Measuring fracture properties using digital image and volume correlation: decomposing the -integral for mixed-mode parameters (original) (raw)

The degradation of material properties over time is one of the core constituents of structural integrity monitoring. For this purpose, the materials’ 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 𝐽-integral method was chosen for obtaining the fracture resistance, with the decomposition method used for determining separate 𝐽𝐼−𝐼𝐼𝐼 components (opening, sliding and shearing: modes I, II and III), and stress intensity factors (𝐾𝐼−𝐼𝐼𝐼), 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 𝐽-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 ‘hybrid’ 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 𝐽-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 𝐽-integral to analytical crack tip fields to which displacement errors are added artificially. It was found that the 𝐽-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 𝐽-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 𝐽-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 𝐽 based decomposition offers a versatile method to extract 𝐾𝐼−𝐼𝐼𝐼 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.