RESEARCH ON NONDESTRUCTIVE MEASUREMENT OF FATIGUE DAMAGE

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THE OBJECTIVE OF THIS PROJECT IS TO ESTABLISH A NONDESTRUCTIVE MEASUREMENT AND EVALUATION OF FATIGUE DAMAGE TO PREVENT FATIGUE FAILURES IN VARIOUS ENGINEERING STRUCTURES.FATIGUE DAMAGE PROCESSES CAN BE CATEGORIZED INTO THREE,ONE IS FATIGUE DAMAGE ACCUMULATION STAGE WHERE MICROSTRUCTURAL CHANGES WILL TAKE PROCESS WITHOUT ANY CRACK FORMATION,SECOND CRACK INITIATION AND SMALL CRACK GROWTH STAGE AND THIRD MACROSCOPIC CRACK GROWTH STAGE WHICH CAN BE INTERPRETED BY FRACTURE MECHANICS APPROACH.IN THIS PROJECTS,TWO NOVEL NONDESTRUCTIVE TECHNIQUES ARE PROPOSED FOR DETECTION OF DAMAGE ACCUMULATION AND CRACK INITIATION AND PROPAGATION ON NUCLEAR PRESSURE VESSEL STEELS,Ni-BASE SUPPER ALLOY AND SINTERED LOW ALLOY STEELS.FIRST PROPOSAL IS INDUCED CURRENT FOCUSING POTENTIAL DROP TECHNIQUE (HERE AFTER DENOTED AS ICFPD) AND SECOND IS ELECTROCHEMICAL METHOD.ICFPD HAS BEEN SUCCESSFULLY APPLIED FOR MONITORING OF FATIGUE DAMAGE PROCESSES AND CAN DETECT DAMAGE ACCUMULATION AS A DECREASE OF MEASURED POTENTIA … More L DUE TO REDUCTION OF MAGNETIC PERMEABILITY CORRESPONDING TO MICROSTRUCTURAL CHANGES BY FATIGUE.AFTER CRACK INITIATION,MEASURE POTENTIAL INCREASES AS EXPECTED FROM AN INCREASE IN CURRENT PATH DUE TO CRACKS.THIS KIND OF VARIATION OF MEASURED POTENTIAL CAN BE INTERPRETED BY A SIMPLE MODEL OF A COMBINATION OF DECREASE IN MAGNETIC PERMEABILITY OF MATERIALS AND INCREASE IN CURRENT PATH CAUSED BY CRACKS.SECOND ONE IS BY ELECTROCHEMICAL METHOD FOR A DETECTION OF FATIGUE DAMAGE ACCUMULATION.DAMAGE ACCUMULATION CAN BE CAUSED BY INCREASE IN DISLOCATION DENSITY AND RESULTANT FORMATION OF DISLOCATION CELL STRUCTURES.THIS METHOD IS BASED ON DETECTION OF THESE POSSIBLE ELECTROCHEMICALLY OR CHEMICALLY ACTIVE SITES BY A PROPER ELECTROLYTE AND POLARIZATION PROCEDURES.THE BEST ELECTROLYTES ARE SODIUM MOLYBDATE SOLUTION FOR PRESSURE VESSEL STEELS AND MIXTURE OF HCL,H2SO4, CUSO4 AND WATER FOR INCONEL 718, RESPECTIVELY.THESE RESULTS CAN PROVIDES A POTENTIAL PROCEDURE FOR NONDESTRUCTIVE EVALUATION OF FATIGUE DAMAGE IN THOSE OPERATING PLANTS SUCH AS NUCLEAR POWER PLANTS,CHEMICAL PLANTS AND AEROSPACE ENGINEERING COMPONENTS. Less