This article investigates metrics to assess and compensate for the degradation of the adhesive layer of surface-bonded piezoceramic transducers for structural health-monitoring applications. Capacitance, resonance frequency, and modal damping parameters are derived from admittance curves using a lumped parameter model to monitor the degradation of the transducer adhesive layer. A pitch-catch configuration is then used to discriminate the effect of bonding degradation on actuation and sensing. It is shown that below the first mechanical resonance frequency of the piezoceramic transducers, the degradation causes a decrease in the amplitude of the transmitted and received signals, while above resonance, in addition to a decrease in the amplitude of the transmitted and received signals, a linear phase shift is observed. A signal-correction factor is proposed to adjust signals based on adhesive degradation evaluated using the measured modal damping. The benefits of the signal-correction factor are demonstrated in the frequency domain for both the A 0 and S 0 modes.

In the present study, a correlation-based imaging technique called Excitelet is assessed to monitor fatigue crack propagation in a riveted aluminum lap-joint, representative of an aircraft component. For this purpose, a micro-machined piezoceramic array is used to generate guided waves into the structure and measure the reflections induced by potential damage. The method uses a propagation model to correlate measured signals with a bank of signals and imaging is performed using a round-robin procedure (full-matrix capture). This allows taking into account the transducer dynamics and finite dimensions, multi-modal and dispersive characteristics of the guided wave propagation and complex interaction between with damage. Experimental validation has been conducted on an aluminum lap-joint instrumented with a compact linear piezoceramic array of 8 circular elements of 3 mm diameter each. The imaging technique is applied to detect crack propagation after fatigue cycling. Imaging results obtained using A 0 mode at 300 and 450 kHz are presented for different crack sizes. It is demonstrated that crack detection and localization can be achieved, while the correlation level indicates the level of reflected energy, and thus damage severity. An accuracy below 5 mm on damage location can be achieved, demonstrating the potential of the correlation-based imaging technique for damage monitoring of complex aerospace structures.

Classical piezoceramic transducer design methods in structural health monitoring based on guided wave propagation rely mostly on the use of the pin-force model, assuming that a piezoelectric actuator can be modelled as a constant shear stress applied at its circumference, whatever the frequency generated. However, the assumptions of this model are only valid for thin piezoelectric elements, weak coupling between the host structure and the transducer, and when the wavelength of the generated guided wave is above the size of the transducer. In order to overcome those limitations, this paper presents an axisymmetric analysis of guided wave generation by a circular piezoceramic, considering the complex shear and normal interfacial stress profiles between the transducer and the host structure. The excitation terms are estimated empirically using a best-fit model and a function derived from measured admittance. The validity of the approach is assessed numerically and experimentally, and the influence of piezoceramic thickness on guided wave generation is accurately modeled for frequencies below the second electro–mechanical resonance frequency.

Guided waves are widely used in structural health monitoring (SHM). Their behaviour is highly sensitive to the mechanical properties of a structure. The performance of damage detection strategies based on guided waves therefore relies on an accurate knowledge of the mechanical properties. This paper presents an integrated characterization technique that identifies the mechanical properties of isotropic structures, namely the elastic modulus and Poisson’s ratio. The approach is based on a modified version of an imaging algorithm (Excitelet), where mechanical properties, instead of geometrical scattering features, are set as the variables to be identified. The methodology, accuracy, repeatability, and robustness are assessed, first via a finite element model (FEM) and then experimentally for an aluminum plate with attached piezoceramic (PZT) transducers. The plate is instrumented with two PZTs located 15 cm from each other in a pitch–catch configuration, distant enough to ensure proper mode discrimination. The algorithm accuracy and robustness with respect to slight variations in the geometrical inputs (PZT to PZT distance and thickness of the plate) are validated within  ± 1% and  ± 2%, respectively, with the FEM. Experimental results are validated within  ± 1% of supplier properties, demonstrating the ability of this approach to allow accurate characterization of a structure in situ without the need for complex and expensive devices or ASTM testing.

In order to reduce operation and maintenance costs of aircraft, in situ structural health monitoring techniques are implemented on critical parts and assemblies. Many of these techniques rely on models considering, with various levels of complexity, the generation, propagation and interaction of ultrasonic guided waves with potential damages, in order to detect, localize and estimate damage severity. Although their potential has been extensively demonstrated on isotropic substrates, their implementation still poses a challenge for composite assemblies for which only quasi-isotropic and cross-ply composites have been considered. This is mainly due to the limitations of the models to properly predict the complex behaviour of guided waves on composites, where the assumptions behind the models actually used for damage imaging do not fully consider the impact of the anisotropy on guided wave generation and propagation. This article presents a comparative analysis of the performances of three model-based damage imaging techniques for composites previously validated on isotropic substrates. The main objective of the study is to address the interest in using more complex analytical formulations to improve the performance of imaging techniques. This is obtained by comparing three imaging techniques, each presenting different levels of complexity in their numerical formulations. Performance of (a) delay-and-sum, (b) dispersion compensation and (c) correlation-based techniques are addressed numerically and experimentally. The analysis is conducted on a unidirectional transversely isotropic laminate instrumented with four circular piezoceramic transducers. A robustness analysis of the models is performed numerically, where the effect of varying stiffness parameters and velocity is addressed. The correlation-based technique is adapted for the first time to composite laminates where the generation is considered using the pin-force model and the propagation is modelled via the use of the global matrix model. Experimental validation is carried out and the results obtained show the benefit of considering the steering effect for well-resolved multi modal damage imaging.