Comparison of transition temperature criteria applied for KLST and ISO-V type Charpy specimens

A great deal of test data have been obtained on reactor pressure vessel steels using the standard Charpy-V test. Although more advanced test methods, based on elastic-plastic fracture mechanics, are both recommendable and already in use in the surveillance programmes of some nuclear power plants (NPPs), Charpy tests are still required, e.g., by regulatory guides. Besides the normal-size (ISO-V) Charpy specimen (10*10*55 mm3), various types of sub-size specimens have been introduced. One standardised sub-size specimen being in use is the so-called KLST specimen, the size of which is 3*4*27 mm3 with 1 mm central notch (DIN 50 115). So far the test data published for the KLST specimen, as well as sub-size specimens in general, is still limited. The results from small specimen testing are typically used for evaluating the fracture behaviour of the ISO-V Charpy specimen and if there are no test results available for the correlation, as there usually is not, a general correlation has to be applied to evaluate the fracture behaviour of the ISO-V specimen. The availability of a sub-size specimen depends therefore significantly on how reliably this relationship has been established. Impact test data measured with different specimens have been correlated using some appropriate criterion (or criteria) and since a total transition curve is normally measured, there are several ones available. The criterion can be a fixed energy or lateral expansion level describing the transition temperature or the level can be derived from the upper-shelf energy (USE). In general, the proposed criterion can be divided into two groups: those derived from the dimensions of the specimens and those derived empirically from experimental data. Test data measured with ISO-V and KLST -type Charpy specimens are discussed and the validity of two proposed, basically different transition temperature criteria and the resulting differences in the temperatures, that are inevitable because of the different size ligaments, studied. Specimens' capability to describe consistently the transition temperature shift characteristic of ferritic steels due to irradiation and recovery annealing is discussed as well. The data consists of the test results published previously [1] and more recent test data measured at VTT with KLST-type specimens for non-irradiated and irradiated FFA, JFL and JRQ pressure vessel steels in an IAEA Co-ordinated research programme.