7 Electromagnetic Acoustic Transducers (EMATs)

1. 7 Electromagnetic Acoustic Transducers (EMATs) 7.1 EMAT Principles 7.2 EMAT Instrumentation 7.3 EMAT Applications

2. Piezoelectricity Quartz (silicon dioxide, SiO2)

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

4. 7.1 EMAT Principles

5. Lorentz force: Ampère's law: Faraday's law: Ohm’s law: Transmission (I F) Reception (v V): Ip B0 B0 Hp Je I V He v F conducting medium Je Je Principle #1: Lorentz Force

6. Signal Signal Signal Time Time Time Principle #2: Magnetization Fm magnetic force µ0 permeability of free space V volume M magnetization H magnetic field y height χ magnetic susceptibility electromagnet Fm specimen y no bias some bias strong bias excitation current magnetization force

7. 10 H = 0 low-carbon steel 0 Fe -10 Magnetostriction [10-6] Co -20 H -30 Ni -40 Magnetic Field [104 A/m] 0 2 4 6 Principle #3: Magnetostriction Spontaneous magnetostriction: Induced magnetostriction:

8. 7.2 EMAT Instrumentation

9. magnetic force: high coupling: “surface” traction: tangential polarization normal polarization B0 n I n I B0 Je Je t t EMAT Polarization

10. N S N B0 S N S B0 B0 Normal-Beam EMATs spiral coil radially polarized shear wave rectangular coil linearly polarized shear wave symmetric coil longitudinal wave

11. meander coil vertically polarized shear wave periodic permanent magnet horizontally polarized shear wave L L S N N B0 S B0 q q Angle-Beam Shear EMATs

12. EMATs with permanent or electromagnets oscillator Vs driver amplifier + _ 20 18 resistance 16 reactance matching network matching network 14 12 10 Impedance [Ω] 8 6 4 2 0 0 0.5 1 1.5 2 2.5 3 Frequency [MHz] EMAT Electronics specimen 7-turn, 10-mm-diameter spiral coil on ferritic steel

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

14. 7.3 EMAT Applications

15. cold-pressed 2024 aluminum, 1.4 MHz, EMAT Texture Assessment by EMATs transmitter receiver Rayleigh wave Textured Specimen cavg = 2,850 m/s, 0.2% per division, η = (cmax – cmin)/cavg η = 0.8 % η 0% (annealed) η = 0.45 % η = 1.6 %

16. 60 55 230 50 45 210 C44 C11 40 Stiffness [GPa] 190 35 30 C66 Stiffness [GPa] 170 25 C33 20 150 200 400 600 800 1000 Temperature [K] 130 200 400 600 800 1000 Temperature [K] SiC/Ti-6Al-4V composite (Ogi et al., 2001) High-Temperature Monitoring

17. 0.05 as-received -1.0 quenched & tempered annealed load 0 unload Birefringence [%] -0.05 Birefringence [%] -1.1 -0.10 0 50 100 150 Stress [MPa] -1.2 -120 -80 -40 0 EMAT Stress [MPa] PZT couplant specimen specimen (Hirao and Ogi et al., 2003) Electromagnetic Acoustic Resonance SCM 440 steel pure titanium

18. Thank You!