Slow Lamb Wave Generation and Reception With a Gold-on-PVDF Interdigitated PVDF Transducer

Interdigitated surface and guided-wave transducers have only recently received attention as possible tools for non-destructive testing. This may be due in part to the increasing attention being paid to piezoelectric polymers as practical transduction materials for structural sensing and actuation. However, much remains to be done to produce a rugged, monolithic device oriented toward these sorts of applications, to characterize and optimize its passive and active response, to develop excitation strategies and signal processing algorithms that in tandem can be employed for arrayed structure monitoring applications. In this paper we confine ourselves to the first two topics and report on the development and proof-of-principle testing of a monolithic interdigitated polyvinyldine fluoride (PVDF) transducer. Specifically, we report on the design and response of an interdigitated transducer with relatively large finger spacings. The finger spacing yield measureable responses in the asymptotically slow single-mode region of Lamb wave dispersion behavior for frequency-thickness products which may be useful for nondestructive testing of many mechanical and civil structural systems.

Optical Transmission Properties of Silicon Wafters: Theoretical Analysis

As a nondestructive measurement method, laser scattering has been preliminarily applied to detect subsurface damage in silicon wafers, but the quantitative correlation between scatter images and subsurface damage depth has not been established yet. In order to assess subsurface damage depth in silicon wafers, a systematic study has been carried out. In the authors’ another paper, a detailed experimental investigation on optical transmission percentage of silicon wafers was presented. As a follow up, this paper will describe a method to calculate the “skin depth” of silicon wafers from the experimental data of optical transmission percentage. And also, how to apply this “skin depth” on assessment of subsurface damage depth will be discussed.

Gearbox Fault Detection Using Empirical Mode Decomposition

Local faults in a gearbox cause impacts and the collected vibration signal is often non-stationary. Identification of impulses within the non-stationary vibration signal is key to fault detection. Recently, the technique of Empirical Mode Decomposition (EMD) was proposed as a new tool for analysis of non-stationary signal. EMD is a time series analysis method that extracts a custom set of bases that reflects the characteristic response of a system. The Intrinsic Mode Functions (IMFs) within the original data can be obtained through EMD. We expect that the change in the amplitude of the special IMF’s envelope spectrum will become larger when fault impulses are present. Based on this idea, we propose a new fault detection method that combines EMD with Hilbert transform. The proposed method is compared with both the Hilbert-Huang transform and the wavelet transform using simulated signal and real signal collected from a gearbox. The results obtained show that the proposed method is effective in capturing the hidden fault impulses.

Ultrasonic Signal Attenuation in Syntactic Foams Filled With Rubber Particles

Ultrasonic imaging is a non-destructive evaluation technique, which is used to obtain density profile, phase distribution and three-dimensional profiles of cracks and defects in a material. Although this technique is used for a variety of metals and non-metals, it is difficult to use it for testing of porous materials and foams due to high attenuation of ultrasonic waves in these materials. Syntactic foams are hollow particle filled composites that have recently emerged as attractive material for use in applications requiring low weight, low moisture absorption and high insulation properties. The present paper focuses on determining the attenuation coefficient in syntactic foams and its correlation with porosity distribution. Eight types of foam samples are tested in the study. A combination of four types of microballoons and two types of rubber particles is used. Volume fractions of microballoons and rubber particles are maintained at 0.63 and 0.02, respectively, in all samples. Pulse Echo ultrasonic test method is used and results are compared to determine the effect of constituent particles on the ultrasound signal attenuation. Coefficient of attenuation is observed to increase with decrease in density of foam samples and with decrease in size of rubber particles.

Ultrasonic Wave Interaction for NDE of Inhomogeneous Prestress in Materials

The problem of ultrasonic nondestructive evaluation (NDE) of inhomogeneous prestress field is studied theoretically for an elastic material and plane strain. The nonlinear effects of ultrasonic wave propagation and interaction are shown to be additional sources of information for NDE. A model problem of simultaneous propagation of two ultrasonic longitudinal waves in physically nonlinear elastic material undergoing two-parametric prestress state is solved. The corresponding analytical solution is derived. Results of numerical simulations lead to the conclusion that it is possible to propose an algorithm for NDE of the inhomogeneous prestress field on the basis of the recorded data of boundary oscillations evoked by simultaneous propagation of two ultrasonic waves. The analysis of the oscillating boundary profiles permits to determine qualitatively the presence of prestress in the material and to distinguish special cases of the prestress state, for example, to evaluate quantitatively the parameters of the prestress.

IR Thermograph Inspection on Adhesion Integrity

The infrared thermography offers a flexible, non-contact and non-destructive tool for investigating the adhesion integrity in different applications. The IR thermography technique has been demonstrated to have wide applications to testing and quality assurance in automobile manufacturing processes. This paper addresses the capability of IR thermography in evaluating the adhesion integrity for two cases which occurred during automobile assembly processes. They are the coated surfaces adhesion integrity inspection and the adhesion strength in welded composite polymer plastic joints. Analytical models for the above two cases will be discussed in comparison to the corresponding thermographs. IR thermography technique can be used with one of or combination of the following mechanisms: the transmitted, the pulsed reflected signals. This paper discusses what type of processing scheme can be best applied to each of these signals.

Advanced Signal Processing Techniques for Multi-Damage Detection With an Improved Embedded Ultrasonic Structural Radar Algorithm and Piezoelectric Wafer Active Sensors

The embedded ultrasonic structural radar (EUSR) algorithm was developed by using piezoelectric wafer active sensor (PWAS) array to detect defects within a large area of a thin-plate specimen. EUSR was verified to be effective for detecting a single crack either at a broadside or at an offside position. However, the damage location was not very precise. This algorithm is improved by using advanced signal processing techniques. The improvement includes: 1) EUSR is able to provide better image of the specimen under monitoring; 2) it is able to detect multiple defects such as several cracks; 3) it is also able to identify different damage types. This paper starts with an introduction of embedded ultrasonic structural radar algorithm. Then the application of using Hilbert transform for extracting the envelopes of the wave packages is discussed. This can eliminate or reduce the effect of side robes so that EUSR produces better images. The improvement of EUSR detectability is concluded through the comparison to the previous results, followed by the experiments to verify the multi-damage detection of EUSR. Finally, we present the results of how EUSR can distinguish different types of damage. This system is implemented by developing a graphical user-friendly interface program in LabView. We conclude with a description of our vision for an even more powerful EUSR for structural health monitoring and embedded nondestructive evaluation.