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Fatigue Assessment of Friction Stir Welded Joints in Aluminium Profiles
KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Materials Technology.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Friction stir welding (FSW) is a low heat input solid state welding technology. It is often used for fabrication of aluminium alloys in transportation applications including railway, shipbuilding, bridge structures and automotive components. In these applications the material is frequently subject to varying load conditions and fatigue failure is a critical issue. In most cases standard codes and fatigue guidelines for aluminium welded joints address only welded structures with conventional welding methods but not those with FSW procedure. In the scope of this thesis fatigue life assessment of friction stir welded components was performed using theoretical approaches along with finite element method (FEM). The further aim of this study was to generate a basis for standardization of fatigue assessment of friction stir welded joints.

Friction stir welded hollow aluminium panels of alloy 6005A are investigated. The panels are used for train wall sides, train floors, deck and bridges. Each panel is made of several profiles that are joined with the friction stir welding method. Fatigue bending tests were performed for profiles in these panels. Fatigue cracks and failure appeared at notches in the profiles. With FEM simulations critical positions for crack initiation and failure were identified. The method of critical distance was used to analyse and estimate the fatigue life. It was shown that the failure location and fatigue limit could be predicted for both base metal and weld location. Choice of welding procedure (clamping condition) can significantly influence the fatigue life. It was shown that for some panels the critical distance method was not able to explain the failure in the weld. In this case fracture mechanics together with residual stress analysis were used successfully to predict the failure.

Assuming homogeneous material properties throughout the weld and the base material, FEM analysis for T and overlap joints as well can provide a reasonable fatigue prediction. This suggests that the same assumption can be extended to complex components for failure analysis of the friction stir welded joints when using the critical distance method.

Fatigue assessment of friction stir welded joints was also performed using standard codes Eurocode 9 and IIW. Fatigue curves of traditional fusion welded joints were used. The results are in reasonable agreement with experimental data and FEM predictions.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2012. , vii, 45 p.
Keyword [en]
Friction stir welding, Fatigue, Aluminium alloys, Critical distance, Finite element method, Standard codes
National Category
Manufacturing, Surface and Joining Technology
Identifiers
URN: urn:nbn:se:kth:diva-92157ISBN: 978-91-7501-291-9 (print)OAI: oai:DiVA.org:kth-92157DiVA: diva2:512432
Public defence
2012-04-26, Sal F3, Lindstedsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20120330Available from: 2012-03-30 Created: 2012-03-27 Last updated: 2012-04-02Bibliographically approved
List of papers
1. Fatigue Strength of Friction Stir WeldedAluminium Profile for Train Car Application
Open this publication in new window or tab >>Fatigue Strength of Friction Stir WeldedAluminium Profile for Train Car Application
2008 (English)Conference paper, Published paper (Refereed)
Abstract [en]

Friction stir welded aluminium alloys are used for many applications in transportation. In theseapplications the material is frequently subject to varying load conditions, making fatigue failure acritical issue. In the scope of this paper, the fatigue performance of friction stir welded profiles ofAl-alloy 6005A has been investigated. A profile that is used for railway car wall side panels waschosen. The profiles were joined together with the friction stir welding method at both sides ofthe profile. 3-point fatigue bending tests were performed for the profiles. Stress ranges givingfailure after 105 to 106 cycles with the stress ratio R=0.1 were used.With FEM simulations critical positions for crack initiation and failure were identified in the parentmetal. These positions were all at narrow radii in the profiles. In fact, the fatigue failures alwaysoccurred at these positions. The maximum stress varied somewhat between the positions. Inspite of this, the cracking and failure took place at six different positions indicating that the profilehad a balanced design.The maximum von Mises stress at the failure positions as determined with FEM was about 50%higher than the corresponding uniaxial fatigue data for the same number of cycles. Thissuggests that the cracks initiate at places with high stresses but propagate into areas with lowerstresses.The FSW joint was a lap-butt joint with a sharp notch (interface between the work pieces) next tothe weld nugget. Since both sides of the profile were welded, such sharp notches appeared onboth sides. In most profiles cracks initiated and propagated form the notch where the centre loadwas placed. However, failure never took place there, i.e. complete fracture was not observed.The direction in which the cracks propagated was perpendicular to the plane of the profiles andperpendicular to the maximum stress direction. On the opposite side to the load position nocracking was observed

Place, publisher, year, edition, pages
TWI, 2008
Keyword
friction stir welding, aluminium alloy, profile, fatigue, railway car wall side panel
National Category
Manufacturing, Surface and Joining Technology Other Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-49556 (URN)978-1-903761-06-9 (ISBN)
Conference
7th Int. Symp. on Friction Stir Welding, Awaji Island, Japan,
Note

QC 20120109

Available from: 2012-01-09 Created: 2011-11-28 Last updated: 2017-02-22Bibliographically approved
2. Fatigue analysis of friction stir welded aluminium profile using critical distance
Open this publication in new window or tab >>Fatigue analysis of friction stir welded aluminium profile using critical distance
2010 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 32, no 2, 302-309 p.Article in journal (Refereed) Published
Abstract [en]

A friction stir (FS) welded extruded aluminium alloy 6005A, which is used for train wall sides, was fatigue tested. The friction stir weld was a lap-butt joint with a sharp notch (interface between the work pieces) next to the weld nugget. Fatigue cracks and failure appeared at notches in the profile. In most profiles, cracks also initiated at the sharp notch at the weld, but the propagation was slow and complete fracture never took place there. Finite element method (FEM) stress analysis combined with the theory of critical distance was used to estimate the fatigue limit. Results from the analysis for the fatigue limit were within 3-28% of the observations. The stress analysis correctly predicted that failure would not occur in the welds.

Keyword
Friction stir welded profile, Fatigue analysis, Theory of critical distance, Finite element method, Stress-distance curve
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-46675 (URN)10.1016/j.ijfatigue.2009.06.019 (DOI)000272295900009 ()2-s2.0-70350570428 (Scopus ID)
Note
QC 20111105Available from: 2011-11-05 Created: 2011-11-04 Last updated: 2017-12-08Bibliographically approved
3. Critical distance method to estimate the fatigue life time of friction stirwelded profiles
Open this publication in new window or tab >>Critical distance method to estimate the fatigue life time of friction stirwelded profiles
2012 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 37, 60-68 p.Article in journal (Refereed) Published
Abstract [en]

The fatigue failure of friction stir (FS) welded aluminium panels of alloy 6005A has been analysed. Thesepanels are produced with two main joint geometries: namely half overlap and hourglass. Presence ofcrack like notches (interface between the welded work pieces) and blunt notches (corners and bendsin the base metal) have been studied with finite element method (FEM) stress analysis combined withthe critical distance method. It was shown that the failure location and fatigue limit could be predictedfor three of the four types of profiles considered. Choice of the welding procedure (clamping condition),however, can significantly influence the fatigue life and prediction accuracy. When different welding procedurewas used the weld failure was not reproduced by model and failure location was not predictedcorrectly. In this case fracture mechanics approach along with residual stress analysis has been used toanalyse the failure. It is also shown that local plastic deformation on the crack tip is induced by clampingresulting in tensile residual stresses at the crack tip. Taking stress intensity factor of the residual stressinto account, the position of the failure can be explained

Place, publisher, year, edition, pages
Elsevier, 2012
Keyword
Friction stir welding, Fatigue analysis, Theory of critical distance, Interface notch, Residual stresses
National Category
Manufacturing, Surface and Joining Technology
Identifiers
urn:nbn:se:kth:diva-49553 (URN)10.1016/j.ijfatigue.2011.10.003 (DOI)000299911100007 ()2-s2.0-84655174945 (Scopus ID)
Note
QC 20111201Available from: 2011-12-01 Created: 2011-11-28 Last updated: 2017-12-08Bibliographically approved
4. Influence of fabrication stresses on fatigue life of friction stir welded aluminium profiles
Open this publication in new window or tab >>Influence of fabrication stresses on fatigue life of friction stir welded aluminium profiles
2012 (English)In: Journal of Materials Processing Technology, ISSN 0924-0136, E-ISSN 1873-4774, Vol. 212, no 7, 1488-1494 p.Article in journal (Refereed) Published
Abstract [en]

Influence of clamping and welding procedure on fatiguelife of a frictionstir (FS) weldedaluminiumprofile has been analysed. Two series of aluminiumprofiles (series A and B) have been fatigue tested. The different clamping conditions in series A and B gave rise to different failure positions that could not be explained neither by the effective notch stress method, nor by the critical distance methods. The clamping in series A introduced plastic deformation at the weld notch that gave rise to tensile residual stresses. The failure position could be explained from the value of the stress intensity factor provided that the residual stresses were considered.

Keyword
Friction stir welding, Clamping loads, Residual stresses, Stress ratio, Fatigue failure
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-92217 (URN)10.1016/j.jmatprotec.2012.02.008 (DOI)000304020800005 ()2-s2.0-84858304916 (Scopus ID)
Note
QC 20120329Available from: 2012-03-29 Created: 2012-03-29 Last updated: 2017-12-07Bibliographically approved
5. Eurocode 9 to estimate the fatigue life of friction stir welded aluminium panels
Open this publication in new window or tab >>Eurocode 9 to estimate the fatigue life of friction stir welded aluminium panels
2012 (English)In: Engineering structures, ISSN 0141-0296, E-ISSN 1873-7323, Vol. 45, 307-313 p.Article in journal (Refereed) Published
Abstract [en]

Eurocode 9 is a standard that covers the design of building and engineering structures made from wrought and cast aluminium alloys. A part of the Eurocode 9 handles the design of aluminium structures susceptible to fatigue. Eurocode 9 has data for aluminium alloys and welded structures for conventional welding methods (fusion welding) except for friction stir welding processes. The present study compares fatigue test results from friction stir welded joints with fatigue curves of traditional fusion welded joints which are presented in Eurocode 9. The results are in reasonable agreement with experimental data and FEM predictions. This suggests that Eurocode 9 can be used for estimating the fatigue strength of friction stir welded joints.

Keyword
Eurocode 9, Fatigue assessment, Friction stir welding, Nominal stress
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-92222 (URN)10.1016/j.engstruct.2012.06.039 (DOI)000309787400024 ()2-s2.0-84866146119 (Scopus ID)
Note

QC 20121017

Available from: 2012-03-29 Created: 2012-03-29 Last updated: 2017-12-07Bibliographically approved
6. Effective notch stress and critical distance method to estimate the fatigue life of T and overlap friction stir welded joints
Open this publication in new window or tab >>Effective notch stress and critical distance method to estimate the fatigue life of T and overlap friction stir welded joints
2012 (English)In: Engineering Failure Analysis, ISSN 1350-6307, E-ISSN 1873-1961, Vol. 25, 250-260 p.Article in journal (Refereed) Published
Abstract [en]

The interface notch is a general characteristic of friction stir welded joints and appears in many types of the welds. Indeed interface notch most likely is a location that fatigue failure can occur there. This paper is concerned with the fatigue of friction stir welded joints. In the present paper fatigue of T and over lap joints were modelled base on critical distance method. Two material properties Delta K-th, the fatigue threshold stress intensity factor and Delta sigma(0), the fatigue limit were used in order for estimating the fatigue life at interface location. This is shown that assuming homogeneous material throughout the weld and the base material in FE analysis still can provide a reasonable fatigue prediction for friction stir joints when using critical distance method. This suggests the same procedure can be used for complex component when failure appears in the interface notch. In order to verify it the critical distance prediction method was also used for actual component where the failure occurred in the weld. The predicted fatigue limit deviated only 15% from the experimental result.

Keyword
Fatigue failure, Interface notch, Friction stir welding, Critical distance method
National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-92224 (URN)10.1016/j.engfailanal.2012.05.019 (DOI)000308770000026 ()2-s2.0-84862681128 (Scopus ID)
Note

QC 20121130

Available from: 2012-03-29 Created: 2012-03-29 Last updated: 2017-12-07Bibliographically approved

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Citation style
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