Four types of high Mn TWIP(Twinning Induced Plasticity) steels were fabricated by varying the Mn and Al content, and the tensile properties were measured at various strain rates and temperatures. An examination of the tensile properties at room temperature revealed an increase in strength with increasing strain rate because mobile dislocations interacted rapidly with the dislocations in localized regions, whereas elongation and the number of serrations decreased. The strength decreased with increasing temperature, whereas the elongation increased. A martensitic transformation occurred in the 18Mn, 22Mn and 18Mn1.6Al steels tested at −196 oC due to a decrease in the stacking fault energies with decreasing temperature. An examination of the tensile properties at −196 oC showed that the strength of the non-Al added high Mn TWIP steels was high, whereas the elongation was low because of the martensitic transformation and brittle fracture mode. Although a martensitic transformation did not occur in the 18Mn1.9Al steel, the strength increased with decreasing temperature because many twins formed in the early stages of the tensile test and interacted rapidly with the dislocations.

This paper is concerned with a test method that can be used to investigate the parameters of the Johnson-Cook constitutive model. These parameters are essential for accurately analyzing material behavior under impact loading conditions in numerical simulation. Ti-6Al-4V alloy (HCP crytal structure) was used as a specimen for the experiments. In the 10−3-103/ s strain rate range, three types of experimental methods (convention, compression and tension) were employed to compare the differences using MTS-810, SHPB and SHTB. Finite element analysis results when applying these parameters were displayed along with the experiment results.

In the present study, the tensile properties and dynamic strain aging of an Fe-24.5Mn-4Cr-0.45C alloy were investigated in terms of strain rate. During tensile testing at room temperature, all the stress-strain curves exhibited serrated plastic flows related to dynamic strain aging, regardless of the strain rate. Serration appeared right after yield stress at lower strain rates, while it was hardly observed at high strain rates. On the other hand, strain-rate sensitivity, indicating a general relationship between flow stress and strain rate at constant strain and temperature, changed from positive to negative as the strain increased. The negative strain-rate sensitivity can be explained by the Portevin Le Chatelier effect, which is associated with dynamic strain aging and is dependent on the strain rate because it is very likely that the dynamic strain aging phenomenon in high-manganese steels is involved in the interaction between moving dislocations and point-defect complexes.

본 연구는 실험계획법(예: 반응표면계획법) 및 하모니 검색 알고리즘을 통하여 다양한 아스팔트 콘크리트 포장 구조체에 있어 피로균열의 공용성 인자인 인장변형률을 예측하는 모델을 개발하는 방법에 대한 연구이다. 인장변형률을 산정하기 위하여 한국건설기술연구소에서 개발한 유한요소 축대칭해석 프로그램인 KICTPAVE를 이용하여 아스팔트 층과 린콘크리트 층의 접속면에서 발생되는 변형률을 구하여 데이터베이스(D/B)화 하였다. 아스팔트 포장에서 입력변수인 층별 탄성계수 및 두께를 다양한 조건에서 KICTPAVE 프로그램을 수행하여 훈련용 D/B(Training Set)인 변형률의 값들을 구축한 후 반응표면계획법에 근거하여 회귀방정식을 정의하였으며 방정식에 필요한 계수값을 결정하기 위하여 하모니 검색 알고리즘을 이용하였다. 최종적으로 결정된 회귀방정식의 계수값들의 정확성을 검증하기 위해서 훈련용 D/B가 아닌 다른 조건의 입력변수를 이용하여 검증용 D/B(Testing Set)를 구축하고 이를 이용하여 개발된 모델을 검증하였다.

본 논문에서는 3차원 유한요소법과 층탄성프로그램인 BISAR를 통해 얇은 아스팔트 콘크리트 표층의 피로균열 수명에 영향을 줄 수 있는 아스팔트 표층에서의 예측 인장변형률 결과를 광폭타이어와 바이어스 프라이 타이어를 이용하여 비교하였다. 본 논문에서는 11R22.5와 10×20 bias ply 타이어의 접지압력 분포도를 분석하였으며, 서로 다른 해석방법을 이용하여 아스팔트 표층 하단부와 상단부에서의 예측인장변형률을 비교하였다. 분석결과, 두 타이어의 접지압 분포는 유사했지만 11R22.5광폭타이어가 10×20 bias ply타이어와 비교해 상당히 큰 연직방향 접지압력을 보였다. 타이어 중앙에서 아스팔트 콘크리트 표층 하단부에서의 예측 인장변형률은 타이어 접지면적을 측정하여 충탄성프로그램인 BISAR에 적용한 BM해석법이 컸으며, 타이어 가장자리로부터 3.5cm 떨어진 곳에서의 상층부 예측 인장변형률은 3차원 접지압을 이용한 3차원유한요소법에 의한 해석이 가장 큰 값을 보였다.

본 논문에서는 축방향 인장 부재의 균열거동과 철근콘크리트 부재의 인장강화현상을 고려하기 위한 새로운 해석적 기법을 제시하였다 균열 후 거동 규명을 위하여 부착응력-슬립의 관계나 부탁 응력의 분포를 가정하는 기존의 해석방법과는 달리, 철근과 콘크리트의 변형률 분포 함수를 다항식으로 가정하여, 이를 바탕으로 일축 인장부재의 균열 해석 기법을 구성하였다. 제시한 균열 해석모델은 기존의 해석기법과 비교하여, 철근콘크리트 구조물의 유한요소해석을 위한 균열 후의 평균 응력-변형률 관계를 정의하거나, 부재의 길이방향으로 철근과 콘크리트가 분담하는 하중 및 슬립량 산정시 매우 효율적이다. 제안된 모델을 이용하여 얻어진 균열하중과 보강철근의 신장률 값을 다른 해석기법 및 실험값과 비교한 결과 만족할만한 정확도를 보여주었다.

취성재료인 탄소섬유보강폴리머(CFRP)의 시편시험에서 총변형량과 유효길이로서 유도되는 환산변형률을 도입하고, 환산변형률의 장점을 기술하였다. 일반적으로 재료의 인장물성을 결정하기 위해 스트레인 게이지 측정값을 사용하지만, 취성특성을 가지는 CFRP에서는 항상 유효한 것은 아니다. 그 이유는 취성재료에서는 응력재분배를 할 수 없으며, 스트레인 게이지의 측정값은 국부거동만을 나타기 때문이다. 따라서 환산변형률은 취성재료의 인장인장특성의 평균값을 측정하고 변형률과 측정값을 검증하는 보조지표로서 효과적으로 사용될 수 있다. 또한 환산변형률은 1) 제작 오차(편차) 와 세팅 오차(정렬 불량)에 의해 발생하는 초기 내부 변형률에 기인한 영향과 2) 불균일 변형분포로 인한 부분파단 이후 거동을 명확히 가시화하는 장점이 있다.

The flexural behavior of ECC according to the tensile strain capacity was analytically estimated with the layered section method. Moment-curvature curve was first obtained, then load-deflection relation was calculated. The analytical result indicated that there was noticeable increase in deflection at peak load with the higher tensile strain capacity within 3.0%, and it was found that more than 3% strain capacity did not provide higher deflection but helped improve the load resistance.

In this study, the uniaxial tensile tests were conducted to verify the difference of ductile behavior according to steel ratio of RC member. The tests were conducted on concrete compressive strength of 30MPa and SD400-D19 rebars. The steel ratio was classified with 0.59% and 2.87%. From the evaluation results based on limit state strain, both 0.59% steel ratio and 2.87% steel ratio were confirmed to exhibit more than 5% strain presented in korean highway bridge design code. But in case of 0.59% steel ratio, the decrease of limit state strain and failure strain was remarkably confirmed.

This research investigated the effects of matrix strength on the direct tensile behavior of high performance hybrid fiber reinforced cementitious composites (HPHFRCCs) at high strain rates. 3 different type matrixes were used (56 MPa, 81 MPa and 180 MPa). And macro fiber was long hooked fiber (H, =0.3 mm,=30 mm) and micro fiber was short smooth fiber (S, =0.2 mm,  =13 mm). The volume content of macro fibers was 1.0% and the volume content of micro fibers was 1.0%. The high matrix strength clearly increased the tensile strength and peak toughness of HPHFRCCs even at high strain rates (74 ~ 161 /sec).

In this study, longitudinal strain distribution of rebar in the 1-directional tension test are evaluated with an image processing method (IPM). From these results, it is estimated that the necking region can make the over estimation of elongation of rebar, therefore, the estimation except the necking region can be suggested to evaluate the true elongation of rebar.

This research investigated the effects of adding micro fibers on the direct tensile behavior of ultra-high-performance hybrid-fiber-reinforced concrete (UHPHFRC) at high strain rates. Macro fiber was long smooth fiber (LS, Df=0.3mm, Lf=30mm) and micro fiber was short smooth fiber (SS, Df=0.2mm, Lf=13mm). The volume content of macro fibers was 1.0% and the volume content of micro fibers varied between 0.0 and 1.0%. The addition of micro fibers clearly increased the tensile strength of UHPHFRCs even at high strain rates.

This study was intended to estimate the axial deformation of joint between pavement modules in the rapid-constructible modular pavement system, and to investigate the applicability of two-phase composites for a joint material, which was composed of cement paste, epoxy, or polyurethane as a matrix and sand as particles.