粘性土试样高度对抗拉强度的影响

针对粘性土抗拉强度测试问题,本文设计改装了土样拉伸试验仪,可完成单轴拉伸、巴西劈裂和轴向压裂三种抗拉强度试验。通过不同方法测定土体的抗拉强度,研究了试样高度对各方法强度测量值的影响及其规律。试验结果表明,试样高度对轴向压裂试验测试结果的影响最大,对巴西劈裂试验测试结果影响很小,几乎不影响单轴拉伸试验测试结果。轴向压裂试验中,抗拉强度测试值随试样高度h增大而减小,随衬垫直径增大而增大。巴西劈裂试验中,试样抗拉强度测试值随试样高度增大略有增大。     

径向压缩下圆环砂岩样的力学特性研究

为了研究径向压缩下圆环试样孔壁处的应力特征,利用厚度为34 mm,外径为50 mm的完整圆盘及不同内径（8～30 mm）的圆环砂岩试样,在巴西劈裂试验中测量孔壁的应变变化,分析试样的力学特性。试验结果表明：圆环试样的峰值载荷随着内径的增大而逐渐减少。圆盘和内径较小的圆环试样达到峰值载荷时出现了失稳破坏,载荷迅速跌落：当内径大于 16 mm,圆环试样达到峰值载荷后,载荷略有下降,但是试样并没有出现失稳破坏,而是持续压缩一定时间后才破裂。圆环最弱部位拉应力不是材料参数,而是一个结构参数,且随圆环内径而变化,基于弹性理论的Hobbs公式不能用于计算岩石抗拉强度。孔壁岩石的破裂只能是达到拉伸变形极限才会破裂,不能以拉伸应力作为破坏指标。研究结果为理解岩石在压拉组合下的力学特征提供了参考。     

双圆环直接拉伸试验试样最优尺寸范围研究

直接拉伸试验是测量岩石抗拉强度最直接有效且最有理论和实际价值的方法,但在传统直接拉伸试验中试样加工难度大,测试设备要求高。为克服传统直接拉伸试验加载困难的缺点,准确测定岩石的抗拉强度,设计研发了多直径岩芯双圆环直接拉伸试验机,从试验和数值模拟两方面重点研究了双圆环直接拉伸试验中试样尺寸的影响和最优取值范围,包括外环直径与试样直径比r1/R、内环直径与试样直径比r2/R、内环直径与外环直径比r2/r1三个比值的最优范围。研究结果发现,外环直径和内环直径满足r1/R=0.62±0.08、r2/R=0.45±0.12、r2/r1=0.64±0.06时内外环根部与内外环重叠部位最大应力值相近,获得的抗拉强度与理论值较一致,与其他试验相比,双圆环直接拉伸试验的结果标准差和振荡系数最小。     

新型土工单轴拉伸试验装置的研制及应用

针对目前土工单轴拉伸试验水平的不足,研制了一套新型土工单轴拉伸试验装置,该套装置主要由试样制备、加载、控制与数据采集4个部分组成。试验装置通过燕尾槽与双滑动底板的设计,可以制备不同拉伸段长度的试样并分别开展相应的单轴拉伸试验;通过试样形式与所对应拉伸夹具的设计,解决了试样端部在拉伸过程中出现的松弛与应力集中问题;通过双导轨拉伸装置的设计,避免了试样在拉伸过程中出现的应力偏心现象;通过双级变速箱的设计使最小拉伸速率可达到0.001 mm/min,能够准确描述材料单轴拉伸破坏的演化过程,并能准确测试材料的抗拉强度以及全过程的拉应力-位移关系曲线。基于所研制的试验装置开展了黏性土的单轴拉伸试验,试验结果表明:黏性土的单轴拉伸破坏形式不是纯脆性破坏,而是在抗拉强度后存在一个软化阶段,此时仍具有一定的承载能力;随着试样拉伸段长度的增大,抗拉强度呈对数减小,峰值位移呈对数增大;随着拉伸速率的增加,抗拉强度呈对数增加,峰值位移呈线性增加;抗拉强度与峰值位移均随压实度的递增呈线性增加;随着含水率的递增,试样的抗拉强度先增大后减小,即存在一个峰值,而峰值位移呈线性增加。     

岩石单轴抗拉强度的几何形状效应研究

提出了一种模拟岩石破坏过程的细胞自动机模型,以力、位移作为基本状态变量,基于基本的力学规律构造了细胞自动机模型的矢量局部更新规则,克服了以往规则确定随意性以及标量化的缺点。利用该模型模拟了岩石的单轴破坏过程,研究了非均质性对单轴直接拉伸破坏过程的影响,并研究了几何形状（长宽比）对单轴拉伸强度的影响,研究结果加深了对岩石单轴抗拉强度的了解,对试验有一定的指导作用,应进一步加强岩石的直接拉伸试验研究。     

单轴拉伸法测定土的抗拉强度试验研究

采用单轴拉伸法研究了影响土的抗拉强度测试方法的因素,探讨了加荷速度、样品直径、高径比对土的抗拉强度测试值的影响,试验结果表明,样品直径对土的抗拉强度测试结果影响不大,加荷速度和高径比是影响土的抗拉强度的测试结果的重要因素。给出了单轴拉伸法测试土的抗拉强度的加荷速度和高径比的建议值。     

围岩强度和变形参数的分布特征及可靠性分析

从康家湾铅锌金矿Ⅲ-1号矿体上盘围岩取大量岩样,分别加工制作了50个压缩和拉伸试验的试样。利用RMT-150B伺服试验系统对试样进行单轴抗压、抗拉试验,各获得了50个试验结果。采用假设检验法,分别对50个单轴抗压强度和50个抗拉强度进行检验,结果表明,它们分别服从正态分布和对数正态分布;对50个 、 和50个C、 ,进行不放回抽样,组成50组E、 、C、 。利用FLAC计算软件,对硐室围岩中的应力进行了计算,分别获得了50个最大主应力和50个最小主应力;采用同样假设检验法,证明它们分别服从对数正态和正态分布;根据单轴抗压、抗拉强度及围岩中的最大主应力、最小主应力概率密度函数,计算了硐室围岩不发生拉伸破坏和压缩破坏的可靠度;并对硐室围岩抗剪强度的校核,得出了该地下硐室围岩稳定的结论。     

土体抗拉强度试验研究方法的进展

理论上土体的抗拉强度与抗压和抗剪强度一样是描述土体力学性质的重要指标之一,也是研究土体张拉破坏特性的基础。由于土体抗拉强度在数值上相对较小,且难以准确测量,在岩土工程领域常常被忽视。随着工程中的张拉破坏问题越来越突出,土体抗拉强度特性引起许多学者的关注,相关研究成果也越来越多。文中对土体抗拉强度试验研究方法进行了系统的归纳和总结,对比分析了各种方法的优缺点,认识到,（1）土体抗拉强度试验方法总体上可分为直接法和间接法两大类,直接法是在试样两端直接施加拉力直到试样发生张拉破坏,根据破坏时的最大拉力及对应的破裂面面积计算出土体的抗拉强度。间接法主要通过一些理论假设,把压应力转换成相应的拉应力并基于一些理论公式计算土体抗拉强度;（2）按试样受力条件,直接法可分为单轴拉伸和三轴拉伸,一般都需要开发专门的拉伸试验设备,以实现拉力荷载的施加及其在试样内的有效传递。常用的方式有粘结、锚固、模具夹持及摩擦力传递等,都各有优缺点,但模具夹持法相对而言更具操作性。间接法中比较有代表性的有巴西劈裂试验、土梁弯曲试验和轴向压裂试验等,一般较适应于刚度较大的土体如化学固化土。最后,笔者提出了今后该课题的研究重点,包括制定土体抗拉强度试验方法规范及标准,研发简单易操作的土体拉伸试验设备,拉伸试验过程中土吸力的测量及控制方法,土体拉伸过程中应变场的准确获取方法及土体张拉特性的数值模拟研究等。     

巴西圆盘试验问题与三维抗拉强度准则

巴西圆盘抗拉强度公式是基于弹性力学平面问题解析解,利用最大拉应力准则得到的,但结果与直接拉伸试验得到的强度有较大的差异。根据10种岩石（煤）的直接拉伸试验与弧形劈裂试验数据,分析了弧形加载面积对应圆心角对试样3维应力状态的影响规律,并采用三维抗拉强度准则来分析巴西试样,结果表明此准则更适合于巴西圆盘试验。     

重塑黏土抗拉特性试验研究

利用应变控制式拉伸试验仪研究重塑黏性土的单轴抗拉特性,探究了含水率、干密度、高径比对试样应力应变曲线、抗拉强度、峰值应变以及拉伸能量的影响,并分析了其内在机理,讨论了土体抗拉强度的来源。试验结果表明:（1）含水率和干密度对抗拉强度影响很大,呈线性相关,具体表现为:含水率升高,抗拉强度先增大后减小;干密度越大,抗拉强度越大。在高径比相同的情况下,高度和直径的变化对抗拉强度影响不大;（2）峰值应变由应力应变曲线形态决定,因而主要受土样的抗拉强度、可塑性以及破坏方式影响;（3）应力应变曲线大致可分为四类,曲线形态各不相同;（4）拉伸能量由应力应变曲线形态决定。含水率影响试样的抗拉强度和塑性,干密度影响试样的抗拉强度和内部结构,因而两者对拉伸能量的影响很大。高径比不变时,试样高度和直径的变化对拉伸能量影响不大。     

Use of the Boussinesq Equation for Determining the Distribution of Stress Within a Diametrical Point Load Test

Summary The maximum tensile stress at failure for a dry specimen, as determined by the Boussinesq equation for the diametrical point load test, was found to be in very good agreement with the diametrical point load tensile strength (Is) as defined by ISRM (1985). The force at failure for specimens of different geometry was used to determine the stress distribution along the line of loading. Distinctive tensile stress gradients dominate almost 84 percent of the specimen radius regardless of the size of the specimen. The maximum tensile stress is located away from the centre of the specimen at a distance approximately 76 percent along the specimen radius, measured from the centre. The stress magnitude at the centre of the specimen is small and represents about 13 percent of the maximum tensile stress calculated, which suggests that the initiation of the fracture is not from the specimen centre. At the zone of contact between the specimen and the loading cones there exists great compressive stress in areas where much material destruction occurs under the loading platen cones. The value of this compressive stress varies from specimen to specimen and, for the material used in these experiments (Oolitic limestone), ranges from 5.3 to 7.2 times the dry unconfined compressive strength of the material. According to the ISRM Suggested Method for Point Load Test, Is ₍₅₀₎ is approximately 0.8 times the uniaxial tensile strength. The maximum tensile stress revealed by the Boussinesq equation (Bs) was correlated with Is ₍₅₀₎ and found to be in the order of 0.9 times the uniaxial tensile strength.     

Three-Dimensional Numerical Investigations of the Failure Mechanism of a Rock Disc with a Central or Eccentric Hole

The diametrical compression of a circular disc (Brazilian test) or cylinder with a small eccentric hole is a simple but important test to determine the tensile strength of rocks. This paper studies the failure mechanism of circular disc with an eccentric hole by a 3D numerical model (RFPA3D). A feature of the code RFPA3D is that it can numerically simulate the evolution of cracks in three-dimensional space, as well as the heterogeneity of the rock mass. First, numerically simulated Brazilian tests are compared with experimental results. Special attention is given to the effect of the thickness to radius ratio on the failure modes and the peak stress of specimens. The effects of the compressive strength to tensile strength ratio (C/T), the loading arc angle (2α), and the homogeneity index (m) are also studied in the numerical simulations. Secondly, the failure process of a rock disc with a central hole is studied. The effects of the ratio of the internal hole radius (r) to the radius of the rock disc (R) on the failure mode and the peak stress are investigated. Thirdly, the influence of the vertical and horizontal eccentricity of an internal hole on the initiation and propagation of cracks inside a specimen are simulated. The effect of the radius of the eccentric hole and the homogeneity index (m) are also investigated.     

Indirect tensile test assessments for rock materials using 3-D disc-type specimens

Indirect test methods are usually the preferred approach for determining the tensile strength of brittle and quasi-brittle materials such as ceramics, rocks, and concrete. In this paper, the tensile strength of a rock material (Iranian Harsin marble) was obtained indirectly by means of two disc type samples. The test samples were the well-known Brazilian disc and semi-disc specimen under three-point bend loading. Since the existing formula for determining the indirect tensile strength is based on 2-D stress assumption which ignores the effects of specimen thickness, a general 3-D tensile strength formulation was derived for each sample by employing finite element analysis. A series of tensile strength tests were conducted on Harsin white marble using disc and semi-disc specimens with different thicknesses in parallel to finite element analysis. The test results using 3-D formulation showed that the tensile strength of the tested marble depends on the thickness/diameter ratio of the specimen and generally decreases from 11.5 to 7 MPa on increasing the thickness of test samples. The average values of tensile strength were about 8.8 MPa for the Brazilian disc specimen and 9.8 MPa for the semi-disc specimen. The good agreement existing between the results of the two specimens suggests that the semi-disc specimen can also be used for determining the rock tensile strength in addition to using the conventional Brazilian test.     

The effect of specimen size and stress rate for the Brazilian test—A statistical analysis

Summary The Brazilian test is a widely accepted method for the determination of the tensile strength of intact rock. Specifications for the Brazilian tensile strength test have been established by the American Society for Testing and Materials (ASTM), ASTM D 3967-86 and a suggested approach is provided by the International Society for Rock Mechanics (ISRM). The ASTM and ISRM allow a relatively wide range of values for specimen geometry defined in terms of length to diameter ratio and loading rates defined as either time to failure or stress rate.A statistical study was carried out on a coal measure sandstone to determine whether the tensile strength determined by the Brazilian test is independent of the specimen geometry and the stress rate.     

岩石巴西圆盘试验中的空间拉应力分布

指出目前人们使用了40多年的岩石巴西圆盘试验拉伸强度公式来自二维弹性力学解答,该公式不适用于实际情况所对应的三维弹性力学问题。为了得到三维条件下试样内部的应力分布规律,采用三维有限元分析软件Marc对试样进行了弹性受力分析,结果表明,试样横截面上的拉应力分布规律与二维条件下的情况相类似,但横截面上的应力值沿试样厚度方向是有变化的,越靠近两端面,水平拉应力越大。分析发现,对于高径比为1、泊松比为0.25的巴西圆盘试样,按二维公式计算的抗拉强度比真实值小23.3 %     

平台中心角对岩石抗拉强度测定影响的数值分析

岩石抗拉强度是岩石的一个重要力学参数,在测试岩石抗拉强度的平台巴西盘劈裂试验中,平台中心角对测试结果会产生重要影响,但测试中对此角度尚无严格规定,从而造成抗拉强度测试结果有很大的离散性。从细观的角度着手,应用数字图像技术对岩石材料内部不同细观介质的空间分布进行精确测量和数值表达,建立与有限元网格之间的相互映射关系,并引入到原有的岩石破坏过程分析系统RFPA中,建立了能反映岩石非均匀性的平台巴西盘数值模型。通过对花岗岩断面图像进行处理,运用数值模型研究了平台中心角对平台巴西盘轴线应力分布、劈裂破坏模式和抗拉强度测定的影响,确定平台中心角的合理值应在20～30?的范围内,这与试验结果相一致,从机制上进一步分析验证了平台中心角的取值范围,为测试中平台中心角的选取提供了理论依据。