This research considers the effects of diffraction, attenuation, and the nonlinearity of generating sources on measurements of nonlinear ultrasonic Rayleigh wave propagation. A new theoretical framework for correcting measurements made with air-coupled and contact piezoelectric receivers for the aforementioned effects is provided based on analytical models and experimental considerations. A method for extracting the nonlinearity parameter β11 is proposed based on a nonlinear least squares curve-fitting algorithm that is tailored for Rayleigh wave measurements. Quantitative experiments are conducted to confirm the predictions for the nonlinearity of the piezoelectric source and to demonstrate the effectiveness of the curve-fitting procedure. These experiments are conducted on aluminum 2024 and 7075 specimens and a β11(7075)/β11(2024) measure of 1.363 agrees well with previous literature and earlier work. The proposed work is also applied to a set of 2205 duplex stainless steel specimens that underwent various degrees of heat-treatment over 24h, and the results improve upon conclusions drawn from previous analysis.
Quantitative evaluation of the microstructural state of a specimen can be deduced from knowledge of the sample's absolute acoustic nonlinearity parameter, β, making the measurement of β a powerful tool in the NDE toolbox. However, the various methods used in the past to measure β each suffer from significant limitations. Piezoelectric contact transducers are sensitive to nonlinear signals, cheap, and simple to use, but they are hindered by the variability of the interfacial contact between transducer and specimen surface. Laser interferometry provides non-contact detection, but requires carefully prepared specimens or complicated optics to maximize sensitivity to the higher harmonic components of a received waveform. Additionally, laser interferometry is expensive and relatively difficult to use in the field. Air-coupled piezoelectric transducers offer the strengths of both of these technologies and the weaknesses of neither, but are notoriously difficult to calibrate for use in nonlinear measurements. This work proposes a hybrid modeling and experimental approach to air-coupled transducer calibration and the use of this calibration in a model-based optimization to determine the absolute β parameter of the material under investigation. This approach is applied to aluminum and fused silica, which are both well-documented materials and provide a strong reference for comparison of experimental and modeling results.
We present the proof-of-principle experiments of a high-speed actuation method to be used in tappingmode atomic force microscopes (AFM). In this method, we do not employ a piezotube actuator to move the tip or the sample as in conventional AFM systems, but, we utilize a Q-controlled eigenmode of a cantilever to perform the fast actuation. We show that the actuation speed can be increased even with a regular cantilever. © 2014 AIP Publishing LLC. [http://dx
The authors describe a method of actuation for atomic force microscope (AFM) probes to improve imaging speed and displacement range simultaneously. Unlike conventional piezoelectric tube actuation, the proposed method involves a lever and fulcrum "seesaw" like actuation mechanism that uses a small, fast piezoelectric transducer. The lever arm of the seesaw mechanism increases the apparent displacement range by an adjustable gain factor, overcoming the standard tradeoff between imaging speed and displacement range. Experimental characterization of a cantilever holder implementing the method is provided together with comparative line scans obtained with contact mode imaging. An imaging bandwidth of 30 kHz in air with the current setup was demonstrated.
Nonlinear Rayleigh wave measurements can be used to determine damage precursors and measure microstructural material changes while only requiring one-sided access to a specimen. These measurements are extremely sensitive to coupling conditions between the specimen surface and the generating and receiving transducers used in propagation and measurement of the Rayleigh waves. Air-coupled detection offers many advantages to traditional contact techniques because it mitigates these experimental coupling concerns with a reasonable cost as contrasted with other non-contact methods. However, to use these devices to measure absolute nonlinearity in a specimen requires a detailed understanding of the transducers and the experimental setup. This work provides a combined numerical and analytical approach to understanding the received signal and uses this information to determine a framework for calculation of absolute measures of material nonlinearity.
Dr. MacNair serves as Director of Laboratory Development in the Woodruff School, and manages Junior and Senior level laboratories in Mechanical Engineering. He develops innovative laboratory experiences based on lessons-learned from the maker movement and real-world industrial challenges, and is building an "ecosystem" of academic laboratory equipment and curriculum resources which allows universities to collaborate on the development and execution of effective undergraduate laboratory experiences. Dr. MacNair joined the Woodruff School in 2015 after working for the Georgia Tech Research Institute, and as an Educational Consultant for Enable Training and Consulting and National Instruments before that. He received his BS in Mechanical Engineering in 2008 and his PhD in Robotics in 2013, both from Georgia Tech. In his non-work hours, David serves as founder and President of the Atlanta Maker Alliance (Atlanta Leadership for the Maker Movement) as well as Executive Director of the Roswell Firelabs (a community education-focused maker space). He also guides the development and investment of various Atlanta-based foundations and non-profits targeting K-12 education.
A new method of actuating atomic force microscopy (AFM) cantilevers is proposed in which a high frequency (>5 MHz) wave modulated by a lower frequency (~300 kHz) wave passes through a contact acoustic nonlinearity at the contact interface between the actuator and the cantilever chip. The nonlinearity converts the high frequency, modulated signal to a low frequency drive signal suitable for actuation of tapping-mode AFM probes. The higher harmonic content of this signal is filtered out mechanically by the cantilever transfer function, providing for clean output. A custom probe holder was designed and constructed using rapid prototyping technologies and off-the-shelf components and was interfaced with an Asylum Research MFP-3D AFM, which was then used to evaluate the performance characteristics with respect to standard hardware and linear actuation techniques. Using a carrier frequency of 14.19 MHz, it was observed that the cantilever output was cleaner with this actuation technique and added no significant noise to the system. This setup, without any optimization, was determined to have an actuation bandwidth on the order of 10 MHz, suitable for high speed imaging applications. Using this method, an image was taken that demonstrates the viability of the technique and is compared favorably to images taken with a standard AFM setup.
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