A novel and effective approach for nondestructive in situ structural health monitoring of a hard-to-inspect location is presented in this article. Laser vibrometry is used to confirm that Lamb waves, generated by low-profile surface bonded piezoceramic transducers, are able to create circumferential creeping waves on the free surface of cylindrical cavities. The behavior of the propagating ultrasonic stress waves around the cavity and their interaction with simulated fatigue damage emanating from the top of the cavity is visualized. The transformation to a cylindrical co-ordinate system is found to be useful in visualizing wave scattering due to the presence of an open notch. Nondestructive evaluation of the structure is shown to be viable with the strength of the scattered spiralling creeping wave field maintaining its form and growing in amplitude as the notch length in increased. An informed selection of the excitation frequency is recommended as the A 0 Lamb mode is found to be more efficient than the S 0 Lamb mode in generating creeping waves around the cavity. Results from a receiving low-profile piezoceramic transducer are used to substantiate the laser vibrometry measurements and further establish the potential of the technique.

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A three-dimensional finite element model for analysis of ultrasonic-guided wave scattering from a hidden notch on a through-thickness circular hole is presented in this article. The structure is representative of an aircraft wing spar with a fuel weep hole. In contrast to the high-frequency bulk-wave wedge transducer methodology traditionally used for weep hole inspection, the length scales of the in situ technique are such that the wavelength of the incident guided waves are comparable with the hole diameter and larger than the defect length. The ability to accurately model acousto-ultrasonic elastic wave dynamics is crucial for future developments in this research field. Three-dimensional laser vibrometry is used to measure the deformation of a low-profile surface-bonded piezoelectric disc. The measurements provide the input conditions for a force-actuated computational model. The computational scattered wave field from a notch on the blind side of the hole is compared with experimental data for model validation. It is concluded that the boundary conditions for wave scattering from the hidden notch are a complex combination of the incident wave field and the dispersive edge-guided Rayleigh-type flexural wave that is formed on the free surface of the hole.

The in situ health monitoring of defects on the blind side of open holes using ultrasonic plate waves is a challenging problem. Scattering phenomena in this hard-to-inspect region can be used indicate the presence of the defect. This is especially advantageous if these phenomena give rise to the scattering of a wave mode that is unique to the interaction between the incident wave mode and defect. When the defect in question is located within an inaccessible structure, an understanding of how the incident ultrasonic elastic wave field can be scattered from this hidden defect propagates to the accessible surface is important. This paper presents a series of computational investigations to highlight the essential physics that explains the scattering phenomena by a defect located on the blind side of an open hole. The work presented is relevant to the monitoring of defects located in hard-to-inspect regions of future unitized metallic and composite structures. The outcomes advance the knowledge base of inspection of hard-to-access regions with actuators and sensors placed in easily accessible locations.

A computational frequency analysis of secondary wave mode generation is considered in this paper. Secondary wave mode conversion at the tip of a through‐thickness slit in a circular isotropic plate is analysed similar to a classical Sommerfeld's diffraction problem. The analysis is useful for quantitative non‐destructive evaluation of fatigue cracked plate structures that involve propagation of a single wave mode (a vertically polarised symmetric Lamb mode) and measurement of the amplitude of a new mode (a horizontally polarised shear mode). A two‐dimensional finite element model is first introduced as a baseline reference for the non‐dispersive scenario. This model is used to study the effect that P‐wave angle of incidence has on the mode converted S‐wave from the tip of the slit. A dispersive three‐dimensional finite element model of the circular plate is then introduced at a number of frequency‐thickness products within the range 0.5–3 MHz.mm−1. Within this frequency–thickness range, significant changes to the through‐thickness displacement profiles of the incident S0 Lamb mode occur. The secondary SH0 plate wave mode generated by mode conversion from the 45o incident S0 Lamb mode at the tip of a slit is analysed to establish the efficiency of the mode conversion. Visualisations of the wave fields demonstrate that at frequency–thickness products higher than the first symmetric Lamé mode (where there is no in‐plane surface displacement), the in‐plane through‐thickness displacement profile of the incident S0 Lamb mode becomes incompatible with that of the SH0 plate mode, and the mode conversion phenomenon is thus severely compromised.