The field of Non Destructive Testing (NDT) has shown tremendous growth in the last half century. Its ability to provide safe volumetric inspection of materials with limited access areas makes ultrasonic testing (UT) one of the fastest growing technique in NDT. However, the Achilles heel of Ultrasonic Testing (UT) has always been the transferring of energy from the UT probe into the material subject of the inspection. Because air limits the transfer of ultrasound, the transducers need to be coupled to the material using a liquid that promotes the transfer of energy into the material.

1. Metal Producing and Processing
February 2005 Page 1 of 3 Innerspec®
Technologies, Inc.
Ultrasonic Inspection Without Couplant
1. Introduction
The field of Non Destructive Testing (NDT) has shown tremendous growth in the last half century. Its
ability to provide safe volumetric inspection of materials with limited access areas makes ultrasonic testing
(UT) one of the fastest growing technique in NDT.
However, the Achilles heel of Ultrasonic Testing (UT) has always been the transferring of energy from the
UT probe into the material subject of the inspection. Because air limits the transfer of ultrasound, the
transducers need to be coupled to the material using a liquid that promotes the transfer of energy into the
material.
2. Technology Comparison
One of the most significant UT developments of the last 20 years is the advent of non-contact solutions
that do not require coupling, such as Electro Magnetic Acoustic Transducers (EMAT), used for ultrasonic
testing of metals. An EMAT consists of a magnet and a coil of wire and relies on electro-magnetic
acoustic interaction for elastic wave generation. Using Lorentz forces and magnetostriction, the EMAT
and the metal test surface interact and generate an acoustic wave within the material thus the material
becomes its own transducer. Figure 1 shows a technology comparison between traditional Piezoelectric
UT and EMAT UT.
Figure 1. Technology Comparison
Piezoelectric
UT
EMAT UT
Magnet
EMAT Coil
Circuit
Ultrasonic WaveUltrasonic Wave
Eddy Currents
Lorentz
Force
Magnetic Field
Couplant
Crystal

2. Metal Producing and Processing
February 2005 Page 2 of 3 Innerspec®
Technologies, Inc.
EMATs have all the benefits of ultrasonic testing, but because the sound is generated in the part
inspected they enjoy some unique advantages:
• Dry Inspection (no couplant). Not having couplant permits more reliable readings (no couplant errors)
and makes this technology easier to automate and integrate in production. High inspection speeds
and high temperatures are also a fundamental advantage of EMATs.
• Insensitive to Surface Conditions. EMATs are not sensitive to oxides, oil, water or uneven surfaces
and can inspect through thin coatings of material.
• Unique Wave Modes. Because they do not depend on liquid to transmit the sound, EMATs can
generate any type of wave mode including Horizontally polarized Shear energy (SH). Shear
Horizontal energy does not mode convert when striking surfaces that are parallel to the direction of
polarization. This is key to inspecting austenitic welds and other materials with dendritic grain
structures. (e.g. some Stainless Steels). Another type of waves that can be easily generated with
EMATs are guided or plate waves (Lamb or SH at 90º).
The main disadvantage of EMAT is the low efficiency of the transducer which require high currents and
very precise electronic designs to generate and detect the signals. These disadvantages are becoming
less relevant with the advent of new electronics and software tools that enhance complex signal
processing in real time.
3. In-Line Inspection
EMATs are especially well suited for in-line integration, where the objective is to have automated, real-
time inspection of 100% of the material. Systems that are commercially available include:
• Weld Inspection. EMAT UT is especially advantageous for weld inspection when compared with
piezoelectric UT. The shear wave is most commonly used for ultrasonic weld inspection. Shear
Vertical (SV) and Shear Horizontal (SH) both have particle vibrations perpendicular to the wave
direction. Conventional ultrasonic inspectional utilizes the SV wave, with an angle of between 30o and
60o from the normal beam. Maintaining the position of the probe is critical to obtaining an accurate
inspection. A limitation of SV waves in weld inspection is the inability to cover the full vertical volume
of the material. At some points defects may even limit complete inspection. On the other hand, an
EMAT can generate a Shear Horizontal Wave at 90º that fills up the full volume of the material and
permits inspection of the full cross-section of the weld in one pass (without rastering motion).

3. Metal Producing and Processing
February 2005 Page 3 of 3 Innerspec®
Technologies, Inc.
• Full Volumetric Inspection of Plates and Tubes. Taking advantage of guided waves, EMAT systems
can perform full volumetric inspection with a limited number of probes and detect defects in all
directions.
• Cast Inspection. Ultrasonic EMAT can be used for flaw detection, thickness or velocity measurements
used for nodularity testing. The flexibility in sensor design, permits the overlapping of transducers to
achieve complete coverage and maximum sensitivity in one inspection pass.
4. In-the-Field Inspection
With new, more efficient electronics and software, EMAT systems are becoming more common for Field
Inspection. Instruments that are commercially available include:
• Hand-held Thickness Gages. EMATs have recently entered the portable instrument arena with
instruments that incorporate the latest electronics technology capable of running complex algorithms
in real-time. The temate®
Thickness Gage provides couplant-free measurements for up to 10 hours
running exclusively on battery power.
• In-Service Tube & Pipe Inspection. Designed for inspection of petrochemical pipes in the field. The
newest system by Innerspec Technologies is capable of performing full circumferential and
longitudinal inspection of pipes in the field in a compact but powerful package.
5. Summary
Created as a non-contact, dry alternative to piezoelectric transducers, Ultrasonic EMAT systems are no
longer limited to laboratories and high-end applications and are now widely used in industrial
environments and in-service operations.
EMAT’s ability to provide volumetric ultrasonic inspection without the inconvenience and problems
associated with liquid couplants, and the capability to easily generate Shear Horizontal energy and guided
waves make it the technique of choice for demanding applications where speed, reliability and quality of
readings is paramount to the success of the inspection.