Local strain for fatigue strength of welded structures

2001 ◽  
Vol 36 (6) ◽  
pp. 605-610 ◽  
Author(s):  
V Dattoma ◽  
C Pappalettere
Keyword(s):  
Fatigue Strength ◽  
Welded Joints ◽  
Structural Integrity ◽  
Strain Curve ◽  
Local Strain ◽  
Fatigue Design ◽  
Curve Versus ◽  
Weld Toe ◽  
Stress Ratios ◽  
Tip Radius

Field criteria, which are usually applied in fracture mechanics to ensure the structural integrity of cracked components, are extended to the fatigue design of welded joints, whose weld toe can be assimilated to a notch with a small tip radius. In particular, fatigue strength in terms of strain rather than of stress has been determined by applying stress cycles with different stress ratios R = σmin/σmax. Finally, for the evaluation of the fatigue strength of welded joints in structural steel, a strain curve versus different R ratios is given to be compared with the service measured local strain at the weld toe.

Evaluation of Fatigue Strength of Welded Joints by Local Strain Measurements at Weld Toe

2007 ◽  
Vol 348-349 ◽  
pp. 701-704
Author(s):  
C. Casavola ◽  
C. Pappalettere
Keyword(s):  
Fatigue Strength ◽  
Working Conditions ◽  
Welded Joints ◽  
Local Strain ◽  
Industrial Applications ◽  
Welded Structures ◽  
Strain Measurements ◽  
Weld Toe ◽  
Strain Magnitude ◽  
The Many

The fatigue strength of real welded structures neither can be covered by formulae or theoretical arguments, nor simply deducted by studying a complex assembly through its elementary components because of the many interactions among different factors governing the phenomenon. In view of this, the support of experimental observations and practical procedures to evaluate the stress/strain magnitude that could lead some components to unsafe working conditions should be preferred, especially in industrial applications where fast and reliable responses are strongly needed.

Fatigue Assessment of Welded Joints Taking Into Account Effects of Residual Stress

2010 ◽  
Author(s):  
Nur Syahroni ◽  
Stig Berge
Keyword(s):  
Residual Stress ◽  
Fatigue Strength ◽  
Welded Joints ◽  
Fatigue Testing ◽  
International Association ◽  
Hot Spot ◽  
Mean Stress ◽  
Element Analysis ◽  
Structural Rules ◽  
Fatigue Design

Residual stress may have a significant effect on the fatigue strength of welded joints. As a non-fluctuating stress, it has an effect similar to that of the mean stress. Recently the International Association of Ship Classification Societies (IACS) has issued Common Structural Rules (CSR) for respectively tankers (IACS 2006a) and bulk carriers (IACS 2006b). The effect of mean stress in fatigue design is taken into account in both sets of rules. However, the treatment is quite different, in particular with regard to residual stress and shakedown effects. In the present paper a comparative study of fatigue design procedures of the IACS rules is reported, with emphasis on residual stress effects. Testing was carried out with longitudinal attachment welds in the as-welded condition. The initial residual stress was measured by a sectioning method using strain gages. Hot spot stress was determined experimentally by strain gauges and numerically by finite element analysis using different types of elements. Fatigue testing was carried out and SN-curves were plotted according to the relevant stress as specified by the rules. In order to investigate the shake-down effect of residual stress, testing was performed for several pre-load conditions which could be taken to represent maximum load levels in a load history. The aim of the study is to contribute towards better understanding of the effect of residual stress and shakedown on fatigue strength of welded joints.

Differences in Fatigue Strength Between Arc and Laser Hybrid Welded Joints

2008 ◽  
Vol 24 (03) ◽  
pp. 139-146
Author(s):  
H. Remes ◽  
P. Varsta
Keyword(s):  
Fatigue Strength ◽  
Welded Joints ◽  
Fatigue Tests ◽  
Notch Depth ◽  
Butt Joints ◽  
Fatigue Design ◽  
Notch Stress ◽  
Notch Geometry ◽  
Laser Hybrid ◽  
Fatigue Endurance

This paper presents the results of fatigue tests, including tests of laser hybrid and arc welded butt joints, for two plate thicknesses, 6 and 12 mm. Pure laser welded joints were also tested. The S-N curves based on nominal stresses for the different welded joints are presented. The results were further analyzed using the notch stress approach, where the fatigue notch factors were determined from the measured geometries of the welded joints. Unexpected differences in the S-N curves based on the notch stresses were found between the laser hybrid and arc welded joints and between the laser hybrid and pure laser welded joints. The reasons for this difference were studied with the help of extensive measurements of weld notch geometries. Significant differences in the geometries were observed. Taking into account the notch geometry and the notch depth, the notch stress approach partially explains the differences between the fatigue endurance limits of the laser hybrid and arc welded joints. The applicability of the notch stress approach to the fatigue design of laser hybrid welded joints is also discussed.

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

2008 ◽  
Vol 222 (12) ◽  
pp. 2351-2363 ◽  
Author(s):  
M-H Kim ◽  
S-W Kang
Keyword(s):  
Fatigue Strength ◽  
Stress State ◽  
Hot Spot ◽  
Mesh Size ◽  
Offshore Structures ◽  
Nominal Stress ◽  
Medium Size ◽  
Effective Measure ◽  
Fatigue Design ◽  
Weld Toe

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.

Fatigue Design of Welded Joints by Local Approaches: Comparison between Fictitious Notch Rounding and Strain Energy Averaging

2007 ◽  
Vol 348-349 ◽  
pp. 449-452 ◽  
Author(s):  
Paolo Lazzarin ◽  
Filippo Berto ◽  
D. Radaj
Keyword(s):  
Welded Joints ◽  
Strain Energy ◽  
Control Volume ◽  
Local Strain ◽  
Stress Distributions ◽  
Good Correspondence ◽  
Load Carrying ◽  
Weld Toe ◽  
Weld Root ◽  
Coarse Meshes

The paper demonstrates the close correspondence between two local approaches to assess the fatigue strength of welded joints: Radaj’s approach based on fictitious notch rounding and a recently proposed approach based on the local strain energy density (SED) averaged over a given control volume. This volume surrounds the weld root or weld toe, both modelled as sharp (zero radius) V-notches with different opening angles. The two approaches are applied to load carrying and non-load carrying cruciform joints and the theoretical fatigue notch factors Kf are compared. The SED averaged over the control volume is determined from finite element models with very fine meshes, as typically designed to evaluate the intensity of the asymptotic stress distributions, and also from coarse meshes, showing a surprisingly good correspondence.

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