Michael Pedrick

All-fiber optic ultrasonic structural health monitoring system

Structural Health Monitoring (SHM) is required for early detection of damage in structural components to improve the safety, reduce the cost, and increase the performance and efficiency of aircrafts. Currently available techniques have a number of deficiencies prohibiting wide spread of SHM in aerospace applications. In this contribution we will present the initial results of development at Luna Innovations of an all-fiber optic ultrasonic airframe SHM system that will be able to address the deficiencies of solutions suggested/developed to date. In this contribution we will present the details on design, development and testing of the prototype fiber optic SHM system.

A Novel Technique With a Magnetostrictive Transducer for In Situ Length Monitoring of a Distant Specimen

A Magnetostrictive sensor was used to generate sound waves in a specimen through thirty feet of wire. Many hardware aspects are discussed such as boundaries, materials, acoustic horn design, and sound propagation characteristics which facilitated the generation of sound energy in the specimen. Temperature effects on velocity and length were calculated and a model was developed to determine length from a time of flight measurement. The specimen was heated in an oven to various temperatures and times of flight were measured and compared to the model. Results show agreement between the measured values and the model as well as the ability for a high precision length measurement.Copyright

Low attenuation waveguide for leaky surface waves

A surface wave on a liquid/solid interface is well-known to radiate acoustic energy into the liquid and is therefore rapidly attenuated. In this work, we have been able to show by experiments and calculations that the proximity of another surface (layer 1 to layer 3 and layer 3 to layer 1) sustains the surface wave through long distances for layers of both plates and concentric tubes. In addition, even when the surface wave is reflected from a distant edge, the returning wave is sustained in the multi-layer system and can be easily detected. This is apparently one of the first observations of leaky surface waves traveling over large distances, in this case over a thousand wavelengths. The effect is modeled on the basis of a cooperative phenomenon between two interfaces separated by a water layer. The effect represents a valuable result in the wave propagation of acoustic surface waves and opens the door to many applications.

Sensitivity comparisons of layered Rayleigh wave and Love wave acoustic devices

Due to their high sensitivity, layered Surface Acoustic Wave (SAW) devices are ideal for various film characterization and sensor applications. Two prominent wave types realized in these devices are Rayleigh waves consisting of coupled Shear Vertical and Longitudinal displacements and Love waves consisting of Shear Horizontal displacements. Theoretical calculations of sensitivity of SAW devices to pertubations in wave propagation are limited to idealized scenarios. Derivations of sensitivity to mass change in an overlayer are often based on the effect of rigid body motion of the overlayer on the propagation of one of the aforementioned wave types. These devices often utilize polymer overlayers for enhanced sensitivity. The low moduli of such overlayers are not sufficiently stiff to accommodate the rigid body motion assumption. This work presents device modeling based on the Finite Element Method. A coupled-field model allows for a complete description of device operation including displacement profiles, frequency, wave velocity, and insertion loss through the inclusion of transmitting and receiving IDTs. Geometric rotations and coordinate transformations allow for the modeling of different crystal orientations in piezoelectric substrates. The generation of Rayleigh and Love Wave propagation was realized with this model by examining propagation in ST Quartz both normal to and in the direction of the X axis known to support Love Waves and Rayleigh Waves, respectively. Sensitivities of layered SAW devices to pertubations in mass, layer thickness, and mechanical property changes of a Polymethyl methacrylate (PMMA) and SU-8 overlayers were characterized and compared. Experimental validation of these models is presented.

Measurement of Absolute Acoustic Strain by Non‐Contact Technique

Some ultrasonic applications require non‐contact techniques because the target material is not easily accessible. In such cases laser‐based and air‐coupled ultrasonic techniques play a major role but commonly significant transmission loss is known to occur especially at higher frequencies. Therefore, it becomes imperative to know the amount of absolute acoustic strain achieved for a given application. In this paper, we report on the use of laser‐based techniques to measure absolute strain on the face of vibrating rods excited under various scenarios. These include contact and air‐coupled excitation at frequencies at resonance, as well as a factor of 100 below and above the resonance. The limit of our out‐of‐plane displacement measurement appears to be about 5 nanometers. Strains as high as 10−6 have been obtained. The paper will describe the details of the ultrasonic techniques and some of the applications. The data are compared to theoretical and simulated strain calculations.

Charged Particle Detection: Potential of Love Wave Acoustic Devices

An investigation of the dependence of film density on group and phase velocities in a Love Wave Device shows potential for acoustic‐based charged particle detection (CPD). Exposure of an ion sensitive photoresist to charged particles causes localized changes in density through either scission or cross‐linking. A theoretical model was developed to study ion fluence effects on Love Wave sensitivity based on: ion energy, effective density changes, layer thickness and mode selection. The model is based on a Poly(Methyl Methacralate) (PMMA) film deposited on a Quartz substrate. The effect of Helium ion fluence on the properties of PMMA has previously been studied. These guidelines were used as an initial basis for the prediction of helium ion detection in a PMMA layer. Procedures for experimental characterization of ion effects on the material properties of PMMA are reviewed. Techniques for experimental validation of the predicted velocity shifts are discussed. A Love Wave Device for CPD could potentially prov...

Ultrasonic nondestructive evaluation in thin‐walled concrete for flaw detection

Accurate inspection techniques for today’s infrastructure have become an area of great interest. Several ultrasonic techniques for testing and evaluating concrete have been established and show great promise. However, much of the work is concerned with concrete piles or thick‐walled specimen (greater than 500 mm). Vast amounts of concrete piping in sewer systems and water mains require testing techniques applicable to thinner‐walled systems (less than 100 mm). This work aims to extend ultrasonic inspection techniques to thin‐walled systems. Impact‐echo and resonant ultrasonic spectroscopy (RUS) techniques are explored. Finite‐element models have been developed to describe the propagation characteristics in different specimen. Several specimens have been tested experimentally to determine the effects of steel reinforcements, pipe curvature, and simulated defects. The influence of sensor configuration, transducer characterization, and data capture on measurement accuracy and inspection time is examined. The...

Advancement of wave generation and signal transmission in wire waveguides for structural health monitoring applications

As per the recent advances in remote in situ monitoring of industrial equipment using long wire waveguides (~10m), novel applications of existing wave generation techniques and new acoustic modeling software have been used to advance waveguide technology. The amount of attainable information from an acoustic signal in such a system is limited by transmission through the waveguide along with frequency content of the generated waves. Magnetostrictive, and Electromagnetic generation techniques were investigated in order to maximize acoustic transmission along the waveguide and broaden the range of usable frequencies. Commercial EMAT, Magnetostrictive and piezoelectric disc transducers (through the innovative use of an acoustic horn) were utilized to generate waves in the wire waveguide. Insertion loss, frequency bandwidth and frequency range were examined for each technique. Electromagnetic techniques are shown to allow for higher frequency wave generation. This increases accessibility of dispersion curves providing further versatility in the selection of guided wave modes, thus increasing the sensitivity to physical characteristics of the specimen. Both electromagnetic and magnetostrictive transducers require the use of a ferromagnetic waveguide, typically coupled to a steel wire when considering long transmission lines (>2m). The interface between these wires introduces an acoustic transmission loss. Coupling designs were examined with acoustic finite element software (Coupled-Acoustic Piezoelectric Analysis). Simulations along with experimental results aided in the design of a novel joint which minimizes transmission loss. These advances result in the increased capability of remote sensing using wire waveguides.

Design and Performance of a Broadband 10MHz Transducer for Elevated Temperature, Leave-in-Place Applications

This paper describes the novel design of an ultrasonic normal beam transducer for prolonged use in elevated temperature environments. Through the use of a Carbon/Carbon composite backing layer, prolonged exposure to elevated temperature had minimal effect on transducer performance. The conductive nature of the Carbon/Carbon allowed for an innovative electrical coupling technique. A clamping mechanism combined with the use of an annealed gold quarter-wave matching layer allowed for joint-free, dry coupling. This simple design allows for easy field assembly and eliminates temperature dependencies in the acoustic coupling. The transducer was tested initially at room temperature for reference data. Further tests after 100+ hours of exposure to a 77°C environment showed little overall change in the transducer performance. The transducer showed consistent −6dB bandwidths on the order of 54–67%, along with negligible change in centerline frequency. The insertion loss as a function of temperature showed an increase from approximately 6.8dB to 8.5dB over a temperature range from 25°C to 85°C. Regression lines show bandwidth changes of −0.01% per °C and insertion loss changes of 0.03dB per °C. These results show potential use for a transducer of this design at even higher temperatures.Copyright