No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Structural Stress Determination at a Hot-Spot
Abstract
Stress analysis by a finite element analysis program sometimes causes singularities in hot-spots. In a failure assessment, the structural stress, membrane and bending stress should be determined in a highly stressed spot (hot-spot). When the structural stress at a hot-spot is disturbed by singularities, the stress result not only diverges by reducing the element size but, also shows a decreased value compared to nominal stress because the stress is substituted into the von Mises equivalent stress equation. In the three-dimensional finite element model, it is hard to avoid the singularity problem at hot-spots particularly when it is subjected to loads in three axial directions. For the alternative, this study converts the 3D model into two 2D plane models to remove the singularity and to obtain reasonable structural stress values excluding the peak stress. The structural stress estimating approaches applied in the 2D model were examined for whether they could avoid the structural stress reduction near the hot-spot with mesh insensitivity. The implemented approaches are stress linearization, single point away method, stress equilibrium, stress extrapolation and the nodal force method. The results computed by each approach are compared and reconstructed to 3D stress by the Cauchy stress matrix. This study found that the difference in structural stress in 2D were eliminated after the 3D stress reconstruction.
... Design standards/guidelines such as Det Norske Veritas (DNV) [44] and International Institute of Welding (IIW) [45] have their own recommended HSS extrapolation methods. Moreover, the extrapolation method includes linear extrapolation and quadratic extrapolation [46]. The extrapolation equations are shown below. ...
With increasing freight volume and speed in heavy-haul railways, the impact of train loads on the orthotropic steel deck (OSD) has significantly increased, resulting in fatigue damage of bridges. In this study, the dynamic responses and fatigue assessment of OSD in heavy-haul railway bridges were carried out based on a coupled train-track-bridge analysis and hot spot stress (HSS) method. First, using Dongting Lake Bridge on Hao-Ji Railway as an example, a finite element (FE) model of OSD was established. Then, a simplified method of train loads is proposed to improve the calculation efficiency. The dynamic response results from the coupled train-track-bridge analysis and the moving train load method were compared. The most fatigue-prone detail was determined to be the rib-diaphragm joint weld at the notch. Subsequently, the HSS results of OSD for different scenarios were compared and analyzed. Moreover, the fatigue life of OSD was obtained. Due to multiple stress cycles and large local stresses caused by the heavy-haul train load, the fatigue life of OSD fatigue-prone detail is estimated to be 25.74 years. Finally, the effects of different parameters (e.g., axle weight, train speed, and track irregularity) on the fatigue life of OSD were analyzed. This study provides a reference for the design and evaluation of OSD in heavy-haul railway bridges.
This paper presents an evaluation of the safety involved when performing fatigue assessment of multiaxially loaded welded joints. The notch stress approach according to the IIW is used together with 8 different multiaxial criteria, including equivalent stress-, interaction equation- and critical plane approaches. The investigation is carried out by testing the criteria on a large amount of fatigue test results collected from the literature (351 specimens total). Subsequently, the probability of achieving a non-conservative fatigue assessment is calculated in order to evaluate the different criteria quantitatively. Large variation in safety is observed, especially for non-proportional loading.
The presence of nozzle openings in process vessels gives rise to stress concentration due to localization of stresses around openings. Introduction of second nozzle in vicinity of first nozzle will generate its own stress contours and will interact with stress contours of primary nozzle. To investigate the effect of a secondary nozzle, computational simulation has been carried out. The simulation is validated with established benchmarks. Geometry of vessel and nozzle openings, size of reinforcement and other geometric parameters like centre to centre distance, axial distance, etc. affecting maximum stress have been studied. Result for each case is plotted and discussed in detail.
Paper presents review of the existing equations for stress intensification factors widely used in the engineering practice. Based on a significant number of data points obtained by means of numerical (i.e. finite element) analysis it has been shown that the existing equations may yield non-conservative results which may lead to underestimate of piping stresses. New equations have been proposed which are a better fit to both numerical data and the experimental data used to establish the original stress intensification factors.
A novel principal stress-based high cycle fatigue (HCF) model is proposed for preloaded threaded fasteners under cyclic tensile-shear loads. The model uses the fastener principal stress amplitude in order to construct a singular multi-axial S-N curve from the conventional uniaxial S-N curve with zero mean stress of bolt along with some experimental data. An material testing system (MTS) fatigue testing system is used first to generate the fastener preload by applying a direct tensile-shear load using a special fixture. Subsequently, the same system is used for applying combined cyclic tensile-shear loading of the fastener at various levels of mean stress. Results show that for the same level of axial stresses, the multi-axial loading would significantly reduce bolt fatigue life as compared to that of uniaxially loaded bolt. Moreover, only one S-N curve would be able to predict the multi-axial HCF of preloaded threaded fasteners, when the maximum principal stress amplitude is used. Detailed discussion of the proposed model results and test data are provided.
The effective notch stress approach for fatigue assessment of welded joints according to the IIW-recommendations is a widely accepted procedure and has already become part of many technical rules and guidelines. The method provides additional means for evaluation of welded joints besides classical nominal and structural stress concepts. In contrast to the latter concepts, effects of many local weld parameters can only be studied analytically using the effective notch stress approach. The effective notch stress approach requires use of a standardized idealization of the weld detail and use of continuum mechanics theory for stress analysis (i.e. use of solid elements for finite element models). The idealization with respect to geometry includes modeling of all weld toes and weld roots using a reference radius together with an associated S-N curve. For joints facing low stress concentration, like butt welds or welds at thin sheets, the method requires checking the fatigue behavior of the base material as well. This sometimes is not considered in using the effective notch stress method, because it is not so obvious. Explanation of background information and a reanalysis of an example from literature is presented here for explanation and demonstration of this concept and of specific needs for welds with low stress concentration. Also the selection of the size of the reference radius will be discussed in this paper by using different radii for the case investigated here. Finally the application limits of the S-N curves as given in the IIW-recommendations is discussed since the selected example is slightly out of the range of application of the S-N curves.
In a related work previously carried out by the authors, finite element analysis of cylindrical vessel-cylindrical nozzle juncture based on the use of thin shell theory, due to the fact that the intersecting nozzle sizes are moderate-to-large, have been presented. Such analysis becomes invalid in cases when the nozzles are small in sizes which may result in nozzles whose configuration violates the validity of shell assumption. As a result, use of solid elements (based on theory of elasticity) in modeling the cylindrical vessels with small-diameter nozzles is presented in the present paper. Discussions of the numerical experiments and the results achieved are, first, given. The results are then compared with the prediction by other models reported in the literature. In order to arrive at the overall design charts that cover all the possible ranges of nozzle-to-vessel diameter ratio, the charts for the vessels with moderate-to-large-diameter nozzles are augmented with those of cylindrical vessels intersected by small-diameter nozzles developed in this work.
The ASME Boiler and Pressure Vessel Code (code) provides guidelines for the classification of linear elastic stresses into primary, secondary, and peak stresses. Although these guidelines cover a wide range of pressure containing components, the guidelines are sometimes difficult to employ for three-dimensional components with complex geometry. This paper uses the m(alpha)-tangent method, an assessment of constraint in the component, based on limit load multiplier estimates as a stress classification tool. The method is applied to several practical pressure vessel components from simple to relatively complex geometric configurations. The results compare well with those obtained by conventional techniques, e. g., inelastic finite element analysis. [DOI: 10.1115/1.4000199]
A key problem in engineering applications of "design by analysis" approach is how to decompose a total stress field obtained by the finite element analysis into different stress categories defined in the ASME Code III and VIII-2. In this paper, we suggest a two-step approach (TSA) of stress classification and a primary structure method (PSM) for identification of primary stress. Together with the equivalent linearization method (ELM), the stress classification problem is well solved. Some important concepts and ideas discussed by Lu and Li [Lu, M. W., and Li, J. G., 1986, ASME PVP-Vol. 109, pp. 33-37; Lu, M. W., aid Li, J. G., 1996, ASME PVP-Vol. 340, pp. 357-363] are introduced. They are self-limiting stress, multi-possibility of stress decomposition, classification of constraints, and primary structures. For identification of peak stress, a modified statement of its characteristic and a "1/4 thickness criterion'' are given. [S0094-9930(00)00201-8].
This paper summarizes work done by the Pressure Vessel Research Council (PVRC) on three-dimensional stress criteria, using primarily elastic two and three-dimensional analytical techniques, finite element analysis. The focus of the work was to recommended guidelines on evaluation of elastic stresses relative to the ASME Boiler and Pressure Vessel Code (Code) defined failure modes as they relate to stress limits. The guidelines are developed such that they may be used to update and expand the procedures for evaluating the required stress limits in Code Section VIII, Division 2 and Code Section III, Subsection NB. This paper summarizes the recommendation using ten using ten guidelines. The project addresses eleven example problems, nine of which include finite element analyses. These example problems validate the recommendations. The detailed information is published in the Welding Research Council Bulletin 429, `3-D Stress Criteria Guidelines for Application.'
The ASME code [1] identifies the modes of failure that must be addressed to ensure acceptable pressure vessel designs. The failure modes addressed in this paper are precluded by limits on the primary and primary plus secondary stress. Both involve the transition from elasticity to plasticity. Their evaluation requires the computation of membrane and bending stresses (the linearized stresses). The original techniques for evaluating the limits were based on beam and shell theory. Since beam and shell theory were the basis of the then-current tools, the transition from analysis results to failure assessment was straightforward. With the advent of finite elements (FE), the transition from the stress distribution to the failure modes requires a different path. For three-dimensional finite element (3D FE), the path is obscure. Since the development of FE, the ASME Code has made no additions to clarify the correlations between FE stress distributions and the failure modes. The authors believe that the Code should provide guidance in this area.
In design, it is common to use values for stress concentration factors (SCFs) obtained from textbooks. These textbook values have generally been obtained from photoelastic experiments for which Poisson's ratio will probably have a value greater than 0.3 and for frozen stress analysis a value approaching 0.5. However, when designing machine parts made from other isotropic materials, e.g. metals and structural ceramics, Poisson's ratio will probably have a lower value, which will affect the value of the SCF.
In this paper, various stress-raising features have been analysed using the finite element method to determine the stress concentration factor. It is shown that Poisson's ratio can have a significant effect on the SCF.
Highly localised through-thickness stress concentrations, higher than the strength of the material, may occur when a linear elastic finite element analysis of a composite structure is performed. Such stresses may be caused by real geometrical or material discontinuities or by artefacts in the model. The objective of this paper is to present a validated approach to determine when these high stresses will not lead to failure by delamination or matrix cracking and can therefore be ignored. Named as the High Stress Concentration (HSC) method, the approach presented in this paper is found to provide good results when applied to several finite element analyses, and is also in agreement with experimental data.
The equilibrium-equivalent structural stress method has been recently developed through several joint industry projects as a robust method to analyze welded components using finite element methods. This method has been proven effective in correlating a large amount of published fatigue test results in the literature. The authors employed this equilibrium-equivalent structural stress method for a competition organized by the SAE Fatigue Design & Evaluation (FD&E) Committee to predict the fatigue life of a rectangular hollow section joint. Among all the methods used by many participants, the authors’ life prediction was selected as the best, based on the actual test results. This paper provides the details of the SAE FD&E fatigue challenge problem definition, test results, and our structural stress approach. The predicted mean life by the equilibrium-equivalent structural stress method utilizing the proposed ASME Div. II Structural Stress curve was in excellent agreement with the actual mean life of the experimental results. Other stress indexes (such as the maximum principal stress and the von Mises stress) had maximum stress at a location different from the actual crack location. Furthermore, the equilibrium-equivalent structural stress showed significant mesh tolerance with only a few percent difference in stresses when halving the mesh size.
A mesh-size insensitive structural stress definition is presented in this paper. The structural stress definition is consistent with elementary structural mechanics theory and provides an effective measure of a stress state that pertains to fatigue behavior of welded joints in the form of both membrane and bending components. Numerical procedures for both solid models and shell or plate element models are presented to demonstrate the mesh-size insensitivity in extracting the structural stress parameter. Conventional finite element models can be directly used with the structural stress calculation as a post-processing procedure. To further illustrate the effectiveness of the present structural stress procedures, a collection of existing weld S-N data for various joint types were processed using the current structural stress procedures. The results strongly suggests that weld classification based S-N curves can be significantly reduced into possibly a single master S-N curve, in which the slope of the S-N curve is determined by the relative composition of the membrane and bending components of the structural stress parameter. The effects of membrane and bending on S-N behaviors can be addressed by introducing an equivalent stress intensity factor based parameter using the structural stress components. Among other things, the two major implications are: (a) structural stresses pertaining to weld fatigue behavior can be consistently calculated in a mesh-insensitive manner regardless of types of finite element models; (b) transferability of weld S-N test data, regardless of welded joint types and loading modes, can be established using the structural stress based parameters.
BPVC.Section II. Materials Part D-Properties
- ASME
Simplified Stress Linearization Method, Maintaining Accuracy