Rapid development of carbon nanotubes (CNTs) reinforced to polymer composites has been recently noticed in many aspects. In this work, the latest developments on fatigue and fracture enhancement of polymer composites with CNTs reinforcement with diverse methods are thoroughly compiled and systematically reviewed. The existing available researches clearly demonstrate that fatigue fracture resistance of polymer composites can be improved accordingly with the addition of CNTs. However, this work identifies an interesting research gap for the first time in this field. Based on the systematic reviewing approach, it is noticed that all previously performed experiments in this field were mostly focused upon studying one factor only at a time. In addition, it is also addressed that there were no previous studies reported a relationship or effect of one factor upon others during examining the fatigue fracture of carbon nanotubes. Moreover, there was no adequate discussion demonstrating the interaction of parameters or the influence of one parameter upon another when both were examined simultaneously. It is also realized that the scope of the conducted fatigue fracture studies of carbon nanotubes were mainly focused on microscale fatigue analysis but not the macroscale one, which can consider the effect of environment and service condition. In addition, the inadequacy of fatigue life predicting models via analytical and numerical methods for CNT-reinforced polymer composites have also been highlighted. Besides, barriers and challenges for future directions on the application of CNT-reinforced polymer composite materials are also discussed here in details.

Through constructing statistical fracture network model based on discrete element method, the evolution characteristics of the fracture aperture had been directly simulated and evaluated caused by redistributed stress after the borehole excavation. This study focuses on the size effect of the discrete element method for the analysis of the effective distance of fracture aperture change after the borehole excavation. A two-dimensional trace-type domain with a maximum size of 1.1 m2 was created using a discrete fracture network with stochastic information of KURT. A total of eight domains with different sizes were constructed from the largest domain area to the 0.4 m2 analysis area. The aperture change ratio which can be depending on the domain size was examined. The ratio was investigated by comparing the aperture size before and after the simulation of borehole excavation. In addition, the effective range of aperture changes was analyzed by comparing the re-distribution distance from the center of the borehole. Based on dimensional analysis, input variables (borehole radius, occurrence distance of aperture changes, domain size) were modeled using exponential distribution form. Through the analysis model, two dimensionless variables were derived to investigate the expected distance of the aperture changes and appropriate DFN domain size for simulating bole excavation. As an application example of the 3-inch borehole simulation, the analysis model predicted that the range of aperture changes could occur within a radius of about 0.98 m from the borehole center, and the suitable size of the model had been inferred as about 5 × 5 m for minimizing the domain size effect.

본 연구는 재래 철근콘크리트공법의 거푸집 및 철근보강재를 FRP 판으로 대체한 개념이다. FRP판에 리브를 갖게 하여 FRP 판과 콘크리트 합성을 개선하고 거푸집의 강성증가를 유도하여 영구거푸집 및 철근보강재로 활용하는 방안이다. 본 연구는 전단경간비가 짧고 리브가 있는 FRP 판과 콘크리트 합성보의 휨/전단 파괴거동과 균열형태를 비교 분석하였다. 콘크리트의 경우 CDP 모델을 사용하였고, 외연적 비선형 유한요소해석 결과와 기존 실험결과를 정점 하중 및 균열형태에 대해 비교 분석하였다. 유효균열방향 개념을 사용하여 콘크리트 균열패턴을 시각적으로 표현하였다. 인장 등가소성변형률이 0 보다 큰 곳에서 균열이 시작된다고 가정하고, 균열평면에 수직인 벡터의 방향은 최대 주소성변형률의 방향과 평행한 것으로 가정하였다. 이 방향을 콘 크리트의 균열으로 생각하여 실험에서 확보한 균열형태와 비교 분석하였다. ABAQUS/Explicit 의 CDP 모델은 FRP 합성구조체의 비선형 거동 및 균열형태 모사가 가능한 것으로 판단된다. 초기강성의 불일치는 리브가 있는 FRP 판과 콘크리트 사이의 미세균열 및 접착력 등의 문제로 인해 발생한 것으로 사료되며 1차 정점 하중 및 균열형태를 적절히 추적할 수 있으므로 앞으로 다양 한 FRP 합성구조시스템의 거동 및 균열해석에 이용 가능할 것으로 판단되나 보다 다양한 파괴 매카니즘에 대한 지속적인 연구가 필요하다고 사료된다.

While efforts have been made to address fall-related injuries in older adults, the problem is unsolved to date. The purpose of this review is to provide a guideline for fall and injury prevention programs in older adults, based on evidence generated over the past 30 years. Research articles published between 1990 and 2020 have been searched on PubMed, using keywords, including but not limited to, falls, hip fracture, injuries, intervention, older adults, prevention, hip protector, vitamin D, safe landing strategy, and exercise. Total of 98 articles have been found and categorized into five intervention areas: exercise program, hip protector, safe landing strategy, vitamin D intake, and compliant flooring. Furthermore, the articles have been rated based on their study design: class 1, randomized controlled trials; class 2, nonrandomized controlled trials; class 3, experimental studies; class 4, all other studies. Exercise programs have shown to decrease the risk of fall, and associated injuries. Hip protectors, safe landing strategy, and vitamin D intake were effective in reducing a risk and incidence of hip fracture during a fall. Furthermore, compliant flooring has also decreased hip fracture risk without affecting balance. An integrated approach combining exercise program, wearing a hip protector, teaching safe landing strategies, scheduled vitamin D intake, and compliant flooring installation, is suggested to address fall-related injuries in older adults.

In turbopump type liquid rocket engines, ignition and starting are known to be the most unstable and risky section among all operating sections of the projectile. The operation of the liquid rocket engine is the process of ignition and combustion of the main combustor after the turbo pump is driven into a stable section due to the turbine driving of the turbo pump and the ignition and combustion of the gas generator by the pyro starter. In this process, the driving of related components directly influences each other, so each component must be operated with sufficient reliability. In particular, if the igniter does not supply sufficient ignition energy at a predetermined time, an explosion may occur due to stagnation of the fuel/oxidant mixture, so reliability is more important. In this study, the fracture analysis of the gas generator igniter rupture disk according to the shape was performed using computational analysis. As a result, comparative analysis was performed to obtain the optimal dimensions according to each variable condition.

This paper aims to experimentally and numerically explore fracture mechanism characteristics of ultra-thin chopped carbon fiber tape-reinforced thermoplastics (UT-CTT) hat-shaped hollow beam under transverse static and impact loadings. Three distinct failure modes were observed in the impact bending tests, whereas only one similar progressive collapse mode was observed in the transverse bending tests. The numerical model was to incorporate some hypothetical inter-layers in UT-CTT and assign them with the failure model as cohesive zone model, which can perform non-linear characteristics with failure criterion for representing delamination failure. The dynamic material parameters for the impact model were theoretically predicted with consideration of strain-rate dependency. It shows that the proposed modeling approach for interacting damage modes can serve as a benchmark for modeling damage coupling in composite materials.

Interfacial adhesion between carbon fiber and epoxy resin mostly determine the mechanical properties of the carbon fiber/ epoxy composites and the chemical structures of epoxy resin and hardener plays an important role. In this regard, stereoisomerism of epoxy hardeners, such as 3,3′ and 4,4′-DDS (diaminodiphenylsulfone), can have significant influence on the fracture toughness of the cured epoxy and related carbon fiber composites. Therefore, this study aims to investigate the influence of stereoisomerism of epoxy hardeners on fracture toughness of the carbon fiber/epoxy composites. Triglycidyl aminophenol (TGAP) are selected as epoxy resin and 3,3′- and 4,4′-DDS are selected as epoxy hardener. Wetting behaviors and fiber matrix adhesion of TGAP/DDS mixtures onto carbon fiber are investigated and fracture toughness (KIC) of TGAP/ DDS mixtures are also investigated. Then, the mode II fracture toughness test of the carbon fiber/TGAP/DDS composites are carried out to investigate the influence of hardener stereoisomerism on fracture toughness of the resulting composites. Wetting and fiber matrix adhesion to carbon fiber of TGAP/3,3′-DDS was better than those of TGAP/4,4’-DDS and KIC of TGAP/3,3′-DDS was also better than that of TGAP/4,4′-DDS. As a result of the synergistic effect of better wetting, fiber matrix adhesion, and fracture toughness of TGAP/3,3′-DDS, the mode II fracture toughness of the carbon fiber/ TGAP/3,3’- DDS composites was almost twice of that of the carbon fiber/ TGAP/4,4′-DDS composites. Based on the results reported in this study, stereoisomerism of the epoxy hardeners can influence the fracture toughness of the resulting composites as well as that of the resin itself. In other words, only small difference, such as the spatial arrangement of the molecular structure of epoxy hardeners can cause huge difference in the mechanical properties of the resulting composites.

In this study, the experiments and analyses were carried out in order to investigate the fracture characteristics on the adhesive at the specimen bonded with aluminum and aluminum-foam. The same conditions were given for the experiments and analyses. The results are investigated by the graph of reaction force according to displacement. It was found that the experimental and the analytical data were very similar to each other. On the basis of the data, the reliability of the analysis data could be confirmed. The notches were produced at the distances of 40, 110, 150, and 190 mm from the front of the test specimen, and the maximum reaction force was compared accordingly. It was found that the highest reaction force was generated at the front end of the adhesive and the lowest reaction force was found at the middle of the adhesive interface. Finally, when the equivalent stress in the test specimen was examined, it was found that the highest stress was obtained at the distance of 110 mm. It can be deduced. As the notch formation point are similar to the point when stress is dispersed as the adhesive is peeled off, it is possible to infer the high stress compared to other test specimens.