Alexei K. Kromine

Photoacoustic probes for nondestructive testing and biomedical applications

Fiber-optic photoacoustic sources for nondestructive testing and biomedical applications are described. The photoacoustic sources consist of a pulsed laser, a fiber-optic cable, and a generation head. The generation head is a miniature hermetically sealed chamber, which can be embedded into solid structures or immersed in liquid media. The face of the chamber acts as a target for laser irradiation. Bulk ultrasonic waves generated inside of the target are transmitted into the medium. The proposed systems offer wide ultrasonic range (0.5-15 MHz), easy control over directivity of the ultrasonic beam, high efficiency of generation, and the ability to operate in a harsh environment. Sources with different radiation patterns with respect to the optical axis of the fiber, such as normal, sideways, as well as focused, have been devised. We present a proof-of-concept experiment using these sources in combination with fiber-optic ultrasonic receivers.

Laser Ultrasonic Array System for Real-Time Cure Monitoring of Polymer-Matrix Composites

A laser ultrasonic system for real-time cure monitoring of a graphite-epoxy composite is proposed. The system contains an array of fiberized laser ultrasonic sources, and an embedded fiber optic Sagnac ultrasonic sensor, and is integrated with a Resin Transfer Molding machine. The use of an optical switch allows ultrasonic generation at several locations of the composite part. Ultrasound generated by the laser source is transmitted into the composite and is detected by the embedded sensor. The cure state is inferred from measurements of ultrasonic velocity. The results of ultrasonic measurements during manufacturing of a composite specimen are presented. The laser ultrasonic cure monitoring system described in this paper has the ability tooperate in a high temperature and high pressure environment; is small enough to be incorporated into molds; and measures the cure state at several locations within the composite part.

Ultrasonic imaging of small surface-breaking defects using scanning laser source technique

The Scanning Laser Source (SLS) technique is based on monitoring the changes in the laser generated ultrasonic signal as the source is scanned over the area of inspection. The SLS imaging scanning system includes a portable Nd-YAG laser, free space or fiberized scanners, an ultrasonic detector, and signal processing software. High resolution ultrasonic images of small EDM notches and fatigue cracks on flat and curved specimens are presented. A Mass-Spring Lattice Model is adopted as the numerical method for the simulation and visualization of the SLS technique. The SLS imaging system offers an effective solution for high resolution non-contact inspection of critical components of complicate shape.

Detection of subsurface defects using laser based technique

The Scanning Laser Source (SLS) technique for the detection of sub-surface defects is presented. This approach monitors the changes in the time and frequency domain signals of laser generated ultrasound resulting from the changed conditions under which the ultrasound is generated over areas with and without defects. Results are presented for detection of small sub-surface defects using a fiberized laser based system. The SLS technique allows detection of subsurface defects smaller than the ultrasonic wavelength.

Sagnac-type fiber-optic array sensor for detection of bulk ultrasonic waves

In this paper, we describe a fiber optic array sensor suitable for detection of bulk ultrasonic waves. This sensor is based on an intrinsic fiber optic Sagnac interferometer. The fiber array is formed by multiple folding of a continuous length of an optical fiber into flat coils. Depending on the orientation of the fiber array with respect to the ultrasonic wave, the proposed sensor can act as a conventional in-phase detector or as a narrowband detector. In the narrowband mode, the center frequency of detection can be tuned by adjusting the spacing of the fiber array elements to be equal to the ultrasonic wavelength of interest. This feature distinguishes this array sensor from conventional hydrophones in which a receiver is typically much smaller than the acoustical wavelength. It is shown that the array sensor provides an enhanced signal-to-noise ratio (SNR) compared with a single element detection scheme. Results are presented for detection of ultrasonic waves in water arising from both piezoelectric and laser ultrasonic sources. Potential areas of application of this sensor include process monitoring, smart structures, bio-medical ultrasound, and chemical sensing.

Applications of scanning laser source technique for detection of surface-breaking defects

A new laser ultrasonic approach, the Scanning Laser Source (SLS) technique, is presented for detection of small surface-breaking defects. In this approach we do not monitor the interaction of a generated ultrasonic wave with a flaw, as in the case of traditional pitch-catch or pulse-echo methods, but rather monitor the changes in the laser generated ultrasonic signal as the source is scanned over a defect. Changes in the amplitude and frequency content of the laser-generated ultrasound are observed resulting from the changed conditions under which the ultrasound is generated over areas without and with a surface- breaking crack. These changes are quite readily detectable using existing ultrasonic detectors. The SLS system includes a fiberized portable Q-switched YAG:Nd laser, which can be combined with either convention PZT transducers or laser interferometers. Results are presented for detection fo small EDM notches and fatigue cracks on flat and curved specimens and thin plates including real structures such as an aircraft turbine disk. It is shown that the SLS technique has several advantages over the conventional pitch-catch approach, including: (i) enhanced signal-to-noise performance, (ii) detection of defects with size smaller than the ultrasonic wavelength (at least 0.125 mm length and 0.06 mm depth), (iii) ability to detect defects of various orientations with respect to the scanning direction, (iv) inspection of surfaces with complex geometry such as bore holes and turbine disk slots.

Scanning Laser Source Technique and its Application to Turbine Disk Inspection

Laser-based ultrasonic (LBU) techniques provide a number of advantages over conventional ultrasonic methods such as higher spatial resolution, non-contact generation and detection of ultrasonic waves, and ability to operate on curved and rough surfaces [1]. In earlier papers, the present authors have described fiberized tunable laser ultrasonic sources [2,3], and fiber-optic heterodyne and Sagnac interferometers for detection of ultrasound [4,5]. In this paper we present a new laser based technique--Scanning Laser Source (SLS) technique--for the detection of small surface-breaking cracks on rough and curved surfaces. This technique allows detection of flaws by monitoring the variations of ultrasonic amplitude and frequency (flaw signature) as the laser ultrasonic source is scanned across the object and passes over any defects. Typical flaw signatures for different kinds of surface breaking defects are obtained. The results of application of the SLS technique for inspection of a turbine disk are presented.

Distributed photoacoustic system for cure monitoring of composites

In this paper we describe a laser ultrasonic system for real-time monitoring of the degree of cure of a graphite-epoxy composite part during manufacturing. The system is integrated with a Resin-Transfer Molding (RTM) machine, and contains (i) a fiberized laser ultrasonic source, and (ii) an embedded ultrasonic sensor based on an intrinsic fiber optic Sagnac interferometer. Bulk ultrasonic waves generated by the laser source are transmitted into the composite structure and are subsequently detected by the embedded ultrasonic sensor. The degree of cure can be obtained from measurements of ultrasonic velocity and attenuation in the composite part. The use of an optical switch in the fiber optic delivery system of the laser ultrasonic source allows ultrasonic generation at several locations of the composite part. In this paper we discuss the design of the laser ultrasonic source and the sensor optimized for cure monitoring applications, and their integration with the RTM mold. The results of ultrasonic measurements during manufacturing of a composite specimen are presented. Our results show that laser ultrasonics offer distinct advantages for manufacturing of modern composite structures including the ability to operate in a high temperature and high pressure environment and provide distributed sensing that can cover critical areas of a component.

Scanning laser source technique for detection of surface-breaking and sub-surface cracks

A new approach is presented for ultrasonic detection of small surface-breaking and subsurface cracks using laser-based techniques. This approach does not monitor the interaction of a generated ultrasonic wave with a flaw, as in the case of traditional pitch-catch or pulse-echo methods, but rather monitors the changes in the generated ultrasonic signal as the laser source passes over the area of inspection. Changes in amplitude and frequency of the laser generated ultrasound are observed resulting from the changed conditions under which the ultrasound is generated over uniform and defective areas. These changes are quite readily detectable using existing laser detectors. The main advantages of the proposed scanning laser source (SLS) technique are: (i) enhanced signal-to-noise performance compared to the conventional pitch-catch mode of operation, (ii) ability to detect smaller defects, (iii) ability to detect defects of various orientations with a respect to the scanning direction, (iv) compatibility with...

Laser Ultrasonic enabled “Smart” Mold for composite parts manufacturing

A Laser Ultrasonic “Smart” Mold, which provides in situ information about the degree of cure of a composite part during manufacturing is described. The LUSM contains a laser ultrasonic source and an embedded fiber optic ultrasonic detector. The degree of cure can be obtained from measurements of the ultrasonic velocity and attenuation in the composite part. The results of ultrasonic measurements during manufacturing of a composite specimen are presented. The main advantages of the proposed technique are the ability to operate at high temperatures and high pressures, and the ability to provide distributed sensing that covers most critical areas of a component.