Mesh-insensitive Structural Stress Method Research Articles

Master S-N Curve Fatigue Life Prediction of a Railroad Car Bogie Based on Mesh Insensitive Structural Stress Method

Fatigue life prediction of a welded structure is a complex phenomenon due to the nature of fatigue and the welding process. Additionally, Finite Element Method (FEM) results are extremely sensitive to the size of elements. Therefore, it is difficult to adopt a method to estimate the fatigue life, especially for welded structures. Besides, mesh size independence is a critical issue to perform fatigue life prediction methods that eliminates the need for excessive element numbers in the mesh. This paper investigates the Master S-N Curve Approach (MCA) using the output parameters of the mesh insensitive Structural Stress Method (SSM). MCA based on SSM employs structural stresses recovered from nodal forces and nodal moments. To recover these inputs, FEM model should be established properly. Thus, boundary conditions and applied loads were prepared for the model according to the BS EN 13749:2021. The submodeling technique in ANSYS software was used to analyze the bogie structure. To justify the mesh independence for the model, different mesh sizes were tested. In a specific range for shell bodies, SSM was shown to provide sufficient mesh independence feature. Furthermore, MCA was compared with Hot Spot Stress Method and Nominal Stress Method based on their fatigue life estimations.

Quantitative fatigue evaluation of complex welded connections and application in a train traction motor frame component

Welded connections are common in rail vehicle structures and associated equipment. It is well known that fatigue behaviors of welded structure are controlled by fatigue capability of welded joints. Although there exists numerous joint design guidance in Codes and Standards as well as best practices, they are largely empirical and difficult to apply in fatigue assessment of complex joints. In this paper, we adopt a mesh-insensitive structural stress method recently stipulated by ASME Code for fatigue evaluation of pressure vessels and piping components. Starting with stress concentration evaluation of two typical, but complex welded connections of relevance to some rail vehicle structures, we examine how the joint design principle inferred from the two examples can be effectively applied to an actual traction motor frame design and its effective fatigue improvement method. The experimental validation process is also presented for the proposed fatigue improvement technique.

An Optimization of Master S-N Curve Fitting Method Based on Improved Neighborhood Rough Set

In this paper, a novel master S-N curve fitting method based on improved neighborhood rough set is proposed. The neighborhood relationship is used to granulate the fatigue information system of aluminum alloy welded joints, and the neighborhood rough set model is constructed from the information perspective. Based on conditional entropy, a neighborhood rough set attribute reduction algorithm (NRSBCE) is proposed. By means of this algorithm, comprehensive mathematical analysis model for fatigue life influencing factors is established and optimization of the S-N curve fitting method is introduced. In this work, the fatigue decision system of the aluminum welded joints is constructed firstly. The key influencing factors of the welded joints are obtained by means of NRSBCE algorithm subsequently. After that, the concept of fatigue characteristic domain is introduced according to the reduction result of the NRSBCE algorithm. Then, the master S-N curve fitting is carried out according to the divided fatigue characteristic domain. Correspondingly, a set of S-N curve clusters are fitted. Finally, fatigue analysis system for welded joints based on the fatigue characteristic domain is designed and developed. In the system, the equations of master S-N curve based on the mesh-insensitive structural stress method and the master S-N curve cluster based on the fatigue characteristic domain could be both obtained by uploading the practical fatigue test data of the aluminum alloy welded joints. This work helps to further reduce the dispersion degree of the fatigue sample data of the aluminum alloy welded joints thus provide reference for fatigue design.

High strength welding of Ti to stainless steel by spot impact: microstructure and weld performance

Vaporizing foil actuator (VFA) spot welding, a type of spot impact welding, was used to weld a titanium alloy (Ti-1.2ASN) to a stainless steel (436 SS). The interfacial microstructures and fracture surfaces were characterized using scanning electron microscopy (SEM). Lap shear tests that strained the samples to failure with digital imaging correlation (DIC) were conducted to study the mechanical performance of these welds. A mesh-insensitive structural stress method was used to understand the stress distribution and model the failure modes of VFA welds in ABAQUS. Despite experimental scatter in this developing joining method, most samples failed through the base metal, but multiple failure modes coexisted, including interface failure. These failure modes were used to classify the results. The failure process can be best understood through the lens of the spatial variation that is natural in this type of weld. The center is naturally unwelded, and there is an annulus of high strength material surrounding this unwelded zone that has a wavy morphology. The mesh-insensitive structural stress method could naturally provide a link between the joint structure and the mechanical properties of the spot impact welds. This could show that despite varied failure modes and nugget strength, strength itself is not usually affected adversely by the size of the central unbonded zone.

The application of load identification model on the weld line fatigue life assessment for a wheel loader boom

The external loads on the wheel loader are essential for structural fatigue life assessment. However, the external loads are hard to be acquired, because the contact force between the soil and the working device is very complex. Most researchers obtain the external loads using dynamic simulation and theoretical analysis, whereas there is a big gap between the simulation and the reality. In order to obtain the precise external loads of the wheel loader, a load identification model verified by test is proposed in this paper. The mathematical relationship between the external loads and loads at the hinged points of the wheel loader is deduced from mechanical balance equation, and the load identification theoretical model is established. Based on the load identification model, a load test system is designed and the test is carried out. Based on the time history of strain measurement near the fatigue weak points and the Miner's damage theory, the relative damage is calculated to ensure that the relative damage ratio of the weak life point between the measured and simulated value is less than 0.8. Finally, the external loads obtained by the load identification model are taken as the input condition, and the life of the boom weld structure is evaluated by the mesh-insensitive structural stress method (MSS method). The results show that the calculated weak point life can truly reflect the actual weld failure position and the error ratio between predicted and actual lives is within one magnitude, which can meet the needs of engineering applications.

Fatigue assessment of welded joints in API 579-1/ASME FFS-1 2016 - existing methods and new developments

The 3rd Edition of API 579-1/ASME FFS-1 2016 Fitness-For-Service includes a new Part 14 dedicated to fatigue assessment. An important section in this part covers the fatigue assessment of welded joints. In this paper, an overview of the fatigue methods for welded joints is provided and extensions are recommended. First, an overview is given of the classical fatigue method used in the ASME B&PV Code based on smooth bar fatigue curves in conjunction with a fatigue strength reduction factor. In addition, the mesh insensitive structural stress method is outlined using an equivalent stress parameter based on fracture mechanics considerations in conjunction with a master S-N curve based on the analysis of over 2000 high and low cycle S-N test data. The resulting master S-N curve approach is applicable to high cycle fatigue and low cycle fatigue if a Neuber correction is introduced. In this paper, a new structural strain method is presented to extend the early structural stress based master S-N curve method to the low cycle fatigue regime in which plastic deformations can be significant while an elastic core is present. With this new method, some of the inconsistencies of the pseudo-elastic structural stress procedure can be eliminated, such as its use of Neuber’s rule in approximating structural strain beyond yield. The earlier mesh-insensitive structural stress based master S-N curve method can now be viewed as an application of the structural strain method in the high cycle regime, in which structural strains are linearly related to traction-based structural stresses according to Hooke’s law. Thus, both low cycle and high cycle fatigue behavior can now be treated in a unified manner. In the low-cycle regime, the structural strain method characterizes fatigue damage directly in terms of structural strains that satisfy a linear through-thickness deformation gradient assumption, material nonlinear behavior, and equilibrium conditions. A PVRC Joint Industry Project is currently sponsoring work on the structural strain method that will lead to its incorporation in the next edition of API 579-1/ASME FFS-1.

Reliability-based assessment of steel bridge deck using a mesh-insensitive structural stress method

This paper aims to conduct the reliability-based assessment of the welded joint in the orthotropic steel bridge deck by use of a mesh-insensitive structural stress (MISS) method, which is an effective numerical procedure to determine the reliable stress distribution adjacent to the weld toe. Both the solid element model and the shell element model are first established to investigate the sensitivity of the element size and the element type in calculating the structural stress under different loading scenarios. In order to achieve realistic condition assessment of the welded joint, the probabilistic approach based on the structural reliability theory is adopted to derive the reliability index and the failure probability by taking into account the uncertainties inherent in the material properties and load conditions. The limit state function is formulated in terms of the structural resistance of the material and the load effect which is described by the structural stress obtained by the MISS method. The reliability index is computed by use of the first-order reliability method (FORM), and compared with a target reliability index to facilitate the safety assessment. The results achieved from this study reveal that the calculation of the structural stress using the MISS method is insensitive to the element size and the element type, and the obtained structural stress results serve as a reliable basis for structural reliability analysis.

Incorporating mesh-insensitive structural stress into the fatigue assessment procedure of common structural rules for bulk carriers

This study introduces a fatigue assessment procedure using mesh-insensitive structural stress method based on the Common Structural Rules for Bulk Carriers by considering important factors, such as mean stress and thickness effects. The fatigue assessment result of mesh-insensitive structural stress method have been compared with CSR procedure based on equivalent notch stress at major hot spot points in the area near the ballast hold for a 180 K bulk carrier. The possibility of implementing mesh-insensitive structural stress method in the fatigue assessment procedure for ship structures is discussed.

Robust Mesh Insensitive Structural Stress Method for Fatigue Analysis of Welded Structures

The design of welded structures and careful fatigue design evaluation is especially important in structural design since the stress concentrations at the welds have significant impact on the overall fatigue lives of the structures. Currently, the nominal stress or hot spot stress methods are widely used in the field for fatigue design evaluation. However, design based on these methods requires constructing separate S-N curves from the fatigue testing based on the joint geometry, loading, thickness, etc. It is also difficult to evaluate the fatigue performance using finite element analysis, where the calculated stress can vary according to element size, type, etc. To overcome these challenges, Battelle has developed a novel, mesh insensitive structural stress method (Verity ® ). The stresses are calculated using the balanced nodal forces and moments obtained at the weld toe location from the finite element solutions. Working with industry partners, Battelle has also developed a unified master S-N curve that combines the effects of joint geometry, loading modes and thicknesses. Battelle's structural stress procedure has been adopted as design weld fatigue codes by ASME Sec. VIII Div. 2 (ASME Boiler and Pressure Vessel Code) and API 579/ASME FFS-1 (Fitness-for-Service Evaluation). As a case study, the fatigue life prediction of a rectangular hollow section joint using a new structural stress method is presented in this paper.

Fatigue of Tubular Joints: Hot Spot Stress Method Revisited

A series of well-known tubular joints tested in UKSORP II have been re-evaluated using the mesh-insensitive structural stress method as a part of the on-going Battelle Structural Stress JIP efforts. In this report, the structural stress based analysis procedure is first presented for applications in tubular joints varying from simple T joints, double T Joints, YT joints with overlap, and K joints with various internal stiffening configurations. The structural stress based SCFs are then compared with those obtained using traditional surface extrapolation based hot spot stress methods. Their abilities in effectively correlating the fatigue data collected from these tubular joints are demonstrated. These tests are also compared with the T curve typically used for fatigue design of tubular joints as well as the structural stress based master S-N curve adopted by ASME Section VIII Div 2. Finally, some of the implications on fracture mechanics based remaining life assessment for tubular joints are discussed in light of the results obtained in this investigation.

Analysis of Solder Fatigue in Electronic Packaging Using a Mesh-Insensitive Structural Stress Method

Solder fatigue is one of the major product reliability concerns in electric packaging design. In this paper, the application of a mesh-insensitive structural stress method, previously developed by the authors, is demonstrated for characterizing fatigue of solder joints under thermal fatigue and mechanical fatigue conditions. Three sets of experimental data from literature are analyzed to demonstrate the effectiveness of the structural stress method in its ability to correlate test data over different packaging designs, test methods, etc. As evidenced in this study, the meshinsensitive structural stress method offers a simple but effective means to correlate the fatigue behavior of solder joints among various test conditions. The results show that the structural stress method can be used as an effective alternative to existing methods in the literature, in reliability design of electronic packaging.

A Robust Structural Stress Parameter for Evaluation of Multiaxial Fatigue of Weldments

Abstract In this paper, the applicability of a new mesh-insensitive structural stress method is examined for the evaluation of multiaxial fatigue behaviors in welded joints. The mesh-insensitive structural stress method proves to be effective in characterizing stress concentration effects in multi-axial fatigue of welded components. In seeking an effective structural stress based fatigue parameter, some well-known published test data were analyzed. These include those published by researchers, e.g., from the Electric Power Research Intitute (EPRI), the Fraunhofer-Institute for Structural Durability (LBF), the University of Illinois at Urbana-Champaign (UIUC), and the Welding Institute (TWI) under bending, torsion, and in-phase and out-of-phase loading combinations. An effective structural stress parameter has then been formulated in a form of a proposed modified Gough's ellipse for considering both in-phase and out-of-phase multiaxial loading conditions. The aforementioned multiaxial fatigue data from a total of five independent sources can be effectively correlated with a structural stress based parameter presented in this paper, without introducing any other material-related empirical parameters. The present method is rather simple to use for day-to-day finite element based fatigue evaluation of welded joints in actual structures subjected to multiaxial loading conditions.

A Robust K Estimation Scheme using Mesh-Insensitive Structural Stresses

In this paper, the applications of a new structural stress method, referred as a mesh-insensitive structural stress method developed recently, in estimating stress intensity factors for welded joints will be discussed. The new structural stress method is based on the considerations of separating an actual stress state into two parts: (1) equilibrium-equivalent part and (2) self-equilibrating part. Both parts are consistent with the far-field stress state with respect to a hypothetical crack location within fracture mechanics context. Therefore, the stress intensity factors (K) can be estimated once the mesh-insensitive structural stresses become available. The method is particularly suited for applications in welded joints in complex structures. In this paper, a brief description of the mesh-insensitive structural stress procedure is reviewed. A robust K estimation procedure is then presented. The new K solution procedures were then validated against a series of well-known K solutions for welded joints in the literature. Finally, discussions are given in terms of the implications of the present solutions in applications in complex structures.

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Fatigue design rules for welds in the ASME Boiler and Pressure Vessels Code are based on the use of Fatigue Strength Reduction Factors(FSRF) against a code specified fatigue design curve generated from smooth base metal specimens without the presence of welds. Similarly, stress intensification factors that are used in the ASME B31.1 Piping Code. are based on component S-N curves with a reference fatigue strength based on straight pipe girth welds. But the determination of either the FSRF or stress intensification factor requires extensive fatigue testing to take into account the stress concentration effects associated with various types of component geometry, weld configuration and loading conditions. As the fatigue behavior of welded joints is being better understood, it has been generally accepted that the difference in fatigue lives from one type of weld to another is dominated by the difference in stress concentration. However, general finite element procedures are currently not available for effective determination of such stress concentration effects. In this paper, a mesh-insensitive structural stress method is used to re-evaluate the S-N test data, and then more effective method is proposed for pressure vessel and piping fatigue design.

Assessment of Asme’s Fsrf Rules for Vessel and Piping Welds using a New Structural Stress Method

Fatigue design rules for welds in the ASME Boiler and Pressure Vessel Code are based on the use of Fatigue Strength Reduction Factors (FSRF) against a Code-specified fatigue design curve generated from smooth base metal specimens without the presence of welds. Similarly, Stress Intensification Factors (SIF) that are used in the ASME B31 Piping Codes are based on component S-N curves with a reference fatigue strength based on straight pipe girth welds. Typically, the determination of either the FSRF or SIF requires extensive fatigue testing to take into account the stress concentration effects associated with various types of component geometry, weld configuration, and loading conditions. As the fatigue behaviour of welded joints is being better understood, it has been generally accepted that the difference in fatigue lives from one type of weld to another is dominated by the difference in stress concentration. However, general finite element procedures are currently not available for effective determination of such stress concentration effects. This is mainly due to the fact that the stress solutions at a notch (e.g., at weld toe) are strongly influenced by mesh size at and near a weld, resulting from notch stress singularity. In this paper, a mesh-insensitive structural stress method is used to re-evaluate the S-N test data. Its applications in consistently representing the stress concentration effects on fatigue S-N data for pipe girth welds are demonstrated. A single master S-N approach is presented by means of a mesh-insensitive structural stress parameter formulated within the context of fracture mechanics. The major findings are as follows: (a) The mesh-insensitive structural stress method provides a simple and effective mean for characterising stress concentrations at vessel and pipe welds (b) The structural stress based parameter provides an effective measure of stress intensity at welds, which can be related to fatigue lives. (c) Once the mesh-insensitive structural stress is used, the S-N data processed thus far can be reasonably consolidated into one narrow band. Therefore, single master S-N curve for vessel and piping welds can now be established, regardless of piping weld types or geometries (straight pipe girth welds, different types of flange welds, elbow welds, mitre bends, etc.), and can be used to general a master fatigue design curve.

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The 3rd Edition of API 579-1/ASME FFS-1 2016 Fitness-For-Service includes a new Part 14 dedicated to fatigue assessment. An important section in this part covers the fatigue assessment of welded joints. In this paper, an overview of the fatigue methods for welded joints is provided and extensions are recommended. First, an overview is given of the classical fatigue method used in the ASME B&PV Code based on smooth bar fatigue curves in conjunction with a fatigue strength reduction factor. In addition, the mesh insensitive structural stress method is outlined using an equivalent stress parameter based on fracture mechanics considerations in conjunction with a master S-N curve based on the analysis of over 2000 high and low cycle S-N test data. The resulting master S-N curve approach is applicable to high cycle fatigue and low cycle fatigue if a Neuber correction is introduced. In this paper, a new structural strain method is presented to extend the early structural stress based master S-N curve method to the low cycle fatigue regime in which plastic deformations can be significant while an elastic core is present. With this new method, some of the inconsistencies of the pseudo-elastic structural stress procedure can be eliminated, such as its use of Neuber’s rule in approximating structural strain beyond yield. The earlier mesh-insensitive structural stress based master S-N curve method can now be viewed as an application of the structural strain method in the high cycle regime, in which structural strains are linearly related to traction-based structural stresses according to Hooke’s law. Thus, both low cycle and high cycle fatigue behavior can now be treated in a unified manner. In the low-cycle regime, the structural strain method characterizes fatigue damage directly in terms of structural strains that satisfy a linear through-thickness deformation gradient assumption, material nonlinear behavior, and equilibrium conditions. A PVRC Joint Industry Project is currently sponsoring work on the structural strain method that will lead to its incorporation in the next edition of API 579-1/ASME FFS-1.