Approaches for fatigue assessment of welded joints in automotive industry

The numerical fatigue assessment of seam welded joints is of significant importance in automotive industry. In order to use the fatigue concepts, which are available in the literature, in series calculations, they usually have to be implemented into commercial software programmes. In the following contribution the possibilities available are discussed with respect to the needs in automotive industry. No specific commercial software will be discussed in detail. At the end open questions for developments necessary in the future will be addressed.     

Fatigue analysis of non-load-carrying fillet welded cruciform joints

The goal of this investigation was to study the effect of local geometrical variations of the weld on the fatigue strength. Therefore the fatigue behaviour of non-load-carrying cruciform fillet welded joint under tensile loading has been studied parametrically. Several two-dimensional (2D) finite element models of the joint were analysed using plane strain linear elastic fracture mechanics (LEFM) calculations in order to get the magnification function M_k. A maximum tangential stress criterion was used to predict the crack growth direction under mixed mode K_I-K_II conditions. The derived M_k solution was then applied both for continuous weld toe cracks and also for semi-elliptical toe cracks at the deepest point of the crack front. An experimental crack aspect ratio development curve was used for propagating semi-elliptical cracks. The as-welded condition was assumed with the result that no crack initiation period was considered and stress ranges were fully effective. The Paris crack growth law was used to predict the growth rate. The effects of weld toe radius, flank angle and weld size on the fatigue strength were systematically studied. Finally, predicted fatigue strength values corresponding to different assumed crack sizes were compared with the available test results.     

A method of determining geometric stress for fatigue strength evaluation of steel welded joints

This paper presents a new method for evaluating the geometric or structural stress in welded constructions. The method is based on the computed stress value 1-mm below the surface in the direction corresponding to the expected crack path. The total stress distribution along the crack path direction is considered to be the sum of the geometric stress caused by the structural geometry change and the non-linear local stress produced by the weld itself. Linear elastic fracture mechanics is used to correlate the total stress based crack propagation life and the local stress based crack propagation life to explain the geometric stress evaluated 1-mm below the surface. Validity of the method is further verified by analyzing fatigue test results for several typical welded joints reported in literature. When compared to the surface extrapolation technique for structural hot spot stress evaluation, the proposed method has the additional advantage in that it is able to account for the size and thickness effect observed in welded joints.     

Recent developments in local concepts of fatigue assessment of welded joints

Several lately proposed modifications or variants of the structural stress or strain concepts, of the notch stress or strain concepts (also termed ‘local stress or strain concepts’) and of the fracture mechanics concepts of fatigue assessment of welded joints are reviewed, whereas the wider context is presented in a recently republished and actualised standard work. The structural stress concepts described first are based on a linearisation of the stress distribution across the plate thickness or along the anticipated crack path and, alternatively, on the structural stress 1 mm in depth below the weld toe. The structural stress is defined and set against design S–N curves. A further structural stress concept is presented for welded joints in thin-sheet steels and aluminium alloys. Among the elastic notch stress concepts, the variant with the reference notch radius, ρ_r = 1 mm, recently verified also for welded joints in aluminium alloys with plate thicknesses t ? 5 mm and the variant with a small-size reference notch radius, ρ_r = 0.05 mm, applicable to welded joints in thin-sheet materials, are outlined. The elastic–plastic notch strain concept is applied to a spot-welded tensile-shear specimen starting from a small-size keyhole notch at the nugget edge. The novel notch stress intensity factor (NSIF) approach relating to crack initiation and extrapolated to final fracture of seam-welded joints in steels and in aluminium alloys is reviewed. A more recently developed crack propagation approach for spot welds is finally described.     

Improving fatigue life for aluminium cruciform joints by weld toe grinding

The increase of fatigue life in aluminium cruciform joints by weld toe grinding was the focus of the current study. The test data are presented by both a nominal stress range approach and by the more refined structural and notch stress range approaches. The influence of the weld toe angle, weld leg length and weld toe radius on the structural and notch stress concentration factor (SCF) was systematically studied by means of finite element analysis. Experimental data based on 18 pieces of as-welded and 13 pieces of weld toe-ground specimens made of 12 mm thick plates showed a significant improvement in fatigue life in aluminium by grinding the weld toe and confirmed the permitted improvement in fatigue life by design codes.     

Notch stress concepts for the fatigue assessment of welded joints - Background and applications

Among modern fatigue design concepts for welded structures, the linear-elastic notch stress concept gains increasing industrial acceptance. There are two variants of this concept, one for thick walled (t ? 5 mm) welded joints with the reference radius r_ref = 1.00 mm, which is already included in the fatigue design recommendations of the IIW and applied for the assessment of big welded structures, and one for thin walled (t < 5 mm) welded joints with the reference radius r_ref = 0.05 mm, which is more and more used in the automotive industry.The concept with r_ref = 1.00 mm is based on the micro-support theory of Neuber with the fictitious radius r_ref = 1.00 mm, derived by Radaj. The background of the concept with r_ref = 0.05 mm is the relationship between the stress-intensity factor and the notch stress according to Creager and Paris as well as Irvin’s theory of crack blunting. Besides these two theories, the definition of both of these radii has also an experimental background; they are observed in many welded joints.In the present paper, first the background and then different applications of both concept variants are described: the application of the reference radius of r_ref = 1.00 mm for MAG-welded offshore K-nodes (t = 30 mm) and sandwich panels for ship decks (t = 5 mm), and the application of r_ref = 0.05 mm for spot-welded automotive doors (t = 1 mm) and MAG-welded automotive trailing links (t = 3-4 mm). The sandwich panels were evaluated additionally with r_ref = 0.05 mm. Calculations and experimental results are compared and the reliability of the notch stress concept variants underlined. Additionally, recommendations for the slope of design lines distinguishing between thin and thick dimensions are given, i.e. k = 3.0 and 5.0 (normal stress, shear stress) for thick and stiff structures, k = 5.0 and 7.0 for thin and flexible structures.     

Measuring fatigue cracks in fillet welded joints

Accurate measurement of short (<1 mm depth) elliptical fatigue cracks that grow from the toes of fillet welds has proved to be an obstacle to the application of fracture mechanics principles to welding fatigue. This paper reports a DC potential drop technique which allows continuous measurement of the depth of such elliptical cracks. A delicate compromise between sensitivity and accuracy, combined with superior electrical stability displayed by the measurement apparatus, has allowed detection of: 1 — crack growth less than 0.01 mm and; 2 — crack growth rates less than 10^?7 mm/cycle for cracks less than 1 mm deep.Preliminary results have indicated the relative importance of stress ratio, defect size and material variation on the growth of these short elliptical cracks. When the weld toe is subject to high stress ratios the phenomenon may be considered propagation dominated whereas low stress levels increase the influence of threshold and initiation mechanisms.     

A structural stress‐based critical plane method for multiaxial fatigue life estimation in welded joints

This paper aims at proposing a new fatigue life estimation model that is preferably adapted to welded joints subjected to multiaxial loading. First, a mesh‐size insensitive structural stress is defined that enables to characterize the stress concentration effect appropriately. Second, the multiaxial stress state and loading path influence are taken into account in the lifetime prediction model by adopting a suitable critical plane method, originally proposed by Carpinteri and co‐authors. Experimental verification is conducted for a given welded joint geometry under different loading conditions, including uniaxial, torsional and multiaxial loads. The reliability and effectiveness of the new method are validated through substantive fatigue testing data.     

A structural stress definition and numerical implementation for fatigue analysis of welded joints

A mesh-size insensitive structural stress definition is presented in this paper. The structural stress definition is consistent with elementary structural mechanics theory and provides an effective measure of a stress state that pertains to fatigue behavior of welded joints in the form of both membrane and bending components. Numerical procedures for both solid models and shell or plate element models are presented to demonstrate the mesh-size insensitivity in extracting the structural stress parameter. Conventional finite element models can be directly used with the structural stress calculation as a post-processing procedure. To further illustrate the effectiveness of the present structural stress procedures, a collection of existing weld S-N data for various joint types were processed using the current structural stress procedures. The results strongly suggests that weld classification based S-N curves can be significantly reduced into possibly a single master S-N curve, in which the slope of the S-N curve is determined by the relative composition of the membrane and bending components of the structural stress parameter. The effects of membrane and bending on S-N behaviors can be addressed by introducing an equivalent stress intensity factor based parameter using the structural stress components. Among other things, the two major implications are: (a) structural stresses pertaining to weld fatigue behavior can be consistently calculated in a mesh-insensitive manner regardless of types of finite element models; (b) transferability of weld S-N test data, regardless of welded joint types and loading modes, can be established using the structural stress based parameters.     

Reliability updating of welded joints damaged by fatigue

The paper introduces a probability-based fatigue assessment model for welded joints in steel bridges. The approach is based on a modelization of the fatigue phenomenon issued from the principles of fracture mechanics theory. The safety margin includes the crack growth propagation and allows us to treat fatigue damage in a general manner. Damaging cycles and non-damaging cycles are distinguished. The reliability calculus is performed by a FORM technique. The sensitivity study of the different parameters shows that some variables can be taken as deterministic. Applications are made on a welded joint ‘bottom plate/stiffener’ of a typical steel bridge. The model is then used for taking into account inspection results. A sensitivity analysis of different non-destructive inspection (NDI) methods is carried out for measuring their uncertainty levels. The different types of inspection results (no detection, detection with no measurement, detection with measurement) are analysed and a general methodology for updating reliability levels is given. The results show their ability to be inserted in a maintenance strategy for optimizing the next inspection time, the need to repair or to replace the joint, and, the eventual possibility of no action.     

Probabilistic prediction of high cycle fatigue reliability of high strength steel butt‐welded joints

The aim of this paper is to develop a probabilistic approach of high cycle fatigue (HCF) behaviour prediction of welded joints taking into account the surface modifications induced by welding and the post‐welding shot peening treatment. In this work, the HCF Crossland criterion has been used and adopted to the case of welded and shot peened welded parts, by taking into account the surface modifications which are classified as follows: (i) the compressive residual stresses, (ii) the surface work‐hardening, (iii) the geometrical irregularities and (iv) the superficial defects. The random effects due to the dispersions of: (i) the HCF Crossland criterion material characteristics (ii) the applied loading and (iii) the surface modifications parameters are introduced in the proposed model. The HCF reliability has been computed by using the ‘strength load’ method with Monte Carlo simulation. The reliability computation results lead to obtain interesting and useful iso‐probabilistic Crossland diagrams (PCD) for different welding and shot peening surface conditions. To validate the proposed method, the approach has been applied to a butt‐welded joint made of S550MC high strength steel (HSS). Four types of specimens are investigated: (i) base metal (BM), (ii) machined and grooved (MG) condition, (iii) As welded (AW) condition and (iv) as welded and shot peened (AWSP) condition. The comparison between the computed reliabilities and the experimental investigations reveals good agreement leading to validate the proposed approach. The effects of the different welded and post‐weld shot peened specimen's surface properties are analysed and discussed using the design of experiments (DoE) techniques.     

Rapid method for low cycle fatigue properties: thickness effect on the fatigue crack initiation life of welded joints

Welded assemblies are commonly used in the shipbuilding industry. Because of the combination of stress concentration and cyclic loading, welded joints could be a critical area for fatigue damage. Thus, knowing stress and strain histories at the critical points of the structure is necessary, particularly when a confined plasticity occurs, to determine the fatigue life of welded assemblies. To avoid time‐consuming nonlinear finite element analyses (FEA), simplified estimation methods of the elastic–plastic strain/stress can be used. In a previous work, an approach to estimate stress state at critical points was developed and employed in the case of double‐notched specimens. The present paper focuses on welded joints in order to validate this strategy with the aim to estimate the fatigue crack initiation life of T‐joints. To go further, a parametric approach has been adopted to take into account the local geometries of welded joints and to determine the constraint operator without any FEA. The results predicted by this approach are compared with experimental fatigue results.     

Interim fatigue design recommendations for fillet welded joints under complex loading

Current fatigue design methods for assessing welded steel structures under complex combined or multiaxial loading are known to be potentially unsafe. This has led to a number of research projects over the past 10 years. Some progress has been made in developing better methods, but they are not yet suitable for general design. This paper presents an interim solution based on a review and analysis of relevant published data; all referring to fatigue failure from a fillet weld toe. These indicate that Eurocode 3/IIW S – N curve FAT80/3 (negative inverse slope of 3) is suitable for combined normal and shear stresses acting in phase, and possibly for out-of-phase (i.e. non-proportional loading) bending and shear if the shear stress is not due to torsion. However, a shallower curve FAT80/5 is necessary for out-of-phase torsion and bending or tension. Both curves are used in conjunction with the nominal maximum principal stress range occurring during the loading cycle.     

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.     

A local strain method for the evaluation of welded joints fatigue resistance: the case of thin main-plates thickness

Current procedures for evaluating fatigue strength of welded structures may not be consistent with the real fatigue behaviour of welded joints. A local strain method for the prediction of the WELded joints FAtigue REsistance (WELFARE), by local strain measurements at the weld toe, was recently proposed on the basis of fatigue tests on more than 10 series of welded joints (T, cruciform, angular and butt joints) in structural steel, with 10–25 mm main‐plate thickness. This paper reports fatigue test results obtained from 30 cruciform and butt welded joints (3–5 mm thick) under two load ratios (0.1 and ?1) in order to extend the applicability of the method to thin welded joints.     

The use of the JV parameter in welded joints: Stress analysis and fatigue assessment

This paper investigates the use of the J_V parameter, a path-independent integral, for the evaluation of the elastic local stress parameters in welded details and for the estimation of their fatigue life.First, the stress intensity factors (SIF) of an embedded crack lying along the bisector of a sharp V-notch is calculated by means of the J_V without modelling the crack and by keeping the same external load and boundary conditions of the cracked model. Furthermore, the notch stress intensity factors (NSIFs) of the welds can be calculated after a post processing procedure of FE analysis with the advantage of using coarse meshes.Second, a correlation between the fatigue life of welded details and the J_V parameter is shown. In fact, careful analysis of the fracture surface of fillet welds taken from literature and of new fatigued joints indicates that the first stage of fatigue crack propagation follows the bisector line of the local V-notch as only mode I would be present. Therefore, since the J_V evaluated on a suitable integration path represents the SIF of an embedded crack lying along the bisector, the J_V is used for the fatigue life assessment of welded details. The critical characteristic length of a suitable integration path for welded joints made of steel and aluminium alloys has been calculated. These critical characteristic lengths were used for the evaluation of two fatigue general scatter bands, mainly based on fatigue data of non-load-carrying cruciform joints characterised by a V-notch angle of 135°. Further, fatigue life data of steel and aluminium alloy welded connections have also been investigated when both mode I and mode II loadings are singular.     

A fracture mechanics analysis on the fatigue behaviour of cruciform joints of duplex stainless steel

The paper presents the results of an experimental and numerical study on the fatigue behaviour of cruciform load carrying joints made from the duplex stainless steel and failing from the weld root through the weld metal. Fatigue crack growth (FCG) data, obtained in specimens of the weld metal, are presented, as well as threshold data, both obtained for R= 0 and 0.5. The influence of stress ratio is discussed, and the FCGR results are compared with data for low carbon structural steels. S–N data were obtained in the joints, both for R= 0.05 and 0.5, and the fatigue cracking mechanisms were analysed in detail with the SEM. It was found that the cracks propagated very early in the lifetime of the joints, under mixed mode conditions (I + II), but the mode I component was found to be predominant over mode II. The geometries of the cracks were defined in detail from measurements taken in the fracture surfaces. A 2D FE analysis was carried out for the mixed mode inclined cracks obtained at the weld root, and the J‐integral formulations were obtained as a function of crack length and crack propagation angle. The values of the crack propagation angle, θ_i, were obtained for the J_max conditions, and it was found that, in the fatigue tests, the cracks propagated in directions very close to the predicted directions of maximum J. K_I and K_II formulations were obtained, and the K_I data were compared with the formulations given in the PD6493 (BS7910) document, and some differences were found. A more general formulation for K under mixed mode conditions was derived. The derived K solutions were applied to predict the fatigue lives of the joints under crack propagation, and an extremely good agreement was found with the experimental results obtained in the fatigue tests.