NON-DESTRUCTIVE ULTRASONIC TESTING METHOD
An alternative method of non-destructive ultrasonic testing that consists in that the wave is generated first in the tested element and not in the piezoelectric transducer. Thanks to this, we can carry out the testing in conditions unavailable for the classical ultrasonic methods.
The EMAT generates ultrasonic waves in the element being tested using two interacting magnetic fields. The field with a relatively high frequency, which is generated by the electrical coils, acts on the field with low frequency or a static field generated by permanent magnets in order to produce a Lorentz force field in a manner similar the operation of an electric motor. Introduction of such an interference into the crystalline network causes generation of an elastic wave. In the process of interacting of elastic waves, in the presence of a magnetic field, currents are generated in the EMAT receiver coil circuit.
Advantages of the technology
The EMAT technology does not require a couplant to transfer waves, which makes it ideally suited for inspection of objects of high temperatures and for applications in automated inspection stations. In the case of the EMAT technology, the state of the surface of the tested material does not affect the course of the inspection, which enables inspections of surfaces that are polluted or covered with corrosion or rough ones. Due to the manner of generating the wave, Snell’s law is not applicable, and the position of the probe against the tested area does not affect the direction of wave propagation. This makes probes easier to operate, especially in automated environments.
– option of signal standardisation for the purpose of automatic and continuous self-calibration
– no couplant or contact with the inspected surface
– the practical distance of the coil from the inspected surface is between 0-3 mm
– a greater distance (even up to 10 mm) can be achieved depending on the material inspected and the devise used and the type of test
– ideal for automated testing and testing in high temperatures
– no impact of the state of the surface on the test result (coatings, oil, corrosion)
Kinds of waves
With the advantage of the EMAT technology we can induce any kind of the waves that are used in ultrasonic testing, including waves that are difficult to generate or even unachievable in the case of classical ultrasonic transducers. The below table presents the kinds of waves and the techniques available for various applications.
INDUSTRIAL AND QUALITY STANDARDS
Testing based on the EMAT technology can be performed in accordance with the standards: ISO, AWS, API, MIL-STD and other. So far, Innerspec has developed examination systems compliant with the following standards:
– API 5CT (ISO11960) and API 5L8 for thin-walled elements (OCTG)
– EN10160 for flat elements
– MIL-STD 2154 ultrasonic inspection of wrought metals
– CGA C-20 for high-pressure cylinders
– EMAT technology is also described in detail in the guidelines of the American Society for Testing
– ASTM E1774-96 Standard Guide for Electromagnetic Acoustic Transducers (EMATs)
– ASTM E1816-96 Standard Practice for Ultrasonic Examinations Using Electromagnetic Acoustic Transducer (EMAT) Technology
– ASTM E1962-98 Standard Test Methods for Ultrasonic Surface Examinations Using Electromagnetic Acoustic Transducer (EMAT) Technology
Portable high-power ultrasonic flaw detector has been designed for ultrasonic examinations that require high voltage and long high-energy excitations, such as non-contact examinations (EMAT, airborne coupling), examinations of materials characterized by strong attenuation or for application of low frequencies. The device enables generation of a signal of 1200 V and 8 kW peak power at pulse repeatability up to 300 Hz. PowerBox H is fitted with an acquisition module and a broadband transmitter/receiver, which make it possible to perform flaw detection testing, thickness measurements as well as examinations of the state of the material structure, both under laboratory conditions and in field. Pulses and pulse packets of frequencies between 100 kHz and 6 MHz allow generating the full range of ultrasonic waves including volume waves and guided waves, in the pulse-echo and pitch-catch modes.