1Sonic Sensors of EMAT Ultrasonics Inc. SLO CA USA

Non-Contact EMAT Non-Contact EMAT TransductionTransduction

N

SI

Ie

A current, I, in a wire near a metal surface induces an equal and opposite eddy current, Ie in the surface of the metal.

When the eddy current occurs in a magnetic field a force is created in the metallic surface.

F = I x BThis force can generate elastic waves, ie. ultrasonic Guided Waves

B

F

2Sonic Sensors of EMAT Ultrasonics Inc. SLO CA USA

EMATs are VersatileEMATs are Versatile Surface Waves: Rayleigh Surface Waves: Rayleigh

WaveWave

Surface SH wavesSurface SH waves

Angle Beam waves: SH or SVAngle Beam waves: SH or SV

Through Thickness Shear Through Thickness Shear WavesWaves

Through Thickness “L” WavesThrough Thickness “L” Waves

SV

SH

LS

3Sonic Sensors of EMAT Ultrasonics Inc. SLO CA USA

EMAT are Exceptional EMAT are Exceptional for Guided Wavesfor Guided Waves

Often called Plate Waves

Sensitive to Plate Thickness

There are many different kinds of Plate Waves

These depend on the displacement through the plate thickness

EMATs are great for SH waves

SHn

Shear Horizontal

Propagation

Propagation

Propagation

Sn

Symmetric

An

Anti-symmetric

4Sonic Sensors of EMAT Ultrasonics Inc. SLO CA USA

EMATs offer Superior EMATs offer Superior Control of Guided WavesControl of Guided Waves

FREQUENCY in SteelEMAT = M120, Wavelength = 6mm

0.4

0.450.5

0.550.6

0.650.7

0.750.8

0.85

0.90.95

11.05

1.11.15

1.21.25

1.3

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 0.55

Plate Thickness (inches)

FREQ

(MHz

)

S0 S1 S2

S3 S4 A1

A2 A3 Rayleigh

There are MANY guided waves to possible in a single thickness plate

Isolating a single mode can be very difficult with a piezo transducer

EMATs isolate guided waves easily!

EMAT defines the wavelength

Choose the frequency at the defined EMAT wavelength, and only one mode generated and detected.

M120 EMAT only generates and receives these modes in a 3/8 inch thick plate

5Sonic Sensors of EMAT Ultrasonics Inc. SLO CA USA

NDE Indication (sized between accept & reject curves)

Length Depth

UpstreamSide of Weld

DownstreamSide of Weld

Automated AnalysisAutomated AnalysisExample: Sizing of defects in Pipe Girth Weld