The emergence of leadership in coordination-intensive groups

Although group coordination was introduced to psychology in the early 1990s, it was not until the advent of nonlinear dynamical systems (NDS) that it was possible to gain an understanding of how the process of implicit learning and self-organization take place in conditions where no hierarchical (management) influences are involved. This experimental study examined how leaders might emerge from coordination-intensive task groups where verbal interaction is not possible. NDS and game-theoretical research indicate that the presence of leaders is not required to produce coordination. Thus the question remains as to whether leaders would emerge from coordination-intensive task groups in the similar manner to how they emerge from other types of groups. In the experiment, 13 4-person groups were allowed to discuss the coordination (card game) task while performing it; 13 other groups worked nonverbally. Split-plot ANOVA showed that verbalizing groups performed better than nonverbalizing groups overall and showed more acute coordination learning curves. Nonlinear regression for temporal dynamics within verbalizing and nonverbalizing groups showed asymptotic stability for initial coordination learning and transfer to a coordination rule of equal difficulty, but a chaotic function was observed when the teams switched to a more difficult coordination rule. A questionnaire measured leadership emergence at the end of the game along with other social contributions to the groups' efforts. The average level of leadership emergence for individuals did not differ between verbal and nonverbal conditions, although differences in other social contributions were observed. This experiment illustrates, furthermore, how the nonlinear science paradigm produces new hypotheses concerning verbalization that would not have been formulated otherwise. New avenues of study concerning coordination, leadership, and hierarchies are also discussed.     

Origins of group coordination: nonlinear dynamics and the role of verbalization

Coordination occurs when two or more people do the same or complimentary tasks simultaneously; its explanation game theory, nonlinear dynamics, and implicit learning theory. In the experiment, 12 four-person groups were allowed to discuss the coordination (card game) task while performing it; 12 other groups worked nonverbally. One to three group members were replaced during the game. Split-plot ANOVA showed that verbalizing groups performed better than nonverbalizing groups overall and showed more acute coordination learning curves, but verbalization did not compensate for the replacement of personnel. Groups that changed one or two players showed positive coordination transfer, but groups that changed three players did not. Nonlinear regression for temporal dynamics within verbalizing and nonverbalizing groups showed asymptotic stability for initial coordination learning and transfer to a difficult rule, a chaotic function when replacements were introduced, and asymptotic stability again when the team with replacements switched to the difficult rule.     

The game of futsal as an adaptive process

Some researchers have described team sports as complex, open, and hierarchical systems. This study aimed to investigate and describe how the game of futsal could be characterized as a dynamic adaptive process. One game, which included participation by two amateur teams, was analyzed by examining players' individual (space occupied, skills with and without ball) and collective actions (attacks and defenses). Data were collected through time-continuum notation, and were analyzed through frequencies and clustering, using trend analysis and multiple comparisons, and Ward's minimum variance method with Euclidean distance, respectively. Results revealed four attack patterns for each team, with four defense patterns for one (Blue), and seven for the other (Red), and they showed within-pattern variability. All were performed in an unpredictable manner, with no absolute correspondence between attacks and defenses. The futsal game as an adaptive process was characterized by changing intra- and inter-patterns.     

考虑海底接触的钢悬链式立管触地点动力响应以及疲劳损伤分析

基于考虑接触的钢悬链式立管SCR(Steel Catenary Riser)触地点处的结构特性,分别采用了考虑管土分离的线性截断模型以及包含土体吸力效应的帽盖模型来描述P-y曲线。通过改变上端浮体的垂荡运动幅度、土体吸力系数以及海床刚度,对SCR触地点处的动力响应以及疲劳损伤特性进行了分析。分析结果表明,SCR触地点的垂向位移、弯矩、等效应力以及疲劳损伤均随着浮体垂荡运动幅度的增加而呈上升趋势。SCR触地点的垂向位移随着土体吸力系数的增大由高幅低频响应转变为低幅高频响应。SCR触地点的疲劳损伤随着海床刚度的增加呈现先稳定再增加再稳定的趋势。     

Numerical analysis of progressive damage in nonwoven fibrous networks under tension

Understanding a mechanical behaviour of polymer-based nonwoven materials that include large-strain deformation and damage can help to evaluate a response of nonwoven fibrous networks to various loading conditions. Here, a nonwoven felt made by thermal bonding of polypropylene fibres was used as a model system. Its deformation and damage behaviour was analysed by means of experimental assessment of damage evolution based on single-fibre failure and finite-element simulations. Tensile tests of nonwoven fabrics were carried out to characterise their damage behaviour under in-plane mechanical loading. It was found that progressive failure of fibres led to localization of damage initiation and propagation, ultimately resulting in failure of the nonwoven felt. To obtain the criteria that control the onset and propagation of damage in these materials, tensile tests on single fibres, extracted from the felt with bond points attached to their ends, were performed. A finite-element model was developed to study damage initiation and propagation in nonwovens. In the model, structural randomness of a nonwoven fibrous network was implemented by means of direct introduction of fibres according to the orientation distribution function. The evolution of damage in the network was controlled by a single-fibre failure criterion obtained experimentally. The proposed numerical model not only captured the macroscopic response of the felt successfully but also reproduced the underlying mechanisms involved in deformation and damage of nonwovens.     

单轴压缩下页岩类脆性材料细观损伤演化的离散元分析

了解广泛存在的类似页岩的脆性材料各向异性对工程安全具有重要意义。本研究将页岩视为粘结颗粒材料,基于离散单元方法研究了横观各向同性脆性页岩的损伤演化。再现了不同层理角的页岩试样的破坏模式,并对比了实验和数值模拟的抗压强度和弹性模量。引入微裂纹的概念,通过定义裂纹密度函数,系统地研究了单轴压缩条件下,页岩层理角对细观结构的影响。此外基于平均配位数建立了配位数变化与细观损伤的联系,并根据配位数的变化与裂纹数量将加载过程分为三个阶段,分析了不同阶段配位数与裂纹数量的对应变化关系。研究表明,页岩的裂纹密度随着层理角的增加而增加,而试样的平均配位数在加载过程中先上升后剧烈下降,颗粒集合体在单轴压缩条件下的应力应变及裂纹数量曲线与平均配位数曲线有良好的一致性。该研究揭示了横观各向同性脆性岩石的破坏过程和内在机理,将为页岩类脆性材料的工程应用提供理论指导。     

基于提升小波变换和互相关函数的梁式桥损伤检测

为了有效地提取梁式桥的损伤特征,提出了一种将提升小波变换与互相关函数幅值向量相结合的损伤识别的方法。首先利用提升小波变换将车载作用下梁桥结构的加速度响应分解到单阶模态上,再对提升重构后得到的第一阶响应进行互相关处理,利用提出的损伤指标来进行损伤定位。通过数值仿真和模拟实验,验证了方法的有效性,讨论了噪声及测点稀疏对识别结果的影响,结果表明该方法具有良好的噪声鲁棒性。另外,在综合考虑经济性和识别结果准确性的情况下,给出了合理的测点布置方式。     

损伤介质中波传播特性的研究

本文研究了弹性波在损伤介质中传播的基本理论,建立了基本动力学方程式,并根据其边界条件和连续性条件求得末端接收波位移的时域曲线。接收波则综合携带了损伤介质内部特性信息,可通过这些信息研究损失介质损坏的程度及位置。通过实例计算,定量地分析了结构内部损伤度、损伤位置和损伤区域大小对接收波幅值和波到达时间的影响。通过分析可知损伤对波传播的影响较大;并且可以根据给定的曲线图,直接由测得的接收波峰值及到达时间探测到结构内部损伤大小与分布。这为无损检测的定量化反分析提供了可靠的依据。     

Biothermomechanics of skin tissues

Biothermomechanics of skin is highly interdisciplinary involving bioheat transfer, burn damage, biomechanics and neurophysiology. During heating, thermally induced mechanical stress arises due to the thermal denaturation of collagen, resulting in macroscale shrinkage. Thus, the strain, stress, temperature and thermal pain/damage are highly correlated; in other words, the problem is fully coupled. The aim of this study is to develop a computational approach to examine the heat transfer process and the heat-induced mechanical response, so that the differences among the clinically applied heating modalities can be quantified. Exact solutions for temperature, thermal damage and thermal stress for a single-layer skin model were first derived for different boundary conditions. For multilayer models, numerical simulations using the finite difference method (FDM) and finite element method (FEM) were used to analyze the temperature, burn damage and thermal stress distributions in the skin tissue. The results showed that the thermomechanical behavior of skin tissue is very complex: blood perfusion has little effect on thermal damage but large influence on skin temperature distribution, which, in turn, influences significantly the resulting thermal stress field; the stratum corneum layer, although very thin, has a large effect on the thermomechanical behavior of skin, suggesting that it should be properly accounted for in the modeling of skin thermal stresses; the stress caused by non-uniform temperature distribution in the skin may also contribute to the thermal pain sensation.     

接触爆炸荷载作用下溢流坝的抗爆性能

以黄登重力坝的溢流坝为研究背景,考虑混凝土的高应变率效应,运用Lagrange-Euler耦合算法建立大坝-库水-空气-炸药全耦合数值模型,研究溢流坝在接触爆炸荷载作用下的抗爆性能。分析满库与空库时溢流坝在爆炸冲击波作用下的动力响应及损伤程度,并进一步研究满库时大坝在不同炸点的水下接触爆炸荷载作用下的动力响应及损伤分布。研究结果表明,满库时水下爆炸比空库时爆炸的动力响应及损伤程度大得多;溢流坝的抗爆薄弱部位主要集中在溢流道顶部及坝体上游折坡处。研究溢流坝的抗爆性能时应重点研究满库时水下爆炸对大坝的破坏特性。     

Theory of spall damage accumulation in ductile metals

Spall damage is a type of fracture produced when large tensile stresses are developed in material bodies as a result of the interaction of stress waves. The damage develops independently at various points in the body because of the short duration of stress application, but interacts in a complicated way with the stress wave. A theory of these phenomena is presented for the case where the basic response of the material is viscoplastic, and where the damage takes the form of a distribution of small rounded voids. This theory is specialized to the case of rectilinear motion and an example problem solved numerically.     

YB3航空有机玻璃平板鸟撞动态响应实验研究

为验证鸟撞有机玻璃平板数值计算模型的正确性,同时为进一步设计研制具有高抗鸟撞能力的航空有机玻璃风挡提供有价值的数据,本文采用动态数据采集系统,对YB3航空有机玻璃平板在两种不同速度下的鸟体撞击动态响应全过程进行了详细研究。测量了YB3平板上三个点的位移、七个点的应变、鸟体撞击方向四个支反力等物理量随时间的变化历程,同时利用高速摄像系统记录了鸟体撞击下平板动态变形和破坏的全过程。分析了鸟撞实验结果的有效性,同时分析了有机玻璃被鸟体击穿后的破坏模式。重复性实验结果的一致性表明了本文实验测试结果的有效性和可靠性。     

循环爆破开挖下隧道围岩振动效应与损伤演化的模型实验

针对推进式循环爆破开挖下隧道围岩振动效应与损伤演化问题,按照相似比理论进行模型实验研究,实验模型采用1:15比例浇筑制成。通过模拟隧道推进式循环爆破开挖方式,以同一测点处爆破前后岩体声速变化评价隧道围岩损伤程度,探寻爆破参量变化对振动效应的影响,探索围岩损伤演化与爆破次数之间的关系。研究结果表明:在最大段药量大致相同情况下,起爆段数对萨道夫斯基公式的介质系数K影响很小,而对萨道夫斯基公式的衰减系数α影响较大;隧道在推进式循环爆破开挖下,同一深度距离爆区相同的测点,其声速降低率存在较大差异,围岩的爆破损伤范围在深度和广度方面均具有典型的各向异性特征;当爆炸参量基本相同时,不同循环爆破开挖下测点的累积声速降低率呈非线性增长趋势;在推进式循环爆破加载下,围岩爆破累积损伤量D与爆破次数n之间存在非线性演化特性,不同的测点具有各自的爆破累积损伤扩展模型,距离爆源越近爆破损伤扩展越快,围岩爆破累积损伤效应具有典型的非线性演化特性和各向异性特征。     

The principle of virtual power: from eliminating metaphysical forces to providing an efficient modelling tool

In a period of a few decades, the formulation known as the principle of virtual power (PVP) has gained a prominent place among the most efficient tools in the thermomechanics of continua. Strongly marked by a “continental” (French-Italian) influence, it has successfully incorporated the basic invariances of modern continuum mechanics while capturing the spirit of twentieth-century analysis (generalized functions or distributions) in which it became synonymous of weak formulation. It proved to provide the surest and safest way to formulate complex theories of continua (so-called “generalized continuum mechanics”, theory of coupled fields, etc) and approximate or generalized theories of structural members and the associated natural boundary conditions while preparing the way for the full thermomechanical formulation, providing the best setting for the proof of various mathematical theorems, and paving the way for modern numerical methods. The present contribution, illustrated by many examples of varying complexity, emphasizes the role of Paul Germain (1920–2009) in this formulation. The author, himself an active contributor and a never tired propagandist of the method, has participated in these developments during four decades and presents here his witness but critical viewpoint, highlighting the difficult points and also the esthetically pleasing ones where necessary.     

位移幅值对铜镁合金微动磨损行为的影响

在研制的多功能微动磨损试验机上,开展了不同位移幅值下铜镁合金微动磨损试验,以研究位移幅值对铜镁合金微动磨损行为的影响. 微动过程中记录摩擦系数曲线与F_t-D-N曲线,利用光学显微镜(OM)、扫描电镜(SEM)、能量色散X射线光谱仪(EDS)及三维形貌仪对损伤区域进行了微观分析. 结果显示:随着位移幅值的增加,铜镁合金微动运行状态由部分滑移进入完全滑移,未发现混合滑移状态;部分滑移区中呈现由弹性变形协调逐渐向塑性变形协调转变的趋势. 磨损体积随位移幅值的增加而增加,在完全滑移区中体积损失非常严重. 在弹性变形协调的部分滑移状态下,接触表面损伤轻微,而由塑性变形协调的部分滑移状态下,接触中心出现较大切应力,疲劳裂纹扩展至接触表面导致材料剥落,接触边缘有磨粒磨损和氧化磨损的痕迹. 在完全滑移状态下,接触表面损伤主要为疲劳剥层,磨粒磨损和氧化磨损.      

Changes in Intracellular Calcium during Compression of C2C12 Myotubes

In recent years, damage directly due to tissue deformation has gained interest in deep pressure ulcer aetiology research. It has been shown that deformation causes muscle cell damage, though the pathway is unclear. Mechanically induced skeletal muscle damage has often been associated with an increased intracellular Ca²⁺ concentration, e.g. in eccentric exercise (Allen et al., J Physiol 567(3):723–735, 2005). Therefore, the hypothesis was that compression leads to membrane disruptions, causing an increased Ca²⁺-influx, eventually leading to Ca²⁺ overload and cell death. Monolayers of differentiated C2C12 myocytes, stained with a calcium-sensitive probe (fluo-4), were individually subjected to compression while monitoring the fluo-4 intensity. Approximately 50% of the cells exhibited brief calcium transients in response to compression, while the rest did not react. However, all cells demonstrated a prolonged Ca²⁺ up-regulation upon necrosis, which induced similar up-regulations in some of the surrounding cells. Population heterogeneity is a possible explanation for the observed differences in response, and it might also become important in tissue damage development. It did not become clear however whether Ca²⁺-influxes were the initiators of damage.     

Combined neural network and reduced FRF techniques for slight damage detection using measured response data

Summary  This paper deals with structural damage detection using measured frequency response functions (FRF) as input data to artificial neural networks (ANN). A major obstacle, the impracticality of using full-size FRF data with ANNs, was circumvented by applying a data-reduction technique based on principal component analysis (PCA). The compressed FRFs, represented by their projection onto the most significant principal components, were used as the ANN input variables instead of the raw FRF data. The output is a prediction of the actual state of the specimen, i.e. healthy or damaged. A further advantage of this particular approach is its ability to deal with relatively high measurement noise, which is a common occurrence when dealing with industrial structures. The methodology was applied to detect three different states of a space antenna: reference, slight mass damage and slight stiffness damage. About 600 FRF measurements, each with 1024 spectral points, were included in the analysis. Six 2-hidden layer networks, each with an individually-optimised architecture for a specific FRF reduction level, were used for damage detection. The results showed that it was possible to distinguish between the three states of the antenna with good accuracy, subject to using an adequate number of principal components together with a suitable neural network configuration. It was also found that the quality of the raw FRF data remained a major consideration, though the method was able to filter out some of the measurement noise. The convergence and detection properties of the networks were improved significantly by removing those FRFs associated with measurement errors. Received 9 March 2000; accepted for publication 12 December 2000     

Mixed testing strategies in the quality check problem for stabilization algorithms

The maximin testing problem for the stabilization quality of linear systems with discrete initial and parametric disturbances is considered. A testing procedure is implemented by reducing the original dynamic game to a geometric game. The transition to a mixed expansion of the geometric game is proposed in the absence of saddle points. An optimal mixed disturbance strategy is shown to be applicable.     

不同加载状态下TA2钛合金绝热剪切破坏响应特性

一般认为绝热剪切现象在宏观上表现为材料动态本构失稳,即热软化大于应变硬化.本文采用帽型受迫剪切试样研究TA2钛合金的动态力学特性和本构失稳过程.首先对剪切区加载应力状态进行理论和数值分析,通过合理设计帽型试样,剪切区变形可近似按剪切状态处理;结合二维数字图像相关法(two-dimensional digital image correlation,DIC-2D)直接测试试样剪切区应变演化,给出帽型受迫剪切实验的等效应力-应变响应曲线.进一步,利用Hopkinson压杆对TA2钛合金开展动态压缩及帽型剪切对比试验研究,比较压缩、剪切试验得到的等效应力-应变曲线,采用"冻结"试样方法分析试样中绝热剪切局域化演化过程,探讨不同加载状态下TA2钛合金的绝热剪切破坏现象及其动态力学响应特性.实验结果表明,在塑性变形初始阶段,动态压缩及剪切加载下的等效应力-应变曲线符合较好,但随塑性损伤发展及绝热剪切带形成,两者出现分离,表明损伤及绝热剪切演化过程与应力状态相关.剪切试样实验得到的本构"软化"特性能够反映绝热剪切带起始、破坏演化过程的力学响应特性,而在动态压缩实验中,即使试样中已出现双锥形的绝热剪切带及局部裂纹分布,其表观等效应力-应变曲线并不出现软化特征,动态压缩实验无法得到关于绝热剪切起始、发展以及破坏的本构软化响应特性.