1、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 re
2、ceived 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 sta
3、tic 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 t
4、his 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 wh
5、ich 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-fr
6、equency 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 magnetostricti
7、ve 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 capabil
8、ity.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 c
9、oil.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
10、 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-performa
11、nce 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 field and cost-effective in comparison wi
12、th permanent magnet.Our objective in the present research is to investigate the effect of the bias magnetic field generated by these optimized yokes on the measured signal output.Especially,we focus on flexural wave-based damage detection in a solid cylinder.The ball drop method has been used as a s
13、tandard method to generate elastic flexural waves8-9.The ridge analysis procedure is employed to estimate instantaneous frequencies by the continuous Gabor wavelet transform(GWT)9 that is utilized as an effective and powerful time-frequency analysis tool for identifying rapidly-varying dispersive wa
14、ve signals.Analysis:A.Bias Magnetic System Design Optimization In this work,we will consider the measurement of flexural waves in a long solid ferromagnetic cylindrical waveguide shown in Fig.1.The voltage output of the coil shown in Fig.1 results from the magnetic field change of the ferromagnetic
15、cylinder at x=xs by the elastic flexural wave.One can show3 that the voltage output V(xs,t)can be written as 21(,)(,)(,)bssAx tV x tEncBxy ydAt x=s (1)where Bb is the magnetic flux density along the x axis.In Eq.(1),c1 is a constant and n is the number of turns of a coil.Youngs modulus and the rotat
16、ion of the cylinder cross section are denoted by E and,respectively.From Eq.(1),we immediately see that the maximum sensor output V(x,t)for flexural wave measurement will be obtained when the integral is maximized.Therefore,if we maximize the following integral f(xs),the performance of the sensor will be maximized:()(,)bsxsAf xBxy y=dA (2)Fig.1.The application of the magnetostrictive sensor consisting of a coil and bias magnets for flexural wave measurement in a long ferromagnetic cylinder.In or
