Non- Destructive method for measuring Fastener Preload via Electromagnetic acoustic resonance 2020-01-0011
We describe a novel, non-destructive fastener preload measurement technique based on electromagnetic acoustic resonance, an emerging ultrasonic spectroscopy technique for nondestructive and noncontact materials characterization, relying on the use of electromagnetic-acoustic transducers and the synchronous envelope detection circuitry for processing the received reverberation signals excited by continuous wave excitation method achieved through narrow band radio-frequency (RF) sweeps. The transduction occurs through the Lorenz force mechanism and, for ferrous metals, the dynamic response of magnetostriction and the magnetic force as well. High signal to noise ratio is achieved by receiving the overlapping coherent echoes at resonant frequencies. The spectral response can be interpreted and the propagation velocities can be correlated to stress applied by fasteners to the area medially surrounding the fasteners.
The acoustoelastic effect involves a change in the velocity of acoustic waves when a material is under mechanical stress. This is state of stress in the structure immediately surrounding fasteners is commonly referred to as preload and is a considered a reaction to the tightening action of the fastener. The acoustoelastic effect of material is well-understood and can be exploited to measure preload directly in the surrounding structure at the surface of the structure near the fastener. This approach departs from prior application of ultrasonic stress measurement in tightened fastener stress which rely on analysis of the ultrasonic time of flight of and ultrasonic wave thru the fastener itself rather than the structure. The ultrasonic measurement is often performed by means the time of flight in the fastener before and after tightening which leads to calibration of individual bolts. Accurate measurement of the preload exerted by a fastener during tightening can be exploited for accurate stress measurement both during the tightening process and after the tightening cycle.