摘要:

Guided wave focusing has been developed as a promising technique for defect detection in pipeworks, where non‐axisymmetric flexural modes are tuned so that ultrasonic energy can be focused at a target point in a pipe. If a defect is located at the target point, large amplitude reflected waves can be observed. In this study, the focusing phenomenon is analyzed using a semi‐analytical finite element method where a region of a pipe is divided in the thickness direction into the cylindrical subdivisions and is analytically treated in the circumferential and longitudinal directions. Visualization of the calculation results reveals that focusing occurs gradually in the vicinity of the target point. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570144

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Three‐dimensional finite element (FE) wave propagation models have been used to examine the effect of defect geometry and frequency on the reflection coefficients of longitudinal guided waves in 3″ Schedule 40 steel pipe. An insight into the propagation characteristics of guided waves in rails is also presented. The dispersion curves for a rail are discussed and modelling results of the variation of reflection coefficient with defect depth is presented for a BS113A type rail. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570145

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

A laser‐ultrasonic system has been installed on a seamless tubing production line of The Timken Company and is being used to measure on‐line the wall thickness of tubes during processing. The seamless process consists essentially in forcing a mandrel through a hot cylindrical billet in rotation and typically results in fairly large wall thickness variations that should be minimized and controlled to respect specifications. The system includes a Q‐switched Nd‐YAG laser for generation of ultrasound by ablation, a long pulse very stable Nd‐YAG laser for detection coupled to a confocal Fabry‐Perot interferometer, a pyrometer to measure tube temperature and two laser Doppler velocimeters to measure the coordinates of the probing location at the tube surface. The laser, data acquisition and processing units are housed in a cabin off line and connected to a front coupling head located over the passing tube by optical fibers. The system has been integrated into the plant computer network and provides in real time thickness data to the plant operators. It allow much faster mill setups, has been used since its deployment for inspecting more than 100,000 tubes and has demonstrated very significant savings. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570146

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Two‐wave mixing based interferometry has been demonstrated to be a powerful technique for non‐contact, broadband and speckle insensitive measurements of the small surface displacements produced by ultrasonic waves propagating in an object. When the object is in rapid motion along the line‐of‐sight of the probing laser or when the laser beam is rapidly scanned on a wavy surface, the two‐wave mixing photorefractive interferometer loses sensitivity to the point it could become useless. To circumvent the Doppler frequency‐shift produced by this relative motion, we propose a dynamic compensation scheme. We report a particularly simple scheme to implement this concept by monitoring the low‐frequency output signal of a balanced two‐wave mixing demodulator whose output is proportional to the frequency difference between the pump and signal beams, and feeding this signal back to the acousto‐optic shifter. With this new concept, the two‐wave mixing interferometer can operate on objects in rapid motion while maintaining its sensitivity to low frequency ultrasound. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570147

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

In work reported at last year’s QNDE meeting, a first step towards the development of a model for the Scanning Laser Source (SLS) technique was presented, which provides an analytical formulation for the transient response of an isotropic, homogeneous, linearly elastic half‐space submitted to a pulsed laser line source operating in the thermoelastic regime. The formulation takes into account optical penetration into the material and thermal diffusion from the source, and is therefore a suitable representation not only for the far field, but also for the field near the laser source, where these effects become significant. In the present paper, we report the progress made in the numerical analysis by the Boundary Element Method of the interactions of the previously obtained laser generated field with surface‐breaking cracks. Some preliminary simulations of SLS experimental observations are presented. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570148

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

When an intensive pulsed laser or ion beam impinging a material surface gradually loses its energy and heating the material along the passing path of the beams, a pulsed ultrasonic wave can be excited due to a thermal source distributed along the depth direction. We deduce an expression and calculate the epicenter displacement of ultrasound. Investigation results show that: the waveform of displacement depends on the distribution depth, the type of thermal source, the properties of samples, and the ratio between the thickness of the absorption layer and the duration of excitation beam. The detail process, results and discussions are presented. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570149

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

This paper describes a non‐contactElectroMagneticAcousticTransducer (EMAT) that can be used to generate both Lamb and Rayleigh waves on metal samples. The generated waves are wideband and low frequency with a dominant frequency content centred on approximately 200kHz extending to around 500kHz. Detection of the waves is achieved using a linear coil detection EMAT. The transducers (generator & detector) have been used on both aluminium and steel, but operate more efficiently on aluminium due to its lower electrical resistance and density when compared to steel. Some considerations are described for the design of the generation EMAT including applications where the dynamic field from the coil alone is used to obtain the Lorentz interaction with the sample surface eddy current. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570150

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

An arc‐shaped line array slit is used for the laser generation of focused laser‐ultrasonic wave. The spatially expanded Nd:YAG pulse laser is illuminated through the arc‐shaped line array slit on the surface of a sample to generate the ultrasonic wave of the same pattern as the slit. Then the generated ultrasonic wave is focused at the focal point of which distance from the slit position is dependent on the curvature of slit arc. The relationship between the characteristics of the generated ultrasonic wave including the focusing performance and several design parameters such like as slit width and slit interval are investigated. By using the focused wave we can upgrade the inspection ability for the small size defect with the improvement of spatial resolution. Also this method can be combined with the scanning mechanism to get image just like we can get by the scanning acoustic microscope(SAM). © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570151

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

Symmetric and antisymmetric Lamb modes are excited in low‐frequency‐thickness regime by illuminating a thin plate with an array of Q‐switched Nd:YAG laser‐generated line sources. The propagation of laser‐generated Lamb waves is detected by measuring the out‐of‐plane displacements in a non‐contact manner using the fiber optic Sagnac interferometer. The laser generation and non‐contact detection of narrowband Lamb waves at the selected operating points with relatively small rate of change in group velocity is presented. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570152

出版商:AIP    

年代:1903

数据来源: AIP

摘要:

We present measurements of high frequency ultrasound generated by a femtosecond laser pulse. Focussed on thin aluminum foils (8 – 80 &mgr;m), the measured ultrasonic spectrum extends up to a detection limit of 800 MHz. The high frequency ultrasonic echoes allow measurements of foil thickness down to 8 &mgr;m, the minimum foil thickness used in this set of experiments. © 2003 American Institute of Physics

ISSN:0094-243X    

DOI:10.1063/1.1570153

出版商:AIP    

年代:1903

数据来源: AIP