A new method and instrument for quickly measuring key mechanical parameters of rock

Abstract

The application discloses a novel method and corresponding measuring device for rapidly measuring key mechanical parameters of rock, including scratch hardness, Brinell hardness, Young's modulus and Mohr-Coulomb parameters (uniaxial compressive strength and internal friction angle). The equipment system comprises a portable frame, which contains a set of horizontal linear driving equipment, equipped with a set of linear encoders for accurately measuring horizontal displacement; the vertical direction contains a set of servo vertical driving equipment, equipped with a set of incremental magnetic encoders for accurately measuring vertical displacement, and a set of linear variable differential transformers (LVDT) for accurately sensing the slight concave-convex fluctuation of the rock surface; the servo motor drives the hemispherical composite piece through a screw rod; the size of the force in the horizontal and vertical directions of the scratch is measured by a bidirectional force sensor installed above the composite piece. The horizontal linear motor and the vertical servo motor are both loaded at a uniform speed; according to the size of the measured force, the scratch hardness and the Brinell hardness of the rock can be calculated; according to the theoretical solution of the slip line field hemispherical tooth, the Mohr-Coulomb parameters of the rock, including the uniaxial compressive strength (or cohesive force) and the internal friction angle, can be inversely calculated. When the vertical servo motor is loaded alone, the Brinell hardness of the rock can also be calculated; the Young's modulus can be calculated by the stiffness during unloading.

Description

Technical Field

[0001] This invention relates to the measurement of basic mechanical parameters of rocks, and more specifically, to a novel rapid measuring device and method. Background Technology

[0002] In numerous rock-related industries, including mining, oil and gas, tunnels, caverns, dams, foundations, slopes, and underground shelters, many designs involve the basic mechanical properties of rocks, such as rock hardness, Young's modulus, and Mohr-Coulomb parameters (uniaxial compressive strength / cohesion and internal friction angle), involving an extremely large economic scale.

[0003] Due to its simplicity, ease of use, and low cost, hardness testing has been widely used in rock engineering. Currently, there are various rock hardness indices, broadly categorized into three types: 1) Scratch hardness, including the Mohs scratch test (1812) and the Cerchar wear test (Suana and Peter, 1982); 2) Indentation hardness, including Brinell hardness (1900), Rockwell hardness (1914), Vickers hardness (Smith and Sandland, 1922), and NCB cone indenter hardness (1969); and 3) Dynamic rebound hardness, including Shore hardness (1910), Schmidt hardness (1951), and Leeb hardness (1978). However, each hardness index is based on experience, not on the physical properties of rocks, so their measured values ​​are only relative. For the past half-century, academia and industry have invested significant human, material, and financial resources in attempting to establish relationships between various hardness values ​​and rock physical parameters, but these efforts remain largely empirical.

[0004] Among the fundamental mechanical properties of rocks, the most common parameters include Young's modulus and Mohr's Coulomb parameters (cohesion and internal friction angle). The traditional measurement method is the three-pressure test (the most classic and important mechanical test in rock mechanics). This involves core sampling, cutting, grinding, standard sample preparation, sealing, fixing, installing an LVDT (Low Volume Demand Thruster), slowly adding hydraulic oil to the predetermined confining pressure and maintaining it constant, applying a constant axial strain rate (usually very slow, such as 0.1% / minute) until the sample reaches its peak (or fails), and recording the changes in axial stress and axial (and radial) strain throughout the process. Due to the heterogeneity of rocks, it is generally recommended to repeat the experiment at least 2-3 times under each confining pressure; then, repeat the above experimental steps at least 3-5 times under different confining pressures, plot the rock Mohr envelope, and finally calculate the Mohr parameters. This process is time-consuming and expensive, and it is a destructive experiment for the rock core (an irreparable loss for deep, precious rock cores); at the same time, it cannot achieve real-time parameter acquisition on-site, making it difficult to meet timeliness requirements. A single experiment (excluding core fabrication and preceding processes) takes about half a day and costs approximately 10,000 RMB. A complete set of experiments takes about three to five days and costs several hundred thousand RMB. Therefore, there is an urgent need in engineering for a cost-effective, non-destructive, rapid strength testing technology. If coreless field testing could be further developed, it would be of great help to many engineering projects.

[0005] The scratch method for measuring rock strength was first proposed by Professor Detourney of the University of Minnesota in the 1990s during experiments. His team discovered that when cutting rock at a shallow depth using a rectangular, sharp diamond composite disc, the rock's "intrinsic" mechanical specific energy and uniaxial compressive strength were close. Furthermore, using a composite disc with a wear surface, they found that the friction angle on the wear surface was somewhat close to the rock's internal friction angle. This discovery was patented in EU 91201708.4 (1991) and USA 5,670.711 (1997), the former focusing on the method and the latter on the equipment. Detourney's team manufactured the first such scratch device (date unknown); subsequently, Schlumberger, the world's largest oilfield services company, manufactured a second similar device in 2014, naming it the Mechanical Profiler Tester; his graduate student Richard returned to Belgium and founded EPSLOG, manufacturing a third similar device in 2015, named Wombat.

[0006] Compared to the traditional three-dimensional pressure test method for determining the Mohr-Coulomb parameters of rocks, the Minnesota scratch method has the following advantages: 1. Simple operation and rapid experiment (30 seconds for half a day); 2. Non-destructive to the rock core, a minimally invasive test (cutting depth is generally only about 1 mm); 3. Simple scratch instrumentation; 4. Lower sample preparation requirements; 5. High repeatability and low error in multiple tests. However, compared to the traditional three-dimensional pressure test method for determining the Mohr-Coulomb parameters of rocks, the Minnesota scratch method also has obvious shortcomings: a. The method actually lacks theoretical basis and is only a summary of experimental observations; b. Although under certain conditions, the mechanical specific energy of a single-toothed tip ( ) and rock compressive strength ( While there is a strong correlation, many factors actually affect mechanical specific energy, including: 1) back slope angle, 2) depth of cut, 3) aspect ratio (rectangular teeth), 4) horizontal cutting speed, and 5) sampling point length. The selection of these parameters will affect the specific values ​​measured. c. Its experiments are mainly for measuring the single compressive strength of rocks, and there is very little verification of the internal friction angle of rocks. This is actually because it is inaccurate, which is why it has not been widely publicized. This is due to the problem with the method itself, which is also its important weakness. d. Its published strength comparison data are basically limited to medium-hard to soft sandstone. For hard rock, the Minnesota scratch method will actually fail because the cutting force of hard rock scratching always fluctuates greatly and cannot find a stable stage, while the Minnesota scratch method requires a stable cutting force. This is another major problem with the Minnesota scratch method. In addition, its equipment is claimed to be mobile, but in reality, its three types of actual equipment are not truly mobile, let alone coreless field testing. Summary of the Invention

[0007] The overall objective of this invention is to retain the advantages of the scratch method for measuring rocks while overcoming the significant shortcomings of the current Minnesota method. Specifically, regarding the equipment, one objective is to provide a fast and truly portable scratch testing device; another objective is that this portable device can perform coreless measurements; regarding the method, a third objective is to provide a measurement method derived from first theory, rather than based on simple empirical observation; a fourth objective is to reduce factors affecting the measurement; a fifth objective is that this method can not only measure the hardness of rocks, but also calculate the uniaxial compressive strength and internal friction angle of the rocks from the hardness, and can also measure the Young's modulus of rocks through separate uniaxial indentation or uniaxial compressive strength tests; and a sixth objective is that this method is applicable not only to medium-hard rocks, but also to very hard rocks.

[0008] Figure 1The overall design of the system is summarized in five parts: Part 1 is the vertical servo loading system, including a servo motor 10, a lead screw (covered), and a drive slider 30; Part 2 is the engraving execution device, including a rigid body connection 20, a ball-type linear variable differential transformer (LVDT) 40, an incremental magnetic encoder 50 and an attached magnetic scale 51, a two-dimensional bidirectional force sensor 60, a composite sheet clamp 70, and a hemispherical composite sheet 80; Part 3 is the transverse (horizontal) linear drive system, including a rock clamp 100, a rock sample 110, a linear encoder 140 and a linear scale 141, a horizontal support frame and guide rail 150, a horizontal linear motor unit 160, a rock support plate 170, and a pair of support vertical plates 180; Part 4 is the servo support and steering system, including a pair of wide and thick vertical plates 190, a steering compass 200, a handle 201, and four pairs of fixing bolt and nut assemblies 192, and a large fastening nut 210; The fifth part is the system support and sliding system, which includes a system bracket 120, movable casters 130, and fixed base 131.

[0009] During the scratch test, the horizontal linear motor unit 160 drives the overall vertical servo system to load at a constant speed (approximately 100-200 mm / s) to the left along the horizontal slide bar. Simultaneously, the vertical servo motor 10 drives the lead screw to load the slider 30 downwards at a constant speed (approximately 0.1-0.5 mm / s), and the scratching actuator moves downwards along with it. When the vertical displacement reaches a set value (e.g., approximately 0.1-0.5 mm), the servo motor adjusts to the initial vertical cutting position, and the constant speed loading is repeated. Because the vertical system is driven to the left by the third servo support, the hemispherical composite sheet 80 will leave a periodic mark of increasing depth on the rock surface, such as... Figure 2 As shown.

[0010] In order to achieve the goal of coreless measurement Figure 3 The diagram shows the physical relationship of the vertical plate 190, the steering compass 200, the handle 201, the four pairs of fixing bolt and nut assemblies 192, and the tightening nut 210 (the bolts and nuts have mating threads, which are not shown in the diagram). The vertical plate 190 has eight 16mm through holes evenly distributed around its circumference, serving to adjust the direction. During adjustment, first loosen the four pairs of M16 bolt and nut assemblies 192, then use the handles 201 to rotate the entire vertical servo loading system to the desired direction angle, and then tighten the four pairs of bolt and nut assemblies 192. There is no need to worry about the vertical loading system falling during operation, as the servo system is tightened into the four M16 threaded holes in the center of the compass 200 by bolts, and the compass 200 is fixed by the inner shaft and the nut 210. Figure 4 Seven other different loading azimuth angles are shown. At these seven azimuth angles, the composite sheet acts directly on the inner wall of the culvert or mine rock.

[0011] The second part of the engraving execution system employs two displacement reading systems. The first system, a ball-type linear variable differential transformer (LVDT) 40, uses tiny spherical balls to sense minute variations in the rock surface's unevenness. The displacement Δh currently read by the LVDT actually reflects the degree of unevenness on the rock surface. The second system, an incremental magnetic encoder 50 and an attached magnetic scale 51, is used to measure the actual cutting depth of the composite section. For core samples, a relatively smooth and flat surface is required; however, for field measurements, it is difficult to ensure that the inner walls of culverts or mine rocks are uniformly smooth and flat. In this case, the displacement read by the first LVDT is fed back to the central control system, which then adjusts the servo motor accordingly based on Δh. The adjustment method is as follows: Assuming the horizontal speed is constant at u (mm / s), the current vertical speed is actually v (mm / s), the horizontal distance between the center of the ball and the center of the hemispherical composite piece is fixed at L (e.g., 30mm), and the time it takes for the composite piece to reach the current horizontal position of the ball is L / u; within this time interval, the target vertical displacement of the composite piece is Lv / u, but due to the surface unevenness, the actual displacement deviates by Δh; to compensate for this deviation, the slider 30 needs to be actively adjusted upwards or downwards by Δh (depending on the sign of Δh), and the adjusted speed of the slider is v + uΔh / L; based on this, the servo motor speed is adjusted according to the ratio of the lead screw diameter and the thread pitch. This adjustment is completed in a very short time, and the cutting depth of the composite piece after adjustment takes into account the influence of the rock unevenness. However, this adjustment is only a fine-tuning and cannot be used to significantly reduce the requirements for the smoothness and straightness of the rock surface, otherwise it will cause a large human error.

[0012] The following is a brief introduction to the method for measuring rock parameters. This method is based on the slip line theory and derives the cutting forces (horizontal forces) in two directions for hemispherical teeth at minimal cutting depths. and vertical force The theoretical analytical solution, which includes the radius of the composite hemisphere. Current depth of entry The cohesion of rocks The internal friction angle of the rock And the coefficient of friction at the interface between the composite sheet and the rock. Here, the contribution term of the hemispherical composite is only its radius. One method significantly simplifies the influence of five geometric parameters on the rectangular composite sheet with chamfered and worn surfaces in the Minnesota method, greatly improving measurement accuracy. At minimal depths of cut (hence the name "scratch"), the rock naturally enters a fully fractured mode under the action of the hemispherical teeth, avoiding the uncertainties of depth of cut and chamfering in the Minnesota method. In this fully fractured mode, the friction angle at the contact surface between the rock and the composite sheet is approximately equal to the internal friction angle of the rock. Mathematically, this means that two known cutting forces... and Then the parameters of two unknown rocks can be completely determined. and .

[0013] In fact, experiments have also verified that, for five types of rocks with completely different hardness, at extremely small cutting depths, the horizontal force... and vertical force The depth of cut is linear in the initial stage of scratching. Figure 5 Experimental curves showing the cutting force and depth of cut for a certain type of marble are presented. Therefore, we define the scratch hardness as... The vertical hardness is , in and The slope of the distribution in the linear stage of the cutting force (e.g.) Figure 5 (As shown). Furthermore, it can be easily proven that... This is precisely the definition of Brinell hardness, therefore the vertical hardness we obtain... The Brinell hardness obtained from traditional indentation tests has the same meaning. Actual measurement data also confirms the similarity between the two. The internal friction angle of a rock is given by the formula... Calculation, where The cohesion of rocks = . Attached Figure Description

[0014] Figure 1 Overall design of the portable scratch instrument Figure 2 Schematic diagram of scratches on the core sample surface Figure 3 Solid figure showing the physical relationship of the vertical plate, steering compass, handle, bolt and nut assembly, and tightening nut. Figure 4 Seven different loading azimuth angles Figure 5 Experimental curves of cutting force and depth of cut for a certain marble-limestone. Figure 6 Example of rock sample scratches Figure 7 Comparison of Mohr parameters and three-dimensional pressure test results for scratch measurement of a certain marble. Figure 8 Comparison of results from limestone scratch test and three-dimensional pressure test Figure 9 Comparison of scratch test and three-dimensional pressure test results of a certain granite Figure 10 Unloading stiffness in rock indentation test.

Claims

1. The scratch instrument features a portable design.

2. The design for coreless field testing of the scratch instrument includes a multi-directional connection design for the compass and bracket.

3. The scratch instrument uses two sets of precise displacement measuring devices in the vertical direction. One set measures the depth of the composite sheet, and the other set measures the unevenness of the rock surface to correct the speed of the servo motor.

4. Lateral drive uniform loading is approximately 0.1-0.5 mm / s, and vertical drive uniform loading is approximately 100-200 mm / s.

5. The scratch uses a hemispherical tooth shape, rather than other tooth shapes, and the material is polycrystalline diamond or cemented carbide composite sheet.

6. This scratch tester simultaneously measures transverse scratch hardness and Brinell hardness.

7. Based on the theoretical analytical solution of the slip field, the method and formula for calculating the cohesion and internal friction angle of rocks from scratch hardness and Brinell hardness.

8. This system can also drive a vertical loading system independently for indentation experiments, and use the method and formula for calculating Young's modulus using the indentation unloading stiffness.

9. Vertical drive can also be applied at a constant speed independently, thus serving as a traditional Brinell hardness test.

10. Vertical drive can also independently and uniformly load and compress traditional rock samples, replacing uniaxial compression tests (especially suitable for field use).

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