A unified approach to crack fields and crack propagation

Thesis

Understanding the mechanical behaviour of materials requires detailed characterisation of crack-tip fields and propagation mechanisms, particularly at small length scales. This thesis develops a generalised, field-based methodology for evaluating fracture parameters and propagation mechanisms using full-field techniques including optical DIC, SEM-DIC, and HR-EBSD. A literature review traces the development of fracture mechanics from global, geometry-based fracture toughness concepts to local, field-based approaches, highlighting the limitations of conventional methods for short fatigue cracks and microscale testing. The first study analyses short fatigue cracks in Zircaloy-4 using high-resolution optical DIC, confirming the feasibility of extracting fracture parameters from field data and showing that short fatigue cracks exhibit distinct crack-tip driving force characteristics. The method is then extended to single crystal nickel-alloy, where concurrent SEM-DIC and HR-EBSD capture both displacement and lattice strain fields of a short fatigue crack in situ for the first time, enabling direct evaluation of energy integrals from measured data. Finally, a microscale fracture toughness testing method based on indentation cracking in silicon integrates HR-EBSD strain mapping and energy-based analysis to extract mode-specific stress intensity factors, retaining the practicality of indentation testing while enhancing its interpretability. Overall, the work offers a robust alternative to classical approaches, grounded in full-field local measurements.

Authors: Su, X

• DOI: 10.5287/ora-nbqqbandv
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Keywords: fracture toughness; stress intensity factor; fatigue; in situ; indentation; HR-EBSD; crack; full-field techniques
• Subjects: Alloys--Fatigue; Fatigue; Fracture mechanics