• Virkkunen, Iikka
• Kuutti, Juha

Abstract

Repeated exposure to rapid temperature transients causesgradual damage in material. This iscalled thermal fatigue. Thermal fatigue is an importantdegradation mechanism in nuclear power plantcomponents and can limit the plant lifetime where thermalloads are present, e.g., due to turbulent mixingor change in plant operating conditions. The effects ofthe thermal load cycles include residual stresses,hardening or softening of the material and, finally,crack initiation and growth.Traditionally, thermal fatigue crack growth rates areestimated from the stress intensity factorscalculated from uncracked stress distributions and theParis' law. In the low-cycle regime, the use ofweight function based stress intensity factor solutionsderived under linear elastic assumptions isquestionable due to considerable plasticity. On the otherhand, numerical contour integral techniques areill-suited for thermal cyclic loading.In this work, the use of the crack opening displacementas the crack driving force parameter isevaluated through simulations of a low-cycle thermalfatigue experiments. The use of the crack tipopening displacement avoids the traditional limitationsin the numerical evaluation of the J-integral. Theunique relationship between the crack openingdisplacement and J-integral is derived and the crackdriving force is used in a crack growth assessment. Theresults show that the crack driving forcecalculated from the uncracked stress distributionsoverestimates the crack driving force significantly (ascompared to values calculated from the crack openingdisplacement). The crack growth rate calculatedwith the Paris' law is in good agreement with theexperimental results, when the crack driving force iscomputed from the crack opening displacement.

Topics

• impedance spectroscopy
• experiment
• simulation
• crack
• fatigue
• plasticity