• Structural Engineering and Mechanics
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• 제64권6권
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• pp.819-826
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• 2017

The effects of thermocycling procedure and material shade on the mechanical properties and wear resistance of resin-based dental restorative materials are investigated. The modulus of elasticity, hardness, plasticity index and wear resistance are determined for the conventional composite, the nanohybrid composite and the nanofilled dental composites. Disc-shape samples are prepared from each material to investigate the effects of thermocycling procedure on the mechanical properties and wear resistance of different types of dental restorative materials. In this respect, a group of samples is thermocycled and the other group is stored in ambient conditions. Then nano-indentation and nano-scratch tests are performed on the samples to measure their mechanical properties and wear resistance. Results show that the A1E shade of the dental nanocomposite possesses higher modulus of elasticity and hardness values compared to the two other shades. According to the experimental results, the mean values for the modulus of elasticity and hardness of the A1E shade of the nanocomposite are 13.71 GPa and 1.08 GPa, respectively. The modulus of elasticity and hardness of the conventional dental composite increase around 30 percent in the oral environment due to the moisture and temperature changes. The wear resistance of the dental composites is also significantly affected by moisture and temperature changes in the oral conditions. It is observed that thermocycling has no significant effect on the hardness, plasticity index and wear resistance of the nanohybrid composite and the nanocomposite dental materials.

• Composites Research
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• 제36권1호
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• pp.16-24
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• 2023

본 논문에서는 복합재료의 섬유와 기지사이의 경계면 손상을 고려한 멀티스케일 점진적 피로 손상 모델을 제안한다. 먼저 점진적인 경계면 손상을 고려하기 위해 서로 다른 4개의 경계면 상태를 정의한 미소구조 모델을 도입하였다. 각각의 상태에 대한 부피분율은 피로 하중의 사이클 수가 증가함에 따라 온전한 상태의 계면에서 완전 박리 상태의 계면으로의 전환이 일어난다. 손상된 경계면의 에쉘비 텐서(Eshelby's tensor)를 계산하기 위해 선형 스프링 모델이 사용되었으며 균질화 방법을 통해 복합재료의 유효 물성을 얻었다. 또한 복합재료의 피로거동을 묘사하기 위해 교번 응력에 대한 섬유, 기지, 그리고 섬유-기지 간의 계면 각각에 대한 손상 변수들이 정의되었고 이를 chaotic firefly 알고리즘을 통해 손상 변수를 특성화 하였다. 제안된 모델은 유한요소해석프로그램 ABAQUS의 UMAT subroutine으로 구현되어 AS4/3501-6 복합재료의 단일방향 라미네이트(unidirectional laminate) 시편들([0]₈, [90]₈,[30]₁₆)을 통해 성공적으로 검증되었다.

• Steel and Composite Structures
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• 제24권4호
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• pp.455-465
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• 2017

This paper presents a series of ultimate and fatigue experimental investigation on concrete-filled rectangular hollow section (CRHS) X joints with Perfobond Leister rib (PBR) under tension. A total of 15 specimens were fabricated, in which 12 specimens were tested under ultimate tension and 3 specimens were investigated in fatigue test. Different parameters including PBR stiffening, brace-to-chord ratio (${\beta}$) and inclined angle (${\theta}$) were considered in the test. Each joint was tested to failure under tension load. Obtained from test result, PBR was found to improve the tension strength and fatigue durability of CRHS joint substantially. Concrete dowel consisted by PBR and concrete inside the chord stiffened the joint, which leaded to a combination failure mode of punching shear and chord plastification of CRHS joint under tension. Finite element analysis validated the compound failure mode. Stress concentration on typical spot of CRHS joint was mitigated by PBR which was observed from fatigue test. Initial fatigue crack presented in CRHS joint with PBR also differentiated with the counterpart without PBR.

• Steel and Composite Structures
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• 제42권6호
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• pp.843-853
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• 2022

The fatigue behavior of welded open rib-to crossbeam joints (ORCJ) in orthotropic bridge structures is investigated using a traction structural stress method. The fatigue behaviors of welded open rib-to crossbeam joints have been a subject of study for decades for ensuring operational safety and future design improvement. A mesh-insensitive combination of traction structural stresses in ORCJ was obtained considering the effect of in-plane shear stress and validated by fatigue test results. The proposed method is advantageous for predicting fatigue cracks that initiate from the crossbeam cutout and propagate along the crossbeam. The investigations carried out with the proposed approach reveal that the normal structural stress decreases with the propagation of fatigue cracks, while the ratio of shear stress to normal stress increases. The effect of shear structural stress is significant for the analysis of fatigue behavior of ORCJ in multiaxial stress states.

• Steel and Composite Structures
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• 제28권3호
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• pp.319-329
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• 2018

The field around additional cutout of the floor beam web in orthotropic bridge deck was subjected to high stress concentration, especially the weld toe between floor beam and U shaped rib and the free edge of the additional cutout. Based on different considerations, different geometrical parameters of additional cutout were proposed in European, American and Japanese specifications, and there remained remarkable differences among them. In this study, considering influence of out-of-plane deformation of floor beam web and U shaped rib, parameter analysis for additional cutout under typical load cases was performed by fine finite element method. The influence of additional cutout shape and height to the stress distribution around the additional cutout were investigated and analyzed. Meanwhile, the static and fatigue test on this structure details was carried out. The stress distribution was consistent with the finite element analysis results. The fatigue property for additional cutout height of 95mm was slightly better than that of 61.5 mm.

• Steel and Composite Structures
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• 제50권6호
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• pp.689-703
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• 2024

Construction of high-rise structures, formwork systems that can be installed quickly, resistant to external loads, can be used more than once, have become a necessity. Timber and composite timber materials are preferred in the formation of such formwork systems due to their durability, ease of assembly, light weight and easy to use more than one time. Formwork beams are the most commonly used structural component in the formation of such formwork systems, and these beams can be damaged for different reasons during their lifetime. In this study, H20 top P type timber formwork beams with 1800 and 2450 mm length which is among the products of DOKA(c) company is damaged under the effect of static loading up to a high load level of 85% of the maximum ultimate capacity and after being retrofitted using anchored CFRP strips, performance and behavior of the beams under the influence of various loading types such as static, fatigue and impact are investigated experimentally. Two different lengths of retrofitted timber formwork beams were tested by applying monotonic static, fatigue and impact loading and comments were made about the effects of the retrofit method on performance under different loading types.

• 한국산학기술학회논문지
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• 제13권7호
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• pp.2865-2871
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• 2012

본 논문에서는 접착제로 접합된 DCB(이중외팔보) 시험편의 피로 해석을 수행하였다. 두께가 25 mm 및 40 mm인 두 모델들의 피로수명과 피로손상의 해석 결과를 비교해보면, 두께 25 mm인 모델이 두께 40 mm인 모델에 비하여 수명과 손상이 불리한 것으로 나타났으며 불규칙 피로하중에서는 'SAE transmission'에서 가장 불리한 값을 나타냈다. Rainflow matrices에서는 'SAE bracket history'에서 가장 취약한 것으로 나타났다. Damage matrices에서는 상대적으로 두께 25 mm인 모델이 불리한 모습을 보였지만, 상대적인 손상 가능성에서는 두께 40 mm인 모델이 더 높은 것으로 나타났다. 또한 'SAE transmission'에서 가장 안정한 경향을 보이고 있고 상대적인 손상으로서 약 1.1 에서 1.8% 정도로 가장 작은 것으로 나타났다. 본 연구에서 얻어진 해석 결과를 접착제로 접합된 실제 복합재 구조물에 적용시켜 파괴거동을 분석하고 그 기계적인 특성을 파악할 수 있다.

• 비파괴검사학회지
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• 제27권1호
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• pp.48-57
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• 2007

사용 중인 항공기 구조를 모사하여 리벳구멍 주위에 피로균열이 존재하는 AA2024-T3 박판을 CFRP 복합재 패치로 접합하여 보수한 후 피로균열 성장거동을 음향방출(AE)을 측정하면서 관찰하였다. 패칭의 효과로 균열성장속도의 뚜렷한 감소와 균열이 인접한 리벳 구멍으로 전파되는 시간을 지연시키는 효과가 있음을 확인하였다. 그리고 패칭 후 균열성장에 의해 탐지된 신호와 패치와 알루미늄 사이의 접합계면 분리에 의한 신호의 구별 가능성을 다변량 자료분석 기법인 주성분분석을 통해 알아보았다. 그 결과 균열성장에 따른 AE신호는 계면분리에 따른 AE신호에 비해 중심주파수가 높고 작은 에너지를 가지는 반면, 계면분리 신호는 균열성장 신호에 비해 상승시간이 길고 중심주파수가 상대적으로 낮으며 비교적 큰 에너지를 가진 것으로 나타났다. 따라서 AE신호의 유형인식 방법은 AE 발생원의 위치표정 결과와 결합할 경우 패치 접합으로 보수된 구조에서의 피로균열 성장거동을 예측하는 방법으로 충분히 활용될 수 있다.

• Steel and Composite Structures
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• 제31권4호
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• pp.409-417
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• 2019

The choice of individual material for industrial application is primarily based on knowledge of its behavior in similar applications and similar environmental conditions. Contemporary design implies knowledge of material behavior and knowledge in the area of structural analysis supported by large capacity computers. Bearing this in mind, this paper presents and analyzes the experimental results related to the mechanical properties of the material considered (30CrNiMo8/1.6580/AISI 4340) at different temperatures as well as its creep and fatigue behavior. All experimental tests were carried out as uniaxial tests. The test results related to the mechanical properties are presented in the form of engineering stress-strain diagrams. The results related to the creep behavior of the material are shown in the form of creep curves, while the fatigue of the material is shown in the form of stress - life (S - N) diagram. Based on these experimental results, the values of the following properties are determined: ultimate tensile strength (${\sigma}_{m,20}=696MPa$), yield strength (${\sigma}_{0.2,20}=355.5MPa$), modulus of elasticity ($E_{,20}=217GPa$) and fatigue limit (${\sigma}_{f,20,R=-1}=280.4MPa$). Results related to fatigue tests were obtained at room temperature and stress ratio R = -1.

• Structural Engineering and Mechanics
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• 제84권6호
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• pp.823-829
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• 2022

This study considered the experimental parameters (Nano-graphene oxide reinforced polycarbonate, GFRP) under low-velocity impact load and vibration analysis. The effect of nano-graphene oxide (NGO) on a polycarbonate-based composite was studied. Two test procedures were adopted to obtain experimental results, vibration analysis. The mechanical tests were performed on damaged and non-damaged specimens to determine the damaging effect on the composite specimens. After the test was carried out, the effect of NGO was measured and damping factors were ascertained experimentally. 0. 2 wt% NGO was determined as the optimum amount that best affected the Vibration Analysis. The experiments revealed that the composite's damping properties were increased by adding the nanoparticles to 0.25 wt% and decreased slightly for the specimens with the highest nanoparticles content. Cyclic sinus loading was applied at a frequency of 3.5 Hz. This paper study the frequency effect of 3.5khz frequency damage on mechanical results. Found that high frequency will worthlessly affect the fatigue life in NGO/polycarbonate composite. In 3.5 Hz frequency, it was chosen to decrease the heat by frequency. Transmission electron microscopy (TEM) micrographs were used to investigate the distribution of NGO on the polycarbonate matrix and revealed a homogeneous mixture of nano-composites and strong bonding between NGO and the polycarbonate which increased the damping properties and decreased vibration. Finally, experimental modal analysis was conducted after the high-velocity impact damage process to investigate the defect on the NGO polycarbonate composites.