COMPARISON OF FRACTURE TOUGHNESS VALUES DETERMINED BY STANDARDIZED METHOD AND SMALL PUNCH TEST

摘要

Large forgings and castings (e.g. steam turbine rotors) reveal significant differences in local mechanical properties as a result of differences in local chemical composition and thermomechanical treatment. In cases, where the critical crack length is to be assessed and/or the remaining lifetime of the component evaluated, local mechanical properties must be taken into consideration, what can be impossible to realize by means of classic mechanical destructive testing, which need relatively large volumes of representative material.Small Punch Test (SPT) [1] is a relatively new and promising test method making it possible to determine basic mechanical properties and also fracture toughness values of metallic structural materials. The most important advantage of the method is the nearly nondestructive withdrawal of test material of the respective component and the small size of test specimen what is interesting in cases of remaining lifetime assessment, when a sufficient volume of the representative material cannot be withdrawn of the components.In opposite, the most important disadvantage of the method stems from the necessity to correlate SPT results with the results of classic test procedures and to build up a database of material data in service. The database should comprise not only original (virgin) basic materials and weld metals data but also material properties degraded by service conditions. The correlations between the SPT results and the results of tensile tests, fracture toughness values and time to rupture characteristics at creep temperatures etc. are necessary for the remainig lifetime assessment of structure in long-time service.The paper describes the comparison of ultimate strength and fracture toughness values determined by standardized methods [2], [3] and the small punch test of several low-alloyed steels and Al-alloys.