On buckling of orthotropic compressed plates with circular holes

Buckling and post-buckling behaviour of square plates having central circular holes is analysed with the aid of the finite element method. Four different degrees of orthotropy pertinent to real materials are examined. Both uniaxial and biaxial compression are considered for various types of boundary conditions. Also, buckling of polar orthotropic annular plates is analysed for different boundary conditions and material properties. In the case of small holes a perturbation method is applied to both kinds of plates.     

Localized plastic buckling deformation in axially compressed cylindrical shells

Summary A linear partial differential equation is derived to describe growth of an axisymmetric perturbation in the plastic buckling of axially compressed cylindrical shells. Simple J ₂ flow theory is used along with rigid-plastic material behavior. An asymptotic solution is then constructed for large values of a parameter which characterizes localization of an initial imperfection. The perturbation remains localized. The solution is compared with experimental results.     

Two distinct buckling modes in carbon nanotube bending

By using controlled SPM manipulation, carbon nanotubes have been continuously bent into a series of increasing angles, and two distinct buckling modes corresponding to "abrupt" and "gradual" buckle formation were observed through recording the height increment at the bend site during the loading process. Molecular dynamics simulation also found the two buckling modes in different types of carbon nanotubes, and their atomistic mechanism was revealed. Finally, the dependence of the critical buckling condition on diameters of carbon nanotubes was tentatively studied.     

Three-dimensional thermal buckling analysis of functionally graded materials

Three-dimensional thermal buckling analysis is performed for functionally graded materials. Material properties are assumed to be temperature dependent, and varied continuously in the thickness direction according to a simple power law distribution in terms of the volume fraction of a ceramic and metal. The finite element model is adopted by using an 18-node solid element to analyze more accurately the variation of material properties and temperature field in the thickness direction. Furthermore, the assumed strain mixed formulation is used to prevent locking as well as maintaining kinematic stability of the finite element model for thin plates and shells. The thermal buckling behavior under uniform or nonuniform temperature rise across the thickness is analyzed. Numerical results are compared with those of the previous works. In addition, the changes of critical buckling temperature due to the effects of temperature field, volume fraction distributions, and system geometric parameters are studied.     

On inhomogeneous straining in compressed sylvinite

Spatiotemporal distributions of local components of the distortion tensor of quasi-plastic materials—saliferous rocks (sylvinite)—have been studied under active compressive straining conditions using double-exposure speckle photography techniques. The strain localization patterns are presented and the features of macroscopic strain inhomogeneity are considered for inelastic behavior of the material. Results obtained for the slow wave processes in deformed saliferous rocks are compared to analogous data available for ionic crystals.     

Avoided crossing of rattler modes in thermoelectric materials

Engineering of materials with specific physical properties has recently focused on the effect of nano-sized 'guest domains' in a 'host matrix' that enable tuning of electrical, mechanical, photo-optical or thermal properties. A low thermal conductivity is a prerequisite for obtaining effective thermoelectric materials, and the challenge is to limit the conduction of heat by phonons, without simultaneously reducing the charge transport. This is named the 'phonon glass-electron crystal' concept and may be realized in host-guest systems. The guest entities are believed to have independent oscillations, so-called rattler modes, which scatter the acoustic phonons and reduce the thermal conductivity. We have investigated the phonon dispersion relation in the phonon glass-electron crystal material Ba(8)Ga(16)Ge(30) using neutron triple-axis spectroscopy. The results disclose unambiguously the theoretically predicted avoided crossing of the rattler modes and the acoustic-phonon branches. The observed phonon lifetimes are longer than expected, and a new explanation for the low kappa(L) is provided.     

Study of critical buckling loads and modes of cross-ply laminated annular plates

Buckling of composite annular plates under uniform internal and external radial edge loads have been investigated using energy method. Trefftez rule is used in the stability equations. The symmetric buckling of symmetric cross-ply laminates is considered. In this paper, buckling behavior for the three laminates (90/0)_2s, (90/0₂/90)_s and (90₂/0₂)_s are studied. Influence of some parameters such as thickness, stacking sequence, type of supports and the ratio of hole to sheet radius on buckling loads and modes are investigated. The results of the energy method are compared with the results of numerical method. Based on the results, in the plates with clamped boundary conditions the symmetric buckling assumption is not accurate, contrary to other boundary conditions.     

Size effect on buckling strength of eccentrically compressed column with fixed or propagating transverse crack

The strength and size effect of a slender eccentrically compressed column with a transverse pre-existing traction-free edge crack or notch is analyzed. Rice and Levy’s spring model is applied to simulate the effect of a crack or notch. An approximate, though accurate, formula is proposed for the buckling strength of the column of variable size. Depending on the eccentricity, the crack at maximum load can be fully opened, partially opened or closed. The size effects in these three situations are shown to be different. The exponent of the power-law for the large-size asymptotic behavior can be −1/2 or −1/4, depending on the relative eccentricity of the compression load. Whether the maximum load occurs at initiation of fracture growth, or only after a certain stable crack extension, is found to depend not only on the column geometry but also on its size. This means that the definition of positive or negative structural geometry (as a geometry for which the energy release rate at constant load increases or decreases with the crack length) cannot be extended to stability problems or geometrically nonlinear behavior. Comparison is made with a previous simplified solution by Okamura and coworkers. The analytical results show good agreement with the available experimental data.     

Initial buckling of curved panels of generally layered composite materials

An initial buckling analysis for cylindrically curved panels made of generally layered composite materials is presented. Four kinds of boundary conditions and the combination of axial compression and shear forces are considered. Two coupled, fourth-order partial differential equations are solved by the use of multiple Fourier series, in which more exact constants within the characteristic beam functions are introduced so that better orthogonality of the series and, therefore, more exact buckling loads are obtained. The influence of curvature, fibre angles, stacking sequence and panel aspect ratios is investigated. An interesting relationship between the critical axial load and shear forces is found for mid-plane symmetric panels. Comparison of present work with experimental results shows fairly good agreement.     

功能梯度材料薄球壳热屈曲问题

研究了由金属和陶瓷组成的功能梯度材料薄球壳热屈曲问题。用张量方法推导得到轴对称球壳稳定性方程。将热本构方程应用到球壳稳定性方程中,得到以位移表示的球壳热屈曲方程组。分别考虑均布外压和温度作用,采用伽辽金法计算分析简支球壳的热屈曲问题,给出薄球壳厚度、物性参数变化、内外表面温差变化引起的临界温度变化趋势和临界压力变化趋势。     

Topology optimization of periodic microstructures with a penalization of highly localized buckling modes

The problem of determining highly localized buckling modes in perfectly periodic cellular microstructures of infinite extent is addressed. A double scale asymptotic technique is applied to the linearized stability problem for a periodic structure built from linearly elastic microstructures. The obtained stability condition for the microscale level is then used to establish a comparative analysis between different material distributions in the base cell subjected to the same strain field at the macroscale level. The idea is illustrated by some two‐dimensional finite element examples and used to design materials with optimal elastic properties that are less prone to localized instability in the form of local buckling modes at the scale of the microstructure. Copyright © 2002 John Wiley & Sons, Ltd.     

Simulation of the elastic response and the buckling modes of single-walled carbon nanotubes

The mechanical behavior of carbon nanotube (CNT) is one of the basic problems on the nanotube composite and nano machinery. Molecular dynamics is an effective way of investigating the behavior of nano structures. The compression deformation of single-walled carbon nanotubes (SWCNTs) is simulated, using the Tersoff–Brenner potential to describe the interactions of atoms in CNT. From the MD simulation for some SWCNTs whose diameters range from 0.5 nm to 1.7 nm and length ranges from 7 nm to 19 nm, respectively, we get the Young’s modulus from 1.25 TPa to 1.48 TPa. The Young’s modulus of CNT decreases as the radius of CNT increases. The Young’s modulus of zigzag CNT is higher than that of armchair CNT. The results also show that there are two different buckling modes for SWCNTs. The difference between the buckling behavior in macroscopic scale and that in nano scale is studied.     

Geometries and materials for subwavelength surface plasmon modes

Plasmonic waveguides can guide light along metal-dielectric interfaces with propagating wave vectors of greater magnitude than are available in free space and hence with propagating wavelengths shorter than those in vacuum. This is a necessary, rather than sufficient, condition for subwavelength confinement of the optical mode. By use of the reflection pole method, the two-dimensional modal solutions for single planar waveguides as well as adjacent waveguide systems are solved. We demonstrate that, to achieve subwavelength pitches, a metal-insulator-metal geometry is required with higher confinement factors and smaller spatial extent than conventional insulator-metal-insulator structures. The resulting trade-off between propagation and confinement for surface plasmons is discussed, and optimization by materials selection is described.     

左手材料薄板波导中的传导模

推导了左手材料薄板波导中传导模的色散方程,分析了波导中存在传导模与波导材料和包敷材料参数取值范围之间的关系.数值算例证实了这些关系.     

An acoustic-emission characterization of the failure modes in polymer-composite materials

A new methodology for the analysis of failure modes in composite materials by means of acoustic emission techniques has been developed. A single-carbon-fiber composite based on a polyester matrix, has been used as a simple model. The occurrence of fiber-breakage during tensile loading tests has been observed by a polarized light microscope and concurrently detected by a resonant acoustic probe. The resonant probe has been used as a trigger for the reading of fiber failure events. Single acoustic emission events from a wide-band probe has been recorded for FFT Analysis. The single-fiber specimen, having a unique failure mode, has advantages for the standardization of AE techniques for the quantitative analysis of failures in polymer-composite materials.

The same procedure can be exploited to investigate other failure modes namely, fiber matrix solidus debonding and matrix cracking.     

The use of initial imperfection approach in design process and buckling failure evaluation of axially compressed composite cylindrical shells

Thin-walled cylindrical shells are susceptible to buckling failures caused by the axial compressive loading. During the design process or the buckling failure evaluation of axially-compressed cylindrical shells, initial geometric and loading imperfections are of important parameters for the analyses. Therefore, the engineers/designers are expected to well understand the physical behaviours of shell buckling to prevent unexpected serious failure in structures. In particular, it is widely reported that no efficient guidelines for modelling imperfections in composite structures are available. Knowledge obtained from the relevant works is open for updates and highly sought. In this work, we study the influence of imperfections on the critical buckling of axially compressed cylindrical shells for different geometries and composite materials (Glass Fibre Reinforced Polymer (GFRP), Carbon Fibre Reinforced Polymer (CFRP)) and aluminium using the finite element (FE) analysis. Two different imperfection techniques called eigenmode-affine method and single perturbation load approach (SPLA) were adopted. Validations of the present results with the published experimental data were presented. The use of the SPLA for introducing an imperfection in axially compressed composite cylindrical shells seemed to be desirable in a preliminary design process and an investigation of a buckling failure. The knockdown factors produced by the SPLA were becoming attractive to account for uncertainties in the structure.     

Temperature measurements of expansion products from shock compressed materials using high-speed spectroscopy

Results from spectral radiance measurements using optical multi-channel analyzer over the visible and near infrared regime provide estimates of temperature from expansion products resulting from shocked materials. Specifically, we have made spectral radiance measurements over the wavelength regime of 300–1500 nm. Experiments conducted on aluminum, cerium, and Composition-B high explosive span a wide regime of E/E_v, where E is the internal energy increase of the shocked material, and E_v, is the specific energy required to vaporize the material. For the materials investigated, the ratio is ∼1, 3 and 5 for aluminum, cerium, and Composition-B, respectively. The basic assumption made to deduce these temperatures is that the debris cloud is radiating as a blackbody with emissivity of one and independent of the wavelength. We are also assuming that the probe is monitoring the debris, which is at a single temperature and that there is no spatial temperature gradient. Temperatures at or above the boiling point are confirmed for aluminum and cerium, while the results for Composition-B provide the time-dependent temperature expansion history for shocked Composition-B over the stress regime of 28–130 GPa. These are the first measurements of temperature obtained from the expansion products from materials that have been shocked to very high pressures.