QUBE:- QUasi-Brittle fracture: a 3D Experimentally-validated approach

Lead Research Organisation: University of Bristol
Department Name: Interface Analysis Centre

Abstract

Ductile materials, like metals and alloys, are widely used in engineering structures either by themselves or as reinforcement. They usually can sustain a lot of plastic damage before failing. Engineers understand quite well the ways that metals fail and how tolerant they are to damage, so efficient and less massive structures may be designed with well-defined margins of safety or reserve strength to cope with extreme events. By comparison, elastic brittle materials such as glasses and ceramics can fail without prior warning, so much larger safety margins are needed.
Quasi-brittle materials are an important class of structural materials. They are brittle materials with some tolerance to damage and include concrete, polygranular graphite, ceramic-matrix composites, geological structures like rocks and bio-medical materials such as bone and bone replacements. Although their damage tolerance is much less than many metals and alloys, it can be quite significant compared to brittle materials such as ceramics and glasses. But this is not accounted for very well when engineers design with, or assess, quasi-brittle materials, as there is not an adequate understanding of the role on their damage tolerance of factors such as the microstructure of the material or the state of stress. Quasi-brittle materials are usually treated as fully brittle, taking little or no account of their damage tolerance, so assessments incorporate very significant safety margins, leading to designs that may be inefficient and unnecessarily bulky. Even when some assessment of damage tolerance is included, the microstructure can change as the material ages over time, and we need ways to measure the effects of this and to predict what it will do to the safety of the structure. This project aims to develop a method to predict the performance and evaluate the integrity of structures and components made from quasi-brittle materials. This will extend opportunities for their use in engineering applications, enabling more efficient design with greater confidence in safety.
Quasi-brittleness is a property that emerges from the material's microstructure. A quasi-brittle material can be made from a connected network of very brittle parts (for instance, a porous ceramic). It exhibits a characteristic "graceful" failure as parts break locally when loaded sufficiently, which gives it damage tolerance. The "gracefulness" of the failure is affected by the random variations of strength and stiffness of the network and the form of the connections. Such networks represent a key part of the microstructure of the material, and to understand quasi-brittle fracture we need to construct models that properly describe the microstructure. There is a need to understand and define the mechanisms that control the fracture at the small and the large scale within these quasi-brittle materials. This will allow us to capture sensitivity to microstructure differences and degradation, and to produce general models that are suitable for the wide range of quasi-brittle materials and applications.
Three-dimensional models that are faithful to the microstructure can be created using modern 3D microscopy methods, such as X-ray computed tomography. But these models are far too complex to simply scale up to structures very large relative to the microstructure. There is no computer than can do this, yet. We will develop modelling methods that sufficiently represent the complexity of quasi-brittle microstructures over a wide range of length scales, such as cellular automata finite elements. We will use advanced tomography and strain mapping techniques to observe how damage develops and to test and refine our models. We will then use this and the understanding that we gain to design new material tests and characterisation methods so that our methods may be used in a wide range of materials, from concretes to advanced nuclear composites, bone replacement biomaterials and geological materials.

Publications

10 25 50
 
Description The Characteristics both physical and mechanical have been evaluated and are the subject of papers in preparation
Exploitation Route Further considerations of quasi brittle materials
Sectors Aerospace, Defence and Marine,Energy

 
Description Build public confidence that it is safe to continue to operate the AGR fleet of nuclear power plant in the UK.
First Year Of Impact 2016
Sector Energy
Impact Types Societal

 
Description Modelling at the test specimen length-scale deformation and fracture of unirradiated and irradiated Gilsocarbon graphite
Amount £30,000 (GBP)
Organisation EDF Energy 
Sector Private
Country United Kingdom
Start 02/2016 
End 10/2016
 
Description Modelling the deformation and fracture of irradiated Gilsocarbon graphite
Amount £27,000 (GBP)
Organisation EDF Energy 
Sector Private
Country United Kingdom
Start 09/2015 
End 01/2016
 
Description Modelling the mechanical properties of Gilsocarbon graphite
Amount £27,000 (GBP)
Organisation EDF Energy 
Sector Private
Country United Kingdom
Start 06/2014 
End 01/2015
 
Title Resistivity measurement under load and temperature of graphite 
Description Measurement of resistivity of tensile test specimens over a temperature range developed in conjunction with Dr Bryan Roebuck at NPL 
Type Of Material Improvements to research infrastructure 
Year Produced 2017 
Provided To Others? No  
Impact Provides new insights on the fracture and deformation of reactor core type graphite 
 
Description Multiscale modelling of mechanical properties of quasi-brittle nuclear graphite 
Organisation Delft University of Technology (TU Delft)
Country Netherlands 
Sector Academic/University 
PI Contribution Abstract Commercial graphites are used for a wide range of applications. For example, Gilsocarbon graphite is used within the reactor core of advanced gas-cooled reactors (AGRs, UK) as a moderator. In service, the mechanical properties of the graphite are changed as a result of neutron irradiation induced defects and porosity arising from radiolytic oxidation. In this paper, we discuss measurements undertaken of mechanical properties at the micro-length-scale for virgin and irradiated graphite. These data provide the necessary inputs to an experimentally-informed model that predicts the deformation and fracture properties of Gilsocarbon graphite at the centimetre length-scale, which is commensurate with laboratory test specimen data. The model predictions provide an improved understanding of how the mechanical properties and fracture characteristics of this type of graphite change as a result of exposure to the reactor service environment. Concluding Comments In Section 5, we have described and discussed results of a multi-scale model used for predicting changes in elastic properties of Gilsocarbon graphite due to service exposure to combined neutron irradiation and radiolytic oxidation. The model is founded on two underlying assumptions: (1) a good description of the material microstructure is necessary; and (2) material properties measured at the appropriate length-scale are needed to describe the deformation and fracture process in a complex multi-phase material such as Gilsocarbon graphite. The model is informed by experimental measurements of 'true' material properties. In this work, mechanical properties (elastic modulus and fracture strength) of virgin and irradiated Gilsocarbon graphite are obtained using a micro-cantilever testing technique. By testing micrometre length-scale specimens, this technique enables material properties of specific features of the microstructure to be obtained via minimisation of the influence of porosity and defects on the measurement results. This is essential for multi-phase materials that exhibit heterogeneities and porosities at multiple length-scales, such as nuclear graphite [9] and cement paste [22], [23], [24], [25]. These measurements provide the necessary input data at the micro-length-scale. In addition, a microstructural model representative of the Gilsocarbon graphite and the changes affecting it over time has been invoked. Therefore, porosity is explicitly included in the microstructural model. This procedure avoids the need for fitting parameters, so the simulation results are fully predictive and dependent on a good microstructural model. This makes data collection and property prediction cheap and efficient. Two different approaches, with varying levels of detail, have been presented: (1) a fully microstructural approach, which has been used to simulate uniaxial tension experiments on the millimetre length-scale; and (2) a statistical microstructure-informed approach, which has been used to simulate three-point bending experiments at the centimetre length-scale, with size corresponding to experiments. Simulation results have been compared to experimental data. Uniaxial tension simulations have shown excellent agreement between simulated elastic moduli and those measured for various in-service conditions. In addition, simulations have shown a shift between relatively brittle to more quasi-brittle behaviour with increasing irradiation and mass loss, signified by an increased contribution of the post-peak work of fracture, accompanied by widening of the fracture process zone. The crack propagation mode changes with increasing irradiation due to strengthening of the filler particles. While virgin condition cracks do penetrate through the filler particles, following irradiation this is not the case. For three-point bending simulations, a simplified statistical microscale approach has been adopted. While this does not explicitly use the material microstructure as input, it is informed by smaller-scale simulations. In a statistical way, this makes the proposed approach significantly less computationally expensive and, therefore, more suitable for larger specimens. Simulation results have shown good agreement with experimental data in terms of flexural strength. Although simplified, this approach is able to reproduce the main behaviours of the more detailed microstructurally-based approach (such as widening of the damage zone with increasing porosity), but with less detail.
Collaborator Contribution Delft: FE modelling
Impact Modelling deformation and fracture of Gilsocarbon graphite subject to service environments By:Savija, B (Savija, Branko)[ 1 ] ; Smith, GE (Smith, Gillian E.)[ 2 ] ; Heard, PJ (Heard, Peter J.)[ 2 ] ; Sarakinou, E (Sarakinou, Eleni)[ 2 ] ; Darnbrough, JE (Darnbrough, James E.)[ 2 ] ; Hallam, KR (Hallam, Keith R.)[ 2 ] ; Schlangen, E (Schlangen, Erik)[ 1 ] ; Flewitt, PEJ (Flewitt, Peter E. J.)[ 2,3 ] JOURNAL OF NUCLEAR MATERIALS Volume: 499 Pages: 18-28 DOI: 10.1016/j.jnucmat.2017.10.076
Start Year 2015
 
Description Multiscale modelling of mechanical properties of quasi-brittle nuclear graphite 
Organisation EDF Energy
Department EDF Energy Nuclear Generation
Country United Kingdom 
Sector Private 
PI Contribution Abstract Commercial graphites are used for a wide range of applications. For example, Gilsocarbon graphite is used within the reactor core of advanced gas-cooled reactors (AGRs, UK) as a moderator. In service, the mechanical properties of the graphite are changed as a result of neutron irradiation induced defects and porosity arising from radiolytic oxidation. In this paper, we discuss measurements undertaken of mechanical properties at the micro-length-scale for virgin and irradiated graphite. These data provide the necessary inputs to an experimentally-informed model that predicts the deformation and fracture properties of Gilsocarbon graphite at the centimetre length-scale, which is commensurate with laboratory test specimen data. The model predictions provide an improved understanding of how the mechanical properties and fracture characteristics of this type of graphite change as a result of exposure to the reactor service environment. Concluding Comments In Section 5, we have described and discussed results of a multi-scale model used for predicting changes in elastic properties of Gilsocarbon graphite due to service exposure to combined neutron irradiation and radiolytic oxidation. The model is founded on two underlying assumptions: (1) a good description of the material microstructure is necessary; and (2) material properties measured at the appropriate length-scale are needed to describe the deformation and fracture process in a complex multi-phase material such as Gilsocarbon graphite. The model is informed by experimental measurements of 'true' material properties. In this work, mechanical properties (elastic modulus and fracture strength) of virgin and irradiated Gilsocarbon graphite are obtained using a micro-cantilever testing technique. By testing micrometre length-scale specimens, this technique enables material properties of specific features of the microstructure to be obtained via minimisation of the influence of porosity and defects on the measurement results. This is essential for multi-phase materials that exhibit heterogeneities and porosities at multiple length-scales, such as nuclear graphite [9] and cement paste [22], [23], [24], [25]. These measurements provide the necessary input data at the micro-length-scale. In addition, a microstructural model representative of the Gilsocarbon graphite and the changes affecting it over time has been invoked. Therefore, porosity is explicitly included in the microstructural model. This procedure avoids the need for fitting parameters, so the simulation results are fully predictive and dependent on a good microstructural model. This makes data collection and property prediction cheap and efficient. Two different approaches, with varying levels of detail, have been presented: (1) a fully microstructural approach, which has been used to simulate uniaxial tension experiments on the millimetre length-scale; and (2) a statistical microstructure-informed approach, which has been used to simulate three-point bending experiments at the centimetre length-scale, with size corresponding to experiments. Simulation results have been compared to experimental data. Uniaxial tension simulations have shown excellent agreement between simulated elastic moduli and those measured for various in-service conditions. In addition, simulations have shown a shift between relatively brittle to more quasi-brittle behaviour with increasing irradiation and mass loss, signified by an increased contribution of the post-peak work of fracture, accompanied by widening of the fracture process zone. The crack propagation mode changes with increasing irradiation due to strengthening of the filler particles. While virgin condition cracks do penetrate through the filler particles, following irradiation this is not the case. For three-point bending simulations, a simplified statistical microscale approach has been adopted. While this does not explicitly use the material microstructure as input, it is informed by smaller-scale simulations. In a statistical way, this makes the proposed approach significantly less computationally expensive and, therefore, more suitable for larger specimens. Simulation results have shown good agreement with experimental data in terms of flexural strength. Although simplified, this approach is able to reproduce the main behaviours of the more detailed microstructurally-based approach (such as widening of the damage zone with increasing porosity), but with less detail.
Collaborator Contribution Delft: FE modelling
Impact Modelling deformation and fracture of Gilsocarbon graphite subject to service environments By:Savija, B (Savija, Branko)[ 1 ] ; Smith, GE (Smith, Gillian E.)[ 2 ] ; Heard, PJ (Heard, Peter J.)[ 2 ] ; Sarakinou, E (Sarakinou, Eleni)[ 2 ] ; Darnbrough, JE (Darnbrough, James E.)[ 2 ] ; Hallam, KR (Hallam, Keith R.)[ 2 ] ; Schlangen, E (Schlangen, Erik)[ 1 ] ; Flewitt, PEJ (Flewitt, Peter E. J.)[ 2,3 ] JOURNAL OF NUCLEAR MATERIALS Volume: 499 Pages: 18-28 DOI: 10.1016/j.jnucmat.2017.10.076
Start Year 2015
 
Description Graphite Core Committee (EDF Energy, UK) 
Form Of Engagement Activity A formal working group, expert panel or dialogue
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Industry/Business
Results and Impact Specialist expert group.
Year(s) Of Engagement Activity 2015,2016,2017