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.

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.

In this study, thermoplastic polyurethane (TPU) parts were fabricated using fused filament fabrication (FFF) by varying key process parameters, namely extruder temperature (210–230 °C), layer thickness (200–400 µm), and printing speed (30–50 mm/s). A Box–Behnken experimental design was used to systematically evaluate the combined influence of these parameters on surface roughness (R_a), dimensional deviation (DD), and ultimate tensile strength (UTS). After fabrication, all specimens were subjected to a Tetrahydrofuran (THF)-based chemical smoothing process to modify surface characteristics. Surface roughness measurements showed a substantial reduction after chemical smoothing, with values decreasing from an initial range of 13.17 ± 0.21–15.87 ± 0.23 µm to 4.01 ± 0.18–7.35 ± 0.16 µm, corresponding to an average decrease of approximately 50–72%. Dimensional deviation improved moderately, from 260–420 µm in the as-printed condition to 160–310 µm after post-processing, representing a reduction of about 20–38%. Mechanical testing revealed a consistent increase in UTS following chemical smoothing, with values improving from 30.24–40.30 ± 0.52 MPa to 33.97–47.94 ± 0.36 MPa, yielding an average increase of approximately 10–24%. Then, the experimental data were used for multi-objective optimization of the FFF process parameters, using a non-dominated sorting genetic algorithm (NSGA-II) implemented in Python 3.11, to identify best parameter combinations that provide a balanced surface quality, dimensional accuracy, and mechanical performance.

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.

Welded lap joints play a vital role in a wide range of engineering structures such as pipelines, storage tanks, pressure vessels, and ship hulls. This study aims to investigate the propagation of ultrasonic guided waves in steel welded lap joints for the baseline-free inspection of joint defects using the mode conversion of Lamb waves. The finite element method was used to simulate a single lap joint with common defects such as corrosion and disbonding. To identify the propagating wave modes, a wavenumber–frequency analysis was conducted using the 2D fast Fourier transform. The power loss of the transmitted modes was also determined to identify damage in the lap joints. The results indicate that the A0 incident in pristine conditions experienced significant transmission losses of about 9.5 dB compared to an attenuation of 2.8 dB for the S0 incident. The presence of corrosion was found to reduce these transmission losses by more than 28%. In contrast, introducing disbonding in the lap joint increased the transmission loss of the S0 incident, while a negligible loss was observed for the A0 incident. The mode-converted S0 (MC-S) and mode-converted A0 (MC-A0) incidents were found to exhibit a unique sensitivity to the presence of corrosion and disbonding. The results indicate that MC-S0 and MC-A0 as well as Lamb mode incidents interact differently in terms of corrosion and disbonding, providing a means to identify damage without relying on baseline signals.

In this study, a biodegradable Mg-Zn-Nd-Gd alloy was processed via multi-directional forging (MDF) to evaluate its microstructural evolution, mechanical performance, and corrosion behavior. Electron backscattered diffraction (EBSD) analysis was conducted to evaluate the influence of grain size and texture on mechanical strength and corrosion resistance. The average grain size decreased significantly from 118 ± 5 μm in the homogenized state to 30 ± 10 μm after six MDF passes, primarily driven by discontinuous dynamic recrystallization (DDRX). Remarkably, this magnesium (Mg) alloy exhibited a rare synergistic enhancement in both strength and ductility, with ultimate tensile strength (UTS) increasing by ~59%, yield strength (YS) by ~90%, while elongation improved by ~44% unlike conventional severe plastic deformation (SPD) techniques that often sacrifice ductility for strength. This improvement is attributed to grain refinement, dispersion strengthening from finely distributed Mg₁₂Nd and Mg₇Zn₃ precipitates, and texture weakening, which facilitated the activation of non-basal slip systems. Despite the mechanical improvements, electrochemical corrosion testing in Hank’s balanced salt solution (HBSS) at 37 °C revealed an increased corrosion rate from 0.1165 mm/yr in homogenized condition to 0.2499 mm/yr (after six passes of MDF. This was due to the higher fraction of low-angle grain boundaries (LAGBs), weak basal texture, and the presence of electrochemically active fine Mg₇Zn₃ particles. However, the corrosion rate remained within the acceptable range for bioresorbable implant applications, indicating a favorable trade-off between mechanical performance and degradation behavior. These findings demonstrate that MDF processing effectively enhances the strength–ductility synergy of Mg-rare earth alloys while maintaining a clinically acceptable degradation rate, thereby presenting a promising route for next-generation biomedical implants.