• Proceedings of the Acoustical Society of Korea Conference
• /
• spring
• /
• pp.377-380
• /
• 2002

Acoustic Characteristics of Nitrile Butadiene Rubber with Carbon Black ContentAcoustic and mechanical properties of Nitrile Butadiene Rubbers (NBR) with the variation of the carbon black content were investigated. NBR where the acrylonitrile content is $33\%$ based on the mole percent has been prepared with fixed sulfur content for vulcanization. Acoustic measurement of the prepared rubbers were peformed in the frequency region of $300\;\~\;1000\;kHz$. Their mechanical properties such as density, hardness were also measured. Increase of the carbon black content in the rubber resulted in enhancement of the mechanical property and linear increase of the sound speed as function of the carbon black content. Interestingly, attenuation of the sound speed was only affected by the existence of the carbon black and not by the amount of carbon black in the experiment range of this article. In this study, it was found that the amount of carbon black content in the NBR was correlated with the acoustic properties and can be estimated nondestructively by the measurement of the specific acoustic property.

• 한국해양학회지
• /
• v.25 no.1
• /
• pp.26-35
• /
• 1990

Sound velocities and attenuation coefficients of marine surface sediments were calculated from insitu acoustic experiments on 4 nearshore areas off Pohang, Pusan Yeosu, and Kunsan around the Korean Peninsula. The relationship between these values and physical properties of sediments was examined and attenuation mechanism was analysed using the estimated gas content. Sound velocities and attenuation coefficients ranging from 1470 to 1616 m/sec and 0.0565 to 0.6604 dB/kHz-m, respectively, are well related to sediment types. The attenuation coefficient is maximum in coarse silts, and the sound velocity increases with density. The gas content estimated less than 8 ppm increases with the decreasing sediment grain size. When the sediment size is greater than fine sand, sound attenuation is mostly due to friction losses, and probably negligible viscous loss remains unchanged with the varying physical properties of sediments. The maximum attenuation in coarse silts result from both friction loss and cohesion of finer sediments between the contacts of silt grains. The cohesion begins to be the dominant dissipative process with decreasing grain size from medium and fine silts.

• Journal of Mechanical Science and Technology
• /
• v.19 no.7
• /
• pp.1414-1423
• /
• 2005

This paper develops and evaluates several strategies for structural vibration control with the objective of attenuating broadband noise inside a rectangular enclosure. The strategies evaluated include model-independent collocated control, model-based feedback control and a new 'modal-estimate' feedback strategy. Collocated control requires no knowledge of model parameters and enjoys the advantage of robustness. However, effective broadband noise attenuation with colocated control requires a large number of sensor-actuator pairs. Model-based con-trollers, on the other hand, can be theoretically effective even with the use of a single actuator. However, they suffer from a lack of robustness and are unsuitable from a practical point of view for broadband structural vibration applications where the dynamic models are of large order and poorly known. A new control strategy is developed based on attenuating a few structural vibration modes that have the best coupling with the enclosure acoustics. Broadband attenuation of these important modes can be achieved using a single actuator, a limited number of accelerometers and limited knowledge of a few modal functions. Simulation results are presented to demonstrate the effectiveness of the developed strategy.

• Journal of Korean Tunnelling and Underground Space Association
• /
• v.24 no.1
• /
• pp.39-55
• /
• 2022

This study carried out the laboratory tests for AE signal attenuation to determine the attenuation coefficient (α) of silo concrete in Gyeongju low and intermediate-level disposal environments. The concrete samples were prepared by satisfying the concrete mixing ratio used in the Gyeongju disposal silo, and these samples were additionally exposed depending on the temperature conditions and saturation and, dry condition. As a result of attenuation tests according to the transmission distance on three concrete specimens for each disposal condition, the AE amplitude and absolute energy measured on the saturated concrete were higher than that of the dry concrete in the initial range of the signal transmission distance, but the α of the saturated concrete was higher than that of the dry concrete. Regardless of the saturation and dry conditions, the α tended to decrease as the temperature increases. The α had a more major influence on the saturation and dry condition than the temperature condition, which means that the saturation and dry condition is the main consideration in measuring the signal attenuation of a concrete disposal structure. The α of concrete in the disposal environment expect to be used to predict the integrity of silos concrete in Gyeongju low and intermediate-level disposal environments by estimating the actual AE parameter values at the location of cracks and to determine the optimum location of sensors.

• Journal of Ocean Engineering and Technology
• /
• v.22 no.1
• /
• pp.53-57
• /
• 2008

Friction welding was performed to investigate mechanical properties for Ni-base superalloy with 15 mm diameter solid bar. The main friction welding parameters were selected to endure good quality welds on the basis of visual examination, tensile tests, impact energy test, Vickers hardness surveys of the bond of area and heat affected zone. And then, the nondestructive technique to evaluate the weld quality was carried out by acoustic emission(AE) and ultrasonic attenuation coefficient. The tensile strength of the friction welded joint was shown up to 90 % of the Alloy718 base metal under the condition of the heating time over 5 sec. The optimal welding conditions were n=2,000 rpm, $P_1=200$ MPa, $P_2=200$ MPa, $t_1=8$ sec and $t_2=5$ sec when the total upset length was 4.4 mm.

• Journal of Power System Engineering
• /
• v.20 no.6
• /
• pp.58-63
• /
• 2016

A stainless steel has high corrosion resistance because of nickel in material, so it is used as materials for transportation and storage of hydrogen. In this study, TIG(tungsten ingot gas) welding was carried out on the stainless steel using the storage vessel of hydrogen. The microscopic structures at each region of TIG welded material such as HAZ, weld and base metals using optical microscope were observed. And the damage behavior of stainless steel that underwent the hydrogen charging using nondestructive evaluation was also studied. Ultrasonic test, which is the most generalized nondestructive technique, was applied to evaluate the relationship between the ultrasonic wave and mechanical properties at each zone of TIG welded stainless steel. The velocity and attenuation coefficients of ultrasonic wave didn't show a remarkable difference at each region of welded stainless steel. However, the attenuation coefficient was the highest at the weld zone when hydrogen charged stainless steel. In addition, acoustic emission test was also used to study the dynamic behavior of stainless steel experienced both hydrogen charging and weld. Lots of AE event at elastic region of stress-strain curve were occurred both the hydrogen charged specimen and the free specimen.

• The Journal of the Acoustical Society of Korea
• /
• v.14 no.6
• /
• pp.40-47
• /
• 1995

A system for measurement of ultrasonic velocity, attenuation and complex modulus of materials by using the spectral analysis method of pulses has been constructed and its performances are estimated. The system has a mechanical scanning part of an acoustic microscope with a ZnO plane wave transducer of the resonant frequency of 85MHz. Ultrasonic velocity has been obtained by the intervals of maxima (or minima) on the power spectrum of a pulse train reflected from the surface and bottom of a specimen, and attenuation has been obtained by the power spectra of three pulses reflected from the surface and the bottom of a specimen and the surface of a standard specimen. The measured results for materials such as fused quartz, polyester show that the system has very high accuracy.

• Korean Journal of Fisheries and Aquatic Sciences
• /
• v.23 no.6
• /
• pp.451-456
• /
• 1990

Underwater acoustic navigation and position fixing systems have been extensively used not only in surface position fixing but also in underwater position fixing. Tn recently, application of these systems has been in the field of underwater inspection of offshore platforms, where it is vital to track the position of an unmanned submersible or diver carrying underwater cameras and nondestructive testing equipment. But these systems are included the fixing errors as results of a signal with additive noise, the attenuation of sound and the interference effects due to multipath reflection and forward scattering. In this paper to improve the position fixing by the supershort baseline acoustic position system, a method to apply the Kalman filter to the fix of the system is proposed and the digital simulation under noise condition is conducted. The optimal positions by the Kalman filter are compared with original positions, and it is confirmed that the results of the pro-posed method are evidently more accurate.

• 한국지구과학회:학술대회논문집
• /
• 2005.09a
• /
• pp.42-50
• /
• 2005

The electromagnetic (light) waves are limited to penetrate the media, ie, water and sea-bottom layers, due to high energy attenuation, but acoustic (sound) waves play as the good messenger to gather the underwater target information. Therefore the acoustic methods are applied to almost of ocean equipments and technology in terms of in-water and sub-bottom surveys. Generally the sound character is controlled by its frequency. In case that the sound source is low frequency, the penetration is high and the resolution is low. On the other hand, its character is reversed at the high frequency. The common character at the both of light and sound is the energy damping according to the travel distance increase.

• Smart Structures and Systems
• /
• v.13 no.1
• /
• pp.155-171
• /
• 2014

Various monitoring techniques are now available for structural health monitoring and Acoustic Emission (AE) is one of them. One of the major advantages of the AE technique is its capability to locate active cracks in structural members. AE crack locating approaches are affected by the signal attenuation and dispersion of elastic waves due to inhomogeneity and geometry of reinforced concrete (RC) members. In this paper, a novel technique is described based on signal processing and sensor arrangement to process multisensory AE data generated by the onset and propagation of cracks and is validated with experimental results from an in-situ load test. Considering the sources of uncertainty in the AE crack location process, a methodology is proposed to capture and locate events generated by cracks. In particular, the relationship between AE events and load is analyzed, and the feasibility of using the AE technique to evaluate the cracking behavior of two RC slab strips during loading to failure is studied.