Studying Transformation Toughening in Zirconia Polycrystals and Fracture in Polymeric Fibre Composites Using Image-based Strain Measurement Techniques

Abstract

This research aims to improve digital image correlation (DIC) to accurately measure microscale heterogeneous material deformations that are hard to reveal using the traditional subset-based DIC. Using different computer vision techniques, substantial improvement was achieved to overcome challenges in applying DIC. The first challenge is when the DIC is applied to polymeric composites. High errors have resulted in hiding the strains at the interface region and distorted rest of the field. The second challenge is when the DIC is applied to fine-grained polycrystals with a size of less than 5 Microns. The strain results are homogenised across grain boundaries, which limits the DIC application to study the crystal plasticity. Also, the grain boundaries are covered by the speckle pattern, which hinders studying boundaries-related mechanics. A technique was developed to address the first challenge, which is related to composites applications, called "phase-specific DIC". This technique revealed strain results at the phase interfaces with acceptable accuracy and outputted a higher accuracy strain field in the phases with high strain gradients. It relies on defining the phase boundaries using one of computer vision techniques. Then, calculation masks divide the image field into homogeneous fields, so high-order DIC is applied to each. Using this technique, a study was conducted to measure an in-situ interfacial strain in a CNT-grafted fibre polymeric composite. The interfacial strains were revealed to show the interfaces' exact debonded parts. The direction and intensity of the interfacial strains explained the mechanism behind the interfacial shear strength enhancement in grafted fibre composites, which was observed in the literature experiments. When the matrix starts to debond, the CNT forest plays a vital role in transferring the load through and around the fibre that is grafted to it, utilising the CNT-fibre remaining bond and the enhanced matrix's stiffness by the CNT forests. This mechanism was observed to be active even after the fibre was fully debonded from the matrix. Also, it was found that the CNT stiffens the matrix against tensile loads but softens it against compressive loads due to the nanotubes microbuckling inside the matrix. A novel technique was developed to address the second challenge, which is related to fine-grained polycrystalline applications, called "grain correlation". The technique does not rely on a speckle pattern. Instead, it correlates the grains by defining their boundaries rather than subsets of image pixels. It generates a grain-specific strain field, revealing the anisotropy inherent in polycrystals. The correlation is calculated between the centroids of each grain in the reference and deformed images as a point set registration problem, which is a computer vision technique. Using this technique, a study was conducted to investigate the strain distribution in Zirconia-based ceramic during transformation toughening. A 3-point bending test was conducted in an SEM machine, during which in-situ imaging and Electron Backscattered Diffraction (EBSD) were done. The strain field was measured for each grain using the developed technique, and stresses were calculated. It was observed that the transformation occurred just before microcracking, which shows that the microcracking is a subsequent mechanism after transformation. Also, a grain of product phase bridged the crack due to its higher toughness, which increased the transformation area locally, demonstrating the synergy between the two toughening mechanisms. An FE model was developed so the transformation effect on the strain field was revealed. It was shown that the transformation significantly increases the heterogeneity that aggravates the strain mismatch between grains, which explains the requirement to have a tougher product phase for the transformation to positively affect the fracture toughness of the ceramic. Show more