基于损伤塑性模型的混凝土双K断裂参数确定

利用混凝土损伤塑性模型,对混凝土三点弯曲梁(TPB beam)损伤-断裂过程进行数值模拟;再通过数值模型,从损伤起始临界时刻定义起裂荷载,由峰值荷载时刻损伤因子的分布确定裂缝失稳扩展时刻的临界等效有效裂缝长度,提出了确定混凝土起裂断裂韧度、失稳断裂韧度的新方法.研究结果表明:基于损伤塑性模型的混凝土三点弯曲梁数值模型,...     

FRP增强预制裂缝混凝土的断裂性能

为了探讨纤维增强复合材料(FRP)增强预制裂缝混凝土的断裂性能,得到单层FRP对不同裂缝深度混凝土的最佳增强效果,通过相同尺寸、不同初始缝高比(0.2、0.3、0.4、0.5)的FRP增强预制裂缝混凝土三点弯曲梁断裂试验,基于FRP-混凝土界面黏结滑移模型,得到了FRP增强预制裂缝混凝土三点弯曲梁的断裂性能参数.结果表明:FRP增强预制裂缝混凝土的荷载-裂缝口张开位移(P-CMOD)曲线的峰值荷载大于普通混凝土的峰值荷载,且FRP增强预制裂缝混凝土峰值荷载随着初始缝高比的增加先增大后减小;初始缝高比从0.2变化到0.5时,FRP增强预制裂缝混凝土的起裂韧度可认为是常数;随着初始缝高比的增加,FRP增强预制裂缝混凝土的失稳韧度呈先增大后减小的趋势,当初始缝高比为0.4时,失稳韧度达到最大值,表明此时增强效果最佳;相比于普通混凝土失稳荷载后的脆性断裂破坏,FRP增强混凝土断裂破坏过程中的延性得到明显提高.     

混凝土三点弯曲梁失稳韧度预测方法研究

基于采用起裂韧度扩展准则的三点弯曲梁断裂全过程数值模拟,研究了混凝土失稳韧度的预测方法及其受黏聚力分布的影响.该方法将起裂韧度作为裂缝起裂及扩展的判断标准,通过理论分析及数值迭代计算得到峰值荷载、临界有效裂缝长度及失稳韧度,摆脱了传统方法需要测量临界状态下的断裂参数才能得到失稳韧度的束缚.利用不同尺寸和不同最大骨料粒径的三点弯曲梁试验对本计算方法进行了验证.分析了不同黏聚力分布模型对失稳韧度及裂缝断裂全过程曲线的影响.研究表明本方法可以较好地预测混凝土失稳韧度,黏聚力分布模型对失稳韧度计算结果影响较小。     

预设裂缝沥青混合料梁三点弯曲试验及扩展有限元模拟

对含预设裂缝沥青混合料梁进行三点弯曲试验,探讨梁底部裂缝位置改变对于裂缝扩展行为的影响。基于随机骨料生成和投放算法建立沥青混合料梁细观异质模型,并采用扩展有限元方法对其进行了不同骨料粒径以及骨料、空隙共存形式下裂缝扩展行为分析,探讨骨料粒径的改变、空隙的加入对于梁内部应力分布、抵抗裂缝扩展以及引起路面沉陷的影响。研究内容对于梁裂缝扩展影响因素的分析及路面裂缝扩展行为数值模拟具有一定的参考价值。     

含直裂缝混凝土梁断裂过程区试验研究与分析

为了研究带预制裂缝的素混凝土梁在荷载作用下裂缝尖端微裂区扩展规律,采用三点弯曲梁试验方法借助于数字图像相关法观测混凝土的损伤、断裂过程。采用有限断裂力学模型和修正最大切应力准则分析不同缝高比及梁高对含裂缝的混凝土起裂荷载、极限荷载、断裂过程区及裂缝尖端名义应力的影响。试验结果与理论分析对比表明:两种理论模型预测的断裂过程区长度比较接近且与试验值有较好的相关性。     

应力波作用下三点弯曲梁破坏过程数值模拟

岩石类准脆性材料三点弯曲梁冲击荷载作用下的破坏受应力波加载速率以及应力波峰值的影响。使用新发展的并行RFPA2D-Dynamic数值模拟工具对三点弯曲梁冲击荷载作用下的破坏过程进行了数值模拟,研究了不同加载速率和加载峰值的破坏规律。数值模拟结果与物理实验结果具有较好的一致性。在低加载速率下,三点弯曲梁发生拉伸破坏,且只出现一条裂纹;在较大加载速率时出现多条平行拉伸裂纹;在大加载速率时出现拉伸、剪切复合裂纹。应力波峰值较小时形成单一的拉伸裂纹,而随着加载峰值的提高,形成多条平行拉伸裂纹。     

基于细观数值模拟的混凝土断裂过程区特性研究

基于细观数值模拟方法研究了混凝土裂缝尖端断裂过程区特性。采用多尺度方法对不同尺寸的三点弯曲梁试件进行建模,并利用非局部损伤模型模拟了断裂过程区的演变过程。结合现有试验对数值计算方法进行了验证。研究表明:基于数值模拟方法得到的荷载—裂缝口张开位移曲线与试验结果吻合较好。峰值荷载时刻对应的临界断裂过程区长度随着试件高度的增加而逐渐增加,临界损伤区域面积亦相应增加,临界断裂过程区长度与试件高度的比值随试件高度的增加趋于稳定。     

循环荷载作用下花岗岩疲劳力学性质及其本构模型

 循环荷载作用下岩石力学性质研究对完善岩石力学基本理论和指导相关工程建设具有重要意义。通过花岗岩三轴循环荷载试验,系统研究花岗岩疲劳力学特性,提出花岗岩疲劳力学模型。研究结果表明：(1) 岩石残余应变和变形模量与循环次数之间关系与岩石体积变形状态相关;(2) 在应力–应变全空间内,花岗岩疲劳性质分为3个区域,不同区域内微观机制不同;(3) 岩石疲劳破坏门槛值应为剪缩和剪胀区域分界点对应的峰值偏应力;(4) 循环荷载作用下岩石疲劳势有别于单调加载时塑性势,循环荷载作用下岩石表现出比单调加载时更强的抵抗体积变形能力;(5) 提出基于内变量理论的岩石疲劳本构模型,试验数据与模拟预测对比显示模型较好地反映出岩石疲劳力学性质。     

岩石类材料裂纹扩展贯通的近场动力学方法模拟

在连续体内引入预制裂纹的数学描述,运用近场动力学方法对含有预制裂隙(不同倾角的单裂缝和不同岩桥倾角的三裂缝)的岩石类材料试件的单轴压缩试验进行数值模拟。结果表明,随着裂缝倾角的增加,翼型裂纹出现的位置逐渐向预制裂隙的两端移动,次生共面剪切裂纹与次生倾斜裂纹出现的时间与形态也随之改变;不同倾角的岩桥呈现的断裂贯通形式有着明显的差异,但裂纹均是在预制裂缝尖端首先产生,随后逐渐扩展贯通,最终导致试件的整体失稳破坏。数值模拟与室内试验结果的对比分析表明近场动力学方法可以很好地模拟岩石类材料的裂纹扩展贯通形态,反映裂隙扩展的基本力学机制;作为一种新的非局部数值模型,其在岩石材料与岩体工程数值研究领域具有广阔的应用前景。     

体外预应力节段预制试验梁力学性能数值分析

提出一种有限元模型,用于体外预应力节段预制试验梁力学性能数值分析。该有限元模型用ANSYS通用分析软件中的SOLID65单元建立试验梁的3D实体模型,以接触单元模拟节段之间的干接缝,用节点耦合法实现体外预应力束与混凝土梁之间的连接。考虑混凝土材料非线性和几何非线性,通过数值模拟分析揭示试验梁的结构行为、混凝土接缝处的应力变化规律和裂缝张开情况。该有限元模型研究了预应力配束形式、体外预应力二次效应和转向处滑移对试验梁弯曲力学性能的影响。数值分析揭示了在荷载300kN以前,节段接缝处于受压状态;在300kN以后,裂缝张开是影响试验梁力学性能的主要因素。研究结果可用于预制节段桥梁的设计。     

Mode I fracture toughness determination with straight notched disk bending method

A new method called the straight notched disk bending method is developed for mode I fracture toughness determination using rock cores. Disk specimens of andesite and marble having a single straight edge notch were subjected to three-point bending loads. Dimensionless stress intensity factor estimations and fracture toughness tests were conducted for different notch lengths, span lengths, thicknesses and diameters of the cylindrical rock specimens. Stress intensity factors were computed by three-dimensional finite element modeling and the results were presented for a wide range of specimen geometrical parameters. Results of experiments were compared to the results of well-known mode I fracture toughness testing methods. For specimens having thickness equal to the radius, mode I fracture toughness was lower and close to the results obtained by semi-circular bending method. When thickness was increased and doubled, mode I fracture toughness increased and approached to the value found by the suggested cracked chevron notched Brazilian disk method. Advantages of the new method included easy specimen preparation and testing procedure, stiffer specimen geometry, smaller fracture process zone, and flexibility of the specimen geometry for the investigation of the size effect behavior.     

混凝土等效断裂韧度的解析方法

基于虚拟裂缝模型,提出一种计算混凝土三点弯曲梁等效断裂韧度的解析方法,并将虚拟裂缝面上黏聚力分布曲线简化为双线性.采用该方法,只要测得混凝土弹性模量Ec和抗折强度fr,就可以计算梁的极限衙载P()和临界等效裂缝长度ac,进而求得等效断裂韧度Kic,而不需要进行断裂试验.     

Peak shear strength prediction for discontinuities between two different rock types using a neural network approach

The peak shear strength of discontinuities between two different rock types is essential to evaluate the stability of a rock slope with interlayered rocks. However, current research has paid little attention to shear strength parameters of discontinuities with different joint wall compressive strength (DDJCS). In this paper, a neural network methodology was used to predict the peak shear strength of DDJCS considering the effect of joint wall strength combination, normal stress and joint roughness. The database was developed by laboratory direct shear tests on artificial joint specimens with seven different joint wall strength combinations, four designed joint surface topographies and six types of normal stresses. A part of the experimental data was used to train a back-propagation neural network model with a single-hidden layer. The remaining experimental data was used to validate the trained neural network model. The best geometry of the neural network model was determined by the trial-and-error method. For the same data, multivariate regression analysis was also conducted to predict the peak shear strength of DDJCS. Prediction precision of the neural network model and multivariate regression model was evaluated by comparing the predicted peak shear strength of DDJCS with experimental data. The results showed that the capability of the developed neural network model was strong and better than the multivariate regression model. Finally, the established neural network model was applied in the stability evaluation of a typical rock slope with DDJCS as the critical surface in the Badong formation of China.

     

动载荷作用下含偏置裂纹 三点弯曲梁破坏过程的数值模拟

 偏置裂纹三点弯曲梁被广泛应用于研究I–II复合型裂纹的扩展过程。利用岩石破裂过程分析(RFPA)程序，对含偏置裂纹三点弯曲梁在动载荷作用下的破坏过程进行数值模拟，研究偏置裂纹的位置(用g 来表达)对三点弯曲梁破坏模式的影响。数值模拟结果表明：当g ≤0.745时，偏置裂纹尖端首先起裂，并沿着一定的角度向上扩展，最终贯通整个试样的高度；当g ＞0.745时，在偏置裂纹发生扩展的同时，在梁底部中心位置也萌生出一条中心裂纹，但最终只有中心裂纹贯通整个试样的高度。数值模拟在现了实验中观测到的2种典型破坏模式及其对应的临界g 值。     

New artificial neural networks for true triaxial stress state analysis and demonstration of intermediate principal stress effects on intact rock strength

Simulations are conducted using five new artificial neural networks developed herein to demonstrate and investigate the behavior of rock material under polyaxial loading. The effects of the intermediate principal stress on the intact rock strength are investigated and compared with laboratory results from the literature. To normalize differences in laboratory testing conditions, the stress state is used as the objective parameter in the artificial neural network model predictions. The variations of major principal stress of rock material with intermediate principal stress, minor principal stress and stress state are investigated. The artificial neural network simulations show that for the rock types examined, none were independent of intermediate principal stress effects. In addition, the results of the artificial neural network models, in general agreement with observations made by others, show （a） a general trend of strength increasing and reaching a peak at some intermediate stress state factor, followed by a decline in strength for most rock types; （b） a post-peak strength behavior dependent on the minor principal stress, with respect to rock type; （c） sensitivity to the stress state, and to the interaction between the stress state and uniaxial compressive strength of the test data by the artificial neural networks models （two-way analysis of variance; 95% confidence interval）. Artificial neural network modeling, a self-learning approach to polyaxial stress simulation, can thus complement the commonly observed difficult task of conducting true triaxial laboratory tests, and/or other methods that attempt to improve two-dimensional （2D） failure criteria by incorporating intermediate principal stress effects.     

含偏置缺口玄武岩原位三点弯曲细观断裂研究

 通过含偏置缺口的玄武岩试件在扫描电镜(SEM)下的三点弯典实时加载试验,得到不同偏置缺口的荷载–位移曲线以及高倍率裂纹扩展照片。一方面,根据矿物颗粒在岩石中总是以一定三维尺度存在,认为裂纹不仅在表面非线性扩展,而且沿厚度方向也是非线性扩展,结合断裂力学中表面能S的计算公式,给出一个考虑岩石非均值和裂纹纵向非线性扩展的表面能密度表达式,并用这个表达式成功解释裂纹扩展中的一些随机现象;另一方面,通过对不同偏置量的荷载–位移曲线对比分析,给出含偏置缺口玄武岩三点弯典荷载–位移曲线的3个典型阶段,并解释各个阶段的微观机制,提出用偏置量影响系数计算偏置缺口三点弯典试样的断裂性能并给出其计算公式,通过对不同偏置缺口试件峰值荷载以及断裂耗能的分析计算,得到峰值荷载与断裂耗能的偏置量影响系数表达式。     

A generic method for rock mass classification

Rock mass classification (RMC) is of critical importance in support design and applications to mining, tunneling and other underground excavations. Although a number of techniques are available, there exists an uncertainty in application to complex underground works. In the present work, a generic rock mass rating (GRMR) system is developed. The proposed GRMR system refers to as most commonly used techniques, and two rock load equations are suggested in terms of GRMR, which are based on the fact that whether all the rock parameters considered by the system have an influence or only few of them are influencing. The GRMR method has been validated with the data obtained from three underground coal mines in India. Then, a semi-empirical model is developed for the GRMR method using artificial neural network (ANN), and it is validated by a comparative analysis of ANN model results with that by analytical GRMR method.     

Compression,Shear and Bending Performance of X-type Lattice Truss Panel Structure by Theoretical Method and Simulation

X-type lattice truss panel structure (LTPS) with high strength-weight ratio has anisotropic mechanical properties. This paper proposes a theoretical mechanical model to give predictions of the compression, shear and bending performance of LTPS including the effective compressive/shearing modulus, effective compressive/shearing strength and failure loads under three-point bending. These properties are expressed as analytical expressions involving all of the geometrical parameters. The numerical simulation is also performed, and the effects of the geometrical parameters are discussed. The results show that the predicted effective modulus and strength by the theoretical method agree well with the simulated results. The effective shear modulus in 23-direction is obviously higher than that in other directions. The geometrical parameters have significant effects on the mechanical properties. The shear yielding strength increases with the increase of truss width and thickness, while it decreases with the increase of truss length and platform length. The truss thickness has the greatest effect on yielding strength, followed by the truss width and the truss length. The changes of the yielding strength with the increase of the angle from horizontal and deflected angle are not monotonic. The failure loads of three-point bending by face sheets yielding and shear induced core collapse decrease, when the span increases. The proposed analytical expressions are available to the engineering application and more complicated calculations of structures made of X-type cores.     

不同深度玄武岩的三点弯曲细观破坏实验研究

 现有深部岩石力学研究大多是从某一深度取出岩样,然后施加不同围压用于模拟不同深度,由此来研究不同深度情况下岩石的物理力学行为这一情况,然而实际中如果深度发生变化,不仅围压发生变化,岩石自身的微细观结构也会发生变化。针对此情况,选取北京门头沟一个有特点的区域,该区域从地表到深度1 100多米都有玄武岩。借助带加载装置扫描电镜(SEM)系统实时观察中心预制缺口不同深度玄武岩、偏心单裂纹及双裂纹玄武岩的细观破坏行为,揭示深度导致岩石力学性能变化的细观机制。研究发现,三点弯曲玄武岩试样中裂纹多数从预置缺口处萌生,进而发展成一条连续主裂纹。不同深度玄武岩的破坏荷载及断裂能随埋深呈增加趋势,主要原因是随着深度增加玄武岩更为致密,孔隙率小;相同偏心距下,偏心单、双2组试样峰值荷载和断裂能都几乎相同,且都随偏心距的增大而线性增加,主要原因是偏置裂纹试件缺口附近受到剪切力和弯矩的双重影响,但随着偏心距离的增加,弯矩影响在逐渐减少,这导致了峰值荷载和断裂能增加。     

钢纤维体积率对高强混凝土断裂性能的影响

通过对相对切口深度为0.2、0.3、0.40、.5的钢纤维高强混凝土三点弯曲试验,研究钢纤维体积率对高强混凝土断裂性能的影响。结果表明:在相对切口深度为0.20、.3、0.4、0.5的条件下,随着钢纤维体积率的增大,钢纤维高强混凝土断裂韧度和断裂能均显著增加。通过对试验结果的统计分析,分别建立断裂韧度、断裂能与劈拉强度之间的关系式。