DIRECT MEASUREMENT OF BOLT TENSION UTILIZING MAGNETOSTRICTION

A NEW METHOD FOR MEASURING BOLT TENSION DIRECTLY IS PROPOSED WHICH EXPLOITS THE RELATIONSHIP BETWEEN STRESS AND MAGNETIC FLUX DENSITY IN MAGNETOSTRICTIVE MATERIALS. A CONVENTIONAL WASHER IS EITHER REPLACED OR SUPPLEMENTED BY A WASHER MADE FROM A MAGNETOSTRICTIVE ALLOY. A WRENCH, MODIFIED TO INCLUDE A HALL EFFECT PROBE AND A PERMANENT MAGNET, IS UTILIZED TO MONITOR THE CHANGE IN MAGNETIC FLUX DUE TO COMPRESSION OF THE WASHER. THIS REPRESENTS AN IMPROVEMENT OVER CURRENT TECHNIQUES, IN THAT THE COMPRESSIVE FORCE ON THE WASHER IS A DIRECT MEASURE OF THE TENSION IN THE BOLT. CONSEQUENTLY, CALIBRATION TO ACCOUNT FOR BOLT PARAMETERS SUCH AS LENGTH, STRESSED LENGTH, DIAMETER, VELOCITY OF SOUND, TENSILE STIFFNESS, AND SURFACE CHARACTERISTICS IS NOT NECESSARY. THE OBJECTIVE OF PHASE I IS TO DETERMINE THE FEASIBILITY OF THIS APPROACH AND TO DEVELOP A OPTIMUM CONFIGURATION. THE ANTICIPATED RESULT IS A BOLT TENSION MEASUREMENT TECHNIQUE WHICH IS EASY TO USE, ACCURATE, APPLICABLE TO ALL JOINT CONFIGURATIONS AND BOLT TYPES, AND WHICH DOES NOT REQUIRE BOLT PREPARATION OR CALIBRATION ON AN INDIVIDUAL BASIS.