DETECTION OF FATIGUE CRACKS IN MILITARY BRIDGES AND STRUCTURES

UNDER STEDILY REPEATED CYCLIC LOADING IMPOSED BY LIVE LOADS, FATIGUE CRACKS IN BRIDGES CAN BE INITIATED AT SMALL DEFECTS. BECAUSE DEFECTS CAN ELIMINATE THE INITIAL STAGE OF CRACK GROWTH, THE FATIGUE LIFE IS GREATLY REDUCED, ESPECIALLY IF FRACTURE IS CONTROLLED BY CRACK INITIATION. THEREFORE, THE FATIGUE CRACK GROWTH PROCESS IS REDUCE TO CRACK PROPAGATION AND UNSTABLE CRACK GROWTH. FATIGUE STUDIES INDICATE THAT WELDED COMPONENTS IN BRIDGES ARE MORE SUSCEPTIBLE TO FATIGUE FRACTURE THAN GIRDERS OR TRUSSES. DEFECTS FORMED DURING THE WELDING PROCESS ACT AS STRESS RISERS TO DEVELOP HIGH STRESS FIELDS WHICH LEAD TO MICROCRACKS AND SUBSEQUENT FAILURE UNDER CYLIC LOADING. SINCE CURRENT NOT TECHINIQUES FOR LOCATING CRACKS IN BRIDGES REQUIRE CONTACT AND SINCE MANY WELDED JOINTS ARE DIFFICULT TO INSPECT BECAUSES OF THE LOCATION, A TECHNIQUE WHICH WOULD BE NONCONTRACTING AND SIMPLY OPERATED IN FIELD ANALYSES UNDER TYPICAL, WORKING ENVIRONMENTS COULD SIMPLIFY THE ANALYSIS. THIS PROPOSED RESEARCH WILL INVESTIGATE THE FACTORS NECESSARY TO DEVELOP A NEW AND INNOVATIVE TECHNIQUE USING INFRARED THERMOGRAPHY TO LOCATE CRACK PROPAGATION ZONES IN WELDED COMPONENTS.