Impact of structure and relaxation on fatigue and micromechanical properties of oxide glasses - the role of volatiles and bonding state

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As subcritical crack growth (SCCG) can reduce tensile strength of glasses by many orders of magnitude, the potential for improvement of fatigue behaviour is most intriguing in developing ultra-strong glasses. An essential bottleneck is the basic understanding of the numerous interplaying pressure- temperature- and water- affected relaxation phenomena at the crack tip and related toughening strategies.Therefore, the present proposal aims to advance the basic understanding of structural relaxation effects and local properties caused by increased volatile concentration and tensile stresses at the crack tip. For this sake, glass topology will be designed physically by preparation of glasses of different P-T histories as well as compositionally by incorporation of volatiles (the focus will be on H2O) into the compressed glass structure. Both are expected to have a strong influence on the viscoelastic properties (complex moduli) of the near-tip glass structur.Therefore the project is composed in the first funding period by three interrelated experimental tasks: Task 1) Synthesis of glasses with different volatile contents and different P-T histories and characterization of structural and bulk mechanical properties of the glasses, Task 2) Studies of sub-Tg relaxation processes in these glasses by mechanical spectroscopy, Task 3) Experimental investigation of crack formation and propagation using in situ and ex situ (indentation) techniques. Cooperation within SPP 1594 will provide basis to implement time-dependent viscoelastic properties into theoretical descriptions of fracture mechanics and will deliver sub-Tg onset temperatures for modelling topological constraints down to room temperature. The project aims therefore to advance the basic understanding of the local relaxation mechanism controlling SCCG and micro-mechanical properties in oxide glasses as they are a key for structural toughening designs to develop SCCG-free glasses and glass surfaces.