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.
Laser Ultrasonic Study of Crack Tip Diffraction
Ultrasonic diffraction pattern at a crack tip and crack depth sizing based on the diffraction are studied by using laser‐generated bulk waves. Directivity patterns oriented essentially along the normal for the diffracted longitudinal wave and essentially at 45 (deg) for the diffracted shear wave for any incident angles is obtained. A laser‐ultrasonic system combined with time‐of‐flight (TOFD) analysis is produced and demonstrates accurate crack sizing on an artificial slot having a variable depth from 0 mm to 10 mm. An improvement of signal‐to‐noise ratio by split spectrum processing (SSP) is also suggested.
SAFT Data Processing Applied to Laser-Ultrasonic Inspection
By relying on optics for providing the transduction of ultrasound, laser-ultrasonics brings practical solutions to a variety of nondestructive evaluation problems that cannot be solved by using conventional ultrasonic techniques based on piezoelectric transduction [1,2]. Laser-ultrasonics uses two lasers, one with a short pulse for the generation of ultrasound and another one, long pulse or continuous, coupled to an optical interferometer for detection. Laser-ultrasonics allows for testing at a large standoff distance, inspection of moving parts on production lines and inspection in hostile environments, such as the one encountered in the steel industry. The technique features also a large detection bandwidth, which is important for numerous applications, particularly involving material characterization. Another feature of laser-ultrasonics, particularly useful for inspecting parts of complex shapes, is the generation of an acoustic wave propagating normally to the surface, independently of the shape of the part and of the incidence angle of the optical generation beam. This characteristic feature occurs either when the ablation mechanism is used for generation or when light from the generation laser penetrates sufficiently deep below the surface. This last condition occurs usually with many polymer-based materials and on materials with painted surfaces.