Fracture Toughness Of Welded Joints Research Articles

Microstructure, tensile properties and fracture toughness of friction stir welded AA7075-T651 aluminium alloy joints

Abstract The main objective of this investigation is to study the microstructure, tensile properties and fracture toughness of friction stir welded (FSW) butt joints of 10 mm thick AA7075-T651 plates. The microstructural features of stir zone (SZ), thermos-mechanically affected zone (TMAZ), heat affected zone (HAZ) were analyzed using optical microscopy technique. The tensile properties were evaluated using smooth and notch tensile specimens and compared to base metal properties. The microhardness survey was done across the weld cross section and correlated to the failure of tensile specimens. Compact tension (CT) specimens were used to evaluate the fracture toughness of welded joints. The fractured tensile and CT specimens were analyzed using scanning electron microscopy (SEM). Results showed that the FSW AA7075-T651 specimens welded using axial load of 12 kN, tool rotation speed of 750 rpm and welding speed of 30 mm/min exhibited 412 MPa tensile strength and 9% elongation. It showed 88 and 89% of base metal strength elongation. The joints showed fracture toughness of 23 MPa m1/2 which is 80% of base metal fracture toughness. The superior tensile and fracture toughness properties of joints are mainly attributed to the evolution of finer grains in SZ due to the stirring action of FSW tool.

Effect of H2S Corrosion on the Fracture Toughness of the X80 Pipeline Steel Welded Joint.

To analyze the causes and mechanisms affecting the fracture toughness of X80 pipeline steel welded joints against H2S, the fracture toughness of different zones of X80 pipeline steel welded joints in both air and saturated H2S solution was investigated. The fracture toughness of welded joints degraded significantly in the saturated H2S solution, where the crack tip opening displacement (CTOD) characteristic value in the coarse grain heat-affected zone (CGHAZ) and weld metal (WM) was only 8% and 12% of that in air, respectively. However, the sub-critical grain heat-affected zone (SCHAZ) showed better resistance to H2S corrosion, with the CTOD characteristic value reaching 42% of that in air. The resistance of the welded joint to H2S corrosion was sensitive to microstructures. The grain boundary ferrite (GBF) presented in WM, and the angle of grain boundary orientation in CGHAZ was not conducive to hindering crack propagation. Moreover, the formation of the resultant hydrogen cracks owing to the H2S corrosion also reduced the fracture toughness of the welded joint.

Fracture toughness of TA31 titanium alloy joints welded by electron beam welding under constrained condition

Electron beam welding (EBW) of titanium alloy was carried out by simulating the constraint conditions in actual welding. The fracture toughness of welded joints was studied. It is found that the fracture toughness decreases a lot, only 70% of the fracture toughness of base metal. The fracture surface was mainly ductile fracture mechanism with a large number of dimples by SEM. It was found that the crack propagation path was straight and mixed mode of intergranular and transgranular fracture by OM. OM and TEM observations show that the weld is mainly composed of fine and disordered acicular martensite with a large number of dislocations and twins. The grain size of constrained weld is finer and more disorderly than that of non-constrained weld, and there is no obvious grain orientation observed by EBSD. The crack propagation behavior of constrained weld was analyzed by the model.

Numerical determination and experimental validation of the fracture toughness of welded joints

Most of the serious weldment failures lead to catastrophic consequences in terms of damage of other equipment, loss of production, and risks to workers’ health and safety. Hence, there is motivation to find margin between safety and disaster. This necessitates guaranteeing the integrity of a welded structure even if a crack is present. Therefore, influence of the material inhomogeneity and residual stresses on deformation and fracture behavior needs to be described precisely. Fracture mechanics parameters J - integral and CTOD have attracted great interest in recent years.Two three point bending specimens of base metal (BM) and weld metal (WM) were tested according to standard BS EN ISO 15653. Than, a two dimensional finite element analysis was performed in Ansys Workbench software to calculate J-integral, and compare with experimental results. The aim of this work is to explore the possibility of using the test results in the reliable prediction of weldment fracture.

Experimental Study on the Fracture Toughness of Welded Joints of API X90 High-grade Pipeline Steels Using Single-Edge-Notched Tension Specimens

The fracture toughness and R-curves of welded joints of API X90 pipeline steels were experimentally studied. Single-edge- notched tension (SENT) specimens and a normalization method were adopted. Joints welded by using three technologies (A, B, and C) were investigated. The results show that the SENT specimen could be used to test the fracture toughness and R-curves of the welded joint; the welded joint of welding method B presented the poorest fracture toughness; and the R-curves and fracture toughness of the heat-affected zone were seriously affected by the weld zone and base metal. A new idea to test the fracture toughness and R-curves of the heat-affected zone was also proposed.

Evaluation of inhomogeneity in tensile strength and fracture toughness of underwater wet friction taper plug welded joints for low-alloy pipeline steels

The microstructural characteristics of underwater wet friction taper plug welded joints for API X52 pipeline steel were investigated and tensile strength and facture toughness of the welded joints were experimentally evaluated. The microstructure of welded joints is considerably inhomogeneous. The tensile strength of welded joints shows heterogeneity in thickness direction. All of the tensile specimens taken from upper side of welded joints fracture at base material, while those taken from the bottom are broken at bonding interface or base material. The fracture toughness of welded joints deteriorates seriously, as compared with base material. The welded joints with notches in weld center have higher fracture toughness than those in bonding zone and the axial force has few influences on fracture toughness of welded joints. The bonding zone between plug and base plate should play a role as fissure defect for the whole welded joint, considering that it is tens of microns and much softer than its vicinity. In addition, the coarse grain, quenched martensite and Widmanstätten ferrite in weld region should also be responsible for the reduction of fracture toughness.

Effect of crack length on fracture toughness of welded joints with pronounced strength heterogeneity

The integrity assessment of repaired welds is dependent on accurate characterization of their fracture behavior and limit load estimation. The final weld consists of multiple microstructures due to different weld consumables used for repair welding. As a result, a large degree of heterogeneity is to be expected. The variation of mechanical and fracture properties within the weld influences the fracture behavior and limit load capacity of repaired weld. This motivated the authors of this work to adapt existing testing methods in order to characterize the fracture behavior of repair welds and to develop limit load solutions which include the effects of weld heterogeneity. This work focuses on the idea of implementing T-stress in characterization of fracture toughness at the onset of crack tip blunting. By normalizing J-integral by stress biaxiality coefficient β, obtained from T-stress solution, a new fracture toughness parameter is derived which tends to be dependent only on the distance from crack tip to interface between two mismatched weld materials.

Effect of welding technology on the mechanical properties of welded joints in pipes for deep water offshore gas pipelines

Results of investigations into the dependence of mechanical properties of welded joints of pipes for offshore pipelines on welding conditions are presented. In particular, the results show that the mechanical properties of pipes of the strength categories K60 (X70) and K56 (X65) are influenced mainly by the heat input, the fracture toughness of welded joints, and pipes of a high strength category are lower by 30–35% than in the case of the K56 (X65) pipes. The hardness of the weld metal in the upper layers is 15–17% higher for automatic submerged arc welding and in the lower layers for gas-shielded automatic arc welding. In inspecting the mechanical properties of the welded joints, special attention must be given to the longitudinal welded joints in the pipes.

Numerical analysis on the welding residual stress and fracture toughness of the heavy thick steel welded joints by welding processes

This study examined the welding residual stress and fracture toughness of 78mm thick steel electro gas welding (EGW) and flux cored arc welding (FCAW) welded joints by numerical analyses of the thermal elasto-plastic behavior and fracture toughness(KIC). The residual stress, fracture toughness characteristics and production mechanism on the welded joints were clarified. Moreover, the effects of the welding process (EGW and FCAW) on the welding residual stresses and fracture toughness of welded joints were evaluated. The results showed that the new welding process (EGW) appears to be an effective substitute for the existing welding process (FCAW) in a thick steel plate with high strength.

An evaluation of the fracture toughness of a thin-walled pressure vessel with account for the structural and mechanical inhomogeneity of the welded joint

A finite-element model has been developed for calculating the fracture toughness of welded joints with account for the random distribution of the structural and mechanical properties of the metal within a weld area. The influence of the coefficient of variation of the yield strength upon the value of the J-integral for a weld crack has been shown using the proposed model. The fracture toughness calculations of a welded thin-walled pressure vessel have been carried out and correction functions for the J-integral have been determined based on the experimental data on the random distribution of the yield strength in the weld area.

Fracture Toughness of Different Locations of Spiral Submerged Arc Welded Joints in API X80 Pipeline Steels

API X80 pipeline steels have been widely used in long-distance transportation of oil or natural gas under high pressure due to their high transportation efficiency, low energy loss and low production cost. While the fracture toughness of welded joints are harder to measured. In this paper, J-integral resistance curves of Spiral Submerged Arc Welded (SSAW) joints were examined at different locations of the BM, WM and HAZ. The grain coarsening induced by welding thermal cycling leaded to weaker toughness in WM and HAZ than the BM. The optical microscope (OM) and scanning electron microscope (SEM) were used to analyze the fracture mechanism.

Fracture toughness of welded joints of a steam turbine low pressure rotor

In order to ensure safety and reliability of a steam turbine welded rotor, the present investigation focused on evaluation of the crack resistance parameter of the base metal (BM), weld metal (WM), and heat affected zone (HAZ) of a turbine rotor welded joint constituent at both room temperature and 300℃. The property of crack resistance was evaluated in terms of fracture energy JIc or plane strain fracture toughness KJIc. Three-point bend experiments were performed to obtain a J-integral based crack resistance curve ( J–R curve) from a single specimen in accordance with ASTM E1820, from which the fracture toughness was determined. To construct a J–R curve, the corresponding crack extension was calculated by incorporating the crack opening displacement measured by a clip-gage, in a compliance method. From the experimental results, the WM was found to have the lowest KJIc values whereas the HAZ was found to have the highest KJIc values at both room and high temperature although the HAZ had little or no crack extension during the experiments. It was also observed that the HAZ samples fractured with crack jumped toward the fusion line. Microstructural root-cause analysis of the abnormal HAZ behavior was performed and discussed.

Fracture Assessment of Cracked Welded Structures Considering the Heterogeneity of Welded Joints

The crack-like defects often cause the failure of welded structures. Most of the fracture assessment procedures that are used for solving this problem are based on deterministic approaches, for example Zerbst (2007). But the experimental results show that the mechanical properties and fracture toughness of welded joints are significantly changing with a random variation. Taking this into account the fracture assessment of welded structures requires improved numerical approaches. A numerical model of heterogeneous welded joint with randomly varying yield strength was proposed. The results show that the mechanical heterogeneity leads to higher J-integral values for the defects. This implies that neglecting the mechanical heterogeneity of welded joints results in underrating of the risk of failure of welded structures. The proposed numerical estimation of the fracture toughness is used for some typical welded joints and thin-walled pressure vessel. The relations between the variation of yield strength in cracked welded joints and J-integral are presented as the correction functions for calculated parameter of crack resistance.

Study on Fracture Toughness of welded joints for heavy-thick steel plates by Centre-Notched small size specimen

A centre-notched small size tension test specimen was developed to evaluate the toughness against unstable brittle fracture from fatigue crack, which is initiated and propagated from the surface of the butt-welded joints Qof heavy-thick steel plates. Centre-notched small size tension test specimens were made from butt-welded joints of 70 mm heavy-thick steel plates and employed for the tests using this plate thickness as the specimen width. In this paper, the test results are evaluated and investigated.

A ductile failure model applied to the determination of the fracture toughness of welded joints. Numerical simulation and experimental validation

A ductile-failure model for analysing the fracture behaviour of welded joints has been implemented. Finite element analyses of mismatched welded joints have been performed using the computational cell methodology applied to SE(B) specimens. Different crack lengths, material mismatching, and widths of weld metal have been considered. Ductile parameters have been experimentally and numerically obtained. The influence of geometry and material mismatching on the fracture behaviour of cracked welded joints has been validated by means of a testing program. In addition, the experimental results have been explained through the crack tip constraint, which has been numerically determined.

Evaluation of HAZ Fracture Toughness of Welded Joints with Strength Mis-Matching by the Local

In this study, the local approach was applied to evaluate the cleavage fracture resistance ofHAZ-notched specimens with strength mis-matching between the base and weld metals. The main purpose of this study is to demonstrate that the critical Weibull stress at fracture initiation of HAZ-notched specimens can be material property independent of strength mis-matching.For the fundamental study to discuss the effect of strength mis-matching near the notch on the fracture resistance by the local approach, homogeneous specimens and model specimens with only strength mismatching near the notch were used. By using the critical Weibull stress obtained for the homogeneous specimen, strength overmatch effect and undermatch effect on the critical CTOD value were predicted. Close agreement was observed between the critical CTOD value predicted and that obtained by the experiment. As a result, independence of strength mis-matching on the critical Weibull stress was verified.The Weibull stress for the HAZ-notched specimens with strength mis-matching (match and overmatch specimens) was evaluated in consideration of that almost all specimens have fracture initiation points in the CGHAZ (LBZ : Local Brittle Zone). It was demonstrated that the cleavage fracture resistance for the HAZ-notched specimens controlled by the CGHAZ can be uniformly treated by using the Weibull stress irrespective of the existence of strength mis-matching.

Influence of strength matching and crack depth on fracture toughness of welded joints

In the present paper, fracture toughness for the specimen of welded joints with different strength matching and crack depth is investigated by finite element computer simulation and a fracture mechanics test. This investigation focuses especially on the effects of strength matching and crack depth on the triaxial stress state at a crack tip, crack tip opening displacement (CTOD), and J-integral fracture toughness. The results show that there exists the fracture toughness peak (FTP) in the curve of fracture toughness vs crack depth. The location of the fracture toughness peak is influenced by the yield strength for a homogeneous material, and it is also influenced by the strength matching for welded joint. The increase in the strength of weld metal could be of benefit to the fracture-resistant property of the weld metal, if the toughness of the weld metal is similar to the parent metal.

Correlation of microstructure and fracture properties in weld heat- affected zones of thermomechanically controlled processed steels

This article presents a correlation study between the microstructural parameters and fracture properties in the weld heat-affected zones (HAZs) of high-strength low alloy (HSLA) steels,i.e., a normalized steel and four thermomechanically controlled processed (TMCP) steels. The influence of the local brittle zone (LBZ) on toughness was investigated by means of simulated HAZ tests as well as welded joint tests. The intercritically reheated coarse-grained HAZ ex-hibited the lowest impact energy over the testing temperature range, indicating that this region was the LBZ. By comparing the volume fraction of martensite islands with impact energy val-ues, this LBZ was attributed mainly to the significant increase in the amount of martensite. Niobium was also found to have a deleterious effect on the HAZ fracture toughness because of martensite hardening. This suggests that the formation of martensite islands must be controlled by proper design of chemical compositions to reduce the carbon equivalent and by using the proper welding conditions to limit cooling rates in order to optimize the fracture toughness of welded joints of TMCP steels.

High-strain fatigue characteristics and fracture toughness of welded joints in components of water turbines

High-strain fatigue characteristics and fracture toughness of welded joints in components of water turbines

Characteristics of low cycle fatigue and fracture toughness of welded joints of water turbine components: Gregor, V. and Polak, P. Zvaranie Feb. 1989 38, (12), 37–40 (in Slovak)

Design and performance analysis of hydraulic turbines are two very crucial aspects of any hydro power project which ensure economical and efficient functioning of these plants. In this context, numerical hydrodynamic analysis of water turbines has been bringing revolutionary changes in the process since the last decade. In the recent years, advancements in computational resources have made the job quite easier and more reliable but brought new challenges as well. Appropriate use of various models and numerical solution schemes in order to capture different hydrodynamic phenomena is highly debatable, even today, due to the lack of generality and robustness. This paper illustrates various utilities of computational fluid dynamics (CFD) in simulating hydraulic turbine operations. The work is mainly aimed at critically reviewing various computational methodologies for achieving different objectives of hydraulic design and performance evaluation of water turbines. Regarding this, different objectives of computational investigations of hydraulic turbines such as derivation of performance characteristics, analysis of various unsteady phenomena, prediction and analysis of cavitation and determination of various losses are discussed in detail. A discussion on boundary conditions and turbulence modeling for capturing different hydrodynamic phenomena at design and off-design operations of hydraulic turbines is one of the major scopes of the paper. Also, various optimization techniques in combination with CFD for hydraulic performance optimization of the turbines are briefly discussed. This work is a dedicated attempt to bring various aspects of computational hydrodynamic investigation of hydraulic turbines on a single platform. Having done the extensive literature review of the CFD utilities for the turbines along with disseminating the best practices, certain research gaps and loose ends are identified and given as future scopes which are also the major highlights of the work.