Laser Ultrasonic Thermoelastic/Ablation Generation with Laser Interferometric Detection in Graphite/Polymer Composites

Ultrasonic signals have been generated and detected in graphite/polymer composites by optical methods. A Doppler interferometric technique was used for detection. The output voltage of this type of interferometer is proportional to the surface velocity of a sample area which is illuminated by cw laser light. Ultrasonic signals were generated by thermoelastic and ablation processes which occur as a consequence of laser pulses incident on the opposite surface of the sample. The evolution of the magnitude and shape of the detected signals was measured as a function of the pulse energy of the generating laser. Low-energy laser pulses generated ultrasound without causing obvious surface damage. At higher energies surface damage was observable in post inspection but could also be detected by observing (through protective goggles) bright flashes near the illuminated area. The energy at which these processes first occur is qualitatively referred to as the ablation threshold. Changes in the observed waveform were evident at energies above the ablation threshold. The higher-energy waveforms were found to consist of a superposition of a thermoelastic component and an ablatic component, whose relative magnitudes changed with laser power. A delay in the initiation of the ablatic wave relative to the thermoelastic wave was observed to be of the order of 0.3 μs, consistent with observations in pure polymer. [1] Photoelectric detection measurements of the ablation plume also showed a clear threshold and a time scale for growth of the ablation products with a characteristic time scale on the order of 0.3 μs

A Linear Systems Approach to Laser Generation of Ultrasound in Composites

Laser ultrasonic generation and detection systems have been shown to be effective in the inspection and evaluation of both metals and composite materials [1–3]. Advantages of these noncontact systems include rapid scanning capability, the inspection of parts with complex geometries, and the ability for use in hostile environments. Unfortunately, laser ultrasonic systems are somewhat less sensitive than conventional contact piezoelectric systems. In order to increase the sensitivity, careful consideration must be paid to the choice of both generation and detection laser systems. Although the sensitivity of current laser ultrasonic systems has been shown to be sufficient for several applications, small improvements may allow for a more wide-spread use.</p

Field Inspection of Ceramic Matrix Composites

Ceramic Matrix Composites (CMCs) are high temperature refractory materials particularly suited to exhaust impinged, signature controlled structures such as those on military aircraft. One of the barriers to their more widespread introduction is the detection of damage in service and the repair of that damage. This work describes a preliminary study of field capable inspection techniques for CMCs.</p

Progress Towards a Fiber-Based Laser-Ultrasonics System for Rapid NDE of Large-Area Composites

State-of-the-art integrally stiffened composite materials, manufactured for use in the next generation of commercial and military aircraft, are being increasingly used for structural components such as wings and fuselages. However, due to the complexity of the manufacturing process, small variations in the shape of integrally stiffened composite structures often occur. Thus, a prioriknowledge of the part shape often does not provide sufficient tolerance to allow an automated conventional ultrasonic inspection. Many of the advantages of laser-based ultrasonics, including its noncontacting nature and applicability to rapid scanning of contoured and integrally stiffened structures, have been described previously [1–5]. To further extend the utility of laser-based ultrasonics, enable limited access inspections and also provide an upgrade/retrofit path for existing ultrasonic scanning systems, it is desirable to reduce the size of current laser-based ultrasound (LBU) system scan heads and provide both generation and detection laser beam delivery via optical fibers. A promising approach is the use of a scanning head based on a Cassegrain optical collection system. This approach minimizes the load carrying requirements of the scanning assembly and is also well-suited for integration with fiber optics to allow the delivery and reception of the ultrasonic generation and detection laser beams via long lengths of optical fiber. This provides increased mobility of the LBU scan head and allows the ultrasonic generation and detection lasers and other sensitive equipment to be housed in a clean environment which potentially can be located hundreds of meters from the inspection area. The use of a pulsed CO2 laser has been reported previously for generation of ultrasonic waves in composite materials [4]. However, the CO2 laser wavelength (λ = 10.6 μm) and the high peak power laser pulses precludes the use of fiber-optic beam delivery over all but very short lengths (< 1.5 m) of specialized optical fiber. Consequently an alternative generating laser has been sought that can be transmitted efficiently over standard quartz optical fiber. An alexandrite laser, which is tunable over the 720–800 nm wavelength range, is being investigated for this application. Progress towards the implementation of a fiber-based LBU system for rapid NDE of large-area composites, and the use of an alexandrite laser for ultrasonic generation in composite materials are described below.</p

From Country to City - Agricultural Land Loss and Economic Development in Dongguan

organized by the Centre for Urban and Regional Studies, Zhongshan University and sponsored by the Commission on Urban Geography, Geographical Society of China and Departmen

The Assessment of Surface Coatings Using Laser Ultrasound Detected with a Wideband Confocal Fabry-Pérot Interferometer Operating in Reflection Mode

Coating technology is used in a variety of applications, for example to improve the wear properties or to improve the bonding properties of a surface. The properties of such surfaces depend on the mechanical properties of the coating, namely its Young’s modulus and its thickness. To be able to predict the performance of such coatings it is necessary to be able to measure these properties in a nondestructive way. Conventional ultrasound is one nondestructive method that has been used for coating measurement [1]. It has the disadvantage of needing a transducer to be placed in direct contact with the material surface, often with the use of a couplant. This is sometimes inconvenient and risks contaminating the surface. Laser ultrasound offers a complete noncontact technique that prevents any surface contamination. It also offers scanning possibilities and laser sensor systems can possess very broad frequency bandwidths [2,3].</p

Effect of two perioxide bleaching systems on tetracycline stained teeth.

The generation and detection of ultrasound with laser beams has become a viable technique in nondestructive testing of materials [1,2]. The main advantage of the technique is its intrinsic non-contact nature. Its main limitation seems to be its fairly low efficiency as compared with that of other standard NDE techniques. An interesting recent development [3–5] consists in using optical fibers to guide the laser light and illuminate the sample under investigation in virtually any desired source configuration. Another inherent advantage of the use of optical fibers is that the optical bench is completely decoupled from the sample under investigation, thus rendering the technique practical for in-situ measurements. The objective of the present study is (1) to present some preliminary experimental results complementary of those of Vogel [4] on the generation of ultrasound with an array of optical fibers; (2) to discuss the possibility of generating directional surface waves in a very narrow frequency band, thus increasing the signal-to-noise ratio; and (3), to discuss the feasibility of the directional detection of ultrasound by using an array of optical fibers as a receiver, also with the goal of increasing the signal-to-noise ratio