潜孔锤破岩系统动力学特性研究

为深入研究潜孔锤破岩过程中推进力、激振频率等对潜孔锤破岩系统特性的影响,基于弹簧、阻尼、滑动器等元件建立了考虑潜孔锤与岩石局部接触、完全接触状态的动力学模型。分别建立不同状态下系统的动力学方程,并基于阶跃函数建立破岩系统的连续无量纲微分方程组,采用龙格-库塔法求解获得不同状态下的潜孔锤钻头位移图、速度图、相空间图以及庞加莱截面图。结果表明:破岩系统的运动状态随推进力改变而改变,当系统处于周期-1状态时,系统相对较稳定且破岩效率最佳;随着激振力频率增大,系统稳定的区间范围随之增加,但破岩效率反而降低。破岩系统动力学特性研究结果为潜孔锤在实际工作中的参数确定提供理论依据。     

Dynamic crushing strength of hexagonal honeycombs

Based on the repeatable collapsing mechanism of cells’ structure under dynamic crushing, an analytical formula of the dynamic crushing strength of regular hexagonal honeycombs is derived in terms of impact velocity and cell walls’ thickness ratio. It is consistent with the equation obtained from the shock wave theory that regards cellular material as continuum, in which the key parameter is approximately measured from the “stress–strain” curve of the cellular material. The effect of unequal thickness of cell walls on the honeycomb's dynamic crushing strength is discussed, and the result shows that the dynamic crushing strength of the hexagonal honeycomb with some double-thickness walls is about 1.3 times of that of the hexagonal honeycomb without double-thickness wall. All of the analytical predictions are compared with the numerical simulation results, showing good agreements.     

Dynamic energy absorption characteristics of hollow microlattice structures

Hollow microlattice structures are promising candidates for advanced energy absorption and their characteristics under dynamic crushing are explored. The energy absorption can be significantly enhanced by inertial stabilization, shock wave effect and strain rate hardening effect. In this paper we combine theoretical analysis and comprehensive finite element method simulation to decouple the three effects, and then obtain a simple model to predict the overall dynamic effects of hollow microlattice structures. Inertial stabilization originates from the suppression of sudden crushing of the microlattice and its contribution scales with the crushing speed, v. Shock wave effect comes from the discontinuity across the plastic shock wave front during dynamic loading and its contribution scales with v². The strain rate effect increases the effective yield strength upon dynamic deformation and increases the energy absorption density. A mechanism map is established that illustrates the dominance of these three dynamic effects at a range of crushing speeds. Compared with quasi-static loading, the energy absorption capacity at dynamic loading of 250 m/s can be enhanced by an order of magnitude. The study may shed useful insight on designing and optimizing the energy absorption performance of hollow microlattice structures under various dynamic loads.     

Dynamic axial crushing of circular tubes

A series of axial crushing tests on steel circular cylindrical shells loaded either statically or dynamically is reported and compared with various theoretical predictions and empirical relations. A modified version of Alexander's theoretical analysis for axisymmetric, or concertina, deformations gives good agreement with the experimental results when the effective crushing distance is considered and provided that the influence of material strain rate sensitivity is retained in the dynamic crushing case.     

The influence of cell micro-topology on the in-plane dynamic crushing of honeycombs

The influence of the cell micro-topology on the in-plane dynamic crushing of honeycombs is studied by means of explicit dynamic finite element simulation using ANSYS/LS-DYNA. Firstly, under the assumption that the edge length and thickness are the same, the dynamic properties of the honeycombs filled by cells with different shapes (equilateral triangular or quadratic cells) and micro-arrangements (regular or staggered arrangement) are numerically analyzed. The full-scale in-plane dynamic crushing of the specimen, as well as the micro-structure transformation during the deformation, is discussed. Based on these, the influence of the cell micro-arrangement on the energy absorption ability of the honeycombs is clarified. The results show that owing to the differences in the micro-topology, triangular or quadratic honeycombs display different local deformation properties during the crushing. The variation of the cell arrangement patterns changes the local dynamic evolution characteristic of stress waves. ‘>’ and ‘<’ mode local deformation bands form at the sides of the stagger-arranged honeycombs, which results in lateral compression shrinkage during the crushing. The plateau stresses also increase with the impact velocity by a square law. The empirical equations for honeycombs filled with different cells (equilateral triangular or quadratic cells) and micro-arrangements (regular or staggered arrangement) at high impact velocities are formulated in terms of impact velocity, and the cell geometrical (edge length and thickness) and topology (edge connectivity) parameters.     

Experimental investigation into dynamic axial impact responses of double hat shaped CFRP tubes

This paper aims to explore the dynamic responses and crashing characteristics of double hat shaped tubes made of weave carbon fiber reinforced plastic (CFRP). Experimental investigations were carried out into three different thicknesses and lengths of the composite tubes fabricated by the bladder molding process. Three distinct failure modes, classified as progressive end crushing, mid-length collapse and overlap opening, were observed in the dynamic crushing tests. Unlike continuous splaying fronds observed in the quasi-static tests, dynamic tests exhibited a number of fragment segments in the progressive end crushing mode. It is shown that the ply number was a critical parameter affecting the failure mode and energy absorption capability. The increase in ply number led to increases in the peak load and specific energy absorption (SEA); whereas the tubal length seemed insensitive to energy absorption capability. Compared to the quasi-static cases, the dynamic impact tests resulted in the higher peak load (increased from 46 % to 125 %) and lower SEA (reduced from 21 % to 33 %) for the tested tubes.     

Dynamic response of a multilayer prismatic structure to impulsive loads incident from water

The dynamic crush response of a low relative density, multilayered corrugated core is investigated by combining insights from experiments and 3D finite element simulations. The test structures have been fabricated from 304 stainless steel corrugations with 0°/90° lay-up orientation and bonded by means of a transient liquid phase method. Characterization of the dynamic crushing of these structures has revealed that at low rates, interlayer interactions induce a buckling-dominated soft response. This softness is diminished at high rates by inertial stabilization and the response of the structure transitions to yield-dominated behavior. Unidirectional dynamic crushing experiments conducted using a dynamic test facility reveal a soft response, consistent with lower rate crushing mechanisms. The 3D simulation predictions of crushing strain, pulse amplitude/duration and impulse delivery rate correspond closely with the measurements. The application of core homogenization schemes has revealed that by calibrating with a multilayer unit cell, high fidelity continuum level predictions are possible. Moreover, even simplified hardening curves based on equivalent energy absorption provide remarkably accurate predictions of the crush strains and the impulse transmitted through the core. The multilayered structures investigated here significantly reduced the transmitted pressures of an impulsive load.     

循环荷载频率和幅值对有砟道床动力行为的影响

为研究有砟道床在长期反复列车荷载作用下的动态响应及劣化规律,利用离散元分析软件PFC并考虑道砟的真实外形,建立可考虑道砟破碎劣化的有砟道床离散元计算模型,研究分析了不同频率、不同幅值循环荷载作用下有砟道床的振动响应、道砟受力以及道砟摩擦耗能、破碎行为。结果表明:道砟的振动加速度随动荷载频率和幅值的提高而非线性增长;道砟所受的接触力主要受荷载幅值影响,受荷载频率影响较小,荷载幅值提高,道砟接触力增大;提高荷载频率和幅值均会增加道砟的摩擦耗能,在荷载作用初期,道砟的摩擦耗能有一个突增过程;动荷载的幅值对道砟的破碎行为占主导因素,荷载幅值较低时道砟稳定在较低破碎水平,荷载幅值较高时道砟破碎现象加剧并受荷载频率影响,此时荷载频率越高道砟破碎越严重。     

Crush dynamics of square honeycomb sandwich cores

Square honeycombs are effective as cores for all‐metal sandwich plates in that they combine excellent crushing strength and energy absorption with good stiffness and strength in out‐of‐plane shear and in‐plane stretch. In applications where sandwich plates must absorb significant energy in crushing under intense impulsive loads, dynamic effects play a significant role in the behaviour of the core. Three distinct dynamic effects can be identified: (i) inertial resistance, (ii) inertial stabilization of webs against buckling, and (iii) material strain‐rate dependence. Each contributes to dynamic strengthening of the core. These effects are illustrated and quantified with the aid of detailed numerical calculations for rates of deformation characteristic of shock loads in air and water. A continuum model for high rate deformation of square honeycomb cores is introduced that can be used to simulate core behaviour in large structural calculations when it is not feasible to mesh the detailed core geometry. The performance of the continuum model is demonstrated for crushing deformations. Copyright © 2005 John Wiley & Sons, Ltd.     

Numerical Study on Hybrid Tubes Subjected to Static and Dynamic Loading

The commercial finite element program LS-DYNA was employed to evaluate the response and energy absorbing capacity of cylindrical metal tubes that are externally wrapped with composite. The numerical simulation elucidated the crushing behaviors of these tubes under both quasi-static compression and axial dynamic impact loading. The effects of composite wall thickness, loading conditions and fiber ply orientation were examined. The stress–strain curves under different strain rates were used to determine the dynamic impact of strain rate effects on the metal. The results were compared with those of a simplified analytical model and the mean crushing force thus predicted agreed closely with the numerical simulations. The numerical results demonstrate that a wrapped composite can be utilized effectively to enhance the crushing characteristics and energy absorbing capacity of the tubes. Increasing the thickness of the composite increases the mean force and the specific energy absorption under both static and dynamic crushing. The ply pattern affects the energy absorption capacity and the failure mode of the metal tube and the composite material property is also significant in determining energy absorption efficiency.     

CRUSHING BEHAVIOUR OF ALUMINIUM HONEYCOMBS UNDER IMPACT LOADING

Understanding the crushing behaviour of honeycombs under dynamic loading is useful for crash simulations of vehicles and for design of impacting energy absorbers. Available experimental techniques, however, are not always able to provide satisfactory precision for tests on honeycombs under impact loading. This paper presents a new application of the Split Hopkinson Pressure Bar (SHPB) for testing honeycombs. Viscoelastic bars are used to improve the accuracy of measurements, and a generalised two-strain measurement method is applied to obtain a sufficient measurable maximum crush (up to 80%). Original experimental data (especially in the in-plane crushing directions) under impact loading are then reported. Differences between quasi-static and dynamic results are discussed.     

组合蜂窝材料面内冲击性能的研究

基于三角形和六边形蜂窝结构面内冲击性能的研究,该文探讨了面内冲击荷载作用下组合Kagome蜂窝结构的变形机制和能量吸收特性。首先,在保证蜂窝结构胞元厚度与边长尺寸比值不变的前提下,分析了不同形状胞元及其组合结构的动态冲击性能,给出了试件宏观及微观胞元结构的动态演化过程。在此基础上,探讨了冲击速度和相对密度一定情况下单位质量不同蜂窝结构的能量吸收特性。其结论将对蜂窝材料微拓扑结构的动力学优化设计提供指导。     

Gradient design of metal hollow sphere (MHS) foams with density gradients

Metal hollow sphere (MHS) structures with a density gradient have attracted increasing attention in the effort to pursue improved energy absorption properties. In this paper, dynamic crushing of MHS structures of different gradients are discussed, with the gradients being received by stacks of hollow spheres of the same external diameter but different wall thicknesses in the crushing direction. Based on the dynamic performance of MHS structures with uniform density, a crude semi-empirical model is developed for the design of MHS structures in terms of gradient selections for energy absorption and protection against impact. Following this, dynamic responses of density graded MHS foams are comparatively analyzed using explicit finite element simulation and the proposed formula. Results show that the simple semi-empirical model can predict the response of density gradient MHS foams and is ready-to-use in the gradient design of MHS structures.     

Experimental study of dynamic crushing of thin plates stiffened by stamping with V-grooves

Experimental study was made for the dynamic crushing of thin plates stiffened by stamping with 1–3 parallel V-grooves in a clamped-end condition. The length of the blanks was set at the same value of 300 mm, while the width was of 70, 140 and 210 mm at a thickness of 1 mm, otherwise specified. The depth of V-grooves was set at values of 4, 7 and 10 mm. In both static and dynamic tests, the results show that the ability of the plates to resist crushing and to absorb deformation energy was substantially enhanced by stamping with V-grooves. Stamped with 1 or 2 grooves of smaller values in depth, the plates 210 mm wide had larger resistance to dynamic crushing than those of greater values in depth did. In general, increases in number and depth of grooves caused an increase in the plates’ capacity of energy absorption. However, the ability of the plates to resist crushing and to absorb deformation energy cannot be determined only in accordance with the second moment of area of cross section, because the plates sufficiently wide stamped with any number of grooves deformed plastically before buckled in dynamic tests and, the interaction between deep grooves during deformation also enhanced the capacity of energy absorption.     

Modeling loading rate effect on crushing stress of metallic cellular materials

A mesoscale numerical model based on Voronoi tessellation is developed to investigate the loading rate effect on the crushing stress of cellular materials. The crushing stresses at both the impact and stationary ends of the Voronoi structures are simulated. The influences of the impact velocity, specimen size, inertia, and rate dependence of the base material on the crushing stress are discussed. The underlining reason for the argument on the rate dependence of cellular materials is clarified by comparing the current simulation results with the numerical results based on a continuum model and a shock wave theory. The conflicting observations from previous experimental studies on the dynamic behavior of cellular materials by different researchers are explained by the simulation results as well as the shock wave theory.     

Dynamic crushing of honeycombs and features of shock fronts

The in-plane dynamic crushing of 2D hexagonal-cell honeycombs has been simulated using finite elements to explore the dynamic response of cellular materials and to investigate the features of the crushing front and to examine the assumptions employed in a one-dimensional shock theory [Reid SR, Peng C. Dynamic uniaxial crushing of wood. Int J Impact Eng 1997;19:531–70; Tan PJ, Reid SR, Harrigan JJ, Zou Z, Li S. Dynamic compressive strength properties of aluminium foams. Part II – shock theory and comparison with experimental data and numerical models. J Mech Phys Solids 2005;53:2206–30]. It has been demonstrated that progressive cell crushing is observed to propagate through the material in a ‘shock’ like manner when the crushing velocity exceeds a critical value. The simulations show that there exists a zone at the shock front across which there are essentially discontinuities in the material ‘particle velocity’, ‘stress’ and ‘strain’ as defined herein. At supercritical crushing velocities the thickness of this zone remains about one cell size, which varies little with the crushing velocity and the relative density. Densification strain increases as crushing velocity increases and asymptotes to a limit once a shock front forms. It has also been shown that the one-dimensional shock theory [Reid SR, Peng C. Dynamic uniaxial crushing of wood. Int J Impact Eng 1997;19:531–70; Tan PJ, Reid SR, Harrigan JJ, Zou Z, Li S. Dynamic compressive strength properties of aluminium foams. Part II – shock theory and comparison with experimental data and numerical models. J Mech Phys Solids 2005;53:2206–30], which was based on an equivalent rigid-perfectly plastic-locking stress–strain curve, tends to overestimate slightly the crushing stress and energy absorbed.     

VRB变厚板典型结构件性能对比试验

目的研究VRB与等厚板成形件在吸能特性和抗弯性方面的差异。方法通过准静态压溃、动态压溃及三点弯曲试验,获得并对比分析VRB变厚板的帽型梁典型结构件性能。结果变厚帽型梁的特殊结构在进行压溃变形时能起到诱导槽的作用;VRB帽型梁相比同质量的等厚板帽型梁,降低了峰值力,且吸能效果更优;随着帽型梁厚度(质量)的增加,冲击速度的提高,峰值力升高,吸收的能量增大;变厚板帽型梁的过渡区位置、过渡区长度、厚度分布等结构参数对抗弯承载性能有较大的影响。结论变厚板与等质量的等厚板成形件相比,有更好的结构性能特性。     

密度对聚丙烯泡沫材料动态冲击性能的影响

目的 以聚丙烯材料为研究对象,研究密度对其动态冲击性能的影响。方法 对4种不同密度的聚丙烯材料进行动态压缩试验,分析接触力、位移和应变以及吸收能、比吸能的变化,从而研究动态条件下不同密度的聚丙烯材料的动态冲击性能。结果 当密度一定时,最大接触力、最大位移、最大应变、比吸能随冲击能的增大而增大。当冲击能一定时,最大接触力随密度的增大而增大,最大位移、最大应变、比吸能随密度的增大而减小。结论 考虑到轻量化以及成本因素,在安全范围内,选择缓冲材料时可以选择密度较小的聚丙烯材料。     

六韧带手性蜂窝结构的动力学响应特性研究

利用显式动力有限元ANSYS/LS-DYNA数值研究了六韧带手性蜂窝结构的面内冲击动力学特性。在保证圆环节点半径不变的前提下,通过改变韧带长度和胞元厚度,首先建立了六韧带手性蜂窝的有限元模型,具体讨论了冲击速度和胞元微结构参数对手性蜂窝材料的面内宏/微观变形行为、密实应变、动态平台应力和比能量吸收能力的影响。研究结果表明,随着冲击速度的增加,六韧带手性蜂窝结构表现为3种宏观变形模态：“> <”型模式、“过渡”模式和“I”型模式。在中、低速冲击载荷下,能够明显观察到拉胀材料在轴向压缩时独特的“颈缩”现象,其主要与韧带绕着圆环中心节点的旋转变形有关。通过引入无量纲“动态敏感因子”,还研究了六韧带手性蜂窝材料的面内动态冲击强化效应。     

动静组合加载下不同负温人工冻结粉质黏土强度和变形特性分析

研究动静组合加载下不同负温冻土的强度和变形特性对提高冻土开挖破碎效率、确保冻土工程稳定具有重要的理论和工程意义.借助改进的分离式Hopkinson压杆试验系统,研究了不同负温下人工冻结粉质黏土的静态和动态应力-应变曲线、抗压强度、变形模量和破坏特征.试验结果表明:单轴动态压缩条件下,随着试验温度的降低,人工冻结粉质黏土...