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.

Qualification of the Notch Stress Approach for the Fatigue Assessment of Welded Pressure Equipment and Power Plant Components

2017 ◽  
Author(s):  
Jürgen Rudolph ◽  
Ralf Trieglaff ◽  
René Stößlein ◽  
Fabian Hauser
Keyword(s):  
Test Data ◽  
Welded Joints ◽  
Low Cycle Fatigue ◽  
Hot Spot ◽  
Pressure Vessels ◽  
Assessment Procedure ◽  
Element Analysis ◽  
Hot Spot Stress ◽  
Fatigue Assessment ◽  
Notch Stress

The fatigue assessment of welded joints in different engineering disciplines is usually based on nominal, structural or notch stresses on one hand (elastic concept using component fatigue curves of load controlled test data) and local strains on the other hand (elasto-plastic concept using material fatigue curves of strain-controlled push-pull test data of un-notched and polished standard specimens). The concepts of the first mentioned group are implemented in widespread standards and recommendations such as [1] to [3]. The fatigue assessment procedure of the European standard for unfired pressure vessels (EN 13445-3, Clause 17 & 18 and related annexes) [4] is currently under revision with one focus on the elaboration of user friendly fatigue assessment options for welded components [5]. The current state of the art focuses on the application of an adapted structural hot spot stress approach to the fatigue assessment of welded pressure equipment [5]. Although this is a significant step forward, the implementation of a notch stress approach can furtherly increase the fatigue assessment options by detailed weld seam analysis. The paper focuses on respective methodological proposals and application examples of typical welded joints. The finite element analysis as part of the procedure has to be harmonized with the requirements of the assessment procedure. Of course, the compatibility of the hot spot stress approach and a notch stress approach has to be guaranteed for individual examples. The direct comparison of the different approaches allows for a qualitative evaluation of methods. The application of an appropriate master fatigue curve FAT100 and the limitations with regard of stress/strain ranges in the low cycle fatigue (LCF) regime as well as the fatigue assessment of welded joints with mild weld toe notches is the subject of special considerations. The latest recommendations of German Welding Society (DVS) [6] constitute a reference for the last two subjects raised.

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.

Fatigue strength assessment of ship structures accounting for a coating life and corrosion degradation

2016 ◽  
Vol 7 (2) ◽  
Author(s):  
Yordan Garbatov
Keyword(s):  
Finite Element ◽  
Fatigue Strength ◽  
Fatigue Damage ◽  
Hot Spot ◽  
Limit State ◽  
State Function ◽  
Fatigue Reliability ◽  
Hot Spot Stress ◽  
Notch Stress ◽  
Fatigue Limit State

Purpose Fatigue strength and reliability assessment of complex double hull oil tanker structures, based on different local structural finite element approaches, is performed accounting for the uncertainties originating from load, nominal stresses, hot spot stress calculations, weld quality estimations and misalignments and fatigue S-N parameters including the correlation between load cases and the coating life and corrosion degradation. Design/methodology/approach Ship hull wave-induced vertical and horizontal bending moments and pressure are considered in the analysis. Stress analyses are performed based on the nominal, local hot spot and notch stress approaches. A linear elastic finite element analysis is used to determine the stress distribution around the welded details and to estimate structural stresses of all critical locations. Fatigue damage is estimated by employing the Palmgren-Miner approach. The importance of the contribution of each random variable to the uncertainty of the fatigue limit state function is also estimated. The probability of fatigue damage of hot spots is evaluated taking into account random coating life and corrosion wastage. Fatigue reliability, during the service life, is modelled as a system of correlated events. Findings The fatigue analysis showed that the fatigue damage at the hotspot, located at the flange of the stiffener close to the cut-out, is always highest in the cases of the structural hot spot stress and effective notch stress approaches, except for the one of the nominal stress approach. The sensitivities of the fatigue limit state function with respect to changes in the random variables were demonstrated showing that the uncertainty in the fatigue stress estimation and fatigue damage are the most important. Fatigue reliability, modelled as a parallel system of structural hot spots and as a serial system of correlated events (load cases) was evaluated based on the Ditlevsen bounds. As a result of the performed analysis, reliability and Beta reliability indexes of lower and upper bounds were estimated, which are very similar to the ones adopted for ultimate strength collapse as reported in literature. Originality/value This paper develops a very complex fatigue strength and reliability assessment model for analysing a double hull oil tanker structure using different local structural finite element approaches accounting for the associated uncertainties and the correlation between load cases and the coating life and corrosion degradation. The developed model is flexible enough to be applied for analysing different structural failure modes.

Fatigue Assessment of Trimaran Structure Based on Simplified Procedure

2012 ◽  
Vol 525-526 ◽  
pp. 333-336
Author(s):  
Hui Long Ren ◽  
Shehzad Khurram ◽  
Chun Bo Zhen ◽  
Khurram Asifa
Keyword(s):  
Fatigue Damage ◽  
High Speed ◽  
Hot Spot ◽  
Mathematical Formulation ◽  
Direct Calculation ◽  
Assessment Procedure ◽  
Hot Spot Stress ◽  
Fatigue Assessment ◽  
Strength Assessment ◽  
Simplified Procedure

In recent years, Trimaran platform design has got the attention of naval architects owing to its superior seagoing performance. Trimaran structure experiences severe loads due to its unique configuration and high speed, causing stress concentration, especially in cross deck region and accelerate fatigue damage. This paper presents fatigue strength assessment of Trimaran structure by simplified procedure. A methodology is proposed to evaluate fatigue loads and loading conditions by load combinations of direct calculation procedure of Lloyds Register Rules for Classification of Trimaran (LR Rules). Global FE analysis, in ANSYS, is performed to investigate the stress response. The stress range is computed by hot-spot stress approach, and its long term distribution is specified by Weibull distribution. Fatigue damage of selected critical details is calculated using mathematical formulation of simplified fatigue assessment procedure of Common Structure Rules (CSR).

Analysis of Recent Fatigue Data Using the Structural Stress Procedure in ASME Div 2 Rewrite

2006 ◽  
Vol 129 (3) ◽  
pp. 355-362 ◽  
Author(s):  
Pingsha Dong ◽  
Jeong K. Hong ◽  
Abílio M. P. De Jesus
Keyword(s):  
Test Data ◽  
Hot Spot ◽  
Comparative Assessment ◽  
Structural Stress ◽  
Hot Spot Stress ◽  
Classification Approach ◽  
Fatigue And Fracture ◽  
Fatigue Data ◽  
Fatigue Evaluation ◽  
Assessment Procedures

In support of the ASME Div 2 Rewrite, a master S-N curve approach has been developed using a mesh-insensitive structural stress procedure for fatigue evaluation of welded components. The effectiveness of the master S-N curve approach has been demonstrated in a number of earlier publications for many joint types and loading conditions for pipe and vessel components as well as plate joints. To further validate the structural stress method, a series of recent test data (small weld details and a full-scale vessel) published by De Jesus et al. (2004, Fatigue and Fracture of Engineering Materials and Structures, 27, pp. 799–810) were analyzed in this paper. A comparative assessment of various existing procedures and their effectiveness in correlating the fatigue test data by De Jesus is also presented. These assessment procedures include current ASME Sec. VIII Div 2, weld classification approach in PD 5500, and the surface extrapolation-based hot spot stress approach in recently approved European EN 13445 Standards.

Analysis of Hot Spot Stress and Alternative Structural Stress Methods

2003 ◽  
Author(s):  
P. Dong ◽  
J. K. Hong
Keyword(s):  
Hot Spot ◽  
Relative Effectiveness ◽  
Sufficient Conditions ◽  
Offshore Structures ◽  
Structural Stress ◽  
Plate Structures ◽  
Hot Spot Stress ◽  
Necessary And Sufficient ◽  
Assessment Procedures ◽  
Calculation Procedures

There is a growing need for robust finite element based fatigue assessment procedures for welded joints in offshore/marine structures. Although widely accepted in tubular structures, the applications of the conventional hot spot stress (HSS) procedures in plate structures have proven to be problematic in some joint types. There are a series of on-going international efforts that are intended to address some of the issues to improve the consistency of the HSS calculation procedures for plate structures. In this regard, alternative structural stress (SS) procedures that share some similarities to conventional HSS methods have been developed. The SS method has been shown to be effective in calculating structural stresses directly at failure locations such as at weld toes with minimum mesh-sensitivity. In this paper, detailed analysis and assessments of the conventional HSS and the alternative SS procedures are presented. Starting with the fundamental definitions of both HSS and SS parameters, the underlying mechanics associated with the two types of the stress analysis methods will be discussed by considering series of typical joint types, particularly on some of the important stress concentration characteristics that separate tubular joints from plate joints. Then, the necessary and sufficient conditions required for a stress definition relevant to fatigue and its calculation procedures will be demonstrated for both methods. Areas of concerns and improvements for both methods will be demonstrated by using a series of selected joint types typical of those in marine/offshore structures. Finally, the corresponding S-N data will be used to demonstrate the relative effectiveness of HSS and SS methods in consolidating the data from drastically different joint types into a single S-N curve.

scholarly journals Research on Fatigue Properties of Typical Welded Joints of DH36 Steel at −60 °C

2020 ◽  
Vol 10 (11) ◽  
pp. 3742
Author(s):  
Weidong Zhao ◽  
Guoqing Feng ◽  
Wenchao Liu ◽  
Huilong Ren
Keyword(s):  
Stress Concentration ◽  
Welded Joints ◽  
Room Temperature ◽  
Polar Region ◽  
Hot Spot ◽  
Fatigue Properties ◽  
Test Results ◽  
Hot Spot Stress ◽  
Confidence Levels ◽  
Better Than

As the development of the polar region continues to increase, the fatigue properties of structures at low temperature are increasingly receiving researcher attention. This study aimed to investigate the fatigue properties of T-welded and cruciform welded joints at −60 °C. Logarithmic S–N curves based on the hot-spot stress of the T-welded and cruciform welded joints at 50% and 95% confidence levels were obtained at −60 °C. The test results showed that the fatigue properties of T-welded joints were almost 2–7% better than those of the cruciform welded joints at −60 °C. Factors that affected the fatigue properties of welded joints, such as the stress concentration factor, microstructure, Vickers hardness profiles, and fractography, were also studied and the test results showed that the fatigue properties of cruciform welded joints at −60 °C were 57.215% better than at room temperature.

Applicability Evaluation of SS (Structural Stress) Method on Actual Project

2008 ◽  
Vol 580-582 ◽  
pp. 633-636 ◽  
Author(s):  
Kwang Seok Kim ◽  
Joong Kyoo Kang ◽  
Joo Ho Heo ◽  
Sung Geun Lee
Keyword(s):  
Fatigue Life ◽  
Hot Spot ◽  
Fatigue Analysis ◽  
Correlation Factor ◽  
Structural Stress ◽  
Hot Spot Stress ◽  
Notch Stress ◽  
Loading Case ◽  
Application Data ◽  
Applicability Evaluation

The structural stress (SS) method developed by BATTELLE has been studied based on small or mid-size scale specimens. In order to apply the new method, such as SS, on an actual project, it should have application results on actual project. However, SS method didn’t have a lot of application data compared to class procedure using hot spot stress (HSS). In order to find out whether the SS method, for the evaluation of fatigue life, can give reasonable results when it is applied under the same loading suggested by classification societies, it was compared with fatigue lives derived by class. ABS & DNV’s simplified fatigue analysis method were adopted to check the validity of SS method. Before applying complicated loading of class, static loading case was applied, since the class method has their own correlation factor for wave loading. And then, simplified fatigue analysis was performed with more complicated loading cases. From the results of fatigue life calculation, it can be said that SS shows reasonable fatigue lives with respect to HSS or notch stress based fatigue lives.

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