Cure Monitoring of Adhesive for Composite/Metal Bonded Structure Based on Highly Nonlinear Solitary Waves

In this paper, a nondestructive evaluation technique based on highly nonlinear solitary waves (HNSWs) is proposed to monitor the curing process of adhesive for composite/metal bonded structure. HNSWs are mechanical waves with high energy intensity and non-distortive nature which can form and propagate in a nonlinear system, such as a one-dimensional granular chain. In the present study, a finite element model of the one-dimensional granular chain is established with the commercial software Abaqus, to study the reflection behavior of HNSWs at the interface between the particle at the end of chain and the sample. The simulation results show that the time of flight (TOF) of the primary reflected solitary wave decreases with the stiffness of the sample increases, and the amplitude ratio (AR) between the primary reflected solitary wave and the incident solitary wave increases. An HNSWs transducer based on the one-dimensional granular chain is designed and fabricated. The relationship between the characteristic parameters of the primary reflected solitary wave (TOF and AR) and the curing time of adhesive for a composite/metal bonded structure is experimentally investigated. The experiment results suggest that the TOF decreases and the AR increases as the epoxy cures. The experimental results are in good agreement with the simulation results. This study provides a new characterization method for monitoring the curing process of adhesive for composite/metal bonded structure.

Theoretical Model for Piezoelectric Energy Harvesting Device Based on Composite Granular Chain of Spheres

In this study, a simple and high-performance piezoelectric energy harvesting devices (PEHD) based on composite granular chain of spheres (CGCS) is investigated. The CGCS is constructed by inserting a light granular chain into the middle of a heavy granular chain. When an impact imposed to the CGCS, the energy of the impact will be carried by solitary wave propagating in the chain. The existence of the heavy-light interface and light-heavy interface makes the middle section of chain a container to trap the energy of the solitary wave. Therefore, the solitary wave will reflect back and forth in the container and experience slow energy attenuation. Piezoelectric wafer is embedded into one of the spheres of the container to act as a PEHD. Theoretical model of the proposed PEHD is given to explain the energy conversion process from external impact to the output voltage of the piezoelectric wafer. The bridge between the solitary wave-induced stress and the electric field is highlighted. Experiments are performed in CGCS to observe the solitary wave-induced voltage of the piezoelectric wafer and the measured waveform agree the theoretically prediction results. Finally, the effects of the differences in material properties of between the light and heavy spheres and the segment number of composite chain on the collected energy are investigated for improving the efficiency of capture energy. It is suggested that increasing the numbers of composite segments and enlarging the differences between the light and heavy sphere is helpful to improve the performance of CGCS-based PEHD.