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

1.1.1项目概述英国目前的先进气体冷却反应堆(AGR)的计划使用年限即将结束。然而，从经济和环境的角度来看，延长核电站寿命(PLEX)都是非常可取的，因为核电站有可能产生大量的无排放电力，而且在不久的将来，新建的电站不太可能与当前的发电能力相匹配。在继续运行被证明是安全的条件下，延长使用寿命是可能的。对PLEX的分析的一部分是确定高温AGR装置中再热裂纹造成的损坏，如锅炉管和集箱，该集箱由316H奥氏体不锈钢制成。这种类型的裂纹是由焊接引起的残余应力驱动的，导致材料蠕变并最终破裂。裂纹扩展是通过实验室蠕变裂纹扩展(CCG)试验来预测的，主要是在紧凑拉伸试件上。目前，对裂纹完全形成后CCG在这些试验中的行为有了相对较好的了解，但对裂纹的萌生阶段知之甚少。来自实验室测试的早期裂纹扩展数据要么被忽视，要么可能被误解，这意味着没有完全确定裂纹的萌生。因此，无论是初期生长还是早期生长都没有得到准确的描述。目的和目的本PHD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。为实现这一目标而制定的目标是：积累和分析进一步的CCG实验数据。试验利用电位降(PD)技术来测量裂纹扩展。在这些测试中将使用新开发的将噪音降至最低的硬件，并将使用改进的方法对数据进行分析，这些方法应该会改进对裂纹萌生的检测。还将使用数字图像相关(DIC)来分析应变场。使用载荷线响应的偏转分区改进C*的实验估计。C*的实验确定取决于蠕变引起的载荷线位移(LLD)速率。使用有限元分析(FEA)中的单轴拉伸数据而不是Ramberg Osgood参数来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*。蠕变和塑性相互依存的特征。虽然挠度划分假定蠕变变形和塑性变形是相互独立的，但其中一个不可避免地会影响另一个的行为。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)是否表征启裂和早期扩展。随后，结合上述三项任务的综合结果，开发综合方法论来描述550℃下316H中裂纹的萌生和早期扩展。

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.