Residual Stress Monitoring in Magnetostrictive Cylinders and Tubes

Results on the determination of residual stresses in magnetostrictive cylinders and tubes, using the magnetostrictive delay line (MDL) principle, are presented in this article. Stresses were induced in selected cross section regions in the under-test cylinders and tubes by means of localized induction heating and consequent water quenching. Three different MDL arrangements are used for this purpose. In all cases, coils are used as excitation and detection means. The first one employs the steady-in-position excitation coil, with the search coil moving along the length of the cylinder or tube to determine the amplitude of the front-end magnetoelastic waves and therefore to observe the localized stress fields. In the second one, the excitation and search coils assembly is fixed at a given distance between the two coils, while the assembly is displaced along the cylinder and the tube, measuring the delay time changes, being direct information on the stress field in the volume between the two coils. The third arrangement employs steady-in-position excitation and search coils, with the search coil been a long solenoid along the length of the under-test cylinder or tube, indicating the areas and the size of stress fields. Localized, alternating field B-H hysteresis loop and eddy current measurements, using coupled primary and secondary coils have also been performed for comparison reasons. From all these measurements, the fixed assembly of excitation and search coils appeared to be promising for actual industrial-field applications.