본 연구의 목적은 청소년들의 스마트폰 과의존과 피로회복 간의 관계에서 불안의 매개효과를 검증하여, 청소년의 스마트폰 과의존 중재의 근거자료를 확보하는 것이다. 연구 대상자는 ‘제16차 청소년 건강행태 온라인조사’를 이용한 청소년 54,948명이다. 연구변수 간의 관계는 상관분석, 요인분석, 매개효과 분석을 실시하였다. 연구 결과 청소년의 스마트폰 과의존은 피로회복에 영향을 미치는 직접효과와, 불안을 경유하여 피로회복에 영향을 주는 간접효과가 나타났다. 청소년의 스마트폰 과의존이 매개변인인 불안에 유의한 영향을 주었고, 피로회복에 유의한 영향을 주었다. 또한 또한 불안은 스마트폰 과의존과 피로회복 간의 관계에서 매개효과가 있는 것으로 확인되었다. 따라서 스마트폰 과의존 청소년을 대상으로 피로회복 을 향상하기 위해 불안을 관리할 필요가 있다.

The purpose of this study is to develop the analysis procedures for the evaluation of the structural integrity of the spent fuel in normal condition of transport at sea. Spent nuclear fuel must be transported from the wet storage facility in the nuclear power plant to the intermediate storage facility, and the structural integrity must be maintained in vibration and shock loads during the transportation. In general, the transport of spent nuclear fuel is performed in three kinds of modes: road, rail, and sea. During transport, the spent nuclear fuel is subjected to repeated vibration and shock loads by road surfaces, railroad tracks, and waves of the sea. It should be evaluated whether the structural integrity of the spent fuel is maintained under these load conditions. All nuclear power plants in Korea are located in coastal sites, and the interim storage facility for spent nuclear fuel is highly likely to be decided as a coastal site as well. Therefore, the main mode of the spent nuclear fuel transport is expected to be maritime transport by ships. In this study, the analysis procedure was developed to evaluate the safety of spent fuel at maritime transport by ships, and the procedure for evaluating the integrity of spent fuel under normal conditions of maritime transport were proposed. CFD analysis using SeaFEM was performed for the vibration analysis of the ship by waves, and the structural vibration analysis of the transport system was simulated using the developed in-house codes. The fatigue durability of the cladding was also evaluated using the developed fatigue analysis program and the fatigue analysis used the strain data obtained from the structural analysis. It was concluded that the value of the fatigue damage on the spent fuel cladding during normal conditions of maritime transportation is close to “0” and the structural integrity of the spent fuel is maintained in the same condition.

Diesel engine is used many industrial fields such as ship, power plant and big-sized vehicles and so on. Roto cap is one of the parts of system of intake and exhaust valve. Roto cap consists of body, disc spring, spring & steel ball, retainer and stop ring. Disc spring is known as taking cyclic load and cyclic load leads to fatigue damage. This study aims to investigate the stability of disc spring due to fatigue damage. As the results, the fatigue life of disc spring according to cylic load could be predicted using fatigue analysis. Consequently, disc spring showed the stability of about 1.7~2 times for criterion load of 1370N.

The main purpose of this study was to examine the correlation between the consumption of red ginseng-based ‘SSR’ for 30 days and the reduction in human fatigue, blood component changes, and immune cell activity in 35 human subjects. ‘SSR’ is composed of zinc oxide, folic acid, and D-α-tocopherol with red ginseng as the main component. According to the protocol criteria of the study, 35 subjects who understood the purpose of the study and signed an informed consent form were selected. The fatigue survey was conducted through a questionnaire, and after taking ‘SSR', a decreased tendency of physical, mental, and neurosensory fatigue was observed. In hematological analysis, no significant changes were observed in the levels of WBC, RBC, and hemoglobin; however, AST (SGOT) and ALT (SGPT) levels were statistically significantly decreased. In immunological analysis, it was observed that the proliferative effect of T cells (CD3+CD4+) was greater than that of NK cells (CD16+CD56+). The collected data were subjected to t-test analysis using the SPSS 25.0 statistical program. The result from this study proposes that ‘SSR’ can be used as a functional food material as it reduces human fatigue and enhances immune function.

In this study, the structural and fatigue analyses were carried out according to the shape of the self-made car frame. As a result of structural analysis, all models are shown to have the weak strength and large deformation, as the equivalent stress increases at the forward part of the impact force. It can be seen that model 3 is deformed less than other models 1 or 2. And model 3 with the truss structure prevents the great deformation from the collision. In case of irregular fatigue loads, the fatigue life of the ‘Sample history’ increased by about 59.3 times compared to the ‘SAE bracket history’ under extreme fatigue load conditions, indicating that the fatigue load condition of the ‘Sample history’ were stable. The fatigue life and deformation of model 3 among all models are significantly different to models 1 and 2. If the research results are applied to the design of self-made cars, it will be useful for improving the durability and preventing the damage. The results of this study can be effectively utilized to investigate the values of stresses and deformations, and fatigue lives without the experiments of fracture and fatigue according to the shape of the car frame.

The fatigue characteristics of glass fiber reinforced plastic (GFRP) composites were studied under repeated loads using the finite element method (FEM). To realize the material characteristics of GFRP composites, Digimat, a mean-field homogenization tool, was employed. Additionally, the micro-structures and material models of GFRP composites were defined with it to predict the fatigue behavior of composites more realistically. Specifically, the fatigue characteristics of polybutylene terephthalate with short fiber fractions of 30wt% were investigated with respect to fiber orientation, stress ratio, and thickness. The injection analysis was conducted using Moldflow software to obtain the information on fiber orientations. It was mapped over FEM concerned with fatigue specimens. LS-DYNA, a typical finite element commercial software, was used in the coupled analysis of Digimat to calculate the stress amplitude of composites. FEMFAT software consisting of various numerical material models was used to predict the fatigue life. The results of coupled analysis of linear and nonlinear material models of Digimat were analyzed to identify the fatigue characteristics of GFRP composites using FEMFAT. Neuber’s rule was applied to the linear material model to analyze the fatigue behavior in LCF regimen. Additionally, to evaluate the morphological and mechanical structure of GFRP composites, the coupled and fatigue analysis were conducted in terms of thickness.

Engine components subjected to cyclic thermal and mechanical loads may experience low-cycle or high-cycle fatigue failures. In particular, both of these failures can easily occur in aluminum cylinder heads, which are exposed to high temperatures and combustion pressures. Predicting the fatigue characteristics of the cylinder head are very important in the design stage of engine development. In this study, a finite element analysis was performed to predict the low-cycle thermal fatigue around exhaust ports of the cylinder head. Temperature distributions are obtained through the heat transfer analysis considering thermal cyclic test. The analysis result involves large plastic deformations, indicating compressive stresses at high temperatures and subsequently turn into tensile stresses at cold conditions. And the results showed that the critical regions such as exhaust port with large plastic strains coincided well with crack locations from thermal cyclic test. Next, design changes were made to the critical areas of the exhaust ports, and the results showed that the durability was improved by about 60% over the initial model and there were no problems in the thermal fatigue test.

Pipe for water supply is one of the important parts that supply water to home, factory and so on. Water leakage in pipe for water supply due to deterioration, ground sinking and earthquake leads to enormous economical loss. Therefore, pipe for water supply should be designed to satisfy the requirement of, for instance, structural stability and fatigue durability. The purpose of this study was to investigate the fatigue durability of flexible joint for relaxing the impact due to earthquake and ground sinking. For this purpose, flexible joint was simulated using dynamic characteristics and fatigue life. As the results, the problem of fatigue durability may occur when flexible joint and pipe for water supply are treated as rigid body in simulation. Thus it means that the role of packing in flexible joint is very important and packing should be designed as optimal conditions that are considered fatigue durability as well as waterproof.

철도 분기기는 결선부와 곡선부의 존재로 인하여 선로에서 가장 손상이 많이 발생하는 구간이며, 구조적 건전성을 유지관리하기 위하여 매년 상당한 비용이 투입되고 있다. 레일연마는 이러한 분기기의 레일 손상을 감소시켜 유지관리 비용을 저감할 수 있는 방안 중 하나로 알려져 있으나, 현재까지 국내에서는 분기기 레일 연마를 통한 유지관리 비용 절감 효과를 정 량적으로 연구한 사례가 거의 없는 상황이다. 이 논문에서는 분기기 레일 연마를 통한 유지관리 비용 절감 효과를 다물체 동적 해석프로그램인 VI-Rail을 이용하여 검토하였다. 레일 연마 효과는 레일 표면의 조도를 이용하여 모사하였으며, VI-Rail 프로 그램의 Flextrack 모델을 이용하여 차량-궤도 상호작용해석을 수행하였다. 수치해석은 60kg #8 분기기와 EMU 차량 모델을 이용 하여 수행하였으며, 레일 표면 조도는 실제 궤도에서 측정된 값을 기반으로 작성한 PSD 함수를 이용하여 모사하였다. 해석 결 과 레일 연마는 분기기 레일의 피로 수명을 6.5% 가량 증가시키는 것으로 나타났다.

Recently, automobile washing methods have been carried out using steam of high temperature and high pressure instead of water. Therefore, it is necessary to secure the structural stability of the steam tank. In this study, it is necessary to reduce the weight of the steam tank by reducing the thickness of the existing steam tank by about 25%. The safety of the product design was verified through simulation to ensure the robustness of the product by securing the structural stability and fatigue analysis at high temperature and pressure of the steam tank according to the weight reduction. For newly developed products compared to existing models.

There are many studies to extend the distance traveled by electric vehicles. However, much research has been done to increase the capacity of the battery. In this study, some engines for power generation, in which a battery is charged with energy by mounting a small internal combustion engine in which an engine is mounted in an electric vehicle and the battery is charged with energy, are being studied. Therefore, since such an engine is operated at a high load, the camshaft and the camshaft are emphasized to have high load strength and durability to withstand fatigue.

In this study, the fatigue and vibration analysis were performed by using Solidworks program to investigate the damage percentage, life cycle and vibration mode depending on the types and positions of load applied to the table (Cases 1, 2, 3, 4). The farther the point of action of the load was, the more the fatigue damage and stability of the table were greatly reduced. The life cycles of Case 1 and 4 were over 100,000 cycles and the fatigue damage was less than 70%. From the vibration analysis, five modes and natural frequencies of Case 1 were confirmed. As the natural frequency increases, the shape of the corresponding mode is predicted not to be deformed.

Fatigue crack growth retardation of 304 L stainless steel is studied using a neutron diffraction method. Three orthogonal strain components(crack growth, crack opening, and through-thickness direction) are measured in the vicinity of the crack tip along the crack propagation direction. The residual strain profiles (1) at the mid-thickness and (2) at the 1.5 mm away from the mid-thickness of the compact tension(CT) specimen are compared. Residual lattice strains at the 1.5 mm location are slightly higher than at the mid-thickness. The CT specimen is deformed in situ under applied loads, thereby providing evolution of the internal stress fields around the crack tip. A tensile overload results in an increased magnitude of the compressive residual stress field. In the crack growth retardation, it is found that the stresses are dispersed in the crack-wake region, where the highest compressive residual stresses are measured. Our neutron diffraction mapping results reveal that the dominant mechanism is by interrupting the transfer of stress concentration at the crack tip.

The purpose of this study was to examine the relationship of fatigue and stress in seafarers. Ninety healthy adults were recruited and cumulative fatigue, stress index and stress resistance of seafarers were measured using autonomic nervous system analysis device (SA-6000; Medicoa Co. Ltd. Korea). The stress index had a significant correlation with the degree of resistance (p<.01). Cumulative fatigue and the stress index had a strong positive correlation (r=.781) with the stress resistance, cumulative fatigue had a strong negative correlation (r=.782) (p<0.01). Furthermore, the stress index had a strong negative correlation with the degree of resistance (r=.924) (p<.01). The present study indicate that there are significant correlation between the cumulative fatigue and stress index, cumulative fatigue and stress resistance, as well as stress index and stress resistance in the seafarers (p<.01).

Stress and fatigue are general physical aspects of our daily lives. It has been shown that physical therapists have different levels of job stress and fatigue according to the type A/B behavior patterns. This study collected data from 212 physical therapists between October 28 and November 23, 2016 using an anonymous, self-administered questionnaire. The study results showed the proportion of physical therapists with the Type A behavior patterns(TABP) was 18% greater than that of physical therapists with the Type B behavior patterns(TBBP). In this study, physical therapists with TABP were compared with physical therapists with TBBP. The results indicated that physical therapists with TABP were more inclined to experience higher levels of overall job stress and fatigue from the following stress factors: physical environment, job requirement, and job autonomy. Therefore, the stronger the tendency toward TABP, the stronger the feeling of job stress and fatigue from physical environment, job requirement, and job autonomy. Those with a tendency toward TBBP showed positive correlations between job requirement and the total job stress score; thus, the stronger the tendency toward TBBP, the stronger the feeling of overall job stress and fatigue from job requirement. This study suggests that it is necessary to manage the job stress and fatigue of physical therapists with both TABP and TBBP and to manage the job stress and fatigue of physical therapists with the type A behavioral pattern.

In the present study, the structural and fatigue analysis on the shape change of an automatic press are investigated for prediction of operation safety and reliability of the automatic press along the thickness(t) and length(L) of head, and corner shapes(case 1, 2, 3). The equivalent stress and deformation characteristics of the automatic press were studied by computerized analysis method for the bushing production of the seat frame. An external stress of 14.0 MPa was applied to predict the operation stability and the fatigue limit of the structure. As the thickness of the header increased and the length of the header decreased, the load stability applied by the piston improved and the maximum stress and deformation were reduced. In addition, due to the change in shape of the corners, the load applied at the cross-sectional area of the corners decreases, and then the maximum stress and deformation appearing in the header are reduced. That is, the change of corner shapes affects the equivalent stress and deformation. That is the change of corner shapes affects the equivalent stress and deformation. From the fatigue and vibration analysis, fatigue failure does not occur even when the number of alternating operation of the automatic press increases, and the natural frequency is predicted for dynamic characteristics.

A fatigue analysis considering dynamic effects yields a more accurate fatigue life prediction than static fatigue analysis because it considers effects of inertia, flexibility and resonance that occur in the structure up to its natural frequency. However, the dynamic fatigue analysis of bogie frames of the rolling stock is not yet taken into account in the norm EN 13749. Therefore, in order to assure the safety of the rolling stocks, it is important to examine the fatigue analysis of that considering dynamic effects under applied load histories. Moreover, since the bogie frame consists of various welded joints, it is necessary to evaluate fatigue life of that considering welding properties as well as dynamic effects. In this study, under load histories converted from measured acceleration histories, static and dynamic fatigue analysis of the welded bogie frame are performed respectively.