Ultrasonic Imaging of Copper Material Using Harmonic Components
2nd International Conference on NDE in Relation to Structural Integrity for Nuclear and Pressurized Components, New Orleans,
USA, 24-25 May, 2000
Harmonic imaging is a well established technique for enhancing the quality of ultrasound images in medicine. Harmonic imaging exploits the properties of nonlinear propagation of ultrasound as it travels through blood and tissues. This paper presents an attempt to apply this technique in the NDE for improving the quality of ultrasound B-scans acquired in immersion inspection of copper. It appears that when transmitting sound pulses in one frequency range in the immersed solid the received echoes contain a broad frequency spectrum with harmonic components. The harmonic components, although much weaker than the echo in the fundamental frequency band, can be combined to enhance the signal to noise ratio and the resolution in B-scan images. A novel method for processing these signals that makes use of noncoherent detection (NCD) scheme is presented in the paper.
In the proposed method individual A-scans are processed using an adaptive noncoherent detector aimed at enhancing the signal to noise ratio in the resulting B-scan. First, the algorithm estimates the noise model from the acquired ultrasonic data, and second, it optimizes parameters of a simple model for the flaw echoes. An index defining signal to noise ratio serves as the optimization criterion. The resulting filter is used for processing the ultrasonic A-scans.
The method is illustrated using an ultrasonic data acquired in the inspection of electron beam welds in copper canisters for spent nuclear fuel. Firstly, the presence of higher components in the signal received from copper specimens is proved in an setup consisting of two ultrasonic transducers with different center frequencies. A narrow band transmitting has a lower center frequency and while a broad band transducer with higher center frequency is used as a receiver. Secondly, the signals acquired in this set up are processed by the proposed algorithm with the aim of enhancing the echoes from voids and porosity masked by backscattering from the weld structure.