Ultrasonic systems employing embedded piezoelectric transducers have seen increased interest in recent years. The ability to sense, actuate, and analyze the wave propagation modes in engineering structures has been fundamental to the advancement of ultrasonic structural health monitoring (SHM). This paper presents a study into the sensing and actuation properties of shear-mode (d35) piezoelectric transducers made of lead zirconate titanate (PZT) that are internally embedded in the bondline of laminate structures. The manuscript presents analytical analysis, finite element simulation, and experimental validation building from an individual piezoelectric element to a full laminate structure. The validated model was then used to perform a parametric study into the effects of d35 PZT transducer size on the strength of actuation and sensing output signal. The selectivity of d35 PZT sensors was also investigated by generating multiple wave modes in the laminate structure and inspecting the output signals. The d35 PZT sensors were found to selectively detect only certain modes of the wave propagation providing a fundamental hardware filter that could be employed to simplify signal analysis and processing. The results of this study indicate that d35 PZTs embedded in the bondline have multiple properties that can potentially be employed for ultrasonic SHM.

There has been recent interest in embedding sensor networks into bondlines to create intelligent materials that can detect, report, and potentially respond to their state. This article presents an initial, large-scale investigation into embedding shear-mode (d15) lead zirconate titanate piezoelectric transducers into the bondline of laminate structures, near the centerline, for the ultrasonic detection of joint defects. The study included analysis of dispersion curves, multiphysics numerical simulations, and experimental results for an aluminum–epoxy–aluminum laminate structure. Analysis of the time of flight and displacement profiles confirmed that antisymmetric waves were generated and propagated through the structure. Simulations were performed for models containing disbonds, through-thickness cracks, and voids with experimental validation of a specimen containing a void to evaluate the effectiveness of d15 lead zirconate titanates embedded in a bondline as actuators and sensors for damage detection. The results were examined by looking at the cross-sectional deformation in simulations, and the signal changes were evaluated by calculating the root mean square deviation damage index and inspecting attenuation and phase shift in pristine and damaged structures. It was found that the d15 shear-mode lead zirconate titanates actuated and sensed antisymmetric waves that were sensitive to all types of damages considered.

A major inhibition to the widespread use of laminate structures is the inability of nondestructive testing techniques to effectively evaluate the bondline integrity. This work proposes and analyzes a bondline-integrity health monitoring approach utilizing shear-mode (d15) piezoelectric transducers. The d15 transducers were embedded in the bondlines of symmetric laminate structures to monitor and evaluate the bondline integrity using ultrasonic inspection. The d15 piezoelectric transducers made of lead zirconate titanate (PZT) enabled ultrasonic inspection of bonds by actuating and sensing antisymmetric waves in laminate structures. Design considerations, fabrication process, and experimental methods for testing a laminate specimen are presented. Designs included bondline-embedded d15 PZT piezoelectric transducers with surface-mounted transverse (d31) piezoelectric transducers for signal comparison. Defects in the bondline were created by a quasi-static three-point bending test, with results showing the ability of d15 piezoelectric transducers to detect bondline damage. Two damage indices based on Pearson correlation coefficient and normalized signal energy were implemented to evaluate the presence of damage and its severity. The experimental results demonstrate the ability of bondline-embedded d15 piezoelectric transducers to be used as actuators and sensors for ultrasonic health monitoring of bondline integrity. A comparison between surface-mounted d31 PZT and bondline-embedded d15 PZT sensors was also conducted. It was seen that signals sensed by bondline-embedded d15 PZTs showed higher distortion due to bondline defects compared with the sensed signals from the surface-mounted d31 PZT.

Structural health monitoring of thin plate and beam structures using ultrasonic guided wave techniques has been widely studied and demonstrated advanced capabilities dependent on detailed analysis of specific guided wave modes. A common setup employs the d31 electromechanical coupling of piezoelectric wafer active sensors mounted on the surface of a beam or plate. Analysis of output signals from these basic systems is complicated because they represent multiple superposed ultrasonic wave modes that propagate at different velocities, are dispersive, and undergo reflection, refraction, and mode conversion. Multiple techniques have been pursued to overcome this complication. This article presents recent research into the use of shear-deforming lead zirconate titanate piezoelectric transducers, employing the d15 electromechanical coupling property, embedded within beam-like structures to selectively actuate and sense specific ultrasonic wave modes. The internally located transducers actuated and sensed transverse shear, coupled to bending and antisymmetric waves. A combination of results from finite element simulations and experiments found that d15 transducers located at the neutral axis of a beam exclusively coupled to antisymmetric wave modes and did neither directly actuate nor sense symmetric wave modes. Further study was performed to evaluate the effects of off-neutral-axis location on the mode selectivity and found that off axis location of the d15 transducer did not diminish the coupling to antisymmetric wave modes, but introduced coupling to symmetric wave modes. Additional study was performed to assess the ability of structural health monitoring systems employing shear-deforming d15 lead zirconate titanates located at the neutral axis to detect common forms of damage in laminate structures. The combination of selective actuation and selective sensing provides a powerful tool for signal analysis in ultrasonic structural health monitoring of thin plates and beams.

The use of the wavelet transform has been gaining widespread acceptance over the last decade as a valuable tool for damage detection. This paper investigates the use of wavelets for detecting mixed-mode, also known as combined mode, cracks in truss structures. The propagation of an open, mixed-mode crack is simulated by using a macroscopic model of damage that is combined with a finite element model of the Warren truss. The natural modes of the truss with varying levels of damage are then used to determine crack location on a specific member of the truss. A damage detection algorithm is developed and the influence of multiple parameters such as truss geometry, crack geometry, number of truss members, etc. is investigated. A direct correlation between damage severity and the magnitude of wavelet coefficients is found for a predefined damage location. It is observed that the proposed damage detection algorithm can be used to successfully detect mixed-mode cracks even in the presence of noise, and even when a relatively coarse sampling of natural modes is used. Multiple simulations are presented and some shortcomings of the proposed algorithm are also discussed in detail.