In this paper the Semi-Analytical Finite Element (SAFE) method for modeling guided wave propagation is extended to account for linear viscoelastic material damping. Linear viscoelasticity is introduced by allowing for complex stiffness constitutive matrices for the material. Dispersive characteristics of viscoelastic waveguides, such as phase velocity, attenuation, energy velocity and cross-sectional wavestructures are extracted. Knowledge of the above-mentioned dispersive properties is important in any structural health monitoring attempt that uses ultrasonic guided waves for long range inspection. The proposed damped formulation is applied to several waveguides with different mechanical and geometric properties. In particular, a viscoelastic isotropic plate, a railroad track and a pipe are studied.

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Visual inspection procedures remain the primary method of infrastructure assessment throughout the USA, but their shortcomings are numerous. In addition to their widely acknowledged variability and subjectivity, the large scale of civil infrastructure systems presents expensive access and time requirements that constrain the frequency of visual inspections and result in poor temporal resolution, which hampers effective decision-making. To overcome this challenge, the research reported herein aimed to assess the ability of computer algorithms together with imagery collected by unmanned aerial vehicles (UAV) to extract accurate and quantitative information to help inform infrastructure management decisions. Techniques such as homography and lens distortion correction are used in this article in a post-processing framework that allows the use of color images obtained by UAVs for actual damage quantification measurements. The experiments described in this article utilize a UAV with a mounted camera and provide measurements from a representative infrastructure mockup with several simulated damage scenarios. Deformation measurements, change detection (related to structural features and the size of deterioration), and crack pattern identification were all analyzed. The results indicated that the developed post-processing algorithms were able to extract quantitative information from UAV captured imagery.

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The monitoring of adhesively bonded joints by ultrasonic guided waves is the general topic of this paper. Specifically, composite-to-composite joints representative of the wing skin-to-spar bonds of unmanned aerial vehicles (UAVs) are examined. This research is the first step towards the development of an on-board structural health monitoring system for UAV wings based on integrated ultrasonic sensors. The study investigates two different lay-ups for the wing skin and two different types of bond defects, namely poorly cured adhesive and disbonded interfaces. The assessment of bond state is based on monitoring the strength of transmission through the joints of selected guided modes. The wave propagation problem is studied numerically by a semi-analytical finite element method that accounts for viscoelastic damping, and experimentally by ultrasonic testing that uses small PZT disks preferably exciting and detecting the single-plate s0 mode. Both the models and the experiments confirm that the ultrasonic energy transmission through the joint is highly dependent on the bond conditions, with defected bonds resulting in increased transmission strength. Large sensitivity to the bond conditions is found at mode coupling points, as a result of the large interlayer energy transfer.

A fundamental understanding of the response of piezoelectric transducer patches to ultrasonic waves is of increasing interest to the field of structural health monitoring. While analytical solutions exist on the interaction of a piezoelectric actuator with the generated Lamb waves, the behavior of a piezoelectric sensor has only been examined for the limited case of a piezo-actuated Lamb wave in a pitch-catch configuration. This paper focuses on the fundamental response of surface-bonded piezoelectric sensors to ultrasonic waves. The response to both Rayleigh waves and Lamb waves is examined, starting with harmonic excitation fields and moving to broadband and narrowband excitation fields. General oblique incidence of the wave on rectangular sensors is treated first; parallel incidence is then derived as a particular case. The solutions are developed analytically for the harmonic and the narrowband excitations, and semianalytically for the broadband excitation. The results obtained can be used to design ultrasonic sensors that are either particularly sensitive to a given mode or possess unique directivity behavior which, in turn, can greatly simplify current algorithms for damage detection and localization.