Fracture Behavior Of Welded Joints Research Articles

Impact toughness behaviour of A516 GR. 60 steel welded joints

Impact fracture behaviour of welded joints made of A516 Gr. 60 is analysed as a commonly used carbon structural steel for welded structures. Impact testing on Charpy instrumented pendulum is performed using specimens with a notch at positions: the base metal (BM), heat-affected-zone (HAZ), and weld metal (WM). The total impact energy is presented as a sum of energies for crack initiation and crack propagation to make the analysis more sophisticated and comprehensive. The effect of temperature up to -60 °C was also analysed to prove the A516 Gr. 60 steel usability at subzero conditions.

Fracture behavior analysis of X80 pipelines welded joints with unequal wall thickness

Pipeline transportation is one of the main methods for transporting oil and natural gas worldwide. The structural safety of welded joints of large diameter pipes with cracks and unequal wall thickness has become a research focus due to discontinuous geometrical structure and cracks in weld seams. In this paper, the tensile experiment and analysis of specimens with unequal wall thickness across welding seam based on the digital image correlation technology are carried out. Moreover, the influence of transition length and transition form on fracture behavior of welded joints of unequal wall thickness pipeline is obtained. In addition, a three-dimensional finite element model of welded joint of unequal wall thickness pipe with oblique line transition form and circular arc transition form is established. and the accuracy of the full-scale pipeline finite element model modeling method is verified by small-scale experiment data. On this basis, the influence of geometrical parameters of the transition section and wall thickness ratio, pipe diameter, and weld strength matching coefficient on the crack driving force of welded joints with unequal wall thickness is obtained. From the perspective of strain-based design, a method for obtaining the minimum transition length section and avoiding girth weld fracture is proposed. The research results guide the design and safety evaluation of welded joints with unequal wall thickness.

Direct microwelding of dissimilar glass and Kovar alloy without optical contact using femtosecond laser pulses

In the current microwelding process using femtosecond (fs) laser between dissimilar materials, surface polishing and pressure assistance, so-called optical contact, are believed necessary. In this paper, direct welding of soda lime glass and Kovar alloy using a fs laser is investigated to overcome the limit of optical contact. The processing of fs laser welding is comprehensively studied by varying the laser power, welding velocity and the number of welding. The shear joining strength is as high as 2 MPa. The cross-section of glass-Kovar alloy joints, the elemental diffusion and the fracture behavior of welded joints were studied. The results show that the fs laser irradiates the surface of Kovar alloy, micron/nanometer-sized metal particles are generated. These particles perform the role as an adhesive part in the welding process. It is believed that the Si atoms diffuses to Kovar alloy from the glass and partially replaces the Fe2+ ions on the surface of Kovar alloy, indicating that the mixing and interdiffusion of materials have occurred during the welding process. Finally, the welded sample was tested and has excellent water resistance and sealing property. Furthermore, to justify that this method can be applied to other stack ups, the glass-copper, the glass-Al6063 and sapphire-ceramic are also welded together. This work greatly simplifies the fs laser microwelding process and promotes its industrial applications, such as optoelectronic devices, medical devices and MEMS.

Improving Microstructural and Mechanical Properties of AA2024 Base Metal by Adding Reinforced Strip Width of AA7075 via Vertical Compensation Friction Stir Welding Technique

Improving microstructural and mechanical properties of AA2024 base metal reinforced with AA7075 via vertical compensation friction stir welding (VCFSW) have become a very vital research area to reinforce the weak part of nugget zone in welded joints. So this paper presents a novel and simple method to improve microstructural and mechanical properties especially ductility and fracture locations of welded aluminum joints by VCFSW technique. Anyhow, most of the welded parts are subjected to the forming processes so improvement in the property of ductility and fracture behavior of welded joints is required. One solution to increase ductility and improve fracture behavior of welded joints is the use of interlayer compensation strip width as a reinforcement which has superior mechanical properties between two blanks of base metal during friction stir welding process. The effect of changing interlayer compensation width was investigated on the microstructural characterization (by an optical microscope, scanning electron microscope and transmission electron microscope) and mechanical properties (using microhardness, tensile and bending tests). Microstructure observations revealed that the tailor-welded joint without compensation interlayer contains micro-voids in the stir zone, while in tailor-welded joints with compensation interlayer, the voids completely disappear on the microscale. X-ray diffraction detected that the use of compensation interlayer of AA7075 between two AA2024 blanks led to the appearance of two precipitates of intermetallic phases, Al2Cu and Mg2Zn, on grain boundaries of aluminum which improved ductility and fracture behavior of welded joints. Ductility enhanced till 54.4% at compensation interlayer width 2 mm when compared to the joint without compensation interlayer. Lower hardness values were detected at nugget zone in joint without compensation interlayer. By contrast, highest hardness values in the softened regions were obtained in compensation interlayer joints. Finally, the highest bending angle occurred without any cracks in case of compensation interlayer of 2 mm at ultimate bending load.

Effect Range of the Material Constraint-I. Center Crack.

Material constraints are important factor effects on the fracture behavior of welded joints. The effect range of the material constraint is an important and interesting issue which needs to be clarified, including whether the effect range of a material constraint exists or not, who will affect it, and whether the material constraint is affected by the no adjacent area or not. In this study, different basic models which reflect different single metallic welded joints, bimetallic welded joints and dissimilar metal welded joints were designed, and the fracture resistance curves and crack tip strain fields of the different models with various material constraints were calculated. Based on the results, the questions above were answered. This study has significance for developing solid mechanics, optimizing joint design, structure integrity assessment, and so on.

Development of Creep Damage in Similar Weld Joints of P92 Steel Pipe

The microstructure and creep behaviour of the welded joints of P92 steel pipe were investigated in order to determine the influence of orbital heat welding technology on the creep resistance. Creep specimens were machined from the welded joints. Tensile creep tests of welded joints were performed at 873 K using different stresses. The microstructure of tested specimens was investigated by scanning electron microscope Tescan equipped with an electron-back scatter diffraction. The creep results showed that the creep fracture strain of the welded joints decreases with decreasing value of applied stress. Microstructure investigation showed that fracture behaviour of welded joints is influenced by an enhanced cavity formation at grain boundaries in the heat-affected zone causing lower fracture ductility.

On lap shear strength of friction stir spot welded AA6061 alloy

The present work investigates the influences of different tool pin shapes along with tool rotational speed and dwell time on lap shear strength of friction stir spot welded AA6061 alloy. Suitable experimental design and subsequent analyses are carried out to study the effect of process parameters and determine the optimal parameter settings. Results indicate that shear strength is significantly influenced by tool rotational speed followed by pin shape and dwell time. The fracture surfaces are examined both at a macroscopic and microscopic level to study the fracture behavior of welded joints. It is observed that for most of the joints, crack initiated from the joint periphery and failure of the joints mainly occurred due to thinning of the upper sheet. Primary mode of failure for most of the joints is observed as shear fracture with presence of micro-voids/dimples elongated along the loading direction. Significant amount of tool wear and profile damage is also observed with some traces of material transfer from the workpiece to tool pin. Towards the center of the joint tearing of the adhered metal is also noted. Finite element (FE) analysis is carried out to simulate the temperature, stress and effective strain distribution on workpiece surface for different tool pin profiles. FE results indicate that more heat is generated for circular tool pin profile; however, more strain is generated for square tool pin due to plastic deformation of base metal by the tool pin corners and improves the mechanical strength of welded joints.

Investigation of resistance spot welds between DP450 steel and aluminum alloys

Abstract In this study, the resistance spot weldability of DP450 steel and aluminum alloys (AA1050, AA6061 and AA7075) was investigated and the interfacial characteristics of fracture zones was examined via microanalysis, scanning electron microscopy and energy dispersive spectroscopy (EDX). In addition, tensile shear load tests and hardness tests were performed to determine the mechanical properties and fracture behavior of welded joints. These results suggest that the tensile shear strength of the DP450/AA1050 and DP450/AA7075 joints was not affected by the reaction layer formed at the welding interface. This indicates that the spot weldability of DP450/AA6061 is better than DP450/AA1050 and DP450/AA7075 Al alloys.

The microstructural evolution and mechanical properties of resistance spot welded Fe–31Mn–3Al–3Si TWIP steel

In this study, similar resistance spot weldability of an Fe–31Mn–3Al–3Si Twinning Induced Plasticity (TWIP) steel was investigated. The microstructural evolution was studied by means of optical microscopy (OM) and scanning electron microscopy (SEM) in order to evaluate the effects of experimental conditions on spot welded (RSW) joints. In addition, tensile–shear load bearing capacity of welded materials and failure modes were precisely inspected to determine the mechanical properties and fracture behavior of welded joints. The results illustrated that due to the expulsion phenomenon, optimum welding parameters for the experimental TWIP steel are shifted to lower values. Meanwhile, OM indicated that the fusion zone (FZ) held a dendritic structure; however, heat affected zone (HAZ) exhibited a significant grain growth in the narrow band. At upper welding parameters, shrinkage cavities and, in certain specimens, in microscopic scale inter-dendritic micro-pores were observed.

Microstructural Analysis of Ballistic Tests on Welded Armor Steel Joints

The deformation and fracture behavior of welded joints made from quenched and tempered steel closely conforming to AISI 4340 were investigated. Due to weld thermal cycles and under matching fillers, the welded armor steel joints showed poor ballistic performance compared with the base metal (BM). The problems encountered in the past were reduced by depositing a soft austenitic stainless-steel buttering layer in between the BM and the hardfaced layer. This method showed enhancement in ballistic performance and good weld integrity. In this investigation, an attempt is made to investigate the microstructure after ballistic testing on the weld metal zone consisting of the hardfaced interlayer. The results reveal the microstructural characteristics before and after ballistic testing of armor steel welds fabricated using the shielded metal arc welding (SMAW) process.

Influence of the moisture content on the fracture characteristics of welded wood joint. Part 1: Mode I fracture

Abstract Friction welding is a joining technique for wood materials. The positive aspects of this technique are the speed of processing and the absence of chemical or mechanical agents, but the welded joints are not water resistant. To understand better the effect of moisture on the fracture behavior of welded joints, their fracture characteristics have been investigated. The double cantilever beam specimens were tested, which permit to compute the mode I energy release rate of a welded joint. The results confirm the negative effect of moisture on the fracture properties of the joint. The data concerning the maximal tensile strength of the joining material were collected by uniaxial tests and implemented in a finite element model to establish a cohesive law, which describes the behavior of welded pieces in terms of moisture content.

Effect of aging treatment on the microstructure and mechanical properties of the similar and dissimilar 6061-T6/7075-T651 RSW joints

The similar and dissimilar resistance spot weldability of 6061-T6 and 7075-T651 aluminium alloys was investigated in this study. Resistance spot weldability of commercial and aged samples in different variations was examined. The microstructure, microhardness, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD) and electron probe micro-analyses (EPMA) were conducted to study the effects aging heat treatment had on similar and dissimilar resistance spot welded (RSW) joints. In addition, tensile shear tests were performed to determine the mechanical properties and fracture behaviour of welded joints. The tests concluded that pre-weld aging heat treatment improves mechanical properties.

Evaluation of Fracture Toughness Test Data for Multilayer Dissimilar Joint Welds Using a Weibull Stress Model

Abstract Fracture behaviour of welded joints with dissimilar weld metals (WMs) has been investigated in this paper. A low-toughness WM and a high-strength WM with a moderate toughness were layered alternately in the thickness direction. Fracture tests were conducted with 3-point bend (3PB) specimen and tension plate with a through-thickness crack. The 3PB specimen showed a multistage fracture. The first fracture was originated from the low-toughness WM. On the other hand, no multistage fracture occurred in the tension plate, although the low-toughness WM was responsible for brittle fracture initiation in the same manner as in the 3PB specimen. The fracture toughness of the multilayer dissimilar joint was apparently larger than the welded joint made with the low toughness WM only. The weakest-link model has been applied for analyzing those toughness properties of the multilayer dissimilar joint. The estimated fracture toughness by the weakest-link model was not necessarily consistent with the toughness data, which was because of the local stress elevation in the low-toughness WM area close to the high-strength WM. Because of this, a modified Weibull stress model was used to estimate fracture toughness values for the multilayer dissimilar joint WM and to account for an active fracture zone length scale in the multilayer weld joint materials.

Structural Changes, Evolution of Damage Parameters and Crack Propagation Behaviour in Welded Plastic Pipes

The microstructure as well as the local mechanical and fracture behaviour of welded joints in plastic pipes made form polyethylene and material zones outside of the welded joints have been analysed using recording microhardness testing, laser extensometry and crack resistance curve tests. In has been found that the mechanical basic properties and damage kinetics are clearly depending on the welding parameters and additional notching.

Metallurgical investigation of the aging process on tensile fracture welded joints in pipeline steel

The effects of aging in the tensile fracture behavior of welded joints of API5L-X52 pipe steel were studied by accelerating aging at 250 °C for different periods of time. The weld metal, heat affected zone and base metal, showed an increase in yield strength while the strain-hardening exponent decreased at early stages of aging. A maximum strength and minimum hardening exponent was found at 500 h due to peak-aging. Subsequently, both properties exhibited an opposite behavior due to over-aging. Tensile fractured specimens for the three different zones exhibited ductile failure, presenting microvoid morphology associated with the coalescence of microcavities. An increase in void density and a reduction in diameter during short periods in the fractured specimens were observed. The maximum density and minimum diameter of voids were obtained at 500 h and were linked to the improvement of strength and precipitation of nanoparticles. Afterward, the fractured surfaces exhibited a reduction in density and the diameters of voids were larger, having been induced by the deterioration of strength and coarsening of particles.

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.

Modelling Local Brittle Zones in Welds Using the Finite Element Method

This paper reports the results of a numerical study concerning the influence of local brittle zones intersecting the crack front on the fracture behaviour of welded joints. This work was performed using the numerical simulation of the three point bending test of weld samples with different amount of brittle structures at the crack front. Using 3D finite element discretization it was possible to simulate welded samples with very small fractions of brittle zone at the crack front, such as 5 %. Comparing the results of samples with increasing proportion of brittle zone it was observed a significant decrease in the crack growth resistance with increasing amounts of brittle material. This decrease in crack growth resistance was obtained even for samples with very small amounts of brittle material at the crack front.

Metallography of 9Cr steel power plant weld microstructures

The microstructural characterisation of welded joints subject to creep and the formation of the heat affected zone (HAZ) in tungsten alloyed high temperature 9% chromium steels is extremely important in modern power station construction. In these investigations, creep specimens were made available taken from fabrication welds in steel castings. The results of the hardness measurements carried out are presented, together with the qualitative and quantitative results of metallographical investigations. Special attention is paid to softening in the HAZ and the effect which this has on the creep resistance and fracture behaviour of welded joints. TEM investigations with the aid of energy filtering (EFTEM) were used to clarify the mechanisms taking place. The results of the metallurgical investigations will be compared with the results obtained in actual welds.

Effect of dynamic loading on fracture behavior of welded joints of structural steel

Effect of dynamic loading on fracture behavior of welded joints of structural steel

Investigation of fracture toughness of SAE 1020 welded using rutile covered electrodes, and determination of mismatch factor

AbstractThe aim of the present work is to examine the yield strength mismatch between the weld metal and the base metal, which affects the deformation and fracture behaviour of welded joints. Specimens prepared from SAE 1020 steel plates were welded using three rutile covered electrodes having different tensile strengths. Fracture toughness of the welded zones was investigated based on the behaviour of cracks caused by fatigue. The mechanical properties of the weld metal and heat affected zone (HAZ) were examined, and graphs were obtained to compare crack progress values Δa (mm) and welding material toughness J (N mm-1), which is a fracture mechanics parameter. Microhardness investigation was carried out on the weld metal, base metal, and HAZ transitions via measurements along lines on four different zones of the specimen. Micrographs of the specimens were also taken.