Effects of material properties on the fragmentation of brittle materials

We present a fundamental investigation of the influence of material and structural parameters on the mechanics of fragmentation of brittle materials. First, we conduct a theoretical analysis (similar to Drugan’s single wave problem, Drugan, W.J. (2001), Journal of Mechanical and Physics Solids 49, 1181–1208.) and obtain closed form solutions for a problem coupling stress wave propagation and single cohesive crack growth. Expressions for a characteristic fragment size s ₀ and a characteristic strain-rate are given. Next, we use a numerical approach to analyze a realistic fragmentation process that involves multiple crack interactions. The average fragment size s is calculated for a wide variety of strain-rates and a broad range of material parameters. Finally, we derive an empirical function that relates the normalized fragment size s/s ₀ to the normalized strain-rate and that fits all of the numerical results with a single master curve.     

乌头碱电喷雾质谱裂解机制的研究

为研究乌头碱的质谱裂解机制,采用蠕动泵进样,在正离子模式下,获得乌头碱的一级和二级质谱。通过QTRAP-MSn检测发现乌头碱的21种裂解碎片,分析乌头碱的3类裂解途径,首次在二级质谱中发现丢失C6H5COOH离子。不同裂解机制特征离子碎片的发现及结构初步鉴定,为乌头碱在较低浓度下的快速定性和定量鉴别提供依据。     

Fracture and fragmentation of thin shells using the combined finite–discrete element method

A model for fracture and fragmentation of multilayered thin shells has been developed and implemented into the combined finite–discrete element code. The proposed model incorporates an extension of the original combined single and smeared fracture approach to multilayered thin shells; it then combines these with an interaction algorithm that is based on the original distributed potential contact force approach. The developed contact kinematics preserves both energy and momentum balance, whereas the developed fracture model is capable of modelling complex fracture patterns such as fracture of laminated glass under impact. Copyright © 2013 John Wiley & Sons, Ltd.     

Scaling of energy dissipation in crushing and fragmentation: a fractal and statistical analysis based on particle size distribution

An extensive experimental investigation on concrete specimens under crushing and fragmentation over a large scale range (1:10) – exploring even very small specimen dimensions (1 cm) – was carried out to evaluate the influence of fragment size distribution on energy density dissipation and related size effect. To obtain a statistically significant fragment production as well as the total energy dissipated in a given specimen, the experimental procedure was unusually carried out up to a strain of approximately –95%, practically corresponding to the initial fragment compaction between the loading platens. The experimental fragment analysis suggests a fractal law for the distribution in particle size; this simply means that fragments derived from a given specimen appear geometrically self-similar at each observation scale. In addition, clear size effects on dissipated energy density are experimentally observed. Fractal concepts permit to quantify the correlation between fragment size distribution and size effect on dissipated energy density, the latter being governed by the total surface area of produced fragments. The experimental results agree with the proposed multi-scale interpretation satisfactorily.     

基于弹塑性剪滞理论的单丝复合材料段裂过程的蒙特卡罗模拟

单丝复合材料段裂试验(SFCFT)中,随着外载荷的增加,纤维出现了随机脆断的现象,并在一定的载荷下纤维的段裂数达到饱和状态(即纤维段裂数目不再增加),该试验常用于表征纤维与基体间界面性能。针对该试验,本文中充分考虑了组分材料的真实性能(即基体材料的弹塑性性能),利用弹塑性剪滞理论进行纤维与基体间的应力传递分析,初步获得较真实的纤维轴向应力及界面剪应力分布形式;在此基础上,考虑纤维强度分布的非均匀性,利用蒙特卡罗(Monte Carlo)方法对试验中纤维的随机段裂过程进行了模拟预报,获得载荷与纤维的段裂数的关系。模拟预报与试验结果比较吻合,表明该应力分析及模拟方法的有效性。     

水下爆炸载荷作用下加肋圆柱壳应变响应研究

采用加肋圆柱壳结构模型模拟潜艇结构,对其在水下爆炸冲击载荷作用下的响应过程进行试验研究。结果表明:采用应变测量的方法对结构进行模态分析是可行的;受附连水质量的影响,加肋圆柱壳结构的低阶湿模态频率较干模态频率显著下降;在爆炸载荷作用下,距离爆炸点越近结构承受的冲击作用越严重,越容易产生塑性破坏;应变的最大响应是冲击波载荷和一次脉动压力载荷共同作用的结果,第二次脉动压力对结构的动态应变响应贡献不明显。     

Stone fragmentation by ultrasound

The presence of kidney stone in the kidney causes discomfort to patients. Hence, removal of such stones is important which is commonly done these days, non-destructively, with lithotripters without surgery. Commercially, lithotripters like extra-corporeal shock wave lithotripters (ESWL) made by Siemens etc are in routine use. These methods are very cumbersome and expensive. Treatment of the patients also takes comparatively more time because of more number of sittings. Some delicate nerves and fibres in the surrounding areas of the stones present in the kidney are also damaged by high ultrasonic intensity used in such systems. In the present work, enhancement of the kidney stone fragmentation by using ultrasound is studied. The cavitation bubbles are found to implode faster, with more disintegration efficiency of the lithotripters, which give better treatment to the patients.     

Relationship between fiber flaw spectra and the fragmentation process: A computer simulation investigation

The single-filament-composite (SFC) fragmentation test can be utilized to provide quantitative information on the fiber strength distribution and the fiber/matrix interface shear stress, which are important properties that control the performance of fiber composites. An accurate interpretation of the fragmentation data, however, is difficult owing to the stochastic nature of the fragmentation process, as well as the complex interplay between the fiber flaw strength variation and the stress transfer zones about every broken flaw. In this work we have developed a computer simulation approach that models the fragmentation process by explicitly incorporating considerations regarding the strength and spatial distributions of the flaws. The effect of stress variation along the fiber length is accounted for by incorporating specific fiber-loading models. From the simulation it is demonstrated that the fragmentation data may be used to produce a rough sketch of the underlying flaw strength spectrum. An examination of the fragment size statistics suggests that appropriate analysis of the SFC data may be utilized to detect the occurrence of matrix/fiber interface yielding or debonding. An alternative methodology for mapping out the flaw strength distribution by means of a multiple-long-fiber failure test is also presented for comparison purposes.     

水下爆炸冲击波的数值模拟与试验研究

采用ANSYS/LS-DYNA有限元分析软件,对小当量TNT炸药在无限水域内爆炸进行数值模拟,并将数值计算结果与实验室内实测数据以及传统的经验公式计算结果进行对比。分析发现:用传统的经验公式计算小当量TNT炸药在水下爆炸时的爆炸冲击波压力峰值,与实测值相比存在较大误差;运用有限元显式动力分析方法计算得到的水下爆炸冲击波压力峰值与实测值相吻合,并且水下爆炸冲击波波形相近。对数值计算值进行拟合得到水下爆炸冲击波压力峰值的计算公式,可以对同等条件下的水下爆炸试验进行估算。     

A functional for shells of arbitrary geometry and a mixed finite element method for parabolic and circular cylindrical shells

In this study a higher‐order shell theory is proposed for arbitrary shell geometries which allows the cross‐section to rotate with respect to the middle surface and to warp into a non‐planar surface. This new kinematic assumption satisfies the shear‐free surface boundary condition (BC) automatically. A new internal force expression is obtained based on this kinematic assumption. A new functional for arbitrary shell geometries is obtained employing Gâteaux differential method. During this variational process the BC is constructed and introduced to the functional in a systematic way. Two different mixed elements PRSH52 and CRSH52 are derived for parabolic and circular cylindrical shells, respectively, using the new functional. The element does not suffer from shear locking. The excellent performance of the new elements is verified by applying the method to some test problems. Copyright © 2000 John Wiley & Sons, Ltd.     

A three‐dimensional atomistic‐based process zone model simulation of fragmentation in polycrystalline solids

A three‐dimensional atomistic‐based process zone model (APZM) is used to simulate high‐speed impact induced dynamic fracture process such as fragmentation and spall fracture. This multiscale simulation model combines the Cauchy–Born rule, colloidal crystal process model, and micromechanics homogenization technique to construct constitutive relations in both grains and grain boundary at mesoscale. The proposed APZM has some inherent advantages to describe mechanical behaviors of polycrystalline solids. First, in contrast to macroscale phenomenological constitutive models, the APZM takes into account atomistic binding energy and atomistic lattice structure. In particular, the electron density related embedded atom method (EAM) potential has been adopted to describe interatomistic interactions of metallic polycrystalline solids in bulk elements; second, a mixed type of EAM potential and colloidal crystal depletion potential is constructed to describe heterogeneous microstructure in the process zone; third, the atomistic potential in both bulk material and process zone has the same atomistic origin, and hence, the bulk and process potentials are self‐consistent. The simulation of dynamic fracture process of a cylinder made of aluminum powder metallurgy (P/M) alloy during high‐speed impact/penetration is carried out, and numerical results demonstrate that APZM finite element method has remarkable ability to accurately capture complex three‐dimensional fragmentation formation and damage morphology. Copyright © 2012 John Wiley & Sons, Ltd.     

Computational p-element method on the effects of thickness and length on self-weight buckling of thin cylindrical shells via various shell theories

This paper is concerned with the development of a global p-element method for the analysis of self-weight buckling of thin cylindrical shells. Such buckling problems occur when cylindrical shells are subject to high-g acceleration, for instance the launching of rockets and missiles under high propulsive power. The cylindrical shells may have any combination of free, simply supported and clamped ends. A p-element computational method has been developed based on various thin shell theories including Donnell, Sanders and Goldenveizer-Novozhilov models. The strain energy for the global element during buckling is formulated and an eigenvalue equation is derived. Unlike the conventional buckling problem where the eigenvalue is directly solved, a pre-determined buckling parameter is fixed at the outset for a geometric-dependent stiffness and a recursive numerical procedure is developed to compute the effect of critical buckling length. The critical buckling length is found to be proportional to thickness to a power of approximately 0.9. The effects of shell thickness and length on buckling parameter are also investigated. Comparison of results from various shell theories indicates solutions of the Sanders and Goldenveizer-Novozhilov shell theories are in excellent agreement while the Donnel shell theory is good for buckling of short cylindrical shells.The work described in this paper was fully supported by grants from City University of Hong Kong [Project Nos. 7001186 (BC) and 7100256 (BC)].     

岩体中传爆点振效应破岩机理探讨

岩体爆破机理研究工作开展的十分艰难,现有的主要理论均不能全面彻底地解释岩体爆破中的现象、描述岩体的破碎过程。而几乎所有的理论都是从岩体出发来展开研究,忽略了对炸药作用的研究。总结前人在岩体爆破破碎机理研究上的学说、论断、论点,结合爆破实践,以炸药作用为出发点,提出传爆点振效应破岩机理,且按照作用源划为三类破岩模式。并尝试用该理论来解释岩体爆破中的各种现象,更加有效、充分、客观地揭示岩体爆破破碎机理的物理实质。     

层合薄壁圆柱壳1:1内共振研究

针对一端固定,一端自由的层合薄壁圆柱壳模型,根据Donnell’s非线性简化壳理论建立其非线性振动方程。采用Galerkin方法对非线性振动方程进行离散化,应用平均法对系统包含两个相邻轴向模态的非线性振动响应进行了解析分析,与数值模拟进行了比较,并得到了不同参数对层合薄壁圆柱壳复杂的振动响应的影响。结果表明,1）由于所选的两个相邻轴向模态频率相距较近,能量在两个模态之间相互传递,系统存在1:1内共振现象;2）系统复杂的振动响应受激振力大小和非线性项的影响比较大,而对于阻尼不敏感。     

Numerical simulation of the fiber fragmentation process in single-fiber composites

This paper attempts to simulate the process of fiber break in single-fiber composite fragmentation test (SFCF). The ensued stress redistribution in fiber, matrix and interface after the fiber break is also researched. A new simulation method based on the user subroutine: “User subroutine to redefine field variables at a material point (USDFLD)” in the general finite element method (FEM) software ABAQUS is proposed. The subroutine is used for the definition of the fiber material constitutive model and is programmed in FORTRAN. It is called by ABAQUS. So the damage mode of the fiber break is simulated by the method. The forms of stress redistribution in fiber, matrix and interface are also obtained in the simulation. Then the T300/epoxy single-fiber composite is fabricated and the single-fiber composite fragmentation test is done. The simulation method and results are proved to be appropriate by the comparative analysis with the experiment.     

岩体中传爆点振效应破岩机理探讨

岩体爆破机理研究工作开展的十分艰难,现有的主要理论均不能全面彻底地解释岩体爆破中的现象、描述岩体的破碎过程。而几乎所有的理论都是从岩体出发来展开研究,忽略了对炸药作用的研究。总结前人在岩体爆破破碎机理研究上的学说、论断、论点,结合爆破实践,以炸药作用为出发点,提出传爆点振效应破岩机理,且按照作用源划为三类破岩模式。并尝试用该理论来解释岩体爆破中的各种现象,更加有效、充分、客观地揭示岩体爆破破碎机理的物理实质。     

变形程度对曲面扁壳类汽车覆盖件刚度的影响研究

为进一步研究变形程度对曲面扁壳类覆盖件刚度的影响规律,并为提高和改善汽车覆盖件的刚度提供理论依据,建立了能够代表汽车覆盖件曲面特点的柱面扁壳、双曲扁壳件和球面扁壳试验模型.通过分析零件胀拉成形过程特点,得出零件变形程度与成形件特征之间的关系:板厚及成形件的回弹对刚度的影响较为显著,板厚越大、回弹越小,板材的刚度越好;由胀拉成形深度增加而引起的回弹量减少及变形程度增加而导致的试件成形均匀性的提高对刚度提高的贡献大于厚度减薄的影响.     

Inverse formulation for geometrically exact stress resultant shells

The inverse elastostatic method deals with a class of problems in which a deformed configuration of an elastic body is known while the initial stress‐free configuration or the stress in the deformed state is to be determined. The method is imperative for certain problems in engineering applications. Computational methods of inverse elastostatics have been established for elastic continua. In this paper, we present an inverse method for thin‐wall structures modeled as geometrically exact stress resultant shells. The theoretical basis and the details of implementation are discussed. Numerical examples involving both isotopic and anisotropic materials are presented. The practical utility of the method is demonstrated using an example of human aneurysm stress analysis. Copyright © 2007 John Wiley & Sons, Ltd.     

Impact resistance and fragmentation behaviour of glassceramics

Impact tests were carried out on five glassceramics tiles. The ballistic resistance was evaluated by the depth of penetration of the projectile into the backing. The fragmentation behaviour of the glassceramics showed that the glassceramics compared with with alumina and displayed some advantages over it.     

内部爆炸加载条件下钢管变形与破碎研究

用高速摄影研究了钢管膨胀历程,并对破碎机制进行了分析,结果表明,材料动态塑性是决定破片最终速度的主要因素,圆管膨胀时外表面形成以剪切为主的裂纹,并向内扩展,接近内表面的绝热剪切带伴随着圆管的膨胀而扩展,并未出现失稳现象,但在正应力的作用下成为优先的断裂通道,决定破片的尺寸和形态,中碳Si-Mn贝氏体钢与50SiMnVB钢的破碎性能相近,但贝氏体钢动态塑料性很更好,破碎的膨胀量较大,更有利于提高破片初速,是弹比较理想的候选材料。