Weld Toe Research Articles

Fatigue fracture assessment of 10CrNi3MoV welded load‐carrying cruciform joints considering mismatch effect

AbstractFatigue experiments and numerical simulations based on the linear elastic fracture mechanics (LEFM) theory were conducted on the evenmatched (EM) and undermatched (UM) 10CrNi3MoV load‐carrying cruciform welded joints (LCWJs). The study firstly experimentally investigated the fatigue crack growth rate (FCGR) of base metal, EM, and UM weldments. The corresponding Paris parameters as essential input data are provided to assess the fatigue crack propagation behavior for weld toe and weld root failure caused by notch stress concentration variations. On the one hand, the stress intensity factors (SIFs) at weld toe and weld root were calculated considering the effects of LCWJ specimen geometries, initial crack types, and sizes. The comparisons between simulated results and standards analytical solutions were executed, which exhibit good accordance. It proved that the fatigue fracture simulation procedure based on LEFM is appropriate for the fatigue assessment of LCWJs. Eventually, it conducted the parametric analysis by predicted S–N curves, which included in the weld length, initial crack shape, initial crack size, penetration length, and materials fracture parameter, to explore some safety assessment reference lines for both failure modes of LCWJ.

Fatigue life extension of existing welded structures via high frequency mechanical impact (HFMI) treatment

High Frequency Mechanical Impact (HFMI) is one of the post-weld treatment methods. In this study, comparative axial fatigue tests were conducted on as-welded and HFMI-treated welded transverse attachment details. The test results demonstrated the efficiency of HFMI-treatment in fatigue life extension of cracked welded structures, providing that the existing crack size is less than 1.2 mm. Cracks were created in some specimens through fatigue testing before HFMI-treatment, while other specimens were not subjected to any fatigue loading prior to treatment. Many of the treated specimens ran-out after 10 million cycles of loading when tested at a stress range of 150 MPa. Therefore, the stress range was increased to 180 MPa or 210 MPa. No remarkable difference was found between the fatigue strength of the crack-free and the cracked treated specimens. It was found that the induced compressive residual stress can exceed the material yield limit, and reach a depth larger than 1.5 mm in most of the cases. The induced compressive residual stress, the local material hardening, the increase in weld toe radius, the change in crack orientation and the shallowness of the crack size were the causatives of the obtained long fatigue lives of the HFMI-treated specimens. Besides, linear elastic fracture mechanics calculations were conducted to predict the fatigue lives of as-welded and HFMI-treated details. The results were in agreement with the experiment. Moreover, the calculations showed that the initial crack size, the clamping stress and the induced compressive residual stress were the main factors behind the scatter in fatigue lives.

Stress magnification effect of initial deformation on the notch stress field and fatigue strength of thin plate welded joints

Compared with thick plate welded joint, the welding joint of thin plate will produce initial deformation due to its low bending rigidity. The existence of initial deformation will cause the welded structure to produce secondary bending effect, which will produce greater stress magnification effect at the weld toe and seriously affect the fatigue strength of thin plate welded joints. Therefore, based on the correction formula of thick plate, considering the influence of initial deformation and geometric nonlinearity of thin plate, this paper deduces the stress magnification factor formula at the weld toe of T-shaped and cruciform specimens. The accuracy of the revised formula is further verified by comparing the notch stress calculated by the modified formula with the FE results. Finally, the modified formula is applied to the notch stress and fatigue evaluation of typical thin plate welded joints respectively. The results show that the proposed notch stress calculation formula can fully consider the stress amplification effect of thin plate structure, and can be used to quickly evaluate the notch stress field and fatigue strength of thin plate welded joints.

Fatigue damage of welded high-strength steel details improved by post-weld treatment subjected to critical cyclic loading conditions

High-frequency mechanical impact (HFMI) treatment has proven its potential in many aspects, however the improvement under the conditions of high stress ratios, stress ranges and variable amplitude loading (VAL) has not been sufficiently addressed. In that respect, the aim of this work is to verify the indications and trends in the recently published literature and to understand the local weld toe mechanisms related to several cyclic loading conditions. The investigations were performed for three types of welds subjected to high stress ratios, ranges as well as VAL in order to represent the real-life scenarios, where the aim was to include the state of compressive residual stresses, cyclic behaviour of material and eventually estimate the fatigue life.The induced beneficial stresses were relaxed after applying compressive overloads (OL). However, a tensile OL indicated a potential benefit, by reducing the extent of residual stress relaxation. With the increasing stress ratio (R) and maximum stress (σmax), the improve- ment due to the HFMI decreased 59% as compared with the cases for OL = 0.8fy. The estimated damage decreased rapidly from R = −1 to R = 0.2, however it decreased slowly from R = 0.2 to R = 0.5, indicating that the effect of HFMI was reduced for high stress ratios due to the stress relaxation. The remaining improvement was then governed by the modification of weld geometry and the strain hardening effect. The fatigue damage was decreased from as-welded (AW) to HFMI states. For R = −1, longitudinal attachments were the most improved joint with 86%, while transverse attachments and butt joints were improved by 73% and 61%, respectively. The fatigue damage was increased when relatively larger block loads applied. Hence, the HFMI improvement was reduced 20% for transverse and longitudinal attachments and 14% for butt joints. The damage for σmax = 0.6fy in HFMI was similar to the damage for σmax = 0.3fy in AW, despite a doubling of the maximum CAL, which indicated promising potential of the HFMI improvement.

Effects of the Number of Welding Passes on Microstructure and Properties of 2219-C10S Aluminum Alloy TIG-Welded Joints

Effects of the number of welding passes on microstructure and properties of 2219-C10S aluminum alloy tungsten inert gas (TIG) welded joints were analyzed in detail. The first capping weld had a solid solution effect on the backing weld. The second capping weld had a solid solution effect on the backing weld close to the capping weld and had an aging effect on the backing weld away from the capping weld. The sizes and quantities of the second phases in partially melted zone (PMZ) and over-aged zone (OAZ) increased slightly with the number of welding passes increased. The hardness of OAZ decreased slightly and its width increased slightly. The larger strain distribution range before tensile fractured of the joints was narrower and the range of plastic deformation of the joints was not significantly improved. The eutectic structure and θ phase at weld toes were not conducive to the tensile properties of joints. The tensile properties of double-pass welded joints were relatively ideal.

Crack detection via strain measurements in fatigue testing

AbstractFatigue cracks have appeared as a significant issue for joints and connections in existing steel structures in the last decades. Therefore, those are a major inspection and maintenance matter for any steel structure's operator. This emphasises the importance of using a reliable detection method to determine the crack size and assessing the severity of such a crack on the structural integrity of a structure. In this article, the effectiveness of strain measurement in detecting fatigue cracks in transversal non‐load carrying welded attachment subjected to out of plane axial loading is studied. Numerical analysis and experimental investigations allowed to correlate the decrease in strain measured by attached gauges to the crack depth at the weld toe. In addition, different strain evolution patterns were found during fatigue testing, and the fracture surfaces of the specimens were observed to interpret these patterns. Moreover, the crack position with respect to the weld toe surface was predicted via strain measurements.

Fatigue strength distribution and probabilistic evaluation on stainless steel welded components under mixed mode loading

Plane sheet specimens and their welded components were examined through fatigue tests, and the data were evaluated by the base line regression model. The three-parameter Weibull estimations showed the most accurate approximation of the measured values with the median rank treatment. The fatigue strengths, limits, lives and failure probabilities under different loading conditions became predictable. The fatigue strengths and limits under mixed mode loading were compared with the Gough–Pollard relation, and could be predicted from the load ratio of the torsion to bending. The notch factors of welded components were 1.24–2.55 at the load ratio of 0.31. The fatigue strengths and scatters mainly depended on the weld flank angle, and had excellent agreement with the weld shape parameter which included the influence of the weld toe radius. They varied with the material factor which included the influences of the non-uniform microstructure, weld imperfection, etc.

Stress concentration factors of brace in-plane loaded bird-beak SHS gap K-joints

Experimental and numerical investigations have been performed to study the stress concentration factors (SCFs) of brace in-plane loaded bird-beak gap K-joints. Four bird-beak SHS gap K-joint specimens were tested. Hot spot strain distributions along the weld toes at crown and saddle area were determined, from which the strain concentration factors (SNCFs) can be extrapolated. Parametric study was then implemented through numerical analyses to explore the influences of the three key non-dimensional parameters (i.e. β, 2γ, τ) on the SCFs, and the SCF design formulae were established correspondingly. Besides, the comparisons between the SCFs of bird-beak gap K-joints and conventional ones were also carried out. The results revealed that the brace crown area possessed the greatest SNCFs within entire bird-beak joint. The diamond bird-beak gap K-joints generally possess less stress concentrations than the square bird-beak counterparts with the same member geometries. Moreover, it is shown that the SCFs of conventional gap K-joints are much greater than those of the bird-beak counterparts investigated in this study.

Generalized SCF Formula of Out-Of-Plane Gusset Welded Joints and Assessment of Fatigue Life Extension by Additional Weld.

The existing S-N curves by effective notch stress to assess the fatigue life of gusset welded joints can result in reduced accuracy due to the oversimplification of bead geometries. The present work proposes the parametric formulae of stress concentration factor (SCF) for as-welded gusset joints based on the spline model, by which the effective notch stress can be accurately calculated for fatigue resistance assessment. The spline model is also modified to make it applicable to the additional weld. The fatigue resistance of as-welded and additional-welded specimens is assessed considering the geometric effects and weld profiles. The results show that the error of SCFs by the proposed formulae is proven to be smaller than 5%. The additional weld can increase the fatigue life by as great as 9.4 times, mainly because the increasing weld toe radius and weld leg length lead to the smaller SCF. The proposed series of S-N curves, considering different SCFs, can be used to assess the welded joints with various geometric parameters and weld profiles.

Evaluation of the Stress Concentration Factor in Butt Welded Joints: A Comparative Study

Surface cracks in butt-welded joints usually occur in places with increased stress concentrations. The stress concentration factor (SCF) can be calculated using an empirical equation, with five geometric parameters of a butt-welded joint (thickness of the base material, toe radius, weld toe angle, weld width, and reinforcement height). However, in an industrial environment, it is impractical and sometimes even impossible to measure all five geometric parameters with sufficient accuracy. In this study, eight experiments on butt-welded joints were performed. All samples were scanned with a 3D scanner, and the geometric sizes of the welded joints were measured using computer software. A modified empirical expression proposed by Ushirokawa and Nakayama was used to calculate the SCF; the expression was adjusted in such a way that the SCF was calculated by knowing only the toe radius. In addition, four new expressions were proposed for the calculation of the SCF by knowing the toe radius in relation to the weld toe angle; the expressions were then compared and analyzed. Additionally, the values of the stress concentrations in the butt-welded joints were obtained using a finite element method (FEM). The SCFs calculated using the four methods were compared and further discussed. Our data suggested a new accurate and straightforward approach for calculating the SCF by knowing only the weld toe radius.

Failure investigation of a fractured vent line

During a routine inspection, the sound of venting gas was heard in the area of the compression module on board a five-year-old natural gas platform in the South China Sea. The sound was traced to a fractured 25 mm vent line pipe on a main process gas line. Analysis of the failure showed that a fracture occurred due to a fatigue failure mode. The fracture was initiated under high stress concentration at the weld toe influenced by a small amount of undercut typically found in such welds. This paper provides a summary of the investigation, methods used, key results and analysis which led to the identification of the root cause of failure.

On the performance of monopile weldments under service loading conditions and fatigue damage prediction

AbstractThick weldments used in offshore structures frequently act as fatigue crack initiation sites due to stress concentration at weld toe as well as weld residual stress fields. This paper investigates the cyclic deformation behavior of S355 G10+M steel, which is predominantly used in offshore wind applications. Owing to the vast size difference of monopile structure and weld cross‐section, a global–local finite element (FE) method was used, and the weld geometry was adopted from circumferential weld joints used in offshore wind turbine monopile foundations. Realistic service loads collected using supervisory control and data acquisition (SCADA) and wave buoy techniques were used in the FE model. A nonlinear isotropic–kinematic hardening model was calibrated using the strain controlled cyclic deformation results obtained from base metal (BM) as well as cross‐weld specimen tests. The tests revealed that the S355 G10+M BM and weld metal (WM) undergo continuous cyclic stress relaxation. Fatigue damage over a period of 20 years of operation was predicted using the local stress at the root of the weldments as the life limiting criterion. This study helps in quantifying the level of conservatism in the current monopile design approaches and has implications towards making wind energy more economic.

A two-scale finite element model for the fatigue design of large welded structures

Weld toes and weld roots of continuously welded structures subjected to cyclic loading are critical zones in terms of the fatigue resistance. The finite element method coupled with a fatigue criterion is commonly used to ensure the correct sizing and fatigue design of welded structures. However, weld geometries are often simplified or idealized to limit computational cost. In this work, a numerical two-scale approach is proposed in order to calculate a non-local multiaxial equivalent stress at the weld toe and the weld root from a global finite element shell model.The influence of the parameters of the proposed model on the stiffness behaviour is investigated for three welded structures and for different loading cases. A comparison in terms of stiffness with other models from the literature is also proposed. The results show that the stiffness behaviour is not affected by the parameters of the proposed approach and that it is the most robust model for the different geometries and loading cases studied. The variation in the non-local multiaxial equivalent stress as a function of the parameters of the proposed approach was also studied. The comparison with full solid finite element models makes it possible to define minimum values for the different parameters studied and validates the potential of the proposed approach for the fatigue design of welded structures.

Experimental study on the determination of welding residual stress in rib-deck weld by sharp indentation testing

Implementing accurate fatigue evaluation of rib-deck welds in orthotropic steel decks (OSDs) has become a focus in recent years. In light of the considerable effect of the welding residual stress (WRS), the surface WRS at weld toe or root (i.e., potential crack initiation position) should be accurately determined, which is a challengeable task. In this study, an evaluation framework combining the sharp indentation testing and finite element analysis (FEA) was proposed. Experimental works on the determination of crack initiation position on a microscale, microhardness and WRS, as well as the numerical work simulating welding process, were carried out. The measured WRS was also compared with the values determined by the hole-drilling method. Results show that the proposed evaluation framework is feasible to determine the surface WRS at weld toe and root. The weld root crack usually initiates at the heat affected zone (HAZ), while the weld toe crack initiates at either the weld metal (WM) or the HAZ. Accordingly, both microhardness and WRS at the potential crack initiation positions are presented and discussed. The results are of importance for accurately evaluating the fatigue durability of rib-deck welds.

Using reinforce plate to control the residual stresses and deformation during local post-welding heat treatment for ultra-large pressure vessels

Larger residual stresses and deformations are inevitably generated during local post-weld heat treatment (PWHT) for ultra-large pressure vessels, which may lead to cracking in weld joint. In this paper, a new control method with reinforced plate was proposed to prevent cracking during local PWHT for ultra-large pressure vessels. And the evolution of stresses and deformation during local PWHT and the reason of cracking were discussed. The results show that bigger inconsistent deformation along axial direction and larger axial stresses in weld toe are the main reason to cause cracking. The hoop, axial and maximum principal stresses in welded joint can be reduced by reinforced plate. And the maximum radial deformation during holding stage of PWHT was decreased by reinforced plate because of the decreased elastic and plastic strain. The maximum radial deformation was decreased by more than 50%. The inconsistent deformation at heating stage can be improved, so the stress concentration in weld toe resulting in cracking could be also improved. This work will provide a good guidance for the manufacture of ultra-large pressure vessels.

A rigid-flexible coordinated method to control weld residual stress and deformation during local PWHT for ultra-large pressure vessels

Local post welding heat treatment (PWHT) is a feasible method to eliminate weld residual stress for ultra-large pressure vessels. This paper is focused on the discussion of weld residual stress and deformation for the typical ultra-large mechanical penetration inserted plate (MPIP) during the local PWHT. The reason why the MPIP subjected to local PWHT causes cracking is clarified. A rigid-flexible coordinated control method is proposed to prevent cracking during local PWHT for the ultra-large pressure vessels. The results show that the inconsistent deformation and greater stress concentration in weld toe are the main reason to result in cracking during PWHT. The axial stresses and radial deformation in the welded joints can be reduced by the rigid-flexible coordinated control method. The axial stress is changed to compressive stress, and the radial deformation is reduced by about 58%. And the inconsistent deformation in welded joints during the heat treatment can be obviously improved, which is of great significance to ensure the safe service for ultra-large pressure vessels.

Residual Notch Stress Intensity Factors in Welded Joints Evaluated by 3D Numerical Simulations of Arc Welding Processes.

Approaches based on calculating Residual Notch Stress Intensity Factors (R-NSIFs) assume the weld toe to be a sharp V-notch that gives rise to a residual singular stress distribution close to the weld toe. Once R-NSIFs are determined, they might be included in local fatigue criteria for the structural strength assessment of welded joints based on NSIFs due to external cyclic loading. However, the numerical calculation of R-NSIFs through finite element (FE) simulations of the welding process requires extremely refined meshes to properly capture the residual stress singularity. In this context, the Peak Stress Method (PSM) has recently been adopted to estimate R-NSIFs due to residual stresses by means of coarse meshes of 2D 4-node plane or 3D 8-node brick elements. The aim of this work is to investigate the applicability of the PSM to estimate R-NSIFs in a butt-welded joint using coarse meshes of 3D 10-node tetra elements. The R-NSIF distribution at the weld toe line is estimated by applying the PSM to coarse meshes of 3D 10-node tetra elements, and the results are in agreement with those obtained using 3D 8-node brick elements. Accordingly, the PSM based on tetra elements further enhances the rapid estimation of R-NSIFs using coarse meshes and could be effective in analyzing complex 3D joint geometries.

Effect of Corroded Surface Morphology on Ultra-Low Cycle Fatigue of Steel Bridge Piers.

Corrosion is a common form of durability degradation of steel bridges. Corrosion morphology affects stress distribution under cyclic loads and causes strain concentrations in pits, thus affecting the mechanical properties of steel structures, including ultra-low cycle fatigue (ULCF). To precisely simulate corrosion morphology and investigate the ULCF failure mechanism of corroded steel piers, a sculpting method was applied to mesh units using three-dimensional surface morphology data of corroded steel specimens. Moreover, the ULCF crack-initiation life was numerically predicted using the finite element model based on the cyclic void growth model (CVGM). The cumulative equivalent plastic strain, cyclic void growth index, and critical void growth index of corroded steel piers with different corroded morphologies were compared. Results reveal that, regardless of whether the pier is corroded, fatigue cracks tend to initiate at the weld toe at corners when exposed to cyclic loads under an oblique direction at the pier top. Additionally, the ULCF crack-initiation life in a corroded pier is less than that in an uncorroded pier, and it is significantly affected by a reduction in the pier wall thickness. Corrosion pits affect the position of ULCF crack initiation in a steel pier and cracks tend to initiate at the bottom of pits with large depth-to-diameter ratios. In the case of minor corrosion, the corrosion morphology affects the seismic performance of piers to a small extent.

Influence of porosity on the fatigue behaviour of welded joints

In this paper, the influence of a single pore on the fatigue behaviour of welded joints has been examined by means of the implicit gradient approach. The pore has been considered a stress raiser that interacts with the weld toe or weld root. As an example, in the case of a cruciform joint with a load-carrying fillet weld, the influence of a single spherical pore is examined. Furthermore, the fatigue behaviour of an aluminium laser welded joint with ellipsoid equivalent pores is considered and a comparison is made with the fatigue strength of an aluminium arc welded joint.

Constraint-based failure assessment for pipelines with coplanar circumferential cracks under biaxial loading

The presence of coplanar or multiple cracks emerges as a common problem in many engineering structures, e.g., at the toe of circumferential welds of pipelines subjected to cyclic actions and in other piping systems for the nuclear power plants. The failure assessment diagram provides a convenient means to examine the competing failure between the fracture failure and plastic collapse failure, often for structural components with a single crack. The existing failure assessment diagram relies on the leading term characterizing the near-tip stress solutions, and ignores the geometry or plasticity-induced constraint variations. This paper aims to integrate the crack interaction effect between two coplanar cracks, measured by a constraint-based crack interaction factor, into the option 3 failure assessment curve in the engineering standard. This study examines three types of circumferential coplanar cracks in a pipe: two coplanar embedded cracks; two coplanar surface cracks; and a surface crack interacting with an embedded crack. Using a modified J solution based on a proposed crack interaction factor, this investigation quantifies the plasticity-driven increase in the near-tip opening stress for coplanar cracks. The stress field estimated based on this modified J solution agrees closely with the numerically computed stress field near the coplanar crack tips at large deformation levels. This work subsequently integrates the modified J solution into the failure assessment diagram. The failure assessment diagram based on the equivalent crack size recommended in BS7910 appears to be un-conservative for a number of coplanar cracks considered in this study.