1、Torsional wave experiments with a new magnetostrictivetransducer configurationYoon Young Kim,Chan Il Park,Seung Hyun Cho,and Soon Woo HanSchool of Mechanical and Aerospace Engineering and National Creative Research Initiatives Centerfor Multiscale Design,Seoul National University,Shinlim-Dong,San 56
2、-1,Kwanak-Gu Seoul 151-742,KoreaReceived 20 April 2004;revised 13 March 2005;accepted 15 March 2005!For the efficient long-range nondestructive structural health inspection of pipes,guided waves havebecome widely used.Among the various guided wave modes,the torsional wave is most preferredsince its
3、first branch is nondispersive.Our objective in this work is to develop a newmagnetostrictive transducer configuration to transmit and receive torsional waves in cylindricalwaveguides.The conventional magnetostrictive transducer for the generation and measurement oftorsional waves consists of solenoi
4、d coils and a nickel strip bonded circumferentially to test pipes.The strip must be premagnetized by a permanent magnet before actual measurements.Because ofthe premagnetization,the transducer is not suitable for the long-term on-line monitoring of pipesburied underground.To avoid the cumbersome pre
5、magnetization and to improve the transductionefficiency,we propose a new transducer configuration using several pieces of nickel strips installedat 45 with respect to the pipe axis.If a static bias magnetic field is also applied,the transduceroutput can be substantially increased.Several experiments
6、 were conducted to study the performanceof the proposed transducer configuration.The proposed transducer configuration was also appliedfor damage detection in an aluminum pipe.2005 Acoustical Society of America.DOI:10.1121/1.1904304#PACS numbers:43.20.Mv,43.20.Gp,43.35.Cg AJZ#Pages:34593468I.INTRODU
7、CTIONThe guided-wave technology has received much atten-tion recently as a powerful tool for the nondestructive in-spection of cylindrical waveguides such as pipes andtubes.16Since guided waves can travel over several metersalong the waveguide axis,the guided-wave technology isvery efficient in insp
8、ecting a large portion of a waveguide.Several wave modes such as longitudinal,torsional,and flex-ural modes can be utilized for nondestructive inspection,butthe torsional wave mode is preferred because its first branchis nondispersive and favorable for signal processing.There-fore,an efficient gener
9、ation of the torsional wave is perhapsan important issue in the torsional wave-based nondestruc-tive evaluation technology.As far as the generation and measurement of the tor-sional wave are concerned,two approaches are available:one approach based on piezoelectric transducers7and theother based on
10、magnetostrictive transducers.8Each approachhas its own advantages and disadvantages,but we are con-cerned with the development of a new magnetostrictivetransducer because magnetostrictive transducers are cost ef-fective and easy to install.The magnetostrictive transducersuse the coupling effect betw
11、een the elastic deformation andthe magnetic field of ferromagnetic materials.Although mag-netostrictive transducers including sensors!have been ap-plied and studied in many cases,914their applications fortorsional wave generation were made only recently byKwun.8The configuration of Kwuns transducer
12、for generatingtorsional waves in a cylindrical waveguides is shown in Fig.1a!.The transducer consists of a nickel strip and a solenoidcoil surrounding the strip.The strip is bonded circumferen-tially to a test specimen such as a pipe and a permanentmagnet is rubbed on the nickel strip for premagneti
13、zation.Any material exhibiting strong magnetization can be used asthe strip material,but nickel is easily available and cost ef-fective.!The premagnetization will induce static magneticfield strength in the circumferential direction,which is indi-cated by HSin Fig.1b!.When alternating current is sen
14、tthrough the solenoid coil,the alternating magnetic fieldstrength(HD)is also developed on the nickel strip in the zaxis direction.If the magnitudes of HDand HSare almostthe same,the resulting magnetic strength vector will point inthe direction that is about 45 from the z axis,as illustratedin Fig.1b
15、!.By developing a new magnetostrictive transducer togenerate torsional waves,Kwun8made a breakthrough in thefield.Nevertheless,the configuration of his transducer hassome drawbacks.First,the nickel strip always needs be pre-magnetized before the transducer is used.When the trans-ducer is to be used
16、for long-term on-line monitoring of un-derground pipes,the strip needs periodic magnetization,which is difficult to achieve.Second,if the magnitudes ofHDand HSare not of the same order,undesirable wavemodes are also generated in addition to the torsional wavemode.To overcome the above-mentioned drawbacks,a newtransducer configuration shown in Fig.2 is proposed.Severalpieces of nickel strips are attached to the test specimen withthe alignment angleaequal to 45.Since the relative per-meability of
