R Moskovic

Models to describe brittle and ductile fracture in ferritic steels

Abstract Theoretical models have been developed to describe the various fracture mechanisms that occur in polycrystalline α-iron and ferritic steels over a range of temperature which spans the ductile-to-brittle transition. At low temperatures, the models enable the proportions of transgranular cleavage and intergranular brittle failure to be explored for different ratios of the fracture energies for the two mechanisms. This allows, for example, the effect of embrittlement arising from grain-boundary segregation of minor alloying and impurity elements to be investigated. In addition the effect of texture on the predictions obtained has been considered and the results are presented. At higher temperatures, the models have been developed to accommodate ductile fracture. They have also been extended to consider the influence of prior creep cavitational damage at grain boundaries on both low- and high-temperature fracture processes and the corresponding fracture energies. The effect of this prior damage on subsequent ductile failure is explored. The predictions show that, if fewer than about 20% of the grain boundaries are fully cavitated, then there is little change in either the mechanism of the fracture process or the fracture strength of the material. These predictions are compared with measured upper-shelf fracture toughness values obtained for a Cr-Mo-V ferritic steel containing proportions of up to about 40% of prior creep damage.

Contribution of multiscale materials modelling for underwriting nuclear pressure vessel integrity

Abstract The continued safe operation of high integrity structures such as Magnox steel reactor pressure vessels is assured by using arguments which are usually based upon deterministic methodologies. These require models to provide the input parameters together with the necessary mechanistic understanding of the changes in mechanical or physical properties of the steels with service life. The use of modelling to provide key parameters for input to the fracture mechanics assessments is discussed, in particular, the application of statistical procedures to describe the required materials mechanical property data. However, there is a need to underwrite these data with an understanding of the underlying physical mechanisms. Here mechanistic models have been derived which span many orders of magnitude of length scale. At the atomic scale these provide an insight into the production of point defect damage in steels as a result of exposure to a neutron spectrum at various temperature and neutron doses. In addition, equilibrium and nonequilibrium segregation of alloying and impurity atom species to grain boundaries is modelled. Both are necessary for understanding the changes in the fracture mode and the mechanical properties for these steels after extended periods of service mode. At the microscale, models are adopted to develop insights into the fracture processes. The benefits of these models to the arguments developed within safety cases which underwrite safe continued operation of these reactor pressure vessels are described.

Understanding fracture behaviour of PGA reactor core graphite: perspective

Abstract Magnox reactors are cooled by carbon dioxide gas. The pile grade A (PGA) graphite moderator bricks in the reactor core loose mass and become more porous during service due to the radiolytic oxidation caused by energy deposition, mainly gamma radiation. In addition, neutron irradiation brings about strengthening by irradiation hardening and dimensional change. In this perspective, experimental data related to the attendant microstructural changes and the associated initiation and propagation of cracks within the graphite are revisited. These results are compared with the predictions of multiscale finite element modelling based upon an idealised microstructure. The discussion considers the quasi-brittle characteristics of the PGA graphite over a range of service exposure conditions.

Fracture Process Zones in Polygranular Graphite in Bending

The formation of fracture process zones in polygranular reactor core moderator graphites subjected to four-point bending has been investigated. The three-dimensional digital image correlation technique has been combined with resistance strain gauge measurements to evaluate, both the localised and the global displacements during testing. The non-linear load-displacement characteristics prior to peak load are correlated with the localised displacements which can extend up to ~3mm (process zone) from the tensile surface of the specimen. At peak load a macro-crack propagates rapidly along an irregular path controlled by the direction of the applied tensile load and the microstructure of the graphite. These cracks arrest prior to complete separation of the specimen. Localised tensile process zones extend for distances of up to ~3mm ahead of the tips of these cracks.

An experimental and theoretical consideration of the effect of prior creep damage on the heat affected zone fracture toughness of CrMoV steel

Material encompassing the heat affected zone has been removed from a region of a manual metal arc weldment in a CrMoV steel. This zone contained creep cavities on grain boundaries oriented normal to the direction of the maximum principal stress arising from the stress relief heat treatment. Fracture toughness tests using reconstituted Charpy geometry specimens with side grooves were undertaken at temperatures of 150 and 350 °C to assess the effect of the creep cavitation on the measured fracture toughness. The results are compared and discussed with respect to the predictions of a theoretical two-dimensional model.

Quasi-Brittle Fracture Concepts to Improve Structural Integrity Assessments of High Oxidation Weight Loss Graphite Components

Quasi-brittle fracture behaviour is characterised by the shape of the load-displacement curve and the associated mechanisms. The curves commonly exhibit non-linearity in the rising part followed by a progressive loss of load carrying capacity post peak. The behaviour of quasi-brittle materials is subject to wide variations, in the load-displacement response, both tensile and compressive, which are associated with stochastic and deterministic size effects, component geometry, load regime and load rate. The behaviour is observed in materials with brittle, heterogeneous, porous microstructures such as concretes, porous ceramics, reactor core graphites and many geological materials. Non-linearity up to the peak load is associated with the formation of distributed micro-cracks that accommodate strain and modify the ability of the material to store strain energy. In this paper, we explore how experimental observations from a range of quasi-brittle materials may help to provide improved understanding of how the microstructure can influence the fracture behaviour of reactor core moderator graphite by modifying the elastic strain energy storage capacity. This is briefly discussed with respect to the impact on the structural integrity of irradiated reactor core graphites.

Fracture Toughness Properties of the Heat-Affected-Zone in Ferritic Steel Weldments

For safety critical applications of plant it is nec essary to ensure that conservative values of mechanical properties are used for structural integ rity assessments. Assessment procedures require fracture toughness often of the weldments as one of the input parameters. The main constituents of weldments are steel plate, weld metal and the heataffected-zone (HAZ) in the plate. This paper considers the evaluation of upper shelf fracture to ughness properties of HAZ. During fracture toughness testing using three point bend specimens th cracks tended to deviate from higher strength coarse grain HAZ into lower strength regions compri sing weld metal or fine-grained HAZ. The tests in which cracks started in the HAZ and propagated ma inly through this region yielded values of J 0.2 in the range 118MPam 1/2 to 177MPam. These experimental results are discussed with re spect to the predictions of a finite element model that consider s particularly the path followed by the crack within the weldment.

An Overview of the Surveillance Measurements Made to Support the Integrity of Magnox Steel Reactor Pressure Vessels

As part of the requirement to demonstrate safe operation of the steel reactor pressure vessels for the UK Magnox fleet, surveillance samples were installed at the time of construction. These were representative of the steels and weld metals used for the manufacture of the vessels. The history of sample removal and mechanical property testing is briefly reviewed. The factors leading to changes in the fracture resistance and tensile properties will be discussed. The link is described between the mechanisms leading to changes in mechanical properties and the statistically based approach adopted to predict values of fracture toughness for the neutron dose and temperatures at key vessel locations. Predictions are compared with test results obtained from samples removed from a decommissioned reactor and also surveillance and accelerated radiation samples removed towards the end of reactor life.