Recent developments in small punch testing: Tensile properties and DBTT

Abstract

Neutron irradiation and temper embrittlement in nuclear power plants (NPPs) lead to microstructural changes in structural materials which induce a shift of the ductile to brittle transition temperature (DBTT) towards higher temperatures. Monitoring of the DBTT in NPP components receives therefore considerable attention — in particular in the context of long term operation. In that context small specimen testing techniques are developed for characterizing structural materials with a limited amount of material.One of the most used of these miniature testing techniques is the small punch (SP) test which is based on disc shaped specimens. Although SP testing has been used for more than 30years, there is still no commonly agreed procedure for deriving basic material properties from SP test data. We describe the current status of the SP test with regard to data evaluation procedures for obtaining yield stress, ultimate tensile strength and DBTT from SP tensile/fracture data. The methods for deriving the quantities characterizing the SP force-deflection curve and their use for determining basic mechanical properties are discussed.Possible reasons for the difference between the DBTT determined from Charpy and SP tests are presented. Data from the present study as well as from the literature suggest that neither notch nor strain rate effects can explain the observed discrepancies.Based on data from ongoing research projects the importance of Finite Element Analysis (FEA) for studying SP tests is presented for the example of tube specimens derived from fuel claddings.Finally an overview over the currently available standards and standardization developments is given.