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

Comprehensive prediction of chromosome dimer resolution sites in bacterial genomes

<p>Abstract</p> <p>Background</p> <p>During the replication process of bacteria with circular chromosomes, an odd number of homologous recombination events results in concatenated dimer chromosomes that cannot be partitioned into daughter cells. However, many bacteria harbor a conserved dimer resolution machinery consisting of one or two tyrosine recombinases, XerC and XerD, and their 28-bp target site, <it>dif</it>.</p> <p>Results</p> <p>To study the evolution of the <it>dif/</it>XerCD system and its relationship with replication termination, we report the comprehensive prediction of <it>dif </it>sequences <it>in silico </it>using a phylogenetic prediction approach based on iterated hidden Markov modeling. Using this method, <it>dif </it>sites were identified in 641 organisms among 16 phyla, with a 97.64% identification rate for single-chromosome strains. The <it>dif </it>sequence positions were shown to be strongly correlated with the GC skew shift-point that is induced by replicational mutation/selection pressures, but the difference in the positions of the predicted <it>dif </it>sites and the GC skew shift-points did not correlate with the degree of replicational mutation/selection pressures.</p> <p>Conclusions</p> <p>The sequence of <it>dif </it>sites is widely conserved among many bacterial phyla, and they can be computationally identified using our method. The lack of correlation between <it>dif </it>position and the degree of GC skew suggests that replication termination does not occur strictly at <it>dif </it>sites.</p

Novel inhibitors of the calcineurin/NFATc hub - alternatives to CsA and FK506?

The drugs cyclosporine A (CsA) and tacrolimus (FK506) revolutionized organ transplantation. Both compounds are still widely used in the clinic as well as for basic research, even though they have dramatic side effects and modulate other pathways than calcineurin-NFATc, too. To answer the major open question - whether the adverse side effects are secondary to the actions of the drugs on the calcineurin-NFATc pathway - alternative inhibitors were developed. Ideal inhibitors should discriminate between the inhibition of (i) calcineurin and peptidyl-prolyl cis-trans isomerases (PPIases; the matchmaker proteins of CsA and FK506), (ii) calcineurin and the other Ser/Thr protein phosphatases, and (iii) NFATc and other transcription factors. In this review we summarize the current knowledge about novel inhibitors, synthesized or identified in the last decades, and focus on their mode of action, specificity, and biological effects

International journal of applied mechanics : IJAM

Abstract not available

A memory efficient slicing algorithm for rapid prototyping

Ultrasonic laser techniques are widely used in non-destructive inspection. Owing to theirs numerous advantages [1], these contactless methods allow characterization of anisotropic and viscoelastic media [2, 3]. Ultrasonic bulk waves, whatever the longitudinal or transverse mode, or their multiple reflections propagating back and forth through the plates, can been detected by interferometric laser techniques. But, how to obtain informations on the stiffness and viscosity tensors? The spectrum ratio between a mode and its reflection allows to get these data. Nevertheless it requires to identify the arrival time of the different modes and of their reflections and to take into account the effect of both dispersion and diffraction. The adopted solution is a parametric method which allows to set free from these problems

Muscular Effects of Headgear-Herbst Appliance Activated step-by-step

When a pulsed laser beam strikes the surface of an absorbing material, ultrasonic waves are generated due to thermoelastic expansion and, at higher laser power densities, ablation of the material. These sound generation mechanisms have been the subject of numerous theoretical [1-3] and experimental [4-6] studies and are now fairly well understood; several reviews have also been published [7-9]. In particular, it has been established that at low power densities the thermoelastic mechanism is well described by a surface center of expansion [1]. This mechanism produces a characteristic waveform whose amplitude is proportional to the energy absorbed from the laser pulse and also dependent on the thermal and elastic properties of the material [1-2]. At higher power densities the melting point of the material is reached, and eventually vaporization of the material takes place [5]. Rapid vaporization leads to ablation of material. Significant ablation occurs only during the laser pulse at power densities near the ablation onset threshold, creating an ultrasonic excitation source with the same time dependence as the laser pulse. At higher laser power densities the ablation process continues after the laser pulse and eventually the ultrasonic source changes from pulse to step like in time dependence [5,9]. In this region plasma absorption also plays a significant role

Involvement of double-stranded RNA protein kinase (PKR) in b-amyloid neurotoxicity

The principles and uses of optical techniques for the generation and detection of ultrasound have been discussed extensively in the literature. Several excellent reviews of the progress in the field of laser ultrasonics are available [1–3]. Laser ultrasonic techniques have several advantages over other inspection methods, making them an attractive option for select applications. Unfortunately, poor sensitivity and high cost of laser ultrasonic systems are currently limiting industrial application. In general, implementation is limited to situations where laser inspection methods present the only available solution, or the few cases that prove cost effective. The goal of the current work is to increase the sensitivity of optical generation and detection systems. The work focuses on the laser/materials interaction that occurs during the generation of ultrasonic waves, and considers a number of methods by which laser ultrasonic systems can achieve greater sensitivity through control of the laser generation source