Many materials are anisotropic. However, there is no widely accepted measure for characterizing the degree of elastic anisotropy. Here, assuming that the limiting case of extreme anisotropy should possess a positive semidefinite stiffness matrix, we propose three criteria to evaluate measures of anisotropy and show that the existing measures in the literature do not satisfy all of the proposed criteria.We then introduce a new measure of anisotropy based on the maximum strain energy ratio that is universally applicable to all material systems. The proposed measure is helpful for understanding the properties and behaviors of materials. Furthermore, this measure can be easily generalized to situations involving multiple fields and nonlinearity. The J-integral based criterion is widely used in elastic-plastic fracture mechanics. However, it is not rigorously applicable when plastic unloading appears during crack propagation. One difficulty is that the energy density with plastic unloading in the J-integral cannot be defined unambiguously. In this paper, we alternatively start from the analysis on the power balance, and propose a surface-forming energy release rate (ERR), which represents the energy available for separating the crack surfaces during the crack propagation and excludes the loading-mode-dependent plastic dissipation. Therefore the surface-forming ERR based fracture criterion has wider applicability, including elastic-plastic crack propagation problems. Several formulae are derived for calculating the surface-forming ERR, and the definition of the energy density or work density is avoided. For any fracture behaviours, the surface-forming ERR is proven to be path-independent. The physical meanings and applicability of the proposed surface-forming ERR, the local ERR, the traditional global ERR, and J-integral are compared and discussed.

views: 44 Month views: 44

The label: Science,Engineering,Solid mechanics

Released on October 21, 2020

Bin Liu

通过分子动力学模拟方法，对离子液体的界面结构与传热过程进行了较为系统的研究。首先基于限域离子液体振动谱演化，揭示了其固液结构转变的内在机制；然后对比范德华及静电作用，初步阐明了离子液体固体界面的微观传热机理，相关的研究成果可以为受限离子液体在相变储能，热量耗散等多个过程提供参考。

views: 33 Month views: 33

The label: Science,Solid mechanics

Released on September 29, 2020

Yanlei Wang

细胞对外界微环境的力学响应主要通过细胞骨架来实现，而细胞骨架的动力学行为使其具有高度的复杂性。研究细胞骨架的力学行为，有助于人们更好地认知和了解细胞的生理学和病理学功能。针对细胞骨架参与的几个细胞力学问题，进行了相关理论建模、分子模拟和有限元模拟。研究的几个问题和主要方法为：（1）建立张拉整体模型，研究在不同加载方式（静态或循环）、不同基底刚度上的极性细胞取向问题。结果表明：细胞骨架的横向支撑部分对细胞取向有重要作用；细胞方位的分布宽度反比于加载幅值等。（2）发展张拉整体模型，探讨单个细胞骨架与群体细胞骨架弹性模量之间的关系，发现群体细胞骨架的弹性模量远高于单细胞的弹性模量，这一结果主要是由于群体细胞中细胞骨架的几何旋转受到了抑制。（3）针对现有AFM压痕实验中发现的细胞骨架弹性模量依赖于压入深度，进行了量纲分析和有限元模拟，得到了一个半定量的压力与压入深度的解析关系。在考虑细胞膜和皮质层表面张力的作用下，我们的解析关系表明：细胞骨架的弹性模量是一个常数，与AFM压入深度无关。相关的理论和模拟结果与大量的实验数据相吻合。

views: 103 Month views: 103

The label: Solid mechanics,Science

Released on September 22, 2020

Guang-Kui Xu

Cell dynamics is of crucial significance for the morphogenesis, self-repair, and other physiological and pathological processes of tissues. Collective cells exhibit greatly different dynamic behaviors from isolated cells. In this lecture, some recent advances in experimental and theoretical researches on collective cell dynamics will be presented, with particular attention paid to the biomechanical mechanisms underlying the morphodynamics of developing embryos and tumors. First, a cell division model is established for the division of interconnecting cells in a biological tissue. Coupled mechanical-chemical mechanisms involved in the multi-phase cell division are taken into account. Second, we explain why spontaneous oscillation of collective cells may occur in such biological tissues as Drosophila amnioserosa during development. It is revealed that the collective cell oscillation in an epithelium-like monolayer results from the dynamic bifurcation induced by feedback between mechanical strains and chemical cues. Further, we investigate, both experimentally and theoretically, the migration of collective cells. We show that migratory cells may behave as a whole either like a viscous solid or fluid, leading to rich patterns with characteristic sizes ranging from several to dozens of cells. On the basis of experimental measurements and theoretical analysis, universal statistical laws are derived for the dynamic features of collective cells.

views: 85 Month views: 85

The label: Science

Released on September 17, 2020

Xi-Qiao Feng

凭借超越天然材料的超常物理性能与波动控制特性，超材料与超表面已发展成为具有重要工程应用价值和广泛应用前景的热点前沿领域。与电磁波和声波相比，弹性波具有更多模态特征，会产生更复杂的散射特性，为超材料/超表面的弹性波控制提出更多挑战。本报告将围绕几种弹性超材料与超表面的结构设计，着重介绍我们关于其在波阵面重塑、非对称传播及隔振“牢笼”等弹性波控制方面的研究。

views: 72 Month views: 72

The label: Science,Engineering,Solid mechanics

Released on September 16, 2020

Bing Li

Metallic glasses (MGs) possess large elastic limit and high strength, but unfortunately they are of limited commercial utility due to their macroscopic brittle nature. Here, we report the recent progress in the improved ductility of MGs via simple structural design. Topics covered include MGs-based chiral nanolattice, and nanoglass (NG) consisting of nanometer-sized glassy grains separated by glass-glass interfaces which can be used to design MGs with unique mechanical properties.

views: 51 Month views: 51

The label: Science,Material science

Released on September 11, 2020

Zhendong SHA

显微拉曼光谱作为一种典型的光谱分析手段，近年来被成功应用于力学领域的实验研究，在微结构工艺残余应力表征、微尺度界面力学行为等方面取得了若干应用成果。围绕拉曼光谱用于力学精细测量的实验理论、分析方法、关键技术、仪器装备以及前沿应用开展研究工作：提出了拉曼光谱应力/应变分量解耦分析与传感测量实验理论，构建了拉曼力学精细表征实验方法新体系；提出了近场、角分辨拉曼等力学测量新技术，突破了"空间分辨率"与"测量置信度"这两个制约光谱力学发展的主要瓶颈；通过发展光-光谱原位实验新方法，解决了多尺度界面的应力定量表征难题，从而发现了纳米复合结构界面应力传递尺寸效应等新现象。

views: 44 Month views: 44

The label: Science,Engineering,Solid mechanics

Released on September 05, 2020

Admin R.E.

Continuum mechanics predicts that the propagation speed of non-equilibrium information in solids is limited by the longitudinal wave speed, so is crack propagation. However, solids are essentially discrete systems. In this paper, via theoretical analysis and numerical simulations, it is demonstrated in a straightforward way that non- equilibrium disturbance (e.g. force, displacement, energy, and so on) can propagate at a supersonic speed in discrete systems, although the magnitude of the disturbance attenuates very quickly. In dynamic fracture, a cascade of atomic-bond breaking events provides an amplification mechanism to counterbalance the attenuation of the disturbance. Therefore, supersonic crack propagation can be realized in a domino way. Another key factor for supersonic crack propagation is to ensure sufficient energy flowing into the crack tip. Since most energy can only be transferred at a speed limited by the longitudinal wave speed, the conditions for the occurrence of supersonic crack propagation are not easily met in most situations, unless there is high pre-stored energy along the crack path or continuous energy supply from the loading concomitantly moving with the crack tip. A quantitative relation between supersonic crack propagation speed and material properties and parameters is given, which implies that knowing all the classical macroscopic quantities is not enough in determining the supersonic crack propagation speed, and the microstructure does play a role. Moreover, it is interesting to note that fracture toughness affects the crack propagation speed in the subsonic regime, but not in the supersonic regime, because the deformation/stress is uniform in front of a supersonic crack where strength criterion dominates.

views: 142 Month views: 142

The label: Science,Engineering,Solid mechanics,Dynamics

Released on September 01, 2020

Bin Liu

Continuous monitoring of blood pressure, an essential measure of health status, typically requires complex, costly, and invasive techniques that can expose patients to risks of complications. Continuous, cuffless, and noninvasive blood pressure monitoring methods that correlate measured pulse wave velocity (PWV) to the blood pressure via the Moens−Korteweg (MK) and Hughes Equations, offer promising alternatives. The MK Equation, however, involves two assumptions that do not hold for human arteries, and the Hughes Equation is empirical, without any theoretical basis. The results presented here establish a relation between the blood pressure P and PWV that does not rely on the Hughes Equation nor on the assumptions used in the MK Equation. This relation degenerates to the MK Equation under extremely low blood pressures, and it accurately captures the results of in vitro experiments using artificial blood vessels at comparatively high pressures. For human arteries, which are well characterized by the Fung hyperelastic model, a simple formula between P and PWV is established within the range of human blood pressures. This formula is validated by literature data as well as by experiments on human subjects, with applicability in the determination of blood pressure from PWV in continuous, cuffless, and noninvasive blood pressure monitoring systems.

views: 95 Month views: 95

The label: Science,Engineering,Solid mechanics,Fluid mechanics

Released on August 29, 2020

Yonggang Huang

划痕会损伤高分子材料表面，影响其功能性与美观性。与直觉想象不同，高分子材料划痕过程伴随着~100°C的剧烈升温。基于线弹性断裂假设的应变能耗散较小，即使完全转化为热量仍然无法描述这一剧烈升温现象。大量研究表明，非晶态脆性PMMA材料的宏观裂纹是由微观韧性银纹演化产生，从银纹微观演化能量角度出发，可以解释刮擦过程的剧烈温升现象。由蒋晗等撰写的关于这一独特现象的复杂宏微观机理的系列论文发表在摩擦学和材料力学领域老牌著名期刊Tribology International和International Journal of Solids and Structures。 Dramatic temperature rise has been observed at the surface facture area during the scratch of brittle poly(methyl methacrylate) (PMMA). However, the strain energy accompanied with the macroscopic linear elastic fracture is inadequate to explain the significant temperature change. To describe this phenomenon, the crazing related energy dissipation mechanism considering the evolution of microscopic crazing is proposed.The frictional heating and macroscopic plastic deformation make their contribution for the temperature rise at relative small scratch load. The energy dissipated by the crazing process plays a dominating role for the scratch-induced fracture. Those findings provide meaningful insight for understanding the mechanism of temperature rise during polymer scratch.

views: 83 Month views: 83

The label: Solid mechanics,Material science,Engineering,Science

Released on August 27, 2020

Han JIANG