[en] All structural components, even class I components of nuclear power plants, contain cracks, although quantity and extent are reduced by a careful finishing and non-destructive testing. The cracks can grow under sustained load, provided that a critical stress intensity factor Ksub(IO) is exceeded. This phenomena occurs even if the influence of corrosion can be excluded. Ksub(IO) is no measure for the extent of stable crack growth. The extent depends on time, stress level and material. Test materials selected for the investigation are the reactor pressure vessel steel 22 NiMoCr 37 (similar to A 533 Grade B class I) and a mild steel St 37. The tests are carried out with CT-Specimens 12,5, 20, 50 and 100 mm thick. Besides the influence of the thickness, that means the stress state, the influences of the initial crack length and the environment (air and water) are investigated, too. (Auth.)