A study of the transduction mechanisms of electromagnetic acoustic transducers (EMATs) on pipe steel materials

• The transduction mechanisms in operation when MS and NB EMATs are used on steels are the MF, SLF and DLF.
• For NB-EMATs, the conductivity of the material is proportional to the critical excitation current.
• The research established the minimum excitation current required to produce acoustic wave by DLF.
• NB-EMATs require less energy to attain the critical excitation current than the MS-EMATs.
• A novel algorithm was developed to decouple and quantify the transduction mechanisms inherent in EMATs.

This research paper presents in detail a novel modeling strategy to decouple and quantify the various transduction forces in operation when EMAT (Normally Biased and magnetostrictive configurations) are used on various grades of pipe steel materials. The strategy established the value of the critical excitation current (CEC) of the EMAT configurations studied. The CEC, when magnetostrictive EMAT configurations (MS-EMAT) are used to generate acoustic waves, was found to be: 260A, 268A, 270A, 274A, 279A, 280A, 286A, 289A, 294A and 305A for EN16, CS70, EN24, L80SS, L80A, TN80Cr3, EN8, EN36, EN3 and J55, respectively, while for Normally Biased EMAT configurations (NB EMAT), the critical excitation currents was found to be 181A, 190A, 197A, 205A, 240A, 122A, 203A, 160A, 231A and 200A for EN16, CS70, EN24, L80SS, L80A, TN80Cr3, EN8, EN36, EN3 and J55 respectively. This implies that beyond the CEC, where the dynamic Lorentz force mechanism dominates, it is possible to develop a coil only EMAT, thereby eliminating the undesirable effect of using a permanent magnet or electromagnet and also reducing the overall size of the EMAT. Furthermore, the research gave an insight into the relationship between the CEC and the electrical and magnetic properties of the steel materials used in this study and also compared the CEC for both the NB and MS-EMAT configurations. A conclusion drawn is that NB-EMATs are more efficient in the generation of acoustic waves as they have a lower CEC than their MS EMAT counterpart.