Environmentally induced damage propagation with localised stresses in CMCs

罗尔斯·罗伊斯高温复合材料公司（RRHTC）生产的SiCf/SiC陶瓷基复合材料（CMC）的现有变体含有基体添加剂，通过形成低粘度高氧渗透率的硼硅酸盐玻璃来控制氧化降解。这些cmc在高温下进行内部氧化，然而这种内部氧化分为两种状态——低温“害虫”状态和高温“自我修复”状态。同样，蒸汽中的高温氧化通过氢氧化硅的挥发导致CMC的表面衰退。该计划将研究CMC的降解机制，并研究温度、湿度、压力（水蒸气的总压力和分压）和局部机械应力区域的变量如何相互作用，以影响内部脆化和/或表面氧化/衰退，从而影响CMC的寿命。具体目标包括：1。量化/限定水蒸汽环境中冷却孔应力升高特征的影响。2. 蒸汽疲劳试样的详细断口特征。3. 探索先进的表征技术，如X-CT和蒸汽环境中的破坏性监测方法，无论是原位还是预暴露。确定环境屏障涂层（EBC）在应力集中时对环境破坏的影响。罗尔斯·罗伊斯和斯旺西大学将合作设计一个测试配置，以研究具有代表性几何冷却孔的CMC材料降解。建议使用熔炉将空心CMC管放置在相当于发动机服务的温度下，并使冷却空气（含有水蒸气的分压）流过管。目的是改变条件，如温度和湿度水平，冷却空气压力和管道的制造，看看哪些变量在特定的时间内导致最大的衰退率和/或脆化变化。罗尔斯·罗伊斯将在需要时向学生提供以前的测试数据和样品，以补充学生评估的新样品。应进行蒸汽疲劳（平面内和层间）试验，以研究局部应力下环境引起的损伤传播。测试将在一系列条件下进行分析，如预曝光、试样几何形状、kt因素、负载与应变控制；以及环境屏障涂料的影响。还将采取协调一致的努力来实施损害监测技术，例如电阻、声发射和图像相关，以进一步了解蒸汽疲劳结果。cmc的未来应用重点是在航空和陆基燃气轮机的高温阶段。在这项研究中开发的信息和方法将为未来CMC材料的发展和寿命技术提供信息。了解各种湿度、压力和温度在一系列测试环境（时间相关、疲劳）下的高度局部应力下产生的损伤机制和影响，有助于发挥这些材料的潜力。

Key Objectives and AimsCurrent variants of SiCf/SiC ceramic matrix composites (CMC) produced by Rolls-Royce High Temperature Composites (RRHTC) contain matrix additions that control oxidation degradation via the formation of a low viscosity high oxygen permeability borosilicate glass. These CMCs undergo internal oxidation at elevated temperatures, however this internal oxidation is split into two regimes - a lower temperature 'pesting' regime and a high temperature 'self-healing' regime. Similarly, high temperature oxidation in steam results in surface recession of the CMC by volatilisation of silicon hydroxide species. This programme will investigate degradation mechanisms of CMCs and examine how the variables of temperature, humidity, pressure (total and partial pressure of water vapour) and areas of localised mechanical stress interact to affect internal embrittlement and/or surface oxidation/recession, and hence CMC lifing. Specific objectives include:1. Quantify/qualify the influence of cooling hole stress raising features in a water vapour environment. 2. Detailed fractography characterisation of steam fatigue specimens. 3. Exploration of advanced characterisation techniques such as X-CT and damaging monitoring methods in steam environments, whether in-situ or pre-exposed.4. Determine the influence of an environmental barrier coating (EBC) on environmental damage at stress concentrations.Novel MethodologiesRolls-Royce and Swansea University will collaborate together to design an implement a test configuration to investigate CMC material degradation with representative geometrical cooling holes. It is proposed that a furnace will be used to place hollow CMC tubes at temperatures equivalent to engine service along with a flow of cooling air (containing a partial pressure of water vapour) through the tube. The aim is to vary the conditions such as temperature and humidity levels, the cooling air pressure and the manufacture of the tubes to see which variables cause the greatest changes in recession rates and/or embrittlement over specific time durations. Previous test data and specimens will be provided by Rolls-Royce to the student when it is applicable to supplement the new specimens evaluated by the student.Steam-fatigue (in-plane and interlaminar) testing shall be carried out to investigate environmentally induced damage propagation with localised stresses. Tests will be analysed over a range of conditions such as pre-exposure, specimen geometries, kt factors, load vs. strain control; as well as the influence of environmental barrier coatings. A coordinated effort will also be pursued to implement damage monitoring techniques, such as electric resistance, acoustic emission and image correlation to further the understanding of the steam-fatigue results.ExploitationCMCs are focussed towards future applications in the high temperature stages of aero and land-based gas turbines. The information and methods developed in this research will inform future CMC material developments and lifing techniques. Understanding the resulting damage mechanisms and effects from various humidity's, pressures and temperatures at highly localised stresses in a range of testing environments (time-dependent, fatigue) can help reach the potential of these materials.