Theory comes from practice and needs to be tested by practice. Experiments are the basis of all scientific research, and the research of rock mechanics also begins with experiments. Although no relevant experimental records have been found in ancient times, Egyptians and Greeks did consider the strength of rocks when they built pyramids and temples thousands of years ago [3]. Dujiangyan built by Qin Zhaowang (306 ~ 25 BC1), Liu Zhu Tomb built by Li Bing and his son (128 ~ 16 BC), Guishan Tomb in Xuzhou, and Sui Daye (58 1 ~ 666 AD) are not difficult to get relevant information on the Internet today with Google and other search tools. Of course, many ancient projects failed. As literature [4] said, all the early activities were of course carried out without the help of modern knowledge. In some cases, the project is a success, usually a dramatic success; ; However, in other cases, we know that they have not succeeded. Many cathedrals were not so lucky as Wells, and collapsed during or shortly after construction.

Leonardo da Vinci's Strength Test of Wire with Different Length [3] may be the earliest known mechanical test record (about AD 1500). Galielo G reported the direct tensile strength and bending strength of hollow beams and solid beams in 1638, and studied the bending strength by cantilever beam end loading method [5].

The first recorded rock mechanics testing machine was made by E.-M.Gauthey in about 1770, and its purpose was to design the pillars of St. Genevieve Church. The compressive strength of cubic rock with a side length of 5cm is obtained by lever system, and it is found that the strength of long cylindrical rock is less than that of cubic rock. /kloc-from the late 8th century to the early 9th century, the design and manufacture of testing machines were inspired by the massive construction of bridges (stone bridges and iron bridges). The design and manufacture of each testing machine will exert the technical level at that time to the limit. The 65438+80' s testing machine has been able to automatically record the load-displacement curve of the sample. 1865, the first commercial laboratory was established in London, with a load of 1000000 lbs. The maximum specimen length of compression test can reach 2 1.5 feet, and the side length of section is 32 inches. 19 10, a testing machine with the maximum compression load of/kloc-0 10000000 lb was installed in the arsenal site in Pittsburgh, and then transferred to the Bureau of Standards in Washington, D.C., and the maximum length of the sample was increased to 30ft [6].

Figure 1- 1 Complete curve of marble under conventional triaxial compression.

The figure on the curve is the confining pressure in MPa.

While the load of the testing machine is increasing, the loading mode of the testing machine is also improving. From mechanical loading to hydraulic loading, from unidirectional loading to pseudo triaxial compression. That is, a cylindrical rock sample is placed in the hydraulic cavity, and the rock sample is laterally loaded by the oil pressure, and the rock sample is axially compressed under the condition of maintaining the confining pressure (also called confining pressure or environmental pressure). The conventional triaxial compression test curve of marble published by von Carmen in 19 1 1 is a landmark work (figure 1- 1), and the highest confining pressure reaches 326 MPa [7]. The test results show that for marble, brittleness is only the performance when the stress is low; However, under high stress conditions (such as geological conditions), rocks can produce great plastic deformation and show ductility. When the confining pressure of some coarse-grained marble reaches 3MPa, it can show ductile deformation characteristics [8].

Mao Kiyoo designed a true triaxial testing machine for three-dimensional unequal pressure loading rectangular cylinder samples, and published a series of articles [9] of 1967, discussing the influence of intermediate principal stress on the strength, deformation, brittleness and ductility of rock samples. Figure 1-2 is a set of typical test results. With the increase of intermediate principal stress, the strength of Dunham dolomite increases, while the plastic deformation decreases during the yield process, and the rock tends to be brittle. Brittle failure consumes less energy, while ductile failure consumes more energy. The test results in figure 1-2 show that increasing the intermediate principal stress has little effect on maintaining the integrity of the rock when the minimum principal stress remains unchanged. Undoubtedly, the actual rock mass is in a complex stress state, and its failure mode needs to be studied.

True triaxial test can be loaded in three directions with solid bearing plate [10]. In order to reduce the influence of interference and friction between loading plates, the true triaxial testing machine later adopted hydraulic loading with minimum principal stress [1 1].

The document [12] introduces the development process, main characteristics and corresponding rock mechanics test results of the triaxial loading testing machine at high temperature and high pressure. Griggs device takes solid lead (Pb) or salt (NaCl) as confining pressure medium, and uses two pistons to generate confining pressure and principal stress difference respectively. The confining pressure reaches 3GPa, the temperature reaches 1500℃, and the creep test at high temperature can be carried out for several months [13, 14]. The cubic press system uses six hydraulic cylinders to carry out true triaxial loading on cubic samples in three directions. For example, in references [15, 16], a 2MN(200 tons) cubic press system is used to test a rock sample with a side length of 42mm, and a temperature of 700℃ is introduced into the rock sample from the indenter. If the cylindrical sample is placed in a solid medium, the cubic pressurization system can also be used for high confining pressure and high temperature tests. Document [17] has carried out triaxial compression tests on timely samples with a diameter of 2.9mm and a length of 8.5 ~ 9.5 mm under the conditions of confining pressure of 7GPa and temperature of 2000℃. According to the hexahedral compression system in reference [18], under the working pressure of 700MPa, the load of the hydraulic cylinder can reach 5 MN(500 tons), and the triaxial compression test can be carried out on samples with a diameter of 8mm and a length of 16mm, with the confining pressure of 3.7GPa, the principal stress difference of 4GPa and the temperature of1000. The confining pressure medium is Pyrophyllite (py).

Figure 1-2 Effect of intermediate principal stress on strength and deformation of dolomite samples

The minimum principal stress σ3= 125MPa, and the number on the curve is the intermediate principal stress σ2 in MPa.

Figure 1-3 Complete curve of rock sample under uniaxial compression [20]

1-chalky gray granite Ⅰ; 2- Indiana limestone; 3- Tennessee marble; 4— Chalk gray granite Ⅱ; 5- basalt; 6- Zolenhofen limestone

1935, after Spaceth W put forward the idea of rigid testing machine, he began to study the whole curve of concrete. After more than 30 years, people have taken various measures to improve the stiffness of the testing machine, including improving the stiffness of the testing machine bracket, installing additional rigid facilities parallel to the rock sample, and reducing the length of the loading cylinder. Finally, mercury is even used as the working medium of the loading cylinder. But it was not until 1966 that Cook N G W got the complete curve of uniaxial compression of rock samples on the rigid testing machine under hydrothermal mixed loading [19]. The collection of the whole curve shows that the explosion damage of the rock is caused by the insufficient stiffness of the testing machine, and the rock can still bear the load after reaching the strength. The symbolic work is that 1968 Wawersik W R improved the testing machine, and obtained a series of complete curves of rock samples under uniaxial compression by manual servo control (Figure 1-3), and pointed out that rocks can be divided into Class I and Class II materials according to the stability of uniaxial compression failure [20]. This view is still controversial.

The characteristics of modern mechanical testing machine are loading control and computer processing of data acquisition. The rigid support and feedback control of the testing machine realize the controllable failure of brittle materials, so as to understand the failure process of rocks after reaching the strength limit, study the bearing and deformation characteristics of rocks during the failure process, and create a new era of rock mechanics research. Figure 1-4a shows the axial stress, axial strain and circumferential strain of coal samples during uniaxial compression, and figure 1-4b enlarges the local curve by 5: 1. During the test, the axial load [2 1] is controlled by the circumferential deformation increase rate of the sample of 4mm/3600sec, and the testing machine samples once every second, with data of ***3600 groups. During the loading process, brittle failure will occur locally in the coal sample, which will suddenly increase the circumferential deformation; In order to maintain a constant circumferential deformation rate, the testing machine will unload the servo control axially to reduce the circumferential deformation, and then continue to load axially.

Figure 1-4 uniaxial compression process of coal sample on servo testing machine

A- the whole process of testing; Local amplification

At present, the microscopic changes of particle structure caused by rock deformation are studied by electron microscope scanning and CT technology. In the process of rock failure, sound and electromagnetic phenomena are also tested by various devices [22 ~ 27].