FRACTURE-TOUGHNESS BY SMALL PUNCH TESTING

The small punch test, currently being used to empirically estimate the material fracture appearance transition temperature (FATT) of a range of components in fossil power plant service, has been further developed for direct estimation of K-lc and J(lc) in a manner that is analytical, material-independent, and requires no prior knowledge of material mechanical properties. The procedure follows an approach that is based on the continuum material toughness concept wherein the criterion for fracture is defined and measured via the continuum stress-strain deformation properties of the material. The procedure specifically involves computing the ''local'' strain energy density accumulated at the location and time of crack initiation in the small punch test specimen using large-strain finite element (FE) analysis. Since the procedure also includes estimation of the material uniaxial tensile stress-strain behavior from the small punch load-displacement curve, both the fracture toughness and the uniaxial tensile behavior are determined from a single test. The procedure has been developed in order to take advantage of the miniature sample removal techniques that can be applied ''nondestructively'' to operating components. Results are presented on a range of steels.