Electromagnetic Acoustic Spectrocopy


3d Lyd

VASI and Resonic are focused on comprehensive inspection of safety critical, and specialty metal products. Our NTD systems and process expertise lie in the following areas:


  • Continuous inspection of small and medium diameter wire, rod, and tubing
  • Automated inspection of forged, stamped, and cast  components, fasteners/bolts, and other critical parts
  • Bonding/adhesion quality measurement of thin sheet metal and composite assemblies


Principles of Operation

The basic principle of the EMAS or more correctly the RM-EMAT (resonant mode electromagnetic transducer) technique is the creation of a very strong & localised electromagnetic field from a driving transducer (manufactured with the use of ultra strong boron based magnets).


The driving transducer transmits ultrasonic acoustic shear-waves through the field into the metal, and the resonant frequencies are picked up by a secondary transducer. None of the tested components need to be in direct contact with the transducers during the testing. Hence the inspection process is "non-contact", making it the only real NDT system that can be fittet into production lines with continous feed of materials, like wires, bolts etc. The principle of operation is very simply, but ingeniously in the sense that any defects will result in attenuation, dampening or complete disappearance of resonance.     


The important distinction between EMAR and traditional Resonant Ultrasound Spectroscopy (RUS or RAS) is in the use of polarised EMAS transducers. 
EMAS’ are not used to force the entire part into resonance but instead create localized, acoustic fields in the areas directly near the sensor faces. With EMAT; resonance are analysed in a much narrower band and generally at higher frequencies. EMAS' are selective in excitation and detection of resonant modes. They are therefore designed to pick-up modes that contain relevant acoustic information about the materials evaluation task.
Enhanced control over size, band, and modes is the basis for a very low noise and high sensitivity resonance based NDT technique.

Hence the system is ideal for what is also known as spectroscopy of metal and metal products.
The real beauty about the system is the enabling of the observer to literally see through metal without the use of radioactive material or other “none- human friendly” or HSE risky techniques.  

The comparison to modern medicine is useful and as any E&R doctor or nurse will confirm; 
The use of ultrasonic diagnostic is a cheap and very precise way of determining the presence of defects (ref the increasing use of MR scans). Medical personnel will use expensive x-ray based techniques as CT (computer tomography), CAT etc, only when they need to determine the defects precisely for an operation or treatment assistance. 

The potential to do a full imaging with EMAS is already demonstrated, and is used in  Resonics systems for scanning high performance engine valves, which are bi-metallic and contains welds. The system can with the new transducer developments be tuned to such perfection that it even picks up hardness differences down to +/-1 RC  (if the volumetric properties are constant). 


In our bolting/fastener case however, we do not really care if the defect is a crack, inclusion or zone of porosity. When it comes to fasteners we will simply take the samples showing signs of defects out of use, and prevent them from entering the assembly process.  
A very important added benefit of the ARIS EMAS systems is the none-contact feature. 
Unlike any other NDT inspection method, the transducers, does not need to be in direct contact with the material. They can e.g. sit 0,2-0,5mm (size/componet dependant) from the bolt diameter and still perform a full scan.
This feature is ideal for automated inspection techniques, but it is also essential in bolt inspections for identifying root-cracks in the threaded sections, which we also have proven detection of in our test lab.    
In terms of automation this enables automated test systems to perform a pass/fail scan of a bolt sample in seconds.