where expertise comes together - since 1996 -

The Largest Open Access Portal of Nondestructive Testing (NDT)

Conference Proceedings, Articles, News, Exhibition, Forum, Network and more

where expertise comes together
- since 1996 -

737 views
Technical Discussions
A. Anev
A. Anev
06:45 Sep-26-1996
Comparison of dry-coupled, air-coupled and EMAT transducers

What is the difference in the accuracy of dry-coupled, air coupled and EMAT transducers generally and regarding near surface and internal imperfections?
Suppose all the transducers work at the accuracy at which they give the best results.
Furthermore do you think polymer transducers can be used as dry-coupled ones?


    
 
 
A. Anev
A. Anev
06:41 Sep-27-1996
Correction Re: Comparison of dry-coupled, air-coupled and EMAT transducers
: What is the difference in the accuracy of dry-coupled, air coupled and EMAT transducers generally and regarding near surface and internal imperfections?
: Suppose all the transducers work at the accuracy at which they give the best results.

I'm sorry. I wanted to say : Suppose all the transducers work at the FREQUENCY (not accuracy) at which they give the best results.

: Furthermore do you think polymer transducers can be used as dry-coupled ones?



    
 
 
Yoseph Bar-Cohen
R & D,
Jet Propulsion Lab (JPL), USA, Joined Nov 1998, 26

Yoseph Bar-Cohen

R & D,
Jet Propulsion Lab (JPL),
USA,
Joined Nov 1998
26
07:56 Sep-27-1996
Re: Comparison of dry-coupled, air-coupled and EMAT transducers
It is possible to produce a dry-coupled transducer that is driven by
polymer piezoelectric materials (PPM). However, this application will
not be an effective use of these piezoelectric materials because of
the spectral range that is required. The useful range of dry-coupled
transducers is below 1MHz, on the other hand PPM transducers are
producing a reasonably high signals only at the MHz range. In
general, PPM are efficient receivers due to their low noise but they
are relatively weak transmitters.

Yosi


: What is the difference in the accuracy of dry-coupled, air coupled and EMAT transducers generally and regarding near surface and internal imperfections?
: Suppose all the transducers work at the accuracy at which they give the best results.
: Furthermore do you think polymer transducers can be used as dry-coupled ones?




    
 
 
Rathgeb/Meier
Rathgeb/Meier
09:32 Sep-27-1996
Re: Comparison of dry-coupled, air-coupled and EMAT transducers
: What is the difference in the accuracy of dry-coupled, air coupled and EMAT transducers generally and regarding near surface and internal imperfections?
: Suppose all the transducers work at the accuracy at which they give the best results.
: Furthermore do you think polymer transducers can be used as dry-coupled ones?

The use of EMAT transducers is restricted to materials with a high
conductivity. The results measured on aluminum or ferritic steel test pieces
are very good, stainless steel test pieces may show worse results. This behaviour
also depends on design details of the EMAT-probe:
- EMAT SH (shear wave horizontal)probes with the electrical coil between the
rows of permanent magnets and the test piece surface work very well also on
stainless steel components, but due to design details, this probe type is
restricted to frequencies below 1 MHz.
- EMAT SH probes with the electrical coil wounded arround a ferritic yoke
allow frequencies up to 2 kHz, but they do not work on stainless steel.
- New develloped Siemens SH probes allow frequencies up to 2 MHz and work
also on stainless steel, but the sensitivity on ferritic steel components
is some dBs better.
- New develloped Siemens SV (shear wave, vertical polarised) probes with
frequencies up to 2 MHz work also on stainless steel components. The
sensitvity on ferritic steel or on aluminium is also some dBs better.
With these probes, used as phased array probes, it is possible to produce
a 0 degree shear wave, a 40 to 45 degree shear wave or a rayleighwave.

Generally, EMAT suffers on a lift off effect: The higher the frequency,
the bigger the lift off. This requires a flat surface for a good "coupling"
of the probe. In the case of the above mentioned SV probe, the lift off effect
(loss of sensitivity) can be corrected by evaluating the amplitude of a
0 degrees wave reflected at the backwall.

EMAT also suffers on a quite long dead zone, due to crosstalk of the transmitter
current to the receiver. Also the sound beam is not so well formed as in the
case of piezo transducers.

Advantages of EMAT are the testing without couplant and the possibility to
choose the orientation of the polarisation! The second means, that one can
avoid the conversion into other wave modes by use of EMAT and one can also
minimize the deflection of the ultrasonic beam in case of a distinct grain
orientation in stainless steel or dissimilar welds by choosing the right
polarisation of the ultrasound.

Dependent on the above mentioned factors (material, frequency, surface, probe
design etc.) it is possible to detect notches with a depth of about 1mm, as well
on the probe near surface (rayleigh wave, lamb wave) as on the probe far
surface (45 degree shear wave). Up to now, precise sizing with EMAT is still
a problem, but this is improoving!

Rainer Meier




    
 
 
A.Anev
A.Anev
09:03 Oct-01-1996
Thank you for the answers
Thank you very much for the answers.
The idea about this workshop is excellent.
I really enjoyed visiting it.
Best regards,
A. Anev


    
 
 

Product Spotlight

IRIS 9000Plus - Introducing the next generation of heat exchanger inspection.

Representing the seventh generation of the IRIS system, the IRIS 9000 Plus has nearly 200 years of c
...
ombined field inspection experience incorporated in its design. This experience combined with a strong commitment to quality and a history of innovation has made Iris Inspection Services® the undisputed leader in IRIS technology.
>

iProbe - USB Phased Array Probe

Turn your PC, Laptop or Tablet into a powerful 32:64 Phased Array system with our USB powered Phas
...
ed Array iProbe and inspection software package. Perform affordable weld inspection and corrosion mapping inspections with this powerful but small system. The probe has a 100V pulser, 8K PRF, dual axis encoder input and achieves outstanding signal quality.
>

Ultrasonic Squirter/Gantry System

TecScan’s NDT Ultrasonic Gantry Systems are industrial Squirter scanners designed for non-destru
...
ctive quality testing and raster scanning of large structures and parts. The Gantry Systems are usually composed of an Industrial Mechanical Scanner and a Control Room with System Workstation.
>

Robotic laser shearography enables 100% inspection of complex, flight-critical composite structures

An article in “Composites World Magazine” showcases Non Destructive Testing of aero-structures
...
with Laser Shearography. Over the years Dantec Dynamics has supplied many solutions for the aerospace industry. Referring to specific customer projects several of these cases are examined to outline the advantages of using Laser Shearography for automated defect detection.
>

Share...
We use technical and analytics cookies to ensure that we will give you the best experience of our website - More Info
Accept
top
this is debug window