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

汽轮机叶片部件疲劳寿命的准确性对发电企业具有重要意义。在新电站建设的规划和调试以及优化退役蒸汽轮机（燃煤）机组的时间方面，公司能够安全地延长运行寿命的程度是一个关键因素。这些涡轮叶片的疲劳行为是由应力集中特征（如杉树根部固定）中的小缺陷的产生和生长控制的。了解这些缺陷在典型服务条件下（包括过载）仍在使用的叶片和圆盘中的生长速度是现在的关键要求。这些部件的疲劳寿命面临的挑战之一是缺乏从汽轮机叶片中使用的实际材料（在其当前老化状态下）获得的特定疲劳裂纹扩展数据，以及了解服务相关过载对这种行为的影响。在临界裂纹尺寸分析中考虑了过载效应，但迄今为止还没有在裂纹扩展阶段考虑过载效应。对于限制超载对临界裂纹尺寸评估的好处与在这些较低超载水平下有益的裂纹延迟机制的可能减少的相对/竞争效应，缺乏更清晰的理解。裂纹扩展速率评价将确定在特征缺口应力场中半椭圆表面断裂裂纹的起裂和早期扩展阶段。这些数据将通过中断测试和缺口根部裂纹演变的复制跟踪来收集。收集的数据将明确地评估裂纹通过塑性变形缺口应力场生长的行为。图1c显示了类似回火马氏体钢中由弦条引起的典型表面断裂裂纹。由于不可避免地向电网提供可变的可再生能源电力，汽轮机部分负荷的增加和相关的启停周期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产生和增长

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