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

Lead Research Organisation: Imperial College London
Department Name: Dept of Mechanical Engineering


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.

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.
Exploitation Route Updating test standards.
Sectors Energy

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 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