Browsing by Author "Molteno, Matthew Robert"

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    Measuring fracture properties using digital image and volume correlation: decomposing the J-integral for mixed-mode parameters
    (Stellenbosch : Stellenbosch University, 2017-12) Molteno, Matthew Robert; Becker, Thorsten Hermann ; Stellenbosch University. Faculty of Engineering. Dept. of Mechanical and Mechatronic Engineering.
    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 ๐ฝ-integral method was chosen for obtaining the fracture resistance, with the decomposition method used for determining separate ๐ฝ๐ผโˆ’๐ผ๐ผ๐ผ components (opening, sliding and shearing: modes I, II and III), and stress intensity factors (๐พ๐ผโˆ’๐ผ๐ผ๐ผ), 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 ๐ฝ-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 โ€˜hybridโ€™ 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 ๐ฝ-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 ๐ฝ-integral to analytical crack tip fields to which displacement errors are added artificially. It was found that the ๐ฝ-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 ๐ฝ-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 ๐ฝ-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 ๐ฝ based decomposition offers a versatile method to extract ๐พ๐ผโˆ’๐ผ๐ผ๐ผ 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.