Mixed-mode fracture: Combination of Arcan fixture and stereo-DIC

To characterise mixed-mode fracture, including mode $III$, a combination of Arcan fixture, that offers both in-plane and out-of-plane loading, and stereo digital image correlation (DIC) has proven valuable for measuring in-plane and out-of-plane surface displacement and deformation fields while allowing for direct stress intensity factor (SIF) extraction throughout the tests, but it comes with many caveats. Using stereo DIC, the mixed mode fracture behaviour of PMMA is analysed using a novel mode decomposition technique that combines experimental and analytical approaches. This technique divides the measured displacement field from digital image correlation into three distinct components: a symmetric field ($u^I$), an in-plane anti-symmetric field ($u^{\mathrm{II}}$), and an out-of-plane antisymmetric field ($u^{\mathrm{III}}$), decomposed by a difference operation on the reflected and non-reflected fields about the crack plane. Then, the strain energy release rate of the crack is calculated for each loading mode using finite elements without knowledge of the sample geometry or nominal loading condition. Our work revealed parasitic loading modes induced by the Archan fixture plus other intrinsic sources related to load accommodation. Nevertheless, the calculated SIFs measured during stable crack growth were normalised using different fracture toughness ($K_{\mathrm{IC}}$) values with $K_{\mathrm{IC}}$ of 1.82 ± 0.32 MPa m^0.5, providing the best fit to the fracture loci with an R² of 0.95. This value agrees well with the ASTM-obtained value of 1.7 MPa m^0.5.