• Advances in nano research
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• v.4 no.1
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• pp.51-64
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• 2016

This paper investigates the effects of thermal load and shear force on the buckling of nanobeams. Higher-order shear deformation beam theories are implemented and their predictions of the critical buckling load and post-buckled configurations are compared to those of Euler-Bernoulli and Timoshenko beam theories. The nonlocal Eringen elasticity model is adopted to account a size-dependence at the nano-scale. Analytical closed form solutions for critical buckling loads and post-buckling configurations are derived for proposed beam theories. This would be helpful for those who work in the mechanical analysis of nanobeams especially experimentalists working in the field. Results show that thermal load has a more significant impact on the buckling behavior of simply-supported beams (S-S) than it has on clamped-clamped (C-C) beams. However, the nonlocal effect has more impact on C-C beams that it does on S-S beams. Moreover, it was found that the predictions obtained from Timoshenko beam theory are identical to those obtained using all higher-order shear deformation theories, suggesting that Timoshenko beam theory is sufficient to analyze buckling in nanobeams.

• Structural Engineering and Mechanics
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• v.70 no.3
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• pp.269-277
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• 2019

Using the non-local elasticity theory, Timoshenko beam model is developed to study the non- local buckling of Triple-walled carbon nanotubes (TWCNTs) embedded in an elastic medium under axial compression. The chirality and small scale effects are considered. The effects of the surrounding elastic medium based on a Winkler model and van der Waals' (vdW) forces between the inner and middle, also between the middle and outer nanotubes are taken into account. Considering the small-scale effects, the governing equilibrium equations are derived and the critical buckling loads under axial compression are obtained. The results show that the critical buckling load can be overestimated by the local beam model if the small-scale effect is overlooked for long nanotubes. In addition, significant dependence of the critical buckling loads on the chirality of zigzag carbon nanotube is confirmed. Furthermore, in order to estimate the impact of elastic medium on the non-local critical buckling load of TWCNTs under axial compression, the use of these findings are important in mechanical design considerations, improve and reinforcement of devices that use carbon nanotubes.

• Journal of the Korean Society of Safety
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• v.1 no.1
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• pp.41-49
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• 1986

Modern technological progress demands the use of materials at high temperature and high pressure. One of the most critical factors in considering such applications-perhaps the most critical one-is creep behavior. In this study the activation energy for the creep rupture (Qf) and the stress dependence of rupture time (n') have been determined during creep of Al 7075 alloy eve, the temporature range of $200^{\circ}C to 500^{\circ}C$ and stress range of 0.64 kgf/$\textrm{mm}^2$ to 9.55 kgf/$\textrm{mm}^2$, respectively, in order to investigate the creep-rupture property. Constant load creep tests were carried out in the enperiment At around the temperature $210^{\circ}C~390^{\circ}C$ and the stress level 1.53~9.55(kgf/$\textrm{mm}^2$), the stress dependence of rupture time(n') had the value of 6.6~6.78 but at 50$0^{\circ}C$, the value of 1.3. Besides at around the temperature of $200^{\circ}C~500^{\circ}C$ and under the stress level of 0.89~8.51 (kgf/$\textrm{mm}^2$), the activation energy for the creepprupture (Qf) was nearly equal to that of the volume self diffusion of pure aluminum (34Kca1/mo1e)

• The Journal of Korean Institute of Communications and Information Sciences
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• v.28 no.7B
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• pp.620-627
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• 2003

Ethernet passive optical networks (EPONs) are an emerging access network technology that provide a low-cost method of deploying optical access lines between a carrier's central office and customer sites. Dynamic bandwidth allocation (DBA) provides statistical multiplexing between the optical network units for efficient upstream channel utilization. To support dynamic bandwidth distribution, 1 propose an cyclic polling-based DBA algorithm for differentiated classes of service in EPONs. And, I show that an interleaved polling scheme severely decreases downstream channel capacity for user traffics when the upstream network load is low. To obtain realistic simulation results, I used synthetic traffic that exhibits the properties of self-similarity and long-range dependence I then analyzed the network performance under various loads, specifically focusing on packet delays for different classes of traffic.

• Nuclear Engineering and Technology
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• v.27 no.6
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• pp.825-832
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• 1995

The effect of loading variables and temperature on fatigue crack growth rate in SA508 Cl.3 nuclear pressure vessel steel was investigated in air environment Crack growth rate tests on compact tension specimen of thickness 12mm were conducted by using sinusoidal waveform. The crack length was monitored by compliance method. Test conditions were at 0.1 and 0.5 of load ratio, at 1 and 10 Hz of loading frequency, and at room temperature to 40$0^{\circ}C$. At the lower temperatures, the fatigue crack propagation was not affected by the frequency and temperature, while at the higher temperatures above 12$0^{\circ}C$, fatigue crack growth rate increased with decreasing loading frequency and increasing temperature. This accelerated fatigue crack propagation was associated with the increase of oxidation rate at the ahead of crack tip. Fatigue crack growth rate increased with in-creasing the load ratio. The effect of load ratio was more significant at the lower temperature, while the dependence on load ratio decreased with increasing temperature. The sensitivity of load ratio to temperature can be explained by crack closure with the oxidation process.

• The Journal of Advanced Prosthodontics
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• v.11 no.3
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• pp.187-192
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• 2019

PURPOSE. The purpose of this in vitro study was to investigate the fracture loads and modes of failure for the full range of natural teeth under simulated occlusal loading. MATERIALS AND METHODS. One hundred and forty natural teeth were taken from mandibles and maxillas of patients. There were 14 groups of teeth with 10 teeth in each group (5 males and 5 females). Each specimen was embedded in resin and mounted on a positioning jig, with the long axis of the tooth at an inclined angle of 30 degrees. A universal testing machine was used to measure the compression load at which fracture of the tooth specimen occurred; loads were applied on the incisal edge and/or functional cusp. RESULTS. The mean fracture load for the mandibular first premolar was the highest (2002 N) of all the types of teeth, while the mean fracture load for the maxillary first premolar was the lowest (525 N). Mean fracture loads for the mandibular and maxillary incisors, and the first and second maxillary premolars, had significantly lower values compared to the other types of teeth. The mean fracture load for the teeth from males was significantly greater than that for the teeth from females. There was an inverse relationship between age and mean fracture load, in which older teeth had lower fracture loads compared to younger teeth. CONCLUSION. The mean fracture loads for natural teeth were significantly different, with dependence on tooth position and the sex and age of the individual.

• Korean Journal of Agricultural Science
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• v.49 no.2
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• pp.269-284
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• 2022

The development of radish collectors has the potential to increase radish yields while decreasing the time and dependence on human labor in a variety of field activities. Stress and fatigue analyses are essential to ensure the optimal design and machine life of any agricultural machinery. The objectives of this research were to analyze the stress and fatigue of major components of a tractor-mounted radish collector under dynamic load conditions in an effort to increase the design dependability and dimensions of the materials. An experiment was conducted to measure the shaft torque of stem-cutting and transferring conveyor motors using rotary torque sensors at different tractor ground speeds with and without a load. The Smith-Watson-Topper mean stress equation and the rain-flow counting technique were utilized to determine the required shear stress with the distribution of the fatigue life cycle. The severity of the operation was assessed using Miner's theory. All running conditions produced more than 10⁷ of high cycle fatigue strength. Furthermore, the highest severity levels for motor shafts used for stem cutting and transferring and for transportation joints and cutting blades were 2.20, 4.24, 2.07, and 1.07, and 1.97, 3.81, 1.73, and 1.07, respectively, with and without a load condition, except for 5.24 for a winch motor shaft under a load. The stress and fatigue analysis presented in this study can aid in the selection of the most appropriate design parameters and material sizes for the successful construction of a tractor-mounted radish collector, which is currently under development.

• Tribology and Lubricants
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• v.26 no.1
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• pp.14-20
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• 2010

Tribological properties of nanoporous structured alumina film was investigated. Alumina film (AAO: anodic aluminum oxide) of $60{\mu}m$ thickness having nanopores of 45 nm diameter with 105 nm interpore-diatance was fabricated by mild anodization process. Reciprocating ball-on-flat sliding friction tests using 1 mm diameter steel ball as a counterpart were carried out with wide range of normal load from 1 mN to 1 N in an ambient environment. The morphology of worn surfaces were analyzed using scanning electron microscopy. The friction coefficient was strongly influenced by the applied normal load. Smooth layer patches were formed on the worn surface of both AAO and steel ball at relatively high load (100 mN and 1 N) due to tribochemical reaction and compaction of wear debris. These tribolayers contributed to the lower friction at high loads. Extremely thin layer patches, due to mild plastic deformation of surface layer, were sparsely distributed on the worn surface of AAO at low loads (1 mN and 10 mN) without the evidence of tribochemical reaction. Delaminated wear particles were generated at high loads by fatigue due to repeated loading and sliding.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.26 no.1
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• pp.55-60
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• 2002

For the development of a new creep test technique, the availability of SP-Creep test is discussed for 1Cr-0.5Mo boiler header material. And some results are also compared with those of 2.25Cr- 1Mo steel which widely uses as boiler superheater tube. The results can be summarized as follows. The load exponents(n) obtained by SP-Creep test for 1Cr-0.5Mo steel are decreased with increasing creep temperature and the values are 15.67, 13.89, and 17.13 at 550$^{circ}C$ ,575$^{circ}C$ and 600$^{circ}C$, respectively. The temperature dependence of the load exponent is given by n = 107.19 - 0.1108T. This reason that load exponents show the extensive range of 10∼16 is attributed to the fine carbide such as M$_{23}$C$_{6}$ in lath tempered martensitic structures. At the same creep condition, the secondary creep rate of 1Cr-0.5Mo steel is lower than the 2.25Cr-1Mo steel1 due to the strengthening microstructure composed by normalizing and tempering treatments. Through a SEM observation, it can be summarized that the primary, secondary, and tertiary creep regions of SP-Creep specimen are corresponding to plastic bending, plastic membrane stretching, and plastic instability regions among the deformation behavior of four steps in SP test, respectively.y.

• Proceedings of the Korean Society for Noise and Vibration Engineering Conference
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• 2009.10a
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• pp.855-863
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• 2009

In this study, a beam-column junction type damper is proposed which saves the inner and outer space for the installation of damping devices and allows easy adjustment of control performance The result of the numerical analysis indicated that the displacement response and base shear of a single degree of freedom system by seismic load, El Centro 1940 was reduced with yield moment of the joint hinge and the specific yield moment ratio $\delta$ of the joint hinge existed for the optimal seismic performance. In addition, the dynamic nonlinear characteristics, effects of yielding and dependence of natural period of bi-linear system with the junction type damper is identified. The analysis of multi-degree of freedom system showed that responses of the controlled structures was reduced significantly as the number of a story increases and yield moment ratio decreases when the system is excited by seismic load and sine wave. On top of that, it was also observed that energy dissipation at the joint connected with the dampers was remarkable during excitation.