Additive manufacturing by electron beam melting is an affordable process for fabricating near net shaped parts of titanium and its alloys. 3D additive-manufactured parts have various kinds of voids, lack of fusion, etc., and they may affect crack initiation and propagation. Post process is necessary to eliminate or minimize these defects. Hot isostatic pressing (HIP) is the main method, which is expensive. The objective of this paper is to achieve an optimum and simple post heat treatment process without the HIP process. Various post heat treatments are conducted for the 3Dprinted Ti-6Al-4V specimen below and above the beta transus temperature (996oC). The as-fabricated EBM Ti-6Al-4V alloy has an α‘-martensite structure and transforms into the α+β duplex phase during the post heat treatment. The fatigue strength of the as-fabricated specimen is 400 MPa. The post heat treatment at 1000oC/30 min/AC increases the fatigue strength to 420 MPa. By post heat treatment, the interior pore size and the pore volume fraction are reduced and this can increase the fatigue limit.

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 immediate effect of neck stabilization exercise and Swiss ball exercise on the recovery from the fatigue of neck muscles induced. The turtleneck posture was set artificially by using Smartphone with healthy adults. Repetitively measured ANOVA was executed to examine the changes in the muscle fatigue of sternocleidomastoid, upper trapezius, and splenius capitis among three-time intervals (at the time of general resting, work and after intervention) in the three intervention groups (neck stabilization exercise group, Swiss ball exercise group, and ordinary rest group). There were no significant differences in the changes of fatigue of sternocleidomastoid muscle among all three intervention groups at the time of general resting, work and after intervention (p>.05). Although there was no significant difference in the changes in the fatigue of upper trapezius and splenius capitis muscles between the intervention groups at the time of general resting and work (p>.05), there was the significant difference between the three intervention groups at the time of work and after intervention (p>.05). This study suggest that Swiss ball exercise is more effective in reducing the muscular fatigue of the neck and shoulder at a turtleneck posture than neck stabilization exercise.

The warm recycling technology has been increasingly used in many countries due to the environmental and financial benefits. In this study, the rheological and fatigue performance evolutions of warm-mix recycled asphalt materials during the secondary service period were evaluated in two scales, mixture and fine aggregate matrix (FAM). A laboratory simulation method was proposed to produce warm-mix recycled asphalt binders with various long-term aging levels for the mixture and FAM tests. The dynamic shear rheometer temperature and frequency sweep test and time sweep test were conducted to characterize the rheological and fatigue behavior of FAMs, respectively. The rheological and fatigue properties of asphalt mixtures were measured by the dynamic modulus test and semi-circular bending test, respectively. Effects of aging levels and recycling plans on different pavement performance were investigated. Performance correlations between the mixture and FAM were finally investigated by the statistical method. It is found that the secondary long-term aging causes the continuous increase in the stiffness and decrease in the viscoelasticity level in each material scale, indicating the improvement of the rutting resistance and the reduction of the fatigue resistance. The warm mix asphalt technology plays a positive role in the fatigue performance with a loss of the rutting resistance. Using the styrene butadiene rubber latex can improve different pavement performance within the whole time-temperature domain. Good performance correlations between the mixture and FAM are developed, indicating that the FAM may be the critical material scale for evaluating the rheological and fatigue performance of warm-mix recycled asphalt pavements.

Among the 8,700 in-service bridges in national highway, the steel bridges cover the 1600 sites and make up approximately the 20 percentage of the total amounts. Due to recent rapid increase both in traffic volume and in frequency of overloaded vehicles, the need for re-evaluating the fatigue life of the steel bridges is increasing. However, the existing fatigue life assessment method are too complicate and difficult to apply to field directly. To improve such deficiency of the existing fatigue life assessment procedures, this study reviews the fatigue life assessment methods in Eurocode and then proposes an easier and simpler fatigue life assessment procedure that could evaluate the remaining fatigue life of the steel bridges using traffic data collected from a high-speed weigh-in-motion system installed in the national highway network. The Proposed fatigue life evaluation method is as follows; 1. Calculation of fatigue resistance 𝛾𝑀f and fatigue strength Δ𝜎𝑐 used in design, 2. Calculation of partial safety factor 𝛾𝐹f for equivalent stress range, 3. Calculation of stress range Δ𝜎𝐸2 using influence line, 4. Calculation of damage equivalent factor λ, 5. Review of Fatigue state and evaluation of fatigue life. The Proposed method can omit the existing complicated and repetitive calculation. Therefore, proposed method can estimate the fatigue damage and the theoretical fatigue life simply by comparing with the existing method.

The recent increase in truck traffic volume and overloading frequency causes a growing need to re-evaluation of fatigue life of steel bridges. However, the traditional fatigue analysis method, that is cumulative damage method(CDM), has limitation to apply to a number of bridges because the required calculation steps are very cumbersome and complicated. This study investigates the feasibility and applicability of proposed simple fatigue evaluation method based on the equivalent damage method (EDM) in Eurocode to estimate the remaining fatigue life for a highway bridge. The Proposed fatigue life evaluation method is as follows; 1. Calculation of fatigue resistance 𝛾𝑀f and fatigue strength Δ𝜎𝑐 used in design, 2. Calculation of partial safety factor 𝛾𝐹f for equivalent stress range, 3. Calculation of stress range Δ𝜎𝐸2 using influence line, 4. Calculation of damage equivalent factor λ, 5. Review of Fatigue state and evaluation of fatigue life. The specifications of the target bridge are as follows; Location : Gyeongbu Expressway, Design Life : 100yr, Construction year : 2006, Total length : 341m, Type of superstructure : continuous steel box girder. The resulting remaining fatigue life estimated by both EDM and CDM have been compared. Remaining fatigue life of target bridge in 2016 was calculated 365yr(EDM using WIM data in 2014), 334yr(CDM using strain gauge data in 2012 and reflecting 2% annual traffic volume increase, Gil and Kang(2012)). As a result of comparison, The remaining fatigue life using proposed method(EDM) was evaluated about 10% higher than the CDM. This result, because traffic volume increase had not reflected when calculation of remaining fatigue life using EDM. Proposed method(EDM) can evaluate remaining fatigue life more easily and simply than CDM.

Conventional airport concrete pavement design uses empirical design method presented by FAA but it is not accurate because it does not consider environment load. In case of mechanistic - empirical design method used overseas, it needs to be modified according to domestic conditions. In this paper, a stress regression model considering environmental load and dual tandem gear load is developed as a mechanical - empirical design process and verified by fatigue model calibration. First of all, literature review was conducted on airports using DT gear as a design aircraft among domestic airports, and the physical properties of concrete pavement layers of each airport were identified. In addition, the environmental load is divided into the temperature load and the moisture load, and the temperature load is calculated from the climatic data of the region where the domestic airport is located, and the moisture load is calculated through the drying shrinkage prediction model developed through the previous study. The stresses occurring in concrete slabs under environmental loads and traffic loads were predicted using FEAFAA, which is a finite element analysis program, and factors predicted to affect concrete pavement were selected for the stress regression.As a result of the sensitivity analysis of the selected factors, the joint spacing, slab thickness, gear load, and road bearing capacity coefficient affected the stress behavior of the slab by more than 5% of the reference stress.According to the mechanical design, the finite element analysis was carried out for the consideration of only the traffic load and the environmental load and the traffic load based on the design factors. Based on the analysis results, multiple regression analysis was performed using the statistical analysis program SPSS and the stress regression equation was calculated. Then, the stress model for the environmental load and the traffic load was calculated and the final stress model with each stress model as the independent variable was derived. Using the calculated stress regression model, the stress was calculated and the bending strength of the concrete was divided to calculate the stress - to - strength ratio, and the appropriate fatigue model was selected and the correction was performed by the least squares method. This study was supported by Incheon International Airport Corporation(BEX00625)

Roller-compacted concrete (RCC) has been widely used for construction of pavements [1]. The strength of RCCP can be obtained from not only hydration of binder but also the aggregate interlock resulted from roller-compaction [2]. For this reason, RCCP normally achieves higher strength compared to conventional concrete pavement with similar cement content. Even though RCCP can be provided a good structural performance, it has been difficult to verify the long-term performance though actual field construction. Therefore, this study aimed to investigate the fatigue characteristics and crack development in RCCP based on full-scale fatigue test and accelerated pavement test. In case of full-scale fatigue tests, fatigue behavior was evaluated by using 1 m × 1 m dimensional RCC slab specimens obtained from the field in order to consider the field variability. Fatigue equation derived from this study shows that the number of load repetitions which causes fatigue failure at the same stress level is slightly larger than that of PCA fatigue equation. In order to evaluate the performance of RCCP, two phases of accelerated pavement test (APT) were conducted. In phase one, the performance of RCCP at two different strengths (35.6 and 30.4 MPa) was evaluated. In phase two, the performance of RCCP with different thickness (5, 7.5 and 10 cm) was investigated. The number of load repetition of fatigue crack occurrence in each section was compared to the estimated fatigue failure determined from fatigue equation of RCCP. The crack development in each section was compared to the AASHTO crack model for JPCP. Overall, it was confirmed that RCCP has equal or better performance compared to JPCP the estimation in term of fatigue cracking. The fatigue equation from PCA and cracking model from AAHTO can be used on RCCP at certain design thickness range.

본 논문에서는 페이즈필드 설계법 기반의 피로 제약 조건 구조물의 위상최적설계를 수행하였다. 페이즈필드 설계법의 도입으로 기존의 위상최적설계법에서 발생하기 쉬운 중간 영역의 크기를 크게 감소시켰다. 수정된 upper bound P-norm의 도입으로 모든 지점의 응력 성분을 고려하면서, 전역적 응력값이 국부적 최대 응력값과 근사한 값을 가질 수 있도록 설정하였다. 또한 기존의 피로 파괴 제약 조건 위상최적설계에서 다루지 않았던 응력 수정 계수에 대한 고려를 위하여 위상최적설계 결과물의 1차 주응력 성분을 고려하여 응력 수정 계수를 도입하고 이에 따라 허용 응력 진폭 값을 수정 하였다. 이를 통하여 인장 응력으로 인한 내구 한도 감소 요인을 반영한 체계적인 설계 방안을 제시하였다.

목적: 본 연구에서는 단초점 콘택트렌즈와 3가지 가입도의 중심부 근용 동시보기 디자인의 멀티포컬 콘 택트렌즈를 착용한 후 근거리에서 조절을 유도하여 콘택트렌즈 타입에 따른 시기능의 변화를 비교하고, 피 로감소용 기능성 콘택트렌즈로서 멀티포컬 콘택트렌즈의 처방에 대한 활용도를 알아보고자 하였다. 방법: 본 연구의 취지를 이해하고 목적과 방법에 동의한 20-30대 성인 남녀 20명(40안, 26.63±4.36세) 을 대상으로 하였다. 평균 구면 굴절력은 –3.38±2.57 D이며, 평균 원주 굴절력은 –0.46±0.31 D, 평균 등 가구면 굴절력은 -3.61±2.64 D였다. 대상자들은 총 4회 방문하였고 매 방문시 단초점 콘택트렌즈, 멀티포 컬 콘택트렌즈 LOW, MID, HIGH를 각각 착용하였고, 조절 부담을 유도하기 위해 40 cm에서 1시간 동안 휴 대폰(4.7 inches)으로 자막이 있는 영상을 시청하였다. 영상을 시청한 후 MEM과 원근 대비시력 100%와 10%, 선명시역, 조절근점, 폭주근점, 조절용이성, 입체시 검사를 실시하였고, 온라인 설문지를 통해 자각적 만족도를 평가하였다. 결과: 멀티포컬 콘택트렌즈의 가입도가 증가함에 따라 MEM은 증가하였고 조절근접은 감소하였다 (p<0.05). 원근거리 대비시력과 선명시역, 폭주근점, 입체시, 조절용이성은 4 종류의 렌즈에서 유의한 차이 가 없는 것으로 나타났다(p>0.05). 단초점 콘텍트렌즈를 착용했을 때 원거리 시력에 대한 만족도가 가장 높 은 것으로 조사되었다. 결론: 멀티포컬 콘택트렌즈의 가입도가 증가하면서 MEM은 증가하고 조절근점은 짧아졌음을 확인하였 다. 멀티포컬 콘택트렌즈를 기능성 콘택트렌즈로 처방하기 위해서는 더 오랜 기간 착용한 후 시기능의 변화 를 추적할 필요성이 있을 것으로 사료된다.

This study aimed to investigate the effect of smartphone size and use on fatigue of the extensor pollicis longus and abductor digiti minimi, and digiti minimi deformity. The 40 people assigned to the experimental group spent at least 6 hours using the smartphone and used the digiti minimi for support. The control group spent at least 3 hours using the smartphone, however did not use digiti mini for support. Questionnaires were used to measure smartphone usage, daily use duration, and method of holding the smartphone device. X-rays of distal interphalangeal joint angle deformity of the digiti minimi were assessed. Surface electromyography was used to analyze muscle fatigue. The angle deformity was significantly different between the experimental group and the control group, and fatigue of the extensor pollicis longus and abductor digiti minimi of the large smartphone users were high. The muscle fatigue was reduced when the smartphone size fit well with the user's hand. This study suggests that the size of the smartphone influence the fatigue of the hand muscles.

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).

목적: 본 연구는 대학생의 눈 피로도에 영향을 미치는 요인을 파악하고, 대학생의 눈피로도를 분석하여 눈 피로도 감소를 위한 기초자료를 제공하고자 한다. 방법: 대학교에 재학 중인 대학생 270명을 대상으로 눈 피로를 평가 설문을 하였고, 맥모니설문으로 건 성안증후군의 유무를 확인하여 한 통계방법으로 분석하였다. 결과: 눈 피로도에 영향을 주는 요인으로는 독서 시간(p=0.00), 독서 거리(p=0.05), 독서 자세(p=0.00), TV 시청 거리(p=0.01), 컴퓨터 사용 시간(p=0.01), 스마트폰 사용 시간(p=0.00)이 유의한 결과로 나타났으며 건성안증후군이 있는 사람과 눈 피로도와의 관계도 유의하게 나타났다(p=0.00). 결론: 독서 시간, 컴퓨터 및 스마트폰의 사용 시간, 독서 거리와 독서 자세가 눈 피로도에 영향을 미치고, 눈 피로도가 높을수록 건성안증후군 점수가 높게 나타났다. 이에 대학생들의 눈 피로도 감소를 위한 적정한 멀티미디어의 사용시간과 거리 및 독서 자세에 관한 후속연구가 필요할 것으로 사료된다.

목적: 본 연구는 건성안 증후군이 눈 피로도와 주의집중력에 미치는 영향을 알아보고자 한다. 방법: 시기능 이상이 없고 콘택트렌즈를 착용하지 않으며 양안 교정시력이 0.8 이상인 19 ~ 83세(평균 연령 40.27세)의 512명을 대상으로 하였다. 일반적인 특성(성별, 연령, 음주 및 흡연 여부), 건성안증후군, 눈 피로도 검사의 설문점수 및 주의집중력의 하위요인별로 검사하여 분석하였다. 결과: 대상자 중 여자가 312명(60.9%)으로 남자보다 많았고 평균 나이는 40.27세이었다. 음주를 하는 응답자가 260명(50.8%)으로 하지 않는 응답자보다 많았으며, 비흡연자가 438명(85.5%)으로 흡연자보다 많 았다. 건성안증후군은 나이, 성별, 흡연, 음주, 눈 피로도와 정적상관을 나타냈으며, 주의집중력의 하위요인 BET, NAR, OET, RED에서 유의한 차이를 나타내었다. 결론: 건성안증후군은 그렇지 않은 사람보다 주의집중력이 낮았고, 건성안증후군이 증가하면 눈 피로도 가 높아졌다.

본 연구는 4주간의 동계훈련기간 중 산수유 추출물을 섭취시켜 간 기능, 근 손상 및 혈중 피로물질에 미치는 영향을 구명하기 위하여 남중 축구선수 20명을 대상으로 동계훈련+산수유 추출물 섭취군(10 명), 동계훈련군(10명)으로 구분하여 실시하였다. 동계훈련은 1일 오전, 오후, 야간운동으로 구분하여 주 6 일, 4주간 실시되었으며, 운동강도는 70-95%HRR로 수행되었다. 산수유 추출물은 매일 아침과 저녁 식사 후 각각 80 mL(1일 160mL)를 4주간 섭취시켰으며, 측정된 자료의 그룹 내 변화는 대응표본 T검정, 그룹 간 차이는 변화율을 산출 한 뒤 독립표본 T검정을 이용하여 비교 분석하였다. 그 결과 혈중 피로물질의 젖산은 동계훈련+산수유 추출물 섭취군에서 유의하게 감소하였으며(p<.05), 동계훈련군 보다 동계훈련+산수유 추출물 섭취군이 유의하게 감소하였다(p<.05). 암모니아는 동계훈련+산수유 추출물 섭취군(p<.05), 동계훈련군(p<.001) 모두 유의하게 증가하였다. 근 손상지표의 CK는 동계훈련+산수유 추출물 섭취군이 유의하게 감소하였으며(p<.05), LDH는 동계훈련+산수유 추출물 섭취군(p<.001), 동계훈련군(p<.05) 모두 유의하게 감소하였다. 간 기능 효소 중 AST(p<.01), ALT(p<.05)는 동계훈련+산수유 추출물 섭취군이 유의하게 감소하였으며, AST는 동계훈련군 보다 동계훈련+산수유 추출물 섭취군이 유의하게 감소하였다. 동계 훈련 시 피로에 쉽게 노출 될 수 있는 선수들에게 있어 산수유 추출물 섭취는 항피로의 역할을 하는데 긍정적인 영향을 미치는 것으로 사료된다.