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Title:Application of normalization method to fracture toughness testing of welds with pronounced strength heterogeneity : doctoral disertation
Authors:ID Štefane, Primož (Author)
ID Gubeljak, Nenad (Mentor) More about this mentor... New window
ID Hertelé, Stijn (Mentor) More about this mentor... New window
Files:.pdf DOK_Stefane_Primoz_2023.pdf (20,57 MB)
MD5: 3AADFE194D81FA23F5D06CAF2920E9C1
 
Language:English
Work type:Doctoral dissertation
Typology:2.08 - Doctoral Dissertation
Organization:FS - Faculty of Mechanical Engineering
Abstract:This doctoral dissertation presents the results of an extensive fracture testing programme of welds with pronounced strength heterogeneity. Purpose of this programme was to determine fracture toughness of heterogeneous welds that contain a midplane crack. Application of standardized fracture testing methods in heterogeneous welds might lead to overestimation or underestimation of fracture toughness and consequentially to inaccurate assessment of structural integrity. Reasons for that are variations in mechanical properties of different material regions in the weld which have a significant impact on development of deformation at the crack tip, and consequently on the crack driving force. Experimental procedures in scope of this research include fabrication of weld sample plates, that were welded with MAG process. The welds were fabricated using two different electrodes, one with higher and one with lower mechanical properties, with respect to base material S690QL in order to replicate extreme variations of mechanical properties in the weldment. Fabricated welds were then characterized in detail using metallography, three-point bend impact testing, indentation hardness measurements and tensile testing of flat miniature and round bar standard tensile specimens. Resistance of welds to stable tearing was investigated by fracture testing of square surface cracked SE(B) specimens containing a weld midplane notch. J-integral has been estimated from plastic work, using the normalization data reduction method that is included in standard ASTM E1820. The advantage of the normalization data reduction method is that no special equipment or complex testing method is needed to measure ductile crack growth during fracture testing. The ductile crack growth is determined directly from the load-displacement record, by applying appropriate calibration function and physical lengths of initial and final cracks that were measured post-mortem with the nine-point method. Several correction factors had to be calibrated in order to successfully implement the normalization data reduction method to fracture testing of welds with pronounced strength heterogeneity. For that reason, parametric finite element analyses were conducted for several weld configurations. Finite element models incorporated plane strain conditions in order to provide calibrated factors that comply with plane strain equations included in ASTM E1820. Additionally, crack tip constraint has been extensively analysed and correlated with the plastic deformation fields. This clarified altered deformation behaviour of modelled welds in comparison with the base material and corresponding effect on fracture toughness. Finally, calibrated factors were applied to computation of J-integral from data that were measured during fracture testing. J-R resistance curves were constructed for the tested heterogeneous welds and compared to the ones of the base material. This directly showed the effect of variations of mechanical properties on the weld fracture behaviour.
Keywords:weld, strength mismatch, fracture, normalization data reduction technique, plastic correction factors, test fixture, SE(B) specimen, J-R resistance curve
Place of publishing:Maribor
Place of performance:Maribor
Publisher:[P. Štefane]
Year of publishing:2022
Number of pages:XIV, 228 str.
PID:20.500.12556/DKUM-83028 New window
UDC:620.178.2:621.791.053(043.3)
COBISS.SI-ID:138416131 New window
Publication date in DKUM:10.01.2023
Views:778
Downloads:202
Metadata:XML DC-XML DC-RDF
Categories:KTFMB - FS
:
ŠTEFANE, Primož, 2022, Application of normalization method to fracture toughness testing of welds with pronounced strength heterogeneity : doctoral disertation [online]. Doctoral dissertation. Maribor : P. Štefane. [Accessed 22 April 2025]. Retrieved from: https://dk.um.si/IzpisGradiva.php?lang=eng&id=83028
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Licences

License:CC BY-ND 4.0, Creative Commons Attribution-NoDerivatives 4.0 International
Link:http://creativecommons.org/licenses/by-nd/4.0/
Description:Under the NoDerivatives Creative Commons license one can take a work released under this license and re-distribute it, but it cannot be shared with others in adapted form, and credit must be provided to the author.
Licensing start date:13.09.2022

Secondary language

Language:Slovenian
Title:Uporaba normalizacijske metode za testiranje lomne žilavosti zvarnih spojev z izrazito heterogeno trdnostjo
Abstract:V doktorski disertaciji so predstavljeni rezultati obsežnega programa lomno mehanskih preizkusov zvarov z izrazito heterogenimi mehanskimi lastnostmi. Namen tega eksperimentalnega programa je bil določitev lomne žilavosti za heterogene zvare. Uporaba standardnih metod za določitev lomne žilavosti v takšnem primeru daje precenjeno ali podcenjeno lomno žilavost zvarov, kar vodi do napak pri oceni celovitosti konstrukcij. To je poseben izziv, ki do danes ni povsem razrešen. Razlog je ta, da regije zvara z različnimi mehanskimi lastnostmi vplivajo na razvoj polja deformacij na konici razpoke in posledično na gonilno silo za razvoj razpoke. Z namenom, da se ta izziv reši, so bili zvari najprej detajlno preučeni, kot je opisano v 2. poglavju te disertacije. Pri eksperimentalnem delu, ki je bilo opravljeno v sklopu te raziskave, so bili najprej pripravljeni vzorčni zvari. Ti so bili izdelani z varjenjem plošč iz nizko legiranega visoko trdnostnega jekla S690 QL, ki se v industriji pogosto koristi za gradnjo zahtevnih konstrukcij. Za varjenje je bil uporabljen MAG postopek. Posebna značilnost izdelanih zvarov je ta, da sta pri izdelavi bili uporabljeni dve varilni žici, ki sta se razlikovali po mehanskih lastnostih. Prva varilna žica je imela višjo mejo plastičnosti, druga pa nižjo v primerjavi z osnovnim materialom. Meje plastičnosti uporabljenih varilnih žic so se v tem primeru razlikovale več kot 30 % glede na mejo plastičnosti osnovnega materiala. Na ta način so bile v vzorčnih zvarih umetno poustvarjene povečane neenakosti v trdnosti posamičnih regij zvara. Natančneje, pol žleba, ki je bil izdelan na ploščah pred varjenjem, je bilo polnjenega z varilno žico, ki ima višjo trdnost. Druga polovica žleba je bila polnjena z varilno žico, ki ima nižjo trdnost. Takšno stanje se v praksi pogosto ustvari pri popravljanju obstoječih zvarov, kjer se za popravilo uporabi drugačna elektroda ali varilna žica kot za izdelavo zvara. Za primerjavo so bili izdelani še zvari, ki so bil polnjeni samo z eno varilno žico. Izdelani zvari so bili detajlno preučeni. Najprej so bile opravljene metalografske preiskave, ki so pokazale na povečano vsebnost bainita in martenzita v mikrostrukturi. Omenjena mikrokonstituenta sta posledica kompleksne termalne zgodovine zaradi varjenja z več varki in pomembno vplivata na trdnost zvarnega materiala. V naslednjem koraku so bile opravljene meritve trdote po Vickersu na vzorcu zvara. Opravljene meritve so pokazale velika nihanja trdote med regijami v osnovnem stanju in pogretimi regijami posamičnih varkov. Povečana trdota je bila opažena tudi v toplotno vplivanem področju (TVP). To nakazuje, da je med varjenjem prišlo do utrjevanja TVP. Dodatno so bile določene lokalne mehanske lastnosti v posameznih regijah zvara z nateznim testiranjem miniaturnih preizkušancev. Preizkusi so pokazali, da mehanske lastnosti, v smislu meje plastičnosti in natezne trdnosti, nihajo v zvaru skladno z izmerjeno trdoto. Hkrati so rezultati preizkusov pokazali, da material zvara z višjo trdnostjo kaže dodatno utrjevanje v pogretem področju. Nasprotno je bilo opaženo pri testiranju materiala zvara z nižjo trdnostjo. V tem primeru pride do mehčanja materiala v pogretem področju. Največji meji plastičnosti in natezne trdnosti sta bili izmerjeni v finozrnatem delu TVP. Dodatno so bili izvedeni še natezni preizkusi standardnih nateznih preizkušancev valjaste oblike. Ti preizkusi so podali povprečne mehanske lastnosti materiala v zvaru in osnovnega materiala. Rezultati teh preizkusov so pokazali relativno dobro ujemanje z rezultati miniaturnih nateznih preizkusov. To pomeni, da je za poenostavljene analize heterogenih zvarov možno koristiti povprečne mehanske lastnosti za izdelavo materialnih modelov, ki se kasneje uporabljajo v simulacijah po metodi končnih elementov ali v analitičnih modelih za oceno celovitosti konstrukcij v skladu s standardom BS 7910. V naslednjem koraku so bili izvedeni preizkusi u
Keywords:zvar, trdnostno neujemanje, lom, normalizacijska metoda, plastični korekcijski faktorji, vpenjalna priprava, SE(B) preizkušanec, J-R odpornostna krivulja


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