Current Issue : July - September Volume : 2019 Issue Number : 3 Articles : 5 Articles
Nondestructive testing methods are used to inspect and test materials and components for discontinuities or differences in\nmechanical characteristics. Phased array signal processing techniques have been widely used in different applications, but less\nresearch has been conducted on contactless nondestructive testing with passive arrays. This paper presents an application of\nbeamforming techniques analysis using a passive synthetic microphone array to calculate the origin and intensity of sound\nwaves in the ultrasonic frequency range. Acoustic cameras operating in the audible frequency range are well known. In order to\nconduct measurements in higher frequencies, the arrangement of microphones in an array has to be taken into consideration.This\narrangement has a strong influence on the array properties, such as its beam pattern, its dynamics, and its susceptibility to spatial\naliasing. Based on simulations, optimized configurations with 16, 32, and 48 microphones and 20 cm diameter were implemented\nin real experiments to investigate the array resolution and localize ultrasonic sources at 75 kHz signal frequency.The results show\nthat development of an ultrasonic camera to localize ultrasonic sound sources is beneficial....
To further understand the fracture behavior of rock materials containing en echelon flaws\nand determine precursor information for the final collapse of damaged mineral assemblies under\ncompression, a series of uniaxial compression experiments using a loading system, an acoustic\nemission system and a video camera was conducted on sandstone specimens containing en echelon\nflaws. The mechanical properties, energy dissipation characteristics and acoustic emission\nparameters of compression failure of selected specimens were successively analyzed. The results\nshowed that crack initiation was accompanied by a stress drop, step-like characteristics on the\nenergy consumption curve and increased crackling noises, which were used as early warning\nsignals before the final collapse happened. In addition, we used the histogram statistics method\nand maximum likelihood method to analyze the distribution of acoustic emission energy and\ndetermined that the acoustic emission energy distributions of sandstone specimens containing en\nechelon flaws followed a power law. With the progress of the experiment, the optimum exponents\nchanged in different stages and gradually decreased as failure was approached, which could also\nbe used as an early warning signal before the final collapse happened. This paper may provide\nsome theoretical basis for monitoring and warning about the collapse and instability of engineering\nrock masses containing en echelon flaws....
Environmental noise is one of the problems modern societies face today. Traffic noise,\nespecially the noise produced from tire/pavement interaction, plays a main role in environmental\nnoise. Pavement rehabilitation with new bituminous mixtures is a good option for combatting\nnoise pollution in urban areas. This paper studies the functional performance of two bituminous\nmixtures of stone mastic asphalt (SMA), fabricated with the same polymer modified binder, but\nwith different maximum aggregate size (MAS) (SMA11 and SMA16). The acoustic absorption, the\ndynamic stiffness, the surface texture and the tire/pavement noise were assessed. The bituminous\nmixture type SMA16 has higher texture levels at nearly every depicted wavelength of the texture\nspectra. This characteristic may lead to its higher average tire/pavement sound level compared to\nthe mixture SMA11. The influence of each texture wavelength on the different frequency bands of the\ntire/pavement noise spectrum was studied, however, this relation is not a simple matter. This paper\nalso presents low-noise pavement labeling methodology (LNP labelingLA2IC). The mixtures SMA11\nand SMA16 are labeled at 50 and 80 km/h. An acoustic label is a valuable tool for construction\ncompanies and urban planners to use in order to define the best option against noise when pavement\nrehabilitation must be carried out....
The nonlinear spring model combined with dislocation dipole theory was applied to\ndescribe the acoustic nonlinearity during the fatigue process in metals. The spring stiffness changes\nwith fatigue degree. For the early stage, spring stiffness approaches infinity, and the heavier\nnonlinearity mainly results from the increase of dislocation density. Further fatigue leads to the\noccurrence of micro-cracks, during which spring stiffness begins to decrease. Abundant micro-crack\nsprouting accelerates the crackâ??s expansion, and spring stiffness drops quickly, which causes the\nobvious decline in the transmitted harmonic amplitudes. Solutions obtained from the nonlinear wave\nequation with dislocation terms were added into the spring model. Varying spring stiffness was\nchosen for simulating the fatigue process. Then, nonlinear harmonic variation during this process\nwas observed, which was classified into three stages: (I) the early dislocation fatigue stage; (II) the\nmicro-crack sprouting stage; (III) the crack expansion stage. Nonlinear acoustic measurements were\ncarried out on an aluminum alloy specimen during its fatigue process until cracks could be seen\nclearly. Harmonic variations in experiments can also be classified into the same three stages as the\nnumerical results, which provides a theoretical and experimental reference for fatigue evaluation in\nmetals using the nonlinear acoustic method....
Nitrogen dioxide (NO2) is a poisonous trace gas that requires monitoring in urban areas.\nAccurate measurement in sub-ppm concentrations represents a wide application field for suitable\neconomical sensors. We present a novel approach to measure NO2 with a photoacoustic sensor using\na T-shaped resonance cell. An inexpensive UV-LED with a peak wavelength of 405 nm as radiation\nsource as well as a commercial MEMS microphone for acoustic detection were used. In this work,\na cell has been developed that enables a â??non-contactâ? feedthrough of the divergent LED beam. Thus,\nunwanted background noise due to absorption on the inside walls is minimized. As part of the\ndevelopment, an acoustic simulation has been carried out to find the resonance frequencies and to\nvisualize the resulting standing wave patterns in various geometries. The pressure amplitude was\ncalculated for different shapes and sizes. A model iteratively optimized in this way forms the basis of\na construction that was built for gas measurement by rapid prototyping methods. The real resonance\nfrequencies were compared to the ones found in simulation. The limit of detection was determined in\na nitrogen dioxide measurement to be 200 ppb...........
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