CN112444449B - An accurate method for calculating the tensile strength of rock based on microscopic failure type analysis - Google Patents
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Abstract
一种基于微观破坏类型分析的精确求解岩石抗拉强度的方法，属于岩石力学与岩土工程领域。首先，岩石试样上下方均匀设置声发射传感器，采用岩石力学试验机对岩石试样进行直接拉伸试验，并进行全过程声发射实时监测。其次，提取声发射信号的主频，统计声发射信号在不同主频带的能量占比，获得低主频集中带声发射信号的能量占比。再次，获得一组岩石试样的低主频集中带声发射信号能量占比与其峰值强度之间的统计关系，进行线性拟合并作出趋势线。最后，将第趋势线向外做延长线与低主频集中带声发射信号能量占比为1的垂直线的交点即为此类岩石的抗拉强度。本发明能够提高岩石抗拉强度求解的准确性，求解的直接拉伸强度可以作为岩石工程的一个保守设计参数。
A method for accurately solving the tensile strength of rock based on microscopic failure type analysis belongs to the field of rock mechanics and geotechnical engineering. First of all, acoustic emission sensors are evenly arranged above and below the rock sample, and a rock mechanics testing machine is used to conduct a direct tensile test of the rock sample, and the whole process of real-time monitoring of acoustic emission is carried out. Secondly, the main frequency of the acoustic emission signal is extracted, the energy ratio of the acoustic emission signal in different main frequency bands is counted, and the energy ratio of the low main frequency concentrated acoustic emission signal is obtained. Thirdly, the statistical relationship between the energy ratio of the acoustic emission signal in the low dominant frequency concentrated zone and its peak intensity of a group of rock samples is obtained, and a linear fitting is performed to draw a trend line. Finally, the intersection point of extending the first trend line to the outside and the vertical line where the energy ratio of the acoustic emission signal in the low dominant frequency concentrated band is 1 is the tensile strength of this type of rock. The invention can improve the accuracy of solving the rock tensile strength, and the solved direct tensile strength can be used as a conservative design parameter of rock engineering.
Description
Technical Field
The invention belongs to the field of rock mechanics and geotechnical engineering, and relates to a method for accurately solving tensile strength of rocks, which is suitable for measuring tensile strength of various rocks encountered in civil engineering, hydraulic and hydroelectric engineering, mineral engineering and traffic engineering.
Background
Rock tensile strength refers to the maximum tensile stress that the rock can withstand when it reaches failure under tensile loading. Rock has a much lower tensile strength than its compressive strength and is typically a brittle material that is not resistant to compression and tension. In the projects of underground chamber construction, rock slope excavation, rock drilling blasting and the like, rock destruction often originates from the initiation and development of crack pulling. The problem of deformation and damage of the rock under the action of tensile load is always a basic scientific problem in the fields of rock mechanics and geotechnical engineering. The tensile strength of the rock is an important mechanical parameter, and the accurate measurement of the tensile strength has important scientific significance and engineering value for the stable evaluation of the rock engineering design, construction and maintenance whole-life process.
The existing tests for rock tensile strength determination include direct tensile tests and indirect tensile tests. The direct tensile test has definite physical significance, relatively accords with the actual condition that the rock bears tensile load, and has relatively stable test result. Therefore, direct tensile testing has found wide application in engineering practice. However, since rock is a natural geological material with a complex internal structure, the rock contains a plurality of microstructures such as cavities and pores, and the microstructure causes the micro-damage process and mechanism of the rock to be complex. Even if the rock macroscopically exhibits tensile failure, it may also microscopically contain shear failure or mixed mode failure. The existing test method for measuring the tensile strength of the rock, including a direct tensile test and a Brazilian split test which are widely used, cannot realize a pure tensile loading condition on the rock sample. Thus, there is no complete tensile failure in the current test methods, and the peak strength obtained directly from the direct tensile test does not represent the "ideal" tensile strength of the rock.
Therefore, the invention provides a method for accurately solving the tensile strength of the rock based on microscopic damage type analysis, and the strength can be used as a conservative design parameter of rock engineering.
Disclosure of Invention
Aiming at the limitations and practical requirements of the existing method, the invention provides a method for accurately solving the tensile strength of the rock according to the statistical analysis of the microscopic damage components of the rock damage, and suggests the strength value as a conservative design parameter of the rock engineering.
In order to achieve the purpose, the invention is realized by the following technical scheme:
a method for accurately solving the tensile strength of a rock based on microscopic damage type analysis is realized based on a rock sample 1, a rock mechanical testing machine, a mechanical control system and an acoustic emission acquisition system; the method comprises the following steps:
in a first step, rock sample 1 was prepared for direct tensile testing.
A cylindrical rock sample 1 is prepared, and an annular induction seam 2 is cut at the middle cross section position of the rock sample 1, so that a better direct tensile test effect can be obtained.
The ratio of the height to the diameter of the rock sample 1 is 2: 1; the width of the annular induction seam 2 accounts for 1-3% of the height of the rock sample 1, and the seam depth accounts for 4-8% of the diameter of the rock sample 1.
And secondly, placing the rock sample 1 in a rock mechanical testing machine, and arranging acoustic emission sensors 3 above and below the rock sample 1, wherein the number of the acoustic emission sensors 3 is more than 4, and the acoustic emission sensors are uniformly and symmetrically fixed on the surface of the cylindrical sample 1. The acoustic emission sensor 3 is connected with an acoustic emission acquisition system, and the rock mechanics testing machine is connected with a mechanics control system. Adopting a rock mechanics testing machine to carry out direct tensile test on a rock sample, and carrying out whole-process acoustic emission real-time monitoring: collecting time and axial force by using a mechanical control system; and the acoustic emission acquisition system is used for recording the waveform data of all acoustic emission signals.
The acoustic emission sensor 3 needs to have a wide frequency response, and the frequency response range should at least be guaranteed to be 10kHz-500 kHz.
And thirdly, extracting the main frequency of the acoustic emission signal. And converting the acquired acoustic emission waveform from a time domain to a frequency domain by adopting Fourier transform to acquire the dominant frequency of all the acoustic emission signals.
And fourthly, counting the energy ratio of the acoustic emission signals in different main frequency bands to obtain the energy ratio of the acoustic emission signals in the low main frequency concentrated band.
The rock acoustic emission signal obtained by indoor test has a wide dominant frequency distribution range. For the convenience of statistical analysis, the main frequency values of the acoustic emission signals need to be subjected to grouping statistics, and the width of each main frequency band is 5-15 kHz. And dividing the acoustic emission signals into different main frequency bands according to the main frequency values, and carrying out energy statistics on the acoustic emission signals of the different main frequency bands. The abscissa is the dominant frequency band number, the ordinate is the energy ratio, a histogram is drawn, and it can be seen that two obvious dominant frequency bands exist, and the dominant frequency bands are defined as a high dominant frequency band and a low dominant frequency band according to the height of the dominant frequency value. And then the energy ratio of the acoustic emission signals in the low main frequency set is counted.
And fifthly, obtaining a statistical relationship between the energy ratio of the low-dominant-frequency concentrated acoustic emission signals of the group of rock samples 1 and the peak intensity of the low-dominant-frequency concentrated acoustic emission signals, drawing a scatter diagram by taking the energy ratio of the low-dominant-frequency concentrated acoustic emission signals as a horizontal coordinate and the peak intensity as a vertical coordinate, and then performing linear fitting and making a trend line.
The group of rock samples refers to rock samples obtained from the same rock core, and the number of the rock samples is more than or equal to 3. Peak intensity (σ) of each rock sample 1max) Is the maximum value (P) of the axial forcemax) Divided by the area (S) of the cross section of the middle of the rock sample 1.
In the formula, σmaxRepresents the peak intensity; pmaxRepresents the maximum value of the axial force; s represents the area of the middle cross section; d represents the diameter of the rock sample 1; l represents the depth of the circular induction gap 2.
And sixthly, taking the trend line obtained in the fifth step as an outward extension line and taking the intersection point of a vertical line with the energy ratio (abscissa) of the low-dominant-frequency concentrated medium-band acoustic emission signal being 1 (namely 100%) as the tensile strength of the rock. That is, when the percentage of energy of the acoustic emission signal in the low main frequency set reaches 100%, the corresponding fitting peak intensity is the tensile strength of the rock.
The invention has the beneficial effects that: the solving method has definite physical significance, improves the accuracy of solving the rock tensile strength, can be used as a conservative design parameter of rock engineering for solving the direct tensile strength, and can be widely applied to rock mechanical property research in the engineering fields of water conservancy and hydropower, transportation, mineral resource exploitation, underground space development and the like.
Drawings
FIG. 1 is a schematic flow chart of a method for accurately solving the direct tensile strength of the rock provided by the invention;
FIG. 2 is a schematic illustration of a direct tensile test of a rock sample;
FIG. 3 is a typical acoustic emission waveform and its frequency spectrum: (a) is a waveform diagram; (b) is a spectrogram.
FIG. 4 is a histogram of Acoustic Emission (AE) signal energy ratios of different main frequency bands of a rock sample.
FIG. 5 is a schematic diagram of the solution of rock tensile strength.
In the figure: 1, a rock sample; 2, circularly inducing a seam; 3, an acoustic emission sensor; 4 a pull head.
Detailed Description
In order to further explain the technical scheme of the invention, the invention is explained in detail by combining the attached drawings and the embodiment.
As shown in fig. 1, a method for solving the direct tensile strength of rock based on micro-destructive component analysis includes the following steps:
(1) sampling on site, preparing a cylindrical standard rock sample 1 with the height-to-diameter ratio of 2:1 according to a specification, cutting an annular induction seam 2 at the middle cross section of the rock sample 1, and preparing the rock sample 1 for a direct tensile test. The rock type that this embodiment adopted is granite, and cylinder rock specimen height is 100mm, and the diameter is 50 mm. And cutting an annular induced seam at the cross section of the middle part of the rock sample, wherein the seam width is 2mm, and the seam depth is 3 mm. The prefabricated induced seam is used for ensuring a good direct tensile test effect, and JGN high-strength viscose is used for bonding the rock sample 1 and the test pull head 4 together as shown in fig. 2, and the rock sample 1 and the two pull heads 4 are always kept on a vertical line. The rock sample 1 with the attached slider 4 was then left standing in the room for one week.
(2) And (3) carrying out direct tensile test on the rock sample by adopting a rock mechanics testing machine, and carrying out real-time monitoring on acoustic emission in the whole process. The rock mechanical testing machine used in this example was MTS 815 (manufactured by MTS Co., USA). During loading, the axial force is measured and recorded in real time. Acoustic emission monitoring employs a PCI-2 acoustic emission real-time three-dimensional localization monitoring system (developed by the acoustic Physics of America (PAC)). The acoustic emission sensor that adopts is Micro30 high accuracy sensor, and it can provide wide frequency response, even can guarantee the effective collection of acoustic emission signal when there is very big noise in experimental environment. The 8 Micro30 acoustic emission sensors 3 were uniformly and symmetrically fixed on the surface of the cylindrical sample, as shown in fig. 2. And vaseline is applied between the rock sample 1 and the acoustic emission transducer 3. And recording the waveform data of all acoustic emission signals in real time by using an acoustic emission acquisition system.
(3) And converting the acquired acoustic emission waveform from a time domain to a frequency domain by adopting Fourier transform. Fig. 3(a) is a typical acoustic emission waveform, and fig. 3(b) is a spectrum diagram thereof. The dominant frequency values of the acoustic emission signals are extracted, as indicated by the dots in fig. 3 (b).
(4) And counting the energy ratio of the acoustic emission signals in different main frequency bands to obtain the energy ratio of the acoustic emission signals in the low main frequency concentrated band. In order to perform grouping statistics on the main frequency values of the acoustic emission signals, the width of the main frequency band set in this embodiment is 10 kHz. As the dominant frequency of the acoustic emission waveform of indoor rock damage is within 500kHz, the range of the 1 st dominant frequency band is (0,10] kHz, the range of the 2 nd dominant frequency band is (10,20] kHz, and so on, and the range of the 50 th dominant frequency band is (490,500] kHz.
(5) A statistical relationship between the energy ratio and the peak intensity of the acoustic emission signal in the low main frequency concentrated band for a set of rock samples 1 was obtained, as shown in fig. 5. The square data points in the graph represent the test results for this set of rock samples 1. The number of rock samples 1 of this example was 5. A linear fit was performed to make a trend line. The linear fitting function of the energy ratio of the signal in the low main frequency central band and the peak intensity of the granite sample in the embodiment is-4.9314 x + 7.8918.
(6) When the energy proportion of the acoustic emission signal in the low main frequency set reaches 100%, the corresponding fitting peak intensity is the tensile strength of the rock. In this embodiment, x is set to 1, which means that the energy ratio of the low-dominant-frequency acoustic emission signal reaches 100%, and the peak intensity y is equal to 2.96 MPa. Thus, the tensile strength of the granite of this example was 2.96MPa, as indicated by the dots in FIG. 5.
The above-mentioned embodiments only express the embodiments of the present invention, but not should be understood as the limitation of the scope of the invention patent, it should be noted that, for those skilled in the art, many variations and modifications can be made without departing from the concept of the present invention, and these all fall into the protection scope of the present invention.
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