Large Slenderness Ratio Research Articles

로켓 모델의 동안정미계수에 대한 풍동시험의 적용

본 논문은 유도 무기나 로켓과 같이 세장비가 큰 비행체의 동안정미계수 획득을 위한 풍동실험장치 및 기법에 관하여 기술하였다. 세장비가 큰 로켓과 같은 비행체의 동적운동을 위한 풍동실험은 풍동 시험부의 크기와 진동 장치의 설치에 대한 문제 등의 어려움이 있어 동안정성 실험을 수행하는데 여러 가지 제약이 따르게 된다. 본 연구에서는 아음속 풍동에서 세장비가 큰 비행체의 동안정 미계수 획득을 위한 풍동실험기법으로 동안정 밸런스를 이용하는 기법이 사용되었다. 본 연구를 통하여 세장비가 큰 비행체의 자유진동 풍동실험 장치가 구축되었으며, 이 장치를 이용한 자료측정 결과는 동안정미계수가 적절하게 획득됨을 보여 주었다. This paper described the wind tunnel testing apparatus and technique to acquire the dynamic stability derivatives of large slenderness ratio air vehicle such as the guided missiles or rockets. There have been few difficulties in conducting wind tunnel testing for slender long rocket due to the size limitation of the test section size and the installation of oscillation equipments. In this study, the dynamic stability balance was used as the wind tunnel technique for obtaining the dynamic stability derivatives. Through the wind tunnel testing, the experimental apparatus for slender air vehicle's oscillation is established. The measured data showed that it is possible to acquire the dynamic stability derivatives of large slenderness ratio rocket, properly.

Preparation and budding growth of whiskers in a homogeneous system

Ca10(PO4)6(OH)2 (HAP) and NH4Al(OH)2CO3·H2O (AACHH) whiskers were prepared by a homogeneous precipitation method based on urea hydrolysis reaction. To clarify the growth process of whiskers in the homogeneous system, XRD and SEM results of the products obtained at different reaction time were investigated in detail. A novel observation about budding growth in preparing both whiskers was described. It was indicated that the growth of whiskers went through three stages, which were oversaturation, nucleation, and budding growth. The growth units of whiskers budded from the surfaces of substrates, which were crystallized flakes for HAP preparation and amorphous spherical nuclei for AACHH preparation. Subsequently, the whiskers grew up accompanying with the disappearing substrates. One-dimensional whiskers with fine morphology and large slenderness ratio were finally obtained. Besides, according to the crystal growth and the interface diffusion theories, the effects of the templates and the budding growth mechanism were discussed.

Compressive strength of circular concrete filled steel tube columns

An experimental study was undertaken to investigate the behaviour of 24 concrete-filled steel tube (CFST) columns, loaded concentrically in compression to failure. Variables in the tests include the length, diameter, strength of the steel tubes and the strength of the concrete. The large slenderness ratio caused all composite columns in Series 1 to fail by overall flexural buckling. Although overall flexural buckling was also experienced in the composite columns of Series 2 tests, the stockier columns failed by crushing of the concrete and yielding of the steel tube. A comparison of the experimental results with the loads predicted by the South African code (SANS 10162-1) and Eurocode 4 (EC4) shows that the codes are conservative by 8.4% and 13.6%, respectively, for Series 1 tests, and 10.5 and 20.2%, respectively, for Series 2 tests. A plot of the compressive load versus the vertical deflection shows the composite columns to be fairly ductile.

The Super-Thin Rod Cylindrical Honing Technology Research

The super-thin rod cylindrical grinding is a problem in the machining, super-thin rod with large slenderness ratio, poor rigidity, large roundness error after grinding, and with low processing efficiency. This study uses cylindrical honing processing super-thin rod parts, and designing the super-thin rod cylindrical honing head, carrying on a honing test. The results show that the super-thin rod cylindrical coarse honing capacity reach up to 0.002mm/double stroke(length 1698mm), surface roughness reach up to Ra 0.8 ~ 0.025μm after honing, roundness error reach up to 2μm. It fully shows that super-thin rod cylindrical honing technology has high precision, low surface roughness, flexible production processing and high efficiency.

Influence of thermal distortion to compression failure of polymer matrix composite panels in fire

This paper discusses the influence of thermal distortion to compression failure load of polymer matrix composite panels in fire. The thermal gradient along the thickness of the panel, caused by low thermal conductivity, induces transverse deformation field even without external mechanical loads, known as thermal distortion. The deformation field of thermal distortion obtained from combined thermal–mechanical analysis is presented in terms of non-dimensional parameters that represent thermal and mechanical properties, including those in phase transition at high temperature range, and dimensional lengths. A failure model is proposed to assess the transition from Euler buckling to plastic compression failure under the influence of thermal distortion. For large slenderness ratio, the panel is failed by Euler buckling. For small slenderness ratio, the panel is failed by plastic failure mechanism, where the analysis quantifies the influence of thermal distortion to the reduction of load-bearing capacity. The compression failure mechanism under the influence of thermal distortion is discussed for developing quantitative approach to design polymer matrix composite structures in fire.

Influence Parameter Research on the Low Cycle Fatigue Life for Welded I-Section Bracings

A survey of past test and numerical simulation studies on the low cycle fatigue behaviour of the welded I-section bracing members subjected to constant amplitude cyclic axial displacement was carried out. Based on the numerical simulation results, some empirical formulas for estimating the low cycle fatigue life are presented and compared with the empirical formulas obtained by tests. It's found that the web plate height-thickness ratio h0/tw has no significant effect on the low cycle fatigue life, and the flange width-thickness ratio b/t and the slendernessare the most important parameters for estimating the low cycle fatigue life. In view of the low cycle fatigue life increasing with the decrease in the flange width-thickness ratio and the increase in the brace slenderness ratio, small flange width-thickness ratio and large slenderness ratio should be adopted in the brace design on the premise of “no buckling under frequent earthquake”.

Buckling Analysis of Double-Limb Lipped Channel Section Member under Axial Load

Double-limb lipped channel section steel member is formed by connecting two single limb members with bolts in order to improve the buckling performance. In order to research the buckling form and ultimate bearing capacity of members with different slenderness ratios under axial load, nonlinear analysis of buckling performance is done to this kind of section using the finite element analysis software ANSYS. The influence on bearing capacity caused by height-breadth ratio of section, height-thickness ratio of web and breadth-thickness ratio of flange is analyzed. The results show that: (1) for larger slenderness ratio, complete buckling occurs to the column mainly and the slenderness ratio has larger influence on buckling bearing capacity, while for smaller slenderness ratio, local distortional buckling occurs more; (2) in a certain range, the increase of height-breadth ratio could raise the ultimate bearing capacity of member, but excessive height-breadth ratio would make the ultimate bearing capacity decrease, (3) the increase of both height-thickness ratio and breadth-thickness ratio would decrease the ultimate bearing capacity.

A model experimental study on backdraught in tunnel fires

Backdraught is a special fire phenomenon in a limited-ventilation space. Research on the occurrence of backdraught in compartment fires has been extensive, but backdraught in subway tunnel fires, which occur in underground spaces with a large slenderness ratio, has received insufficient attention. In the present study, 31 cases, divided into two groups under conditions of natural ventilation and mechanical ventilation, were examined using a 1/8 reduced-scale model tunnel to investigate the critical conditions and characteristics of backdraught occurrence, as well as the differences between tunnel backdraught and compartment backdraught. The existing critical values of the mass fraction for different ventilation conditions are also discussed. The results indicated that the key parameter determining the occurrence of backdraught in subway tunnel fires is the mass fraction of the volatilized unburned fuel in the tunnel. The critical values of the mass fraction in natural ventilation and mechanical ventilation were 8.78% and 11.71%, respectively, with a humidity of 15% in fresh air. With natural ventilation, backdraught occurrence in a single tube tunnel configuration was similar to that in a compartment, and the ignition delay depended on the velocity of the gravity current; with mechanical ventilation, backdraught occurrence in a twin-tube tunnel configuration was different from that in a compartment, and its ignition delay was determined mainly by the ignition source delay. In addition, as the velocity of air flowing into the tunnel increased, so did the intensity of the backdraught. The experimental data were qualitatively validated and analyzed based on the flammability diagram of the fuel, and these results were similar to those obtained in previous experiments. Also, the humidity of the fresh air flowing into the tunnel affected the occurrence of backdraught; it would not occur under conditions of higher humidity even if the mass fraction mentioned above was higher. Furthermore, it may be economical and feasible to install a humidification device in the tunnel ventilation system to humidify the fresh air flowing into the tunnel to inhibit backdraught.

Estimation of Volume Fraction of Martensite Induced in Austenitic Stainless Steel during the Tensile Deformation by the Resistivity and Specimen Configuration Dependency

To estimate the volume fraction of martensite induced in austenitic stainless steel during tensile deformation, the resistivity of a specimen was measured at a temperature of 77, 187 or 293K. Since the distribution of the martensite in the specimen was observed in the specimen having the small slenderness ratio. Accordingly, the specimen having the large slenderness ratio was used to estimate the martensite volume by the resistivity. The negative resistivity region was observed on the resistivity- plastic strain curve regardless of the specimen temperature. It was suggested that the negative resistivity was caused by the formation of e martensite that is a precursor of the martensite and the process of the resistivity was controlled by the formations of the e martensite and the microscopic plastic deformation induced by the martensite. The linear relation between the resistivity and the volume fraction of the martensite was confirmed by the experiment and a simple model. It was clarified that the resistivity could be used as an index of estimating the deformation-induced martensite in austenitic stainless steel.

Large deflection theory of nanobeams

One-dimensional nanostructured materials are often used as beams in many applications such as ultrahigh-frequency resonators and ultrasensitive sensors. Compared with usual macroscopic beams, nanobeams have much higher surface/volume ratios so that their surface energies may play a significant role. Besides, they often bear large deflections due to their typically large slenderness ratios and larger elastic ranges. There is, however, lack of a theory that takes into account of both the above two features owned by nanobeams. In this paper, we present such a theory and give applied examples to show that surface energy and large deflection may individually or jointly have notable effects.

Initial thermo-mechanical post-buckling of beams with temperature-dependent physical properties

The objective of this work is to analyze the elastic buckling and initial post-buckling behavior of slender beams subjected to uniform heating. The beams are assumed to be double-hinged with fixed ends, preventing thermal expansion. Consequently, destabilizing compressive forces arise that may lead to beam buckling. When the temperature is further increased, the beam experiences finite displacements, with the result that the analysis is geometrically non-linear. The modulus of elasticity and the thermal induced strain, key material properties for this problem, are temperature-dependent. Thus, the coefficients of the governing equations are not constant. This suggests the physical non-linearity of the mathematical model. Hence, the analysis is geometrically and physically non-linear. The analysis is sensitive to the beam initial temperature, as the thermal strain is a function of the initial and final temperatures. The material is considered to be linear elastic, and consequently viscoelastic and plastic effects are not taken into account. Furthermore, the beam cross-section properties are assumed to be constant, which is consistent with the small strain formulation. A perturbation method is applied to the governing non-linear differential equations so that the initial post-buckling behavior may be analytically determined when temperatures above the critical temperature are applied to the beam. To illustrate the application of the formulation we present a case study for the aluminum 7075-T6 alloy, a material commonly used in aerospace and naval industries. Nonetheless, it is expected similar behavior for other metallic materials. The curves that define the variation of the modulus of elasticity, the thermal strain and the yield stress with temperature are considered in our analysis. The change in length, reaction forces at the supports and geometric configurations are obtained as a function of temperature and the beam slenderness ratio. The critical buckling loads and temperatures and the initial post-buckling analysis are also calculated in the context of the temperature-independent physical properties. Our results emphasize the importance of modeling the material's non-linearity if accuracy is required. However, from a practical application point of view results are acceptable if temperature-independent physical properties are employed, especially for large slenderness ratios.

Micromechanical Modeling of Honeycomb Structures Based on a Modified Couple Stress Theory

A micromechanics model is developed for hexagonal honeycomb structures by using a modified couple stress theory. The formulation is based on structural analysis of repeating units and a homogenization procedure. Both bending and axial deformations of cell walls are considered here, unlike earlier models. The closed-form expressions for five effective in-plane elastic constants (including two Young's moduli, one shear modulus and two Poisson's ratios) of a honeycomb structure are derived in terms of the Young's modulus of the cell wall material and the slenderness (length-to-thickness) ratio of the cell wall. The formulas for two length scale parameters are obtained in terms of the slenderness ratio and thickness of the cell wall, thereby providing a direct measure of the microstructure of the honeycomb. The constitutive relations for the two-dimensional couple stress continuum equivalently representing the discrete honeycomb structure are provided using the five effective elastic constants and two length scale parameters. The numerical results reveal that for honeycombs with small slenderness ratios the effective Young's moduli and shear modulus are strongly dependent on the slenderness ratio. For low-density honeycombs with large slenderness ratios, the effective elastic constants predicted by the current model agree well with those by an existing model that considers only bending deformations.

SYNTHESIS AND CHARACTERIZATION OF ZNO MICROCRYSTAL TUBES

Zinc oxide microcrystal tubes were prepared by using H202-activated Zn powder and active carbon as raw materials. The prepared samples were characterized by powder X-ray diffraction, scanning electron microscopy and fluorescence spectrophotometer. The results indicate that ZnO microcrystal has tubular structure with large slenderness ratio, and hexagonal cross sections for both intine and ectotheca. When excitated at 380 nm, two photoluminescence bands of ZnO microcrystal tubes appear, one is a strong emission at 383 nm (near UV band-edge) and the other a broad green emission band centered at 500 nm. The intensity of green emission peak is far weaker than that of near UV band-edge emission peak, which suggests low defect density and fine quality of the product

Optimum design of structures subjected to follower forces

In this paper, shape optimization is used to optimize the critical load of an Euler–Bernoulli cantilever beam with constant volume subjected to a tangential compressive tip load and/or a tangential compressive load arbitrarily distributed along the beam. This is achieved by varying appropriately the beam cross-section, thus its stiffness and mass properties, along its length, so that the critical load reaches its maximum or a prescribed value. The problem is reduced to a nonlinear optimization problem under equality and inequality constraints as well as specified lower and upper bounds, which, together with large slenderness ratios, ensure the validity of the Euler–Bernoulli theory and the serviceability of the beam. The evaluation of the objective function requires the solution of the dynamic stability problem of a cantilever beam with variable cross-section. This problem is solved using the analog equation method (AEM) of Katsikadelis for the fourth-order hyperbolic differential equation with variable coefficients, together with a simple and direct iterative method for the evaluation of the critical load based on the eigenvalue sensitivity. Besides its accuracy, this method overcomes the shortcoming of a possible FEM solution, which would require resizing of the elements and re-computation of their stiffness properties during the optimization process. Example problems of various types of follower forces are presented, which illustrate the method and demonstrate its applicability and efficiency.

Giant dielectric constant and resistance-pressure sensitivity in carbon nanotubes/rubber nanocomposites with low percolation threshold

Nanocomposites consisting of methylvinyl silicone rubber (VMQ) with excellent elasticity as polymer matrix and multiwall carbon nanotube (MWNT) with large slenderness ratio and high conductivity as filler were fabricated by a solution method and subsequently hot pressing technology. Studies on dielectric property and resistance-pressure sensitivity of the MWNT/VMQ composites showed that there was a giant dielectric constant and significant resistance-pressure sensitivity as the concentration of MWNT was near a low percolation threshold, fc≈0.012. After the composite applied an enough pressure for long time, the resistance-pressure sensitivity still shows an excellent reproducibility due to the good dispersion and low loading of MWNT.

Effect of interfacial friction during forging of solid powder discs of large slenderness ratio

The paper reports an investigation into the effect of interfacial friction law during the forging of a powder circular disc with large slenderness ratio(L/D), between two flat dies. The deformation pattern during the operation is influenced by many factors, which interact with each other in a complex manner. The relative velocity between the work piece material and the die surface, together with high interfacial pressure and/or deformation modes, creates the conditions essential for adhesion in addition to sliding. The decisive factors are the interfacial conditions, initial relative density of the preform and geometry of the preform. An attempt has been made to determine the most realistic interfacial friction law and die pressures developed during such forging using an upper bound approach. The results so obtained are presented graphically and discussed critically to illustrate the interaction of various interfacial friction laws involved.

High speed forging of solid powder discs of large slenderness ratio

The paper reports an investigation into the forging of a solid powder circular disc with large slenderness ratio(L/D) between two flat dies at high speed. The deformation pattern during the operation is influenced by many factors, which interact with one another in a complex manner. The decisive factors are the interfacial conditions, initial relative density of the preform and the geometry of the preform. An attempt has been made to determine the die pressures developed during such forging, using an upper bound approach. The results so obtained are presented graphically and discussed critically to illustrate the interaction of various process parameters involved.

In-plane dynamic crushing of honeycomb. Part II: application to impact

Finite element simulations were employed to analyse in-plane dynamic crushing of two different hexagonal honeycombs (slenderness ratios L/ t=38 and 167). The response of the honeycomb with the smaller slenderness ratio was studied for impact speeds up to 40.0 m/s which corresponds to a nominal strain rate for the specimen of 500 s −1 . Total plastically dissipated energy was used to quantify the effects of increasing strain-rate, since other measures showed a strong dependence on details of the finite element model. The simulations revealed a strong increase of total dissipated energy with increasing impact speed for velocities larger than the critical speed for wave trapping, v cr . One reason was a higher percent of cells collapsing in a symmetric crush mode (IV). Another reason for the larger total dissipation at higher crushing speeds was the greater irregularity in the folding pattern that developed. Experimental and calculated global force-time curves were compared for the honeycomb with the larger slenderness ratio at impact speeds 3.0 and 7.2 m/s and a satisfactory agreement was found. At these low impact speeds there was no increase of initial collapse or plateau stresses. A comparison of sequences of deformed mesh plots and high speed photos showed good correlation of the general distribution and modes of crushing.

X型制振プレース付剛節架構の機構と弾塑性性状について : (その1)引張接合型プレース架構

In the previous papers, we presented a new effective hysteretic damper and showed it had good restoring force characteristics and enough energy absorption capacity for a severe earthquake ground motion. In this paper, a tension connected braced frame with the hysteretic damper is proposed. The various kinds of loading tests and F.E.M. analyses of one story - one bay scaled frames with the hysteretic damper are performed for verifying the mechanism of the present frame and clarifying the required sectional properties of the brace member. It shows that the tension connected braced frame with the hysteretic damper has a good mechanism, which prevent compressive axial force in the brace, and therefore the brace buckling is not occurred even if the brace has relatively large slenderness ratio. So that the repeated, large drift of the frame causes no hysteresis loop deterioration of the damper.

Free Vibration of Thin-Walled Closed-Section Composite Beams with Optimum and Near-Optimum Coupling

An investigation of the free vibration characteristics for a set of extensiontwist coupled thin-walled single-cell composite beams with optimal and near-optimal coupling is performed using an analytical model, a finite element simulation, and testing. It is found that, despite the relatively large beam slenderness ratio, the applicability of the analytical model is limited to the first bending modes, for which excellent agreement with the finite element and experimental results is obtained. For analytically predicted higher order modes, increasingly significant in-plane warping is shown by the corresponding finite element results, and an increasingly large variance is observed between the two sets of results. The occurrence of shell modes that cannot be captured by the analytical model is also shown by the finite element results. Test data confirm finite element calculations as well as low modes predicted by the analytical method. The sensitivity of natural frequencies to lay-up perturbation angle is established over a frequency range of interest, providing insight into the robustness of the modal characteristics for the global and the local optima, respectively.