Analysis and Interpretation of Early Stage Creep Crack Growth Behaviour in Type 316H Stainless Steel
Lead Research Organisation:
Imperial College London
Department Name: Mechanical Engineering
Abstract
1.1.1 Project Summary
The 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 Objectives
The 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.
The 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 Objectives
The 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.
People |
ORCID iD |
| Catrin Davies (Primary Supervisor) | |
| Michael Jones (Student) |
Studentship Projects
| Project Reference | Relationship | Related To | Start | End | Student Name |
|---|---|---|---|---|---|
| EP/N509486/1 | 01/10/2016 | 31/03/2022 | |||
| 2067508 | Studentship | EP/N509486/1 | 21/10/2017 | 30/09/2021 | Michael Jones |
| Description | Developed a new method for analysing data from a creep crack growth test making use of experimental tensile data to model the material behaviour which can be implemented in numerical crack growth simulations. Determined that experimental testing of notched bars for determining the failure strain vs triaxiality relationship of an alloy is subject to significant inaccuracy. Developed a new method for measuring creep strain of engineering alloys. |
| Exploitation Route | Updating test standards for both creep crack growth and creep/creep-rupture testing. |
| Sectors | Energy |
| URL | https://www.sciencedirect.com/science/article/pii/S0013794419305855 |
| Description | Findings will be used by EDF Energy. |
| First Year Of Impact | 2018 |
| Sector | Energy |
| Title | Advancements to load line displacement rate partitioning method for creep crack growth testing |
| Description | A numerically based method to partition load line displacements (LLD) into elastic, plastic and creep contributions during experimental creep crack growth (CCG) tests has been newly established. This enables, for the first time, the plastic (and subsequently creep) contribution to the load line displacement to be accurately estimated and stress relaxation effects to be considered. This has transformed the CCG test data analysis method, particularly for stainless steels, such that more accurate experimental CCG rate laws are now determined. These CCG rate laws are used by industry to predict the remnant life of their high temperature components. This is particularly important for UK nuclear power industry, which relies on such predictions to develop the safety cases required for life extension of their aging plant. Life extension is essential in the short term to maintain the UK's energy supply (as noted above). The outcomes of this work, performed in collaboration with EDF Energy, contributes to the development of the UK defect assent procedure R5. |
| Type Of Material | Improvements to research infrastructure |
| Year Produced | 2020 |
| Provided To Others? | Yes |
| Impact | This revolutionises how creep crack growth tests should be analysed. |
| Description | EDF Energy High Temperature Centre |
| Organisation | EDF Energy |
| Department | EDF Energy Nuclear Generation |
| Country | United Kingdom |
| Sector | Private |
| PI Contribution | Expertise and facilities for carrying out research, experiments and models contributing to the award. |
| Collaborator Contribution | Provide industrial experience and access to industrial data. |
| Impact | See publications. Two conference publications and one journal publication to date. |
| Start Year | 2017 |
| Description | In2scienceUK online mentoring sessions |
| Form Of Engagement Activity | A talk or presentation |
| Part Of Official Scheme? | No |
| Geographic Reach | Regional |
| Primary Audience | Schools |
| Results and Impact | Carried out two mentoring sessions for year 12 students. In the first session they had the opportunity to ask questions about university and studying engineering/physics. In the second session I gave a talk on engineering reliability, focusing on case studies of the two shuttle disasters and the Chernobyl and Fukushima disasters. This hopefully gave the students a better understanding of engineering. I also had further discussions with one of the students about his choice of where to apply to university. |
| Year(s) Of Engagement Activity | 2020 |
| Description | in2scienceUK Placement |
| Form Of Engagement Activity | Participation in an activity, workshop or similar |
| Part Of Official Scheme? | No |
| Geographic Reach | Local |
| Primary Audience | Schools |
| Results and Impact | I hosted two year 12 students as a part of the in2scienceUK scheme, which aims to empower students from disadvantaged backgrounds to achieve their potential and progress to STEM and research careers. For our first practical activity we welded together bars of chocolate to find out how much the load bearing capacity of the material in bending could be improved using different geometries for the cross section. During the rest of the week we looked at a number of different lab techniques for testing the material behaviour of various steel and aluminium alloys including uniaxial tensile, bend, charpy and fracture toughness, as well as polishing and etching samples to analyse their microstructure. In addition to the experimental beam bend testing we also created an FEA model from scratch of the bend test to obtain values for the maximum axial stress by means of a numerical method, and analysed the scenario using beam theory. This allowed us to compare a number of different results for the maximum stress and discuss which ones were the most accurate and why. I also explained the fundamentals of my research project to the two participants and went through my significant findings to date at that point. The students were able to get an idea of the challenges involved with mechanical testing, and will hopefully be able to use the knowledge gained from their experience to strengthen their application to study at university. |
| Year(s) Of Engagement Activity | 2019 |