LONGITUDINAL STRESS (FORCE) MEASUREMENT OF RAIL UNDER OPERATING CONDITIONS

IN 1982, A SPECIAL COMMITTEE OF THE NMAB ISSUED A REPORT ON THE NONDESTRUCTIVE METHODS THAT MIGHT BE USED TO MEASURE THELOGITUDINAL FORCE IN INSTALLED RAILS. IT SHOWED THAT THE ACOUSTO-ELASTIC EFFECT WHICH INFERS THE STRESS FROM MEASURE MENTS OF SMALL SHIFTS IN THE VELOCITY OF SOUND WAS PROMISINGBUT DIFFICULT TO IMPLEMENT BECAUSE VARIATIONS IN THE METALLURGICAL STRUCTURE OF THE METAL AND UNCERTAINTIES IN THE COUPLING BETWEEN THE RAIL AND THE TRANSDUCER APPEAR TO CAUSE VELOCITY SHIFTS THAT MASK THE DESIRED MEASUREMENTS. THIS PROPOSAL AVOIDS THE COUPLING PROBLEM BY USING COUPLANT-FREE EMATS AND IT MINIMIZES THE ROLE PLAYED BY METALLURGICALTEXTURE BY USING A NEWLY DISCOVERED ACOUSTO-ELASTIC EFFECT THAT IS BASED ON THE SYMMETRY PROPERTIES OF THE STRESS TENS TENSOR RATHER THAN THE VALUES OF ANY METALLURGICAL CONSTANTS. THE PROPOSED PHASE I EFFORT IS INTENDED TO DEMONSTRATE THEFEASIBILITY OF DEDUCING THE LOGITUDINAL STRESS LEVEL FROM A PORTABLE PROBE HELD AGAINST THE WEB OF THE RAIL.