SiGnal Processing Techniques For Nondestructive Evaluation Using Laser Ultrasonics
Laser ultrasonics is a form of ultrasonic evaluation in which a pulsed laser generates an ultrasonic wave in a sample which interrogates a specific feature. When this wave returns to the surface a separate laser interferometer detects the small resulting displacement. This technique is strictly noncontact and is thus suitable for in-process inspection of parts at high temperature or moving at high speed. In many applications the acquired temporal signals have features which differ from those acquired using conventional transducers. In this paper we will describe these features and discuss techniques which have been used to process these signals.
Study of laser ultrasonic in adipose tissue by the optical absorption coefficient of inclusions
Based on the thermo-elastic mechanism of a pulsed laser in adipose tissue with inclusions, the physical model of generation and transmission of the ultrasound was established. The transient responses of the single pulsed laser interacting with adipose tissue were presented by means of numerical simulation, meanwhile the corresponding stress field and the transmission of the resultant ultrasound were also investigated. By analyzing the laser-induced ultrasonic waveforms in different positions, we found that when the laser acts on the adipose tissue with inclusions, the inhomogeneity of the tissue leads to different ultrasound modes. This can be attributed to the difference of the optical absorption coefficient between adipose tissues and inclusions. The larger optical absorption coefficient of inclusion in the adipose tissue, the narrower of the full width at half maximum of laser ultrasound signal waveform. Full width at half maximum of laser ultrasound signal waveform of a pure adipose tissue is wider than that of the adipose tissue with inclusions. The resultant ultrasound signal waveform has larger amplitude in the inclusions with larger optical absorption coefficient than in the pure adipose tissue. The laser ultrasound signal waveform of the adipose tissue with inclusions has a multiple-interface reflected wave. These results can provide valuable information about further research on methods and techniques of ultrasonic noninvasive detection of biological organization.
Brainstem Influences on the Cephalic Circulation: Experimental Data From Cat and Monkey of Relevance to the Mechanism of Migraine
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