Publications

Stress corrosion cracking (SCC) had occurred in early-generation high-level nuclear waste tanks constructed by welding carbon steel. This paper describes an ultrasonic inspection system and its fundamental ability to detect and quantify the length of SCC on thick welded steel plates. The finite element method (FEM) was applied to simulate the welding process to estimate the welding residual stress field. Growth of stress corrosion cracks is driven by crack stress intensities exceeding the subcritical cracking threshold intensity. The subject plate was experimentally inspected with ultrasonic nondestructive evaluation (NDE) techniques to characterize the extent of SCC. The NDE system uses a piezoelectric transducer to generate guided waves in the thick steel plate, and a scanning laser Doppler vibrometer (SLDV) to measure multidimensional time–space wavefield data over a user-defined scanning area in the plate surface. The measured wavefield data can show wave interactions in a localized area in the plate due to the presence of the discontinuities of the SCC. To generate an inspection image that can precisely show the crack’s location and/or the dimension, the wavefield data are further processed to generate inspection image that maps the entire sample plate so the crack can be clearly identified in the plate while its length can be readily estimated. The ultrasonic test results for crack length agree well with the visually estimated length and are close to that predicted by the FEM for cracks in the weld residual stress field.

Network wavefield imaging methods are able to image significantly complex discontinuities or shapes in plate - like structures for superior ultrasonic structural health monitoring ( SHM ) / nondestructive evaluation ( NDE ) . The imaging provides high - resolution location , shape and / or size images of a structure , and for discontinuities with more complicated profiles . Guided wave ( GW ) network wavefield imaging methods combine tomography and wavefield / wavenumber imaging algorithms . Metallic plate damage detection uses guided ultrasonic waves and non - contact laser vibrometry . Guided waves are generated by piezoelectric transducers ( PZT ) . A non - contact scanning laser Doppler vibrometer ( SLDV ) measures the full velocity plate guided wave wave fields . Developed network wavefield imaging algorithms account for multiple - actuator excitations from different angles enclosing the discontinuity , with algorithms using intrinsic wave characteristics such as wavefield , wavenumber , or reconstructed wave energy . Determined locations , sizes and shapes of highlighted areas in wavefield, wavenumber and / or filter reconstructed energy - based images correlate with location , size and shape of damage in metallic plates.

Pitting corrosion presents challenges for ultrasonic nondestructive evaluation due to the small pit dimension. Few Lamb wave-based techniques have achieved the identification of individual pits as subwavelength wave scatterers that are densely packed in a small cluster. In this article, noncontact laser vibrometry-based fence-like arrays with wavefield filteringassisted adaptive imaging algorithms are developed for detecting and identifying small pits in a cluster. Signals of back scattering waves induced by a cluster of subwavelength scatterers are acquired by noncontact laser Doppler vibrometry at sensing points and form a fence-like array surrounding the area of inspection. The signals are then processed by our array imaging algorithms to construct inspection images which take advantage of three techniques, including the wave mode and wave direction filters to extract single-mode back scattering Lamb waves induced by subwavelength scatterers, the pseudo-reversal propagation of back scattering waves to address the dispersion effect and improve the radial imaging resolution, and the adaptive weighting to improve the angular imaging resolution. Moreover, this work introduces the wave diffraction-related Rayleigh and Abbe limits that are conventionally used for characterizing optic lenses, for characterizing the resolution limit of Lamb wave-based arrays, and optimizing the array configuration. To validate our array imaging approach, a proof-of-concept experiment has been performed to detect a cluster of 3 3 3 pits with the pit diameter of 2 mm and the interval of 2 mm in a 3.2-mm thick aluminum plate; the experimental imaging result shows that our method can identify most pits except the one at the center of the pit cluster. We believe this study will benefit the design, characterization, and optimization of Lamb wave-based arrays for subwavelength resolution imaging and enable potential applications for the noncontact inspection of hidden pitting corrosion in civil, petrochemical, nuclear, and aerospace structures.

This paper presents a non-contact air-coupled Lamb wave imaging technique using a twodimensional (2D) cross-correlation method that not only detects the damage but also precisely quantifies for orientations and sizes. The air-coupled transducers (ACT) is used together with a scanning laser Doppler vibrometer (SLDV) for sensing, making a fully non-contact Lamb wave system used for this study. We first show that single-mode Lamb wave actuation can be achieved by the ACT-based on Snell’s law. Detailed study and characterization of the directional ACT Lamb waves are conducted. For damage detection, a 2D cross-correlation imaging technique that uses the damage introduced scattered waves of all directions is proposed for correlating with the incident waves. The frequency-wavenumber filtering technique is used to implement the acquisition of the scatted waves and incident waves, respectively. In the end application to notches with various orientations and various sizes in terms of depth and length is given. The results show the proposed technique can precisely imaging the damages and can quantitatively evaluate the damage size in terms of length and depth.

Composite materials are widely used in aerospace industries due to their light weight, strength, and various other desired properties. However, they are susceptible to various defects occurring during the manufacturing process or in service. Typical defects include porosity, wrinkles, and delamination. Nondestructive means of detection of the defects at any stage are of great importance to ensure quality and safety of composite structures. A nonintrusive removable Lamb wave system and accompanied methodology that is not material dependent are presented in this paper to detect various types of typical defects in laminated composite plates, flat or curved. Through multidimensional data acquisition and processing, abnormality in waves caused by defects is captured and presented in inspection images. The methodologies are demonstrated in two cases: delamination in a curved plate and wrinkles in a flat plate. Overall, the results show that Lamb waves using the piezoelectric transducer and laser vibrometer system can be used for various types of defects inspection in flat or curved composite plates.

Corrosion as common damage in civil, petrochemical, nuclear, and aerospace structures affects the integrity and safety of the structures and may lead to catastrophic failures. This paper presents dispersion curve regression-assisted local wavenumber analysis method, which can analyze the time-space wavefield containing wideband information of wave-damage interaction and further extract the structural information carried by such wavefield for characterizing hidden corrosion damage in an isotropic plate. To acquire the time-space wavefield, a noncontact scanning laser Doppler vibrometer is used. In our analysis method, the acquired time-space wavefield is firstly processed to generate a local wavenumber function. The derived wavenumber function is further analyzed with a dispersion curve regression step, which searches in a set of theoretical frequency-wavenumber dispersion curves for different plate thicknesses and identifies the optimal plate thickness whose theoretical curve best matches the frequency-wavenumber relation contained in the local wavenumber function. By this means, a 3D profile including both in plane and thickness dimensions of the structure can be constructed for corrosion visualization and quantification. The experimental study demonstrates that our method can quantify the profile of hidden uniform corrosion in metal plates in terms of its in-plane shape and size as well as its out-of-plane depth in the subwavelength scale. Moreover, compared to previous Lamb wavebased corrosion inspection methods, our technique allows for noncontact 3D characterization of hidden corrosion from a far distance to the structure. We believe this work will inspire new 3D damage quantification methods that are based on wavefield analysis as well as enable potential applications for the quantification of hidden corrosion in civil, petrochemical, nuclear, and aerospace structures.

Lamb waves have been widely used for damage detection on plate-like structures. However, there are still considerable interests on quantifying damage with complex profile. In this article, quantification of complex damage in plate-like structures using a network of actuators and time-space Lamb wavefield is investigated. The actuator network inspection system is implemented with multiple PZT transducers for Lamb wave actuation in round robin pattern and scanning laser Doppler vibrometer for wavefield sensing. The PZT network is arranged in a way that the target area is fully enclosed and Lamb waves come to the damage from all directions. Waves induced by the damage are subsequently obtained through frequency-wavenumber filtering, using the experimentally acquired dispersion curves presented in the paper. The filtered waves from all wave actuators are then used to generate a synthetic image of the damage being inspected. Two cases of complex damage are evaluated on aluminum plates, mass loss with triangular profile and mass addition with a three-letter cluster profile. Our results show that the damages are not only detected but also their profiles are clearly outlined in the images. We believe the subject methods provide improved evaluation of damage profile for Lamb wavefield based damage quantification.

: Piezoelectric transducers are convenient enablers for generating and receiving Lamb waves for damage detection. Fatigue cracks are one of the most common causes for the failure of metallic structures. Increasing emphasis on the integrity of critical structures creates an urgent need to monitor structures and to detect cracks at an early stage to prevent catastrophic failures. This paper presents a two-dimensional (2D) cross-correlation imaging technique that can not only detect a fatigue crack but can also precisely image the fatigue cracks in metallic structures. The imaging method was based on the cross-correlation algorithm that uses incident waves and the crack-scattered waves of all directions to generate the crack image. Fatigue testing for crack generation was then conducted in both an aluminum plate and a stainless-steel plate. Piezoelectric wafer transducer was used to actuate the interrogating Lamb wave. To obtain the scattered waves as well as the incident waves, a scanning laser Doppler vibrometer was adopted for acquiring time-space multidimensional wavefield, followed with frequency-wavenumber processing. The proof-of-concept study was conducted in an aluminum plate with a hairline fatigue crack. A frequency-wavenumber filtering method was used to obtain the incident wave and the scattered wave wavefields for the cross-correlation imaging. After this, the imaging method was applied to evaluate cracks on a stainless-steel plate generated during fatigue loading tests. The presented imaging method showed successful inspection and quantification results of the crack and its growth.

In this paper, the excitation and propagation of guided waves in multilayer hollow cylinders with piezoelectric wafer active sensor (PWAS) transducers were modeled with the normal mode expansion (NME) method using the semi-analytical finite element (SAFE) formulation. The theoretical development of SAFE for hollow cylindrical structures was introduced and used to obtain guided-wave mode shapes and dispersion curves of multilayer hollow cylinders. The SAFE discretization was applied across the thickness. The layers present in the cylinder were modeled by grouping the elements in the region corresponding to the respective layers. Each finite element region was given the property of the layer that it represented. The number of elements in a layer was determined through convergence studies. The PWAS excitation effect, introduced using the ideal-bonding assumption, was represented by a line-force acting on the PWAS boundary. The SAFE-NME solution obtained in the wavenumber domain was resolved in the physical domain through inverse Fourier transform and residue theorem. Experimental validation of theoretical prediction was performed by comparison with scanning laser Doppler vibrometer (SLDV) measurements from a “6-inch schedule-40” pipe of 11 mm thickness installed with a 7-mm square PWAS transducer for wave excitation. Numerical prediction of the guided wave propagation emanating from the PWAS was first performed and wavefront visualization was obtained. An SLDV area scan of the guided waves generated by the PWAS was then performed and compared with numerical predictions. A good match between experiment and prediction was observed.