The Ultrasonic Measurement of Stress on Ferrous Plate using Pulsed Electromagnet Magnetostrictive Emat Probes
Last year we finished the design and assembly of a semi-automatic system for the measurement of stress in textured ferritic steel, Magneto- strictive EMATs, which have no contributions generated by Lorentz force, were used on ferrous alloys since the absence of the Lorentz force mechanism precludes any velocity errors that might be produced by interference of Lorentz force and magnetostrictively generated signals. Phase shifts due to lift-off were solved by driving the transmitter EMAT’s in series. To make reliable wave-speed measurements in an industrial environment with an economical sampling period of 100 nanoseconds, a Fourier Transform-Phase-Slope technique was implemented which not only solves the zero-crossing problem[1,2], but achieves accuracy approaching the Cramer-Rao bound, Initial data are improved over the previous year’s work, however, further refinements are required to archive the expected error bound of ± 2 ksi. These refinements take the form of reducing the effects of temperature induced velocity changes during the measurement process.
KeywordsLorentz Force Velocity Error Ultrasonic Measurement Ferrous Alloy Time Delay Estimate
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