Tissue engineering has been rapidly developed in oral and maxillofacial reconstruction. Biocompatible scaffold from chemically composites seeded with stem cells is essential and several growth factors for bone formation and angiogenesis are also required. To overcome limited activity of new bone formation with scaffolds, several biomechanical stimulation methods on cells have been made to grow cells in scaffold. Several bioreactors have been developed for real tissue growth in culture laboratory. In addition to biological stimulants like BMP, growth factors and exogenous drugs, biomechanical stimulation technique has also been known as an effective method in cell differentiation. We developed our own bioreactor with tensile mechanical strains. Then we tested with it for detection of suitable biomechanical effect on the cell differentiation and proliferation. And we also compared the results with the effect of low intensity pulsed ultrasound (LIPUS). Mechanical strain group showed more rapid reaction with cell differentiation and proliferation than non-mechanical strain group. Mechanical strain groups stimulated with 0.5∼0.7Hz for 6 hours and 8 hours showed more active cell differentiation than the group with 0.5∼0.7Hz for 2.5 hours tensile strain stimulation. Group of LIPUS also showed more rapid reaction in cell differentiation and proliferation. LIPUS with 3MHz showed more cell reaction than the LIPUS group with 1MHz. Our results showed the positive effect on differentiation and proliferation of cell with mechanical tensile strain, LIPUS both.

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.

The effect of strain aging on the tensile properties of API X70 linepipe steel was investigated in this study. The API X70 linepipe steel was fabricated by controlled rolling and accelerated cooling processes, and the microstructure was analyzed using optical and scanning electron microscopes and electron backscatter diffraction. Strain aging tests consisting of 1% pre-strain and thermal aging at 200 oC and 250 oC were conducted to simulate U-forming, O-forming, Expansion(UOE) pipe forming and anti-corrosion coating processes. The API X70 linepipe steel was composed of polygonal ferrite, acicular ferrite, granular bainite, and bainitic ferrite whose volume fraction was dependent on the chemical composition and process conditions. As the thermal aging temperature increased, the steel specimens showed more clearly discontinuous type yielding behavior in the tensile stress-strain curve due to the formation of a Cottrell atmosphere. After pre-strain and thermal aging, the yield and tensile strengths increased and the yield-to-tensile strength ratio decreased because yielding and aging behaviors significantly affected work hardening. On the other hand, uniform and total elongations decreased after pre-strain and thermal aging since dislocation gliding was restricted by increased dislocation density after a 1% pre-strain.

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.

The purpose of this study was to evaluate the tensile fracture energy absorption capacity of hybrid fiber reinforced cement composite by strain rate. Experiment result, it was confirmed that PVA suppressed the microcrack around the HSF at the strain rate 101/s, which resulted in the improvement of the pullout resistance of the HSF.

CFRP (Carbon Fiber Reinforced Plastic) has high tensile strength, light weight, and excellent corrosion resistance, so it is used for construction such as seismic reinforcement and explosion proof in construction area. Dynamic loads, such as earthquakes and explosions, cause rapid deformation of the material and the material behaves differently from its static condition. Therefore, in this study, tensile tests of CFRP were conducted under static and dynamic loads, and the tensile performance of was evaluated according to the strain rate.

최근 건설분야에서도 스마트(Smart)라는 개념을 도입한 재료 및 구조방식의 개발에 관한 관심이 고조되고 있다. 특히 기존 콘크리트와 같은 시멘트 복합체에 다양한 전도물질을 혼입하여 외력의 작용시 유발되는 변형을 시멘트 복합체의 저항변화로 평가하는 자기 감지형 건설재료 개발 연구가 활발히 진행되고 있다. 통상적으로 시멘트 복합체는 인장변형에 대한 저항능력이 낮기 때문에 주로 압축변형과 전기적인 저항특성의 상관성을 평가하는 연구 즉, 압축변형 감지능력을 갖는 시멘트 복합체 개발에 관한 연구가 대부분이었다. 본 연구에서는 직접인장하에서 0.5% 인장변형시까지 자기 감지능력을 갖는 변형 경화형 시멘트 복합체(SHCC)를 개발하고 또한 철근의 보강이 SHCC의 자기 감지능력에 끼치는 영향을 평가한다.

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 purpose of this study was to evaluate the direct tensile fracture behavior of steel fiber hybrid reinforced cement composite by strain rate. Experiment result, it was confirmed that SSF suppressed the microcrack around the HSF at the strain rate 101/s, which resulted in the improvement of the pullout resistance of the HSF.

In this research, Sensing Performance in Tensile Strain of Strain-Hardening Cement Composites by Containing of Carbon nanotube have been studied. The ultimate strength and strain were improved with increasing in amounts of CNT, and fractional change in resistivity were improved when same tensile strain. and %LE were decreased with increasing in amounts of CNT.

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.

The purpose of this study was to evaluate the direct tensile fracture behavior of fiber hybrid reinforced cement composite by strain rate. Experiment result, it was confirmed that PVA suppressed the microcrack around the steel fiber at the strain rate 101/s, which resulted in the improvement of the pullout resistance of the steel fiber.

In this study, the fiber blending ratio and strain rate effect on the tensile behavior of hybrid fiber reinforced cement composite was evaluated. Hooked steel fiber and polyvinyl alcohol fiber were used for reinforcing fiber. The fiber blending ratio of HSF+PVA were 1.5+0.5, 1.0+1.0 and 0.5+1.5vol.%. As a results, the tensile strength, strain capacity and fracture toughness of the hooked steel fiber reinforced cement composites were improved by the increase of the bond strength of the fiber and the matrix according to increase of strain rate. However, the tensile stress sharply decreased after the peak stress because of the decrease in the number of straightened pull-out fibers by micro cracks in the matrix around hooked steel fiber. On the other hand, PVA fiber showed cut-off fracture at strain rate 10-6/s with multiple cracks. However, at the strain rate 101/s, the multiple cracks and strain capacity were decreased because of the pull-out fracture of PVA fiber. The HSF1.5PVA0.5 shown the highest tensile strength because the PVA fiber suppressed the micro cracks in the matrix around the hooked steel fiber and improved the pull-out resistance of hooked steel fiber. Thus, DIF of strain capacity and fracture toughness of HSF1.5PVA were greatly improved. In addition, the synergistic response of fracture toughness was positive because the tensile stress was slowly decreased after the peak stress by improvement of the pull-out resistance of hooked steel fiber at strain rate 101/s

The purpose of this study was to evaluate the tensile fracture behavior of fiber hybrid reinforced cement composite according to the strain rate. Experiment result, it was confirmed that the number of cracks and the strain capacity tended to decrease with increasing PVA fiber volume fraction at the strain rate of 101/s.

본 연구에서는 유압식 급속재하 시험 장치를 제작하여 변형 속도에 따른 후크형 강섬유 및 폴리아미드 섬유보강 시멘트 복합체의 압축강도 및 인장강도 특성을 평가하였다. 그 결과, 변형 속도가 증가함에 따라 압축강도, 최대 응력 점에서의 변형 및 탄성계수는 증가하였으 며, 섬유 종류 및 혼입률은 변형 속도에 의한 압축강도의 영향은 크지 않았다. 본 연구에서 평가된 압축강도의 DIF는 CEB-FIP model code 2010 에 비해 상회하였으며, ACI-349의 예측값과 유사한 경향이 나타났다. 인장특성의 경우에도 변형 속도가 증가함에 따라 인장강도와 변형능력 이 크게 향상되었다. 후크형 강섬유보강 시멘트 복합체는 변형 속도가 증가함에 따라 섬유와 매트릭스의 부착력이 증가하는 것에 의해 인장강 도와 변형능력이 크게 향상되었으며, 섬유가 매트릭스로부터 인발되는 파괴 특성이 나타났다. 한편, 폴리아미드 섬유보강 시멘트 복합체의 경 우 섬유와 매트릭스의 부착력이 크기 때문에 섬유가 매트릭스로부터 인발되지 않고 끊어지는 파괴 특성이 나타났으며, 폴리아미드 섬유보강 시멘트 복합체의 인장특성에 대한 변형 속도 효과는 섬유의 인장강도에 큰 영향을 받는 것으로 판단되었다. 이러한 결과로부터 폴리아미드 섬 유보강 시멘트 복합체의 인장강도에 대한 변형 속도의 효과는 후크형 강섬유의 부착력에 대한 민감도 보다 큰 것으로 사료된다.

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).

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.