• Title, Summary, Keyword: stiffness

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Influence of Stiffness Coefficients on Optical Performance in Composite Optical Substrate (강성계수가 복합재 광학판 성능에 미치는 영향성 연구)

  • Kim, Kyung-Pyo
    • Journal of the Korea Academia-Industrial cooperation Society
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    • v.18 no.11
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    • pp.762-769
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    • 2017
  • The extensional stiffness in quasi-isotropic laminates is uniform in the radial direction, but the bending stiffness varies radially due to the stacking sequence. This paper addresses the directional dependency of the bending stiffness and its radial variation in three types of quasi-isotropic laminate reflectors consisting of unidirectional fiber composite materials (UDM) and randomly distributed composite materials (short fiber, RDM). The extensional stiffness and bending stiffness in optical reflectors using RDM are uniform, while the bending stiffness in those using UDM varies radially from 11% to 26%. Also, the stiffness sensitivity, such as the bend-twist or bend-torsion effect, due to the differences in the stiffness value in the composite, is large. These factors are problematic in the optical field requiring precision surfaces. Utilizing RDM might be one way to eliminate the presence of bending stiffness in composite mirror substrates.

Computation of Complex Stiffness of Inflated Diaphragm in Pneumatic Springs by Using FE Codes (상용 유한요소해석 프로그램을 이용한 공압 스프링 내 다이아프램의 복소강성 산출)

  • Lee, Jeung-Hoon;Kim, Kwang-Joon
    • Transactions of the Korean Society for Noise and Vibration Engineering
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    • v.16 no.9
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    • pp.919-925
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    • 2006
  • An accurate mathematical model for complex stiffness of the pneumatic spring would be necessary for an efficient design of a pneumatic spring used in vibration isolation tables for precision instruments such as optical devices or nano-scale equipments. A diaphragm, often employed for prevention of air leakage, plays a significant role of complex stiffness element as well as the pressurized air itself Therefore, effects of the diaphragm need to be included in the dynamic model for a more faithful description of dynamic behavior of pneumatic spring. But the complex stiffness of diaphragm is difficult to predict In an analytical way, since it is a rubber membrane of complicated shape in itself. Moreover, the diaphragm should be expandable in response to pressurization inside a chamber, which makes direct measurement of complex stiffness of diaphragm extremely difficult. In our earlier research, the complex stiffness of diaphragm was indirectly measured, which was just to eliminate the theoretical stiffness of pressurized air from the measured complex stiffness of the pneumatic spring. In order to reflect complex stiffness of inflated diaphragm on the total stiffness at the initial design or design improvement stage, however. it is required to be able to predict beforehand. In this paper, how to predict the complex stiffness of inflated rubber diaphragm by commercial FE codes (e.g. ABAQUS) will be discussed and the results will be compared with the indirectly measured values.

Parametric Study on the tendency of Stiffness Variation using Variable Stiffness Mechanism (변수변화에 따른 가변강성 메커니즘의 강성변화 경향성에 관한 연구)

  • Ham, KiBeom;Han, Jiho;Jeon, JongKyun;Park, YongJai
    • Journal of the Korea Academia-Industrial cooperation Society
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    • v.17 no.6
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    • pp.750-758
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    • 2016
  • In general, a system can be stable when it is designed with a rigid material. However, the use of a rigid system can be limited, such as grasping a glass or using a small surgical instrument. To resolve this limitation, a variable stiffness mechanism was developed using a flexible material. Previous research verified the variable stiffness mechanism where flexible segments and rigid segments were connected alternately in series. However, research into the design parameters of the variable stiffness structure is needed to satisfy the desired stiffness. Therefore, a variable stiffness structure was tested by varying the design parameters to confirm the trend of the stiffness variation. When the radius of the structure becomes larger, the stiffness increases. The stiffness increased with decreasing length of the flexible segments. Under the same design parameters, the length of the flexible segments had a greater effect on the stiffness than the length of the rigid segments. In addition, the stiffness was estimated using the pseudo rigid body model and was compared with the experimental results. This parametric study can be used as a design guideline for designing the variable stiffness mechanism to satisfy the desired stiffness.

Effects of Sling Forearm Plank Exercises on Superficial Back Line Muscle Tone and Stiffness

  • Wang, Joongsan
    • Journal of International Academy of Physical Therapy Research
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    • v.10 no.1
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    • pp.1695-1699
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    • 2019
  • Background : Although plank exercises is reported to the changes in muscle activity of the deep muscles and superficial muscles among the core muscles. However, no study has examined the effects of forearm plank exercise on tone and stiffness in the superficial back line muscle. Objective: To compare the effects of sling forearm plank exercises and mat forearm plank exercises on the superficial back line muscle tone and stiffness. Design: Randomized controlled clinical trial (single blind) Methods: The subjects were randomized to sling forearm plank exercise group (N = 8) or mat forearm plank exercise group (N = 8). The measurements were taken for each research group following exercises: the muscle tone and stiffness of upper lumbar muscles, lower lumbar muscles, long head of biceps femoris, and medial part of gastrocnemius among the superficial back line muscles. Results: Sling forearm plank exercise group Indicated statistically significant increases in stiffness of medial part of gastrocnemius (p<.05). However, mat forearm plank exercise group reported no statistically significant in muscle tone and stiffness of all measured muscles. No significant differences in measured variables were found between the groups. Conclusions: These results suggest that the forearm plank exercise performed with an unstable surface in the defined sling can increase the stiffness of calf muscle, but it is unlikely to achieve increases in muscle tone and stiffness of the overall superficial back line muscles.

Evaluation of Stiffness Matrix of 3-Dimensional Elements for Isotropic and Composite Plates (등방성 및 복합재 플레이트용 16절점 요소의 강성행렬 계산)

  • 윤태혁;김정운;이재복
    • Transactions of the Korean Society of Mechanical Engineers
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    • v.18 no.10
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    • pp.2640-2652
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    • 1994
  • The stiffness of 6-node isotropic element is stiffer than that of 8-node isotropic element of same configuration. This phenomenon was called 'Relative Stiffness Stiffening Phenomenon'. In this paper, an equation of sampling point modification which correct this phenomenon was derived for the composite plate, as well as an equation for an isotropic plate. The relative stiffness stiffening phenomena of an isotropic plate element could be corrected by modifying Gauss sampling points in the numerical integration of stiffness matrix. This technique could also be successfully applied to the static analyses of composite plate modeled by the 3-dimensional 16-node elements. We predicted theoretical errors of stiffness versus the number of layers that result from the reduction of numerical integration order. These errors coincide very well with the actual errors of stiffness. Therefore, we can choose full integration of reduced integration based upon the permissible error criterion and the number of layers by using the thoretically predicted error.

Effect of Hysteresis on Interface Waves in Contact Surfaces

  • Kim, Noh-Yu;Yang, Seung-Yong
    • Journal of the Korean Society for Nondestructive Testing
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    • v.30 no.6
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    • pp.578-586
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    • 2010
  • This paper describes a theoretical model and acoustic analysis of hysteresis of contacting surfaces subject to compression pressure. Contacting surfaces known to be nonlinear and hysteretic is considered as a simple spring that has a complex stiffness connecting discontinuous displacements between two solid contact boundaries. Mathematical formulation for 1-D interfacial wave propagation between two contacting solids is developed using the complex spring model to derive the dispersion relation between the interface wave speed and the complex interfacial stiffness. Existence of the interface wave propagating along the hysteretic interface is studied in theory and discussed by investigating the solution to the dispersion equation. Unlike the linear interface without hysteresis, there can exist only one distinct mode of interface waves for the hysteretic interface, which is anti-symmetric motion. The anti-symmetric mode of interface wave propagates with the velocity faster than the Rayleigh surface wave but less than the shear wave depending on the interfacial stiffness. If the contacting surfaces are compressed so much that the linear interfacial stiffness is very high, the hysteretic stiffness does not affect the interface wave velocity. However, it has an effect on the speed of interface wave for a loosely contact surfaces with a relatively low linear stiffness. It is also found that the phase velocity of anti-symmetric wave mode converges to the shear wave velocity in despite of the linear stiffness value if the hysteretic stiffness approaches 0.5.

A simplified geometric stiffness in stability analysis of thin-walled structures by the finite element method

  • Senjanovic, Ivo;Vladimir, Nikola;Cho, Dae-Seung
    • International Journal of Naval Architecture and Ocean Engineering
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    • v.4 no.3
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    • pp.313-321
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    • 2012
  • Vibration analysis of a thin-walled structure can be performed with a consistent mass matrix determined by the shape functions of all degrees of freedom (d.o.f.) used for construction of conventional stiffness matrix, or with a lumped mass matrix. In similar way stability of a structure can be analysed with consistent geometric stiffness matrix or geometric stiffness matrix with lumped buckling load, related only to the rotational d.o.f. Recently, the simplified mass matrix is constructed employing shape functions of in-plane displacements for plate deflection. In this paper the same approach is used for construction of simplified geometric stiffness matrix. Beam element, and triangular and rectangular plate element are considered. Application of the new geometric stiffness is illustrated in the case of simply supported beam and square plate. The same problems are solved with consistent and lumped geometric stiffness matrix, and the obtained results are compared with the analytical solution. Also, a combination of simplified and lumped geometric stiffness matrix is analysed in order to increase accuracy of stability analysis.

Design strategy of hybrid stay cable system using CFRP and steel materials

  • Xiong, Wen;Cai, C.S.;Xiao, Rucheng;Zhang, Yin
    • Steel and Composite Structures
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    • v.13 no.1
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    • pp.47-70
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    • 2012
  • To enhance cable stiffness, this paper proposed a combined application of carbon fiber reinforced polymers (CFRP) and steel materials, resulting in a novel type of hybrid stay cable system especially for the cable-stayed bridges with main span lengths of 1400~2800 m. In this combination, CFRP materials can conserve all their advantages such as light weight and high strength; while steel materials help increase the equivalent stiffness to compensate for the low elastic modulus of CFRP materials. An increase of the equivalent stiffness of the hybrid stay cable system could be further obtained with a reasonable increase of its safety factor. Following this concept, a series of parametric studies for the hybrid stay cable system with the consideration of stiffness and cost were carried out. Three design strategies/criteria, namely, best equivalent stiffness with a given safety factor, highest ratio of equivalent stiffness to material cost with a given safety factor, and best equivalent stiffness under a given cost were proposed from the stiffness and cost viewpoints. Finally, a comprehensive design procedure following the proposed design strategies was suggested. It was shown that the proposed hybrid stay cable system could be a good alternative to the pure CFRP or traditional steel stay cables in the future applications of super long span bridges.

Electrically-induced actuation for open-loop control to cancel self-excitation vibration

  • Makihara, Kanjuro;Ecker, Horst
    • Smart Structures and Systems
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    • v.9 no.2
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    • pp.189-206
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    • 2012
  • This paper focuses on the actuation system combined with a piezoelectric transducer and an electric circuit, which leads to a new insight; the electric actuation system is equivalent to mechanical variable-stiffness actuation systems. By controlling the switch in the circuit, the electric status of the piezoelectric transducer is changed, and consequently a variable-stiffness mechanism is achieved on the electric actuator. This proposed actuator features a shift in the equilibrium point of force, while conventional electrically-induced variable-stiffness actuators feature the variation of the stiffness value. We intensively focus on the equilibrium shift in the actuation system, which has been neglected. The stiffness of the variable-stiffness actuator is periodically modulated by controlling the switch, to suppress the vibration of the system in an open-loop way. It is proved that this electric actuator is equivalent to its mechanical counterpart, and that the electrical version has some practical advantages over the mechanical one. Furthermore, another kind of electrically-induced variable-stiffness actuator, using an energy-recycling mechanism is also discussed from the viewpoint of open-loop vibration control. Extensive numerical simulations provide comprehensive assessment on both electrically-induced variable-stiffness actuators employed for open-loop vibration control.

A Study on the Within Wafer Non-uniformity of Oxide Film in CMP (CMP 패드 강성에 따른 산화막 불균일성(WIWNU)에 관한 연구)

  • Park, Ki-Hyun;Jung, Jae-Woo;Park, Boum-Young;Seo, Heon-Deok;Lee, Hyun-Seop;Jeong, Hae-Do
    • Journal of the Korean Institute of Electrical and Electronic Material Engineers
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    • v.18 no.6
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    • pp.521-526
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    • 2005
  • Within wafer non-uniformity(WIWNU) improves as the stiffness of pad decrease. We designed the pad groove to study of pad stiffness on WIWNU in Chemical mechanical polishing(CMP) and measured the pad stiffness according to groove width. The groove influences effective pad stiffness although original mechanical properties of pad are unchanged by grooving. Also, it affects the flow of slurry that has an effect on the lubrication regime and polishing results. An Increase of the apparent contact area of pad by groove width results in decrease of effective pad stiffness. WIWNU and profile of removal tate improved as effective pad stiffness decreased. Because grooving the pad reduce its effective stiffness and it makes slurry distribution to be uniform. Futhermore, it ensures that pad conforms to wafer-scale flatness variability. By grooving the top pad, it is possible to reduce its stiffness and hence reduce WIWNU and edge effect.