Kaiyuan Li

Reference-free damage detection by means of wavelet transform and empirical mode decomposition applied to Lamb waves

Guided ultrasonic waves are increasingly used in all those structural health monitoring applications that benefit from built-in transduction, moderately large inspection ranges, and high sensitivity to small flaws. This article describes a monitoring system based on the generation and detection of the guided ultrasonic waves from an array of sparse transducers. In a round-robin manner, ultrasonic waves are generated and measured from all possible different pairs of excitation and sensing transducers. The ultrasonic signals are then processed using continuous wavelet transform and empirical mode decomposition to extract few damage-sensitive features that enable the detection and localization of damage. With respect to most of the existing guided ultrasonic wave–based methods, the proposed approach does not require to record data from a pristine structure (baseline data), and damage is inferred by examining the selected features obtained from all the possible combinations of actuator–sensor pairs of the array. In this study, the method is validated using commercial finite element software to model the presence of 10 ultrasonic transducers bonded onto an aluminum plate. The results are promising and ongoing studies are focusing on the experimental validation and the application to other waveguides.

Noncontact monitoring of immersed plates by means of laser-induced ultrasounds

This article presents the results of an experimental and numerical study where guided ultrasonic waves were used for the structural health monitoring/nondestructive evaluation of an immersed aluminum plate. Leaky Lamb waves were generated by means of a pulsed laser and detected by an array of immersion transducers. The signals were then processed using continuous wavelet transform to extract few damage-sensitive features that were fed to an unsupervised learning algorithm based on outlier analysis. The experimental setup was simulated numerically using a commercial finite element software to predict the time of arrival of the propagating modes. In order to assess the capability of the monitoring system to detect damage, four defects were devised on the plate prior to the immersion in water. We found that the noncontact probing system and the signal processing enable the detection of cracks and holes.

Energy harvesting using arrays of granular chains and solid rods

In the last two decades, it has been demonstrated that highly nonlinear solitary waves (HNSWs) can be used in many physics and engineering applications, such as acoustic lenses, impurity detectors, and nondestructive testing. HNSWs are compact nondispersive waves that propagate in nonlinear media such as 1D chains of spherical particles. In this paper, we propose to couple an array of granular particles that support the propagation of HNSWs, to a wafer-type lead zirconate titanate (PZT) transducer in order to harvest the energy of an object tapping the array. This latter is in contact with a polycarbonate block where the nonlinear waves become linear and coalesce at a designed focal point. Here, the PZT converts the acoustic energy into electricity that powers a load resistor. The performance of this harvester is compared to a similar system where the chains are replaced by solid rods. The results demonstrate that the granular system generates more electricity.

On the Reliability of a Solitary Wave Based Transducer to Determine the Characteristics of Some Materials

In the study presented in this article we investigated the feasibility and the reliability of a transducer design for the nondestructive evaluation (NDE) of the stiffness of structural materials. The NDE method is based on the propagation of highly nonlinear solitary waves (HNSWs) along a one-dimensional chain of spherical particles that is in contact with the material to be assessed. The chain is part of a built-in system designed and assembled to excite and detect HNSWs, and to exploit the dynamic interaction between the particles and the material to be inspected. This interaction influences the time-of-flight and the amplitude of the solitary pulses reflected at the transducer/material interface. The results of this study show that certain features of the waves are dependent on the modulus of elasticity of the material and that the built-in system is reliable. In the future the proposed NDE method may provide a cost-effective tool for the rapid assessment of materials’ modulus.

Experimental parametric analysis of an energy harvester based on highly nonlinear solitary waves

We investigate experimentally five different designs of an energy harvester based on mechanical vibration and highly nonlinear solitary waves. The harvester consists of a metamaterial formed by granular chains, an oscillator that taps the metamaterial, a solid in contact with the metamaterial, and a piezoelectric element glued to the solid. The overall principle is that the oscillator taps the metamaterial and creates a train of solitary waves along each chain. At the interface between the chains and the solid, part of the acoustic energy refracts into the solid where it coalesces at a point and triggers the vibration of the solid. Here, a transducer converts the focalized stress wave and the waves generated by the reverberation with the edges into electric potential. In the study presented in this article, we evaluate the effect of certain harvester parameters on the amount of energy that can be extracted. We considered five different designs by changing the oscillator, the dimension of the array, the solid material, and the transducer boundary condition. For each design we computed the power density, and we found that the density obtained with the best design is four orders of magnitude higher than the worst design.

Signal processing for the inspection of immersed structures

In this paper, we present a non-destructive inspection method for immersed waveguide. A laser operating at 532 nm is used to excite leaky guided waves on an aluminum plate immersed in water. The plate has a few artificial defects. An array of immersion transducers is used to detect the propagating waves. A signal processing based on continuous wavelet transform is utilized to extract a few damage-sensitive features that are used in an outlier analysis and in a probabilistic-based imaging method. The experimental results show that the proposed system can be used for the inspection of underwater waveguides.

Energy Harvesting Using Arrays of Granular Chains and Solid Rods

In the last two decades it has been demonstrated that highly nonlinear solitary waves (HNSWs) propagating in chains of granular particles can be used in many physics and engineering applications, including acoustic lenses, impurity detectors, and nondestructive testing. HNSWs are compact nondispersive waves that propagate in nonlinear medium such as 1D chains of spherical particles. In this paper we propose to couple an array supporting the propagation of HNSWs to a wafer-type lead zirconate titanate (PZT) transducer to harvest energy from the vibration of an object tapping the array. The array of granular chains is in contact with a polycarbonate block, where the nonlinear waves become linear and coalesce at a designed focal point. Here, the PZT converts the acoustic energy into electricity to power a load resistor. The performance of this harvester is compared to a similar system where the chains of particles are replaced by solid rods, and the results demonstrate that the granular system generates more electricity. doi: 10.12783/SHM2015/121

On the use of nonlinear solitary waves for energy harvesting

In the last decade there has been an increasing attention on the use of highly- and weakly- nonlinear solitary waves in engineering and physics. These waves can form and travel in nonlinear systems such as one-dimensional chains of spherical particles. One engineering application of solitary waves is the fabrication of acoustic lenses, which are employed in a variety of fields ranging from biomedical imaging and surgery to defense systems and damage detection. In this paper we propose to couple an acoustic lens to a wafer-type lead zirconate titanate transducer (PZT) to harvest energy from the vibration of an object tapping the lens. The lens is composed of a circle array made of chains of particles in contact with a polycarbonate material where the nonlinear waves coalesce into linear waves. The PZT located at the designed focal point converts the mechanical energy carried by the stress wave into electricity to power a load resistor. The performance of the designed harvester is compared to a conventional cantilever beam, and the experimental results show that the power generated with the nonlinear lens has the same order of magnitude of the beam.

Coupling mechanism of granular medium and slender beams

We present a methodology to assess slender beams by means of highly nonlinear solitary waves. This is accomplished by understanding the coupling mechanism between highly nonlinear solitary waves propagating along a granular system and a beam in contact with the granular medium. Nonlinear solitary waves are compact non-dispersive waves that can form and travel in nonlinear systems such as one-dimensional chains of particles. In the study presented in this paper, the waves are generated by the mechanical impact of a striker and are detected by means of sensor beads located along the chain. We investigated numerically and experimentally the effect on the solitary waves of slender beams of different modulus, length, boundary condition, and axial stress. We found that the geometric and mechanical properties of the beam or thermal stress applied to the beam alter certain features of the solitary waves. In the future, these findings may be used to develop a novel sensing system for the Nondestructive Evaluation of beams.