Measuring mechanical properties using digital image correlation: extracting tensile and fracture properties from a single sample.

Huchzermeyer, Richard Lynn (2017-12)

Thesis (MEng)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: Multiple material properties are required to perform structural integrity assessments and reliability estimates on in-service equipment. Conventional material characterization testing approaches do not cater towards the testing of inservice equipment, and therefore ‘near-non-destructive testing’ approaches, in particular the small punch test (SPT), are preferred. The SPT, while capable of determining multiple material properties from a single small sample, does have limitations both in terms the complexity in analysing the resulting data, and the accuracy of the measured properties. These limitations may be addressed through full-field surface displacement analysis techniques facilitated by digital image correlation (DIC). A combined approach to extracting multiple material properties from in-plane (two-dimensional, 2D) surface displacements, measured on a single sample through DIC, has been developed assuming an isotropic linear elastic material. This approach utilizes the virtual fields method (VFM) to obtain Young’s modulus (E) and Poisson’s ratio (v). These tensile stiffness properties (E and v) are in turn input to a non-linear least squares field fitting approach (FF), which is used to obtain the critical stress intensity factor (𝐾𝑓𝑓) associated with a crack or notch in a material. The VFM and FF are applied to two compressively loaded disk shaped sample geometries (containing central notches) as well as an elongated half compact tension sample geometry (W = 25 mm) manufactured from 6 mm thick polymethyl methacrylate (PMMA). The experimental methodology to obtain suitable two-dimensional surface displacement measurements though DIC is described. Furthermore, the implementation of the VFM and the FF is developed on a sample specific basis. Through a comparison to properties determined using standardized ASTM testing, a relative error for the VFM of -1.5 % to 4.6 % in E and 12.9 % to 40.2 % in v is obtained. A concomitant relative error in the FF is determined to be 33 % to 38 % for 𝐾𝑓𝑓. Experimental errors, in particular out of plane rotation, are identified and the limitations of the assumptions made in applying the techniques are examined. Furthermore, the manner in which the error in E and v obtained through the VFM contributes to the associated error in 𝐾𝑓𝑓 identified through the FF is examined. It is found that the FF approach is less sensitive to error in v provided that the error in E is small. The nominally successful combined application of the VFM and the FF to 2D displacement fields measured with DIC on a single sample (assuming a linear elastic isotropic material), strongly motivates for the further development of this approach. The approach could be extended to accommodate out-of-plane deformations measured through DIC and could be developed to extract properties from ductile metallic materials. It is envisioned that this will be addressed in future work, which could lead to the methodology being applied directly to the SPT. A significant first step towards this is presented in this work, which demonstrates the first successful combined application of the VFM and the FF for extracting stiffness and fracture properties from full-field in-plane 2D displacements measured through DIC on a single sample.

AFRIKAANSE OPSOMMING: Veelvuldige materiaaleienskappe word vereis om strukturele integriteitsassesserings en betroubaarheidsberamings uit te voer op toerusting wat reeds in diens is. Konvensionele materiaaleiensskapstoetse is nie geskik vir toerusting wat reeds in diens is nie. Dus word daar gebruik gemaak van “near non-destructive” benaderings in die toetse, en spesifiek word die “small punch test” (SPT) verkies. Die SPT, hoewel in staat om veelvuldige materiaaleienskappe te bepaal vanuit ’n klein monster, het beperkings beide in terme van die kompleksitiet in die analise van die data, en ook die akkuraatheid van die gemete eienskappe. Hierdie beperkings kan aangespreek word deur vol-veld oppervlakverplasing analise tegnieke gefasiliteer deur digitale beeld korrelasie (“digital image correlation” (DIC)). ’n Gesamentlike benadering tot die afleiding van verskeie materiaaleienskappe vanaf in-vlak (twee-dimensionele, 2D) oppervlak verplasings, gemeet op ’n enkele monster deur DIC, is ontwikkel met die veronderstelling van ’n isotropiese lineêrelastiese materiaal. Hierdie benadering gebruik die virtuele veld-metode (“virtual field method” (VFM)) om Young se modulus (E) en Poisson se verhouding (v) te bepaal. E en v is insette tot ’n nie-lineêre “least squares field fitting approach” (FF), wat gebruik word om die kritiese spannings-intensiteitsfaktor (𝐾𝑓𝑓), geassosieer met ’n kraak of keep in die materiaal, te bepaal. VFM en FF word toegepas op twee tipes samedrukkend-gelaaide skyfvormige monsters (wat sentrale kepe bevat) sowel as ’n verlengde halwe kompak-spanning monster vervaardig van 6 mm dikte polymethyl methacrylate (PMMA). Die eksperimentele metodologie om die mees geskikte 2D oppervlakverplasingsmetings deur DIC te verkry word beskryf. Ook word die inwerkingstelling van die VFM en die FF ontwikkel op ’n monster-spesifieke basis. ’n Relatiewe fout-faktor vir VFM van -1.5 % tot 4.6 % in E en 12.9 % tot 40.2 % in v is verkry deur ’n vergelyking met eienskappe bepaal deur standaard ASTM toetsing. ’n Begeleidende relatiewe fout-faktor in FF is vasgestel op 33 % tot 38 % vir 𝐾𝑓𝑓. Eksperimentele foute, in besonder uit-vlak rotasie, word geidentifiseer en die beperkings van die aannames gemaak tydens die toepassing van die tegnieke word ondersoek. Verder word die die bydrag van die foute in E en v, gevind deur VFM, tot die begeleidende fout in 𝐾𝑓𝑓, soos geidentifiseer deur FF, ondersoek. Daar is gevind dat die FF benadering minder sensitief is vir foute in v wanneer die fout in E klein is. Die nominaal suksesvol gekombineerde toepassing van die VFM en die FF tot 2D verplasingsvelde soos gemeet met die DIC op ’n enkele monster (met die veronderstelling van ’n lineêr-elastiese isotropiese materiaal) ten sterkste motiveer vir verdere ontwikkeling van hierdie benadering. Die benadering kan ook uitgebrei word na uit-vlak vervormings gemeet deur DIC en kan ontwikkel word om eienskappe te onttrek van vervormbare metaal materiale. Daar word voorgestel dat dit aangespreek word in toekomstige navorsing, wat kan lei daartoe dat die metodologie direk toegepas word op die SPT. ’n Beduidende eerste stap hiervoor word in hierdie tesis voorgelê. Hierdie tesis beskryf die eerste suksesvol gekombineerde toepassing van die VFM en FF vir die bepaling van styfheid en breuk eienskappe vanaf die vol-veld in-vlak 2D verplasings gemeet deur DIC op ’n enkele monster.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/102805
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