Measuring fracture properties using digital image and volume correlation: decomposing the J-integral for mixed-mode parameters
| dc.contributor.advisor | Becker, Thorsten Hermann | en_ZA |
| dc.contributor.author | Molteno, Matthew Robert | en_ZA |
| dc.contributor.other | Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering. | en_ZA |
| dc.date.accessioned | 2017-11-20T13:19:08Z | |
| dc.date.accessioned | 2017-12-11T10:56:24Z | |
| dc.date.available | 2017-11-20T13:19:08Z | |
| dc.date.available | 2017-12-11T10:56:24Z | |
| dc.date.issued | 2017-12 | |
| dc.description | Thesis (PhD)--Stellenbosch University, 2017. | en_ZA |
| dc.description.abstract | ENGLISH ABSTRACT: The degradation of material properties over time is one of the core constituents of structural integrity monitoring. For this purpose, the materialsโ resistance to brittle fracture is the most widely used property in the determination of safe operation and predicting component life. Measuring such properties by conventional methods presents challenges: large numbers of samples are needed, accounting for mixed-mode loading. Non-contact measurements by digital image correlation shows promise in extracting such parameters from in situ loaded structures, accounting for their complex geometry and loading. This thesis aims to develop a framework that allows for the extraction of mixed-mode brittle fracture properties, using three-dimensional (3D) image correlation techniques: multi-camera Digital Image Correlation (stereo-DIC), and Digital Volume Correlation (DVC). The ๐ฝ-integral method was chosen for obtaining the fracture resistance, with the decomposition method used for determining separate ๐ฝ๐ผโ๐ผ๐ผ๐ผ components (opening, sliding and shearing: modes I, II and III), and stress intensity factors (๐พ๐ผโ๐ผ๐ผ๐ผ), as both methods are resilient to crack tip and displacement errors. The first stage of development is a finite element based method for the removal of experimental displacement artifacts from DIC or DVC results, intended as a pre-processor for the ๐ฝ-integral. Subsequently, a theoretical derivation is provided to link the two volumetric forms: the volume and the path-area integrals. This results in a proposed โhybridโ integral, which benefits from the resilience to crack-front positional errors of the path-area approach, and resilience to random displacement noise of the volume integral. Initial testing of the ๐ฝ-integral based decomposition method was on the surface using stereo-DIC and the Arcan fixture to induce mixed-mode loading. The results are verified with mixed-mode fracture toughness values measured from PMMA and compared to literature and ASTM 1820 tests. The typical image correlation errors close to fracture are quantified by a methodology of applying the ๐ฝ-integral to analytical crack tip fields to which displacement errors are added artificially. It was found that the ๐ฝ-integral is most prone to DIC errors under anti-symmetric (mode II/III) loading. Testing in the volume used X-ray computed tomography to acquire images and DVC for displacement maps. The measurements were verified on two configurations: a SENT polyurethane composite specimen (mode I), and a shear loaded inclined notch in Magnesium alloy-WE43 (mixed-mode). The ๐ฝ-integral was verified against values from finite element fields resulting again in larger errors in mixed-loading. Decomposition of the volume integral requires an approach to separate the anti-symmetric ๐ฝ-integral. Two extensions are proposed: the first using a ratio derived from mode II/III Williams series formulas, and the second using the interaction integral. Both approaches are verified on DVC displacements. This thesis finds that ๐ฝ based decomposition offers a versatile method to extract ๐พ๐ผโ๐ผ๐ผ๐ผ values from non-standard 3D geometries and loadings. However, the pre-processing of fields to minimise errors is essential when mode II-III displacement fields are prominent. Although it is beyond the scope of this thesis, the hope is that this work will assist in the adoption of full-field measurement techniques as a standard testing practice in structural integrity assessments, and lead to better informed maintenance and inspection schedules. | en_ZA |
| dc.description.abstract | AFRIKAANSE OPSOMMING: Die agteruitgaan van material-eienskappe met tyd is een van die kernkomponente van strukturele integriteit monitering. Vir hierdie doel is die materiaal se weerstand teen brosfraktuur die mees algemene gebruikte eienskap vir die bepaling van veilige werking en die voorspel van die komponent se leeftyd. Die meet van sulke eienskappe volgens konvensionele metodes bied uitdagings, bv. groot getalle monsters word benodig en komplekse kragte moet in ag geneem word. Nie-kontakmetings deur digitale-beeldkorrelasie toon belofte om sulke parameters uit in situ gelaaide strukture te onttrek, terwyl komplekse kragte en material vorme in ag geneem word. Hierdie proefskrif beoog om 'n raamwerk te ontwikkel wat die ontginning van gemengde-modus brosfraktuur-eienskappe moontlik maak deur gebruik te maak van driedimensionele (3D) beeldkorrelasietegnieke: Digitale Kamera-Korrelasie (stereo-DKK) en Digitale Volume Korrelasie (DVK). Die ๐ฝ-integrale metode is gekies vir die verkryging van die breukweerstand, met behulp van die ontbindingsmetode vir die bepaling van afsonderlike ๐ฝ๐ผโ๐ผ๐ผ๐ผ komponente (opening, gly en skeer: mode I, II en III) en stresintensiteitsfaktore (๐พ๐ผโ๐ผ๐ผ๐ผ), aangesien beide metodes skerm teen kraakpunt en verplasings foute. Die eerste fase van ontwikkeling is 'n eindige element gebaseerde metode vir die verwydering van eksperimentele verplasing artefakte van DKK of DVK resultate, wat bedoel is as 'n voorverwerker vir die ๐ฝ-integraal. Vervolgens word 'n teoretiese afleiding verskaf om die twee volumetriese vorms te verbind, naamlik die volume en pad-area integrale. Dit lei tot 'n voorgestelde 'hibriede'-integraal. Hierdie โhibriedeโ-integraal vereis โn kompromie tussen die robuustheid van die pad-area-benaderingsfout van die pad-area-benadering en robuustheid van lukrake verplasings geraas van die volume-integraal. Aanvanklike toetsing van die ๐ฝ-integraal gebaseerde ontbindings metode is op die oppervlak met behulp van stereo-DKK, en die Arcan greep om gemengde-mode laai te veroorsaak. Die resultate word geverifieer met gemengde-mode-fraktuur taaiheid waardes, gemeet vanaf PMMA en baseer op literatuur sowel as die ASTM 1820 toetse. Die tipiese beeldkorrelasie foute naby aan fraktuur word gekwantifiseer deur die toepassing van die ๐ฝ-integraal op analitiese kraakpuntvelde waarby verplasingsfoute kunsmatig bygevoeg word. Daar is bevind dat die ๐ฝ-integraal die meeste geneig is tot DKK foute onder asimmetriese (mode II/III) laai. Toets in die volume gebruik X-straal-rekenaartomografie om beelde en DVK vir verplasings kartering te bekom. Die metings is geverifieer op twee konfigurasies: 'n SENT saamgestelde-polyuretaan monster (mode I) en 'n skuinsbelaaide spleet in magnesium-allooi, WE43 (gemengde mode). Die ๐ฝ-integraal is geverifieer teen waardes van eindige elementvelde wat weer groter foute in gemengde laai veroorsaak. Ontleding van die volume-integraal vereis 'n benadering om die anti-simmetriese ๐ฝ-integraal te skei. Twee uitbreidings word voorgestel: die eerste met behulp van 'n verhouding wat afgelei word vanaf mode II / III Williams reeks formules, en die tweede maak gebruik van die interaksie integraal. Albei benaderings word geverifieer op DVK-verplasings. Hierdie proefskrif bevind dat ๐ฝ gebaseerde ontbinding 'n veelsydige metode bied om ๐พ๐ผโ๐ผ๐ผ๐ผ waardes uit nie-standaard 3D-geometrieรซ en ladings te onttrek. Die voorbereiding van velde om foute te beperk, is egter noodsaaklik wanneer die mode II-III verplasingsvelde prominent is. Alhoewel dit buite die omvang van hierdie proefskrif is, is die hoop dat hierdie werk sal help met die aanvaarding van volwaardige metingstegnieke as 'n standaard toetspraktyk in strukturele integriteit assesserings, en lei tot beter ingeligte onderhouds en inspeksieskedules. | af_ZA |
| dc.format.extent | 169 pages : illustrations | en_ZA |
| dc.identifier.uri | http://hdl.handle.net/10019.1/102804 | |
| dc.language.iso | en_ZA | en_ZA |
| dc.publisher | Stellenbosch : Stellenbosch University | en_ZA |
| dc.rights.holder | Stellenbosch University | en_ZA |
| dc.subject | Decomposition method | en_ZA |
| dc.subject | Fracture of materials -- Measurement | en_ZA |
| dc.subject | Digital images | en_ZA |
| dc.subject | Digital image correlation | en_ZA |
| dc.subject | UCTD | en_ZA |
| dc.title | Measuring fracture properties using digital image and volume correlation: decomposing the J-integral for mixed-mode parameters | en_ZA |
| dc.type | Thesis | en_ZA |