The capability of embedded piezoelectric wafer active sensors (PWAS) to excite and detect tuned Lamb waves for structural health monitoring is explored. First, a brief review of Lamb waves theory is presented. Second, the PWAS operating principles and their structural coupling through a thin adhesive layer are analyzed. Then, a model of the Lamb waves tuning mechanism with PWAS transducers is described. The model uses the space domain Fourier transform. The analysis is performed in the wavenumber space. The inverse Fourier transform is used to return into the physical space. The integrals are evaluated with the residues theorem. A general solution is obtained for a generic expression of the interface shear stress distribution. The general solution is reduced to a closed-form expression for the case of ideal bonding which admits a closed-form Fourier transform of the interfacial shear stress. It is shown that the strain wave response varies like sin a, whereas the displacement response varies like sinc a. ...

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Tuned Lamb-Wave Excitation and Detection with Piezoelectric Wafer Active Sensors for Structural Health Monitoring

Review of progress, QNDE

Quantitative nondestructive evaluation

Omni-directional guided wave array transducers contain a circular pattern of elements that individually behave as omni-directional point transmitters or receivers. The data set acquired from such an array contains time-domain signals from each permutation of transmitter and receiver. A phased addition algorithm is developed that allows an omni-directional, B-scan image of the surrounding plate to be synthesized from any geometry of array. Numerically simulated data from a single reflector is used to test the performance of the algorithm. The results from an array containing a fully populated circular area of elements (Type I array) are found to be good, but those from an array containing a single ring of elements (Type II array) contain many large side-lobes. An enhancement to the basic phased addition algorithm is presented that uses deconvolution to suppress these side-lobes. The deconvolution algorithm enables a Type II array to equal the performance of a Type I array of the same overall diameter. The effect of diameter on angular resolution is investigated. Experimental data obtained from a guided wave array containing electromagnetic acoustic transducers (EMAT) elements for exciting and detecting the S/sub 0/ Lamb wave mode in a 5-mm thick aluminium plate are processed with both algorithms and the results are discussed.

Omni-directional guided wave transducer arrays for the rapid inspection of large areas of plate structures

In this paper we review the state of the art in an emerging new technology: embedded ultrasonic non-destructive evaluation (NDE). Embedded ultrasonic NDE permits active structural health monitoring, i.e. the on-demand interrogation of the structure to determine its current state of structural health. The enabling element of embedded ultrasonic NDE is the piezoelectric wafer active sensor (PWAS). We begin by reviewing the guided wave theory in plate, tube, and shell structures, with special attention to Lamb waves. The mechanisms of Lamb wave excitation and detection with embeddable PWAS transducers is presented. It is shown analytically and verified experimentally that Lamb wave mode tuning can be achieved by the judicious combination of PWAS dimensions, frequency value, and Lamb mode characteristics. Subsequently, we address in turn the use of pitch-catch, pulse-echo, and phased array ultrasonic methods for Lambwave damage detection. In each case, the conventional ultrasonic NDE results are contrasted with embedded NDE results. Detection of cracks, disbonds, delaminations, and diffuse damage in metallic and composite structures are exemplified. Other techniques, such as the time reversal method and the migration technique, are also presented. The paper ends with conclusions and suggestions for further work.

/pdf/embedded-non-destructive-evaluation-for-structural-health-jldh78vfv5.pdf

Embedded non-destructive evaluation for structural health monitoring, damage detection, and failure prevention

A simple idea for self-focusing of a linear array has been extended to Rayleigh and Lamb waves. The self-focusing procedure first measures the backscattered signals for a first transmission by a single element of the array. A cross-correlation technique is used to determine the time-of-flight differences of the backscattered signals received by the elements of the array. These time delays are used to adjust the times of excitation of the elements for transmission focusing on the defect. Using the differences in arrival time once more, the backscattered signals after transmission focusing are aligned for reception focusing. Experimental results demonstrate the ability to self-focus on single defects. For multiple defects, the technique has been extended to focus on the defect that produces the largest backscattered signal.

Self-Focusing of Rayleigh Waves and Lamb Waves with a Linear Phased Array

A model is presented for the pulsed laser generation of ultrasound in isotropic layered plates. The stresses and displacements of the plate have been formulated in the Hankel and Laplace transform domains using the Thompson transfer matrix approach. The time domain response has been obtained by numerically inverting the transforms. Several numerical results are presented showing the normal surface displacement in the following configurations: single-layer film on a semi-infinite substrate, two layers on a semi-infinite substrate, and three-layer plates. The model provides a useful tool for the determination of which modes are generated by a laser source in a layered system. It can also be used to determine how sensitive the modes are to small changes in density, thickness, or elastic properties of the layers and to help in the selection of experimental parameters (laser spot size, pulse length, and source to receiver distance) for optimal sensitivity.

Simulation of laser-generated ultrasonic waves in layered plates

A linear array of surface wave transducers has been developed to generate focused surface wave motion. A novel theoretical approach, whereby time-harmonic surface wave motion is represented by a carrier wave that satisfies a reduced wave equation on the surface of the body and supports the subsurface motion, is used to model the beam generated by a single element of the array. Comparison of theoretical and experimental results show that, for a single element, the opening angle of the beam is about 20/spl deg/ and its cross-section can be represented by a Gaussian distribution of the normal displacements. For an eight-element array, the focused beam is subsequently obtained by superposition considerations. For the focused beam comparisons of theoretical and experimental results, in which the latter have been obtained by the use of a laser interferometer, show excellent agreement both for the normal displacements along a radial line and across the width of the beam. The array can be used for self-focusing of surface waves on a surface defect.

Focusing of Rayleigh waves: simulation and experiments

Quantitative nondestructive evaluation (QNDE) of the degradation of adhesive bonds remains one of the most challenging problems in QNDE. The objective of this research was to approach this problem by the detection of nonlinearity due to bond deterioration. The paper starts with experimental observations of the reflection of ultrasonic signals by adhesive bonds. The specimens for these tests had been subjected to cyclic loading which was expected to cause bond deterioration. Differences in the reflections could, however, be observed only if the adhesive bonds were subjected to static loads simultaneously with the ultrasonic testing. The higher the number of fatigue cycles, the lower the required load to display the differences in signals with unloaded cases. The second part of the paper presents a theoretical explanation of these ultrasonic measurements based on the postulate of nonlinear stress-strain behavior of the deteriorated bond. The ultrasonic tests provide the slope of the stress-strain curve and the results can therefore be used to determine the deviation of the stress-strain curve from linear behavior. For the higher numbers of fatigue cycles, this deviation, which is indicative of bond deterioration, starts at smaller load values.

Ultrasonic evaluation of adhesive bond degradation by detection of the onset of nonlinear behavior

For concrete bridge decks, the use of salt in cold weather areas is known to result in damage, such as freeze or thaw damage (microcracks) and surface breaking cracks. In this paper attention is directed to an ultrasonic method to scan for surface-breaking cracks and to determine the crack depth in concrete bridge decks.

https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=3274&context=qnde

A. Cheng

Papers

Self-Focusing of Rayleigh Waves and Lamb Waves with a Linear Phased Array

Simulation of laser-generated ultrasonic waves in layered plates

Focusing of Rayleigh waves: simulation and experiments

Ultrasonic evaluation of adhesive bond degradation by detection of the onset of nonlinear behavior

Depth Determination of Surface-Breaking Cracks in Concrete Slabs Using a Self-Compensating Ultrasonic Technique