CN111238939A - Method and device for determining rock brittleness index - Google Patents
Method and device for determining rock brittleness index Download PDFInfo
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- CN111238939A CN111238939A CN202010187734.0A CN202010187734A CN111238939A CN 111238939 A CN111238939 A CN 111238939A CN 202010187734 A CN202010187734 A CN 202010187734A CN 111238939 A CN111238939 A CN 111238939A
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- G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
- G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
- G01N3/08—Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces
Abstract
The invention discloses a method and a device for determining a rock brittleness index, which are characterized in that a stress-strain curve of a rock test piece is obtained, the pre-peak accumulated energy of the rock test piece is determined according to the stress-strain curve, the elastic energy stored when the rock test piece reaches the residual strength is determined according to the stress-strain curve, the post-peak released energy of the rock test piece is determined according to the stress-strain curve, the brittleness index of the rock test piece is determined according to the determined pre-peak accumulated energy, the elastic energy and the post-peak released energy, the information in the stress-strain curve of the rock test piece is effectively and greatly utilized in the process of quantifying the brittleness index of the rock test piece, the accuracy of quantifying the brittleness index of the rock test piece is improved, and the brittleness of the rock can be more accurately evaluated.
Description
Technical Field
The invention relates to the field of rock mechanics, in particular to a method and a device for determining a rock brittleness index.
Background
The coal bed gas is an important oil gas resource stored in coal rocks of a coal bed, and the coal rocks have good fracturability so as to improve the exploitation efficiency of the coal bed gas. And the brittleness of the coal rock is a key parameter for evaluating whether the coal rock has good compressibility. Thus, the technician would evaluate the friability of the coal rock prior to mining.
At present, a technician may obtain a stress-strain curve of a coal rock in advance, simplify a pre-peak curve (a curve before the coal rock is damaged) and a post-peak curve (a curve after the coal rock is damaged) in the stress-strain curve to obtain two corresponding straight lines, obtain slopes of the two straight lines, i.e., a young modulus and a softening modulus, and divide the softening modulus by the young modulus to obtain a value as a brittleness index of the coal rock. Then, the technician can evaluate the brittleness of the coal rock according to the brittleness index of the coal rock, wherein when the brittleness index is larger than a specified brittleness index threshold value, the brittleness of the coal rock is considered to be larger; when the friability index is not greater than the specified friability index threshold, the coal rock is considered to be less friable.
However, the process of changing the stress and strain of the coal rock is complicated in both the pre-peak curve and the post-peak curve. In the prior art, the change process is simplified, and the brittleness index obtained by calculating according to the slope has large error, so that the brittleness evaluation of the rock is inaccurate.
Disclosure of Invention
In view of the above problems, the present invention provides a method and an apparatus for determining a rock brittleness index, which overcome the above problems or at least partially solve the above problems, and the technical solution is as follows:
a method of determining a rock brittleness index, comprising:
obtaining a stress-strain curve of the rock test piece;
determining the accumulated energy before the peak of the rock test piece according to the stress-strain curve;
determining the elastic energy stored when the rock test piece reaches the residual strength according to the stress-strain curve;
determining the peak post-release energy of the rock test piece according to the stress-strain curve;
and determining the brittleness index of the rock test piece according to the determined pre-peak accumulated energy, the determined elastic energy and the determined post-peak released energy.
Optionally, determining the accumulated energy before the peak of the rock test piece according to the stress-strain curve includes:
determining a plurality of points of the rock test piece in a curve before a peak according to the stress-strain curve, and obtaining the abscissa and the ordinate of each point in the plurality of points in the curve before the peak, wherein the abscissa is a strain value, and the ordinate is a stress value;
inputting the determined abscissa and ordinate of each of the plurality of points in the pre-peak curve into a first calculation formula:
obtaining the accumulated energy before the peak of the rock test piece; in the formula: wpreEnergy accumulated before peak, σ stress, ε strain, εAIs the peak strain.
Optionally, the determining, according to the stress-strain curve, the elastic energy stored in the rock test piece when the residual strength is reached includes:
according to the stress-strain curve, obtaining a first stress value of the rock test piece when the residual strength is reached, and obtaining an elastic modulus corresponding to the residual strength;
inputting the first stress value and the elastic modulus into a second calculation formula:
obtaining the elastic energy stored when the rock test piece reaches the residual strength; in the formula, WrAs said elastic energy, σBIs a first stress value of the rock specimen when the residual strength is reached, EBIs the modulus of elasticity corresponding to the residual strength.
Optionally, the determining the post-peak release energy of the rock test piece according to the stress-strain curve includes:
determining a plurality of points of the rock test piece in a post-peak curve according to the stress-strain curve, and obtaining the abscissa and the ordinate of each point in the plurality of points in the post-peak curve, wherein the abscissa is a strain value, the ordinate is a stress value, and each parameter combination comprises a strain value and a corresponding stress value;
inputting the determined abscissa and ordinate of each of the plurality of points in the post-peak curve into a third calculation formula:
obtaining the peak of the rock test piece and then releasing energy; in the formula, WpostThe energy is released after the peak, and the energy is released,σ is stress, ε is strain, εBIs the strain, ε, of the rock specimen at the time of reaching the residual strengthAIs the peak strain.
Optionally, the determining the brittleness index of the rock test piece according to the determined pre-peak accumulated energy, the determined elastic energy and the determined post-peak released energy includes:
inputting the determined pre-peak accumulated energy, the determined elastic energy and the determined post-peak released energy into a fourth calculation formula:
Bre＝1-exp[-(Wpre-Wr)/Wpost]
obtaining the brittleness index of the rock test piece; in the formula, BreIs the brittleness index, W, of the rock specimenpreIs the accumulated energy before the peak, WrAs the elastic energy, WpostIs the energy released after the peak.
Optionally, after the determining the brittleness index of the rock test piece, the method further comprises:
and evaluating the brittleness performance of the rock test piece according to the determined brittleness index.
Optionally, the evaluating the brittleness performance of the rock test piece according to the determined brittleness index includes:
judging whether the brittleness index is not less than a preset threshold value, if so, judging that the brittleness of the rock test piece is high; otherwise, the brittleness of the rock test piece is small.
A device for determining a rock brittleness index, comprising: a first obtaining unit, a first determining unit, a second determining unit, a third determining unit, and a fourth determining unit, wherein:
the first obtaining unit is used for obtaining a stress-strain curve of the rock test piece;
the first determining unit is used for determining the accumulated energy before the peak of the rock test piece according to the stress-strain curve;
the second determining unit is used for determining the elastic energy stored when the rock test piece reaches the residual strength according to the stress-strain curve;
the third determining unit is used for determining the peak of the rock test piece and then releasing energy according to the stress-strain curve;
and the fourth determining unit is used for determining the brittleness index of the rock test piece according to the determined pre-peak accumulated energy, the determined elastic energy and the determined post-peak released energy.
Optionally, the first determining unit specifically includes: a fifth determining unit and a second obtaining unit, wherein:
the fifth determining unit is used for determining a plurality of points of the rock test piece in a curve before a peak according to the stress-strain curve, and obtaining an abscissa and an ordinate of each point in the plurality of points in the curve before the peak, wherein the abscissa is a strain value, and the ordinate is a stress value;
the second obtaining unit is configured to input the determined abscissa and ordinate of each of the plurality of points in the pre-peak curve into a first calculation formula:
obtaining the accumulated energy before the peak of the rock test piece; in the formula: wpreEnergy accumulated before peak, σ stress, ε strain, εAIs the peak strain.
Optionally, the second determining unit specifically includes: a third obtaining unit and a fourth obtaining unit, wherein:
the third obtaining unit is used for obtaining a first stress value of the rock test piece when the rock test piece reaches the residual strength according to the stress-strain curve and obtaining an elastic modulus corresponding to the residual strength;
the fourth obtaining unit is configured to input the first stress value and the elastic modulus into a second calculation formula:
in (1), obtainingThe elastic energy stored by the rock test piece when the residual strength is reached; in the formula, WrAs said elastic energy, σBIs a first stress value of the rock specimen when the residual strength is reached, EBIs the modulus of elasticity corresponding to the residual strength.
Optionally, the third determining unit specifically includes: a fifth obtaining unit and a sixth obtaining unit, wherein:
the fifth obtaining unit is configured to determine multiple points of the rock test piece in a post-peak curve according to the stress-strain curve, and obtain an abscissa and an ordinate of each of the multiple points in the post-peak curve, where the abscissa is a strain value, the ordinate is a stress value, and each parameter combination includes one strain value and a corresponding stress value;
the sixth obtaining unit is configured to input the abscissa and the ordinate of each of the plurality of points in the determined post-peak curve into a third calculation formula:
obtaining the peak of the rock test piece and then releasing energy; in the formula, WpostRelease energy after peak, σ is stress, ε is strain, εBIs the strain, ε, of the rock specimen at the time of reaching the residual strengthAIs the peak strain.
Optionally, the fourth determining unit is specifically configured to:
inputting the determined pre-peak accumulated energy, the determined elastic energy and the determined post-peak released energy into a fourth calculation formula:
Bre＝1-exp[-(Wpre-Wr)/Wpost]
obtaining the brittleness index of the rock test piece; in the formula, BreIs the brittleness index, W, of the rock specimenpreIs the accumulated energy before the peak, WrAs the elastic energy, WpostIs the energy released after the peak.
Optionally, the apparatus further comprises an evaluation unit, wherein the evaluation unit is configured to:
after the brittleness index of the rock test piece is determined, the brittleness performance of the rock test piece is evaluated according to the determined brittleness index.
Optionally, the evaluation unit is specifically configured to:
after the brittleness index of the rock test piece is determined, judging whether the brittleness index is not smaller than a preset threshold value, if so, judging that the brittleness of the rock test piece is high; otherwise, the brittleness of the rock test piece is small.
According to the method and the device for determining the rock brittleness index, the stress-strain curve of the rock test piece is obtained, the pre-peak accumulated energy of the rock test piece is determined according to the stress-strain curve, the elastic energy stored when the rock test piece reaches the residual strength is determined according to the stress-strain curve, the post-peak released energy of the rock test piece is determined according to the stress-strain curve, the brittleness index of the rock test piece is determined according to the determined pre-peak accumulated energy, the elastic energy and the post-peak released energy, the information in the stress-strain curve of the rock test piece is effectively and greatly utilized in the process of quantifying the brittleness index of the rock test piece, the accuracy of quantifying the brittleness index of the rock test piece is improved, and therefore the brittleness of the rock can be evaluated more accurately.
The foregoing description is only an overview of the technical solutions of the present invention, and the embodiments of the present invention are described below in order to make the technical means of the present invention more clearly understood and to make the above and other objects, features, and advantages of the present invention more clearly understandable.
Drawings
In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the drawings needed to be used in the description of the embodiments or the prior art will be briefly introduced below, it is obvious that the drawings in the following description are only embodiments of the present application, and for those skilled in the art, other drawings can be obtained according to the provided drawings without creative efforts.
FIG. 1 shows a flow chart of a method of determining a rock brittleness index;
FIG. 2 shows a stress-strain plot of a rock test piece;
FIG. 3 shows stress-strain plots for rock coupons M1 and M2;
FIG. 4 shows a flow chart of another method of determining a rock brittleness index;
FIG. 5 is a schematic diagram showing a device for determining the brittleness index of rock;
fig. 6 shows a schematic structure diagram of another rock brittleness index determination device.
Detailed Description
Exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. While exemplary embodiments of the invention are shown in the drawings, it should be understood that the invention can be embodied in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.
As shown in fig. 1, the present embodiment proposes a method for determining a rock brittleness index, which includes the following steps:
s10, obtaining a stress-strain curve of the rock test piece;
specifically, the invention can use a triaxial shear apparatus to perform a confined compression test in a triaxial rock mechanical test on a rock test piece so as to obtain a stress-strain curve of the rock test piece.
It should be noted that the rock specimen in the present invention may be coal rock.
In practical application, the invention can firstly drill a cylindrical rock test piece with phi 25mm multiplied by H50mm on a large block of rock according to the American ASTM D2938 standard, in order to avoid end effect, the periphery of the rock test piece needs to be smooth, the parallel of the upper end surface and the lower end surface should be kept within the range of 0.02mm, and the perpendicularity of the end surfaces and the axis is within 0.05 mm. The invention can use RTR-1500 electrohydraulic servo high-temperature high-pressure dynamic rock triaxial test system to carry out triaxial rock mechanical test on a rock test piece, apply confining pressure of 8MPa to the rock test piece according to actual well depth or simulated well depth (not less than 600 m), then apply axial load and continuously increase the axial load until the rock test piece is damaged, and reduce the applied axial load to a certain value after the rock test piece is damaged to obtain a stress-strain curve of the rock test piece, such as the stress-strain curve shown in figure 2, in this embodiment, the related formula in the following steps is explained by the stress-strain curve shown in figure 2.
It should also be noted that the process of obtaining the stress-strain curve of the rock test piece may use the prior art, and the present invention is not limited to this specific process.
S20, determining the accumulated energy before the peak of the rock test piece according to the stress-strain curve;
the curve before the peak point of the stress-strain curve can be determined as a curve before the peak, and the curve after the peak point of the stress-strain curve can be determined as a curve after the peak. The peak point is the point corresponding to the rock specimen when it is subjected to the maximum axial force during the test. For example, in the stress-strain curve shown in fig. 2, point a is a peak point, and the curve before point a is a curve before the peak.
Specifically, in the process of carrying out the confined compression test on the rock test piece, the axial force applied to the rock test piece is continuously increased before the rock test piece is damaged, and a corresponding point in a stress-strain curve when the rock test piece is damaged is determined as a peak point.
Specifically, the present invention may determine a value obtained by integrating the pre-peak curve as the pre-peak accumulated energy.
Optionally, step S20 may specifically include:
determining a plurality of points of the rock test piece in a curve before a peak according to the stress-strain curve, and obtaining the abscissa and the ordinate of each point in the plurality of points in the curve before the peak, wherein the abscissa is a strain value, and the ordinate is a stress value;
inputting the determined abscissa and ordinate of each of the plurality of points in the pre-peak curve into a first calculation formula:
obtaining the accumulated energy before the peak of the rock test piece; in the formula: wpreEnergy accumulated before peak, σ stress, ε strain, εAIs the peak strain.
Wherein epsilonANamely the strain of the rock test piece at the point peak point.
S30, determining the elastic energy stored when the rock test piece reaches the residual strength according to the stress-strain curve;
wherein, the residual strength is the strength of the rock test piece after the peak point and when the stress is stabilized to a certain value. For example, the state of the rock specimen at the point B shown in fig. 2 corresponds to the residual strength.
Optionally, step S30 may specifically include:
according to the stress-strain curve, obtaining a first stress value of the rock test piece when the residual strength is reached, and obtaining an elastic modulus corresponding to the residual strength;
inputting the first stress value and the elastic modulus into a second calculation formula:
obtaining the elastic energy stored when the rock test piece reaches the residual strength; in the formula, WrAs said elastic energy, σBIs a first stress value of the rock specimen when the residual strength is reached, EBIs the modulus of elasticity corresponding to the residual strength.
It should be noted that the invention can theoretically unload the force applied to the rock test piece when the rock test piece is damaged (the unloading is not performed in the actual test process), and the rock test piece can rebound after the unloading. The unloading curve (stress-strain curve of the unloaded rock) can be rebounded along the elastic modulus of the rock test piece corresponding to the peak point in the compression stage (pre-peak curve stage), but the rock test piece cannot be restored to the state before the limited compression test is performed after the rebound is finished because of the compression and plastic deformation of the original fracture of the rock test piece in the compression stage.
Wherein, the elastic modulus corresponding to the rebound of the rock test piece is E corresponding to the residual strength of the rock test pieceB。
Wherein the tangent modulus of the rock specimen at the peak point is EAThe damage variable of the crack development degree of the rock test piece in the crack propagation stage in the compression stage is D, EB、EAThe relationship to D is:
EB＝(1-D)EA
. Because the plastic zone of the coal rock is short, D can be regarded as zero, namely EBIs approximately equal to EA. For simple calculation, the invention can convert the secant modulus E at the peak pointAoIs determined as the tangent modulus E at the point of the point peakAThe value of (c). Therefore, the invention can convert EAoIs determined as EB。
Wherein, as shown in FIG. 2, the slope of the invention can be E at the passing point BBThen from the intersection of the straight line with the horizontal axis, point B and point εBThe resulting first area is determined as the value of the stored elastic energy of the rock specimen at the time of reaching the residual strength, i.e. the second calculation formula:
and the right part of the equal sign in the second calculation formula is the first area.
S40, determining the peak of the rock test piece and then releasing energy according to the stress-strain curve;
the method can determine the curve behind the peak point in the stress-strain curve of the rock test piece as the curve behind the peak. For example, in the stress-strain curve shown in fig. 2, the present invention may determine the curve after point a as a post-peak curve.
Specifically, the present invention may integrate a part of the post-peak curve from the peak point to the point corresponding to the residual intensity in the post-peak curve, and determine the value obtained after the integration as the post-peak released energy of the rock specimen. As shown in fig. 2, the present invention may integrate a part of the post-peak curve from a peak point a to a point B corresponding to the residual intensity, and determine a value obtained by the integration as the post-peak released energy.
Optionally, step S40 may specifically include:
determining a plurality of points of the rock test piece in a post-peak curve according to the stress-strain curve, and obtaining the abscissa and the ordinate of each point in the plurality of points in the post-peak curve, wherein the abscissa is a strain value, the ordinate is a stress value, and each parameter combination comprises a strain value and a corresponding stress value;
inputting the determined abscissa and ordinate of each of the plurality of points in the post-peak curve into a third calculation formula:
obtaining the peak of the rock test piece and then releasing energy; in the formula, WpostRelease energy after peak, σ is stress, ε is strain, εBIs the strain, ε, of the rock specimen at the time of reaching the residual strengthAIs the peak strain.
And S50, determining the brittleness index of the rock test piece according to the determined accumulated energy before the peak, the determined elastic energy and the determined released energy after the peak.
Optionally, step S50 may specifically be:
inputting the determined pre-peak accumulated energy, the determined elastic energy and the determined post-peak released energy into a fourth calculation formula:
Bre＝1-exp[-(Wpre-Wr)/Wpost]
obtaining the rock specimenA brittleness index; in the formula, BreIs the brittleness index, W, of the rock specimenpreIs the accumulated energy before the peak, WrAs the elastic energy, WpostIs the energy released after the peak.
It should be noted that, because the nonlinear elastic section in the pre-peak curve of the rock specimen is longer (as the curve of the section 0a in fig. 2), the plastic section is shorter (as the curve of the section bA in fig. 2), and the post-peak curve falls in a multi-layer step-wise manner, in order to effectively maximize the utilization of the stress-strain curve of the rock specimen to quantify the brittleness index of the rock specimen, the first ratio relation of the pre-peak accumulated energy, the post-peak released energy and the elastic energy stored when the residual strength is reached of the rock specimen in the fourth calculation formula is constructed according to the energy accumulation and energy conversion processes of the rock specimen in the test process:
(Wpre-Wr)/Wpost
the value obtained by the first ratio relation is greater than 0.
Optionally, according to the test analysis, 1 can be selected from the value range of the first ratio as a boundary value for judging the brittleness of the rock test piece. When the value obtained by the first ratio relation is more than 1, the rock test piece is considered to be fragile, and the brittleness is stronger when the numerical value is larger; when the value obtained by the first ratio relation is greater than 0 and not greater than 1, the rock test piece is considered to be poor in plasticity or brittleness, and the smaller the value is, the smaller the brittleness is. Of course, the boundary value can also be selected by a technician in the value range of the first ratio relationship according to the actual situation, and the specific selection value of the boundary value is not limited by the invention.
Specifically, in order to more intuitively reflect the brittleness index of the rock test piece, the method can adjust the value obtained by the first ratio relation to the interval [0, 1 ] according to the characteristic of the exponential function with the natural constant e as the base in the fourth calculation formula]Value B ofreI.e. the brittleness index. If B isreThe larger the brittleness of the rock test piece is; if B isreThe smaller the brittleness of the rock specimen.
Compared with the prior art, the method and the device have the advantages that the information in the stress-strain curve of the rock test piece is utilized more effectively and greatly, so that the accuracy of the brittleness evaluation of the rock test piece can be improved. For example, for the stress-strain curves of the rock test pieces M1 and M2 obtained by the technician as shown in fig. 3, based on the brittleness indexes of M2 and M1 obtained by the prior art respectively, the brittleness index of M2 is larger than that of M1 but has a small difference, and the brittleness of M2 and M1 is judged to be similar by the prior art; based on the brittleness indexes of M2 and M1 obtained in steps S10 to S50, the brittleness index of M2 is greater than that of M1, and the difference is 0.1, the present invention can judge that the brittleness of M2 is significantly greater than that of M1. From the graph shown in fig. 3, it can be seen that M2 accumulates more energy in the pre-peak curve than M1, and the mechanical energy required for falling in the post-peak curve is less than M1, and the brittleness of M2 is obviously higher than that of M1, so that, combining the calculation results of M1 and M2 brittleness indexes of the present invention, it can be seen that the brittleness index of the rock test piece determined by the present invention is more accurate than that of the prior art.
According to the method for determining the brittleness index of the rock, provided by the embodiment, through obtaining a stress-strain curve of a rock test piece, according to the stress-strain curve, the pre-peak accumulated energy of the rock test piece is determined, according to the stress-strain curve, the elastic energy stored when the rock test piece reaches the residual strength is determined, according to the stress-strain curve, the post-peak released energy of the rock test piece is determined, according to the determined pre-peak accumulated energy, the elastic energy and the post-peak released energy, the brittleness index of the rock test piece is determined, in the process of quantifying the brittleness index of the rock test piece, the information in the stress-strain curve of the rock test piece is utilized more effectively and more greatly, the accuracy of quantifying the brittleness index of the rock test piece is improved, and therefore the brittleness of the rock can be evaluated more accurately.
Based on the method shown in fig. 1, the present embodiment proposes another method for determining the brittleness index of rock, as shown in fig. 4, the method may further include step S60 after step S50.
And S60, evaluating the brittleness performance of the rock test piece according to the determined brittleness index.
Optionally, step S60 may specifically be:
judging whether the brittleness index is not less than a preset threshold value, if so, judging that the brittleness of the rock test piece is high; otherwise, the brittleness of the rock test piece is small.
In practical application, the brittleness of the rock test piece can be evaluated according to the relationship between the brittleness index obtained in the fourth calculation formula and the preset threshold value. The value of the preset threshold may be set by a technician according to an actual situation, which is not limited in the present invention. For example, the preset threshold value is calibrated to the brittleness index threshold value defined in the prior art, that is, the present invention may perform steps S10 to S50 in the stress-strain curve corresponding to the brittleness index threshold value defined in the prior art, and then determine the obtained brittleness index as the preset threshold value.
According to the method for determining the rock brittleness index, which is provided by the embodiment, the brittleness index of the rock test piece is compared with the size relation of the preset threshold value through setting the preset threshold value, so that the brittleness of the rock test piece is specifically evaluated.
Corresponding to the method shown in fig. 1, the embodiment proposes a device for determining a rock brittleness index, which may include: a first obtaining unit, a first determining unit, a second determining unit, a third determining unit, and a fourth determining unit, as shown in fig. 5, wherein:
the first obtaining unit is used for obtaining a stress-strain curve of the rock test piece;
specifically, the invention can use a triaxial shear apparatus to perform a confined compression test in a triaxial rock mechanical test on a rock test piece so as to obtain a stress-strain curve of the rock test piece.
It should be noted that the rock specimen in the present invention may be coal rock.
In practical application, the invention can firstly drill a cylindrical rock test piece with phi 25mm multiplied by H50mm on a large block of rock according to the American ASTM D2938 standard, in order to avoid end effect, the periphery of the rock test piece needs to be smooth, the parallel of the upper end surface and the lower end surface should be kept within the range of 0.02mm, and the perpendicularity of the end surfaces and the axis is within 0.05 mm. The invention can use RTR-1500 electrohydraulic servo high-temperature high-pressure dynamic rock triaxial test system to carry out triaxial rock mechanical test on the rock test piece, apply confining pressure of 8MPa to the rock test piece according to the actual well depth or the simulated well depth (not less than 600 m), then apply axial load and continuously increase the axial load until the rock test piece is damaged, and reduce the applied axial load to a certain value after the rock test piece is damaged, so as to obtain the stress-strain curve of the rock test piece.
It should also be noted that the process of obtaining the stress-strain curve of the rock test piece may use the prior art, and the present invention is not limited to this specific process.
The first determining unit is used for determining the accumulated energy before the peak of the rock test piece according to the stress-strain curve;
the curve before the peak point of the stress-strain curve can be determined as a curve before the peak, and the curve after the peak point of the stress-strain curve can be determined as a curve after the peak. The peak point is the point corresponding to the rock specimen when it is subjected to the maximum axial force during the test.
Specifically, in the process of carrying out the confined compression test on the rock test piece, the axial force applied to the rock test piece is continuously increased before the rock test piece is damaged, and a corresponding point in a stress-strain curve when the rock test piece is damaged is determined as a peak point.
Specifically, the present invention may determine a value obtained by integrating the pre-peak curve as the pre-peak accumulated energy.
Optionally, the first determining unit may specifically include: a fifth determining unit and a second obtaining unit, wherein:
the fifth determining unit is used for determining a plurality of points of the rock test piece in a curve before a peak according to the stress-strain curve, and obtaining an abscissa and an ordinate of each point in the plurality of points in the curve before the peak, wherein the abscissa is a strain value, and the ordinate is a stress value;
the second obtaining unit is configured to input the determined abscissa and ordinate of each of the plurality of points in the pre-peak curve into a first calculation formula:
obtaining the accumulated energy before the peak of the rock test piece; in the formula: wpreEnergy accumulated before peak, σ stress, ε strain, εAIs the peak strain.
Wherein epsilonANamely the strain of the rock test piece at the point peak point.
The second determining unit is used for determining the elastic energy stored when the rock test piece reaches the residual strength according to the stress-strain curve;
wherein, the residual strength is the strength of the rock test piece after the peak point and when the stress is stabilized to a certain value.
Optionally, the second determining unit may specifically include: a third obtaining unit and a fourth obtaining unit, wherein:
the third obtaining unit is used for obtaining a first stress value of the rock test piece when the rock test piece reaches the residual strength according to the stress-strain curve and obtaining an elastic modulus corresponding to the residual strength;
the fourth obtaining unit is configured to input the first stress value and the elastic modulus into a second calculation formula:
obtaining the elastic energy stored when the rock test piece reaches the residual strength; in the formula, WrAs said elastic energy, σBIs a first stress value of the rock specimen when the residual strength is reached, EBIs the modulus of elasticity corresponding to the residual strength.
It should be noted that the invention can theoretically unload the force applied to the rock test piece when the rock test piece is damaged (the unloading is not performed in the actual test process), and the rock test piece can rebound after the unloading. The unloading curve (stress-strain curve of the unloaded rock) can be rebounded along the elastic modulus of the rock test piece corresponding to the peak point in the compression stage (pre-peak curve stage), but the rock test piece cannot be restored to the state before the limited compression test is performed after the rebound is finished because of the compression and plastic deformation of the original fracture of the rock test piece in the compression stage.
Wherein, the elastic modulus corresponding to the rebound of the rock test piece is E corresponding to the residual strength of the rock test pieceB。
Wherein the tangent modulus of the rock specimen at the peak point is EAThe damage variable of the crack development degree of the rock test piece in the crack propagation stage in the compression stage is D, EB、EAThe relationship to D is:
EB＝(1-D)EA
. Because the plastic zone of the coal rock is short, D can be regarded as zero, namely EBIs approximately equal to EA. For simple calculation, the invention can convert the secant modulus E at the peak pointAoIs determined as the tangent modulus E at the point of the point peakAThe value of (c). Therefore, the invention can convert EAoIs determined as EB。
The third determining unit is used for determining the peak of the rock test piece and then releasing energy according to the stress-strain curve;
the method can determine the curve behind the peak point in the stress-strain curve of the rock test piece as the curve behind the peak.
Specifically, the present invention may integrate a part of the post-peak curve from the peak point to the point corresponding to the residual intensity in the post-peak curve, and determine the value obtained after the integration as the post-peak released energy of the rock specimen.
Optionally, the third determining unit may specifically include: a fifth obtaining unit and a sixth obtaining unit, wherein:
the fifth obtaining unit is configured to determine multiple points of the rock test piece in a post-peak curve according to the stress-strain curve, and obtain an abscissa and an ordinate of each of the multiple points in the post-peak curve, where the abscissa is a strain value, the ordinate is a stress value, and each parameter combination includes one strain value and a corresponding stress value;
the sixth obtaining unit is configured to input the abscissa and the ordinate of each of the plurality of points in the determined post-peak curve into a third calculation formula:
obtaining the peak of the rock test piece and then releasing energy; in the formula, WpostRelease energy after peak, σ is stress, ε is strain, εBIs the strain, ε, of the rock specimen at the time of reaching the residual strengthAIs the peak strain.
And the fourth determining unit is used for determining the brittleness index of the rock test piece according to the determined pre-peak accumulated energy, the determined elastic energy and the determined post-peak released energy.
Optionally, the fourth determining unit may be specifically configured to:
inputting the determined pre-peak accumulated energy, the determined elastic energy and the determined post-peak released energy into a fourth calculation formula:
Bre＝1-exp[-(Wpre-Wr)/Wpost]
obtaining the brittleness index of the rock test piece; in the formula, BreIs the brittleness index, W, of the rock specimenpreIs the accumulated energy before the peak, WrAs the elastic energy, WpostIs the energy released after the peak.
It should be noted that, because the nonlinear elastic section is longer, the plastic section is shorter, and the curve after the peak falls in a multi-layer step manner in the curve before the peak of the rock test piece, in order to effectively utilize the stress-strain curve of the rock test piece to the maximum to quantify the brittleness index of the rock test piece, the invention constructs the first ratio relation of the accumulated energy before the peak, the released energy after the peak and the elastic energy stored when the residual strength is reached in the fourth calculation formula according to the energy accumulation and energy conversion process of the rock test piece in the test process:
(Wpre-Wr)/Wpost
the value obtained by the first ratio relation is greater than 0.
Optionally, according to the test analysis, 1 can be selected from the value range of the first ratio as a boundary value for judging the brittleness of the rock test piece. When the value obtained by the first ratio relation is more than 1, the rock test piece is considered to be fragile, and the brittleness is stronger when the numerical value is larger; when the value obtained by the first ratio relation is greater than 0 and not greater than 1, the rock test piece is considered to be poor in plasticity or brittleness, and the smaller the value is, the smaller the brittleness is. Of course, the boundary value can also be selected by a technician in the value range of the first ratio relationship according to the actual situation, and the specific selection value of the boundary value is not limited by the invention.
Specifically, in order to more intuitively reflect the brittleness index of the rock test piece, the method can adjust the value obtained by the first ratio relation to the interval [0, 1 ] according to the characteristic of the exponential function with the natural constant e as the base in the fourth calculation formula]Value B ofreI.e. the brittleness index. If B isreThe larger the brittleness of the rock test piece is; if B isreThe smaller the brittleness of the rock specimen.
Compared with the prior art, the method and the device have the advantages that the information in the stress-strain curve of the rock test piece is utilized more effectively and greatly, so that the accuracy of the brittleness evaluation of the rock test piece can be improved.
The device for determining the brittleness index of the rock provided by the embodiment can utilize the information in the stress-strain curve of the rock test piece more effectively and more greatly in the process of quantifying the brittleness index of the rock test piece, so that the accuracy of quantifying the brittleness index of the rock test piece is improved, and the brittleness of the rock can be more accurately evaluated.
Based on the apparatus shown in fig. 5, the present embodiment provides another apparatus for determining a rock brittleness index, which may further include an evaluation unit, as shown in fig. 6, for:
after the brittleness index of the rock test piece is determined, the brittleness performance of the rock test piece is evaluated according to the determined brittleness index.
Optionally, the evaluation unit may be specifically configured to:
after the brittleness index of the rock test piece is determined, judging whether the brittleness index is not smaller than a preset threshold value, if so, judging that the brittleness of the rock test piece is high; otherwise, the brittleness of the rock test piece is small.
In practical application, the brittleness of the rock test piece can be evaluated according to the relationship between the brittleness index obtained in the fourth calculation formula and the preset threshold value. The value of the preset threshold may be set by a technician according to an actual situation, which is not limited in the present invention.
According to the method for determining the rock brittleness index, which is provided by the embodiment, the brittleness index of the rock test piece is compared with the size relation of the preset threshold value through setting the preset threshold value, so that the brittleness of the rock test piece is specifically evaluated.
It should also be noted that the terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising an … …" does not exclude the presence of other identical elements in the process, method, article, or apparatus that comprises the element.
The above are merely examples of the present application and are not intended to limit the present application. Various modifications and changes may occur to those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application should be included in the scope of the claims of the present application.
Claims (10)
1. A method of determining a rock brittleness index, comprising:
obtaining a stress-strain curve of the rock test piece;
determining the accumulated energy before the peak of the rock test piece according to the stress-strain curve;
determining the elastic energy stored when the rock test piece reaches the residual strength according to the stress-strain curve;
determining the peak post-release energy of the rock test piece according to the stress-strain curve;
and determining the brittleness index of the rock test piece according to the determined pre-peak accumulated energy, the determined elastic energy and the determined post-peak released energy.
2. The method of claim 1, wherein determining the pre-peak accumulated energy of the rock specimen from the stress-strain curve comprises:
determining a plurality of points of the rock test piece in a curve before a peak according to the stress-strain curve, and obtaining the abscissa and the ordinate of each point in the plurality of points in the curve before the peak, wherein the abscissa is a strain value, and the ordinate is a stress value;
inputting the determined abscissa and ordinate of each of the plurality of points in the pre-peak curve into a first calculation formula:
obtaining the accumulated energy before the peak of the rock test piece; in the formula: wpreEnergy accumulated before peak, σ stress, ε strain, εAIs the peak strain.
3. The method of claim 1, wherein determining the stored elastic energy of the rock specimen at the time of reaching residual strength from the stress-strain curve comprises:
according to the stress-strain curve, obtaining a first stress value of the rock test piece when the residual strength is reached, and obtaining an elastic modulus corresponding to the residual strength;
inputting the first stress value and the elastic modulus into a second calculation formula:
obtaining the elastic energy stored when the rock test piece reaches the residual strength; in the formula, WrAs said elastic energy, σBIs a first stress value of the rock specimen when the residual strength is reached, EBIs the modulus of elasticity corresponding to the residual strength.
4. The method of claim 1, wherein determining the post-peak release energy of the rock specimen from the stress-strain curve comprises:
determining a plurality of points of the rock test piece in a post-peak curve according to the stress-strain curve, and obtaining the abscissa and the ordinate of each point in the plurality of points in the post-peak curve, wherein the abscissa is a strain value, the ordinate is a stress value, and each parameter combination comprises a strain value and a corresponding stress value;
inputting the determined abscissa and ordinate of each of the plurality of points in the post-peak curve into a third calculation formula:
obtaining the peak of the rock test piece and then releasing energy; in the formula, WpostRelease energy after peak, σ is stress, ε is strain, εBIs the strain, ε, of the rock specimen at the time of reaching the residual strengthAIs the peak strain.
5. The method of claim 1, wherein determining the brittleness index of the rock specimen based on the determined pre-peak accumulated energy, the elastic energy, and the post-peak released energy comprises:
inputting the determined pre-peak accumulated energy, the determined elastic energy and the determined post-peak released energy into a fourth calculation formula:
Bre＝1-exp[-(Wpre-Wr)/Wpost]
obtaining the brittleness index of the rock test piece; in the formula, BreIs the brittleness index, W, of the rock specimenpreIs the accumulated energy before the peak, WrAs the elastic energy, WpostIs the energy released after the peak.
6. The method of any of claims 1 to 5, wherein after the determining the brittleness index of the rock test piece, the method further comprises:
and evaluating the brittleness performance of the rock test piece according to the determined brittleness index.
7. The method of claim 6, wherein the evaluating the brittleness performance of the rock test piece based on the determined brittleness index comprises:
judging whether the brittleness index is not less than a preset threshold value, if so, judging that the brittleness of the rock test piece is high; otherwise, the brittleness of the rock test piece is small.
8. An apparatus for determining a rock brittleness index, comprising: a first obtaining unit, a first determining unit, a second determining unit, a third determining unit, and a fourth determining unit, wherein:
the first obtaining unit is used for obtaining a stress-strain curve of the rock test piece;
the first determining unit is used for determining the accumulated energy before the peak of the rock test piece according to the stress-strain curve;
the second determining unit is used for determining the elastic energy stored when the rock test piece reaches the residual strength according to the stress-strain curve;
the third determining unit is used for determining the peak of the rock test piece and then releasing energy according to the stress-strain curve;
and the fourth determining unit is used for determining the brittleness index of the rock test piece according to the determined pre-peak accumulated energy, the determined elastic energy and the determined post-peak released energy.
9. The apparatus according to claim 8, wherein the first determining unit specifically includes: a fifth determining unit and a second obtaining unit, wherein:
the fifth determining unit is used for determining a plurality of points of the rock test piece in a curve before a peak according to the stress-strain curve, and obtaining an abscissa and an ordinate of each point in the plurality of points in the curve before the peak, wherein the abscissa is a strain value, and the ordinate is a stress value;
the second obtaining unit is configured to input the determined abscissa and ordinate of each of the plurality of points in the pre-peak curve into a first calculation formula:
obtaining the accumulated energy before the peak of the rock test piece; in the formula: wpreEnergy accumulated before peak, σ stress, ε strain, εAIs the peak strain.
10. The apparatus according to claim 8, wherein the second determining unit specifically includes: a third obtaining unit and a fourth obtaining unit, wherein:
the third obtaining unit is used for obtaining a first stress value of the rock test piece when the rock test piece reaches the residual strength according to the stress-strain curve and obtaining an elastic modulus corresponding to the residual strength;
the fourth obtaining unit is configured to input the first stress value and the elastic modulus into a second calculation formula:
obtaining the elastic energy stored when the rock test piece reaches the residual strength; in the formula, WrAs said elastic energy, σBIs a first stress value of the rock specimen when the residual strength is reached, EBIs the modulus of elasticity corresponding to the residual strength.
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