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• CN 62-1112/TF
• ISSN 1005-2518
• 创刊于1988年

## 单轴压缩试验中减弱端部效应新型方法研究

1.重庆大学煤矿灾害动力学与控制国家重点实验室，重庆 400044

2.重庆大学资源与安全学院，重庆 400044

## Study on a New Method of Weakening End Effect in Uniaxial Compression Test

JIA Jingkai,1,2, HUANG Gun,1,2, WANG Long1,2, CHENG Qiang1,2, ZHEN Libing1,2

1.State Key Laboratory of Coal Mine Disaster Dynamics and Control，Chongqing University，Chongqing 400044，China

2.School of Resources and Safety Engineering，Chongqing University，Chongqing 400044，China

Received: 2020-12-23   Revised: 2021-03-04   Online: 2021-07-14

Abstract

As a difficult point in rock mechanics，end effect has been widely concerned by scholars.The phenomenon of end effect is caused by the friction between the indenter of testing machine and rock specimen in the process of compression，which usually causes the measured compressive strength of rock increased.However，the more intuitive performance of end effect is that the radial strain of rock is larger in the middle and smaller at both ends.According to Saint-Venant’s principle，when the height diameter ratio is greater than 2.0，the influence of the end effect can be ignored，but only the influence of the end effect on the measured uniaxial compressive strength can be ignored，which doesn’t form uniform stress in the specimen.Therefore，the method of weakening the end effect should be further studied.By summarizing the research of domestic and foreign scholars，it is found that there are two main methods to reduce the end effect，they are reducing the end friction and controlling the end deformation.The first method is to add friction reducing agent between the indenter of the testing machine and the end of the rock specimen，but it can’t completely eliminate the end effect.The second method is to use the metal specimen with the same mechanical parameters as the rock specimen as the cushion block，but it is too difficult to find this kind of metal material.Based on the second idea，to reduce the influence of end effect in the measurement of uniaxial compressive strength，a new uniaxial compression test method was proposed.In this method，uniaxial compression test was carried out using the rocks with the same material as the specimen as the cushion block.Two combination forms of （25+50+25）mm and （20+60+20）mm are set for the experiment，and the experimental results are compared with those specimens with the height of 50 mm and 60 mm.The results show that the new experimental method can reduce the influence of end effect on the measurement of uniaxial compressive strength of rock and obtain more uniform radial strain.Compared with the specimens with the height of 50 mm and 60 mm，the uniaxial compressive strength of the specimens with the height of （25+50+25）mm and （20+60+20）mm is reduced by 38.41% and 39.69% respectively，ad-ditionally，the strength is also decreased comparing with the standard specimen.Numerical simulation results show that the uniaxial compressive strength of the combined specimen with or without end friction is close to the uniaxial compressive strength of the standard specimen in ideal state.All of the specimens have uniform radial strain without end friction.The radial strain of the combined specimen is relatively uniform when there is end effect.Numerical simulation results prove that the new experimental method reduces the end effect，but it can’t completely eliminate the end effect.

Keywords： rock mechanics ; end effect ; uniaxial compression ; numerical simulation ; sandstone ; cushion block ; strain distribution

JIA Jingkai, HUANG Gun, WANG Long, CHENG Qiang, ZHEN Libing. Study on a New Method of Weakening End Effect in Uniaxial Compression Test[J]. Gold Science and Technology, 2021, 29(3): 382-391 doi:10.11872/j.issn.1005-2518.2021.03.003

## 1 单轴压缩试验

### 图1

Fig.1   Combined specimen form

Table 1  Test scheme

10500
20600
3206020
4255025
501000

## 2 试验结果及分析

Table 2  Uniaxial compressive strength of rock specimens

### 图2

Fig.2   Radial strain at different positions of specimens

### 图3

Fig.3   Failure pattern of rock in each test

### 图4

Fig.4   Numerical model

Table 3  Parameters of numerical model

$Fmax=cA+tanϕFn-μA$

$kn=ks=10maxK+43GΔzmin$

Table 4  Contact surface parameters of different schemes

1接触面-100

2接触面-1415

### 3.2 模拟结果分析

Table 5  Uniaxial compressive strength of all tests

5058.32
86.67
25+50+2558.09
58.60
10059.27
58.04

### 图5

Fig.5   Section location

（1）无端部摩擦时模型的径向应变分析。无端部摩擦时，A-A′剖面径向应变云图如图6所示。B-B′剖面（模型边缘）上待测试件的径向应变在轴向不同高度上的分布如图7所示。其中零点位置位于待测试件边缘中间点。

### 图6

Fig.6   Radial strain nephogram without end friction

### 图7

Fig.7   Radial strain at different positions without end friction

（2）有端部摩擦时模型的径向应变分析。有端部摩擦时，A-A′剖面径向应变云图如图8所示。从图8（a）中可以看出，单一试件模型端部区域的径向应变较小，中部区域的径向应变较大；由图8（b）可以看出，组合试件模型中待测试件径向应变分布比单一试件模型更均匀，佐证了室内试验结果，提高了其可信度。根据图8（b），容易发现接触面-2对径向应变变化具有较大的影响，两端垫块受到的端部效应影响无法完全通过接触面，进而影响到待测试件的变形，使得待测试件的径向应变分布较为均匀。相较于图8（a）中单一试件模型的表现，组合试件模型中待测试件的径向应变分布仍表现为端部较小、中部较大，符合受到端部效应影响的特征，但其受到的端部效应影响比单一试件模型小，说明增加垫块的试验方式减弱了待测试件受到的端部效应的影响。通过使用与待测试件具有相同物理力学性质的金属垫块进行试验，可以最大限度地消除端部效应，获得均匀的径向应变，上述试验现象表明端部效应并未完全消除。

### 图8

Fig.8   Radial strain nephogram with end friction

### 图9

Fig.9   Radial strain at different positions with end friction

## 4 结论

（1）端部效应在单轴压缩试验中的影响主要表现在影响实测的岩石单轴抗压强度和导致岩石径向不均匀变形上。

（2）分别对比高度为（25+50+25）mm组合试件与50 mm单一试件、（20+60+20）mm组合试件与60 mm单一试件可知，组合试件的单轴抗压强度分别降低了38.41%和39.69%；对比组合试件与标准试件可知，组合试件的单轴抗压强度低于标准试件；对比径向应变发现，组合试件中待测试件的径向变形更均匀。说明采用岩石本身作为垫块，将待测试件与垫块组合起来进行单轴压缩试验，有助于减弱端部效应对试验结果的影响。

（3）组合试件模型在有端部摩擦时获得了与单一试件模型无端部摩擦时极为接近的单轴抗压强度，该强度值与岩石试件理想状态下的抗压强度值也很相近；无端部摩擦时，2种试件均可获得均匀的径向应变，有端部摩擦时，仅组合试件模型中待测试件有较为均匀的径向应变。

http://www.goldsci.ac.cn/article/2021/1005-2518/1005-2518-2021-29-3-382.shtml

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