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 -
810 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?


 
 Reply 
 
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?



 
 Reply 
 
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?




 
 Reply 
 
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




 
 Reply 
 
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


 
 Reply 
 

Product Spotlight

OmniScan™ X3 flaw detector

The OmniScan X3 flaw detector is a complete phased array toolbox. Powerful tools, like total focus
...
ing method (TFM) images and advanced visualization capabilities, enable you to complete your inspection with greater confidence.
>

ISAFE3 Intrinsically Safe Sensor System

ISAFE3 intrinsically safe sensor system of Vallen Systeme is especially targeted at the petrochemica
...
l - as well as oil and gas transportation industry. The sensor system is designed for permanent monitoring or periodic inspection tasks. Sensors are available for different AE-frequency ranges optimized for corrosion and fatigue crack detection and other applications. The ISAFE 3 sensor system consists of an AE-sensor (model ISAS3) certified according to ATEX/IEC for installation in zone 0, gas group IIC, IP68, 20 to +60 °C, and a signal isolator (model SISO3) certified for installation in zone 2. An ISAS3 sensor can be mounted in atmosphere or submerged, e.g. in water or crude oil. It is supported by mounting tools for temporary (magnets) or permanent (welded) installation. ISAFE3 supports automatic sensor coupling test and can be used with any AE signal processor supporting 28V supply at 90 mA peak, e.g. Vallen Systeme ASIP-2/A.
>

Immersion systems

ScanMaster ultrasonic immersion systems are designed for high throughput, multi shift operation in a
...
n industrial or lab environment. These fully integrated systems provide various scanning configurations and incorporate conventional and phased arrays technologies to support diverse applications, such as inspection of disks, bars, shafts, billets and plates. All of ScanMaster immersion systems are built from high accuracy scanning frames allowing for scanning of complex parts and include a multi-channel ultrasonic instrument with exceptional performance. The systems are approved by all major manufacturers for C-scan inspection of jet engine forged discs. Together with a comprehensive set of software modules these flexible series of systems provide the customer with the best price performance solutions.
>

Aerospace Systems - Automated Ultrasonic Inspection

USL are specialists in the design and manufacture of turnkey ultrasonic inspection systems for aer
...
ospace applications. From monolithic composites to complex honeycomb structures. This video shows just a few examples of what is possible, find out more at: www.ultrasonic-sciences.co.uk
>

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