摘要:

The objective of this paper is to show that time-reversal invariance can be exploited in acoustics to accurately control wave propagation through complex media and create a variety of useful instruments. To illustrate the robustness of time reversal mirrors, several experiments conducted in multiply scattering media, in waveguides and in reverberant cavities are presented. They show that, contrary to long-held beliefs, multi pathing and multiple scattering enhances resolution in time reversal focusing, by making the effective size of the time reversal mirror much larger than its physical size. Applications of time-reversal mirrors in pulse echo detection and particularly in nondestructive testing will be discussed. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1373736

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The last 25 years have seen an explosion in the number of new x-ray based imaging methods and applications. In reviewing these new capabilities we see that the ability to generate improved data quality underlies a number of the major advances. Improvement in the quality of x-ray sources, in detector resolution and dynamic range, in data analysis methods both in image processing and in reconstruction techniques have yielded significant advances in image quality. In addition to the improved imaging hardware, the use of computers play a key role in extending capabilities in high speed computed tomography, in real time image enhancement, in automated defect recognition and in the development of accurate x-ray inspection simulations. The ability to generate information and to analyze the complexities introduced by typical inspection demands can quickly outstrip even today’s computational resources. Developments in parallel computing and computer memory are beginning to managing the huge data volumes now routinely produced and open up a number of new applications. Some examples include high-resolution 3D-image generation with the capability to probe micron length scales, dual energy computed tomography providing improved airport security, materials characterization techniques providing new tool for process development, and x-ray image formation modeling. The ability to model the details of the generation of bremsstrahlung radiation and its interaction with the complex geometry of the object under consideration provide for the first time a means to determine and quantify the optimal parameters for an inspection. These advances over the last 25 years represent exciting time in x-ray imaging, the impact of which will be played out in the next decade. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1373737

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

Medical imaging technologies are evolving rapidly in response to clinical needs in major disease categories. The contributions of medical physics, applied mathematics, computer science, and biomedical engineering have resulted in revolutionary medical imaging applications. For example, the detection, diagnosis and treatment of cancer, principles of biomedical image science are being applied to the development of new diagnostic methods and minimally invasive therapies. The potential of these technological advances to improve patient outcomes is great. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1373738

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The coupling of a scatterer polymerization technique with the Multiple Multipole Expansion method allows full-scale simulations of elastic wave multiple scattering in fiber-reinforced composites containing cylindrical scatters of various cross-sections. Results are presented for the case of an incident SH wave impinging on a bounded Ti/SiC composite region, the plane of propagation being orthogonal to the fibers’ axis. Effects of the presence of elliptical cavities are investigated. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1373739

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

Extension to the cuspidal edges of wave surfaces in a plane of material symmetry of anisotropic solids is proposed via three different approaches. Two of the possible extensions stem from the definition of wave arrival used in the stationary phase approximation and the Cagniard-de Hoop technique for the calculation of the line source (2D) Green’s function. The third one is based on ray theory generalized to complex values of the group velocity. The stationary phase method yields an extension but only near to the cusp points. Concerning the two others methods, an extension for any angle of propagation can be obtained and it leads to a closed wave surface. The three methods are compared with experimental data and the extension given by the Cagniard technique best matches experimental measurements. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1373740

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

Several problems of two-dimensional propagation and scattering of ultrasonic waves are simulated and visualized by using a program based on the mass-spring lattice model. The problems are related to reflection, refraction, and diffraction of ultrasonic waves. It is found that all numerical results are in good qualitative agreement with the wave mechanics. Features incorporated into the updated program are explained. Though the present state is far from our ultimate goal to develop a complete simulator of ultrasonic testing, the developed software is useful for educational purposes even at the present stage of development. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1373741

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

Transmission of a longitudinal elastic wave through a periodic particulate composite with a tetragonal arrangement of heavy spherical inclusions in an elastic matrix was measured experimentally. It exhibits several band gaps and the associated jumps in the phase velocity spectrum near two sets of frequencies: frequencies at which enhancement of the rigid body (dipole) resonance of a particle through its shear interaction with the remaining particles within the layer; and frequencies determined by the inter-layer distance. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1373742

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The motivation of this work is to investigate the simulation of a beam which is bounded both spatially and temporally and to look at the response of the plate either near or far from the specular reflection. Using the model it is demonstrated that the dispersion curves built up from a two dimensional Fourier transform correspond either to the leaky Lamb waves dispersion curves (with real frequency and complex slowness) or to the transient Lamb waves dispersion curves (with complex frequency and real slowness) according to the location of the receiver. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1373743

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

The motivation of this work is to study the group velocity of Lamb waves in anisotropic plates in order to understand the characteristics of the propagation of wave packets in arbitrary directions. The group velocity equations have been derived, taking into account the spatial dispersion (variation of the velocity with the direction of propagation) as well as the frequency dispersion. For bulk waves, it is already well known that anisotropy implies a difference between group velocity and phase velocity. It is shown here that this phenomenon occurs for Lamb waves as well, affecting both the velocity and the direction of propagation. An experimental demonstration is also given, using a plate composed of a unidirectional Glass Epoxy composite. The obtained experimental curves are in good agreement with the theoretical calculations. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1373744

出版商:AIP    

年代:1901

数据来源: AIP

摘要:

Reflection of the fundamental modesa0ands0from the end of a plate is studied, in order to identify the extent to which the generation of non-propagating modes influences the field and could therefore affect Lamb wave NDE of the plate. Analytical predictions, Finite Element simulations and experimental measurements are presented for frequencies below thea1cut-off. It is shown that reflection of thea0mode is delayed by 90 degrees in phase and that there is significant additional motion due to non-propagating modes within about 5 plate thicknesses of the end. Reflection of thes0mode occurs without significant evidence of non-propagating modes or phase delay. ©2001 American Institute of Physics.

ISSN:0094-243X    

DOI:10.1063/1.1373745

出版商:AIP    

年代:1901

数据来源: AIP