Effective Notch Stress Concept Research Articles

Fatigue strength of cruciform joints with weld imperfections: A comprehensive numerical study

The paper presents an investigation into the fatigue resistance of load-carrying cruciform joints that exhibit weld imperfections. To accomplish this, a finite-element model of the joint is validated by a comparison to experimental investigations. The model includes both a global and a submodel that integrates the effective notch stress concept into its design. This enables the simulation of various types of weld imperfections, such as lack of penetration, lack of root fusion, undercut, excessive convexity, incorrect weld toe, excessive asymmetry of fillet welds and linear misalignment, to assess their impact on the fatigue strength of the cruciform joint. The analysis reveals that these imperfections lead to an increase in stress at the notches of the cruciform joint, which can be characterized as a function of the joint's geometrical properties. Global geometry influences on the fatigue strength of different cruciform join geometries can be regarded independently from the influence of the weld imperfections that is given by stress concentration factors. The normalised stress concentration factor enables a comparison between imperfection influences and shows, that the notch-softer design with full penetration welding at the cruciform joint is in a relative relation more strongly influenced by imperfections than the design with fillet welds. Also, the undercut represents the imperfection with the strongest effect in a relative relation within the sizes considered.Being able to evaluate the fatigue strength values of steel components with weld imperfections eliminates the need for costly repairs and new fabrications, which would otherwise result in a loss of resources and economic efficiency.

Overload and variable amplitude load effects on the fatigue strength of welded joints

Different load spectra and individual load peaks might substantially relax high residual stresses as well as induce compressive residual stresses in welded components and, consequently, affect the fatigue performance of these joints. Consideration of peak loads and resulting relaxation of residual stresses in fatigue analyses can substantially enhance the accuracy of life prediction. The aim of the current study is to experimentally investigate the fatigue strength of welded joints subjected to different levels of overloads and variable amplitude (VA) loads and to develop local fatigue assessment method to account for the relaxation of residual stresses via a mean stress correction using the 4R method. The 4R method applies a local stress ratio for the mean stress correction considering material strength, residual stresses, applied stress ratio of external loading and local weld geometry in elastic–plastic material behaviour. Fatigue tests were carried out on fillet-welded longitudinal gusset joints made of S700 high-strength steels under applied stress ratio R = 0–0.1. A mild strength steel (S355) and ultra-high-strength steel (S1100) were selected as reference steel grades for the fatigue testing to study the material strength effects. Numerical analyses were conducted to evaluate the fatigue notch factors using the effective notch stress concept with the reference radius of rref = 1.0 mm and theory of critical distance (TCD) using the point method. The experimental results indicated that a substantial improvement in the fatigue strength capacity can be claimed in the joints subjected to tensile overloads, particularly in the studied S700 and S1100 steels. The higher-level overload (0.8fy), corresponding to the nominal cross-sectional area, improved the mean fatigue strength of the welded joints manufactured of high-strength S700 steel by approximately 60%, while the lower overload (0.6fy) improved the mean fatigue strength by 20%. In addition, a use of equivalent nominal stresses for the joints subjected to VA loads resulted in conservative assessments when employing S–N curves obtained for the CA loading. The 4R method, via the local mean stress correction for individual cycles, provided higher accuracy for the fatigue assessments.

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.

Generalisation of the effective notch stress concept for the fatigue assessment of arbitrary steel structures

The paper addresses current implementations and potentials for the future in the field of fatigue assessment using local stress concepts. The background of the effective notch stress concept and its numerical application are described in detail. Common codes and regulations containing local stress approaches have been analysed and compared. A useful approach to apply the notch stress concept not only for welded, but also for unwelded steel components and machined or ground welded joints is presented. It has been shown that this approach coincides with fatigue regulations in EN 1993-1-9. Moreover, typical and innovative application areas for the effective stress concept are summarised. These comprise local weld imperfections and even for additive manufactured steel components the concept can be applied. Finally, an approach using the effective notch stress concept with random radii is presented. The relation between fatigue class and applied radius at the weld toe can be expressed by an exponential relationship.

Determination of notch factors for transverse non-load carrying stiffeners based on numerical analysis and metamodeling

Stress concentration factors (SCFs) at weld toes and weld roots as required for the effective notch stress concept (see [1, 2]) are usually computed using finite element analysis (FEA) which requires a certain amount of effort for model generation, the solving process, and postprocessing. Regression functions of many FEAs within given parameter bounds provide the possibility of a fast prediction of SCFs. This paper provides new and accurate regression formulae for the estimation of notch stresses at idealized weld geometries on the basis of multiple linear-elastic FEAs for the transverse stiffener (non-load carrying T-joint) under tension and bending of the load carrying slab. Regression of sampled finite element results has been performed using (a) second-order polynomial regression with coupling terms (PRC) and (b) artificial neural networks (ANN). The presented formulae are compared with several existing estimations of stress concentration factors. The new methods appear to show a higher quality of prognosis as well as apply to significant larger ranges of the geometrical parameters of the weld joint. The formulae presented here for the transverse stiffener add another welded joint to a series of similar surrogate models presented from Munich University of Applied Sciences in earlier publications and made available for use by the web-based tool SCF-Predictor.

Fatigue assessment of weld seams considering elastic\u2013plastic material behavior using the local strain approach

In this contribution, the notch strain approach is applied to seam welds including butt welds and filet welds. This allows the fatigue assessment for all regimes of fatigue life, including the low cycle fatigue regime (N > 10). Linear-elastic finite element analyses are used to determine the local stresses. The modeling of the geometry to be assessed is similar to that of the effective notch stress concept. Necessary input values besides linear-elastic stresses are estimated from the hardness of the heat-affected zone. Subsequently, the elastic–plastic stresses and strains are estimated, and a service life calculation is carried out. The used algorithm for the notch strain approach is based on the German FKM-guideline “nonlinear” and includes the influence of stress gradients, highly stressed surfaces, and the influence of surface roughness. Up to now, this FKM-guideline has only been approved for non-welded components. However, this article shows that, with a few modifications, it can also be applied to welds.

Investigations on the fatigue behaviour of welded-in stiffeners with gaps

The paper deals with investigations on the fatigue resistance of welded-in stiffeners with gaps. The presented investigations aim to specify limits of acceptable imperfections in terms of gaps in welds, as these do not reduce the fatigue strength in all cases. Finite-element simulations applying the effective notch stress concept have been conducted and confronted with fatigue test results from a detailed fatigue test database. Several geometrical influences regarding the fatigue behaviour of welded-in stiffeners with gaps were analysed and evaluated by performing a numerical study. Finally, interesting details of welded-in stiffeners were worked out and presented, in order to be tested experimentally in a future fatigue test program. It can be concluded that the effective notch stress concept is in fact a valuable tool to assess influences on the fatigue behaviour of the presented detail. The weld toe of the stiffener proves to be the relevant failure location in all investigated configurations, the weld root on the other hand is irrelevant. While the section size has a minor influence on the fatigue strength of the welded-in stiffener, the distance of the weld toes is the main influencing characteristic. The gap size does not play a major role for the fatigue resistance, which is why gaps with a certain size could be accepted in practice. This aspect will be confirmed in the future by conducting additional experimental fatigue tests.

On the weld root fatigue strength and improvement techniques for non-load-carrying transverse attachment joints with single-sided fillet welds and made of mild and ultra-high-strength steels

The present study experimentally and numerically investigates the fatigue strength of non-load-carrying transverse attachment joints prepared with single-sided fillet weld. Constant amplitude fatigue tests are carried out on specimens made of S355 and S1100 steel grades to obtain the fatigue strength of these joints. In addition, the study evaluates fatigue life improvement technique for these joints comprising the use of a curved plate edge shape in the attachment, which results in varying weld penetration and weaved weld root shape. Numerical analyses are carried out to determine the applicability of the effective notch stress concept in assessing the fatigue strength of the studied joints. The experimental results show that the weld root to the base plate is the critical fatigue failure mechanism, and characteristic design fatigue strengths of FAT97.7% = 109 MPa and FAT97.7% = 97 MPa (with a slope parameter of m = 3) in the nominal stress system for the S355 and S1100 grades with a straight root geometry are obtained, respectively. Using a curved plate edge shape and varying weld penetration, the fatigue strength could be improved by up to 25% for the S1100 grade. The numerical analyses reveal that the effective notch stress modeling technique of the fictitious notch radius significantly affects the stress concentration factors, while the fillet-type modeling technique at the weld root showed more accurate results than the keyhole shape.

Fatigue analysis of wire arc additive manufactured (3D printed) components with unmilled surface

First investigations have been carried out on the characterisation of the as-welded surface roughness of unmilled wire arc additive manufactured (WAAM) components, followed by an experimental determination of the fatigue strength. Subsequently, randomly based finite-element (FE) simulations are carried out to expand the findings. The statistical characteristics of the surface provide the input values for the Monte-Carlo (MC) simulation. Here, the fatigue strengths of single-wire and multiple-wire welded components have been compared applying the effective notch stress concept performing over 2200 simulations. Finally, a fatigue detail classification for surfaces of WAAM components is given following the principles of EN 1993-1-9 [1].

Re-evaluation of weld root fatigue strength for load-carrying fillet welded joints using the notch stress concept

The re-evaluation of fatigue strength, applying nominal and notch stress concepts has been carried out, using extracted experimental fatigue data for load-carrying fillet welded joints failing from weld root. Various reference radii, including rref = 1 mm, rref = 0.5 mm, and rref = 0.3 mm are applied to investigate an appropriate reference radius for assessing fatigue strength of weld root using the effective notch stress concept. In addition, a new FAT class with modified slope is proposed to improve the accuracy of the current recommended class for thin joints with rref = 0.3 mm.

Fatigue strength of misaligned non-load-carrying cruciform joints made of ultra-high-strength steel

Misalignments and distortions in welded plate components act as stress raisers and can significantly decrease the fatigue strength of welded connections. This paper investigates the effect of the plate misalignment of transverse attachments on the fatigue behavior of axially-loaded non-load-carrying cruciform (NLCX) joints. Experimental fatigue tests with and without the plate misalignment are carried out for fillet-welded NLCX joints. The test specimens were fabricated of S1100 ultra-high-strength steel grade, and the fatigue tests were conducted using an applied stress ratio of R = 0.1. Numerical finite element analyses were conducted to obtain the stress concentrations induced by the misalignment. In addition, varied geometry parameters were applied to investigate their effect on the magnitude of stress concentrations. Stress concentrations were obtained using the structural hot spot stress method applying linear surface extrapolation and 1 mm below depth methods, and the effective notch stress concept with the reference radius of 1 mm. Experimental fatigue tests showed a decrease of up to 12% in fatigue strength depending on the degree of misalignment. The highest stress concentration was induced when the misalignment to the joint width ratio was e/L = 0.2–0.4. Structural stresses cannot be estimated using the linear surface extrapolation. Instead, structural stress at the 1 mm depth and effective notch stress concept accurately evaluated the misalignment effect on the fatigue performance of NLCX joints, and provided a good correspondence between the theoretical and experimental fatigue strength estimations.

Mid- and High-Cycle Fatigue of Welded Joints in Steel Marine Structures: Effective Notch Stress and Total Stress Concept Evaluations

The characteristic far field response spectrum of welded joints – the governing fatigue sensitive locations in steel marine structures – is predominantly linear elastic, meaning mid- and high-cycle fatigue (MCF and HCF) is most important for design. Using the effective notch stress- and the total stress concept, involving respectively Se and ST as intact- and cracked geometry fatigue strength criterion, one MCF-HCF resistance curve has been obtained for all welded joints. A generalised random fatigue limit model explicitly incorporating the MCF life time and HCF strength limit scatter provides statistically the most accurate fatigue strength and fatigue life time estimates. Similar MCF performance is obtained for Se and ST. Although crack growth dominates the MCF damage process, the results for an initiation related criterion like Se and natural crack growth related criterion like ST are similar. Adopting Se rather than ST as fatigue strength criterion naturally related to the crack initiation dominated HCF region showing the largest data scatter may explain the better effective notch stress concept HCF performance. Since the HCF resistance scatter is relatively large, the MCF-HCF generalised random fatigue limit model design curves show approximately 1-slope behaviour. meaning that for design purposes a linear Basquin model approximation rather than a piecewise continuous bi-linear MCF-HCF formulation according to guidelines, standards and classification notes should be adopted.

Fatigue analysis of welds and bolts in end plate connections of I-girders

End plate connections with prestressed bolts represent economical joints in steel construction. However, the fatigue resistance of these connections is currently insufficiently covered in Eurocode 3-1-9 [1]. The weld and bolt stresses during fatigue loading of end plate connections have been examined in detail. To this end, fatigue tests have been carried out on component and small-scale specimens. In a subsequent numerical investigation, FE-models have been validated by the experimental results. Second, geometric influences were investigated. The fatigue strength of the welds has been assessed by the effective notch stress concept. Finally, a detail category recommendation is given.

Fatigue design of welded double‐sided T‐joints and double‐sided cruciform joints in steel marine structures: A total stress concept

AbstractFatigue is a governing design limit state for marine structures. Welded joints are important in that respect. The weld notch stress (intensity) distributions contain essential information and formulations have been established to obtain a total stress fatigue damage criterion and corresponding fatigue resistance curve; a total stress concept. However, the involved weld load carrying stress model does not provide the required estimates and trends for varying geometry dimensions and loading & response combinations. A new one has been developed and performance evaluation for T‐joints and cruciform joints in steel marine structures shows that in comparison with the nominal stress, hot spot structural stress and effective notch stress concept based results up to 50% more accurate fatigue design life time estimates can be obtained. Taking advantage of the weld notch stress formulations, the effective notch stress concept performance has improved adopting a stress‐averaged criterion rather than a fictitious notch radius‐based one.

Investigations on the fatigue behavior of end plate connections with prestressed bolts

In many areas of our infrastructure, as for instance the construction of crane runway beams, fatigue design of steel structures is a major issue. An important detail in practice, but insufficiently classified in the (Eurocode 3-1-9, 2005) detail catalogue, is the end plate connection with prestressed bolts. Detailed numerical simulations are often necessary for the fatigue check of the weld at the T-stub. The knowledge of the true bolt force required in the detail catalogue can usually also only be taken into account using finite element simulations. For these reasons, the detail ‘end plate connection with different weld configurations and prestressed bolts’ containing the two critical design cases mentioned above has been investigated. The analyzed details comprise butt welds as well as fillet welds and connections with overhanging and flush end plate connections.A literature review of fatigue in details of end plate connections is given. Moreover, the test program on three fatigue tests series is presented. After conducting experimental investigations on small-scale as well as large-scale specimens detailed numerical simulations have been realized. In order to assess the fatigue resistance of the welded connection, the effective notch stress concept has been used. Additionally, fatigue resistances of prestressed bolts have been investigated by recalculating the bolt force progress. After the verification of the numerical models, a parametric study has been conducted by varying geometric characteristics in order to give recommendations on the design of prestressed bolts and welds in end plate connections.

09.05: Remaining fatigue life of steel railway bridges: Improved models for considering realistic behaviour

ABSTRACTMany steel railway bridges will reach the end of their scheduled design service life within the next years. One can also observe some bridges, considerably younger than their scheduled design service life, with insufficient remaining fatigue life and rarely cracks in structural elements. Mostly because of the early applications of the welding technology and sometimes insufficient consideration of fatigue effects, e.g. neglecting of high local secondary stresses. It is a great challenge and very expensive, if a couple of bridges have to be replaced in a short period. Therefore, it is important to have models for realistic fatigue verification to extend the service life.The focus of this paper is to show different approaches – both on the load and the resistance side – to verify an extended service life of steel railway bridges. For example, the big potential of the use of real, measured stress spectra is presented, instead of the common approach according to the Eurocode. In addition, the application of the consequent Miner's rule results in another significant reduction of equivalent stress ranges for application of the fatigue strength curves of the Eurocode.Furthermore, the simplified and conservative assumptions regarding the fatigue resistance in conventional assessment procedures according to the Eurocode (nominal stress approach) are compared with alternative concepts for typical welded joints. For example, the geometric (hot spot) stress concept, the effective notch stress concept and fracture mechanics. The Eurocode already allows the use of the geometric stress concept, but gives little guidance. The practical implementation of these concepts is shown for a representative detail of a steel bridge, considering a variety of geometries.Fracture mechanics approaches are state of the art in mechanical engineering, but still relatively uncommon in civil engineering. These concepts allow for the explicit consideration of real or assumed “non‐detected” flaws or cracks in the component, thereby eliminating the need of an exact knowledge of the past load history, up to the considered defect size. It is shown that the fracture mechanics concept is suitable for steel bridges to determine the remaining fatigue life of critical details.Finally, a multilevel framework is presented to give guidance for engineers to evaluate the service life of a specific steel railway bridge.

An extrapolation method to determine the effective notch stress in welded joints

AbstractThis paper presents the development of an extrapolation method to estimate the notch stress in welded steel joints, in line with the effective notch stress concept described in the International Institute of Welding guideline. The proposed approach provides a convenient method to determine the local notch stress at the weld toes without the need to model a finite notch radius as required in engineering guidelines, which creates significant modeling challenges in the numerical procedure, especially for 3‐D joints with a complex topology. An experimental study validates the feasibility of this extrapolation method through strain measurements on cruciform joints. The numerical study then investigates two types of welded connections: (1) cruciform joints with full‐penetration welds and (2) tube‐to‐plate joints. The proposed extrapolation method provides accurate effective notch stress estimations on both joint types, compared with the effective notch stress calculated following the mesh requirement in the existing guidelines.

Fatigue predictions of welded joints and the effective notch stress concept

Several research papers on a new concept of the effective notch stress of a welded joint were recently published. The toe of the weld is modelled with a prescribed radius. A parameter for the severity of the stress distribution at the weld can then be expressed as a Kt-value to be calculated with FE analysis. The new concept offers interesting applications for designing against fatigue crack initiation and predictions on the fatigue limit. It is argued that the radius concept should be replaced by a ratio of two dimensions and a proposal is made for this purpose. The fatigue assessment should be based on the calculated Kt-value. Limitations of the present codes are discussed.

Fatigue strength evaluation of friction stir welded aluminium joints using the nominal and notch stress concepts

In this study the fatigue strength is investigated for Friction Stir Welded (FSW) overlap and butt welded joints in different thicknesses based on nominal and effective notch stress concepts. The fatigue test results are compared with fatigue strength recommendations according to EN 1999-1-3 and International Institute of Welding (IIW). The results are also compared with available published data and Finite Element Analysis (FEA) is carried out to investigate the effect of plate thickness and nugget size on the fatigue strength of overlap joints. 3–3mm butt welded joints shows the highest fatigue strength in comparison with 3–5mm butt welded and overlap joints. Slopes of the SN-curves for two different joint types differ from the slope recommended by IIW. A specific failure trend is observed in overlap FSW joints. However, the slopes of the SN-curves are in close agreement with slopes found in EN 1999-1-3. The slopes of various published results and test results presented in this study are in good agreement with each other. The suggested fatigue design curves for the nominal and effective notch stress concept have a higher slope than given for fusion welds by IIW.

Design Optimization of Components Based on Material Mechanics Fatigue Concepts

Abstract The paper deals with the fatigue design concepts for engineering components based on material properties and mechanics, i.e. the Short Crack Model (SCM), the Local Strain Approach (LSA), and the Effective Notch Stress Concept (ENSC) for welds. The calculation procedures are presented and their accuracy and robustness are demonstrated on the example of a cab bearing console providing complex geometry (geometry notch, weld with pronounced start and ends) and subjected to operating loading with variable stress amplitudes and mean stresses. Parametrical studies considering two ductile steels were performed. Comparison of calculated curves by means of the SCM and the LSA supported by the PSW T-parameter explores life factors of 3 to 6.7 emphasizing the crucial influence of load sequence effects on fatigue life and the usefulness of the SCM to consider them. The choice of the material influences the lifetime of the console significantly (life factors of ∼5 to ∼11 depending on the applied model). The weld design was assessed by the ENSC to be safe.