考虑初始孔隙性的含瓦斯煤岩力学本构关系与损伤演化研究

冲击地压及瓦斯次生灾害更易在深部复杂的地质构造区域发生,其实质是煤岩介质在变化多样的地应力、采动应力与瓦斯共同作用下的力学平衡系统失稳过程,准确描述含瓦斯煤岩受载过程的力学关系是明确煤岩动力灾害孕育与发生机制的根本。考虑原生裂隙对煤岩力学性质的劣化特性,将其定义为初始损伤,理论推导了以孔隙率表征的初始损伤公式,综合考虑初始孔隙率、吸附瓦斯煤岩基质膨胀、瓦斯运移的软化特性及真三向应力状态,构建了基于非均匀性统计理论的含瓦斯煤岩损伤演化方程及力学本构模型,并定性分析了各参量对煤岩力学性质的影响规律。结果表明:当煤岩初始孔隙率越高时,任意截面的裂纹长度、宽度和条数均增加,有效面积减小,局部更易产生应力集中,损伤曲线在峰值损伤量更高及峰后段增长率略降低,反之,当均质度越高时,其峰值点处的损伤值越低,损伤演化越滞后并集中于峰后阶段发展,相应的损伤演化曲线斜率越大;随瓦斯压力升高,煤岩吸附产生的膨胀应力明显增加,吸附参量a、b值同样对膨胀应力具有促进作用;弹性模量和均质度对煤岩强度及峰后应力降模量均呈现为正相关影响,使煤岩冲击危险性增强;初始损伤和较高的瓦斯压力均促进了煤岩中裂隙的发育,降低了煤岩性质的均一性,使强度和峰后应力降模量减小而塑性特征越发显著并促进煤岩软化,这也是高瓦斯煤层冲击地压低于常规指标发生的原因。

饱和土体固结压缩和蠕变的热力学本构理论及模型分析

岩土热力学模型(thermodynamic soil model,TSM)是基于颗粒固体的非平衡态热力学理论,建立的一种崭新的描述岩土力学问题的统一理论模型。该模型引入"颗粒熵运动"和"弹性弛豫",对土体颗粒层次的耗散机制进行了合理地考虑,这些使得模型能够更深入描述土体的变形和能量耗散机理,从而能够在统一理论框架中描述岩土体复杂多变的物理力学行为。基于该理论模型,研究了饱和土体的固结压缩和蠕变问题,分析了加载速率、应力/应变路径和非单调荷载等因素的影响规律。模拟结果表明:模型具有描述复杂条件下的饱和土体的固结压缩和蠕变特性的能力,具有较高的理论和工程应用价值。

混凝土在多维应力状态下的损伤本构模型

已有的混凝土平面应力有限元分析均是利用单轴应力-应变本构关系,其中经验系数的选取带有相当的主观色彩,因此十分有必要建立混凝土在多轴应力特别是二维应力状态下的本构关系并直接应用于非线性分析.基于连续损伤力学的理论框架,作者建立了理论上较为完备、应用较为简单的确定性混凝土拉剪弹塑性损伤本构模型.模型数值模拟和已有试验结果的对比表明,本文建议模型能够很好的描述双轴应力条件下混凝土材料的弹性区域包络线以及强度包络线,同时应力-应变全过程结果和Kupfer (Kupfer et al.1969)的试验结果吻合良好,能够很好的模拟混凝土双轴受压强度的提高和双轴拉压应力状态下的"拉压软化效应",这对混凝土结构的非线性分析是非常重要的.

分布式光纤裂缝传感技术的试验研究

为探讨光纤—混凝土复合体在开裂过程中形成的力学—光学本构关系,本文采用模型试验方法对混凝土裂缝监测的分布式光纤传感力学—光学本构关系进行了实测标定,将试验结果中的混凝土裂缝宽度和光损耗值绘于坐标图上,分别采用半对数坐标幂函数拟合和普通坐标的多项式拟合2种方法建立了二者之间的函数关系。2种拟合方法结果的比较表明,普通坐标多项式拟合法优于半对数坐标幂函数拟合法。

基于人工神经网络的固体推进剂细观损伤与宏观刚度映射关系

作为一种高夹杂比颗粒增强聚合物基复合材料,固体推进剂的宏观力学性能主要由其细观结构决定。外加载荷下,初始缺陷或细观颗粒团聚均可诱发局部应力集中,导致颗粒-基体细观界面脱粘,材料宏观力学性能劣化。如何构建细观损伤与宏观性能间的映射关系,已成为推进剂细观实验结果合理运用、固体火箭发动机结构灾变准确预报的关键。为此,本文发展了基于连续介质力学框架的人工神经网络,以变形梯度的不变量为输入、自由能为输出,遴选现有自由能函数和损伤增长函数形式为神经网络设计激活函数,使神经网络先验地满足变形连续性、坐标不变性、热力学一致性等要求。基于上述物理相容性,神经网络能在稀疏训练数据条件下快速收敛,还能够自下而上地实现损伤状态的遗传映射。最后,采用有限元分析获取的数据集,验证了该网络模型对不同预损伤下的推进剂在单轴拉伸、等双轴拉伸、纯剪切3种加载条件下的宏观刚度预报能力。

低温下复合材料层合板的层间应力分析

以低温推进剂的纤维增强树脂基复合材料燃料罐为对象,研究在低温环境下复合材料/结构的力学性能。基于细观力学模型,分别建立了用于复合材料贮箱的2种典型复合材料E-Glass/Epoxy和Graphite/Epoxy低温环境下的本构关系。采用有限元法,通过对低温环境下典型铺层的层合板自由边界附近层间应力的分析,讨论了低温对E-Glass/Epoxy和Graphite/Epoxy这2种典型复合材料层合板自由边界附近层间应力分布的影响。结果表明:温度的变化对层合板层间应力分布的影响较明显,且温度越低层合板的最大层间应力值越大。研究成果将对低温推进剂的纤维增强树脂基复合材料燃料罐设计提供一定的参考价值。

Theoretical analysis of thermoviscoelastic contact between friction lining and wire rope in mine friction hoists

Serious accidents of mine hoists caused by high-speed sliding between friction lining and wire rope are often seen in coal mines.In order to solve this problem,we analyzed the contact characteristics between friction lining and wire rope.Then we carried out a dynamic mechanical analysis(DMA) to explain the change in mechanical properties of the friction lining as function of temperature and load frequency and found that temperature has a stronger effect on the mechanical properties than the frequency.We used multiple regression analysis to obtain the thermoviscoelastic constitutive relations of the friction lining.As well we derived the analytic solution for the thermoviscoelastic contact radius and pressure by combining the theory of viscoelastic contact mechanics with thermoviscoelastic constitutive relations.

Research on damage evolution of metal plate based on improved micropolar peridynamic model

To study the damage evolution of the metal plate in elastic and plastic deformation stages, an improved micropolar peridynamic model is proposed to simulate the deformation process and damage evolution of metal materials with variable Poisson’s ratios in the elastic-plastic stages. Firstly, both the stretching and bending moments of the bonds between the material points are added to peridynamic pairwise force functions, and the coordinate transformation of the micro-beam made up of bonds is deduced. Therefore, the numerical calculation implementation of the improved micropolar peridynamic model is obtained. Then, the strain values are obtained by solving the difference equation based on the displacement values of material points, and the stress values can be calculated according to generalized Hook’s law. The elastic and plastic deformation stages can be estimated based on the von Mises yield criterion, and different constitutive equations are adopted to simulate the damage evolution. Finally, the proposed micropolar peridynamic model is applied to simulate the damage evolution of a metal plate with a hole under velocity boundary conditions, and the effectiveness of the model is verified through experiments. In the experiments, the displacement and strain distributions in the stretching process are analyzed by the digital image correlation(DIC) method. By comparing the results, the proposed model is more accurate than the bond-based peridynamic model and the error of the proposed model is 7.2% lower than that of the bond-based peridynamic model. By loading different velocity boundary conditions, the relationship between the loads and damage evolution is studied.

单轴压缩条件下不同冲击倾向性煤岩损伤与破碎能演化规律研究

研究围绕“同一矿井煤岩试样会出现强度相近但峰后应力降模量明显差异,进而引起多种冲击倾向性判定结果和能量演变特征”的现象,以物理实验为基础,分析考虑了孔、裂隙对有效承载面积的削减作用,建立了基于孔隙率的初始损伤表达式,结合非均匀性统计理论构建了煤岩本构关系及损伤演化方程,基于煤岩破碎后块体分布的自相似性,理论推导获得了以分形维数表征的破碎块体新增表面积及耗散能密度计算公式并实验验证。研究表明:随所受载荷增加,煤岩损伤演化曲线呈现为“受载初期平直、临近峰值陡增”的典型“S”型变化,且损伤在峰值点均低于0.5并集中于峰后阶段发展,冲击倾向性增强,其初始和应力峰值损伤越低,损伤越集中于峰后发展;认为煤岩产生损伤演化特点随冲击倾向性变化主要是因孔裂隙发育及不同物质组成所致,孔隙率逐渐增加使试样更易在缺陷部分发生微裂隙扩展并与相邻的缺陷部分汇聚而形成宏观破裂;弱冲击倾向试样宏观煤岩成分具有更多暗煤,反之冲击倾向性增强煤岩中镜煤、亮煤占比增加,两种宏观煤岩组分虽均使煤岩展现出较高的抗压强度,但暗煤韧性大,与之相对应的亮煤、镜煤脆性特征更为显著,表现出峰后软化及损伤差异;冲击倾向性增强,煤岩破碎程度增加即分形维数升高、碎块分布向更小尺寸集中;随碎块分形维数增加,破碎耗散能理论曲线呈斜率升高的“下凹”型增长,更为直观地反映了煤岩冲击倾向性与破碎度、盈余能间的正相关关系。

An experimental investigation into electromyography, constitutive relationship and morphology of crucian carp for biomechanical “digital fish”

Currently, the integrated biomechanical studies on fish locomotion come into focus, so it is urgent to provide reliable and sys- tematic experimental results, and to establish a biomechanical ＂digital fish＂ database for some typical fish species. Accord- ingly, based on the control framework of ＂Neural Control - Active Contraction of Muscle - Passive Deformation＂, the elec- tromyography （EMG） signals, the mechanical properties and the constitutive relationship of skin, muscle, and body trunk, as well as morphological parameters of crucian carp, are investigated with experiments, from which a simplified database of bio- mechanical ＂digital fish＂ is established. First, the EMG signals from three lateral superficial red muscles of crucian carp, which was evolving in the C-start movement, were acquired with a self-designing amplifier. The modes of muscle activity were also investigated. Secondly, the Young＇s modulus and the reduced relaxation function of crucian carp＇s skin and muscle were de- termined by failure tests and relaxation tests in uniaxial tensile ways, respectively. Viscoelastic models were adopted to deduce the constitutive relationship. The mechanical properties and the angular stiffness of different sites on the crucian carp＇s body trunk were obtained with dynamic bending experiments, where a self-designing dynamic bending test machine was employed. The conclusion was drawn regarding the body trunk of crucian carp under dynamic bending deformation as an approximate elastomer. According to the above experimental results, a possible benefit of body effective stiffness increasing with a little energy dissipation was discussed. Thirdly, the distribution of geometric parameters and weight parameters for a single experi- mental individual and multiple individuals of crucian carp was studied with experiments. Finally, considering all the above re- suits, generic experimental data were obtained by normalization, and a preliminary biomechanical ＂digital fish＂ database for crucian carp was established.

Thermodynamic properties and constitutive relations of crystals at finite temperature

The dynamics equation for each individual atom is established directly around the equilibrium state of the system of N atoms based on the inter-atomic potential energy of EAM model.Using the theory of lattice dynamics and periodical boundary condition,the 3N×3N stiffness matrix in eigen equations of vibration frequencies for a parallelepiped crystal is reduced to a 3n×3n matrix of eigen equations of vibration frequencies for a unit lattice.The constitutive relation of the crystal at finite temperature is extracted based on the quantum-mechanical principle.The thermodynamic properties and the stress-strain relationships of crystal Cu with large plastic deformation at different temperatures are calculated,the calculation results agree well with experimental data.

Theoretical model of double-walled carbon nanotube pullout from a composite matrix

A micromechanical model of double-walled carbon nanotube (DWCNT) pullout from a composite matrix is presented with the interfacial residual stress and van der Waals (vdW) forces taken into consideration.The interfacial residual stress induced by thermal expansion coefficient (TEC) mismatch is introduced via thermo-elastic constitutive relations.The influence of vdW interactions between two layers of DWCNT on the interfacial stress distributions of DWCNT and matrix is analyzed.The analytical expressions of interfacial shear stress and the axial stresses of DWCNT and matrix are derived,respectively.Furthermore,the influences of temperature change,interfacial friction coefficient,DWCNT aspect ratio,DWCNT volume fraction and the relative modulus between DWCNT and matrix are illustrated and discussed.