基于0.2定围压比状态下的约束混凝土性能研究

通过对混凝土施加0.2定围压比,使单轴受压混凝土裂缝发展受到约束,研究了混凝土在接近无裂纹发展状态下的受压力学性能。本文以在MTS815.02岩石电液伺服系统上对不同强度等级的混凝土试件在定围压比为0.2时的单调、循环加卸载受压常规三轴试验为基础,研究分析了0.2定围压比对混凝土受压应力-应变规律的影响和围压对混凝土裂缝发展的限制效果,并根据0.2定围压比下混凝土材料的特殊性能,提出了无裂纹发展状态下混凝土上升段表达式及参数。分析结果表明,0.2定围压比下,各强度混凝土本构关系均表现出了类似金属材料的本构关系,且低强混凝土试件应力-应变的提高幅度优于中强混凝土试件。综上所述,0.2定围压比改善了混凝土在受压时无征兆失效(脆性破坏)的力学性能。     

冲击载荷下脆性空心颗粒破碎机理

为考察脆性空心颗粒在冲击载荷作用下的应变率效应和破碎行为的细观机理,以粉煤灰漂珠为研究对象,基于低速冲击实验和有限元数值模拟,对比了典型空心颗粒材料在不同加载速率下的力学响应特性和细观压溃行为,阐释了材料宏观应变率效应产生的细观机理,获得以下结果。（1）在0.001～300 s?1应变率范围,漂珠颗粒的破碎率和Hardin破碎势平均提升了约21%和10%～30%,材料比吸能提升了50%～125%,比吸能的额外增加主要与动态颗粒滑移产生的摩擦耗能相关。颗粒平均尺寸较大的试样体现出更强的应变率效应。（2）初始压溃阶段的应力应变响应特征的数值模拟结果与实验结果较吻合,低速冲击下动态二次压溃现象产生的细观机理为动态颗粒滑移和压紧行为对加载速率的依赖性。(3) 数值模拟表明,冲击加载下产生相同应变时颗粒的损伤程度和范围大于准静态加载,这与实验所得破碎势随应变率增加的结果一致。对比低速冲击实验的相对破碎势分析和细观数值模拟结果可知,脆性颗粒堆积材料在动态冲击下表现出的宏观应变率效应主要归因于颗粒压溃行为的率敏感性和动态加载下颗粒破碎能量利用率的降低。     

水泥砂浆在主动围压下的动态力学性能

水泥砂浆等脆性材料的力学性能与所受围压的大小密切相关。为了研究水泥砂浆在围压下的动态力学性能,研制了适于分立式Hopkinson压杆加载的主动围压装置,最大预加载主动围压大于30 MPa。实验得到了水泥砂浆在不同围压、不同应变率下的轴向应力应变曲线,发现材料在围压作用下抗压强度和韧性大大提高并且整体进入了伪塑性,而材料的应变率效应也是显著的。     

材料在准—维应变下被动围压的SHPB试验方法

本文提出了改进的SHPB被动围压试验方法,可用于研究材料在准一维应变下的动态力学响应。分析中计及了试样的径向变形,并讨论了试验结果与围压套筒材料性质、几何尺寸等的关系,从而可进一步推得材料的动态泊松比υ2、动态杨氏模量E2、动态剪切模量G以及动态压缩强度Y。又分析了试验中的摩擦力效应及其对围压下轴向应力应变关系的影响,完成了对材料弹性常数的修正。     

考虑围压效应的冻结砂土动态本构模型研究

为描述主动围压作用下冻结砂土的动态力学特性，通过在朱-王-唐模型的非线性体上串联塑性体，建立了能够考虑围压效应的冻结砂土动态损伤本构模型；分析了损伤参数对应力-应变曲线特征、屈服点、峰值应力和峰值应变的影响规律，基于冻结砂土动力学试验数据确定了模型参数；通过将模型和试验数据进行对比，并对不同试验条件下模型的预测误差进行分析，验证了模型的适用性和准确性。结果表明，损伤参数对应力-应变曲线弹性阶段和屈服点无明显影响，而对塑性阶段和破坏阶段的影响较为显著，本构模型预测的应力-应变曲线与试验结果具有较好的一致性。模型能够预测围压引起冻结砂土塑性阶段占比大和屈服点明显的特征，且能够描述围压对冻结砂土动态强度的增强效应；不同负温和主动围压条件下，模型对峰值应力和屈服强度的预测效果优于峰值应变和屈服应变。     

V-5Cr-5Ti的常温动态压缩力学性能

采用分离式霍普金森压杆(SHPB)装置,在常温下对V-5Cr-5Ti合金分别进行了应变率效应试验、 限制应变试验和应变累积的动态压缩试验;分析了应变率、应变累积对V-5Cr-5Ti合金动态压缩性能的影响; 并对压缩试验后的试件进行了金相分析。结果表明,V-5Cr-5Ti合金具有明显的应变率和应变历史效应,而 应变历史对材料的动态压缩性能的影响更明显。在高应变率下,应变低于0.20时,出现孪晶组织;而应变高 于0.20时,孪晶组织急剧减少甚至消失。     

混凝土类材料动态压缩强度在多维应力状态下的应变率效应

对混凝土类材料动态压缩应变率效应研究的发展及问题进行了概述,对比不同应力状态下混凝土类材料动态压缩应变率效应的表现特征,揭示了不同加载路径下实测动态强度提高系数的显著差异。研究表明,在高应变率下,基于初始一维应力加载路径的试件将因横向惯性效应导致的侧向围压而演化至多维应力状态,传统霍普金森杆技术无法获得高应变率下基于真实一维应力路径的动态强度提高系数,在强度模型中直接应用实测数据将过高估计材料的动态强度。鉴于应变率效应的加载路径依赖性,将仅包含应变率的强度提高系数模型扩展至同时计及应变率和应力状态的多维应力状态模型,并结合Drucker-Prager准则在强度模型中给予了实现。针对具有自由和约束边界试件开展的数值霍普金森杆实验表明,多维应力状态下的应变率效应模型可以考虑应变率效应随应力状态改变的特点,从而准确预测该类材料的动态压缩强度。研究结果可为正确应用霍普金森杆技术确定脆性材料的动态压缩强度提供参考。     

钢质套筒被动围压下混凝土材料的冲击动态力学性能

为了研究混凝土材料在钢质套筒侧限约束下的动态力学性能参数和破坏规律,采用分离式大直径（75 mm）SHPB实验技术,测试了钢质套筒侧限约束下不同混凝土试件在不同载荷作用下轴向或径向的应力、应变峰值,平均应变率,计算了混凝土材料的损伤值,描述了加载破坏现象,对实验结果进行了分析。结果表明:混凝土材料在被动围压下,延性、抗破坏能力得到加强,具有明显的增强效应。被动围压下SHPB实验中混凝土材料的破坏应变为典型SHPB实验中破坏应变的1.8～2.8倍;破坏应力达到150 MPa以上,为静力学无围压条件下的2～5倍。     

水泥砂浆石在准一维应变下的动态力学性能研究

以改进的SHPB被动围压试验方法 ,研究了水泥砂浆石在准一维应变下的动态力学响应。分析讨论了试验结果与围压套筒材料性质、几何尺寸等的关系。得到了水泥砂浆石的动态泊松比、动态杨氏模量、动态压缩强度。分析了试验中的摩擦力效应及其对围压下轴向应力应变关系的影响 ,完成了对材料弹性常数的修正。     

围压状态下的混凝土本构模型

利用已有不同围压下的混凝土三向压缩试验, 分析研究不同围压条件对混凝土强度和应力-应变关系的影响。试验表明, 把混凝土简单视为脆性材料的观点并不符合实际情况, 塑性特征也是混凝土材料主要力学特征。由热力学定律出发, 本文提出了多次方形式的屈服函数和与围压状态相关联的损伤标准化函数, 提出了考虑围压状态的混凝土本构模型。试验结果与数值计算比较表明, 该本构关系可以很好地描述混凝土在不同围压状态下的力学损伤发展和脆-塑性转换力学特征, 也可以描述拉伸作用下的应力-应变关系, 具有一定的普遍意义。     

三向受压混凝土的动态特性研究

为探究混凝土在三向受压状态下的动态特性,利用自行研制的大型多功能三轴材料试验机,进行不同应变速率(10^-5/s、10^-4/s、10^-3/s、10^-2/s)下混凝土不同定侧压比(1:1、2:1、3:1、4:1)的动态真三轴抗压试验,研究了混凝土在动态抗压下的强度和变形特性。结果表明:混凝土在三向受压状态下表现出明显的应变率效应,峰值应力随着应变速率的增加先减小后增大,峰值应力减小最大幅度为5.42%而后开始增大,最大增幅为18.22%。而峰值应力随着侧压比的增大而增大,到达3:1时应力速率敏感性降低。混凝土的峰值应变在低侧压比(1:1和2:1)时随着应变速率的增加而减小,而在侧压比较高(3:1和4:1)时先减小再增大。随着应变速率的增加,裂纹开始从骨料处产生,塑性应变增大,导致峰值应变增大;在动态加载条件下,峰值应变随着侧压比的增大有先增大后减小的趋势。较高侧压比时混凝土的弹性模量随着应变速率的增加具有增大的趋势,最大增幅为240.66%。应力-应变曲线上升段随着应变速率增大越来越陡峭。试件的破坏形态随着侧压比的增大从柱状破坏变为斜剪破坏。     

Micromechanical analysis of damage in saturated quasi brittle materials

In this paper, we propose a micromechanical analysis of damage and related inelastic deformation in saturated porous quasi brittle materials. The materials are weakened by randomly distributed microcracks and saturated by interstitial fluid with drained and undrained conditions. The emphasis is put on the closed cracks under compression-dominated stresses. The material damage is related to the frictional sliding on crack surface and described by a local scalar variable. The effective properties of the materials are determined using a linear homogenization approach, based on the extension of Eshelby’s inclusion solution to penny shaped cracks. The inelastic behavior induced by microcracks is described in the framework of the irreversible thermodynamics. As an original contribution, the potential energy of the saturated materials weakened by closed frictional microcracks is determined and formulated as a sum of an elastic part and a plastic part, the latter entirely induced by frictional sliding of microcracks. The influence of fluid pressure is accounted for in the friction criterion through the concept of local effective stress at microcracks. We show that the Biot’s effective stress controls the evolution of total strain while the local Terzaghi’s effective stress controls the evolution of plastic strain. Further, the frictional sliding between crack lips generates volumetric dilatancy and reduction in fluid pressure. Applications of the proposed model to typical brittle rocks are presented with comparisons between numerical results and experimental data in both drained and undrained triaxial tests.     

轴向拉伸下氮化钛纳米杆变形机制及力学性能的分子动力学研究

本文使用分子动力学软件包lammps并采用第二近邻改进型嵌入原子法(2NN MEAM)模拟了单晶氮化钛纳米杆的轴向拉伸破坏过程,分析了分别沿[100]、[111]晶向的不同截面尺寸、不同拉伸应变率、不同温度下的氮化钛纳米杆的力学性能,详细描述了氮化钛纳米杆拉伸变形过程。研究发现, 拉伸晶向、截面尺寸、拉伸应变率及温度均会对TiN纳米杆的拉伸变形过程及屈服强度、弹性模量等力学性能产生不同程度的影响。 沿[100]晶向的拉伸,截面尺寸越大,屈服强度越低;而沿[111]晶向,截面尺寸越大,屈服强度越大。应变率越大,屈服强度及屈服应变越大,但对于弹性模量几乎无影响。温度越高,材料的屈服强度、屈服应变及弹性模量越小,断裂应变越大。不同拉伸条件下的氮化钛纳米杆的拉伸过程均包括弹性变形、塑性变形与断裂阶段。[100]晶向的弹性模量都要高于[111]晶向。     

Pressure-shear waves in 6061-T6 aluminum and alpha-titanium

Thepressure-shear plate impact technique is used to study material behavior at high rates of deformation. In this technique, plastic waves of combined pressure and shear stresses are produced by impact of parallel plates skewed relative to their direction of approach. Commercially pure alpha-titanium and 6061-T6 aluminum are tested under a variety of pressure and shear tractions by using different combinations of impact velocities and angles of inclination. A laser interferometer system is used to monitor simultaneously the normal and transverse components of motion of a point at the rear surface of the target plate. The experimental results are compared with numerical solutions based on an elastic/viscoplastic model of the material. Both isotropic and kinematic strain hardening models are used in the computations. The results indicate that unlike the normal velocity profiles, the transverse velocity profiles are sensitive to the dynamic plastic response and, thus, can be used to study material behavior at high strain rates. For the materials tested the results suggest that the flow stress required for plastic straining increases markedly with increasing strain rate at strain rates above 10⁴s^?1. Hydrostatic pressure of the order that exists in the tests (up to 2 GPa) does not affect the plastic flow in 6061-T6 aluminum and appears to have at most a minor effect on the deformation of the titanium.     

Phenomenological modeling of the response of a dense colloidal suspension under dynamic squeezing flow

The split Hopkinson pressure bar experimental technique is used to evaluate the squeezing flow response of a concentrated, discontinuously thickening colloidal suspension of spherical silica particles loaded at high stresses/strain rates. These results provide insight into the transitional behavior of these materials, as well as the post-transitional response under compressive loading. A method of analyzing the strain and strain rate dependent behavior is presented to identify modes of material response (viscous, elastic, etc.). Experimental results are presented as stress–strain–strain rate plots and a surface fitting approach is used to develop a phenomenological model describing the overall response. From this model, it is possible to identify regions of elastic and viscous behavior using a gradient analysis approach. It was found that, after an initial period of viscous deformation, the suspension behaves like a viscoelastic material – this regime corresponds well with transition in which large clusters of particles percolate. This is followed by a third, viscous regime in which the material undergoes viscous deformation. At the highest stresses, a plateau region of plastic deformation has been identified. This approach and the conditions under which it may be applied are described in detail in the paper.     

Contact laws between solid particles

This paper presents a comprehensive study for the contact laws between solid particles taking into account the effects of plasticity, strain hardening and very large deformation. The study takes advantage of the development of a so-called material point method (MPM) which requires neither remeshing for large deformation problems, nor iterative schemes to satisfy the contact boundary conditions. The numerical results show that the contact law is sensitive to impact velocity and material properties. The contact laws currently used in the discrete element simulations often ignore these factors and are therefore over-simplistic. For spherical particles made of elastic perfectly plastic material, the study shows that the contact law can be fully determined by knowing the relative impact velocity and the ratio between the effective elastic modulus and yield stress. For particles with strain hardening, the study shows that it is difficult to develop an analytical contact law. The same difficulty exists when dealing with particles of irregular shapes or made of heterogeneous materials. The problem can be overcome by using numerical contact laws which can be easily obtained using the material point method.     

动力荷载对围压卸载下岩石动态变形模量的影响

利用SHPB岩石动静组合加载实验系统,研究在不同轴压水平下围压以1 MPa/s速率卸载至预加值50%时矽卡岩受频繁冲击作用的动态变形模量变化规律。实验结果表明:高轴压促使岩石内部微裂纹萌发与扩展,降低了岩石抵抗外部冲击的能力。围压的侧向约束阻碍岩石内部裂纹的横向扩展,但在围压卸载时会加剧岩石内部的损伤,这是由于高轴压下,围压卸载导致岩石内部应力重新分布。轴压与围压卸载共同影响着冲击作用下的岩石动态变形模量,通过岩样在冲击荷载下的能量耗散分析岩石动态变形模量的变化规律,有助于了解深部岩体开挖的破坏机制。     

考虑塑性和刚度不匹配的焊接残余应力估算

现有残余应力计算方法未能考虑材料塑性变形和焊接接头刚度不匹配的影响,使得焊接残余应力计算结果和实际残余应力存在较大偏差.在2219-T87铝合金钨极氩弧焊焊接头残余应力测试基础上,提出一种基于非线性有限元和材料弹性模量分区的残余应力—释放应变曲线的残余应力计算方法,研究了材料塑性变形和接头刚度不匹配对焊接残余应力计算的影响.结果表明,焊接接头中非均质材料塑性不匹配可以引起对于残余应力计算的较大误差;材料塑性变形对残余应力的影响大于接头刚度不匹配对残余应力的影响.所提出方法修正了传统方法在焊接接头的残余应力计算中由于未考虑接头非均质材料塑性不匹配而引起的误差.