Characterisation of short crack growth behaviour in a notch stress field under varying block loading cycles in steam turbine blade material

对蒸汽涡轮机机群中的涡轮机叶片部件的精确疲劳寿命的信心对发电公司具有重要意义。公司能够安全地延长运行寿命的程度是规划和调试新电站建设以及优化蒸汽涡轮机（燃煤）机组退役时间的关键输入。这些涡轮机叶片的疲劳行为受应力集中特征（如枞树形根部固定件）中小缺陷的产生和增长的控制。了解这些缺陷在代表性服务条件（包括过载）下保持服务的叶片和圆盘中的增长速度现在是一个关键要求。这些部件疲劳寿命的挑战之一是缺乏从蒸汽涡轮机叶片中使用的实际材料（处于其当前老化状态）获得的特定疲劳裂纹扩展数据，以及了解服务相关过载对该行为的影响。过载效应已被认为是在临界裂纹尺寸分析，但迄今为止，在裂纹扩展阶段。一个更清楚的理解是缺乏限制过载的相对/竞争的影响，在这些较低水平的过载有益的裂纹尺寸评估与可能减少有益的裂纹阻滞机制方面的好处。裂纹增长率评估将建立裂纹萌生和增长的早期阶段的半椭圆形表面断裂裂纹在一个特征缺口应力场。这些数据将通过间断试验和缺口根部裂纹演化的复制跟踪来收集。所收集的数据将明确评估裂纹通过塑性变形缺口应力场扩展的行为。图1c显示了一个典型的表面断裂裂纹，该裂纹起源于类似回火马氏体钢的纵梁。由于可再生能源电力不可避免地向电网提供可变的电力供应，汽轮机的部分负荷和相关的启停周期增加，这给疲劳评估带来了新的挑战。我们还将探索使用涂层修复涡轮机叶片表面的修复和疲劳缓解策略，并评估涂层如何影响疲劳裂纹的萌生和扩展

Confidence in accurate fatigue lifing of turbine blade components in the steam turbine fleet has significant implications for power generating companies. How much the company can safely extend the operating life is a critical input in the planning and commissioning of new power station build and in optimizing the timing of retiring the steam turbine (coal-fired) fleet. The fatigue behavior of these turbine blades is controlled by the initiation and growth of small defects in stress concentration features such as the fir-tree root fixings. Knowing how fast these defects will grow in blades and discs that remain in service under representative service conditions (including overloads) is now a critical requirement. One of the challenges in fatigue lifing these components is the lack of specific fatigue crack growth data obtained from the actual materials (in their current aged state) used in the steam turbine blades and to understand the implications of service relevant overloads on this behaviour. Overload effects have been considered in the critical crack size analysis but not, to date, in the crack propagation phase. A clearer understanding is lacking of the relative/competing effects of limiting the overloads in terms of the benefit to critical crack size evaluations versus the possible reduction in beneficial crack retardation mechanisms at these lower levels of overload.Crack growth rate evaluations will establish crack initiation and early stages of growth for semi-elliptical surface breaking cracks in a characteristic notch stress field. This data will be gathered using interrupted tests and replica tracking of the crack evolution in the notch root. The data gathered will explicitly evaluate the behavior of cracks growing through a plastically deforming notch stress field. Figure 1c shows a typical surface breaking crack that has initiated from a stringer in a similar tempered martensitic steel. Increased part-loads and the associated stop-start cycles on steam turbines due to the inevitable variable supply of renewables-based power to the grid offer new challenges to fatigue evaluations. We will also explore repair and fatigue mitigation strategies using coatings to repair turbine blade surfaces and evaluate how the coatings affect the initiation and growth of fatigue cracks