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

由于亚临界裂纹扩展（SCCG）会使玻璃的抗拉强度降低许多数量级，因此在开发超高强度玻璃方面，改善疲劳行为的潜力是最令人感兴趣的。一个重要的瓶颈是对裂纹尖端的许多相互作用的压力-温度和水影响的松弛现象和相关的增韧策略的基本理解。因此，本研究旨在促进对裂纹尖端挥发性浓度和拉应力增加引起的结构松弛效应和局部特性的基本理解。为此，将通过制备不同P-T历史的玻璃以及通过将挥发物（重点将放在H2O上）掺入压缩玻璃结构来物理地设计玻璃拓扑结构。两者都有望对近尖端玻璃结构的粘弹性性能（复模量）产生强烈影响。因此，该项目在第一个资助期由三个相互关联的实验任务组成：任务1)合成具有不同挥发分和不同P-T历史的玻璃，并表征玻璃的结构和整体力学性能；任务2)利用机械光谱研究这些玻璃中的亚tg弛豫过程；任务3)使用原位和非原位（压痕）技术研究裂纹形成和扩展的实验研究。SPP 1594的合作将为将随时间变化的粘弹性特性应用到裂缝力学的理论描述中提供基础，并将提供亚tg的起始温度，用于模拟室温下的拓扑约束。因此，该项目旨在促进对氧化玻璃中控制SCCG和微观力学性能的局部松弛机制的基本理解，因为它们是开发无SCCG玻璃和玻璃表面的结构增韧设计的关键。

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.