Structural integrity

 

PDF icon    MecaSens 2013 

 

Key Contact:

Robert Harrison, Programme Leader 


Phone: +61 9717 3150

Email: rph@ansto.gov.au

 

Purpose
  

The Structural Integrity Program carries out research on materials for use in extreme environments typical of the conditions likely to be experienced in advanced power generation systems - such as supercritical steam, Generation IV and fusion nuclear plants.

  

The research is also applicable to high technology engineering applications such as the aerospace industry. 
  

The extreme environments considered include high temperature, high radiation and high stress.

 

The aims of the research are to:

 

  • determine the effects extreme conditions have on the integrity of the materials and welds within the structures of plants
  • improve the understanding of the effect of neutron irradiation on structural materials and how these changes affect the integrity of the structures, and to
  • design and fabricate new materials for such applications.


One of the key areas of research is understanding how residual stresses develop in welds due to thermo mechanical effects (including the evolution of microstructure).

 

Research outcomes are published in internal reports, through scientific papers in appropriate high-ranking journals and presentations at international conferences.

 

Four themed areas of the Structural Integrity Program:


 

Contacts:

 

Structural Integrity – Bob Harrison (rph@ansto.gov.au)
Radiation damage – Dave Carr (djc@ansto.gov.au)
Multi scale modelling of Structural Materials – Phil Bendeich (pbx@ansto.gov.au)
High temperature materials/RemLife – Warwick Payten (wmp@ansto.gov.au)
New materials development – Daniel Riley (dry@ansto.gov.au)

 
The current and future research programmes are supported by extensive experimental and characterisation facilities. These include facilities for:

 

Irradiating structural materials with ions using the ANTARES or STAR accelerators and neutron beams using the OPAL research reactor
Mechanical testing of materials in the temperature range -196°C to +1,500°C. These include tensile, fracture, creep and creep-fatigue testing facilities.
Transmission and scanning electron microscopy, and X-ray diffraction. A focussed ion beam instrument will be added shortly.


Funding has recently been confirmed for the design and construction of extensive hot cell facilities to enable radioactive materials to be examined, machined and tested. Funding has also been approved for the design phase of a new electron microscopy building.

 



Radiation damage   


This theme concentrates on the effects of radiation on structural materials for applications in future power generation systems. By studying these effects, changes can be made to the manufacturing processes and the microstructure of the materials to improve their resistance to radiation damage.

 

Neutron or ion beams are employed to irradiate the test materials. Any resulting damage is then evaluated using a variety of experimental techniques; including transmission electron microscopy, mechanical testing and neutron or x-ray diffraction/scattering.

 

When neutron activation is a problem due to the samples becoming radioactive, ion beams using helium and/or other heavier ions may be used to irradiate the materials. A relatively new technique, soon to be commissioned at ANSTO, called focussed ion beam milling allows very small samples to be machined from solids. These samples can then be tested to measure mechanical property changes and as they are very small (down to ~5 µm x 5 µm x 40 µm) are suited to neutron irradiated experiments.

 

 

Radiation damage of stainless steele

Stainless steel micro-cantilever specimen machined by focussed ion beam milling.  Samples are extracted from the ion beam radiation-damaged layer to evaluate radiation-induced mechanical property changes.

 



Multi scale modelling of Structural Materials   


Modelling within the Structural Integrity programme occurs across a wide range of length scales These atomic level Molecular Dynamics (MD) modelling, microstructural scale Elasto-Plastic Self-consistent (EPSC) modelling and macro scale Finite Element modelling (FEA). 

 

Large -scale molecular dynamics (MD) modelling work is being used to study the effect of radiation on a variety of structural materials including zirconium alloys, SiC, graphite and various titanium/aluminium alloys. Pure silicon has been used as a model system for testing and validating developed tools and visualization techniques.

 

The figure below shows a MD model of stacking fault tetrahedra-like vacancy cluster forming in the centre of a 20 keV displacement cascade in L10 TiAl intermetalics at T=100K. Blue, green, yellow and orange spheres denote Al self-interstitials, Ti self-interstitials, Ti vacancies and Al vacancies, respectively.

 

 

Materials Image 1
Multi scale modelling of structural materials illustration

 

 

The EPSC method is typically used to describe material behaviour at a crystallographic level by modelling internal elastic lattice strains with resulting dislocation and twinning events etc. It is also useful in predicting material behaviours including the evolution of texture, elastic constants, and strain hardening during plastic deformation.

 

This occurs during events such as simple plastic deformation or thermo-mechanical processes such as welding, events that are commonly simulated using the FEA method.

 

FEA modelling is used to predict the behaviour of components and structures.

 

A major effort within the programme is focussed on modelling residual stress and distortion in welded components.

 

This includes the development of validated models based on physically relevant constitutive modelling which enables the results of the models to be used directly in Structural Integrity assessment.

 

Our constitutive model development includes phase transformation during thermal processing. Variables such as heating rates and initial grain sizes can be used to predict the timescale for the formation of both desirable and undesirable phases. The formation of various phases and the temperatures that they occur at can also have a significant influence on the formation and distribution of residual stresses.

 

Activities in this area will be directed towards the following programs:

 

The European NeT programme in which the research is fundamental welding simulation research oriented towards Generation IV nuclear systems
US Nuclear Regulatory Commission involvement and British Energy/Electricite de France collaboration involving simulation of welds in power reactors
Provision of support to OPAL Operations

 

Residual stress developed in the wire cut austenitic steel sample

Post-weld equivalent plastic strain as predicted by isotropic, kinematic, and mixed hardening models across the plate thickness at mid-length: comparison of the predicted plastic strain along BD, identifying the number of applied thermal cycles (tc). Note that material subject to either melting or annealing experiences monotonic tension (mt) during cool-down, and is subject to thermal cyclic loading during deposition of consecutive passes provided the temperature does not exceed the annealing temperature of the material.

 



High temperature materials   


The main objective of research in this area is to reduce the conservatism inherent in the current high temperature assessment methods for advanced power generation systems by providing a more accurate assessment of the condition of the material and any defects that may be present. The main research areas are to:

 

Develop and undertake mechanical test programmes for candidate materials.
Validate the behaviour of existing materials using the latest theoretical developments in creep-fatigue life prediction methodologies.
Develop assessment methodologies that extend the relevant international codes such as R5 and ASME.


Various residual stress measurement techniques, for example, x-ray, contour and neutron methods, are employed in support of these research areas.


These activities rely heavily on collaboration with a number of organisations. The main collaborations are with the ANSTO Bragg Institute, Electricity de France, Rolls Royce, TWI, Serco Assurance, Imperial College, Bristol University, the Open University and EPRI Creep-Fatigue Expert Group.

 


 

RemLife Software   

 

Remlife, developed within the High Temperature Materials theme, is a software package used to assess the damage to equipment caused by operation and cycling of conventional and nuclear power plants. Remilfe simulates the effects of unit cycling on (i) the damage sustained by the equipment, (ii) the economic implication of the historical operating patterns and (iii) the effect that (i) and (ii) would have on the future life of the station.

 

It allows a relatively detailed assessment to be performed in hours rather than the days that is required for a detailed remaining life calculations.

 

A unique feature of the software is that it simulates operational strategies based on cycling profiles such as two shifting, hot banking and load reductions. Optimal start-up and shifting profiles are provided allowing the assessment can be assessed to both the damage sustained by the high-temperature equipment and the economic benefits or not of the chosen cycling regime.


 

Screen for imputting data on a steam header into remlife software
Screen for inputting data on a steam header into the Remlife Software

 

 

The calculations of remaining life are based on the ductility exhaustion method using the R5 code. This assessment procedure was developed by British Energy, BNFL Magnox Generation and Serco Assurance. It incorporates, augments and where necessary replaces the provisions of ASME III subsection NH and the French Code RCC-MR.

 

 

Materials Image 2
HP Turbine Rotor Maximum Secondary Stresses


 



New materials development   


Research under this theme is directed towards developing and fabricating materials for use in extreme environments.

 

High temperature environments require the use of specialised materials to not only maintain integrity but also provide insulation. Work is currently being undertaken on porous refractory carbides.

 

Ultra-high temperature ceramics (UHTCs) are able to withstand these extreme conditions and are promising materials for other high temperature applications. By introducing a controlled porous structure into a UHTC material it is hoped to improve the material’s insulating properties.

 

The University of Sydney and ANSTO's Materials Institute are collaborating to produce a range of refractory carbide foams with potential for high temperature applications.

 

One possible application is its use in high speed aircraft where considerable skin friction raises the temperature of the leading edges to beyond 3000C.

 

At present there is no material that can both withstand these temperatures and maintain dimensional stability.

 

The recent collaboration has resulted in a novel process for templating zirconium carbide foams. Specialised processing has produced a controlled macro-porosity (as shown below) suitable for reducing the thermal conduction through the material.

 

 

Electron backscattered diffraction mapping

Electron backscattered diffraction mapping (or EBSD mapping) provides a method for assessing the underlying microstructural information of materials.

 

The image below shows a novel EBSD map from an investigation of a large area of pure titanium metal. As can be seen in the figure, the size, the orientation and the distribution of unique crystallographic grains have been determined for the entire 10 mm diameter disc. This technique provides a routine and rigorous method of analysis that is essential when correlating the performance of a material with its underlying microstructure.

 

Microscture Materials image 1
Novel EBSD map from a large area of pure titanium metal