Adaptive Heterodyne Interferometer for Ultrasonic NDE

For NDE applications, the remote generation and detection of ultrasound by laser present many advantages over traditional piezo-electric based methods. They provide nonintrusive, point generation and detection with a large frequency bandwidth. For example, it can be used on surfaces of complex geometry and elevated temperature on a production line. Ultrasound generation using absorption of pulse laser energy is well known. Various interferometers for optical detection of ultrasound have been described in the literature [1]. In order for the interferometer to be attractive for NDE applications, the interferometer must also be able to operate, without loss of sensitivity, in an environment where large amplitude low frequency vibrations are generally present. Furthermore, the interferometer must be able to achieve a good sensitivity on rough surfaces. The laser light reflected on a rough surface is characterized by speckles. The random intensity and phase distributions of these speckles require the use of interferometer with large étendue. In addition, the possibility of using an optical fiber in the path of the probe beam without loss in the interferometer sensitivity is highly desirable for applications where access to the specimen is limited. The confocal Fabry-Pérot interferometer [1] has been shown to be well adapted for NDE applications. Unfortunately, since the elimination of the optical side-bands is based on the optical filtering action of the confocal cavity, it is sensitive mostly to high frequencies for a reasonable cavity size, typically above 1MHz for a 1 meter cavity length

In-Plant Demonstration of High-Temperature Emat System on Continuous Caster Strand

The Pacific Northwest Laboratory (PNL) has been working with the Office of Industrial Programs of DOE and the American Iron and Steel Institute (AISI) since 1983 to develop a sensor that can determine the internal temperature of hot steel bodies. Research projects utilizing the relationship between acoustic velocity and temperature have been the primary technique investigated. The techniques most recently used have been laser-generated stress waves and EMAT systems [1,2,3]. Development of an ultrasonic sensor that can measure the acoustic velocity at high temperatures has been the major effort over the past few years. The internal temperature of steel bodies can be determined by measuring the time-of-flight through a known distance to calculate the velocity, and from the relationship between temperature and velocity, the average internal temperature can be determined [4,5]

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

Analysis of Laser Ultrasonic Measurements of Surface Waves on Elastic Spheres

In conventional ultrasonic nondestructive evaluation studies, piezoelectric transducers are used to generate sound waves in solids via a couplant that transmits the mechanical motions. In recent years, a different method of generating sound in solids, pulsed laser heating, was introduced by White [1,2]. This method is noncontacting, requires no coupling medium, and operates directly on the surface of the specimen. Noncontacting ultrasonic detection using laser interferometers of several types has also been developed [3]. Laser techniques can achieve essentially point source and point detection of ultrasonic motion through focusing. Laser ultrasonics can, therefore, be used on objects with complex shapes, e.g. curved surfaces, and are applicable to material shapes more commonly found in industry. Often the goal of ultrasonic measurements is to determine material properties such as Lame’s elastic constants. The conventional approach measures longitudinal and shear wave speeds between two parallel flat surfaces. The work reported here demonstrates the versatility of laser ultrasonics by directly measuring the surface motion of a solid sphere generated by ablation from a pulsed laser beam. The results compare well with elastodynamic theoretical calculations, where the ablation source is approximated as a normal impulse on the surface. This work suggests that an algorithm could be formulated to measure elastic properties of targets with curved surfaces

Optical Detection of Ultrasound by Two-Wave Mixing in Photorefractive Semiconductor Crystals Under Applied Field

The optical detection of transient surface motion has many practical applications which include, in particular, the vibration monitoring of engineering structures (aircraft, power plants,...) and the detection of ultrasound produced by piezoelectric transducer or by pulse laser excitation. This last application where ultrasound is generated and detected by lasers, presents many advantages over conventional piezoelectric based methods. First, laser-ultrasonics is a remote sensing technique. Consequently it can be used, for example, for inspecting hot materials and products moving on a production line. Second, surfaces of complex shapes can also very easily be probed. For many applications, these advantages compensate the usually lower sensitivity of the laser-based technique compared to piezoelectric transduction

Noncontact Temperature Measurements of Hot Steel Bodies Using an Electromagnetic Acoustic Transducer (EMAT)

A noncontacting sensor system for measuring the average internal temperature of hot steel objects has been developed. The system uses a pulsed ruby laser for the generation of the acoustic wave and a pulsed Electromagnetic Acoustic Transducer (EMAT) as the receiver in a through-transmission technique. The pulsed EMAT design has been successfully tested to 1300° Celsius on a nine-inch-long stainless steel sample. The system measures the time-of-flight of the acoustic wave, which is coupled with the part dimension to determine the average acoustic velocity. From a calibrated relationship between velocity and temperature, the average sample temperature is determined.</p

Laser-Based Ultrasonics on Gr/Epoxy Composite a Systems Analysis

Critical issues are examined in the application of laser generation and detection of ultrasound to the inspection of large area air-frame composites. Among these issues are surface roughness, signal-to-noise ratio, insensitivity to the path length between the part and detector, and wide-band versus narrow-band generation. Supporting experiments are reported on broad-band and narrow-band generation in Gr/Epoxy panels and angular reflectance measurements on painted and unpainted Gr/Epoxy. On the basis of these measurements, a laser-in/laser-out systems analysis is carried out for a 10 mm diameter delamination about 1 cm deep. The analysis assumes that a Spherical Fabry-Perot interferometer is used for detection and a 10 nsec laser pulse with a peak power of 13 MW/cm2 for generation. The estimates indicate a S/N ≈ 20 dB for a detection probe laser power of about 400 mW.</p

Landslides and sediment delivery to a drainage system: Some observations from Hong Kong

The delivery of sediment from hillslopes to a drainage system has been examined for Hong Kong. Landslides may influence sediment production in small upland drainage basins by the direct delivery of material to the channel, subsequent fluvial action on materials exposed in debris trails and erosion as the failure moves downslope, sometimes to become channelised. Evidence of the direct delivery of sediment by landslides is presented from a small drainage basin. On undeveloped terrain in Hong Kong, in general, less than half of the landslides deliver debris to the channel system. Examples of landslide debris volume exceeding that of source area failure in Hong Kong are presented and this relates to channelisation of debris flows. Revegetation of landslide debris trails may be quite rapid and this restricts the time for fluvial action on the exposed materials. The temporal aspect of sediment delivery has been documented. Rainfall events that are guaranteed to generate large numbers of landslides are relatively rare. © 2004 Elsevier Ltd. All rights reserved.link_to_subscribed_fulltex

Evidence-based practice in oral and maxillofacial surgery. Audit of one training center

Objectives: To evaluate the proportion of evidence-based interventions in the field of Oral and Maxillofacial Surgery (OMS) in a regional training centre. We also evaluated the levels of evidence present in the OMS literature, and the current availability of evidence in various diagnostic categories. Methods: A prospective clinical audit was carried out within the discipline of OMS, University of Hong Kong, in February 2005, for a period of six months to investigate the extent of evidencebased practice. Consecutive diagnosis and intervention pairs were identified and recorded through standardized charts in randomly selected clinical sessions. A corresponding literature search using Medline and the Cochrane Library was performed to identify best current evidence, then analysed and graded according to the evidence. Results: Two hundred and seventy three out of 500 cases were eligible for evaluation while the rest were excluded based on four defined exclusion criteria. A majority of interventions (n = 195, 71.4%) were found to be evidence based. Seventy eight (28.6%) interventions were found to be not evidence based. Among the evidence, a majority (56.1%) were level 5 which are mainly case series, and 36% were level 3 or above, which are randomized control trial (RCT) (level 3), meta-analysis of RCTs (level 2) or systematic review of RCTs (level 1). There was no statistically significant difference in the proportion of evidence-based practice between specialists and trainees in OMS who treated the patients. Conclusion: This study demonstrated that most interventions prescribed in this OMS training centre were evidence based and the proportion was comparable to that reported by other specialties.link_to_subscribed_fulltex

Mandibular distraction osteogenesis: basic research on animal model

Ultrasonic techniques are being currently used in a wide range of applications. However, there are situations where it is difficult to utilize ultrasound for NDT purposes. They are severely limited in non-contact applications due to attenuation in air. Since the present day technology requires a physical contact or a couplant between the transducer and the specimen surface to minimize diffraction effects, the temperatures sensitive piezoelectric transducers are not suited for hostile operating environments and extreme temperature gradients. Another drawback of piezoelectric transducers is the narrow banded source signals. Generation of very high frequency ultrasound becomes difficult and expensive. Complex contoured specimens are, also, difficult to handle, since traditional ultrasound is sensitive to the normalization of the incident ultrasonic beam. As with all mechanical devices, the rate of scanning of these transducers is very slow