1. 
 Varias, A.G., et al.
(författare)

Simulation of hydrogen embrittlement in zirconium alloys under stress and temperature gradients
 2000

Ingår i: Journal of Nuclear Materials.  Elsevier BV NorthHolland.  00223115 . 18734820. ; 279:23

Tidskriftsartikel (refereegranskat)abstract
 A finite element model is presented for hydrogen embrittlement of zirconium alloys. The model takes into account the coupled processes of hydrogen diffusion, nonmechanical energy flow, hydride precipitation, hydride/solid solution deformation and fracture.The model has been tested successfully against the Sawatzky experiment and exact analytical solutions on hydrogen diffusion and hydride precipitation under a temperature gradient. Based on this model, the hydrogen embrittlement of a Zircaloy2 cracked plate, is studied under tensile stress and temperature gradient. The initial and boundary conditions are according to those encountered in the fuel cladding of light water reactors during operation. The strong effect of crack tip stress intensification on hydrogen diffusion and initial hydride precipitation is shown. The problem is also studied by considering neartip material damage which affects hydride precipitation significantly.


2. 
 Varias, A.G., et al.
(författare)

Temperature and constraint effects on hydride fracture in zirconium alloys
 2000

Ingår i: Engineering Fracture Mechanics.  Pergamon.  00137944 . 18737315. ; 65:1

Tidskriftsartikel (refereegranskat)abstract
 The fracture of hydrides in zirconium alloys is under consideration. According to the present boundary value problem, a hydride platelet lies ahead of a semiinfinite crack, along the crack plane. The surrounding material is elasticplastic zirconium alloy. The platelet is either continuous or split into two parts, connected by a ductile matrix ligament. At distances from the crack tip, which are large compared to the hydride and the plastic zone size, the KT field is applied and mode I, plane strain and contained yielding conditions prevail. Hydride platelet failure initiation and growth is simulated by using a decohesion crack growth model and the stress intensity factor, which causes fracture, is estimated at various temperatures as well as under various constraint conditions. Comparison of the calculated temperature effect on toughness with the experimental one is satisfactory. Fracture toughness decreases with Tstress. This effect is attributed to the interaction of the KT field with hydride expansion, during precipitation. The reduction becomes more important at elevated temperatures and moderates the benefits on fracture toughness, caused by temperature increase. In addition to the detailed finite element results, analytical estimates on fracture toughness are presented, based on a cohesive zone model


3. 
 Dufournaud, O, et al.
(författare)

ElasticPlastic Deformation of a Nuclear Fuel Cladding Specimen under the Internal Pressure of a Polymer Pellet
 2002

Konferensbidrag (refereegranskat)abstract
 During the operation of light water reactors, corrosion of nuclear fuel cladding results into the generation of hydrogen. With operation time increase, the hydrogen concentration in the cladding may exceed its terminal solid solubility and brittle zirconium hydrides may precipitate. Indeed hydrides are present in high burnup fuel cladding, which is therefore more susceptible to failure. The Expansion Due to Compression (EDC) test has been developed for the study of irradiated and hydrided cladding failure, under conditions of pellet cladding mechanical interaction, which are expected during a reactivity initiated accident (RIA) or an inpile test. During the EDC test, a piece of cladding tube is circumferentially loaded in tension due to the expansion of a polymer pellet, axially compressed inside the tube. A finite element simulation of the EDCtest is presented. The objective of the study is: (i) to understand the deformation of the cladding, during the experiment, including the effect of cladding material properties, and (ii) to provide information, necessary for the development of failure criteria. The distributions of important field quantities with respect to the damage of the cladding are derived together with the evolution of their maximum values, during loading. It is shown that, before cladding yielding as well as after substantial plastic deformation, the radial displacement, on the external surface, and the total energy per unit volume, when appropriately normalized, vary along the cladding axis according loadindependent distributions. This characteristic of cladding deformation is used for the development of failure criteria.


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9. 
 Varias, A G, et al.
(författare)

Hydrideinduced embrittlement and fracture in metals  effects of stress and temperature distribution
 2002

Ingår i: Journal of the Mechanics and Physics of Solids;7.  Elsevier Science Ltd..  00225096. ; :7

Tidskriftsartikel (refereegranskat)abstract
 A mathematical model for the hydrogen embrittlement of hydride forming metals has been developed. The model takes into account the coupling of the operating physical processes, namely: (i) hydrogen diffusion, (ii) hydride precipitation, (iii) nonmechanical energy flow and (iv) hydride/solidsolution deformation. Material damage and crack growth are also simulated by using decohesion model, which takes into account the time variation of energy of decohesion, due to the timedependent process of hydride precipitation. The bulk of the material, outside the decohesion layer, is assumed to behave elastically. The hydrogen embrittlement model has been implemented numerically into a finite element framework and tested successfully against experimental data and analytical solutions on hydrogen thermal transport (in: Wunderlich, W. (Ed.), Proceedings of the European Conference on Computational Mechanics, Munich, Germany, 1999, J. Nucl. Mater. (2000a) 279 (2–3) 273). The model has been used for the simulation of Zircaloy2 hydrogen embrittlement and delayed hydride cracking initiation in (i) a boundary layer problem of a semiinfinite crack, under mode I loading and constant temperature, and (ii) a cracked plate, under tensile stress and temperature gradient. The initial and boundary conditions in case (ii) are those encountered in the fuel cladding of light water reactors, during operation. The effects of neartip stress intensification as well as of temperature gradient on hydride precipitation and material damage have been studied. The numerical simulation predicts hydride precipitation at a small distance from the cracktip. When the remote loading is sufficient, the neartip hydrides fracture. Thus a microcrack is generated, which is separated from the main crack by a ductile ligament, in agreement with experimental observations


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