 Home |
|  Home |
473KB
|
THE EFFECT OF THE BIAS MAGNETIC FIELD ON THE MAGNETOSTRICTIVE SENSOR PERFORMANCE IN FLEXURAL WAVE-BASED DAMAGE DETECTION IN A CYLINDER Woochul Kim1 and Yoon Young Kim 1Seoul National University, Seoul, South Korea
Abstract: Flexural wave measurements for non-destructive evaluation in waveguides have received much attention recently. The main motivation to use dispersive flexural waves is that longitudinal waves employed for non- destructive evaluation often go through mode conversion into flexural waves as in curved regions. The objective of this investigation is to investigate the effect of the static bias magnetic field on the measured output of a magnetostrictive non-contact sensor for flexural waves. Unlike measuring longitudinal waves by the magnetostrictive sensor, the sensor output for flexural wave measurement is affected significantly by the applied bias magnetic field distribution. In this work, we will consider a few bias magnetic configurations to reject the longitudinal waves but capture only the flexural waves, and investigate their performance through several flexural wave experiments. Specially, we will consider the measurements of flexural waves in cracked solid cylinders in which the flexural waves are generated by ball drops perpendicular to the cylinders. Through these experiments, it will be addressed that small cracks are difficult to diagnose without using an optimal bias magnetic system. The signals measured by the magnetostrictive sensors were analyzed in the time-frequency plane for accurate damage assessment.
Introduction: When time-varying mechanical loads are applied to a ferromagnetic material, the magnetic field distribution within the material changes. This phenomenon is known as the inverse magnetostriction effect or the Villari effect1-2. The magnetostrictive or magnetomechanical sensor measures elastic strain waves in ferromagnetic materials based on this phenomenon. Other sensors, based on electromechanical or piezoelectric principles may also be used to measure elastic waves, but only the magnetostrictive sensor has the non-contact measurement capability. In addition, the magnetostrictive sensor has a simple configuration: the sensor simply consists of coils surrounding the specimen and one or more bias permanent magnets. The change in the magnetic flux density within the ferromagnetic material is converted to the voltage change in the surrounding coil. Thus far, applications of magnetostrictive sensors have mostly focused on the measurement of longitudinal and torsional waves in ferromagnetic waveguides. Recently, however, the sensor has also been used for the measurement of flexural vibrations and waves3. A recent paper4 shows that when the sensor is employed for flexural wave measurement, the location of the bias magnets must be carefully selected. Cho et al5. proposed an optimal shape of the permanent magnet and Kim and Kim6-7 also proposed the bias magnetic system using optimized yoke shape obtained by topology optimization for high-performance magnetostrictive sensors applicable for the measurement of flexural waves propagating in ferromagnetic waveguides. The magnetostrictive sensor using yokes instead of permanent magnet have merits in that it can be adjusted the strength of the bias magnetic |
| © NDT.net |