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Static and fatigue analyses of welded steel structures: some aspects towards lightweight design
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures.
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The objectives of this thesis comprise of overcoming the challenges in designing lightweight welded structures such as material selection, choice of fatigue design methods, and increased performance by using improvement techniques. Material selection of welded joints is dependent on the filler and base material strengths. Partially and fully penetrated cruciform and butt welded joints were designed in under-matching, matching, and over-matching filler materials. Base material steel grades were S600MC, S700MC, and S960. Current design rules are developed for welds in steel up to yield strength of 700MPa. Therefore, design rules in Eurocode3, AWS d1.1, and BSK 07 were verified and recommendations for developing design rules for designing welded joints in S960 were concluded. Numerical methodology for estimating static strength of welded joints by simulating heat affected zone was also developed.

Another objective of the thesis work was to overcome the challenges in selection of fatigue design methods. The available design curves in standards are developed for uniaxial stress states, however, in real life the welds in mechanical structures are subjected to complex multiaxial stress states. Furthermore; weld toe failures are frequently investigated, weld root failures are seldom investigated. Therefore, in this work the multiaxial fatigue strength of welded joints failing at the weld root was assessed using experiments and various nominal and local stress based approaches. Butt welded joints with different weld seam inclinations with respect to applied uniaxial loading were designed to assess the root fatigue strength in higher multiaxial stress ratio regime. The fatigue strength of multi-pass tube-to-plate welded joints subjected to internal pressure only and combined internal pressure and torsion in and 90° out of phase loading was also investigated. Test data generated in this thesis was evaluated together with the test data collected from literature.

Last objective of the thesis included investigation of the increased performance in fatigue strength by post weld treatment methods such as HFMI. The behavior of residual stresses induced due to HFMI treatment during fatigue loading is studied. Numerical residual stress estimations and residual stress relaxation models are developed and the effect of various HFMI treatment process parameters and steel grade on the induced residual stress state is investigated. Specimens studied were non load carrying longitudinal attachments and simple plates. Residual stresses in both test specimens were measured using X-ray diffraction technique.

Place, publisher, year, edition, pages
Stockholm: KTH School of engineering sciences , 2017. , p. 33
Series
TRITA-AVE, ISSN 1651-7660 ; 2017:04
Keywords [en]
Fatigue strength, welded joints, static strength, high strength steel
National Category
Vehicle Engineering Aerospace Engineering
Research subject
Vehicle and Maritime Engineering; Aerospace Engineering
Identifiers
URN: urn:nbn:se:kth:diva-200829ISBN: 978-91-7729-270-8 (print)OAI: oai:DiVA.org:kth-200829DiVA, id: diva2:1070954
Public defence
2017-04-07, Sal F3, Lindstedsvägen 26, Stockholm, 09:00 (English)
Opponent
Supervisors
Note

QC 20170206

Available from: 2017-02-06 Created: 2017-02-03 Last updated: 2017-02-06Bibliographically approved
List of papers
1. Ultimate strength and failure modes for fillet welds in high strength steels
Open this publication in new window or tab >>Ultimate strength and failure modes for fillet welds in high strength steels
2012 (English)In: Materials and Design, ISSN 0261-3069, Vol. 40, p. 36-42Article in journal (Refereed) Published
Abstract [en]

A study has been carried out on the effect of strength mismatch in weld metal and penetration ratio on the ultimate strength capacity of fillet welds and their failure modes. The ultimate strength capacity evaluated with nonlinear Finite Element Analysis (FEA) and testing is compared with the ultimate strength capacity predicted by standards: Eurocode3, BSK 07 (Boverkets handbok om StalKonstruktioner) and AWS (American Welding Society) D1.1 Structural welding code-Steel. Test results are used to establish correct material properties, to be input into the finite element model. A criterion for selection of consumables has been developed, when two different grades of high strength steels are to be joined. The results show that fully penetrated joint with under-matched filler material is more ductile and the ultimate strength capacity of base plate can be achieved. It is observed that joints with under-matched filler material are more sensitive to penetration ratio. The test data correlates reasonably well to the results predicted by FEA software and design codes. It is also concluded that joint preparation has an effect on the ultimate strength capacity of the joint.

Keywords
fillers, finite element method, high strength steel, standards, steel testing
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-98313 (URN)10.1016/j.matdes.2012.03.048 (DOI)000304545000004 ()2-s2.0-84859648579 (Scopus ID)
Note
QC 20120625Available from: 2012-06-25 Created: 2012-06-25 Last updated: 2017-02-03Bibliographically approved
2. Load Carrying Capacities of Butt Welded Joints in High Strength Steels
Open this publication in new window or tab >>Load Carrying Capacities of Butt Welded Joints in High Strength Steels
2015 (English)In: Journal of engineering materials and technology, ISSN 0094-4289, E-ISSN 1528-8889, Vol. 137, no 4, article id 041003Article in journal (Refereed) Published
Abstract [en]

The aim of this study is to investigate the influence of yield strength of the filler material and weld metal penetration on the load carrying capacity of butt welded joints in high-strength steels (HSS) (i.e., grade S700 and S960). These joints are manufactured with three different filler materials (under-matching, matching, and over-matching) and full and partial weld metal penetrations. The load carrying capacities of these mentioned joints are evaluated with experiments and compared with the estimations by finite element analysis (FEA), and design rules in Eurocode3 and American Welding Society Code AWS D1.1. The results show that load carrying estimations by FEA, Eurocode3, and AWS D1.1 are in good agreement with the experiments. It is observed that the global load carrying capacity and ductility of the joints are affected by weld metal penetration and yield strengths of the base and filler materials. This influence is more pronounced in joints in S960 steel welded with under-matched filler material. Furthermore, the base plate material strength can be utilized in under-matched butt welded joints provided appropriate weld metal penetration and width is assured. Moreover, it is also found that the design rules in Eurocode3 (valid for design of welded joints in steels of grade up to S700) can be extended to designing of welds in S960 steels by the use of correlation factor of one.

National Category
Materials Engineering
Identifiers
urn:nbn:se:kth:diva-173958 (URN)10.1115/1.4030687 (DOI)000360707000003 ()2-s2.0-84934922510 (Scopus ID)
Note

QC 20151006

Available from: 2015-10-06 Created: 2015-09-24 Last updated: 2017-12-01Bibliographically approved
3. The multiaxial weld root fatigue of butt welded joints subjected to uniaxial loading
Open this publication in new window or tab >>The multiaxial weld root fatigue of butt welded joints subjected to uniaxial loading
2016 (English)In: Fatigue & Fracture of Engineering Materials & Structures, ISSN 8756-758X, E-ISSN 1460-2695, Vol. 39, no 10, p. 1281-1298Article in journal (Refereed) Published
Abstract [en]

In this study, the fatigue strength of inclined butt welds subjected to a proportional multiaxial stress state generated by uniaxial loading is studied in nominal and local stress concepts. The local methodologies studied included principal stress hypothesis, von Mises stress hypothesis and modified Wöhler curve method. Nominal methodologies included modified Gough-Pollard interaction equation, the design equation in Eurocode3 and the interaction equation in DNV standard. Results are evaluated along with data published in relevant literature. It is observed that both local and nominal stress assessment methods are able to estimate multiaxial fatigue strength. No obvious difference in fatigue strength is observed in the nominal stress concept, but the notch stress concept is able to capture a decrease in fatigue strength in shear-dominated joints. It is concluded that modified Wöhler curve method is a suitable tool for the evaluation of fatigue strength in joints dominated by both normal and shear stresses.

Place, publisher, year, edition, pages
Blackwell Publishing, 2016
Keywords
Fatigue strength, Multiaxial stress ratio, Multiaxial stress state, Weld inclination, Welded joints, Butt welding, Shear stress, Welding, Welds, Interaction equations, Local stress concept, Multi-axial fatigue, Multiaxial stress, Nominal stress concept, Normal and shear stress, Stress analysis
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-186757 (URN)10.1111/ffe.12444 (DOI)000383726900009 ()2-s2.0-84962688225 (Scopus ID)
Note

QC 20160530

Available from: 2016-05-30 Created: 2016-05-13 Last updated: 2017-11-30Bibliographically approved
4. Multiaxial weld root fatigue of fillet welded tube to plate joints subjected to non-proportional multiaxial stress state
Open this publication in new window or tab >>Multiaxial weld root fatigue of fillet welded tube to plate joints subjected to non-proportional multiaxial stress state
(English)Article in journal (Refereed) Submitted
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-200919 (URN)
Note

QC 20170206

Available from: 2017-02-05 Created: 2017-02-05 Last updated: 2017-02-06Bibliographically approved
5. Behavior of compressive residual stresses in high strength steel welds induced by high frequency mechanical impact treatment
Open this publication in new window or tab >>Behavior of compressive residual stresses in high strength steel welds induced by high frequency mechanical impact treatment
2014 (English)In: Journal of Pressure Vessel Technology-Transactions of the ASME, ISSN 0094-9930, E-ISSN 1528-8978, Vol. 136, no 4, p. 041404-Article in journal (Refereed) Published
Abstract [en]

Residual stress state plays an important role in the fatigue life of welded structures. The effect can be beneficial or detrimental, depending on the nature of residual stresses. High frequency mechanical impact (HFMI) treatment is a postweld fatigue improvement technique for welded joints. In this research work the behavior of compressive residual stresses induced in welded joints in high strength steels (HSS) by HFMI treatment has been investigated. Longitudinal nonload carrying attachments in HSS are tested with constant amplitude (CA) and variable amplitude (VA) fatigue loading. Stress concentration factors have been calculated using finite element analysis (FEA). Residual stresses have been measured at different cycles during fatigue testing using X-ray diffraction technique. It is observed that the induced residual stresses are quite stable with some relaxation in CA and VA loading. The overloads in VA loading seem to be more detrimental. Relaxation of residual stresses is more obvious in VA tests.

Keywords
Compressive residual stress, Fatigue improvement, Mechanical impacts, Relaxation of residual stress, Residual stress state, Stress concentration factors, Variable amplitudes, X-ray diffraction techniques
National Category
Other Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-145381 (URN)10.1115/1.4026651 (DOI)000338507600009 ()2-s2.0-84898064883 (Scopus ID)
Note

QC 20140520

Available from: 2014-05-20 Created: 2014-05-19 Last updated: 2017-12-05Bibliographically approved
6. Residual stresses in welded components following post-weld treatment methods
Open this publication in new window or tab >>Residual stresses in welded components following post-weld treatment methods
2013 (English)In: Procedia Engineering, Elsevier, 2013, p. 181-191Conference paper, Published paper (Refereed)
Abstract [en]

Weld seam is the weakest point of the structure. Weld defects, weld geometry such as toe radius, and residual stresses, which are typically tensile in the critical area, weaken the fatigue strength of the weld seam. These factors become more important with the welding of high strength steels (HSS) because structures containing welded seams loose the benefit from the strength of the steel. The weld seam fatigue strength can be improved locally with several different methods. In this paper residual stresses of the weld area and their modifications for improving the fatigue strength are discussed. The residual stresses of the toe area are changed from tensile up to compressive stresses, which are known to improve fatigue strength, with several methods. High tensile stresses can be relaxed mechanically or thermally as postweld heat treatment. Mechanical treatments are more common especially in large structures. Typical mechanical methods are burr grinding, TIG remelting, hammer peening and needle peeing. A new method is laser re-melting method. All these methods also change the geometry of the weld toe by improving it. This paper concentrates to needle peening (high frequency mechanical impact, HFMI), and presents some results from laser remelting tests. Both surface measurements and depth distributions of stress measured by X-ray diffraction (XRD) method are presented.

Place, publisher, year, edition, pages
Elsevier, 2013
Series
Procedia Engineering, ISSN 1877-7058 ; 66
Keywords
Fatigue strenght improvement, HFMI, High stregnth steel, Residual stress, Welding
National Category
Mechanical Engineering
Identifiers
urn:nbn:se:kth:diva-137313 (URN)10.1016/j.proeng.2013.12.073 (DOI)000338327000018 ()2-s2.0-84894490402 (Scopus ID)
Conference
5th International Conference on Fatigue Design, Fatigue Design 2013; Senlis; France; 27 November 2013 through 28 November 2013
Note

QC 20140107

Available from: 2013-12-12 Created: 2013-12-12 Last updated: 2017-02-03Bibliographically approved
7. Residual stress state induced by High Frequency Mechanical Impact Treatment in different steel grades -numerical and experimental study
Open this publication in new window or tab >>Residual stress state induced by High Frequency Mechanical Impact Treatment in different steel grades -numerical and experimental study
2017 (English)In: International Journal of Mechanical Sciences, ISSN 0020-7403, E-ISSN 1879-2162, Vol. 123, p. 34-42Article in journal (Refereed) Published
Abstract [en]

High frequency mechanical impact treatment is observed to increase the fatigue strength of welded joints. This technique induces compressive residual stresses, increases the local hardness, and reduces the stress concentration by modifying the weld toe radius. The goal of this study was to investigate residual stresses induced by ultrasonic impact treatment in S355, S700MC, and S960 grades steel experimentally and numerically. Plate specimens were manufactured and treated with different treatment intensities i.e. vibration amplitudes of the Sonotrode. The indentation depths were measured by the aid of a laser scanner and residual stresses using X-ray diffraction technique. The effect of steel grade and treatment intensity on the induced compressive residual stress state was firstly studied experimentally. In addition, displacement controlled simulations were carried out to estimate the local residual stress condition considering the effect of different material models. Both the numerically estimated and experimentally measured residual stresses were qualitatively in good agreement. Residual stress state in S355 and S700MC can be estimated well using combined strain rate dependent material model. No significant effect of the treatment intensity is observed on the indentation depth and residual stress state for S355 grade steel. The indentation depth decreases with the increase in the yield strength of the steel.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Residual stress state; HFMI; Steel; X-ray diffraction; Finite element analysis; Fatigue strength
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-200920 (URN)10.1016/j.ijmecsci.2017.01.027 (DOI)000397695700003 ()2-s2.0-85011102713 (Scopus ID)
Note

QC 20170206

Available from: 2017-02-05 Created: 2017-02-05 Last updated: 2017-04-28Bibliographically approved
8. Stability of High Frequency Mechanical Impact (HFMI) post-treatment induced residual stress states under cyclic loading of welded steel joints
Open this publication in new window or tab >>Stability of High Frequency Mechanical Impact (HFMI) post-treatment induced residual stress states under cyclic loading of welded steel joints
(English)Article in journal (Refereed) Submitted
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-200921 (URN)
Note

QC 20170206

Available from: 2017-02-05 Created: 2017-02-05 Last updated: 2017-02-06Bibliographically approved

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