How it works
How It Works
EMAR is a contactless, resonance spectroscopy NDT technique for use on metals and non-metallic materials that are electrically conductive. At its core it is a real time measurement of material elasticity and, like traditional UT, velocities of sound. But unlike UT or any other NDT technique, EMAR is also a measurement of the complete spectrum of physical and chemical properties that affect elastic behavior in materials.
EMAT Sensors + Acoustic Resonance
A specialized probe design and a spectroscopy method work together in EMAR and make up the Acoustic Resonance Inspection System line of NDT products.
Resonance Mode-Electromagnetic Acoustic Transducers (RM-EMAT) are ultrasonic probe types that use magnetic fields and a Lorenz force mechanism to excite resonance with no direct contact nor the use of coupling fluids.
Relatives of the RM-EMAT are used in guided wave ultrasonic test systems but the two sensor types are not interchangeable. RM-EMATs lack the ability to create long travel acoustic waves and Guided Wave-EMATs cannot generate the multiple narrow band resonant modes required for spectroscopy analyses.
- A transmitting RM-EMAT sensor is located near the surface of a part or material and forces the part into resonance at a restricted number of its natural resonant modes using a narrow band, swept frequency excitation signal.
- A similar receiving sensor picks-up the resonance events in the material with the same method and presents the modulated signal for processing.
The important distinction between EMAR and traditional Resonant Ultrasound Spectroscopy (RUS or RAS) is in the use of EMAT sensors. EMATs 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 EMATs resonances are analyzed in a much narrower band and generally at higher frequencies. EMATs also are selective in excitation and detection of resonant modes. They are designed to pick-up only modes that contain relevant acoustic information about the materials evaluation task at hand. Enhanced control over size, band, and modes is the basis for a very low noise and high sensitivity resonance based NDT technique.
In practice, an integrated EMAR test procedure will often overcome small active test areas by incorporating motion controlled sensors or moving test subjects. These systems work by examining many small, bounded areas in rapid succession to get a comprehensive look at longer, larger, or more geometrically complex test subjects.
An internal combustion engine valve is a highly engineered and high performance component which serves as a good example of testing by EMAR. Instead of a valve stem resonating as a string or rod along its length, consider that the stem resonance can be thought of as hundreds of smaller strings that vibrate at significantly higher frequencies through its diameter. An EMAR test is a lengthwise scan of the stem in which each of the strings are excited and analyzed individually yielding finer defect detection.
Resonance is through thickness and so it is equally sensitive to surface and internal defects.
A non-destructive test of metals is made by observing changes to resonant signals within a part or from part to part. The analytical methods can be made to include both supervised training algorithms and calibrated quantitative measurements which are extremely responsive to small changes.