Nominal Stress Approach Research Articles

Welding residual stress impact on fatigue life of a welded structure

A welded component has been fatigue tested and then analyzed using both the nominal stress approach and the fracture mechanics approach. The nominal stress approach predicted the wrong failure location as well as a very short fatigue life. A welding simulation was then performed to understand the residual stresses generated in the structure by welding. Results showed a highly compressive stress state at the low life location predicted by the nominal stress approach. A fracture mechanics-based assessment of fatigue was then performed, incorporating welding residual stresses. The results showed excellent agreement with the fatigue test results, emphasizing the important role played by residual stresses in determining the failure location.

Fatigue Tests of Axially Loaded Butt Welds up to Very High Cycles

Fatigue strength curves that are established from fatigue tests provide a basis for the fatigue assessment applying nominal stress approach. In the codes valid for steel structures, like the EC 3, the fatigue strength curves for constant amplitude loading have a knee point in the transition region. The fatigue strength curve beyond this knee point is commonly assumed to be a horizontal asymptote. However, the behaviour of the fatigue strength curve in the area of very high cycles and more importantly the existence of an endurance limit are much discussed. In the case of welded joints the experimental data beyond 107 load cycles is limited due to the possibilities in testing. Testing techniques with high frequencies are necessary to obtain experimental data with very high cycles in a reasonable period of time. In this scope a testing device with approximately 390Hz operates by alternating current magnets and using resonance amplification, which was developed by a third party. This testing device was investigated and advanced for the application of long term tests reaching 5·108 load cycles. Fatigue tests on axially loaded butt welds with constant amplitude loading are conducted in three test series until very high cycles. The fatigue tests include the area of high and very high cycles. The influence of test frequency and stress ratio is investigated.

Effect of high-strength filler metals on the fatigue behaviour of butt joints

Welded structures made of high-strength steel offer benefits in fatigue strength for finite life applications. The high-cycle fatigue limit, however, depends mostly on the geometry, and the metallurgy of the notch is of little account. Therefore, an optimised weld process is required to achieve an improvement in the fatigue strength. This paper contributes to the field of fatigue behaviour of thin-walled, high-strength steel butt joints, with regard to an optimisation of the gas metal arc weld process. An existing methodology was extended to manufacture welded specimens with minimised production scatter. The majority of the butt joint samples were dynamically tested, with the root surface ground flush to plate, to study the effect of the weld process on fatigue. The investigated specimen were carefully analysed by metallographic studies along with hardness, distortion, and weld toe topography measurements. This facilitated in finding a relation between experimental fatigue life and the weld parameters. The nominal stress approach, including a benign, nonconservative thinness correction, and the recommended notch stress concept, were applied to assess the fatigue behaviour of the thin-walled, high-strength steel butt joints. The experimental results showed that in case of high-quality welds with negligable geometric notch factor, a small, but distinct influence of the filler metal on fatigue is observable. The highest fatigue strength for the investigated butt joint design was obtained with a high-strength metal-cored wire filler in combination with a three-component shielding gas.

Fatigue Strength Of Thin- Plated Block Joints With Typical Shipbuilding Imperfections

During design of block joints in ship structures it is assumed that the permissible tolerances for axial and angular misalignment given in shipbuilding standards are not exceeded. This is particularly important for fatigue assessments based on the nominal stress approach which implicitly incorporates stress magnification due to imperfections. Preliminary investigations of thin-plated ship structures (t = 4 mm) showed that (a) it is very difficult to keep the thickness-dependent tolerances for thin plates, and (b) even if the tolerances are kept, the fatigue strength might not be sufficient as the stress magnification incorporated in the nominal stress approach is far exceeded. The present paper highlights the problem in the beginning. The main part describes fatigue tests of components performed in a joint industry project. Stiffened panels were fabricated by different shipyards under realistic shipbuilding constraints. The partly large imperfections (buckles, axial and angular misalignment) were extensively recorded using photogrammetric technique in order to obtain the characteristics of typical pre-deformations. The component tests establishing the link to the fatigue strength were carried out with a stress ratio R = −1 which is typical for naval vessels. Concurrent nonlinear finite element calculations containing the whole test set-up were carried out to obtain relations between pre-deformation and stress increase. After description of the test results, a procedure for the assessment of imperfections is discussed which are measured in the usual way in shipbuilding.

J-integral-based approach to fatigue assessment of laser stake-welded T-joints

The aim of this paper is to investigate the influence of the plate thickness on the fatigue strength of laser stake-welded T-joints under the tension loading condition. Fatigue tests were conducted on specimens with plate thicknesses below 5mm subjected to tension loading with the load ratio R=0. The statistical analysis of the weld geometry showed a normal distribution of the each parameter that was measured. In addition, the parameters had similar proportions in comparison to the specimens with plate thicknesses above 5mm. FE analysis was performed with the aim of determining the stress state in the joint along with the J-integral. If the square root of the J-integral, √ΔJ, is used as the fatigue strength assessment parameter, the fatigue strength obtained at five million cycles is similar as in the case of other steel welded joint types. The investigation concluded that the stress state changes with the reduction of the plate thicknesses and the contribution of fracture mode II becomes significant. However, using √ΔJ as a fatigue strength assessment parameter ensures that the complex state of the mixed fracture mode loading is accurately accounted for. This fact further enables the fatigue strength of laser stake-welded T-joints of any plate thicknesses to be described by means of a narrower scatterband than the one obtained by the nominal stress approach.

On the fatigue performance of Aluminum alloy 2024 scarfed lap joints

A series of fatigue test were carried out on scarfed lap joints (SLJ) using in airfoil siding to explore the effect of structural details, such as rows of rivets, lap angles, on its fatigue performance. Finite element (FE) analysis was employed to explore the effect of lap angle on load transfer and the stress evolution around the rivet hole. At last, the fatigue lives were predicted by nominal stress approach and critical plane approach. Both of the test results and predicted results showed that fatigue life of SLJ was remarkably increased after introducing lap angle into the faying surface. Specimen with the lap angle of 1.68o exhibits the best fatigue performance in the present study.

Fatigue life reliability based design optimization for the missile suspension structure

Purpose – The purpose of this paper is to optimize the key dimensions parameters of the missile suspension structure to ensure the structural fatigue life (>10000 cycles) with the reliability of 0.995.Design/methodology/approach – The design objective is the fatigue life reliability of the structure, while the design variables are the four fatigue‐sensitive dimensions. The nominal stress approach is introduced to predict the fatigue life, and it was verified by comparing with experimental data. The second respond surface method is applied to solve the reliability in a finite element‐supported analysis using software MSC Patran/Nastran. A Sequential quadratic programming (SQP) algorithm is used for structure optimization.Findings – The fillet radius r is the most important factor that influences the fatigue life reliability of the structure. The four optimal dimensions parameters are obtained by a reliability‐based design optimization process with the fatigue life and reliability fulfilling the demands.Ori...

Effect of High-Strength Filler Metals on Fatigue

Fatigue life of welded joints is in general independent from the material strength. High-strength materials are only beneficial in the low-cycle-fatigue region due to their increased yield limit. This property leads to their application, for instance, in welded mobile crane structures. The high-cycle fatigue limit, however, depends mostly on the geometry and the metallurgy of the notch. Therefore, an optimized weld process is required to achieve a certain fatigue strength. This paper contributes to the obtainable fatigue limits for thin-walled, high-strength joints regarding an optimization of the gas metal arc weld process for fillet welds without additional post-treatment. A methodology is designed to manufacture welded specimens with minimized production scatter. The specimens were carefully analysed by metallographic studies, hardness, distortion and geometric weld toe measurements. The detailed analysis enables a profound link between experimental fatigue life and weld process settings. For the assessment of the fatigue life of thin-walled specimens, the nominal stress approach and the notch stress method are used. The thin wall thickness is considered in the nominal approach by a thickness correction factor. The experimental results showed that the highest fatigue strength for the specific specimen design in ‘as-welded’ condition can be obtained when using a high-strength metal-cored wire filler in combination with a three-component mixed gas.

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.

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.

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 strength analysis of a welded railway vehicle structure by different methods

The endurance limit approach with maximum amplitude loading (constant loading amplitude) and the cumulative damage approach with variable amplitude loading are both established methods for the fatigue assessment of the bogie frame of a rolling stock. These methods are presented for the vertical main loading of a bogie frame for a light rail vehicle. Assessments are carried out for the normative load requirements according to the relevant standard EN 13749 [1] and also for a vertical force range spectrum deriving from a multiple body system simulation of this railway vehicle. The multiple body system simulation supplies more realistic loads depending on the actual railway application conditions. These different fatigue assessments allow a comparison of the normative load requirements with realistic load assumptions. In general the stress analysis is based on the nominal stress approach. Additionally, for the area subject to the maximum stress, a notch stress analysis is carried out. The comparison of stress analyses for some selected stress points shows the potential and the advantages of the fatigue assessment with the real loading assumptions and with different evaluation methods.

Re-analysis of fatigue data for welded joints using the notch stress approach

Experimental fatigue data for welded joints have been collected and subjected to re-analysis using the notch stress approach according to IIW recommendations. This leads to an overview regarding the reliability of the approach, based on a large number of results (767 specimens). Evidently, there are some limitations in the approach regarding mild notch joints, such as butt joints, which can be assessed non-conservatively. In order to alleviate this problem, an increased minimum notch factor of K w ⩾ 2.0 is suggested instead of the current recommendation of K w ⩾ 1.6. The data for most fillet-welded joints agree quite well with the FAT 225 curve; however a reduction to FAT 200 is suggested in order to achieve approximately the same safety as observed in the nominal stress approach.

Finite-Element-Based Methods for the Fatigue Design of Bolts and Bolted Joints

Due to the increase of computer performance, modern screw design is more and more based on finite element analyses. Depending on the problem, different possibilities of modelling bolted joints are reasonable. Traditionally, a nominal stress approach is used for the fatigue design of bolted joints. But, in modern lightweight constructions it is more and more common to connect threaded components directly without using a classic bolted connection. In this case a local approach has to be used. In several investigations at the Fachgebiet und Institut fur Werkstoffkunde (Chair and Institute for Materials Technology) in Darmstadt and the Robert Bosch GmbH in Schwieberdingen different concepts for a local approach have been tested for the fatigue design of threaded connections. As a result, an overview can be given, which design concept and which model can be properly used for calculations.

Testing and analysis of fatigue behaviour in edge details: A comparative study using hot spot and structural stresses

At present, the fatigue design of welded structures is primarily based on a nominal stress or hot spot stress (HSS) approach with a series of classified weld S-N curves. Although well accepted by major industries, the nominal stress-based fatigue design approach is relatively cumbersome in terms of securing a series of S-N curves corresponding to each class of joint types and loading modes. Moreover, it is very difficult, if not impossible, to determine the nominal stress at each structural component, particularly in complex ship structures. The HSS-based fatigue design is based on the stress at the weld toes obtained by linear or quadratic extrapolation of stresses over two or three points in front of the weld toe. Finite-element analysis is mostly applied. However, this method has a difficulty of finding a proper stress through the global model, the medium size model and the detail model of ship structure. Besides, the calculated HSS values may vary depending on the extrapolation technique used. Recently, a mesh-size insensitive structural stress (SS) definition that gives a stress state at the weld toe with a relatively large mesh size has been proposed. The SS definition is based on the elementary structural mechanics theory and provides an effective measure of a stress state in front of the weld toe. As an experimental validation of the Battelle SS method in obtaining the fatigue strength of weldments, a series of experiments are carried out for various sizes of weldments. Based on the results from this study, it is expected to achieve the development of a more precise fatigue strength evaluation technique and saving on the time required in the fatigue design of ship and offshore structures.

Variable amplitude fatigue of autofrettaged diesel injection parts

AbstractExperimental and analytical investigations of constant and variable amplitude fatigue life of not autofrettaged and autofrettaged components have been performed. In variable amplitude loading the new standardised COmmon‐RAil‐Load sequence CORAL has been used as well as two‐level‐tests with small cycles at high mean stresses interrupted by large cycles for the evaluation of load sequence effects. The results of the two level tests show that small cycles with amplitudes far below the fatigue limit cause fatigue damage. Life calculations have been performed according to the nominal stress approach with S‐N‐curves and improved Miner’s Rule, linear‐elastic fracture mechanics with 3D‐weight functions,elastic‐plastic fracture mechanics applying an extended strip yield‐model, and explicit 3D‐FE‐simulation of fatigue crack growth with predefined crack fronts.All approaches are appropriate for predicting realistic variable amplitude lives. From a practical point of view the explicit 3D‐FE‐simulation of fatigue crack growth is too time‐consuming. However, such simulations show that the approaches based on linear‐elastic fracture mechanics and elastic‐plastic fracture mechanics with extended strip yield‐model capture the essential physics of fatigue crack growth in a realistic way.

Fatigue assessment of out-of-plane attachments with various angles by using local stress approaches

In order to investigate the effect of the angle between principal stress direction and weld attachment on the fatigue behavior, crack initiation points and fatigue strengths have been examined by using the fatigue test results of out-of-plane attachments with various angles under uniaxial loading. As fatigue assessment methods, the nominal stress approach, the structural hot spot stress approach and the effective notch stress approach are used. It is possible to find fatigue crack initiation points by using the effective notch stress approach, regardless of the angle between principal stress direction and weld attachment. The effect of the angle between principal stress direction and weld attachment on the fatigue crack initiation life cannot be considered when the nominal stress approach and the structural hot spot stress approach are used. However, by using the effective notch stress approach, it is possible to consider the effect of the angle on the crack initiation life.

Influence of Base Plate Flexibility on the Fatigue Performance of Welded Socket Connections

This study investigates the effect of base plate flexibility on the behavior and fatigue performance of welded socket connections commonly used in sign support structures. The results of a three specimen static load testing program, extensive finite-element analysis, and parametric study are discussed. Results clearly show that base plate flexibility, primarily base plate thickness, has a major influence on the stress distribution in the tube wall adjacent to the welded socket connection. Currently, AASHTO gives no specific guidance on the design of the base plates for sign support structures that explicitly address flexibility. Increasing base plate thickness is shown to be a very effective method to improve the fatigue resistance of this detail. Based on this trend, a simple procedure that incorporates the influence of base plate flexibility into the infinite life, nominal stress approach to fatigue design is proposed.

Numerical and experimental investigation of weld root fatigue in fillet-welded structures

For fabrication reasons, a great part of the joints in ship structures are welded with non-penetrating fillet welds. This implies the risk of fatigue failure originating from the weld root under cyclic loading. A fatigue analysis may be performed using the nominal stress approach, which is rather coarse, and the crack propagation approach, which is less practical due to the high computational effort. Alternatives are offered by the effective notch stress approach, which assumes a fictitious radius at the tip of the non-fused root faces and by a newly developed structural stress approach for weld failures. The latter is outlined in the paper and applied to the fillet-welded end of a rectangular hollow section (RHS) and to a doubler plate under out-of-plane loads. These represent typical situations with one-sided welds subjected to local throat bending, where fatigue tests have generally shown weld root failure. From the fatigue lives, a design S-N curve based on the structural weld stress is derived. For further verification, the effective notch stress approach is applied to the RHS-joint showing similar results in relation to the design S-N curve as the structural stress approach.