An apparatus for carrying out high cycle fatigue tests of a specimen under high pressure, including: a pressure vessel; a load train including a first horn and a second horn between which a specimen is to be arranged, wherein the load train is arranged within an internal chamber of the pressure vessel; and a converter configured to apply ultrasonic waves into the load train by exciting the first horn to apply a dynamic stress to the specimen. A base part of the second horn is movably seated in the pressure vessel such that two separated chambers are formed within the pressure vessel with the first chamber for the specimen, wherein both chambers can be fed with gas and charged with different gas pressures in order to apply a static stress to the specimen.