Assessment of Weld Fatigue by Effective Notch Stress Approach

2015 ◽  
Author(s):  
Majid Anvari ◽  
Øyvind Fagnastøl ◽  
Bernt J. Leira
Keyword(s):  
Linear Relation ◽  
Welded Joint ◽  
Hot Spot ◽  
Nominal Stress ◽  
Notch Stress ◽  
Nonlinear Relation ◽  
Effective Notch Stress ◽  
Weld Fatigue ◽  
Weld Root ◽  
Nominal Stress Approach

Applicability of different approaches, i.e. nominal stress-, structural hot spot- and effective notch stress approach, for fatigue assessment of welded structures has been discussed. The effective notch stress approach was validated for a cruciform fillet welded and fully penetration welded joint, according to DNV guideline [1]. The same fillet welded joint was further investigated for varying weld sizes. The results show that with changing weld size, there are considerable differences between calculated stresses, and specially calculated fatigue lives, when comparing with nominal stress approach. Further investigations revealed that there is a nonlinear relation between effective notch stress and weld sizes, while comparison of the notch stress and nominal stress approaches indicate that a linear relation is to be expected. Based on the established methodology for the cruciform joint, another joint, i.e. a knee plate located in a horizontal brace of a drilling vessel, was assessed for weld root fatigue. Based on the proposed equations and the linear relation found between the nominal- and the notch stress S-N curves, a weld size providing a longer fatigue life at the weld root, rather than that at the toe, was proposed.

scholarly journals NUMERICAL METHODS FOR THE FATIGUE ASSESSMENT OF WELDED JOINTS: INFLUENCE OF MISALIGNMENT AND GEOMETRIC WELD IMPERFECTIONS

2017 ◽  
Vol 9 (1) ◽  
pp. 9-24 ◽  
Author(s):  
Andreas TARAS ◽  
Harald UNTERWEGER
Keyword(s):  
Numerical Methods ◽  
Welded Joints ◽  
Numerical Models ◽  
Hot Spot ◽  
Nominal Stress ◽  
Geometric Imperfections ◽  
Hot Spot Stress ◽  
Notch Stress ◽  
Effective Notch Stress ◽  
Nominal Stress Approach

The fatigue design life of welded joints in steel structures is increasingly assessed by using numerical models and methods, such as the structural (hot-spot) stress method and the effective notch stress method. When compared to the classical design approach using nominal stress S-N design curves, these methods offer the advantage of flexibility and a wider scope of application. However, a number of questions arise when these methods are used to assess geometrically "imperfect" welded joints, such as joints with plate misalignments or excessive weld convexity or concavity. In these cases, the classical S-N curves are known to cover imperfections up to the common tolerance classes for fatigue-prone welded joints (e.g. in accordance with ISO 5817 class B). For the numerical methods, differing and conflicting recommendations exist on how to account for the geometric imperfections in the welded joints, with little or no background to these recommendations available. In this paper, a study is presented in which two standard welded joints (butt welds between plates of equal and unequal thickness; T-joints with fillet welds) are analysed with the help of the structural (hot-spot) stress and the effective notch stress approach, considering various levels of geometric imperfection up to the tolerance limits, and the resulting fatigue life predictions are compared to test results from the literature and the nominal stress approach predictions. Since the nominal stress approach curves are based on reliable statistical data and desired survival probabilities for these known, standard cases, this methodology allows one to determine the correct application of the numerical methods to cases with geometric imperfections. This information may be used for a pertinent refinement of design recommendations for these methods, as well as for cases where these methods are applied to fitness-for-purpose assessments - e.g. because the nominal stress approach is not applicable.

Weld Rod Fatigue Analysis Using Effective Notch Stress Method

2018 ◽  
Author(s):  
Kumarswamy Karpanan ◽  
Allison Weber Kirk ◽  
Gerald Hershman
Keyword(s):  
Fatigue Life ◽  
Welded Joints ◽  
Failure Modes ◽  
Welding Process ◽  
Fatigue Analysis ◽  
Fillet Welds ◽  
Notch Stress ◽  
Effective Notch Stress ◽  
Weld Fatigue ◽  
Weld Root

Welds are one of the commonly used joint types and are employed extensively in subsea oil and gas production equipment. Commonly used weld joints in subsea components are fillet, butt, full-penetration, plug, and girth. Fatigue is one of the critical failure modes for welded joints. Welded joints are complex to analyze for fatigue loading due to the microstructure change during the welding process. The welding process also induces residual stress in the heat affected zone (HAZ) surrounding the weld. This, in turn, can adversely affects the fatigue life of the joint. The S-N fatigue approach is commonly used for weld fatigue analysis due to the simplicity of this method. Industry standards such as DNV, IIW, BS-7608, and ASME BPVC Sec VIII Div. -2 or -3 are typical references for this type of analysis. For subsea specific applications, DNV-RP-C203 and BS-7608 are generally used because these two standards provide S-N curves for welds in “air” as well as in “seawater with cathodic protection”. These two codes also provide S-N curves for various weld geometries ranging from simple fillet welds to complex tubular joints. Some of the weld fatigue analysis techniques used in the subsea industry are the: nominal stress approach, structural hot spot stress approach, effective notch stress approach (ENS), structural stress method (ASME VIII-2, -3) and the Fracture mechanics based fatigue crack propagation (FCG) approach. This paper presents the fatigue analysis of fillet welds in bore inserts using the ENS method. In the ENS method, a 1mm radius notch is modelled at the weld root or toe, see Figure 1, which yields a finite weld root stress. The stress analysis is carried out using FEA and the stresses on the notch along with the appropriate fatigue curve are used to estimate the weld root fatigue life.

Round robin study on structural hot-spot and effective notch stress analysis

2008 ◽  
Vol 3 (4) ◽  
pp. 335-345 ◽  
Author(s):  
Wolfgang Fricke ◽  
Andrea Bollero ◽  
Ionel Chirica ◽  
Yordan Garbatov ◽  
Fabrice Jancart ◽  
...  
Keyword(s):  
Stress Analysis ◽  
Hot Spot ◽  
Round Robin ◽  
Notch Stress ◽  
Effective Notch Stress

scholarly journals Tuotantokäyttöön soveltuva edullinen menetelmä hitsin juuren puolen väsymiseliniän arvioimiseksi

2017 ◽  
Vol 50 (3) ◽  
pp. 220-223
Author(s):  
Ilkka Valkonen ◽  
Antti Valkonen
Keyword(s):  
Hot Spot ◽  
Notch Stress ◽  
Effective Notch Stress

Juuren puolen väsyminen on käytännössä erityisen ongelmallinen syystä, että vauriota ei voi havaita ennenkuin särö on edennyt hitsin läpi. Analyysimenetelmistä esimerkiksi ns. Hot-Spot menetelmä ei sovellu juuren puolen väsymisen arviointiin. Eurocode kolmessa on joitain tapauksia, jotka käytännössä ovat juuren puolen säröön liittyviä, mutta yleistä käyttökelpoista menetelmää ei ole aikaisemmin esiintynyt. Viime vuosina on kyseisen ongelman ratkaisuun esitetty tehollisen loven menetelmää (ENS, "Effective Notch Stress"), jossa juurivirhe mallinnetaan ja virheen päissä kytetään 1 mm säteistä kaarta. Elementtimenetelmään on uutena yleisempään tietoisuuteen tullut alkujaan Belytschkon ja hänen ryhmänsä kehittämä XFEM, jolla pystyy mallintamaan särön elementtiverkosta riippumatta. Tämä antaa teoriassa hyvän mahdollisuuden tuotantokäyttöön soveltuvan analyysimenetelmän pohjaksi. Tämän esityksen tarkoituksena on verrata nimellisen jännityksen menetelmää, ENS-menetelmää, sekä XFEM-murtumismekaniikan käyttöä juuren puolen väsymisen arviointiin.

Study of Typical Welded Joint Hot Spot Stress Concentration Factor

2010 ◽  
Vol 160-162 ◽  
pp. 1482-1487
Author(s):  
Bin Jie Wang ◽  
Qiang Li ◽  
Zhi Ming Liu
Keyword(s):  
Stress Concentration ◽  
Stress Concentration Factor ◽  
Concentration Factor ◽  
High Speed ◽  
Welded Joint ◽  
Hot Spot ◽  
Nominal Stress ◽  
Hot Spot Stress ◽  
The Difference ◽  
Fatigue Evaluation

Hot spot stress (HSS) approach is more applicable for fatigue evaluation, because it is more closer to the real stress at the welded notch than nominal stress.This paper studies a typical welded joint, fillet welded joint, which is used widely in high-speed car. Hot spot stress concentration factor is calculated under different nominal stress and loading methods. The S-N curves belong to nominal stress and hot spot stress were analysized. The result show that the difference between the S-N curves under different load style can be reduced greatly by hot spot stress. The hot spot stress characteristic variable was put forward, which is used to describe the influence from the welding size of the joint.

Criteria for Fatigue Failure of Materials: Application in Fatigue Assessment of Structures

2018 ◽  
Vol 26 ◽  
pp. 1-8
Author(s):  
Sergei Petinov ◽  
Ruslan Guchinsky
Keyword(s):  
Fatigue Failure ◽  
Strain Energy ◽  
Hot Spot ◽  
Inelastic Strain ◽  
Fatigue Properties ◽  
Hot Spot Stress ◽  
Fatigue Assessment ◽  
Notch Stress ◽  
Assessment Procedures ◽  
Nominal Stress Approach

Presently in rules for fatigue assessment of structures subjected to intensive alternating service loading the Stress-Life (S-N) criteria are recommended in versions of the Nominal stress approach, Hot-spot stress and Notch-stress approach based on using the stress range a representative of the current damage. The criteria and approaches provide assessment of fatigue properties of structures accompanied with a series of approximations and uncertainties. A physically and mechanically more correct procedures might be provided by the Strain-life and Inelastic strain energy criteria for fatigue failure and approaches, although specific with intrinsic sources of approximations. The nature of approximations in the approaches is briefly commented and feasible means of improvement the fatigue assessment procedures and applications are presented.

scholarly journals Fatigue Failure Analysis of Fillet Welded Joints Used in Offshore Structures

2014 ◽  
Author(s):  
Jonas W. Ringsberg ◽  
Majid Anvari ◽  
Djan Eirik Djavit ◽  
Erik Strande
Keyword(s):  
Fatigue Life ◽  
Fatigue Failure ◽  
Welded Joints ◽  
Parameter Study ◽  
Maritime Industry ◽  
Calculation Methods ◽  
Notch Stress ◽  
Weld Toe ◽  
Effective Notch Stress ◽  
Weld Root

This paper presents a comparison made of different fatigue calculation methods used in the maritime industry today, with the aim of having a higher control of a fatigue failure site. To provide an overview of the different fatigue calculation methods, a comparison study was performed, as well as a local weld parameter study for two typical fillet welded joints. The two methods used for this study were the structural hot spot and effective notch stress method. Two fillet welded joints were provided by Aker Solutions MMO AS, Bergen, Norway. The first joint is a rectangular hollow section from a davit, built as a truss. The second model is a part of a K-joint from a semi-submersible (Aker H3 design). Both joints were analysed using fine 3D finite element models. The two different fatigue life calculation methods yielded different fatigue lives for the weld toe, with inconclusive results regarding their conservatism which is discussed in the paper. An increased weld toe radius gave a higher fatigue life for the weld toe, while the larger weld size increased the fatigue life in the weld root. Any weld size effect regarding fatigue life in the weld toe could not be established. Based on the effective notch stress method calculations, there was an indication of weld root failure for the K-joint of the drilling unit. Fatigue life improvement methods only increasing weld toe fatigue life are not recommended based on these results.

Fatigue study on rib-to-deck welded joint considering weld penetration rate

2019 ◽  
Author(s):  
Xiaochen Ju ◽  
Xiaogang Liu ◽  
Zhibin Zeng ◽  
Xinxin Zhao
Keyword(s):  
Welded Joints ◽  
Welded Joint ◽  
Penetration Rate ◽  
Fatigue Cracks ◽  
Element Model ◽  
Fatigue Tests ◽  
Fatigue Performance ◽  
Weld Penetration ◽  
Weld Fatigue ◽  
Weld Root

<p>The U-shaped rib-to-deck welded joint in orthotropic steel deck is a part with multiple fatigue cracks. The penetration rate which is the ratio of penetration depth to U-shaped rib thickness has an important influence on fatigue performance of partial joint penetration (PJP) welds. In this study, the influence of penetration rate on the fatigue performance of U-shaped rib-to-deck welded joints was studied. Firstly, the finite element model of U-shaped rib-to-deck welded joints with penetration rate of 65%, 75%, 85% and complete joint penetration (CJP) welds were established. The mechanical characteristics of different welding forms under typical loading conditions were analyzed. It was found that with the increase of penetration rate of PJP welds, the stress concentration at the weld root weakened. Then fatigue tests on specimens with different weld penetration rate were carried out. The fatigue cracks of CJP welds all started at the inner welding toe of the U-shaped rib. However, the fatigue cracks of PJP welds mainly started at the welding root of the unfused weld. Fatigue S-N curves for PJP and CJP were regressed, respectively, it could be found that the fatigue performance of CJP welds was superior to that of PJP welds.</p>

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