Numerical investigations on the mechanical properties of metallic hollow spheres structure with perforations under compression

This paper numerically investigates the compressive mechanical properties of the perforated hollow spheres structures with different geometrical and physical properties. In these structures, the metallic hollow spheres are perforated regularly with several holes, which open the inner sphere volume and surface and are bonded in simple cubic, body-centered cubic and face-centred cubic patterns. The 5×5 cells finite element models under uniaxial compression are established by ABAQUS 6.14 software for simulation. The influence of the spheres’ spatial pattern, as well as base materials for the spheres and bonding necks on the structural mechanical properties are evaluated. By changing the wall thickness, hole diameter and bonding radius in the body-centered cubic packing finite element model, the elastic modulus, Poisson's ratio and initial yield stress are calculated and discussed as functions of these geometrical parameters and their average densities.     

金属空心微球的制备及应用现状

金属空心微球不仅具有一般金属空心球的结构和性能等特点，而且还具有小尺寸的独特优势，可望在微电子、微系统和生命科学等诸多前沿领域发挥重大的作用。介绍了金属空心微球的制备和应用现状，并对存在的问题进行了的讨论。     

Finite element modelling of creep deformation in fibre-reinforced ceramic composites

The tensile creep and creep-recovery behaviour of a unidirectional SiC fibre-Si₃N₄ matrix composite was analysed using finite element techniques. The analysis, based on the elastic and creep properties of each constituent, considered the influence of fibre-matrix bonding and processing-related residual stresses on creep and creep-recovery behaviour. Both two- and three-dimensional finite element models were used. Although both analyses predicted similar overall creep rates, three-dimensional stress analysis was required to obtain detailed information about the stress state in the vicinity of the fibre-matrix interface. The results of the analysis indicate that the tensile radial stress, which develops in the vicinity of the fibre-matrix interface after processing, rapidly decreases during the initial stages of creep. Both the predicted and experimental results for the composite show that 50% of the total creep strain which accumulated after 200 h at a stress of 200 MPa and temperature of 1200°C is recovered within 25 h of unloading.     

Finite element modelling of interfacial failure in model carbon fibre-epoxy composites

Finite element analysis has been used to model a single unsized carbon fibre embedded in an epoxy matrix subjected to tensile loading. The predicted fibre strain distribution is compared with experimental data, obtained using the technique of laser Raman spectroscopy, for a number of incremental applied strain levels. Good correlation is obtained on the assumption that the prevailing mode of interfacial failure in the composite involves a conical matrix crack initiating at the fibre end. The geometry of the matrix crack is estimated on the assumption that the crack propagates in a self-similar manner.     

Finite element modelling of saturated porous media at finite strains under dynamic conditions with compressible constituents

Two finite element formulations are proposed to analyse the dynamic conditions of saturated porous media at large strains with compressible solid and fluid constituents. Unlike similar works published in the literature, the proposed formulations are based on a recently proposed hyperelastic framework in which the compressibility of the solid and fluid constituents is fully taken into account when geometrical non‐linear effects are relevant on both micro‐ and macroscales. The first formulation leads to a three‐field finite element method (FEM), which is suitable for analysing high‐frequency dynamic problems, whereas the second is a simplification of the first, leading to a two‐field FEM, in which some inertial effects of the pore fluid are disregarded, hence the second formulation is suitable for studying low‐frequency problems. A fully Lagrangian approach is considered, hence all terms are expressed with reference to the material setting; the balance equations for the pore fluid are also expressed in terms of the chemical potential and the mass flux of the pore fluid in order to take the compressibility of the fluid into account. To improve the numerical response in the case of wave propagation, a discontinuous Galerkin FEM in the time domain is applied to the three‐field formulation. The results are compared with analytical and semi‐analytical solutions, highlighting the different effects of the discontinuous Galerkin method on the longitudinal waves of the first and second kind. Copyright © 2010 John Wiley & Sons, Ltd.     

Finite element modelling of helmeted head impact under frontal loading

Finite element models of the head and helmet were used to study contact forces during frontal impact of the head with a rigid surface. The finite element model of the head consists of skin, skull, cerebro-spinal fluid (CSF), brain, tentorium and falx. The finite element model of the helmet consists of shell and foam liner. The foam is taken as elasto-plastic, the brain is assumed to be viscoelastic and all other components are taken as elastic. The contact forces and coup pressures with helmet on the head are much lower than in the absence of the helmet. A parametric study was performed to investigate the effect of liner thickness and density on the contact forces, pressures and energy absorption during impact. For 4 ms⁻¹ velocity, expanded poly styrene (EPS) foam of density 24 kgm⁻³ gave the lowest contact forces and for the velocities considered, thickness of the foam did not affect the contact forces.     

Nosing of thin-walled hollow spheres using a die: experimental and theoretical investigation

In contrast to end forming of tubes, there is no published work that addresses the manufacturing of thin-walled hollow spheres by nosing using a die. Important characteristics of the process such as the development of plastic instability modes (local buckling), thickening of the tube-wall and occurrence of wrinkling needs to be properly studied. This paper presents a numerical and experimental investigation of the nosing of thin-walled hollow spheres using a die with the purpose of examining the process mechanics, obtaining a better understanding of the modes of deformation and establishing the formability limits in terms of the major process parameters. The paper also presents a new concept of nosing thin-walled hollow spheres that makes use of preforming stages and tube-end preparation schemes in order to successfully extend the formability limits of the process. Theoretical investigation and process development are supported by numerical predictions based on the finite element flow formulation and the overall methodology is assessed by means of experimental tests on industrial AA6060 Aluminium alloy tubes (natural aged) under laboratory-controlled conditions.     

Finite element modelling of wooden structures at large deformations and brittle failure prediction

This paper presents a constitutive wood model that accounts for both hardening associated with material densification at large compressive deformations and brittle failure modes. The model is adapted from previous work by the authors and has been modified to deal with wood behaviours. The main novelty of the model is the coupling between the anisotropic plasticity and the ductile densification. The model developed is successfully implemented in the commercial ABAQUS software. Validation was made for uniaxial compressive loadings and an application on a three-points bending test. The results obtained, for the uniaxial compressive loadings, demonstrate the capability of the model to simulate the wood behaviour at large compressive deformations and show clearly the effect of the densification on the plastic behaviour. The result obtained for the three-points bending test shows a good implementation of the brittle failure criterion and demonstrates the suitability of the developed model to analyse and design wooden structures.     

Finite element computational modelling and experimental investigation of perforated NiTi plates under tension

This paper explores the pseudoelastic deformation behaviour of perforated near-equiatomic NiTi plates by means of finite element modelling and tensile experimentation. The numerical modelling is based on an elastohysteresis model, which takes into account the hysteretic and hyperelastic contributions of material response in the global deformation. The effects of hole size, shape and number on stress–strain behaviour are discussed. The numerical results are compared and validated with experimental data.     

Finite element modelling of the progressive crushing of braided composite tubes under axial impact

Composite tubular structures are of interest as viable energy absorbing components in vehicular front rail structures to improve crashworthiness. Desirable tools in designing such structures are models capable of simulating damage growth in composite materials. Our model (CODAM for COmposite DAMage), which is a continuum damage mechanics based model for composite materials with physically based inputs, has shown promise in predicting damage evolution and failure in composites. In this study, the model is used to simulate the damage propagation, failure morphology and energy absorption in triaxially braided composite tubes under axial compression. The model parameters are based on results from standard and specialized material testing and a crack band scaling law is used to minimize mesh sensitivity (or lack of objectivity) of the numerical results. Axial crushing of two-ply and four-ply square tubes with and without the presence of an external plug initiator are simulated in LS-DYNA. Refinements over previous attempts by the authors include the addition of a pre-defined debris wedge, a distinguishing feature in tubes displaying a splaying mode of failure, and representation of delamination using a tiebreak contact interface that allows energy absorption through the un-tying process. It is shown that the model adequately predicts the failure characteristics and energy absorption of the crushing events. Using numerical simulations, the process of damage progression is investigated in detail and energy absorptions in different damage mechanisms are presented quantitatively.     

Deformation localisation modelling of polymer foam microstructure under compression: A new approach by discrete element modelling

A discrete element model (DEM) has been developed to represent the behaviour of the microscopic structure of polymer cellular material, consisting of closed-cells. In DEM, the polymer foam is represented as an assembly of particles which model the closed-cells. The behaviour of the particle is based on the Gibson model and depends on morphologic and mechanical parameters. The present numerical study demonstrates the effect of deformation localisation on the microstructure. It is noted that the cell morphologic parameters and the distribution of various size cells in the specimen have a significant influence of the local deformation. The effect of macroscopic faults is also studied.     

Finite element modelling of crack propagation in elastic-plastic media

Materials which are cyclically stressed by sliding indenters often undergo fatigue wear, as surface breaking vertical cracks and subsurface horizontal cracks propagate causing eventual loss of material. In this study, the authors model crack propagation in an elastic-plastic material using finite element techniques, and consider the influence of friction, elasticity, plasticity and degree of penetration on the J-integral at the tip of a vertical crack. Crack propagation directions are estimated using J-integral maxima as the determining variable. It is found that the J-integral values, as a measure of strain energy release rate, can be used to estimate the crack propagation angle. Its main advantage lies in the fact that it considers both modes (I, II) of crack propagation. Using the J-integral values, one finds that, in the absence of friction between the indenter and the material, the vertical crack is equally prone to propagation at both 45 and 135° angles. However, one notices that the vertical crack favours the direction opposite to the direction of rolling for non-zero values of friction, i.e. 135°. The effects of both the crack depth and the crack tip plasticity are also investigated. It is found that any experimental findings suggestive of crack orientations closer to the horizontal in the direction opposite to the sliding direction are probably a result of shallow vertical asperities or higher crack tip plasticity.     

A review of the fatigue failure mechanism of metallic materials under a corroded environment

Assessment of the fatigue strength of pipeline steels is essential considering that the components are subjected to cycle loads in service. This paper presents a fatigue life assessment review of failure pipeline steels. Failure or deterioration of pipelines takes place by corrosion and fatigue, which later leads to rupture. Stress-life, strain-life, and linear elastic fracture mechanics crack propagation method has shown to be well accepted as a benchmark model of fatigue assessment. The relation curves are based on different cases of individual characterised fatigue properties. Other methods like probability and statistical-based assessment are employed to provide reliable results in the assessment of fatigue strength. This method deals with scatter data resulting from variations in sample parameters. It shows that choosing an appropriate and accurate method is important; particularly for quantifying the extent to which the fatigue life is reduced. Good predictions subsequently offer successful designs of pipelines and therefore, any unwanted damage can then be avoided.     

From CdS aggregate spheres to PbS hollow spheres: a case study of the growth mechanism in chemical conversion

Monodisperse PbS hollow spheres were successfully prepared via using CdS aggregate spheres as template. The present strategy is based on the different solubilities of CdS and PbS. This process was intensively studied by time-dependent trails which were monitored by transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), and photoluminescence spectroscopy (PL). Reaction temperature was found to play an important role in controlling the diffusion rate of Pb²⁺ ions and the quality of as-prepared PbS crystals, which finally leads to different shape evolution processes from the starting aggregate spheres to the final hollow spheres. Two growth mechanisms defined as kinetics-controlled process (KCP) and thermodynamics-controlled process (TDCP) were, respectively, proposed for the two conversion patterns observed at 30 and 90 °C. Moreover, specific structural evolution including primary crystal size, diameter growth, and shell thickness were also discussed in detail. This work is of great significance in elucidating the underlying mechanism of chemical conversion and could be potentially applied to synthesize other hollow architectures.     

Modeling and characterization of dynamic failure of borosilicate glass under compression/shear loading

In this paper, we study the impact-induced dynamic failure of a borosilicate glass block using an integrated experimental/analytical approach. Previous experimental studies on dynamic failure of borosilicate glass have been reported by Nie et al. [Nie X, Chen WW, Sun X, Templeton DW. Dynamic failure of borosilicate glass under compression/shear loading – experiments. J Am Ceram Soc, in press.] using the split Hopkinson pressure bar (SHPB) technique. The damage growth patterns and stress histories have been reported for various glass specimen designs. In this study, we propose to use a continuum damage mechanics (CDM)-based constitutive model to describe the initial failure and subsequent stiffness reduction of glass. Explicit finite element analyses are used to simulate the glass specimen impact event. A maximum shear stress-based damage evolution law is used in describing the glass damage process under combined compression/shear loading. The impact test results are used in quantifying the critical shear stress for the borosilicate glass under examination. It is shown that with only two modeling parameters, reasonably good comparisons between the predicted and the experimentally measured failure maps can be obtained for various glass sample geometries. Comparisons between the predicted stress histories for different sample designs are also used as model validations.     

Finite element modelling of closed cracks in eddy current testing

The eddy current inspection of small fatigue cracks in Ti–6AL–4V is evaluated in both a finite element model and experiments. The crack was created in a fatigue process and an eddy current measurement was carried out as the resulting crack was subjected to different levels of static load. The signal showed a strong dependency of the time between the creation of the fatigue crack and the eddy current measurement. This dependency is proposed to be related to oxides forming on the crack faces. The oxide is favourable for the detection of fatigue cracks. The narrow width of the fatigue crack is important to consider in eddy current inspection and as static loads are applied across the crack faces, electrical connections arise within the crack, which has a strong influence on the eddy current signal. Four different models of the contact behaviour were implemented within the finite element model. It is shown that the electrical connections that arise within small fatigue cracks, as well as the influence from the narrow opening as tensile loads are applied, can be predicted by a finite element model of the eddy current method.     

Study on energy properties and failure behaviors of heat-treated granite under static and dynamic compression

Abstract

The material testing machine and the split Hopkinson pressure bar (SHPB) were adopted, respectively, to conduct the static and dynamic compression tests on granite specimens heat treated by different temperatures. The effects of strain rate and heat-treatment temperature on the mechanism of energy evolution of the specimen during deformation and failure process were studied. The results show a significant strain rate effect on the granite, with the energy dissipation density increasing with increasing impact velocity (or strain rate), regardless of the treatment temperature. The specimens heat treated at 300?°C and 700?°C have the minimum and maximum energy dissipation densities, respectively. The specimen in the SHPB tests easily broke into pieces or even powder; while under static compression, only macroscopic fracture surfaces and spalling phenomenon on the specimen were detected. The energy dissipation density is inversely proportional to the compressive strength of the specimen. The rate of energy dissipation change is defined, which can be used to identify the stages in the deformation process of rock and to determine the position of the failure point in the stress-strain curve. For both the dynamic and static compression tests, the value of energy utilization ratio is relatively low, with a maximum value of about 35%.     

CFRP薄壁C型柱轴向压缩破坏机制及吸能特性

为研究碳纤维增强树脂基复合材料（CFRP）薄壁C型柱轴向压缩破坏机制及吸能特性,制备了4种铺层方式、3种厚度组合共12种T700/MTM28 CFRP薄壁C型柱试件。考察C型柱低速轴向压缩过程中的失效模式及载荷变化,通过比较初始峰值载荷、平均压缩载荷、比吸能和载荷效率,分析铺层数及铺层角度对C型柱失效模式及吸能特性的影响。结果表明,纯0°铺层C型柱在轴压载荷作用下发生整体失稳,不具备实际意义上的能量吸收作用;0°/90°铺层、±45°铺层、45°/90°/-45°/0°铺层试件均发生了渐进式破坏,呈现出局部屈曲叠缩的失效模式。其中,45°/90°/-45°/0°铺层的C型柱比吸能随铺层数的增加而增加,具有更大的吸能设计与应用潜力。