TECHNIQUE AS A FATIGUE INDICATOR FOR MILITARY BRIDGES

THE DEVELOPMENT OF THE INTERNAL FRICTION DAMPING NON- DESTRUCTIVE EVALUATION TECHNIQUE AS A FATIGUE INDICATOR FOR MILITARY BRIDGES. A NONDESTRUCTIVE TEST TECHNIQUE WHICH HAS THE POTENTIAL FOR MEASURING THE DEGREE OF FATIGUE IN MILITARY BRIDGE STRUCTURES HAS BEEN INDENTIFIED BY US. THE TECHNIQUE IS CALLED INTERNAL FRICTION DAMPING NONDESCTRUCTIVE EVALUATION. THE PHYSICAL PROPERTY THAT IS MONITORED IN THIS TECHNIQUE IS THE RATE AT WHICH AN ENERGY IMPULSE IMPARTED TOA MATERIAL OR STRUCTURE DECAYS DUE TO THE INTERNAL FRICTION OF THE MATERIAL. A CHANGE IN THE RATE OF DECAY REFLECTS CHANGES IN THE BASIC CHEMICAL AND/OR MECHANICAL PROPERTIES OF THE MATERIAL DUE TO AN ACCUMLATION OF SUBTLE MATERIAL VARIABLES. THESE MATERIAL VARIABLES INCLUDE THE GRAIN SIZE, CHEMICAL COMPOSITIONS, INTERSITIAL ELEMENTS, DISLOCATIONS, PRECIPITATE PARTICLES, STRAIN RATE EFFECTS, ETC. BY PROPER MONITORING OF THE ACCUMULATION OF THESE INSIDIOUS VARIABLES, IT IS POSSIBLE TO PREDICT THE REMAININGUSEFUL LIFE OF A MATERIAL BEFORE FAILURE OCCURS. IT IS THE OBJECTIVE OF THE FIRST PHASE OF THIS PROGRAM TO ADAPT THIS TECHNIQUE TO MILITARY BRIDGES BY FIRST IDENTIFYING THE CHARACTERISTICS DAMPING COEFFICIENT VERSUS FATIGUE LIFE CURVE FOR BAR SPECIMENTS OF THE MATERIALS USED IN THE BRIDGES AND THEN DEMONSTRATING ITS SUCCESSFUL APPLICATION BY FATIGUING A FULL SIZED BRIDGE GIRDER AND COMPARING THE DDAMPIG COEFFICIENTS DURING ITS FATIGUE LIFE TO THOSE OBTAINED WITH THE BAR SPECIMENS.