• Journal of the Korean Wood Science and Technology
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• 제24권3호
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• pp.18-27
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• 1996

The feasibility of using stress-wave technique in the transverse direction for the assessment of early stages of decay was investigated using compression test specimens having different annual ring orientations subjected to decay by Tyromyces palustris for various time intervals. Decay detection, quantitative assessment of decay, and the prediction of residual strength of decayed wood with less than five percent weight loss can be feasible using stress-wave parameters (wave velocity, wave impedance, and stress-wave elasticity) and their percent reduction due to decay, measured by stress-wave technique in the transverse direction. The use of stress-wave technique in the transverse direction for the application of this technique to structural members in service is desirable, when considering the easiness of attachment of accelerometers of stress-wave measuring device on the surface of members and also accurate detection of localized decayed areas. In stress-wave technique in the transverse direction, stress-wave parameters measured were different according to the angles between wave propagation path and annual ring, due to the anisotropy of wood structure. Therefore, it is recommended to use percent reduction in stress-wave parameters instead of stress-wave parameters. This evaluation method using percent reduction in stress-wave parameters is ideal when it is impossible to observe annual ring orientation on the transverse surface of wood.

• Nuclear Engineering and Technology
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• 제54권12호
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• pp.4481-4490
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• 2022

In this study, a new rupture disk corrosion test (RDCT) method was developed for real-time detection of stress corrosion cracking (SCC) initiation of Alloy 600 in a primary water environment of pressurized water reactors. In the RDCT method, one side of a disk specimen was exposed to a simulated primary water at high temperature and pressure while the other side was maintained at ambient pressure, inducing a dome-shaped deformation and tensile stress on the specimen. When SCC occurs in the primary water environment, it leads to the specimen rupture or water leakage through the specimen, which can be detected in real-time using a pressure gauge. The tensile stress applied to the disk specimen was calculated using a finite element analysis. The tensile stress was calculated to increase as the specimen thickness decreased. The SCC initiation time of the specimen was evaluated by the RDCT method, from which result it was found that the crack initiation time decreased with the decrease of specimen thickness owing to the increase of applied stress. After the SCC initiation test, many cracks were observed on the specimen surface in an intergranular fracture mode, which is a typical characteristic of SCC in the primary water environment.

• 스마트미디어저널
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• 제12권11호
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• pp.1-9
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• 2023

스트레스는 감당하기 어려운 외부 또는 내부 요인으로부터 유발되는 것으로 현대 사회의 주요한 문제 중 하나이다. 높은 스트레스가 장기적으로 지속되면 만성적으로 발전할 수 있으며, 건강 및 생활 전반에 큰 악영향을 초래할 수 있다. 그러나 만성적인 스트레스를 겪는 사람들은 자신이 스트레스를 받고 있는지 알아차리기 어렵기 때문에 사전에 스트레스를 인지하고 관리하는 것이 중요하다. 웨어러블 기기로부터 측정된 생체 신호를 이용하여 스트레스를 탐지한다면, 스트레스를 효율적으로 관리할 수 있을 것이다. 그러나 생체 신호를 이용하는 데에는 두 가지 문제점이 있다. 첫째로 생체 신호에서 수작업 특징을 추출하는 것은 바이어스를 발생시킬 수 있으며, 두 번째는 실험 주체에 따라 분류 모델 성능의 변이가 클 수 있다는 것이다. 본 논문에서는 데이터의 핵심적인 특징을 표현할 수 있는 합성곱 오토인코더를 이용해 바이어스를 줄이고 앙상블 학습 중 하나인 소프트 보팅을 이용해 일반화 능력을 높여 성능의 변이를 줄이는 모델을 제안한다. 모델의 일반화 성능을 확인하기 위하여 LOSO 교차 검증 방법을 이용하여 성능을 평가한다. 본 논문에서 제안한 모델은 WESAD 데이터셋을 이용하여 높은 성능을 보여주었던 기존의 연구들보다 우수한 정확도를 보임을 확인하였다.

• 한국자기학회지
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• 제16권6호
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• pp.305-308
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• 2006

Magnetic flux leakage(MFL) 신호를 이용하여 비파괴 검사 신호를 획득하는데 주로 사용하는 감지기 또는 금속 덩어리를 의미하는 pig 이용 기술은 일반적으로 천연 가스 전송에 사용되는 강철 파이프라인 상의 대량 부식 흔적을 감지하는데 널리 이용 되어지고 있다. 이를 대체할 수 있는 비파괴 검사 기술의 양상에 대하여 소개 하자면, 지하에 매설된 가스 파이프라인의 바깥 면에 가로축을 따라 난, 부식에 의한 결함에 일반적으로 동반하는 '압력에 의한 부식 흠집(Stress Corrosion Crack : SCC)'을 탐지하는 기술이 있다. 본 논문은 가스 파이프라인 상의 가능성 있는 SCC 검출 장치로서 탐촉자의 원거리에 형성된 다상 회전 전자장 활용 기술에 대하여 논하고 있다. Pig 원형에 대한 묘사와 비파괴 신호 검사용 탐촉자에 대한 설명이 덧붙여져 있으며 결함 검지장치의 유한 요소 모델링과 연관한 부분에 관하여도 발전적으로 논하였다. 테스트 용 pig를 이용한 초기 실험 결과는 본 논문에서 보이고 있는 비파괴 검사 양식이 가스 파이프 라인의 가로축을 따라 발생한 결함에 매우 민감하게 반응하고 있음을 보이고 있다.

• KSII Transactions on Internet and Information Systems (TIIS)
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• 제8권7호
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• pp.2512-2531
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• 2014

We propose a new Distributed Denial of Service (DDoS) defense mechanism that protects http web servers from application-level DDoS attacks based on the two methodologies: whitelist-based admission control and busy period-based attack flow detection. The attack flow detection mechanism detects attach flows based on the symptom or stress at the server, since it is getting more difficult to identify bad flows only based on the incoming traffic patterns. The stress is measured by the time interval during which a given client makes the server busy, referred to as a client-induced server busy period (CSBP). We also need to protect the servers from a sudden surge of attack flows even before the malicious flows are identified by the attack flow detection mechanism. Thus, we use whitelist-based admission control mechanism additionally to control the load on the servers. We evaluate the performance of the proposed scheme via simulation and experiment. The simulation results show that our defense system can mitigate DDoS attacks effectively even under a large number of attack flows, on the order of thousands, and the experiment results show that our defense system deployed on a linux machine is sufficiently lightweight to handle packets arriving at a rate close to the link rate.

• Transactions on Electrical and Electronic Materials
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• 제13권5호
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• pp.215-220
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• 2012

Miniature ultrasonic and tactile sensors on Si substrate have been proposed, fabricated and characterized to detect objects for a dexterous robot. The ultrasonic sensor consists of piezoelectric PZT thin film on a Pt/Ti/$SiO_2$ and/or Si diaphragm fabricated using a micromachining technique; the ultrasonic sensor detects the piezoelectric voltage as an ultrasonic wave. The sensitivity has been enhanced by improving the device structure, and the resonant frequency in the array sensor has been equalized. Position detection has been carried out by using a sensor array with high sensitivity and uniform resonant frequency. The tactile sensor consists of four or three warped cantilevers which have NiCr or $Si:B^+$ piezoresistive layer for stress detection. Normal and shear stresses can be estimated by calculation using resistance changes of the piezoresitive layers on the cantilevers. Gripping state has been identified by using the tactile sensor which is installed on finger of a robot hand, and friction of objects has been measured by slipping the sensor.

• Smart Structures and Systems
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• 제29권4호
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• pp.589-598
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• 2022

Timber structures are susceptible to structural damages caused by variations in moisture content (MC), inducing severe durability deterioration and safety issues. Therefore, it is of great significance to detect MC levels in timber structures. Compared to current methods for timber MC detection, which are time-consuming and require bulky equipment deployment, Lead Zirconate Titanate (PZT)-enabled stress wave sensing combined with statistic machine learning classification proposed in this paper show the advantage of the portable device and ease of operation. First, stress wave signals from different MC cases are excited and received by PZT sensors through active sensing. Subsequently, two non-baseline features are extracted from these stress wave signals. Finally, these features are fed to a statistic machine learning classifier (i.e., naïve Bayesian classification) to achieve MC detection of timber structures. Numerical simulations validate the feasibility of PZT-enabled sensing to perceive MC variations. Tests referring to five MC cases are conducted to verify the effectiveness of the proposed method. Results present high accuracy for timber MC detection, showing a great potential to conduct rapid and long-term monitoring of the MC level of timber structures in future field applications.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2005년도 하계학술대회 논문집 Vol.6
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• pp.574-574
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• 2005

To found out the degradation characteristic of transformer insulation, insulation material was depisited into transformer oil and heated. Due to the thermal stress which added to insulation, the density of carbon dioxide which included in transformer oil was mesured by using the gas density detection equipment of gas sensor and air circulation method. As a result, it didn't match with the transformer supervision standard. But it was found that as thermal stress increased, the density of carbon dioxide propertionally increased.

• Smart Structures and Systems
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• 제10권1호
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• pp.47-65
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• 2012

The presence of crack-like defects in mechanical and structural elements produces failures during their service life that in some cases can be catastrophic. So, the early detection of the fatigue cracks is particularly important because they grow rapidly, with a propagation velocity that increases exponentially, and may lead to long out-of-service periods, heavy damages of machines and severe economic consequences. In this work, a non-destructive method for the detection and identification of elliptical cracks in shafts based on stress wave propagation is proposed. The propagation of a stress wave in a cracked shaft has been numerically analyzed and numerical results have been used to detect and identify the crack through the genetic algorithm optimization method. The results obtained in this work allow the development of an on-line method for damage detection and identification for cracked shaft-like components using an easy and portable dynamic testing device.

• 대한원격탐사학회:학술대회논문집
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• 대한원격탐사학회 2005년도 Proceedings of ISRS 2005
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• pp.253-256
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• 2005

The use of satellite remote sensing in maritime safety and security can aid in the detection of illegal fishing activities and provide more efficient use of limited aircraft or patrol craft resources. In the area of vessel traffic monitoring for commercial vessels, Vessel Traffic Service (VTS) which use the ground-based radar system have some difficulties in detecting moving ships due to the limited detection range. A virtual vessel traffic control system is introduced to contribute to prevent a marine accident such as collision and stranding from happening. Existing VTS has its limit. The virtual vessel traffic control system consists of both data acquisition by satellite remote sensing and a simulation of traffic environment stress based on the satellite data, remotely sensed data. And it could be used to provide timely and detailed information about the marine safety, including the location, speed and direction of ships, and help us operate vessels safely and efficiently. If environmental stress values are simulated for the ship information derived from satellite data, proper actions can be taken to prevent accidents. Since optical sensor has a high spatial resolution, JERS satellite data are used to track ships and extract their information. We present an algorithm of automatic identification of ship size and velocity. This paper lastly introduce the field testing results of ship detection by RADARSAT SAR imagery, and propose a new approach for a Vessel Monitoring System(VMS), including VTS, and SAR combination service.