Structural health monitoring of thin plate like structures based on the analysis of ultrasonic guided waves has demonstrated extensive capabilities including the abilities to detect, locate, characterize, and quantify damage and even compensate for environmental effects.   these capabilities require different levels and types of analysis of changes in the waveforms representing the various different modes of propagation. For example the basic abilities of detection and location can be performed raw signals containing multiple modes and distortions,  but the more detailed analyses require knowledge of the specific waveforms and paths represented in the signal. Unfortunately the output sensor signals generated by these systems regularly represent multiple waveform propagation modes and they are not easily interpreted. Further complicating signal analysis is the fact that the wave modes propagate at different velocities, are dispersive,  and can distort, reflect, refract, or even change modes when they encounter structural heterogeneities.

 

Antisymmetric wave modes have been found to be particularly useful for analysis and challenging to isolate.   Antisymmetric modes are sensitive to different types of damage than symmetric modes and at relatively low frequency have a slower propagation velocity and shorter wavelength than symmetric modes making them sensitive to smaller defects.   Unfortunately the slower propagation velocity also results in larger distortion and later arrivals where they can encounter constructive and destructive interference from symmetric modes that have propagated multiple paths.

 

Several techniques have been developed to support signal analysis including modeling of the wave propagation the structure, sizing transducers to suppress a single mode at a single frequency,  or otherwise designing transducers to selectively actuate and sense a single propagation mode. Past efforts of each of these have encountered limitations that inhibit broad adoption. This paper presents recent work on a novel transduction technique that selectively couple  to anti-symmetric waveforms efficiently and independent of frequency.

 

Recent research has identified the ability to exclusively actuate and sense antisymmetric Lamb wave modes by locating shear deforming piezo-electric transducers at the neutral axis of a structure. This approach was intended to leverage the transverse shear phenomena from established beam theory which is inherently coupled to beam bending and is maximum at the structural  neutral axis.

 

This work will present a combination of matching experiments and finite element simulations.  Further investigation and parametric studies were performed using the validated finite element models to inspect ultrasonic wave interactions with various forms of damage, and evaluate the sensitivity of various established damage indices in a system  that's only actuates and senses antisymmetric wave modes. Additional analysis was performed to inspect the effects of locating shear deforming transducers within a plate like structure but off of the neutral-axis which could be a consequence of design or manufacturing defect. 


Effects of D15 Mode Pzt Transducer Location on Mode Selectivity of Lamb Waves

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

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