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Fatigue properties of cut and welded high strength steels: Quality aspects in design and production
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Lightweight Structures. (KTH Weld Mechanics Group)ORCID iD: 0000-0003-4465-9299
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This doctoral thesis concerns fatigue of welded structures. Welding is one of the world’s most common joining methods and it is frequently used in several structural applications in many fields. Some examples are construction vehicles, loader cranes, trucks, busses, forestry and agricultural machines, bridges and ships. Since these structures are subjected to repeated loading, fatigue is the most common cause of failure.

A novel numerical algorithm has been developed which assesses the welded surface and calculates and quantifies weld quality parameters and the presence of defects which are critical in fatigue applications. The algorithm is designed for implementation in serial production. It will provide robust and reliable feedback on the quality being produced, which is essential if high strength steels are utilized.

Two welding procedures which can increase the weld quality in as welded conditions have been assessed. These procedures utilize welding in different positions and pendling techniques, which can be accomplished using the existing welding equipment. It was found that by using these methods, the fatigue strength can be increased compared to normal weld quality.

Furthermore, two fatigue assessment methods ability to account for increased weld quality in low cycle and high cycle fatigue applications has been studied. One of these methods showed sufficient accuracy in predicting the fatigue strength with small scatter and also account for increased weld quality.

When implementing thinner high strength steels, the overall stress level in the structure increase. Therefore, other locations such as the steel cut edges may become critical for fatigue failure unless they are not designed and manufactured with the same quality as the welded joint. The influence of surface quality on cut edges was studied and the fatigue strength was estimated using international standards and a fatigue strength model for cut edges.

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2016. , 56 p.
Series
TRITA-AVE, ISSN 1651-7660 ; 2016:21
Keyword [en]
Welding, Fatigue assessment, Cutting, Weld quality, Fatigue design, Fracture Mechanics
National Category
Vehicle Engineering
Research subject
Vehicle and Maritime Engineering
Identifiers
URN: urn:nbn:se:kth:diva-188225ISBN: 978-91-7729-027-8 (print)OAI: oai:DiVA.org:kth-188225DiVA: diva2:934626
Public defence
2016-08-26, F3, Lindstedtsvägen 26, Stockholm, 09:00 (English)
Opponent
Supervisors
Projects
WIQLIGHTSTRUCTONWELD
Funder
VINNOVA
Note

QC 20160613

Available from: 2016-06-13 Created: 2016-06-09 Last updated: 2016-06-13Bibliographically approved
List of papers
1. Development of an algorithm for quality inspection of welded structures
Open this publication in new window or tab >>Development of an algorithm for quality inspection of welded structures
2012 (English)In: Proceedings of the Institution of mechanical engineers. Part B, journal of engineering manufacture, ISSN 0954-4054, E-ISSN 2041-2975, Vol. 226, no 6, 1033-1041 p.Article in journal, Editorial material (Refereed) Published
Abstract [en]

A selection of reliable measurement systems that are able to measure the toe radius along the weld bead in a T-jointhas been studied. The results show that the currently used methods and gauges do not provide the required accuracywhen measuring the toe radius. The gauges are handled differently by different operators – even when using a vision system– which makes the current evaluation methods subjective and unreliable.Therefore, an algorithm has been developed that assesses weld bead surface data and automatically identifies and calculatesthe toe radius and the toe angle along the weld. The results obtained by the algorithm are consistent with referencemeasurements and it is possible for the algorithm to be used in improving the process and quality control duringwelding.

Place, publisher, year, edition, pages
Sage Publications, 2012
Keyword
Welded joints, imperfections, quality, surface measurements, inspection
National Category
Production Engineering, Human Work Science and Ergonomics
Research subject
Vehicle and Maritime Engineering; Solid Mechanics; Mathematics; Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-188216 (URN)10.1177/0954405412439138 (DOI)000305483000007 ()2-s2.0-84871941353 (Scopus ID)
Projects
WIQLIGHTSTRUCT
Funder
VINNOVA, 2009-04719
Note

QC 20160609

Available from: 2016-06-09 Created: 2016-06-09 Last updated: 2017-11-30Bibliographically approved
2. Fatigue life assessment of improved joints welded with alternative welding techniques
Open this publication in new window or tab >>Fatigue life assessment of improved joints welded with alternative welding techniques
2014 (English)In: Engineering Failure Analysis, ISSN 1350-6307, E-ISSN 1873-1961, Vol. 42, 10-21 p.Article in journal (Refereed) Published
Abstract [en]

In this study, the fatigue life improvement by adopting the toe weaving technique on non-load carrying cruciform welded joints has been investigated. Fatigue testing was conducted on two batches of specimens welded using double-pass manual welding. One batch had a straight second pass and the other was weaved. The influence of different weaving shape parameters was analyzed by performing crack growth analyses. The fatigue testing shows a slightly improved fatigue life for the two different batches compared to as- welded joints; the improvement is similar for both batches. The crack growth analysis concludes that the batch with the straight second pass should provide slightly higher fatigue life compared to the toe weaved batch. Measurements show a presence of undercuts in the vicinity of the crack initiation site. Nonetheless, an increased fatigue life is obtained, due to the low flank angle created during welding of the second pass, which reduces the stress concentration in the weld toe, prolonging the fatigue life.

National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-147405 (URN)10.1016/j.engfailanal.2014.03.012 (DOI)000336660400002 ()2-s2.0-84897030872 (Scopus ID)
Funder
Vinnova
Note

QC 20140702

Available from: 2014-07-02 Created: 2014-06-27 Last updated: 2017-12-05Bibliographically approved
3. Comparison of local stress based concepts - Effects of low-and high cycle fatigue and weld quality
Open this publication in new window or tab >>Comparison of local stress based concepts - Effects of low-and high cycle fatigue and weld quality
2015 (English)In: Engineering Failure Analysis, ISSN 1350-6307, E-ISSN 1873-1961, Vol. 57, 323-333 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents a study of the ability for two stress based methods, the effective notch stress method and the structural stress approach, to estimate the fatigue life in the low cycle and the high cycle fatigue regime, considering the weld quality. Two different non-load carrying joint configurations were considered, cruciform joint and T-joint. The conducted fatigue analysis shows that both methods are capable to estimate the fatigue life with good accuracy within the low cycle and the high cycle regime. The effective notch stress generally shows a smaller scatter, it also considers increased weld quality with good accuracy, in contrast to the structural stress approach.

Place, publisher, year, edition, pages
Pergamon Press, 2015
Keyword
Fatigue assessment, Structural stress, Effective notch stress, Welded structures
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-174583 (URN)10.1016/j.engfailanal.2015.07.022 (DOI)000361833000026 ()2-s2.0-84939632528 (Scopus ID)
Funder
VINNOVA, 2011-03620
Note

QC 20151207

Available from: 2015-12-07 Created: 2015-10-07 Last updated: 2017-12-01Bibliographically approved
4. Welding procedures for fatigue life improvement of the weld toe
Open this publication in new window or tab >>Welding procedures for fatigue life improvement of the weld toe
2016 (English)In: Welding in the World, ISSN 0043-2288, E-ISSN 1878-6669, Vol. 60, no 3, 573-580 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents the results of an experimental study of gas metal arc welded, GMAW, cruciform joints made of common construction steel S355. The hypothesis is that smooth undercuts in as welded conditions can give enhanced fatigue properties similarly as post treated welds. Undercuts are generally seen as a defect or imperfection. Welders try to avoid these and repair them when they occur, which result in increased production lead time. Post weld improvement methods i.e. grinding or high-frequency-impact treatment (HFMI) as fatigue-enhancing post-treatment methods enforce amongst other effects a certain smooth undercut-shaped groove in the treated weld toe region. The obtained shallower weld toe transition reduces the geometrical notch effect and increases fatigue strength. This paper presents a study whereas welded specimens with a weld toe geometry similar to what is obtained by weld toe grinding or HFMI-treatment, has been produced, fatigue tested and analyzed. The improvement of the fatigue strength is comparable to post-weld treated specimens. It has proven to be an efficient way to achieve high-quality welds without introducing any additional operations in production, thus enabling weight reduction using cost-effective methods.

Place, publisher, year, edition, pages
Springer Publishing Company, 2016
Keyword
Weld toes, Weld shape, Fatigue improvement, Fillet welds, GMA welding
National Category
Vehicle Engineering
Research subject
Vehicle and Maritime Engineering; Solid Mechanics; Production Engineering
Identifiers
urn:nbn:se:kth:diva-188224 (URN)10.1007/s40194-016-0309-9 (DOI)000379434400016 ()2-s2.0-84966378339 (Scopus ID)
Funder
VINNOVA
Note

QC 20160610

Available from: 2016-06-09 Created: 2016-06-09 Last updated: 2017-11-30Bibliographically approved
5. Fatigue assessment of cut edges in high strength steel - Influence of surface quality
Open this publication in new window or tab >>Fatigue assessment of cut edges in high strength steel - Influence of surface quality
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The influence of surface roughness on the fatigue strength in high strength steels which was cut using different cutting processes were studied. Fatigue testing was conducted on S700 and S960 material of different plate thicknesses cut in dog bone specimens using oxygen, plasma, laser and waterjet cutting. The surface roughness was measured on all specimens and residual stress measurements were conducted. Estimations of the fatigue strength were made using the measured surface roughness and the ISO 9013:2002 standard for thermal cutting quality tolerances. The testing proved a 15-70% increased fatigue strength compared to the estimation, proving a weak connection between the surface quality levels in ISO 9013:2002 and the fatigue test results. Different codes and design recommendations (IIW, EC3 and EN 13001) for fatigue strength of cut surfaces were compared with the fatigue test results which clearly showed an increased fatigue strength with enhanced quality and steel grades and where the codes and design recommendations do not allow for any fatigue strength improvement with improved quality and increased strength. 

Keyword
High Strength Steels, Fatigue assessment, Thermal Cutting, Oxygen cutting, Plasma cutting, Laser cutting, Waterjet cutting, Quality, Surface roughness, Welded structures, Cut edges
National Category
Vehicle Engineering
Research subject
Vehicle and Maritime Engineering; Solid Mechanics; Production Engineering; Machine Design
Identifiers
urn:nbn:se:kth:diva-188541 (URN)
Note

QC 201606013

Available from: 2016-06-13 Created: 2016-06-13 Last updated: 2016-06-13Bibliographically approved

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