7 Electromagnetic Acoustic Transducers EMATs 7 1 EMAT

7 Electromagnetic Acoustic Transducers (EMATs) 7. 1 EMAT Principles 7. 2 EMAT Instrumentation 7. 3 EMAT Applications

Piezoelectricity Quartz (silicon dioxide, Si. O 2)

Electromagnetic Acoustic Transducers (EMATs) Key Features: non-contact/no couplant multiple wave modes (including SH) Advantages: Disadvantages: easy automation low sensitivity high speed scanning requires special electronics high reproducibility material dependent high-temperature inspection minimal wear less surface preparation required easy to customize

7. 1 EMAT Principles

Principle #1: Lorentz Force Lorentz force: Ip Ampère's law: Hp Je Faraday's law: He Ohm’s law: conducting medium Transmission (I F) Reception (v V): B 0 I V v F Je Je

Principle #2: Magnetization Fm magnetic force µ 0 permeability of free space V volume Fm M magnetization y H magnetic field y height χ magnetic susceptibility electromagnet some bias strong bias Signal no bias Time excitation current Signal specimen Time magnetization force Time

Principle #3: Magnetostriction Spontaneous magnetostriction: 10 Induced magnetostriction: H Magnetostriction [10 -6] H=0 low-carbon steel 0 Fe -10 Co -20 -30 Ni -40 0 2 4 Magnetic Field [104 A/m] 6

7. 2 EMAT Instrumentation

EMAT Polarization magnetic force: high coupling: “surface” traction: tangential polarization B 0 normal polarization n. I B 0 Je t

Normal-Beam EMATs spiral coil radially polarized shear wave rectangular coil linearly polarized shear wave N N B 0 S symmetric coil longitudinal wave B 0 N S

Angle-Beam Shear EMATs meander coil vertically polarized shear wave periodic permanent magnet horizontally polarized shear wave L L N B 0 S N S q q

EMAT Electronics EMATs with permanent or electromagnets driver amplifier matching network specimen 7 -turn, 10 -mm-diameter spiral coil on ferritic steel Impedance [Ω] oscillator 20 18 16 14 12 10 8 6 4 2 0 resistance reactance 0 0. 5 1 1. 5 2 Frequency [MHz] 2. 5 3 + _ Vs

Impedance Matching transformer (κ ≈ 1) I 2 I 1 V 1 ideal transformer (κ = 1) N 1 N 2 F 11 F 12 , F 21 F 22 V 2

7. 3 EMAT Applications

Texture Assessment by EMATs cold-pressed 2024 aluminum, 1. 4 MHz, EMAT transmitter receiver Rayleigh wave Textured Specimen cavg = 2, 850 m/s, 0. 2% per division, η = (cmax – cmin)/cavg η 0% (annealed) η = 0. 45 % η = 0. 8 % η = 1. 6 %

High-Temperature Monitoring Si. C/Ti-6 Al-4 V composite (Ogi et al. , 2001) 60 230 190 170 C 44 50 Stiffness [GPa] 210 Stiffness [GPa] 55 C 11 C 33 45 40 C 66 35 30 150 25 20 130 200 400 600 800 Temperature [K] 1000

Electromagnetic Acoustic Resonance (Hirao and Ogi et al. , 2003) SCM 440 steel -1. 0 as-received quenched & tempered annealed load unload Birefringence [%] 0. 05 pure titanium 0 -0. 05 -0. 10 0 50 100 Stress [MPa] couplant 150 -1. 1 -1. 2 -120 -80 -40 Stress [MPa] PZT EMAT specimen 0

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