Notch Stress Approach Research Articles

Fatigue enhancement of thin-walled, high-strength steel joints by high-frequency mechanical impact treatment

It is well known that the fatigue strength of welded structures is in general independent from the material strength. In case of high-strength steels, however, a significant improvement in the fatigue behaviour can be realised through post-treatment processes. This paper deals with the effect of high-frequency mechanical impact (HFMI) on the fatigue behaviour of a range of steels starting from mild construction steel (S355) to ultra high-strength steel (S960). The experiments involve fatigue tests at a stress ratio of R = 0.1 on butt welds, T-joints, and longitudinal attachments on 5 mm, thin-walled specimens. The fatigue assessment was performed in accordance to the nominal and the notch stress approach taking the HFMI condition into account. Finally, a novel method is outlined to evaluate the notch stress fatigue behaviour of HFMI-treated joints made of high-strength steel. Applicability of this new HFMI notch stress approach is shown through fatigue assessment of about 330 HFMI post-treated specimens taken from both literature and own test results. Further work focuses on the expansion of the introduced HFMI notch stress model for load spectra influence covering overloads and multiaxial fatigue.

Fatigue strength behaviour of stud-arc welded joints in load-carrying ship structures

The application of stud-arc welding to cyclic loaded primary ship structures was investigated. Thereby M8- and M12 studs were welded on the bulb of Holland profiles using the drawn stud-arc welding with ceramic ferrules. Several fatigue tests were performed, varying the stiffener size, the stud diameter, its position on the bulb, the welding position, weld parameters, residual stress state and the static load in the stud. The obtained fatigue strength is above FAT 80 and only slightly affected by these factors. Hence, the investigated stud-arc welding can be applied at Holland profiles in primary ship structures, provided the quality criteria are met. In order to transfer the results to other geometries and load situations, the applicability of the effective notch stress approach was investigated. The corresponding finite element calculations using idealized weld shapes and a reference weld toe radius of 1 mm showed an insignificant influence of the weld geometry on the notch stress. The determined notch stresses for each test series was above the design S-N curve FAT 225 for the notch stress approach and make it also applicable to stud-arc welding. Further computations were performed using the real weld profile and the stress-averaging approach by Neuber, showing similar results.

An efficient meshing approach for the calculation of notch stresses

For the application of the notch stress approach, local stresses at the notch root have to be calculated. This demands a fine discretization of the finite element model in the notch leading for complex structures to large finite element models which are difficult to handle and take a high amount of calculation time. This paper contains the detailed investigation of the required finite element mesh in the notch area for application of the notch stress approach. Based on small-scale specimens, various meshing rules are applied and compared to each other. As the most important factor for the efficiency of a finite element mesh, the element shape in the notch root was identified. It should be chosen according to the stress gradients: The higher the stress gradients are, the smaller the element edge length should be modeled. The shape of the subsequent elements inside the volume has no major influence on the accuracy of the calculation. Recommendations for practical use are given for various element types, including commonly used tetrahedron and hexahedron element with linear and quadratic shape function. Expected numerical errors for the chosen mesh configuration are indicated.

Influence of the Notch Rounding Radius on Estimating the Elastic Notch Stress Concentration Factor in a Laser Welded Tee Joint

In recent years an increased interest of industry in sandwich-type metal structures can be observed. These structures consist of thin plates of 2.5 mm in thickness separated by stiffeners of different shapes and forms. Welds joining the plates and stiffeners are made on the outer side of the plates using laser welding technique. A locally focused source of heat causes the plate to melt creating a very narrow and elongated joint. As a result, sharp geometric notches are formed on the side of the root of a weld – a place which is inaccessible and cannot be checked. Geometries of individual welded joints vary, sometimes considerably, and this makes their analysis even more complicated. Additionally, the use of laser welding technique influences the formation of untypical distribution of changes in material properties in weld zones. The effect is a joint whose behaviour under load is significantly different from the behaviour of a welded tee joint made with the use of classical methods. Fatigue strength calculations for this type of joints can be conducted based on local stress values in notches, which can be determined with the use of Finite Element Method (FEM). This article analyses the influence of the notch rounding radius on the elastic notch stress concentration factor Kt The aim of the analysis is to evaluate the notch stress concentration according to local notch stress approach.

Effective Notch Stress Method for Fatigue Evaluation of Welded Joints in a Steel Bridge Deck

Effective notch stress, as an approach to evaluate the local stress at a notch (weld toe or root), is defined as the total stress assuming linear-elastic material behavior. This method can be effectively used to evaluate the fatigue performance of welded joints. In this study, finite element analysis results using the effective notch stress method were correlated with fatigue test results of rib-to-deck welded joints in a steel orthotropic bridge deck. Effective notch stress approach provided a good correlation with the crack pattern observed in the full-scale fatigue test. A higher effective notch stress at the critical weld toe than at the weld root was consistent with the dominant crack pattern observed at the weld toe during testing. The effective notch stress at the toe on the deck plate was about 80% higher than that on the rib; no cracks at the weld toe on the rib in the testing were observed. Maximum effective notch stress at the weld root occurred on the upper side of the root notch, which indicates that cracks are more likely to propagate into the deck plate, not into the weld metal. This is also consistent with the observed crack pattern in which the crack from the root propagated upward into the deck plate. No such crack pattern, propagating into the weld metal, was observed in the testing.

Fatigue damage assessment of corroded oil tanker details based on global and local stress approaches

Fatigue damage assessment of double hull oil tanker structural details is performed, based on global and local structural finite element models. The wave-induced vertical and horizontal bending moments, as well as local pressure loads are accounted for in the fatigue damage calculations. Local stress analyses are considered based on the notch stress approach. Time-dependent stresses as a function of corrosion deterioration are analyzed based on nonlinear corrosion wastage during the design life of ship. The effective notch stress approach is applied for analyzing the stress distributions and the fatigue damage of the welded joint at two hotspots. The hotspots are located between the flat bar stiffener of a transverse web frame and the flange of a longitudinal stiffener at the side shell of a tanker ship hull. The details under consideration are modeled separately in a fine mesh employing the sub model techniques. Finally the fatigue damage assessment accounting for corrosion deterioration of the considered hotspots is analyzed.

Modelling and fatigue life assessment of orthotropic bridge deck details using FEM

The fatigue life estimation of orthotropic steel bridge decks using the finite element method is most frequently associated with the application of the structural hot spot stress approach or the effective notch stress approach, rather than the traditional nominal stress approach. The application of these approaches to a welded joint with cut-out holes in orthotropic bridge decks, where it is not easy to distinguish the non-linear stress caused by the notch at the weld toe from the stress concentration effect emanating from the hole in the detail, was investigated. The results of the finite element calculations were compared with the results of the fatigue tests which were carried out on full-scale specimens. The results of the finite element analyses revealed that the structural hot spot stresses obtained from the shell element models were unrealistically high when the welds were omitted. Moreover, the way in which the welds were represented had a substantial influence on the magnitude of the hot spot stress. The results of the analysis when using the effective notch stress approach showed that the agreement between the estimated fatigue life using this approach and the fatigue life obtained from the fatigue tests was good.

Estimate on fatigue welded units of orthotropic slabs of bridge mental spans in the local directions

The aim of this paper was to examine the application of the local stress method in the fatigue life assessment of rib-to-crossbeam joints of orthotropic decks. The hot spot stress approach and the effective notch stress approach were used. The fatigue life predicted by the local and nominal stress method was compared with the experimental results.

노치응력법에 의한 용접 연결부 피로수명 추정에 관한 연구

*School of Naval Architecture and Ocean Engineering, Univ. of Ulsan, Ulsan, KoreaKEY WORDS: Local stress 국부응력, Fatigue effective notch stress 유효노치피로응력, Critical distance 임계거리, Finite fatigue life, 유한피로수명ABSTRACT: This paper analyzes the fatigue-life of welded joints using the notch stress approach. In the notch stress approach, the notch effects are usually approximated by introducing weld-bead parameters fo r the local detailed weld joints. The actual bead shape is comp lex and 3-dimensional. It may also greatly influence the fatigue strength. In this study, the welded shape was modeled using a 3D-scan ner. The critical distance method was adopted in the evaluation of the fatigue ef fective notch stress for the weldments. Fatigue life tests were performed to verify the present method of fatigue life estimation for two types of weld ed plates with longitudinal attachments. The estimated results of the present methods were applied to the results of the experiment. The results of t he analysis showed that the scatter of fatigue-life for the exp erimental data expressed in the nominal stress was significantly reduced by applying the effective fatigue stress of the present study.교신저자 양박달치: 울산광역시 남구 대학로 93, 052-259-2159, pdcyang@mail.ulsan.ac.kr

Fatigue assessment of laserbeam welded PM steel components by the notch stress approach

AbstractThe local fatigue strength of a laserbeam weld of a complex engine component, which joins a PM with a formed sheet component, was assessed by the notch stress concept with the fictitious reference radius of rref = 0.05 mm. First, simplified specimens, following the main geometric dimensions of the parts, were manufactured. On these specimens the fatigue strength was identified by tests and the notch stresses calculated by finite element analysis. Based on these results a design SN‐curve was derived to assess the fatigue strength of the engine component. The numerical assessment of the welded joint was verified by proof tests with the component. The assessment could be improved by considering statistical and stress gradient dependent size effects according to the concept of the highly stressed volume.

Application of the Notch Stress Intensity and Crack Propagation Approaches to weld toe and root fatigue

Several fatigue failures are initiating from the root of non-penetrating welds, which are widely applied in several industrial sectors. The structural optimization and reduction of weld material increase the danger of root cracking. The fatigue assessment during design is mainly based on the nominal, structural hot-spot or notch stress approach. These approaches are only partly applicable to weld roots, give rough estimates such as the nominal stress approach or over-conservative life estimation such as occasionally the notch stress approach when keyhole notches are modelled. An alternative assessment is possible with the crack propagation approach assuming the non-welded root gap and a short crack at the weld toe as initial cracks. A new approach is the notch stress intensity approach (N-SIF approach) which is able to assess the fatigue life of V-shaped notches at weld toes as well as crack-like notches at weld roots using the strain energy density around the notch. Both approaches are applied to different joints where toe and root failures are probable. The results are compared with a previous analysis using the notch stress approach. Fatigue test results are also available for a comparison which allows to draw conclusions with respect to the approaches for an appropriate fatigue strength assessment.

Evaluation of nominal and local stress based approaches for the fatigue assessment of seam welds

In the context of the German joint research project “Applicability of fatigue analysis methods for seam welded components”, fatigue tests were performed by five universities and institutes on welded components, welded parts of larger structures as well as component-like samples of weld details. The sheet thickness t was in the range 1 mm ⩽ t ⩽ 20 mm. The welding parameters for all test coupons and structures tested were chosen according to the industrial production process. Based on the data acquired, nominal, structural and notch stress approaches were analysed with regard to applicability and quality of assessment. The actual weld geometry except the real notch radii was taken into account within the notch stress approach. For the notch radii various values, the reference radii 0.05, 0.3 and 1 mm, were applied. Experimental and numerical results for welded steel components are presented. Approximately equivalent scatter ranges were obtained when applying the various approaches based on the current state of the art. It should be noted that both the nominal and the structural stress approaches are limited in their application compared to the notch stress approach. A comparison of the scatter bands obtained for the various approaches is subject to limitations because it was necessary, in each case, to use different test series as the basis for determining the scatter bands.

Modeling and fatigue assessment of weld start‐end locations based on the effective notch stress approach

AbstractThe present paper contains a methodology for modeling and life assessment of fatigue loaded welded components providing distinct weld start and end locations. The proposed methodology follows the IIW recommendation regarding modeling and finite element meshing of weld toe and root by means of an effective notch radius and uses the corresponding Wöhler curve (FAT class) to assess the durability. Geometrical singularities and, therewith, numerical discontinuities, can be overcome especially when 3D weld root problems are treated. The fatigue life assessment is performed on the basis of normal stresses acting at the failure‐critical weld toe and root locations. Comprehensive experimental database containing stress and fatigue life results derived from motor truck's hypoid rear axles providing complex 3D welds subject to vertical, longitudinal, and torsional loading is used to verify the calculation accuracy of the proposed methodology. The agreement between experimentally determined and calculated fatigue results is satisfactory.

Fatigue strength of laser beam welded automotive components made of thin steel sheets considering size effects

The applicability of and the quality of assessment using the nominal stress, structural stress and notch stress approaches for calculating the fatigue strength of laserbeam welded components made of thin steel sheets has been investigated. For this purpose, the fatigue lives of a longitudinal carrier, an injector and two tube-flange specimens have been determined by tests under constant amplitude loading. Fatigue cracks initiated at sharp root notches on all of these components. While the nominal stress is derived by theory of structural mechanics, the determination of structural and notch stresses is performed using 3D finite element models and solid elements. The structural stress is derived by an extrapolation of surface stress to the fatigue critical notch and the notch stresses by rounding the sharp root notch with a reference radius of r ref = 0.05 mm. For all of the concepts used, the endurable stresses have been compared to the design SN-curves recommended by the International Institute of Welding (IIW). On comparing the quality of assessment of the different concepts, the notch stress approach shows the highest scatter. The highest endurable notch stresses occur in specimens with crack initiation at weld ends. These specimens have a very small highly loaded weld length. The lowest endurable stresses are determined for specimens with a long, equally loaded weld. The reason for these findings can be explained by statistical size effects. For an improved fatigue assessment, an easily applicable method is introduced, which takes into account the highly stressed weld length.

Investigations on different fatigue design concepts using the example of a welded crossbeam connection from the underframe of a steel railcar body

For thin-walled steel structures, welded joints often turn out to be the weak spots under cyclic loading. For the dimensioning of welded safety components in vehicle construction, strength concepts are necessary that are well-defined and well-reviewed, in terms of their predictive accuracy. Within the scope of the research results presented here, the applicabilities of the nominal, structural and notch stress approaches have been examined on the basis of different arc welded and cyclic loaded steel structures, taken from the railway sector. In detail, this is a crossbeam connection from the underframe of a railcar body. Different sheet thicknesses in the range 2.7 mm–4 mm in combination with a misalignment lead to an increase of load in the region of the weld seams. Several different components and specimens with critical regions of failure have been tested under cyclic loading with constant amplitude. With the help of strain gauges, the (technical) crack initiation has been determined. A short review of the possibility of obtaining information about the crack initiation by ultrasonic-burst-phase-thermography is also given. The specimens were the basis for the application and evaluation of the different concepts for the assessment of fatigue life. The numerical determination of the nominal, structural and notch stresses has been performed with finite-element models. For a local approach, according to the notch stress analysis, a submodelling technique has been used. The FE-models have been compared with the experimental data by means of optical 3D deformation analysis and strain measurements with strain gauges. Existing weld seam and specimen tolerances have been included through the use of parametric models. Finally the experimental and computational results have allowed the derivation of structural and notch S– N curves for the crack initiation and the rupture of the specimen.

Fatigue assessment of arc welded automotive components using local stress approaches: Application to a track control arm

The applicability of the structural and notch stress approach is examined on the basis of arc welded and dynamically loaded steel structures, taken from the automotive sector. In detail, this is a transverse control arm. Components and specimens with critical regions of failure are tested under cyclic loading with constant and variable amplitudes. With the help of strain gauges, the crack initiation is determined. The specimens are the basis for the application and evaluation of the different approaches for the assessment of fatigue life. The numerical determination of the nominal, structural and notch stresses is performed with finite-element models. Finally the experimental and computational results allow the derivation of structural and notch Woehler S–N curves.

Fatigue strength investigations of welded details of stiffened plate structures in steel ships

Ships are prone to fatigue due to high cyclic loads mainly caused by waves and changing loading conditions. Therefore, fatigue is an important criterion during design. Different approaches are applied to the fatigue strength assessment. However, the results are varying and the validity of results from small-scale fatigue tests for real structures is sometimes unclear. Therefore, deeper fatigue strength investigations were performed within a German industry-wide joint research project aiming at the harmonization of the approaches. Regarding ship structures, two types of structures were selected for the investigations. The first concerns web frame corners being typical for ro/ro ships from which three models were tested. The associated small-scale specimen is a cruciform joint. The second type is the intersection between longitudinals and transverse web frames, which recently showed fatigue failures in containerships. Five models were tested, three under constant and two under variable amplitude loading. The associated small-scale specimen is a longitudinal attachment on a stiffener top. All large-scale tests showed a relatively long crack propagation phase after first cracks had appeared, calling for a reasonable failure criterion. For the numerical analysis, the nominal, the structural hot-spot as well as the effective notch stress approach have been applied. The latter allows the consideration of the weld shape which could partly explain differences in the observed and calculated failure behaviour. The applicability of the different approaches is quite good if some specific aspects are observed. Insofar the investigations give a good insight into the strength behaviour of complex welded structures and into current problems and opportunities offered by numerical analyses.

Durability of welded aluminium extrusion profiles and aluminium sheets in vehicle structures

Different approaches – the nominal stress, structural hot spot stress and notch stress approach – to analyse the fatigue strength of welded structures made from wrought aluminium alloys were studied. Experimental and numerical investigation was carried out for this purpose on detail specimens and components. The results shown here were generated during the research project “Extrusion profile and sheet metal structures of wrought aluminium alloys in vehicle construction” [1]. The studies show that due to the existing guidelines, welds on structures made from aluminium alloys are sometimes designed very conservatively. It is possible to optimise and reliably design welded joints of thin sheet structures by applying the notch stress approach using the reference radius r ref = 0.05 mm and the reference SN curves derived here.

Contribution to the fatigue assessment of laser welded joints

Laser welded components are widely used in automotive industry enabling high quality joints with minimized heat energy input. This paper contributes to the feasible fatigue levels of disc laser welded joints made of 20MnCr5 and GJS620 using different process parameters. The fatigue behaviour of similar joints made of 20MnCr5 was also studied for two different contact conditions. The influence of the heterogeneous start and stop pilot region of the laser beam to fatigue life is also discussed. Accompanying fracture analysis detected the spot of technical crack initiation at the most stressed root surface. A high process quality for the laser welded joints was confirmed by examining both metallographic sections and the surface topography. Structural weld simulation work was performed to elucidate the local differences between the continuous seam and the overlapping laser weld pilot regions. Investigations showed that the notch stress approach with an effective radius of 0.05mm lead to S/N-curves which generally matched the standard recommendations regarding fatigue.

Evaluation of fillet weld properties and fatigue behaviour in dependence of welding parameters

Numerous different design codes can be used to describe the durability of welded structures. One wide spread approach is the local notch stress approach, which calculates the fatigue lifetime in dependency of the notch stress factor using different effective radii. To calculate the fatigue behaviour by using the local notch stress approach, the RIMS-concept is commonly used [1] . The evaluation of the influence due to the welding process parameters, especially for high-strength steels, the effect of both the geometrical and metallurgical notch is studied in a parametric way for selected weld joints. Experimental fatigue tests have been performed to investigate the link between fatigue life and manufacturing process dependent weld toe notch design. To be able to capture the influence of welding parameters, as energy input per unit length, welding velocity, angle of blowpipe, size and shape of the heat input zone in a numerical way, a local coupled thermo-mechanical simulation is build-up. The complexity of this modelling increases very strong by the temperature dependency of the multitudinous phase material properties. The material and manufacturing properties were adjusted by comparison of the temperature profiles. This experimental based procedure defines the simulation base for more complex welding seams.