件的断裂韧度的影响。首先采用韧性较好的材料进行断裂韧性实验,通过实验得到结果:试件的弹塑性断裂韧度J积分及临界裂纹尖端张开位移CTOD随着试件厚度的增加而线性增加。然后从能量平衡的角度出发,考虑在裂纹扩展过程中的所有的宏观能量耗散机制,根据通过实验验证的假设,化简后,得到最终的结果:一定程度上表征能量耗散的断裂韧度随着厚度的增加而增加,与实验结果吻合得较好。     

Constraint effects at brittle fracture initiation in a cast ferritic steel

The fracture resistance of a cast low carbon manganese ferritic steel intended for containers for spent nuclear fuel has been analysed by combining several approaches. Based on data from three-point bend specimens with shallow and deep cracks the effect of crack tip constraint at brittle fracture initiation has been followed. Q-parameter was used for the constraint quantification. The crack length effect on the fracture toughness–temperature diagram has been analysed and peculiarities of fracture behaviour in the lower shelf region have been explained. The role of cleavage fracture stress in brittle fracture initiation under the influence of crack tip constraint has been analysed.     

Probabilistic treatment of fracture using two failure models

A probabilistic methodology for brittle fracture based on two local failure models is presented. Probabilistic fracture parameters are obtained using a weakest link and a chain-of-bundles formulation. Both models define limiting distributions for the fracture stress described by a two-parameter Weibull distribution. Numerical procedures employing measured toughness data and finite element solutions are also described to calibrate the Weibull parameters. An application of the methodology then follows to predict geometry and stable crack growth effects on the distribution of macroscopic fracture toughness (J_c) for a high-strength steel. Measured fracture toughness values for a high-constraint geometry that exhibit no prior ductile tearing are effectively ‘transferred' to a different geometry having much lower constraint and in which tearing precedes cleavage. The inherent difficulty in predicting the scatter of experimental fracture toughness, as well as constraint and ductile tearing effects, within the scope of conventional procedures appears greatly reduced in the framework presented in this work.     

Ductile tearing assessment of high-strength steel X-joints under in-plane bending

This study reports an experimental investigation of a fatigue-cracked, pre-notched circular hollow section X-joints fabricated from high strength steels (with the yield strength higher than 800 MPa) subjected to brace in-plane bending. The circular hollow section X-joint entails a prefabricated V-notch near the weld toe at the crown position. The experimental procedure applies a fatigue pre-cracking cyclic load followed by a monotonic brace in-plane bending, which leads to brittle through-thickness crack propagation after some amount of ductile tearing. The ductile tearing assessment, integrating the fracture resistance curve obtained from the small-scale fracture specimens and the crack extension in the large-scale tubular joint, predicts closely the load level at which unstable crack extension takes place. The generic level 2A curve outlined in the BS7910 provides an un-conservative estimate on the failure load of the X-joint specimen. The parametric numerical investigation reveals that the strength definition for the cracked joints imposes a significant effect on the shape of the failure assessment curve.     

A Fracture Probability Competition Mechanism of Stress Corrosion Cracking

The stress corrosion cracking (SCC) of austenitic stainless steel was studied via polarization, slow strain rate and scanning electron microscope (SEM) techniques. Many SCC mechanisms have been proposed in which hydrogen embrittlement and passive film rupture-repassivation theories are generally accepted, but they can hardly explain the SCC mechanism of austenitic stainless steel in acidic chloride solution adequately, because the steel is in active dissolution state and cathodic polarization can prevent it from occurring. Our experiment shows that the anodic current increases the creep rate and decreases the plastic strength of the material on single smooth specimen as well as at the SCC crack tip. The fractured surface was characterized as brittle cleavage, while the surface crack of smooth specimen was almost vertical to the tensile strength, which can confirm that the cracks were caused by tensile stresses. A fracture probability competition mechanism of SCC was proposed on the basis of the experimental results combined with the viewpoint of ductile-brittle fracture competition. When the anodic dissolution current is increased to a certain degree, the probability of fracture by tensile stress will exceed that by shear stress, and the brittle fracture will occur. The proposed SCC mechanism can riot only explain the-propagation of SCC cracks but can explain the crack initiation as well. The strain on the surface distributes unevenly when a smooth specimen is deformed, so does the anodic current distribution. The crack will initiate at a point where the anodic current density is large enough to cause the material at a specific point to fracture in brittle manner.     

Dynamic crack propagation in large steel specimens

Dynamic fracture experiments on crack initiation and crack growth in single edge bend specimens are performed. The impact velocity is in the range of 14 to 50 m/s and the specimen size is 320×75 mm with a thickness varying from 18 to 40 mm. The experiments are recorded by high speed photography.Two different steel qualities are investigated and their constitutive characterisation are obtained from uni-axial tension tests and shear tests with strain rates in the range 10⁻⁴ to 10³ s⁻¹ and tension tests at temperatures between −196 and 600°C.One of the materials exhibits a transition from a ductile dimple fracture to a brittle cleavage fracture as the loading velocity increases and as the specimen thickness increases. Scanning electron microscope fractographs show that the density of plastic bridges within cleavage ligaments decreases with increasing impact velocity and with increasing specimen thickness. It is also noted that the local crack propagation direction deflects from the global one in cleavage fracture areas with a high density of plastic bridges.The other material fails in a ductile mode in all the investigated cases.     

CONSTRAINT EFFECTS ON DUCTILE CRACK GROWTH IN CLADDED COMPONENTS SUBJECTED TO UNIAXIAL LOADING

Abstract A two-parameter approach based on the J-integral and the parameter h, the ratio of the hydrostatic stress to the effective stress, was examined for ductile crack growth in cladded specimens. A series of cracked specimen configurations were tested and analysed by FEM to study the crack-tip constraint in different geometries. The test program consisted of homogeneous SEN specimens of a base material (A533-B steel), homogeneous SEN specimens of a cladding material (stainless steel weldment) and cladded specimens containing surface cracks through the cladding. Some issues concerning the cladding/base interface were also discussed from the basis of metallographical and fractographical examinations. While the crack growth initiation of the investigated materials appeared to be insensitive to the crack-tip constraints, the propagation of ductile crack growth was significantly influenced by crack-tip constraints. The crack-tip constraints in different specimen configurations could successfully be characterized by the parameter h. Prediction of crack growth along the crack fronts in two cladded specimens using the developed resistance laws accounting for constraint effects gave promising results.     

Residual stress determination in a dissimilar weld overlay pipe by neutron diffraction

Residual stresses were determined through the thickness of a dissimilar weld overlay pipe using neutron diffraction. The specimen has a complex joining structure consisting of a ferritic steel (SA508), austenitic steel (F316L), Ni-based consumable (Alloy 182), and overlay of Ni-base superalloy (Alloy 52M). It simulates pressurized nozzle components, which have been a critical issue under the severe crack condition of nuclear power reactors. Two neutron diffractometers with different spatial resolutions have been utilized on the identical specimen for comparison. The macroscopic ‘stress-free’ lattice spacing (d_o) was also obtained from both using a 2-mm width comb-like coupon. The results show significant changes in residual stresses from tension (300-400 MPa) to compression (−600 MPa) through the thickness of the dissimilar weld overlay pipe specimen.