Misure di tenacità a frattura su acciai utilizzando velocità di deformazione elevate

Abstract

The knowlegde of dynamic mechanical properties is useful in all cases where the strain rate sensitivity of metallic materials is an issue, and whenever the actual loading conditions for a structure (either in normal operation or under accidental circumstances) are different from static. Furthermore, in some investigations increasing the loading rate is used to simulate other embrittling mechanisms such as thermal aging or neutron exposure. This paper provides an overview of SCK•CEN experience on measuring fracture toughness of steels at elevated loading rates, with specific emphasis on instrumented impact tests on precracked Charpy (PCVN) specimens. After briefly dwelling on the basic mechanisms which explain loading rate effects on cleavage and ductile fracture toughness, the experimental and analytical procedures for measuring fracture toughness at elevated loading rates are addressed, both in terms of official ASTM and ISO test standards and considering standardization efforts currently in progress under SCK•CEN coordination: revision of ASTM E1921 (MasterCurve methodology for measuring fracture toughness in the ductile-to-brittle transition region) and a future ISO standard on instrumented PCVN testing. This latter document is examined in more detail, focussing the attention on the dynamic evaluation of brittle fracture toughness (Impact Response Curve) and the determination of crack resistance curves using multiple and single-specimen techniques. Finally, selected examples from SCK•CEN database of dynamic toughness measurements will be illustrated, mainly relevant to reactor pressure vessel (RPV) steels.