Fracture mechanics-based fatigue life assessment of additively manufactured Ti-6Al-4V

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dc.contributor.advisorBecker, Thorsten Hermann en_ZA
dc.contributor.advisorBlaine, DCen_ZA
dc.contributor.authorMacallister, Nen_ZA
dc.contributor.otherStellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.en_ZA
dc.date.accessioned2024-02-22T15:51:59Zen_ZA
dc.date.accessioned2024-04-26T18:07:32Zen_ZA
dc.date.available2024-02-22T15:51:59Zen_ZA
dc.date.available2024-04-26T18:07:32Zen_ZA
dc.date.issued2024-02en_ZA
dc.descriptionThesis (PhD)--Stellenbosch University, 2024.en_ZA
dc.description.abstractENGLISH ABSTRACT: This dissertation presents a study on fracture mechanics-based fatigue life assessment for Additively Manufacturing (AM). The mature laser powder bed fusion (LPBF) process with the Ti-6Al-4V alloy in particular is selected for study, as it is well suited to the South African context with regard to economic climate, strong AM relationships and abundant mineral titanium reserves available. Furthermore, the Ti-6Al-4V alloy is a staple of aerospace, automotive and biomedical industries which are amongst the largest promoters for using AM technology, and for whom fatigue characterisation remains a prevalent topic as many end-use applications are intended for cyclic loading. Though significant research in fatigue behaviour exists, the conundrum of reliably certifying fatigue life in AM parts persists. This problem stems from the complex relation between AM print parameters, build orientation, surface roughness, inherent defects, residual stresses, meso- and microstructure; and establishing reportable fatigue strength baseline values required by industry. Moreover, as the AM environment promises saving in cost and time, full fatigue testing schemas are undesirable. As such, alternate damage-tolerant methods are becoming increasingly popular, where adopting fracture mechanics-based frameworks accompanied by limited or non-destructive testing could aid in certification. For this purpose, the dissertation first presents a novel version of the fatigue predictive NASGRO model where parameters are established that are unique to LPBF produced Ti-6Al-4V meso- and microstructures. In establishing these parameters for LPBF produced Ti-6Al-4V, the influence of process inherent microstructure, residual stress, and orientational dependant meso- structure is considered through examining near-threshold in combination with steady-state fatigue crack growth rates. The analysis shows that the descriptors of material constraint are sensitive to build orientation and microstructure. Furthermore, the effect of residual stresses is observed to not be severe. In this a clear effect of build orientation and meso- and microstructure is established for selecting NASGRO model parameters. Secondly, the proposed NASGRO formulation is translated into a comprehensive novel damage-tolerant fracture mechanics-based model to estimate fatigue life. Non-uniform defect populations, typical of AM material, in terms of size, shape and location are captured through X-ray tomography and surface profilometry and used as inputs modelled as equivalent crack lengths. The fatigue strength estimations are shown to be sensitive to fatigue crack growth rate threshold parameters and short crack growth mechanic descriptions. Furthermore, by introducing multiple crack initiations, the fatigue estimates are shown as distributions and are sensitive to defect number. Finally, sub-size specimen testing is investigated as a potentially elegant solution to accompany fatigue life assessments for threshold validation. Where results show inconsistent near-threshold fatigue behaviour linked to the microstructure. In this, considering unique meso- and microstructural features of LPBF produced Ti-6Al-4V, the domain and suitability in using sub-size specimens for fatigue crack growth rate threshold testing is discussed. Overall, this dissertation walks the path required in establishing reliable damage tolerant fatigue life estimation approaches for LPBF produced Ti-6Al-4V. Providing fundamental insights into interactions of fracture mechanic mechanisms and descriptions necessary for reliably modelling fatigue behaviour, therefore contributing to the developing frameworks and philosophies in AM to help in certification of fatigue performance of LPBF produced Ti-6Al-4V components.en_ZA
dc.description.abstractAFRIKAANSE OPSOMMING: Hierdie proefskrif bied ʼn studie van breukmeganika-gebaseerde vermoeidheidslewensduurassessering vir laagvervaardiging (LV) aan. Die volwasse laser-poeierbedsmeltingsproses (LPBF-proses) met die Ti-6Al-4V-allooi in die besonder is vir die studie gekies, aangesien dit veral vir die Suid-Afrikaanse konteks met betrekking tot ekonomiese klimaat, sterk LV-verhoudings en volop beskikbare minerale titaanreserwes geskik is. Ook is die Ti-6Al-4V-allooi die materiaal wat mees algemeen in die ruimtevaart-, motor-en biomediese bedryf gebruik word. Hierdie bedrywe is van die grootste voorstanders van die gebruik van LV-tegnologie en vermoeidheidskarakterisering bly vir hulle ʼn belangrike onderwerp, aangesien talle van die eindgebruiktoepassings vir sikliese belasting bedoel word. Hoewel beduidende navorsing in vermoeidheidsgedrag bestaan, bly die betroubare sertifisering van vermoeidheidslewensduur in LV-onderdele steeds ʼn vraagstuk. Hierdie probleem spruit uit die ingewikkelde verwantskap tussen LV-drukparameters, bou-oriëntasie, oppervlakgrofheid, inherente defekte, naspannings, meso-en mikrostruktuur; en die vasstel van die basislynwaardes vir aanmeldbare vermoeidheidsterkte wat deur industrie vereis word. Daarbenewens word volledige vermoeidheidstoetsskemas as ongewens beskou, omdat die LV-omgewing besparings ten opsigte van koste en tyd beloof. As sulks word alternatiewe skadetolerante metodes, waar die gebruik van breukmeganika-gebaseerde raamwerke saam met beperkte of niedestruktiewe toetsing met sertifisering kanhelp, dus toenemend gewild. Vir hierdie doel bied die proefskrif dus eers ʼn nuwe weergawe van die vermoeidheidsvoorspellende NASGRO-model aan, waar parameters vasgestel word wat uniek aan LPBF-vervaardigde Ti-6Al-4V-meso-en mikrostrukture is. In die vasstel van hierdie parameters vir LPBF-vervaardigde Ti-6Al-4V word die invloed van proses-inherente mikrostruktuur, naspanning en oriëntasie-afhanklike mesostruktuur in ag geneem deur die naby-drumpel-in kombinasie met die konstante-vermoeidheidskraakgroeitempo te ondersoek. Die ontleding toon dat die beskrywers van materiaalbegrensing sensitief vir bou-oriëntasie en mikrostruktuur is. Daarbenewens is waargeneem dat die effek van naspanning nie te erg is nie. Hierdeur is die duidelike effek van bou-oriëntasie en meso-en mikrostruktuur vir die kies van NASGRO-modelparameters gevestig. Tweedens word die voorgenome NASGRO-formulering omvorm tot ʼn omvattende nuwe skadetolerante breukmeganika-gebaseerde model om vermoeidheidslewensduur te beraam. Niegelykmatige defekbevolkings, tipies van LV-materiaal ten opsigte van grootte, vorm en ligging, word deur X-straaltomografie en oppervlakprofilometrie vasgelê en gebruik as invoere wat as ekwivalente kraaklengtes gemodelleer is. Die vermoeidheidsterkte-beramings blyk sensitief vir die drumpelparameters van die vermoeidheidskraakgroeitempo en beskrywings van kortkraakgroeimeganika te wees. Daarbenewens word die vermoeidheidsberamings, deur die gebruik van veelvuldige kraakaanvorings, as verspreidings aangetoon en is dit sensitief vir die getal defekte. Laastens word subgrootte eksemplaartoetsing ondersoek as ʼn moontlike elegante oplossing om saam met vermoeidheidslewensduurassessering vir drumpelgeldigheidsbevestiging te gebruik, waar resultate veranderlike naby-drumpel-vermoeidheidsgedrag toon wat met die mikrostruktuur verband hou. Hierin word die domein en geskiktheid van die gebruik van subgrootte eksemplare vir drumpeltoetsing van die vermoeidheidskraakgroeitempo in die lig van die unieke meso-en mikrostruktuurkenmerke van LPBF-vervaardigde Ti-6Al-4V bespreek. Oor die algemeen volg hierdie proefskrif die vereiste baan vir die vestiging van betroubare benaderings tot skadetolerante vermoeidheidslewensduurberaming vir LPBF-vervaardigde Ti-6Al-4V. Dit bied die grondinsigte in die wisselwerking van breukmeganika-meganismes en -beskrywings wat nodig is om vermoeidheidsgedrag betroubaar te modelleer en dra dus by tot die ontwikkeling van raamwerke en filosofieë in LV om te help met die sertifisering van die vermoeidheidsprestasie van LPBF-vervaardigde Ti-6Al-4V-komponente.af_ZA
dc.description.versionDoctorateen_ZA
dc.format.extentxxi, 163 pages : illustrationsen_ZA
dc.identifier.urihttps://scholar.sun.ac.za/handle/10019.1/130452en_ZA
dc.language.isoen_ZAen_ZA
dc.language.isoen_ZAen_ZA
dc.publisherStellenbosch : Stellenbosch Universityen_ZA
dc.rights.holderStellenbosch Universityen_ZA
dc.subject.lcshAdditive manufacturingen_ZA
dc.subject.lcshMaterials -- Fatigueen_ZA
dc.subject.lcshFracture mechanicsen_ZA
dc.subject.lcshPowder bed fusionen_ZA
dc.subject.lcshUCTDen_ZA
dc.titleFracture mechanics-based fatigue life assessment of additively manufactured Ti-6Al-4Ven_ZA
dc.typeThesisen_ZA
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