STUDY ON STRESS CORROSION CRACKING MECHANISM AND CRACK GROWTH VELOCITY BY MEANS OF FREQUENCY ANALYSIS OF ACOUSTIC EMISSION.

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IT HAS BEEN REPORTED THAT THERE ARE TWO TYPES OF STRESS CORROSION CRACKING CONCERNING CRACK GROWTH BEHAVIOUR : CRACK GEOWS (I) DISCONTINUOUSLY AND (II) CONTINUOUSLY.STRESS CORROSION CRACING TESTS WERE CARRIED OUT ON MATERIALS OF BOTH TYPES,A SUS304 STAINLESS STEEL AND A 2014-T652 ALUMINIUM ALLOY,UNDER CONSTANT STRESS INTENSITY FACTOR CONDITIONS USING RING TYPE SPECIMENS AT CONSTANT ELECTROCHEMICAL POTENTIALS IN A 3.5% NaCl SOLUTION.aCOUSTIC EMISSION SIGNALS WERE RECORDED AND ANALYSED DURING THE TESTS.IT HAS BEEN FOUND THAT PREFERENTIAL DISSOLUTION OCCURS AT DEFECTS DUE TO STRESS CONCENTRATION AND THIS LEADS TO A DISCONTINUOUS CRACK GROWTH,IN THE ALUMINIUM ALLOY.THE ACOUSTIC EMISSION CAUSED BY THE DISCONTINUOUS GROWTH HAS RELATIVELY LARGE AMPLITUDE AND LOW FREQUENCY,AND CNA BE DISTINGUISHED FROM THE ACOUSTIC EMISSION CAUSED BY THE GENERATION OF GASES AT THE ELECTRODES.IN THE STAINLESS STEEL,DISSOLUTION OCCURS AT THE CRACK TIP AND CRACK GROWS CONTINUOUSLY.IN THIS TYPE OF CRACK GROWTH,ACOUSTIC EMISSION WAS RECORDED.THE NUMBER OF ACOUSTIC EMISSION EVENTS DECREASED WITH AN INCREASE IN THE AMPLITUDE.THIS ACOUSTIC EMISSION CHARACTERISTIC IS DIFFERENT FROM THAT OF THE DISCONTINUOUS GROWTH.IT HAS BEEN DEDUCED THAT THIS TYPE OF STRESS CORROSION CRACKING CAN BE DISTINGUISHED FROM THE OTHER TYPE BY ANALYSING ACOUSTIC EMISSION SIGNALS.