Imaging of Impact Damage in Composite Materials

Conventional ultrasonic C-scan images are generated from information acquired within “gates” placed at specific temporal locations on low-pass filtered and rectified versions of A-scans. Placing the gates at temporal locations which correspond with interfaces allows the integrity of the interfaces to be examined. However, if the interfaces are closely spaced, as is the case for quasi-isotropic graphite/epoxy composites, the information from upper layers is blurred into the layers below because of the finite time duration of the ultrasonic pulse. This creates a low signal-to-background-level ratio, which causes blurring at and below the first interface

Implementation of Self-Noise Suppression Techniques for Ultrasonic Correlation Systems

Pseudo-random signal correlation techniques can improve the flaw detection capability of ultrasonic NDE systems. While the correlation-based systems provide significant improvement in the signal-to-noise ratio compared to pulsed systems, their performance is limited by the so-called “self-noise” of the system. Self-noise is a result of imperfect autocorrelation characteristics of the excitation signal. Last year, we suggested some techniques for improving the flaw detection capability of continuous-mode ultrasonic NDE systems [1]. These systems use a continuously transmitted coded waveform as an excitation signal, and the received signal is processed through a correlation filter. This year, we present another new approach and demonstrate performance results and the practicability of each approach.</p

Choice of Coded Waveform and Correlation Filter for Self-Noise Suppression in Ultrasonic Correlation Systems

Various ultrasonic correlation systems have been suggested in the past [1–5] in order to improve the flaw detection capability of ultrasonic NDE systems. These systems use a coded waveform for transmission, and the received signal is processed through a correlation filter. Although, these systems provide considerable improvement in the flaw detection capability, their performance is limited by what is called “self-noise” of the system. This paper discusses the self-noise limitation of conventional ultrasonic correlation systems and presents various approaches for self-noise suppression. Theoretically predicted performance has been verified using detailed computer simulations. Finally, comparison of the performance and practicability of each approach is discussed

Multisection vialess baluns with coupled-line impedance transformers

Multisection vialess baluns are proposed by cascading several λ/4 edge-coupled lines. Without needing any via-hole or ground connection, these vialess baluns are simply implemented on microstrip structures. With the backside aperture, the even-mode impedance of coupled microstrip lines is raised beyond 280 Ω. Taking advantage of multlsection coupled line approach, the eventual even-mode impedance can achieve as small as 2 Ω, thus satisfactorily meeting the requirements of λ/4 coupled line baluns with open-circuited terminals. Three- and five-section vialess baluns are designed and the predicted performances are validated by experiment. In a wide frequency range (1.8-3.6 GHz), the measured amplitude imbalance is less than 0.4 dB and phase Imbalance is within ±5°. © 2006 IEEE.link_to_subscribed_fulltex