Analysis and Interpretation of Early Stage Creep Crack Growth Behaviour in Type 316H Stainless Steel

1.1.1项目概要英国现有的先进气冷堆（AGR）即将达到其计划使用寿命的终点。然而，从经济和环境的角度来看，电厂寿命延长（PLEX）是非常可取的，因为核电厂有潜力产生大量的无排放电力，而且在不久的将来，新建造的核电厂不太可能能够与当前的发电能力相匹配。对PLEX分析的一部分贡献是确定高温AGR装置中再热裂纹所造成的损坏，如316H奥氏体不锈钢锅炉管和集箱。这种类型的开裂是由焊接产生的残余应力驱动的，残余应力导致材料蠕变并最终开裂。通过实验室蠕变裂纹扩展（CCG）测试，主要是紧凑的拉伸试样的裂纹扩展进行预测。目前，一旦裂纹完全形成，对CCG在这些测试中的行为有相对较好的理解，但对裂纹的起始阶段知之甚少。来自实验室试验的早期裂纹扩展数据要么被忽略，要么可能被误解，这意味着裂纹的萌生没有完全确定。因此，起始和早期生长均未准确表征。本博士学位的主要目的是进一步了解蠕变裂纹扩展的早期阶段，并试图将其与断裂力学参数C* 或Ct相关联。为实现这一目标而设定的目标是：积累和分析进一步的实验CCG数据。测试利用电位降（PD）技术测量裂纹扩展。新开发的硬件，最大限度地减少噪音将被用于这些测试，数据将使用修改后的方法进行分析，应改善裂纹萌生的检测。数字图像相关（DIC）也将用于分析应变场。使用载荷线响应的挠度划分改进C* 的实验估计。C* 的实验测定取决于蠕变引起的载荷线位移（LLD）速率。在有限元分析（FEA）中使用单轴拉伸数据而不是兰贝格奥斯古德参数划分载荷线响应可以提高蠕变贡献的准确性，从而更好地估计C*。蠕变和塑性相互关系的表征。虽然挠度分配假定蠕变和塑性引起的变形是相互独立的，但一个不可避免地影响另一个的行为。因此，有必要确定蠕变变形对塑性的影响程度，反之亦然。确定裂纹尖端参数（如C* 或Ct）是否表征裂纹萌生和早期扩展。随后，结合上述三项任务的综合研究结果，开发综合方法来描述316 H在550 ℃下的萌生和早期裂纹扩展。

1.1.1 Project SummaryThe current fleet of Advanced Gas Cooled Reactors (AGRs) in the UK is reaching the end of its planned service life. However, plant life extension (PLEX) is highly desirable both from an economic and environmental perspective as nuclear plants have the potential to generate large amounts of emissions free power, and it is unlikely that new build stations will be able to match the current generation capacity in the near future. This life extension is possible on the condition that the continued operation is proved to be safe.Part of the analysis contributing to PLEX is determination of the damage caused by reheat cracking in high temperature AGR plant such as boiler tubes and headers which is made of 316H austenitic stainless steel. This type of cracking is driven by residual stresses as a result of welds which cause the material to creep and eventually crack. The crack growth is predicted through laboratory Creep Crack Growth (CCG) tests, predominantly on Compact Tension specimens. Currently there is a relatively good understanding of the behaviour of CCG in these tests once the crack is fully formed, but much less is known about the initiation stage of the crack. Early stage crack growth data from laboratory tests is either disregarded or likely misinterpreted, meaning that the initiation of a crack is not fully determined. Therefore neither initiation nor early stage growth is accurately characterised. Aims and ObjectivesThe main aim of this PhD is to further understand the early stages of creep crack growth and to attempt to correlate it with a fracture mechanics parameter, either C* or Ct. The objectives set out in order to achieve this are: Accumulate and analyse further experimental CCG data. Tests make use of the Potential Drop (PD) technique for measurement of crack growth. Newly developed hardware which minimises noise will be used in these tests, and the data will be analysed using modified methods which should improve detection of crack initiation. Digital Image Correlation (DIC) will also be used to analyse strain fields. Improve experimental estimate of C* using deflection partitioning of load line response. Experimental determination of C* depends on the load line displacement (LLD) rate due to creep. Partitioning the load line response using uniaxial tensile data in Finite Element Analysis (FEA) as opposed to Ramberg Osgood parameters can improve the accuracy of the creep contribution, leading to better estimates of C*. Characterisation of creep and plasticity interdependency. Although deflection partitioning assumes that deformation due to creep and plasticity are independent of each other, one inevitably affects the behaviour of the other. As such it is necessary to determine to what extent creep deformation affects plasticity and vice versa. Determine whether crack tip parameter such as C* or Ct characterises initiation and early stage growth. Subsequently develop comprehensive methodology to describe initiation and early stage crack growth in 316H at 550degreesC incorporating combined findings from above three tasks.