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Title: Residual stresses, cracking and stress intensity factors for Vickers indentations in ceramics
Author: Sun, Xiaodan
ISNI:       0000 0004 6352 9550
Awarding Body: University of Oxford
Current Institution: University of Oxford
Date of Award: 2015
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Vickers hardness indentations on AI2O3 were studied comprehensively. Beneath indentation, there is a well-defined interface between the plastic zone and the elastic region and the radius of the plastic zone is close to half the indentation diagonal length. The residual stress field around the indentations has been measured by optical fluorescence microscope (FLM) scans. Yoffe's stress model predicted higher stresses than the experimental stress results. the radial crack lengths were measured by FLM scans, and the optical microscope (OM) and SEM methods for crack length measurements were found to significantly underestimate the crack lengths. Post indentation slow crack growth was found on the radial cracks were determined. The radial-median crack system was found for alumina indentations. Lateral cracks were also found for all the indentations, and their depths were close to the half diagonal lengths. the lateral cracks joined the tips of the radial cracks and had the similar growth rates to the radial cracks. The indentation parameters of SiC and 3Y-TZP indentations were also measured. Lateral cracking was observed beneath in SiC indentations but not in case of 3Y-TZP. The depth of the lateral crack of the SiC indentation was found to be close to the half diagonal length. the residual stresses and surface profiles around SiC and 3Y-TZP indentations were measured. the lateral crack lengths of the SiC determined from the surface profiles were close to the radial crack lengths. For TZP indentations, the highest volume fraction of the monoclinic zirconia phase was found near the edge of the indentations. Surface uplifts around indentations were found to have the profiles matching with distributions of the volume fraction of the monoclinic phase. A new Vickers indentation residual stress model has been presented using assumptions based on the experimental findings on alumina indentations. The main novelty of the model is that it predicts the stresses in the presence of cracking, and is therefore testable. The residual stress model has successfully predicted the stress fields around the polycrystalline alumina and SiC indentations but failed on TZP indentations because of the lack of lateral cracking and the tetragonal-to-monoclinic phase transformation. From the residual stress model, a new formula for the indentation stress intensity factor K was derived, which provided good results for threshold stress intensity factor, K10, on alumina, and fracture toughness, KIC, for SiC, and Si3N4. Three adjustable parameters can be added to strengthen existing model.
Supervisor: Todd, Richard Sponsor: China Scholarship Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available