• Structural Engineering and Mechanics
• /
• 제37권1호
• /
• pp.1-15
• /
• 2011

The response of concrete to transient dynamic loading has received extensive attention for both civil and military applications. Accordingly, thoroughly understanding the response and failure modes of concrete subjected to impact or explosive loading is vital to the protection provided by fortifications. Reactive powder concrete (RPC), as developed by Richard and Cheyrezy (1995) in recent years, is a unique mixture that is cured such that it has an ultra-high compressive strength. In this work, the concrete cylinders with different steel fiber volume fractions were subjected to repeated impact loading by a split Hopkinson Pressure Bar (SHPB) device. Experimental results indicate that the ability of repeated impact resistance of ultra-high-strength concrete was markedly superior to that of other specimens. Additionally, the rate of damage was decelerated and the energy absorption of ultra-high-strength concrete improved as the steel fiber volume fraction increased.

• Computers and Concrete
• /
• 제30권1호
• /
• pp.19-32
• /
• 2022

This paper presents an investigation into the failure of RC columns under impact loadings. A numerical simulation of 19 identical RC columns subjected to single and repeated impact loadings was performed. A free-falling hammer was dropped at midspan with the same total kinetic energy input but varying mass and momentum. The specimens under the repeated impact test were struck two times at the same location. The colliding index, defined as the impact energy-momentum ratio, was proposed to explain the different impact responses under equal-energy impacts. The increase of colliding index from low to high indicates the transition of the impact response from static to dynamic and failure mode from flexure to shear. This phenomenon was more evident when the column had a greater axial load and was impacted with a high colliding index. The existence of the axial load had an inhibitory effect on the crack development and increased the shear resistance. The second impact changes the failure mode from flexural to brittle shear as found in the specimen with 20% axial load subjected to high a colliding index. Moreover, a deflection prediction equation based on the impact energy and force was limited to the low colliding index impact.

• International Journal of Naval Architecture and Ocean Engineering
• /
• 제10권6호
• /
• pp.670-682
• /
• 2018

This paper presents the experimental and numerical investigation results of the response of low-temperature steel (LT-FH32 grade steel) beams under repeated impacts at room temperature and a single impact at a sub-zero temperature. After conducting tensile tests at room and sub-zero, repeated impact tests were conducted on two clamped single-beam models at room temperature, and single-impact tests of two other clamped single-beam models were conducted at $-50^{\circ}C$. The single and repeated impact tests were conducted by releasing a knife-edge striker using a drop testing machine. The permanent deflection of the model measured after each impact gradually increased with increasing number of impacts. Under the reduced temperature, the permanent deflection of the models slightly decreased. The numerical analyses were also performed to predict the damage response of the tested single-beam models. A comparison of the numerical prediction with those of experiments showed quite reasonable agreement.

• 한국산학기술학회논문지
• /
• 제21권5호
• /
• pp.648-655
• /
• 2020

현대의 자동소총의 공이는 공이치기에 의해 타격을 받아 장전된 탄약의 뇌관을 기폭 시키는 역할을 한다. 이 과정에서 공이는 충격하중을 받게 되며 소총의 수명주기 동안 반복적인 힘을 받게 된다. 소총의 내구도 시험에서 전체의 96.26% 진행 중 공이가 조기에 파손되는 현상이 발생하였다. 이에 따라 원인분석과 재현시험을 통해 파손현상 사례연구를 실시하였다. 파손이 발생한 공이의 파단면을 현미경 및 SEM 분석결과 반복충격에 의해 표면 원주방향 전체에서 균열이 시작해 심부로 피로균열이 발생했다. 반복충격에 의해 균열이 성장하다 마지막에 피로파괴가 발생하였으며, 노치에 의한 것으로 추정되었다. 검증을 위해 원주방향 0.03mm의 노치를 생성한 공이로 재현시험결과 동일한 형태의 파단면을 가지면서 전체 수명의 64.25%에서 파손되었다. 파손사례연구를 위한 노치형태별 재현시험결과 한쪽 측면 노치 0.3mm, 0.5mm의 공이는 각각 65.53%, 50.76%에서, 6개 지점의 노치 0.03mm는 85.65%에서 파손되었다. 마지막으로 표면 거칠기가 거칠고 툴 마크가 육안으로 확인이 가능한 공이는 내구수명을 만족하며 381㎛의 내부균열이 진행되었다. 본 연구를 통해 노치형태별 파손에 대해 고찰하였으며, 반복충격을 받는 부품의 신뢰성 확보를 위해 노치와 표면 거칠기 품질관리가 중요한 것을 알 수 있다.

• 한국정밀공학회:학술대회논문집
• /
• 한국정밀공학회 2005년도 추계학술대회 논문집
• /
• pp.1025-1028
• /
• 2005

Generally, fiber-reinforced composites have the highest possibilities of impact damages with external object collisions. Also, resulting in fatigue fracture considering the continued impact load. For the reasons mentioned above, the accurate understanding of interactions between the impact of composites and the fatigue load will be essential to understand the safety level of material structures. Furthermore, the composite materials and structures, due to the geometrical effect, vary the life in connection with the impact-fatigue. Therefore, I have reached the point that a focus of this study will be to evaluate fatigue fracture characteristics by the impacts-fatigue load of fiber-reinforced composites. Thus, in this paper, I have tried to work on impacts-fatigue load causing aspects and impact characteristics through impact-fatigue test on HTV-5Hl Black 9250 material made- structure, along with to evaluate the expected lift of real structures, the FEM analysis was carried out.

• 한국해양공학회지
• /
• 제33권1호
• /
• pp.85-91
• /
• 2019

In a cryogenic storage structure, the insulation system is in an environment in which fluid impact loads occur throughout the lifetime of the structure. In this study, we investigated the effect of repetitive impact loading on the mechanical performance of glass fiber-reinforced polyurethane foam. The repeated impact loading test was conducted in accordance with the required impact energy and the required number of repetitive impacts. The impact behavior of glass fiber-reinforced polyurethane foam was analyzed in terms of stress and displacement. After the impact test, the specimen was subjected to a compression test to evaluate its mechanical performance. We analyzed the critical impact energy that affected mechanical performance. For the impact conditions that were tested, the compressive strength and elastic modulus of the polyurethane foam can be degraded significantly.

• 한국산업융합학회 논문집
• /
• 제23권4_2호
• /
• pp.611-616
• /
• 2020

Polycarbonate (PC) materials having electrical insulation properties, are thermoplastic material and are easily processed, have excellent strength and heat resistance characteristics, and also have transparent and hard characteristics. In this study, we tried to derive the ε-N curve of strain-life, which shows the relationship between the strain characteristics and the life of the material when repeated impact loads are applied to the PC plastic material. As the impact load increased to 3.0kg, 4.0kg, 5.0kg, and 6.0kg, the strain also increased linearly to 0.033, 0.041, 0.046, and 0.055. At 3.0kg of mass impact, the test piece broke with 12000 impact cycles, 8400 times at 3.5kg, 7400 times at 4.0kg, 6600 times at 4.5kg, 4700 times at 5.0kg, 3000 times at 5.5kg, and 1000 times at 6.0kg. The number of fractures exponentially decreased as the load gradually increased. Using these results, an ε-N curve for PC plastic was derived.

• 한국의류학회지
• /
• 제34권12호
• /
• pp.2062-2072
• /
• 2010

This research evaluates the change of the water absorbency and drying speed of a quick-drying knit fabric by repeated laundering and laundering conditions and investigates the influence of laundering conditions on the functional properties of the knit fabric. Four factors of laundering conditions were studied: detergent, water hardness, water temperature, and frequency of rotation. Knit fabrics were washed for 25 laundering cycles in a drum-type washing machine with nine different laundering conditions derived from an orthogonal array. The properties of knit fabrics were measured with a drop absorption test, a strip test, and a drying time test. Relaxation shrinkage pointed to a change in the structural characteristics of the knit fabric. Wetting time was faster and wickability was greater in the knit fabrics that underwent 5 laundering cycles; in addition, there were no obvious changes in wetting time and wickability. The detergent was the most important factor in wetting time (40.4%) and wickability (60% or above). Water hardness, water temperature and RPM had less of an effect on wetting time and wickability. There were no significant differences between the levels of laundering conditions (except for detergent) on wetting time and wickability. Drying times with neutral and alkali were slower by repeated laundering; however, there was no obvious change in drying time. Hardness, water temperature and RPM had less of an impact on drying time.

• Structural Engineering and Mechanics
• /
• 제47권6호
• /
• pp.773-790
• /
• 2013

Bending behaviors of corroded reinforced concrete (RC) beams under repeated loading were investigated experimentally. A total of twenty test specimens, including four non-corrosion and sixteen corrosion reinforced concrete beams, were prepared and tested. A numerical model for flexural and cracking behaviors of the beam under repeated loading was also developed. Effects of steel corrosion on reinforced concrete beams regarding cracking, mid-span deflection, stiffness and bearing capacity of corroded beams were studied. The impact of corrosion on bond strength as the key factor was investigated to develop the computational model of flexural capacity. It was shown from the experimental results that the bond strength between reinforcement and concrete had increased for specimen of low corrosion levels, while this effect was changed when the corrosion level was higher. It was indicated that the bearing capacity of corrosion beam increased even at a corrosion level of about 5%.

• Journal of Microbiology and Biotechnology
• /
• 제22권3호
• /
• pp.422-430
• /
• 2012

Previous studies have demonstrated that repeated restraint stress in rodents produces increases in depression and anxiety-like behaviors and alters the expression of corticotrophin-releasing factor (CRF) in the hypothalamus. The current study focused on the impact of Bupleurum falcatum (BF) extract administration on repeated restraint stress-induced behavioral responses using the forced swimming test (FST) and elevated plus maze (EPM) test. Immunohistochemical examinations of tyrosine hydroxylase (TH) expression in rat brain were also conducted. Male rats received daily doses of 20, 50, or 100 mg/kg (i.p.) BF extract for 15 days, 30 min prior to restraint stress (4 h/day). Hypothalamic-pituitary-adrenal axis activation in response to repeated restraint stress was confirmed base on serum corticosterone levels and CRF expression in the hypothalamus. Animals that were pre-treated with BF extract displayed significantly reduced immobility in the FST and increased open-arm exploration in the EPM test in comparison with controls. BF also blocked the increase in TH expression in the locus coeruleus of treated rats that experienced restraint stress. Together, these results demonstrate that BF extract administration prior to restraint stress significantly reduces depression and anxiety-like behaviors, possibly through central adrenergic mechanisms, and they suggest a role for BF extract in the treatment of depression and anxiety disorders.