جستجوی مقالات مرتبط با کلیدواژه « انتشار صوت » در نشریات گروه « فنی و مهندسی »

Marine phenomena that disturb the stratification cause oscillations in the acoustic signals. Among them, Eddies can be effective factors in mixing the ocean and create significant disturbances in the sound field. In this paper, the different parameters of Eddy and the effects of Eddy on sound propagation were analyzed in a hypothetical environment and conditions, and the amount and way of loss of sound signals in the presence of Eddy were also determined.The analyses showed that when the sound source is over the Eddy and near the surface, the sound propagation is limited to the surface sound channel. But when the sound source is in the mid-depth or under the Eddy, the sound propagation expands and dark spots decrease. in this condition  (when the sound source is in the middle depth) with the increase in speed and radius of the Eddy, the amount of loss increases too.

Carbon/epoxy composites have been used frequently in several structures due to their incredible ratio of strengths to weights. In this regard, investigation and classification of damages in composite structures are essential to prevent any probable happenings and to enhance the reliability. Such failures can be generally categorized into 4 groups, including matrix cracking, fiber breakage, debonding of fibers from the matrix, and the delamination. One of the new methods for detection of defects in composites is to utilize the acoustic emission approach. Accordingly, the aim of this article is to investigate and classify the different types of failure mechanisms in open-hole laminate composite specimens under tensile loading using acoustic emission. First, an open-hole specimen was examined under tensile loading based on ASME standards. Then, elastic waves due to failures in the specimen were recorded by wide-band acoustic emission sensors. Two methods have been utilized to detect the failure percent, including Pocket wavelet transform and Fuzzy clustering approaches. Results from these methods were compared to micro-structure images by the scanning electron microscopy. Results showed that about 50% of damages corresponds to debonding, about 30% comes from matrix cracking, and 20% is relevant to fiber breakage. In addition, the difference between two considered methods was 7%. Obtained results in this research indicated the appropriate efficiency of the acoustic emission approach to detect the type of failures and their percent in laminate composites.

High ratio of strength to weight in carbon/epoxy composites causes to their applications in several structures, especially aerospace structures. In addition, to enhance the reliability in such structures, investigating damages in composites is essential. One way to detect defects in composites is to utilize the acoustic emission approach. Thus, the objective of the present research is to find failure mechanisms in open-hole laminate composite specimens with 〖[0_3/〖90〗_2/0_2]〗_s layup under cyclic loadings at different displacement amplitudes, using the acoustic emission. First, the standard specimen was examined and elastic waves due to failures in the specimen were detected by acoustic emission wide-band sensors. Two methods have been utilized to detect the failure percent, including Pocket wavelet transform and Fuzzy clustering approaches. Results from these methods were compared to micro-structure images by the scanning electron microscopy. Obtained results in this research indicated the appropriate efficiency of the acoustic emission approach to detect the type of failures and their percent in laminate composites.

Using aluminum alloys is common in the engine industry due to low weight, high heat transfer and proper ductility. Detecting damages in aluminum structures is also important for higher reliability. One way to detect defects is the acoustic emission (AE) approach. Therefore, the objective of this article is to detect damages in open-hole aluminum alloy under tensile loadings. Firstly, the specimen was tested and elastic waves according to cracks from the hole were measured by AE sensors. As a result, nominal and pure stress concentration factors and the maximum stress around the hole were calculated by two relations and compared. For the fracture frequency in the specimen, the fast Fourier transform was utilized and the AE maximum voltage of the failure was obtained. Then also, the cumulative AE energy was compared to the crack length and a non-linear relation was fitted to experimental data.

The main characteristic of sound propagation in shallow waters is that graph of sound speed is almost constant or it has a down ward fracture. Therefore, the sound propagation is shallow waters areas is that each beam emitted from the sound source during the propagation time is constantly reflected from the seabed reflectivity of the seabed is more important than the reflectivity of the surface of the sea. In this paper, the characteristics of geographical environment in the Persian Gulf and hormuz strait and the way of sound propagation in these areas has been explained. The effect of heat, cold, salinity and water flow on the speed of sound will also be evaluated. The Persian Gulf and its coastal areas with depth less than 100 meters is a type of shallow waters environments.

Sound travels to long distances in the ocean by various forms of ductedpropagation. When sound travels in a duct، it is prevented from spreading indepth and remains confined between the boundaries of the duct. The surfaceduct is a zone bounded above by the sea surface and below by the SLD. Within the surface duct، sound rays are alternatively refracted and reflected. A surface duct exists when the negative temperature gradient within it doesnot exceed a value determined by the effect of pressure on sound speed. Specifically، the surface duct is characterized by a positivesound-speed gradient. At very low frequencies، sound ceases to be trapped in the surface duct. So calculating themaximum wavelength for duct transmission is very important. In this paper we used a profile as a pattern of sound velocity distribution to calculate related parameters. Finally an optimum frequency for minimum scattering of sound channel propagation presented.