固体的统计细观力学--连接多个耦合的时空尺度

从固体力学所面临的新的挑战--多物理、多尺度耦合及其现状的描述开始,以层裂过程为例,说明了这些多尺度非平衡问题的基本困难在于,在固体中不同尺度上有不同的微结构层次及不同的演化物理和速率.接下来,概述了一些针对这一困难的独特的思路及其成果.第3部分强调了一些统计平均方法的范式,以及处理包含多个时间和空间尺度的问题的新思路,特别是非平衡损伤演化导致宏观失效的问题.在第4部分,简要评述了一些连接多个空间和时间尺度的细观力学框架,如位错理论,物理细观力学,Weibull理论,随机理论等,并且阐述了其中蕴含的跨尺度

Catastrophic Rupture Induced Damage Coalescence in Heterogeneous Brittle Media

In heterogeneous brittle media, the evolution of damage is strongly influenced by the multiscale coupling effect. To better understand this effect, we perform a detailed investigation of the damage evolution, with particular attention focused on the catastrophe transition. We use an adaptive multiscale finite-element model (MFEM) to simulate the damage evolution and the catastrophic failure of heterogeneous brittle media. Both plane stress and plane strain cases are investigated for a heterogeneous medium whose initial shear strength follows the Weibull distribution. Damage is induced through the application of the Coulomb failure criterion to each element, and the element mesh is refined where the failure criterion is met. We found that as damage accumulates, there is a stronger and stronger nonlinear increase in stress and the stress redistribution distance. The coupling of the dynamic stress redistribution and the heterogeneity at different scales result in an inverse cascade of damage cluster size, which represents rapid coalescence of damage at the catastrophe transition.

A Nonlinear Threshold Model Applied to Spallation Analysis

Spallation in heterogeneous media is a complex, dynamic process. Generally speaking, the spallation process is relevant to multiple scales and the diversity and coupling of physics at different scales present two fundamental difficulties for spallation modeling and simulation. More importantly, these difficulties can be greatly enhanced by the disordered heterogeneity on multi-scales. In this paper, a driven nonlinear threshold model for damage evolution in heterogeneous materials is presented and a trans-scale formulation of damage evolution is obtained. The damage evolution in spallation is analyzed with the formulation. Scaling of the formulation reveals that some dimensionless numbers govern the whole process of deformation and damage evolution. The effects of heterogeneity in terms of Weibull modulus on damage evolution in spallation process are also investigated.

Directly synthesized strong, highly conducting, transparent single-walled carbon nanotube films

We report the direct synthesis of strong, highly conducting, and transparent single-walled carbon nanotube (SWNT) films. Systematically, tests reveal that the directly synthesized films have superior electrical and mechanical properties compared with the films made from a solution-based filtration process: the electrical conductivity is over 2000 S/cm and the strength can reach 360 MPa. These values are both enhanced by more than 1 order. We attribute these intriguing properties to the good and long interbundle connections. Moreover, by the help of an extrapolated Weibull theory, we verify the feasibility of reducing the interbundle slip by utilizing the long-range intertube friction and estimate the ultimate strength of macroscale SWNTs without binding agent.

Enfoque de los Modelos de Ruptura Dieléctrica desde la Termodinámica Estadística

Resumen: El modelado del proceso de ruptura dieléctrica de aislantes sometidos a un campo eléctrico intenso es importante para el desarrollo de nuevos materiales en las industrias eléctrica y electrónica. Información experimental generada en el laboratorio muestra que el proceso de ruptura dieléctrica involucra la transferencia de energía con la consecuente acumulación progresiva de carga y daño en el material. Ésta, produce finalmente la ruptura macroscópica, creando un camino conductor que inutiliza al material aislante. A partir de experimentos de simulación se han calculado los parámetros característicos (dos y tres parámetros) de la función de distribución de fallas (distribución de Weibull), para distintas familias de árboles. También se ha estudiado la relación intrínseca entre las distintas familias de árboles, mediante una distribución generalizada de Weibull con un índice entrópico q. Este índice q es representativo de la estructura fractal de los distintos árboles.

Miner累积损伤理论的试验验证和统计分析

本文通过对LY12-CZ铝合金板材光滑试件和中心孔试件的常幅加载和变幅加载疲劳试验,对Miner累积损伤理论进行了试验验证,并对累积周比的分散性作了统计分析,发现累积周比的分散性服从Weibull分布,因此本文提出以最小累积周比值或以与一定存活率相应的累积周比值进行安全寿命估计的观点。

加卸载响应比与损伤变量关系研究

首先介绍加卸载响应比的2个基本参量,并从损伤力学的基本理论出发,引入损伤变量,结合加卸载响应比方法的思想,推导用损伤和应变定义的加卸载响应比,并以Weibull分布作为随机分布函数,建立且分析损伤变量与加卸载响应比之间的联系,并进一步考察Weibull指数对两者关系的影响.然后利用岩石破坏声发射实验的数据,用声发射数密度描述岩石试件的损伤演化过程,并由此根据损伤和应变定义的加卸载响应比,结合实验过程中的损伤和应变,计算花岗岩试件的加卸载响应比曲线,并与用Benioff应变作响应量计算的加卸载响应比曲线进行比较,发现两者具有相当程度的一致性,两者的加卸载响应比值都经历了出现异常→升高到最大值→急剧回落→岩石试件破裂或失稳,这一规律与实际地震震例中加卸载响应比的演化趋势是一致的,同时也说明在实际地震预测中用Benioff应变作为响应量来计算加卸载响应比是合理的.最后介绍意大利那不勒斯大学完成的二层楼房的加卸载实验,且对实验结果进行分析.建立的加卸载响应比与损伤变量的关系,不仅为加卸载响应比用来定量研究脆性介质的损伤程度提供更为坚实的基础,而且也可能为评估大型建筑的损伤和预测工程事故开辟新道路.

一种岩石概率型本构及其应用

在岩石材料非均匀特性的基础上,建立了弹塑性概率材料本构.认为材料单元的切线模量与破坏强度都是服从二参数Weibull分布的随机量,通过用户材料子程序将该本构导入到LS-DYNA3D中,推导了程序流程,分析了导入过程中的关键技术.算例验证表明,应用该本构模拟得出的现象与实验观测完全一致,这是确定性本构无法实现的.

Effects of Microstructural Heterogeneity on The Spallation Behavior of Materials

It is of utmost importance to understand the spallation behaviour of heterogeneous materials. In this paper, a driven nonlinear threshold model with stress fluctuation is presented to study the effects of microstructural heterogeneity on continuum damage evolution. The spallation behavior of heterogeneity material is analyzed with this model. The heterogeniety of mesoscopic units is characterized in terms of Weibull modulus m of strength distibution and stress fluctuation parameter k. At high stress, the maximum damage increases with m; while at low stress, the maximum damage decreases. In addition, for low stress, severe stress fluctuation causes higher damage; while for high stress, causes lower damage.

Catastrophic Rupture Induced Damage Coalescence in Heterogeneous Brittle Media

In heterogeneous brittle media, the evolution of damage is strongly influenced by the multiscale coupling effect. To better understand this effect, we perform a detailed investigation of the damage evolution, with particular attention focused on the catastrophe transition. We use an adaptive multiscale finite-element model (MFEM) to simulate the damage evolution and the catastrophic failure of heterogeneous brittle media. Both plane stress and plane strain cases are investigated for a heterogeneous medium whose initial shear strength follows the Weibull distribution. Damage is induced through the application of the Coulomb failure criterion to each element, and the element mesh is refined where the failure criterion is met. We found that as damage accumulates, there is a stronger and stronger nonlinear increase in stress and the stress redistribution distance. The coupling of the dynamic stress redistribution and the heterogeneity at different scales result in an inverse cascade of damage cluster size, which represents rapid coalescence of damage at the catastrophe transition.

平台用高强度钢多次补焊及TIG熔修的疲劳分析

海洋石油平台在服役期内，由于受到各种环境载荷的交变作用，其受力的焊接构件不可避免地会出现疲劳裂纹。对出现的疲劳裂纹进行补焊修复，可以延长平台的使用期，带来巨大的经济和社会效益。本文通过实验研究，考察了多次补焊对海洋平台用高强度钢焊接接头疲劳性能的影响，以及是否可以通过TIG熔修工艺的采用提高补焊构件的疲劳寿命，达到延长补焊后平台的使用寿命，减少补焊次数的目的。本文涉及的主要研究内容如下：1、考察了多次补焊对焊接接头疲劳寿命的影响。通过对平台用高强度钢焊接接头初焊试样，以及重复三次补焊试样的疲劳实验，得到了各次补焊试样在对数正态分布和威布尔分布下的寿命估算式和考虑了存活率的R-S-N曲线。对实验结果进行了比较和分析，从而考察了多次补焊对焊接接头疲劳性能的影响。2、在每次补焊后，由于焊趾外移造成了焊趾角的变化，使得焊趾处的应力集中系数发生了变化。本文采用有限元方法计算了试样的应力场分布，得到了焊趾处的应力集中系数与焊趾角的关系。考虑到应力集中系数的变化后，对多次补焊试样疲劳实验的实验数据进行了重新处理。3、考察了多次补焊熔修对焊接接头疲劳寿命的影响。本文通过对平台用高强度钢焊接接头补焊及补焊熔修试样的疲劳实验，得到了多次补焊和补焊熔修试样对数正态分布和威布尔分布下的寿命估算式和考虑了存活率的R-S-N曲线。对实验结果进行了比较和分析，从而考察了补焊后采用TIG熔修工艺对焊接接头疲劳性能的影响。4、一次补焊后进行熔修处理可以极大的改善试样的疲劳性能，提高试样的疲劳寿命。但这一结果是在平焊位条件下得出了。为了将这一结果用于实际，还要考察实际操作中大量采用的立焊位条件下熔修的效果。本文完成了立焊位条件下补焊及补焊熔修试样的疲劳实验，得到了相应的S-N曲线。5、疲劳实验数据的处理中，大量采用威布尔分布作为数据统计的分布类型。但由于威布尔分布具有三个参数，给求解带来一定的困难。本文提出一种以在威布尔坐标纸上，数据分布的线形相关系数作为判据的求解威布尔参数的方法，并通过程序加以实现。为了对疲劳实验数据的处理更加方便和直观，编制了“疲劳实验数据处理程序“，将对疲劳实验数据的编辑、分布参数的求解、不同存活率下的疲劳寿命估算和各种分布类型图以及R-S-N曲线的绘制工作包括在该程序中。

非均匀脆性介质破坏的非平衡非线性统计演化

在白以龙小组已有工作的基础上，利用他们提出的简化的耦合斑图模型，几种可能的应力重分配模型在该论文中得到了进一步的讨论。通过不同的应力重分配模型，我们发现了该类演化过程中的三个一般规律，这些规律对于类似的动力过程的预报（例如，非均匀介质的破坏）提供了线索。首先，我们采用系综统计的方法，对相同宏观参量的大量样本的强度分布作了考察，结果表明：宏观强度的统计结果可以非常好的拟合为Weibull分布，且其Weibull模数与系统的大小、应力重分配的方式以及细观单元的强度分布相关。其次，在模拟过程中，对演化过程中的能量释放、损伤事件的统计发现，它们存在标度行为，而且这一标度主要归因于灾变点附近的损伤事件。这一现象表明这一转变具有某种临界特征。最后，对于我们模型中的动力学过程，我们发现了灾变预报的线索。从样本演化过程 中的能量释放规律来看，我们发现有两件事是有意义的：一是辨别出主破坏的发生点（在这一时修，系统中的大部分能量得到释放）；另外，给出转变点（整体稳定转化为演化诱致灾变）的预报。对前一个问题，我们通过考察系统在GS和EIC段的应力损伤涨落特征可以给出回答，通常，在EIC段的最大应力涨落（通常出现在主破坏过程中）比在GS过程中的最大应力涨落高一个数量级，根据这一差异，可以设立一个应力涨落的警戒值来判断系统所处的演化状态。对于后者，受到地震预报中采用的加卸载响应比（LURR）的启发，我们通过对系统中的外回转应力或损伤单元施加一个微增扰动，然后，根据系统在扰动前后释放的能量和相应的扰动，就可以得到临界敏感系数，临界敏感系数在灾变点附近迅速增加，在灾变点之迅速下降到1附近-我们称这一特征为临界敏感性。不同的应力重分配模型下得到了类似的现象，由此看来，对于 类似的动力学过程，临界敏感性是一个一般的特征。这一特征可能为我们对非均匀脆性介质的破坏提供线索。

Fumar antecipa a menopausa ? Evidências do Estudo Pró-Saúde

A menopausa é definida pelo momento em que a menstruação cessa permanentemente como conseqüência da falência ovariana. Uma vez estabelecida, há um aumento no risco de doença coronariana, doença de Alzheimer, osteoporose e fraturas e sua antecipação está relacionada a maiores índices de mortalidade. Com o envelhecimento geral da população mundial, as mulheres passaram a viver de um terço a metade de suas vidas no período pós-menopáusico e, conseqüentemente, a pesquisa de condições associadas à menopausa ganham importância. A idade da menopausa e os fatores que a influenciam variam entre os diversos estudos e poucos estudos brasileiros abordam o tema. O fumo tem sido associado à Antecipação da idade da menopausa. O objetivo deste estudo é analisar as diversas dimensões de associação entre o fumo e a idade da menopausa, levando em consideração possíveis relações de dose-resposta. Com base em dados do Estudo Pró-Saúde, foi realizado um estudo seccional utilizando-se para a análise o modelo de sobrevida paramétrico de riscos proporcionais com distribuição de Weibull. Os resultados apontam para uma redução em 32% do risco de menopausa entre fumantes ativas, que a alcançam 2,5 anos mais tarde que nunca fumantes, ajustando-se para escolaridade e paridade. Entre fumantes ativas, no entanto, foram sugeridos aumentos de risco de 123% e 192% para fumantes de 10 a 20 cigarros por dia e de mais de 20 maços-ano respectivamente, quando comparadas às fumantes de menos de 10 cigarros por dia e menos de 10 maços-ano. Nesses grupos, a menopausa foi antecipada em 3,3 anos e 4,4 anos, respectivamente. Em relação ao tempo decorrido entre cessar o tabagismo e a idade menopausa, duração do fumo ou idade de início, não foram encontradas associações. Neste estudo, em possíveis efeitos de dose-resposta, o fumo apresenta-se como fator associado à antecipação da idade da menopausa em alguns aspectos, embora não em outros.

Comportamento mecânico de cerâmica produzida a partir de reciclagem de para-brisa de automóvel

É crescente o interesse nos materiais cerâmicos, devido as suas características como baixa massa específica e maior resistência a ambientes agressivos do que a maioria das ligas metálicas. Este trabalho tem o objetivo de produzir a partir de material descartado, cerâmicas em diferentes temperaturas de sinterização e avaliar a sua tensão de ruptura em ensaio de flexão de três pontos e a confiança desta medida. Devido ao custo de produzir pó de vidro, tanto pelo alto gasto de energia para fundir a matéria-prima como pelo consumo de minerais industriais, foi proposto utilizar vidro de para-brisa obsoleto reduzindo despesas operacionais e definindo um destino econômico e ambiental viável para estes rejeitos. A metodologia consistiu-se na obtenção do pó de vidro com características adequadas para ser conformado e sinterizado. Foram usadas duas composições e quatro tratamentos térmicos para obter oito materiais. Uma composição com apenas o pó oriundo da moagem de para-brisa e outra com este pó mais 4% de óxido de nióbio. A resistência à flexão dos produtos obtidos foi avaliada. Utilizou-se a estatística de Weibull para caracterizar estes resultados. Os resultados obtidos indicam que o material de composição pó de vidro e temperatura final de sinterização de 650C obteve a maior resistência mecânica entre os materiais sintetizados. A adição do óxido de nióbio provoca um decréscimo na resistência mecânica se comparada com o material sem a adição deste óxido. Entretanto, comparando as duas composições na mesma temperatura final de sinterização, a adição de óxido de nióbio provocou um aumento no módulo de Weibull, excetuando-se dois de oito materiais obtidos. As diferentes composições e temperaturas de sinterização afetaram as propriedades mecânicas dos materiais obtidos.

An evaluation of back-calculation methodology using simulated otolith data

I simulated somatic growth and accompanying otolith growth using an individual-based bioenergetics model in order to examine the performance of several back-calculation methods. Four shapes of otolith radius-total length relations (OR-TL) were simulated. Ten different back-calculation equations, two different regression models of radius length, and two schemes of annulus selection were examined for a total of 20 different methods to estimate size at age from simulated data sets of length and annulus measurements. The accuracy of each of the twenty methods was evaluated by comparing the back-calculated length-at-age and the true length-at-age. The best back-calculation technique was directly related to how well the OR-TL model fitted. When the OR-TL was sigmoid shaped and all annuli were used, employing a least squares linear regression coupled with a log-transformed Lee back-calculation equation (y-intercept corrected) resulted in the least error; when only the last annulus was used, employing a direct proportionality back-calculation equation resulted in the least error. When the OR-TL was linear, employing a functional regression coupled with the Lee back-calculation equation resulted in the least error when all annuli were used, and also when only the last annulus was used. If the OR-TL was exponentially shaped, direct substitution into the fitted quadratic equation resulted in the least error when all annuli were used, and when only the last annulus was used. Finally, an asymptotically shaped OR-TL was best modeled by the individually corrected Weibull cumulative distribution function when all annuli were used, and when only the last annulus was used.