Noncontacting laser Ultrasonic Generation and Detection at the Surface of Molten Metal

The use of pulsed lasers for noncontacting generation of ultrasound in solid materials is expanding rapidly [1], as is optical detection of ultrasound [2]. The noncontacting nature of laser ultrasonics is opening new areas of research where physical contact of transducers to the material under study is impossible or inadvisable. One example is in the titanium melting industry. Currently, vacuum arc remelting (VAR) is used to produce much of the nation’s titanium from Kroll process sponge. However, the process provides only limited means of removing oxynitride and carbide inclusions from the melt, which can become stress intensifiers in the ingot. VAR of titanium can be replaced with plasma or electron beam hearth melting, both of which have the potential to eliminate these stress-intensifying inclusions by increasing the residence time of the molten titanium in the hearth so that the oxynitrides dissolve and the carbides settle out of the melt. This process is so important that industry is starting to replace VAR with hearth melting for titanium to be used in critical applications such as rotating turbine parts. The new process has other advantages as well. Processing steps will be eliminated because sponge will no longer need to be consolidated into electrodes and fewer melting steps will be required. The improved quality of the melted product will result in less scrap, and the ability to recycle scrap into high value products will also be a major improvement. The most important aspect, though, is the capability to produce superior ingots with the potential of allowing turbine engines to be lighter and more efficient. However, industry has identified a critical requirement for these hearth melting processes: measurement of the volume of molten metal to ensure sufficient residence time in the melt. Ultrasonic sensing is one possible way for locating the interface between molten and solid metal so that the depth of the molten metal, the volume, and thus the residence time may be determined. Because the titanium hearth operates at high temperatures (1650°C), contacting transducers with buffer rods are not practical; it is also a potential source of melt contamination. Therefore, a totally noncontacting sensor system is needed. This sensing technology would also be widely applicable to other metals, including other reactive and refractory metals, superalloys, and steel.</p

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

Process Monitoring Using Optical Ultrasonic Wave Detection

Certain microstructural features of materials, such as grain size in metals, porosity in ceramics, and structural phase compositions, are important for determining mechanical properties. Many of these microstructural features have been characterized by ultrasonic wave propagation measurements, such as wave velocity and attenuation. Real-time monitoring of ultrasonic wave propagation during the processing stage would be valuable for following the evolution of these features. This paper describes the application of laser ultrasonic techniques to the monitoring of ceramic sintering. Prior to this work, ultrasonic wave measurements of the sintering of ceramics have been made only through direct contact with the material with a buffer rod [1,2]. Recently, several advances have been made using lasers for both generation and detection of ultrasonic waves in a totally noncontacting manner for material microstructure evaluation [3–5]. Application of laser ultrasonic techniques now opens the possibility for real-time monitoring of materials in very hostile environments as are encountered during processing [6]

Pulsed Lasers for Quantitative Ultrasonic NDE

There has been a lot of recent interest in the use of pulsed lasers for the generation of ultrasound in a range of media [1], due to advantages inherent in the generation process. These include the non-contact nature of the method, and the generation of a wide bandwidth over a small source area. The size and shape of the source can also be changed using suitable optics, as can the generation mechanism itself

Probing liquid surface waves, liquid properties and liquid films with light diffraction

Surface waves on liquids act as a dynamical phase grating for incident light. In this article, we revisit the classical method of probing such waves (wavelengths of the order of mm) as well as inherent properties of liquids and liquid films on liquids, using optical diffraction. A combination of simulation and experiment is proposed to trace out the surface wave profiles in various situations (\emph{eg.} for one or more vertical, slightly immersed, electrically driven exciters). Subsequently, the surface tension and the spatial damping coefficient (related to viscosity) of a variety of liquids are measured carefully in order to gauge the efficiency of measuring liquid properties using this optical probe. The final set of results deal with liquid films where dispersion relations, surface and interface modes, interfacial tension and related issues are investigated in some detail, both theoretically and experimentally. On the whole, our observations and analyses seem to support the claim that this simple, low--cost apparatus is capable of providing a wealth of information on liquids and liquid surface waves in a non--destructive way.Comment: 25 pages, 12 figures, to appear in Measurement Science and Technology (IOP

Second Order Speckle Statistics in Optical Coherence Tomography

Peer reviewed: NoNRC publication: Ye

Laser Ultrasound Imaging of Lamb Waves in Thin Plates

Laser ultrasound offers many advantages over conventional piezoelectric ultrasound including the potential for rapid wide-area scanning, non-contacting (no couplant) generation and sensing, and large bandwidth [1,2]. Ultrasonic surface waves may be easily generated by a laser and can travel extended distances when the part is not immersed and loss to a surrounding water bath is eliminated. In addition, the geometric attenuation is significantly less as the sound energy spreads out in a circular annulus rather than in a spherical shell giving rise to an amplitude decay proportional to r −1/2.We have shown that synthetic focusing of laser ultrasound data [3] permits us to use this information to create images of near-surface defects outside the scan area. A single scan line can be used to image the complete surface of part with high speed, resolution, and sensitivity (Figure 1).</p

Photografting of HSS on the surface of LDPE film by two steps

In order to get the surface of polymer materials acidified effectively, the introduction of strong acid group (sulfonic group) to the surface of LDPE film by photografting polymerization in two steps was investigated. Influences of parameters such as reaction time, monomer concentration, pH value in solution and grafting times on polymerization were examined. The obtained results indicate that prolonging photo reduction time and graft reaction time, increasing monomer concentration and pH value in solution, and grafting for more than once help to the improvement of graft extent. The grafted film was characterized by gravimetric method, water contact angle measurement, infrared spectrum analysis and scanning electron microscope observation.link_to_subscribed_fulltex