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FATIGUE ANALYSIS OF INDUSTRIAL

WELDED STRUCTURES


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INTRODUCTION

Welding Technology had a significant impact onindustrial developments.

Connections refers to those locations in a structurewhere elements are joined to reconcile changes ingeometry and/or accommodate fabrication or servicerequirements.

Failures in engineering structures occur predominatelyat component connections.


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WELD DISCONTINUITIES

Weld discontinuities may be divided into three categories:

1) Crack like discontinuities

Cracks

Lack of fusion

Lack of penetration

Overlap

2) Volumetric discontinuities

Porosity

Slag inclusions

3) Geometric discontinuities Undercut

Incorrect profile

Misalignment


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Figure 1:- Imperfections and cracks in welded joints


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DEFECTS

Discontinuities are designated as defects only when

their size, orientation, and distribution exceed

specification limits and their presence affects the

integrity of the component and renders it unfit for itsintended application.

Codes and specifications define acceptance levels for

discontinuities in terms of their type, size, orientationand distribution. Usually crack and crack-like

discontinuities are prohibited.


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METHODOLOGIES OF VARIOUS CODES AND STANDARDS

Intent of these design and construction codes is toensure that safe and reliable structures are produced at

reasonable cost .Various codes differ in methodologies

employed for fatigue life assessment.

The calculated stress fluctuations are compared to the

appropriate material fatigue curve derived from small

smooth specimen test results and fatigue life isdetermined from the stress value from either a mean

data curve or adjusted design curve.


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FUTURE METHODS

The burden in the analytical approach can be over

come by using finite element methods with the help

of commercially available FEA software.

The hot spot method should be developed in future.

The local stresses can be calculated with the Finite

Element Method. Local methods can be applied for

determination of fatigue life of welded joints under

multiaxial fatigue.


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OBJECTIVE OF THE THESIS PROJECT

Based on the use of Finite Element Analysis of the

cracks inserted in the models of the damaged zones.

The aim of the proposed methodology is to predict

crack initiation and crack growth in industrials

structures until failure.

The solutions of SIFs from FEA have been compared

with solutions from IIW literature. ABAQUS finite

element software is used to simulate various weldshapes due to limitation in use of analytical and

empirical solutions.


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CRACK LIKE IMPERFECTIONS

NDT indications are idealized as elliptical cracks for

which the stress intensity factor is calculated accordingly

RECOMMENDATIONS FOR FATIGUE DESIGN OF

WELDED JOINTS AND COMPONENTS

IIW Fatigue Recommendations

IIW-1823-07/XIII-2151r4-07/XV-1254r4-07 Dec. 2008


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Figure 3:- Transformation of NDT indications to elliptic or semi-elliptic cracks


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Table -1: Dimensions for assessment of crack-like imperfections (example)


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STRESS INTENSITY FACTORS

The stress intensity factor K defines the magnitude of

local stresses around the crack tip. This factor dependson loading ,crack size, crack shape and geometricboundaries;

General form,

remote stress applied

a crack lengthf(g) correction factor that depends on specimen and crack

geometry

Stress intensity factor solution have been obtained for wide

variety of problems and published in hand book


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SIGNIFICANCE OF FEA SOFTWARE FOR

CALCULATION OF SIF

Fracture mechanics concerns the interaction of theapplied crack driving force and material fractureresistance.

For simple geometry and linear elastic materials

fracture parameters can be easily calculated basedon existing analytical equations.

For complicated geometry and elastic-plastic

materials finite element method is necessary. The main purpose of these computational exercises

is to use ABAQUS to calculate the fractureparameters of a 2D plane.


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STRESS DISTRIBUTION OVER THE PLATE

THICKNESS

m membrane stress

b shell bending stress

nl non linear stress peak

The membrane stress m is equal to the average stress calculated through the

thickness of the plate. It is constant through the thickness.

The shell bending stress b is linearly distributed through the thickness of the

plate.


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CALCULATION OF STRESS INTENSITY FACTORS BY

PARAMETRIC FORMULAE

First, the relevant applied stress (usually the localnominal or the structural hot spot stress) at thelocation of the crack is determined, assuming thatno crack is present. Ideally, the stress should be

separated into membrane and shell bending stresscomponents. The stress intensity factor (SIF) Kresults as a superposition of the effects of bothstress components. The effects of the crack shape

and size are covered by the correction function Y.The effects of the any remaining stress raisingdiscontinuity or notch (non-linear peak stress) canto be covered by additional factors Mk


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The correction functions Ym and Yb can be found in the literature. For most

cases, the formulae for stress intensity factors given in Table are adequate.

Mk-factors may be found in references .


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Table -3: Stress intensity factors at welds


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SIMPLIFIED PROCEDURE

The simplified procedure makes use of the fatigue

resistance at 2x106 cycles (analogous to FAT classesfor the classified structural details) for a ranges of

crack types, sizes and shapes, of which the data are

presented in Tables. These were obtained byintegration of the crack propagation law for steel,

given in Table 5, between the limits of an initial crack

size ai and a final crack size af of 0.75% of the wall

thickness. In addition, use was made of thecorrection functions and the local weld geometry

correction given in Table


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FATFatigue Class

Furthermore, within the limits imposed by static strengthconsiderations, the fatigue curves of welded joints are independent of

the tensile strength of the material.Each fatigue strength S-N curve is identified by the characteristic fatiguestrength of the detail in MPa at 2 million cycles. This value is the fatigueclass (FAT).

Table -5: Parameters of the Paris power law and threshold data for steel

Table : Stress ranges at 2x106 cycles (FAT classes in N/mm2) of welds containing

cracks for the simplified procedure


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IMPORTANT EQUATIONS


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PHASEII-EXPECTED RESULTS

The ABAQUS is used to simulate various weld

shapes due to limitation in use of analytical and

empirical solutions. The entire fatigue process inwelded joint has been modelled by pure fracture

mechanics approach.


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CONCLUSION

The methodology has to be developed to determine crackinitiation and crack propagation in industrial weldedstructures. The generalization and sub modelling will

allows to perform fast computation while representingcorrectly the local stresses and stress intensity factors.Depending on the type of issues to be solved, part of theoverall methodology can be processed. It's then a set of

tools and methods based on the same principles andhelping the engineer to solve complex and non-linearcrack issues.


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REFERENCES1) John M. Barsom , Stanley T. Rolfe. Fracture and fatigue control in structures:

Application of fracture mechanics, Third Edition (1999).

2) International Institute of Welding (IIW).Recommendations for Fatigue Design ofWelded Joints and Components. InternationalInstitute of Welding, doc. XIII-2151r4-07/XV-1254r4-07.Paris, France, October 2008.

3) Tom Lassen and Naman Recho, Fatigue life analyses of welded structures, ISTE,London, ISBN 1-905209-54-1 (2006).

4) D. Lebaillif, I. Huther, M. Serror, N. Recho , Fatigue Crack Initiation andPropagation: a complete industrial process compared with experiments on industrialwelded structure(2005).

5) D.Lebaillif, E.Petitpas , R.Paquet , M. Serror. Multi-scale approach for crack

initiation and propagation(2007).

6) A.Al Mukhtar, H. Biermann, P. Hbner,In Fatigue Crack Propagation Life Calculationin Welded Joints( 2007).


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