Fatigue Design Of Welded Joints Research Articles

Evaluation of the approach based on the maximum principal stress from the IIW-Recommendation for welded joints under proportional, multiaxial stress states

The IIW-Recommendation for fatigue design of welded joints and components presents two different evaluation approaches for multiaxial stress states under constant amplitude: an approach based on the maximum principal stress, which is only applicable under certain conditions, and the Gough-Pollard approach for the remaining cases. In the course of this work, this maximum principal stress-based approach is analyzed. It is examined in which cases the method is applicable, the advantages and disadvantages compared to the approach according to Gough-Pollard are discussed, and an evaluation according to the von Mises equivalent stress is presented as a third alternative. This is followed by a validation based on literature data on laser-welded steel joints using the nominal stress, hot spot stress, and effective notch stress concept.

Lifetime assessment of the rear axle for an off-road vehicle in a fatigue test bench by means of the Peak Stress Method

In this study the Peak Stress Method (PSM) has been applied to the fatigue assessment of the welds of an axle equipped in an off-road vehicle. In the fatigue design of welded joints, the PSM is a FE-based method to rapidly evaluate the NSIFs relevant to the opening, sliding and tearing modes by means of the linear elastic peak stresses extracted from the FE model. First, an axle, which consists of a central body and two lateral branches composed by several plates welded to a tube, has been fatigue tested under a vertical load in a test bench designed to replicate the frame-axle interface. Then, an FE model of the axle has been calibrated to simulate the behaviour observed during the fatigue test, where the reaction forces and the strains at pre-defined locations have been considered. Once the FE model has been calibrated, the PSM has been applied to the welds of the axle, taking advantage of an automatic procedure, and the crack initiation location has been estimated. Finally, the theoretical estimations have been compared with the experimental datum.

Misalignment effect on the fatigue failure behavior of load-carrying cruciform welded joints

This paper presents an analytical model based on the Strain Energy Density (SED) method to predict the high cycle fatigue failure behavior of load-carrying cruciform welded joints (LCWJ) weakened by axial and angular misalignments. Compared with the Hot Spot Stress (HSS) and Effective Notch Stress (ENS) methods recommended in standard codes, the SED values at weld toe and weld root were carried out systematically considering the welded details and the types of misalignments. Moreover, the analytical model was applied to estimate the magnitudes of fatigue indicators and illustrated the effects of misalignment on the fatigue failure transition region in LCWJ. It is validated effectively as a supplementary analysis method for fatigue life prediction and fatigue design of welded joints compared with the experimental data and published reports.

The Peak Stress Method applied to fatigue lifetime estimation of welded steel joints under variable amplitude multiaxial local stresses

Fatigue actions of most engineering welded structures involve variable amplitude (VA) loading cycles. The fatigue design of welded joints in such structures is based on the use of a fatigue strength criterion for constant amplitude (CA) loading in conjunction with a cumulative damage rule. In the present work, the Peak Stress Method (PSM), has been reformulated to estimate the lifetime of welded joints subjected to VA loadings. The PSM is an engineering finite element (FE)-oriented technique to rapidly estimate the mode I, II, and III notch stress intensity factors (NSIFs) at the weld toe or at the weld root, by taking advantage of the singular linear elastic, opening, sliding, and tearing peak stresses, respectively. Such stresses are computed at the V-notch tip by means of 2D or 3D FE models discretized using coarse meshes, provided that some conditions of applicability of the method are satisfied. The fatigue strength under CA loading is then evaluated by combining the simplicity and rapidity of the PSM in evaluating the NSIFs with a robust and validated fatigue strength criterion such as the one based on the averaged Strain Energy Density (SED), which can be written as a function of the relevant NSIFs. To preserve the simplicity of the method, its extension to VA loading conditions has been achieved by assuming the Miner’s Linear Damage Rule (LDR) as a cumulative damage rule. The proposed method has been validated against new experimental results generated by testing non-load-carrying (nlc) fillet-welded joints with double inclined attachment and made of structural steel under pure axial loading.

Automated fatigue strength assessment of arc-welded structures according to the Peak Stress Method

The Peak Stress Method (PSM) is an approximate, FE-oriented application of the notch stress intensity factor (NSIF) approach to fatigue design of welded joints. This approach is based on the singular linear elastic peak stresses calculated from coarse FE models. The PSM was calibrated by using 2D, four-node plane quadrilateral elements; 3D, eight-node brick elements and 3D, four-node or ten-node tetra elements. By adopting the averaged Strain Energy Density (SED) as a fatigue strength criterion, a design stress, the so-called equivalent peak stress, has been defined and adopted in conjunction with a reference design fatigue curve to estimate the fatigue life of welded structures. The equivalent peak stress has successfully been adopted to assess the fatigue strength of arc-welded joints made of either structural steels or aluminum alloys and subjected to axial, torsion or multiaxial fatigue loadings. In the present work, a subroutine, named ANSYS-PSM, has been developed in the post-processing environment of ANSYS® FE code to automate the PSM for 2D or 3D FE models. After having briefly recalled the theoretical background of the PSM and the procedure to apply it step by step, the paper presents the adopted programming languages and the workflow of the developed subroutine.

Multiaxial fatigue assessment of welded steel details according to the peak stress method: Industrial case studies

In fatigue design of welded joints, the Peak Stress Method (PSM) is an engineering, rapid, finite element-based tool to apply the notch stress intensity factor (NSIF) approach. The PSM presents some advantages, such as: (i) coarse meshes can be adopted, the required FE size being some orders of magnitude larger than that necessary to evaluate the NSIFs from the local stress distributions; (ii) only a single stress value is sufficient to estimate the NSIFs; (iii) 2D as well as 3D FE models can be used and (iv) the design engineer is able to determine the crack initiation point when competition between weld root and weld toe failure exists. Therefore the PSM may be a convenient design tool to adopt in the industry. In the present paper, new fatigue results have been generated by testing plate-to-tube welded steel details taken from industrial case studies under in-phase bending-torsion fatigue loadings. In particular, full-penetration joints adopted in the structure of a roundabout-type carousel and fillet-welded joints for quarter-turn scotch-yoke valve actuators have been tested. Experimental fatigue results have been analysed using the PSM, which proved to identify correctly the fatigue crack initiation location. Finally, a fairly good agreement has been obtained between the experimental results and the relevant PSM-based design curves.

Multiaxial fatigue assessment of welded steel details according to the peak stress method based on tetra elements

The Peak Stress Method (PSM) is an engineering, FE-oriented application of the notch stress intensity factor (NSIF) approach to fatigue design of welded joints, which takes advantage of the singular linear elastic peak stresses from FE analyses with coarse meshes. Originally, the PSM was calibrated by using 3D, eight-node brick elements, taking advantage of the submodeling technique. Recently, the PSM has been calibrated by using ten-node tetra elements, which are able to directly discretize complex 3D geometries without the need for submodels. The PSM was validated for pure axial or bending loadings as well as pure torsion loadings; recently it has been extended to multiaxial loadings by adopting a design stress, the so-called equivalent peak stress, in conjunction with a reference design fatigue curve. After having briefly recalled the calibration of the PSM with tetra elements, the paper presents some applications of the PSM relevant to steel plate-to-tube welded details of industrial interest under in-phase bending-torsion fatigue loadings. Experimental data have been re-analysed using the PSM based on tetra elements. Eventually a good agreement between experimental and theoretical results has been obtained in terms of fatigue crack initiation location as well as total fatigue life.

The peak stress method applied to the fatigue assessment of tube-tube steel joints with weld ends under multiaxial loadings

The Peak Stress Method (PSM) is an approximate, FE-oriented application of the notch stress intensity factor (NSIF) approach to fatigue design of welded joints, which is based on the singular linear elastic peak stresses calculated from FE analyses performed by using coarse mesh patterns. By adopting the averaged strain energy density (SED) as a fatigue strength criterion, a design stress (the equivalent peak stress) can be defined; in conjunction with a reference design curve previously defined, the fatigue strength assessment of welded joints subjected to multiaxial fatigue loadings can be performed. In the present contribution, the PSM has been applied to the fatigue assessment of tube-tube steel joints with weld ends, which have been fatigue tested in a previous contribution under combined loadings: namely pure axial, pure torsion and in-phase as well as out-of-phase axial-torsion loadings, all of which with two load ratios, i.e. R = 0 and R = -1. The experimental fatigue results have been re-converted in terms of equivalent peak stress by adopting a 3D FE model including an idealised weld end geometry. The equivalent peak stress has proved to assess the fatigue crack initiation location in agreement with experimental observations, moreover a quite good agreement has been obtained between the experimental results and the PSM-based design scatter band.

The Peak Stress Method to assess the fatigue strength of welded joints using linear elastic finite element analyses

In fatigue design of welded joints according to the notch stress intensity factor (NSIF) approach, the weld toe profile is assumed to be a sharp V-notch having tip radius equal to zero, while the root side is assumed to be a pre-crack in the structure. The Peak Stress Method (PSM) is an engineering, FE-oriented method to estimate the NSIFs starting from the singular linear elastic peak stresses calculated at the V-notch or crack tips by using a coarse FE mesh. The element type is kept constant and the average element size can be chosen arbitrarily within a given range. The method is used in conjunction with Ansys software. The FE meshes are claimed to be coarse in comparison to those necessary to evaluate the NSIFs from the local stress distributions. Two-dimensional as well as three dimensional FE analyses can be adopted to apply the method. By using the averaged Strain Energy Density (SED, which can be expressed as a function of the relevant NSIFs) as a fatigue strength criterion, a so-called equivalent peak stress is defined to assess either weld toe or weld root fatigue failures in conjunction with a properly calibrated design curve. After presenting the theoretical background of the method, the paper presents a review of applications of the PSM relevant to steel welded joints under uniaxial as well as multiaxial fatigue loadings. Because of the relatively coarse FE analyses required and simplicity of post-processing the calculated peak stresses, the PSM might be useful in the everyday design practice.

Rapid estimation of notch stress intensity factors in 3D large-scale welded structures using the peak stress method

The Peak Stress Method (PSM) is an engineering, FE-oriented application of the notch stress intensity factor (NSIF) approach to fatigue design of welded joints, which takes advantage of the singular linear elastic peak stresses from FE analyses with coarse meshes. Originally, the PSM was calibrated to rapidly estimate the NSIFs by using 3D, eight-node brick elements, taking advantage of the submodeling technique. 3D modelling of large-scale structures is increasingly adopted in industrial applications, thanks to the growing spread of high-performance computing (HPC). Based on this trend, the application of PSM by means of 3D models should possibly be even more speeded up. To do this, in the present contribution the PSM has been calibrated under mode I, II and III loadings by using ten-node tetra elements, which are able to directly discretize complex 3D geometries without the need for submodels. The calibration of the PSM has been carried out by analysing several 3D mode I, II and III problems. Afterwards, an applicative example has been considered, which is relevant to a large-scale steel welded structure, having overall size on the order of meters. Two 3D FE models, having global size of tetra elements equal to 5 and 1.66 mm, have been solved by taking advantage of HPC, being the global number of degrees of freedom equal to 10 and 140 millions, respectively. The NSIFs values estimated at the toe and root sides according to the PSM have been compared with those calculated by adopting a shell-to-solid technique.

Multiaxial fatigue strength assessment of welded joints using the Peak Stress Method – Part I: Approach and application to aluminium joints

The notch stress intensity factor (NSIF) approach for the fatigue strength assessments of welded joints assumes the weld toe as a sharp V-notch with tip radius equal to zero and the weld root as a pre-crack. The Peak Stress Method (PSM) is an approximate, FE-oriented application of the NSIF-based approaches to fatigue design of welded joints and it is based on the singular linear elastic peak stresses calculated from FE analyses carried out by using a proper mesh pattern; more precisely, the element type is fixed, the average element size is kept uniform in the mesh pattern and before running the mesh generation algorithm the size can be chosen arbitrarily within a given range. The PSM allows to adopt rather coarse FE meshes if compared to those required for the NSIFs evaluation from the local stress fields. While originally the PSM was validated for pure axial or bending loadings as well as pure torsion loadings, here the PSM is extended for the first time to analyse the fatigue strength of welded joints subjected to in-phase as well as out-of-phase multiaxial fatigue loadings. By adopting the averaged Strain Energy Density (SED) as a fatigue strength criterion, a so-called equivalent peak stress is defined and it is adopted to assess either weld toe and weld root fatigue failures in conjunction with a properly defined design curve. Some multiaxial fatigue test data taken from the literature and relevant to welded joints made of aluminium alloys are analysed using the Peak Stress Method. The equivalent peak stress has shown to correlate with good approximation all the experimental data.

Assessment of root failures in tube-to-flange steel welded joints under torsional loading according to the Peak Stress Method

In the context of the local approaches to fatigue design of welded joints, the notch stress intensity factor (NSIF) approach assumes the weld toe as well as the weld root profile as sharp V-shaped notches, the V-notch opening angle being greater than zero at the toe side and equal to zero at the root side. The Peak Stress Method (PSM) is an engineering, finite element (FE)-oriented method to rapidly estimate the NSIF local stress parameters by means of linear elastic FE analyses with coarse meshes. By using the averaged Strain Energy Density (SED, which can be expressed as a function of the relevant NSIFs) as a fatigue strength criterion, a so-called equivalent peak stress can be defined and adopted to assess either weld toe and weld root fatigue failures in conjunction with an existing design fatigue curve, that was originally calibrated on experimental results relevant only to weld toe failures. In the present paper new fatigue test results relevant to weld root failures have been generated by testing tube-to-flange fillet-welded steel joints under torsional loading. Good agreement has been obtained between theoretical estimations according to the PSM and experimental fatigue test results.

Fatigue strength assessment of partial and full‐penetration steel and aluminium butt‐welded joints according to the peak stress method

AbstractIn fatigue design of welded joints, the local approach based on the notch stress intensity factors (NSIFs) assumes that the weld toe profile is a sharp V‐notch having a tip radius equal to zero, while the root side is a pre‐crack in the structure. The peak stress method (PSM) is an engineering, FE‐oriented application of the NSIF approach to fatigue design of welded joints, which takes advantage of the elastic peak stresses from FE analyses carried out by using a given mesh pattern, where the element type is kept constant and the average element size can be chosen arbitrarily within a given range. The meshes required for the PSM application are rather coarse if compared with those necessary to evaluate the NSIFs from the local stress distributions. In this paper, the PSM is extended for the first time to butt‐welded joints in steel as well as in aluminium alloys, by comparing a number of experimental data taken from the literature with the design scatter bands previously calibrated on results relevant only to fillet‐welded joints. A major problem in the case of butt‐welded joints is to define the weld bead geometry with reasonable accuracy. Only in few cases such geometrical data were available, and this fact made the application of the local approaches more difficult. Provided the local geometry is defined, the PSM can be easily applied: a properly defined design stress, that is, the equivalent peak stress, is shown (i) to single out the crack initiation point in cases where competition between root and toe failure exists and (ii) to correlate with good approximation all analysed experimental data.

The peak stress method combined with 3D finite element models for fatigue assessment of toe and root cracking in steel welded joints subjected to axial or bending loading

ABSTRACTIn fatigue design of welded joints, the local approach based on the notch stress intensity factor (NSIF) assumes that the weld toe profile is a sharp V‐notch having a tip radius equal to zero, while the root side is a pre‐crack in the structure. The ratio between the mode I NSIF and the linear elastic opening peak stress evaluated at the point of singularity (i.e. the V‐notch tip) by the finite element (FE) method depends only on the type and size of the elements adopted in the discretization. The ratio between the mode II SIF and the linear elastic sliding peak stress evaluated at the crack tip has the same property. This useful property of the opening and sliding peak stresses evaluated by the FE method gives rise to the so‐called peak stress method (PSM). In the present paper, it is adopted as an engineering, FE oriented application of the NSIF approach to fatigue design of welded joints. Originally applied to 2D FE models, the peak stress method is combined here with 3D numerical models to assess the fatigue strength of steel welded joints having complex geometry and characterised by toe as well as root cracking. A properly defined design stress, that is, the equivalent peak stress, is shown to correlate with good approximation all analysed experimental data.

Fatigue strength assessment of tubular welded joints by an alternative structural stress approach

The tubular joints, frequently employed in the offshore industry, are submitted to stresses resulting from elementary loadings: tension/compression, in-plane bending and out-of-plane bending. This work concerns the analysis of the recommendations commonly used for the fatigue design of welded joints submitted to combined loadings. Particularly, it deals with the fatigue behaviour of T-joints submitted to deviated-bending: first, a finite element analysis was developed and a post-processing based on the structural stress approach, as proposed by the International Institute of Welding (IIW). Then, fatigue tests were conducted on T-joints submitted to deviated-bending. Comparisons between experimental and numerical results showed that this kind of recommendations is not systematically conservative. Thus, an alternative approach based on structural stresses and taking into account the multi-axial stress state in the weld toe was developed in order to complete the recommendations for the fatigue design of tubular welded joints.

Application of different concepts for fatigue design of welded joints in rotating components in mechanical engineering

Several concepts are used for the fatigue design of welded joints. In this paper investigations are presented, which were carried out in a joint project between five research institutes [1]. The aim is to investigate currently applied fatigue concepts with respect to their limitations, compatibility and reliability, in order to improve the accuracy of lifetime estimation and to simplify the choice of the optimum fatigue concept. Here, the results of the investigation of welded joints in rotating universal joint shafts are shown [2]. In the critical weld, a structural steel and a quenched and tempered steel are joined. In practice, stresses result from rotating bending, torsion and also residual stresses are sometimes present. Several welding techniques, MAG, TIG and laser welding, and two seam geometries were investigated with regard to their influence on fatigue strength. Experiments were conducted with welded tube specimens representative of the actual component application and with derived flat specimens as detail specimens. The welded sheet thickness was 5.5 mm. Fatigue strength was investigated from 10 4 to 10 7 numbers of cycles. In numerical analyses, nominal stress, structural hot spot stress and elastic notch stress with reference radii of 0.3 mm and 0.05 mm were calculated. In the comparison of the concepts, their respective advantages and disadvantages have been demonstrated. A comparison of the results with the IIW recommendation for fatigue design of welded joints and components [3] has been carried out and improvements have been suggested.

Fatigue design of welded joints using the finite element method and the 2007 ASME Div. 2 Master curve

Fatigue design of welded structures is primarily based on a nominal stress; hot spot stress methods or local approaches each having several limitations when coupled with finite element modeling. An alternative recent structural stress definition is discussed and implemented in a post-processor. It provides an effective means for the direct coupling of finite element results to the fatigue assessment of welded joints in complex structures. The applications presented in this work confirm the main features of the method: mesh-insensitivity, accurate crack location and life to failure predictions.

A Consideration of Allowable Equivalent Stresses for Fatigue Design of Welded Joints According to the Notch Stress Concept with the Reference Radii rref = 1.00 and 0.05 mm

In the literature, allowable stresses (FAT-values) for the fatigue design of welded joints are established according to the notch stress concept with the reference radii rref = 1.00 mm for thick connections (t ≥ 5 mm) and 0.05 mm for thin connections (t < 5 mm). However, it is not clear for which strength hypothesis they are valid. As local equivalent stresses may be calculated by the principal stress or von Mises hypotheses, it is necessary to distinguish between the applied hypotheses. The FAT-values according to the principal stress and von Mises hypotheses are compiled for steel, aluminium and magnesium for the reference radii rref = 1.00 and 0.05 mm. The allowable stresses are derived from normal as well as from shear stresses. However, the values derived from pure normal loading (axial or bending) and from pure torsion are not compatible when the principle stress or the von Mises hypotheses are applied. Therefore, in case of biaxial loading, the stated incompatibility between the values obtained from different loading modes should be overcome by the Gough-Pollard relationship.

Discussion on local approaches for the fatigue design of welded joints

Design of welded structures under fatigue is governed by two conflicting requirements: to minimize weight without compromising structure safety. Any theoretical and/or numerical approaches are necessarily based on some simplifying assumptions which, because of the complexity of fatigue phenomena, could miss some aspects involved in the real structure behaviour. On the other side, experimental approaches provide direct information on structure behaviour. In view of this, the paper will underline the importance of experimental observations and practical procedures to evaluate the stress/strain magnitude that could lead some components to unsafe working conditions. Experimental approaches may greatly support designers in all industrial applications where fast and reliable responses are strongly needed.

Multiaxial fatigue assessment of welded joints – Recommendations for design codes

In the assessment of welded joints submitted to multiaxial loading the calculations method applied, independently of the concept (nominal, structural, hot-spot or local), must consider primarily the materials ductility. While proportional loading can be assessed by von Mises, the principal stress hypothesis, the Findley method or the Gough–Pollard relationship, using any of the mentioned concepts, difficulties occur when the loading is non-proportional, i.e. the principal stress (strain) direction changes. This causes a significant fatigue life reduction for ductile steel welds, but an indifferent behaviour for semi-ductile aluminium welds. This different response to non-proportional loading can be assessed when ductility related mechanisms of fatigue failures, i.e. the mean value of plane oriented shear stresses for ductile materials and a combination of shear and normal stresses for semi-ductile materials, are properly considered. However, as these methods require a good expertise in multiaxial fatigue, for design codes used by non-fatigue experts, simpler but sound calculation methodologies are required. The evaluation of known fatigue data obtained with multiaxial constant and variable amplitude (spectrum) loading in the range N > 10 4 cycles suggests the application of the modified interaction algorithm of Gough–Pollard. In the case of variable amplitude loading, constant normal and shear stresses are replaced by modified reference normal and shear stresses of the particular spectrum. The modification of the reference stresses is based on the consideration of the real Palmgren–Miner damage sum of D PM = 0.5 (for spectra with constant mean loads) and the modification of the Gough–Pollard algorithm by consideration of the multiaxial damage parameter D MA = 1.0 or 0.5, which is dependent on the material’s ductility and on whether the multiaxial loading is proportional or non-proportional. This method is already part of the IIW-recommendations for the fatigue design of welded joints and can also be applied by using hot-spot or local stresses.