Ultrafast high repetition rate absorption spectroscopy of polymer shock compression

A dual-beam transient absorption spectrometer for high repetition rate (80 shocks per second) studies of shock compressed materials is described. The apparatus time response is 100 ps, so the time resolution of the shock compression process is generally limited by the shock transit time across the sample. In turn the sample thickness is limited by the sensitivity of the spectrometer. Using 400 nm thick samples of R640 dye aggregates in \textit{poly} methyl methacrylate (PMMA) and a 4.2 GPa laser-driven shock, transient absorption spectra show a shock induced absorption redshift occurring in 500 ps, considerably longer than the 200 ps shock front transit time (round trip) through the sample. This noninstantaneous shock compression is consistent with the $\sim 300$ ps viscoelastic response of PMMA at 4.2 GPa. Received 30 July 2001 / Accepted 13 March 2002 – Published online 17 June 2002     

Damage initiation and growth in metals. Comparison between modelling and tomography experiments

Damage in heterogeneous model materials was measured using high-resolution X-ray absorption tomography. The material consisted of an aluminium matrix containing 1% and 4% of spherical ceramic particles acting as nucleation sites for an interface decohesion mechanism of damage. The damage initiation stage was quantified using the global population of particles in the 4% material. A strain path change experiment was then applied to the 1% material. The sample was first deformed in tension in order to create elongated cavities and then compressed at 45^° to rotate and close these cavities. The results of a model based on the Rice and Tracey approach accounting for the presence of particles inside the cavities and calculating their rotation with assuming a linear hardening plastic behaviour of the matrix were compared with the observations. The model was modified to account for the damage initiation phase. It was shown to give a good global prediction of the void volume fraction provided that the physical, mechanical and morphological information are corresponding in the experimental and the model cases. The cavity rotation experiment was also shown to compare well with the calculation although only one cavity was sufficiently opened after compression to allow the comparison.     

Prediction scheme for the catastrophic failure of highly loaded brittle materials or rocks

Due to broadly distributed heterogeneities, the multistep character of the development of random damage in brittle materials or rocks, reflecting the discrete nature of the fracturing process, enables us to compare the diffusion of damages with the percolation phenomenon. Based on a discrete scale hierarchy, the fracturing process leading to material failure is then identified with a second-order phase transition, enabling prediction. In this connection, this paper presents a prediction scheme for the catastrophic failure or the lifetime of highly loaded materials or rocks. An experimental measurement method and a modelling of bi-phase materials or rocks are proposed. Although this scheme is introduced in the context of materials science, it also works in rock for the prediction of large natural earthquakes and some artificially induced earthquakes and rock bursts due to damming and mining. Results with strong forecasting potential explain for the first time the origin of complex critical exponents through a structural parameter.     

复合泡沫塑料力学行为的研究综述

复合泡沫塑料是一种重要的防护材料,它在国防工业和民用工业各部门均有许多重要的应用,对这类材料的力学行为进行研究具有重要的学术价值和应用前景.本文对复合泡沫塑料力学行为的研究文献进行了综述.首先,对复合泡沫塑料力学行为研究的早期工作进行了简介.然后,重点介绍了复合泡沫塑料力学行为研究的最新进展,其中也包括作者近期在该领域开展的一些工作;对复合泡沫塑料进行了静、动态压缩实验和细观加载实验,研究了材料的宏观变形规律和细观失效机制;在理论研究方面,探讨了复合泡沫塑料的能量吸收和缓冲特性,从宏、细观力学分析出发研究了复合泡沫塑料有关力学性能的理论预测问题;还利用计算机和通用软件对高密度复合泡沫塑料进行了有限元分析,研究了高密度复合泡沫塑料的失效行为.最后,给出对该领域研究工作的一些展望.      

A directional crack path criterion for crack growth in ductile materials subjected to shear and compressive loading under plane strain conditions

A directional crack growth criterion in a compressed elastic perfectly plastic material is considered. The conditions at the crack-tip are evaluated for a straight stationary crack with a small incipient kink. Remote load is a combined hydrostatic pressure and pure shear applied via a boundary layer. Crack surfaces in contact are assumed to develop homogenous Coulomb friction.The crack opening displacement of an extended kink is examined in a finite element analysis to judge the risk of opening mode failure. It has been found that the direction that maximizes the crack opening displacement of an extended kink tip coincides very well with a prediction of the crack growth direction obtained by using a criterion for continued crack growth direction discussed by the authors elsewhere [Int. J. Fract. 108 (2001) 351].Moreover, the by the model predicted incipient crack growth directions are qualitatively comparable with reported crack paths obtained in ductile materials in a limited number of experiments performed under a combined load of in-plane shear and compression.     

泡沫材料的应变率效应

对泡沫材料的应变率敏感性进行了系统深入的讨论,认定这种材料是应变率敏感材料,这种敏感性主要是由于泡孔的变形特性产生的。泡沫材料变形的局部化、微观惯性和致密性导致压垮应力明显提高,基体的应变率效应及泡孔的形状大小并不能对泡沫材料应变率敏感性起主导作用。     

Ultrasonic investigation of interpenetrating networks of poly(methyl methacrylate) and polyurethane

Interpenetrating networks are the most recent development in polymeric blend materials. Due to the crosslinking of both the continuous and dispersed phases, a high degree of molecular mixing is achieved in these materials. Notwith-standing that poly(methyl methacrylate)-polyurethane (PMMA-PUR) interpene-trating and semi-interpenetrating networks have been extensively investigated by Meyer et al., ultrasonic relaxation technique has been applied here for the first time. These materials were found to be highly ultrasound absorbing.It is observed that ultrasonic absorption has a peak at a particular composition of PMMA-PUR interpenetrating network. The absorption coefficient increases with frequencyf. The absorption is of relaxational nature and is not due to the scattering of ultrasonic waves by the domains of the dispersed phase. At every composition of the interpenetrating network, the/f ² vs.f curve indicates the presence of a relaxation frequency below 2 MHz and that the absorption increases with the temperature at some compositions which indicates the presence of thermal relaxation. An attempt is made to relate the absorption with the relaxation of pendent groups of polyurethane in the continuous phase.     

Measurement of the Permeability Tensor of Compressed Fibre Beds

A new method to measure the permeability tensor of highly compressed fibre beds is developed. The method is based on saturated parallel flow and is evaluated through experiments with various textile materials: press fabrics used in papermaking and fibre reinforcements designed for composites. Since the materials are in the form of sheets, two measuring cells are used, one for the principal in-plane permeabilities and the other for the out-of-plane permeability. A unique feature is that the edge and the bulk flow are measured separately, so that any influence from enhanced or suppressed edge flow may be eliminated. The technique is evaluated with good results in terms of scatter in the measured permeability and the influence of test geometry, pressure, and liquid properties.     

定向凝固多晶铁磁形状记忆合金力-温度耦合的实验研究

铁磁形状记忆合金兼具大输出应变与高响应频率等综合特性,是新一代驱动与传感材料。采用定向凝固技术制成的多晶铁磁形状记忆合金具有较多优越的力学性能。本文对温度和应力耦合作用下的定向凝固多晶铁磁形状记忆合金的力学特性进行了实验测试,分别获得了定向凝固多晶Ni-Mn-Ga试样在不同恒定温度时的应力-应变循环曲线,以及试样在压缩时两个互相垂直方向的数字散斑图。结果表明:同一恒定温度时,随着应力循环次数的增加,其应变值逐渐减小;同一压缩应力时,不同温度作用过程中定向凝固方向的应力,随着温度的升高逐渐减小。本文结果可为铁磁形状记忆合金在工程中的应用提供一定的指导作用。     

Ambiguity of the critical load for spherical shells with shear damageability of the material

The structural-probabilistic approach to the modeling of combined crack formation and material deformation processes is used to develop a technique for solving bifurcation stability problems for thin-walled structural members made of damageable materials under single and repeated loadings. The example of a uniformly compressed spherical shell is used to show that, under repeated loading, thin-walled structural members made of shear damageable materials can lose stability under loads smaller than the upper critical loads. The ambiguity of the critical loads for various damage accumulation processes in the material of thin-walled structures depends on the level and character of loading. This phenomenon can be one possible cause of the experimental data spread and the discrepancy between theoretical and experimental results used to determine the critical loads for spherical and cylindrical shells.     

Fracture and stability of materials and structural members with cracks: approaches and results

A new alternative approach to fracture problems for materials and structural elements with cracks is set out. It is based on the mechanism of local instability near defects. The approach is used to study the fracture of materials compressed along interacting cracks and the fracture of thin structural members with cracks under tension with allowance for local buckling.__________Translated from Prikladnaya Mekhanika, Vol. 40, No. 12, pp. 18–64, December 2004.     

Determination of the critical loads of shells by nondestructive methods

Two methods for determining the location of and load level to produce instability of compressed cylindrical shells are presented. The first relates the variation in the wall normal stiffness as a function of applied compressive force to the critical load. It uses the distribution of stiffness over the surface of the shell as a guide to buckle location. The second method associates the local dynamic mass with instability behavior. The test data presented show that either method will give excellent prediction capability from low-load-level data for shells of orthodox form. Neither method appears to apply to spirally stiffened shells. This is thought to be due to the fact that there is a substantial difference between the buckle pattern under axial compression and the imperfection shape induced by the normal displacement which is used to ascertain the wall stiffness and the dynamic mass.     

An analysis of the process of active enhancement of falling film absorption

 We study here the process whereby active enhancement intensifies absorption rates over vertical flat plates. Towards capturing the essential aspects of this process, we employ a model already developed, introducing new heat and mass transfer coefficients. It is shown that when the enhancement is done by a vibrating screen, it is possible to estimate the absorption rates employing simple parameters such as the plate peak velocity, vibrational frequency and amplitude. The predictions obtained are compared to recent experimental data. Although the prediction with this simple model is not exact, the essential features of the enhancement technique seem to be reflected quite well. Our calculations allow identification of the factor possibly limiting absorption. Estimates of the ultimate absorption rates possible with the limiting factor removed are offered, along with speculative thought as to whether this limiting factor can indeed be neutralized by mechanical agitation. Received on 25 April 2000     

软基体混合胞孔材料的力学性能及抗多次冲击性能

为了探索具有优异吸能性能的软基体混合胞孔材料的力学性能,研究该类材料在多次冲击下的冲击响应和材料的可恢复性,对一种软基体混合胞孔材料—人工软骨仿生超材料（artificial cartilage foam,ACF）进行了不同速度下的单轴拉伸和压缩实验,得到了ACF材料在不同应变率条件下的应力-应变曲线。并利用落锤冲击实验机对ACF材料和另一种软基体混合胞孔材料—发泡聚丙烯材料（expanded polypropylene,EPP）进行了多次冲击下的对比测试,得到了2种材料在单次和多次冲击下的动力学响应。实验结果表明:ACF材料是一种应变率敏感的材料,随着应变率的提升,材料的弹性模量、抗拉强度和抗压强度均逐渐提高;在50 J 冲击能量作用下,ACF材料能够吸收96%以上的冲击能量,远高于EPP材料的70%,ACF材料具有更加优异的吸能性能;5次冲击后ACF材料的最大峰值力、最大变形量和吸能能力几乎不变。相比于EPP材料,ACF材料有良好的可恢复性,且具有稳定的多次抗冲击能力。这些研究为软基体混合胞孔材料在多次冲击防护中的应用提供了实验依据。     

Energy absorption of an egg-box material

Conical frustra made from leaded gun-metal have been compressed axially. Collapse is either by a travelling plastic hinge or by tearing. An analytical model is developed for the travelling plastic hinge in a rigid, ideally plastic solid; its predictions are compared with the observed response, and with those of an axisymmetric finite element analysis. The travelling hinge mechanism is also observed in the compressive collapse of an egg-box material comprising a square array of conical frustra. Collapse mechanism maps are constructed for the egg-box material, and they show the regimes of dominance of elastic buckling, material tearing and the travelling plastic hinge. The maps are useful for selecting egg-box geometries that maximise the energy absorption per unit mass at any prescribed value of collapse stress. The optimisation indicates that the egg-box material has a similar energy absorption capacity to that of hexagonal honeycombs and is superior to that of metal foams.     

A fractal study of sound propagation characteristics in roughened porous materials

The influence of rough elements of porous materials on sound propagation characteristics in roughened porous materials is investigated. Rough surface topography on the roughened porous materials is simulated by the fractal geometry theory, in which relative roughness is defined clearly as a function of the fractal dimension, porosity, average diameter and diameter ratio of the hexagonal elements on pore wall of the porous materials. Based on the sound propagation model of the smooth porous materials, a sound propagation model of the roughened porous materials is built. The effective density and bulk modulus, acoustic impedance and propagation constant, flow resistivity and sound absorption coefficient of the roughened porous materials are derived and discussed as a function of the relative roughness. It is demonstrated that the sound absorption coefficient of the roughened porous materials is improved as the relative roughness increases. The model predictions for the sound absorption coefficient of the roughened porous materials are well agreed with that of the existing test results.     

多孔材料吸能行为对相对密度和冲击速度的依赖性

多孔材料是一种优异的吸能缓冲材料,但由于其变形模式的非单一性以及动态应力应变曲线的难获取性,其吸能行为对相对密度和冲击速度的依赖性关系还并不完全明朗。本文基于不需要提前作本构假定的波传播法,开展了多孔材料的吸能行为研究。采用多孔材料的细观有限元模型进行Taylor冲击虚拟实验,获取全场质点速度时程曲线,结合Lagrange分析法得到多孔材料的局部应力应变信息,进而探讨了动态吸能性能对材料相对密度和冲击速度的依赖性。研究结果表明多孔材料的吸能行为可依据变形模式分为三个阶段。在冲击模式下,多孔材料单位体积吸能与相对密度成线性增加关系,此时惯性起主导作用;在过渡模式下,惯性的主导作用减弱,单位体积吸能量的增加速率随相对密度的增加而减弱;在准静态模式下,多孔材料只能发生微小的变形,其吸能很少。本文进一步获得了区别于多孔材料准静态应力-应变曲线的动态应力-应变状态曲线,并考察了其与相对密度之间的关系。结果表明：随着相对密度的增加,多孔材料的动态压实应变将变小,而动态塑性平台应力将提高。     

Instability of an elastic film on a viscous layer

A flat, compressed elastic film on a viscous layer is unstable. The film can form wrinkles to reduce the elastic energy. In this paper, we are interested in the two-dimensional models for thin films bonded to a viscous layer and in particular we focus on generic instabilities evidenced in this context by Suo and coworkers [Huang, Z., Hong, W., Suo, Z., 2005. Non linear analyses of wrinkles in a film bonded to a compliant substrate. J. Mech. Phys. Solids 53, 2101–2118; Lo, Y.H., 1991. New approach to grow pseudomorphic structures over the critical thickness. Appl. Phys. Lett. 59, 2311–2320]. We present a rigorous linear perturbation analysis for anisotropic materials, that allows the prediction of both the orientation of the corrugations of the thin film, and the wavelength that maximize the growth velocity. Finally, we compare our theoretical estimates to experimental results for a In_0.65Ga_0.35As alloy constraint to InP.     

刹车条件对铁基粉末冶金材料摩擦磨损性能的影响

采用粉末冶金法制备某型号飞机用铁基刹车材料,研究了刹车条件对铁基刹车材料摩擦磨损性能的影响,利用光学显微镜观察铁基刹车材料表面及其磨损表面形貌,利用X射线衍射仪分析其组织的变化情况.结果表明:随着刹车条件(单位面积吸收能量)的增加,铁基刹车材料的摩擦系数减小,磨损量增大,摩擦表面温度升高;磨损前铁基刹车材料由粗大的细片层珠光体和块状铁素体组成,随着吸收能量的增加,细片层珠光体较使用前明显增多和细化;当刹车条件从正常着陆刹车变为超载着陆刹车时,其摩擦表面Fe的高价氧化物比例增大.     

Stability of a Strip Made of a Multilayer Composite with a Curved Structure

The stability of a long plate (strip) made of a layered composite with a curved structure is studied within the framework of the three-dimensional linearized theory of stability. The cases of simply supported and rigidly fixed plates compressed along the layers are considered. The curvature of the layers is allowed for within the framework of the continuum approach. Numerical results are obtained by the finite-element method for a strip consisting of alternating layers of two homogeneous isotropic materials