CN109781548B - Method for testing rock composite fracture toughness based on NDB (NDB) sample by asymmetric three-point bending loading - Google Patents
Method for testing rock composite fracture toughness based on NDB (NDB) sample by asymmetric three-point bending loading Download PDFInfo
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Abstract
The invention provides a method for testing rock composite fracture toughness based on asymmetric three-point bending loading of an NDB (normalized difference B) sample, wherein a used test piece body is a rectangular deep beam (NDB) with a cutting groove, the shape of the test piece body is a cuboid, the cutting groove with the height of a is prefabricated at the middle cross section of the body, the cutting groove extends in the thickness direction of the body and penetrates through the front end face and the rear end face in the thickness direction of the body; the length L of the test piece body is 2 times of the width W of the test piece body, and the thickness B is not less than 0.8W. Based on the NDB sample, the invention provides a method for testing pure I type, pure II type and I-II composite fracture toughness of a rock by using asymmetric three-point bending loading, and the application of tension and shear load is realized by asymmetric loading. The NDB sample used in the invention has a simple structure and is easy to process by using rock blocks; the clamp used for loading is a common three-point bending clamp, only the support is arranged asymmetrically, the experiment loading is very convenient, and the I-II composite fracture toughness test of a pure I type, a pure II type and the whole composite loading interval can be realized.
Description
Technical Field
The invention belongs to the field of rock fracture toughness testing, and particularly relates to a rock composite fracture toughness testing method.
Technical Field
Rock fracture mechanics is an important branch subject in the rock mechanics subject, rock fracture toughness represents the capability of a rock material for resisting crack initiation and propagation, is a core parameter for solving the rock engineering problem by utilizing the fracture mechanics, and reasonably determining the rock fracture toughness is one of the main research works of the rock fracture mechanics at present.
The International Society for Rock Mechanics (ISRM) has proposed 4 test pieces and corresponding test methods to perform rock type I static fracture toughness tests, including: the 4 samples were further processed on a drilled core basis for the chevron-notch Stub (SR) sample under compact tensile loading (1977), the chevron-notch three point bend round (CB) sample (1988), the radial loaded chevron-notch brazilian disc (CCNBD) sample (1995), and the symmetric three point bend loaded through straight notch half disc bend (SCB) sample (2014). The distance between the initial crack length and the critical crack length of the SR specimen is long, so that the critical load is close to the maximum load when nonlinear correction is carried out. In contrast, the distance between the initial crack length and the critical crack length of the CB test piece is short, and the critical load of the nonlinear correction is low. And after the SR test piece is loaded with a load, the crack propagates and advances in a broken line mode, and the crack obviously deviates from the central line. In addition, SR and CB samples have the problems of difficult preparation, complex experimental equipment and loading systems and the like. The CCNBD test piece has complex structure, is difficult to prepare, and can not carry out simplified plane analysis, so that the minimum dimensionless stress intensity factor Y of the test pieceminIt must be calibrated by three-dimensional numerical calculation. For an SCB sample, if two test pieces are cut and manufactured from a complete disc, part of materials are lost to form two nonstandard semicircular discs, and a test result needs theoretical correction; if the sample is processed into a standard SCB sample, only one test piece can be processed by one disc, and the rock material is wasted. In addition, the American Society for Testing and Materials (ASTM) also provides various configuration test pieces and test methods for fracture toughness of materials, such as the Single Edge Notched Beam (SENB) three-point bend test (2001), the compact disc tensile (DCT) test (1986), and the like.
The ISRM and ASTM suggested test methods described above are mostly used for type I fracture toughness testing, however in actual rock engineering, rock fractures are subjected to various load patterns such as type I, type II and type I/II composite loads. In order to accurately predict rock fracture propagation, the rock composite fracture toughness must be accurately measured. The SENB test piece can realize the I/II composite fracture test through asymmetric three-point bending loading/four-point bending loading, but the length of the SENB test piece is large, so that the fracture load is small, therefore, the traditional SENB test piece is more suitable for the fracture toughness test of metal materials, and is not the optimal choice for brittle materials such as rock, concrete and the like. The CB sample has similar problems to the SENB sample. Although the CCNBD and SCB test pieces can achieve the I/II composite fracture toughness test of the entire composite loading interval from pure I type to pure II type by adjusting the crack inclination angle, the crack length, and the like, as described above, the test pieces (especially large-sized test pieces) are difficult to process. The central straight crack Brazilian disc (CSTBD) sample and the central straight crack platform Brazilian disc (CSTBFD) sample are simple in structure, and the fracture toughness tests of pure I type, pure II type and I/II compound type only need to change the included angle between the loading direction and the central crack. However, it has been found that disc-like specimens are difficult to process when conducting large-size specimen fracture toughness tests. The single-side grooving deep beam test piece (NDB) provided by the Chinese patent ZL201510397736.1 adopts inclined cracks and symmetrical loading to realize loading of an I/II composite fracture toughness test by utilizing a simple test piece configuration, but inclined cracks are needed during the composite fracture toughness and II type fracture toughness test, and the prefabrication of large-inclination-angle cracks has difficulty. And the slight error of the inclination angle brings great error of the test result. Therefore, the method has higher requirements on experimental operation and test piece processing precision, and the accuracy of the test result is difficult to control and guarantee.
As mentioned above, the difficulty in processing large-size SR, CB, CCNBD and SCB samples is very prominent; in comparison, the non-circular configurations such as SENB and NDB only need to be cut to process the test piece with the required size from the rock mass, and the method is more operational. However, in order to realize the composite loading, the samples with the traditional configurations of SENB, NDB and the like need to adopt complicated loading or prefabricated inclined cracks, and the requirements on the sample processing level and the loading device are high. Therefore, the research on the novel rock composite fracture toughness testing technology which is simple in configuration, easy to machine and easy to load has important practical value.
Disclosure of Invention
The invention aims to provide a method for testing the composite fracture toughness of rock by asymmetric three-point bending loading based on an NDB (non-inclined crack) sample aiming at the defects of the existing testing technology of the composite fracture toughness of rock, which utilizes the NDB sample of a non-inclined (vertical) crack to realize the fracture toughness testing of pure I type, pure II type and loading in the whole I/II composite interval, avoids the problems of high precision required by the prefabrication of inclined cracks and high prefabrication difficulty of large-inclination-angle cracks, and has easy control and guarantee of testing accuracy.
The method comprises the steps that an NDB sample is used, the body of the NDB sample is a cuboid, a vertical cutting groove which is opened from the center line position of the lower end face of the body and is parallel to the thickness B direction is formed in the body, the cutting groove penetrates through the front end face and the rear end face in the thickness B direction, the length L of the test piece body is 2 times of the width W of the test piece body, the thickness B of the test piece is not less than 0.8W, the depth of the cutting groove is a, and a is not less than 0.3W and not more than 0.7W;
the invention utilizes asymmetric three-point bending loading to carry out pure I type, pure II type and I-II composite fracture toughness tests on rocks, and the distance between a three-point bending left support and the central plane of a cutting groove is S1The distance between the right support and the central plane of the cutting groove is S2And S is2≤S1(ii) a The cutting length a is selected within the range of a/W being more than or equal to 0.3 and less than or equal to 0.7, and S is more than or equal to 0.41Determining the distance S between the left support and the central line within the range of W being less than or equal to 0.91Adjusting the ratio S of the distance between the right support and the central line to the width of the test piece2The value of/W is from S1the/W starts to decrease gradually until the corresponding S of pure form II is obtained2So as to obtain different load combination degrees; comprehensively calibrating the dimensionless stress intensity factor Y of the crack tip of the test piece under asymmetric loading by using finite element numerical software ABAQUS/ANSYS and the likeI、YIIAnd non-dimensionalized nonsingular stress T, and obtaining S corresponding to pure II type loading2And then the distance S between the right supports is adjusted by a three-point bending fixture graduated scale2The difference between the front end distance and the rear end distance of the two adjusted supports is not more than 1%, and the same left support distance S is used in the same group of experiments1。
Further, before testing the fracture toughness of the rock, a finite element numerical software ABAQUS/ANSYS and the like is utilized to comprehensively calibrate a dimensionless stress intensity factor Y of the crack tip of the test piece under asymmetric loadingI、YIIAnd nothingDimensionalizing the nonsingular stress T and obtaining S corresponding to the pure II type loading2The method comprises the following specific steps:
(1) and establishing a corresponding numerical value calibration model. A two-dimensional geometric model of a test piece body is established by using finite element numerical calculation software, a rock material is assumed to be a linear elastic material and is endowed with elastic parameters of the test piece, then a grid is divided (set as a plane strain unit), a node is restrained at a left support (the node is a contact point of the left support and a test sample in the two-dimensional geometric model) to perform vertical and horizontal displacement, a vertical displacement is restrained at a right support, and a reference node load P (vertical) is applied to an upper pressure head loading point. Under the action of a load P, the stress intensity factors K of the type I and the type II of the test pieceI，KIIAnd the nonsingular T stresses can be expressed as:
reversely deducing a dimensionless stress intensity factor Y according to the formulas (1) to (3)I、YIIAnd dimensionless nonsingular stresses T are as follows:
(2) based on the steps(1) The numerical model continuously reduces S by adopting a dichotomy in calculation2And (4) taking values. When the load combination degree M is obtained by calculationeIn the vicinity of 1, the error does not exceed 10-4The magnitude of the load can be approximately considered to reach the pure II type fracture, namely the support interval for realizing the pure II type loading can be obtained, and the calculated result is shown in the table 1. Wherein the load complex is given by:
TABLE 1 different a/W, different S1W when the specimen realizes pure II type fracture S2Size of/W
(3) Determining the cutting groove length a and the left support interval S required by the test based on the numerical model establishing method in the step (1) and the obtained right support interval in the step (2)1Right support spacing from S1Gradually reducing to the critical S determined in the table 1 according to the set step pitch2Calculating the dimensionless stress intensity factor Y one by oneIAnd YIIAnd dimensionless nonsingular stress T. Further, the method of the invention also comprises the following steps:
(4) drawing three auxiliary lines in the vertical direction on the front end face of the test piece body in the thickness direction, wherein one auxiliary line is positioned on the central line of the front end face of the test piece body, the other two auxiliary lines are distributed on two sides, and the distance between the two adjacent auxiliary lines is equal to the distance S between the left support and the right support and the central line1And S2。
(5) And mounting the test piece on the test bed by means of the ruler and the auxiliary lines on the front end surface of the test piece body, so that the three auxiliary lines on the front end surface of the test piece body, the middle auxiliary line is aligned to the load loading pressure head, and the auxiliary lines on the two sides are aligned to the support fulcrum.
(6) And adjusting the testing machine to ensure that the loading pressure head of the testing machine slightly contacts with the test piece, finally checking the mounting position of the test piece, and installing an LVDT displacement sensor after the test piece is confirmed to be correct.
(7) And controlling loading by using an LVDT displacement sensor, and operating a rock mechanics testing machine to load the test piece until the test piece is damaged and loses the bearing capacity completely.
(8) According to the peak load and I/II type dimensionless stress intensity factor Y in the experimentI/YIIThe composite fracture toughness of the rock can be calculated according to the following formula:
further, the rock mechanics tester is operated to load the test piece at a rate preferably less than 0.2mm/min to avoid any dynamic effects that may occur during the test.
Furthermore, the load loading pressure head (3) is a round bar structure loading pressure head, the supports (4) and (5) are round bar structure supports, and the circumference of the upper round bar pressure head is parallel to the central plane of the cutting groove.
Further, the included angle of any two adjacent surfaces of the test piece body is 90 degrees +/-0.5 degrees; the section size deviation of any two positions in one direction of the test piece body is not more than 0.1 mm.
Further, the length L of the specimen body is preferably not less than 10 times the rock grain size and not less than 76 mm.
Further, the length L of the test piece body is controlled to be 2 times of the width W of the test piece body, and the error is not more than 0.04W.
Further, the thickness B is not less than 0.8 times the width W of the body and not less than 30 mm; the width b of the cutting groove is less than 1 mm.
The manufacturing method of the test piece for testing the pure I type, the pure II type and the I-II composite fracture of the rock comprises the following steps:
(1) the sample was cut. Rock pieces collected from an engineering site or in the field are preliminarily cut by a rock cutting machine and processed into a test piece body which is at least 10mm larger than the required size (length, width and height). If the double-cutter cutting machine is used for processing a rock sample, the distance between the two blades can be adjusted more conveniently to control the thickness of each cutting, and the processing is more convenient.
(2) And (5) polishing the sample. And (3) grinding one surface of the preliminarily processed test piece by using a grinding machine, grinding four surfaces vertical to the test piece by using the ground surface as a reference surface, and finally grinding the surface parallel to the reference surface to the required thickness. When a grinding machine is used for grinding, the feed amount is preferably less than 1mm each time. The included angle of any two adjacent surfaces of the processed test piece body is controlled to be 90 degrees +/-0.5 degrees, and the size deviation of the cross section of any two positions in one direction of the body is preferably not more than 0.1 mm.
(3) And (5) prefabricating a crack. It is recommended to use a diamond blade or a very fine wire saw with a thickness of less than 0.3-0.5mm at the bottom middle position of the specimen, and directly machine to the required crack length. And cutting to form a crack with the length of a, wherein the crack is vertical to the lower end face. In the grooving process, the feed amount per pass is preferably less than a/10.
Compared with the prior art, the invention has the following beneficial effects:
1. the method utilizes the vertical grooving NDB sample under asymmetric loading to carry out rock composite fracture toughness test, and enriches the rock fracture toughness test technical system.
2. The NDB test piece used in the invention has a simple structure, is easy to cut and process by using a rock block, avoids core drilling on the rock block with a limited size, is convenient to process, has low requirements on the appearance of raw materials, can realize composite loading without adopting complex loading or prefabricating inclined cracks, and has low requirements on the processing level of a sample and a loading device. Therefore, the method is a novel rock composite fracture toughness testing technology with simple test piece configuration, easy processing and manufacturing and easy loading.
3. Compared with the scheme ZL201510397736.1, the test piece and the test method provided by the invention have the advantages that the distance S between the left support and the right support and the central line is adjusted only under the condition that the test piece is vertically grooved1、S2And the crack length a, the I/II composite fracture toughness tests of pure I type, pure II type and the whole composite fracture interval can be realized. The vertical grooving of the component greatly simplifies the processing difficulty of the test piece, the processing of the test piece can be completed by using a common cutting machine, the processing difficulty of cutting an inclined crack is avoided, and particularly, the inclined included angle beta is overcomeIn particular beta>The defect that the test piece is difficult to process at 60 degrees is overcome, the problem of large error of a test result caused by the processing error of the beta inclination of the test piece is solved, and the loading method is simple and easy to realize.
5. Compared with the scheme of ZL201510397736.1 and the traditional SENB test piece, the test piece and the test method have the advantages that the distance between the support seats is smaller, the stress concentration degree of the crack tip is smaller, the breaking load is larger, and the reliability of the test result is higher.
6. Compared with the traditional SENB sample, the method can avoid two defects of the SENB sample, namely the asymmetric three-point bending loading needs to realize the I/II composite fracture test by moving the crack position, and the pure II type loading cannot be realized; the procedure for achieving pure type II loading is relatively difficult for four-point bending experiments.
7. The ratio of the length L to the width W of the test piece used by the invention is 2.0 +/-0.04, the thickness of the test piece meets the condition that B is more than or equal to 0.8W, the upper limit is not set under the allowable condition of test conditions, and the upper limit of the ratio of the thickness B of the traditional SENB test piece to the length L of the test piece is 2/9. Therefore, compared with the traditional SENB test piece, the test piece provided by the invention is shorter and thicker, and the plane strain condition required by the rock fracture toughness test is compounded.
8. The three-point bending test process of the test piece can be conveniently matched with an acoustic emission and non-contact strain test system DIC to track the crack propagation process and measure the rock deformation at the front edge of the crack.
9. The invention can also be used for the plane strain fracture toughness test of other brittle and quasi-brittle materials (such as concrete, PMMA, ceramics, glass and the like).
Drawings
FIG. 1 is a schematic structural diagram of a test piece for testing fracture toughness of rock according to the invention;
FIG. 2 is a front view of a test piece for rock fracture toughness testing according to the present invention;
FIG. 3 is a side view of a test piece for rock fracture toughness testing according to the present invention;
FIG. 4 is a top view of a test piece for rock fracture toughness testing according to the present invention;
FIG. 5 is a schematic mounting diagram and a loading diagram of a test piece for testing the fracture toughness of the rock, provided by the invention, on a test bed of a rock mechanical testing machine.
FIG. 6 shows a type I dimensionless stress intensity factor Y of a test piece for rock fracture toughness test according to the present invention when the ratio of the crack length a to the test piece width W is 0.3IA numerical calculation result;
FIG. 7 shows a type II dimensionless stress intensity factor Y of a test piece for rock fracture toughness test according to the present invention when the ratio of the crack length a to the test piece width W is 0.3IIA numerical calculation result;
FIG. 8 shows the result of T-x numerical calculation when the ratio of the fracture length a to the test piece width W is 0.3 for the test piece for rock fracture toughness test according to the present invention;
FIG. 9 shows a type I dimensionless stress intensity factor Y of a test piece for rock fracture toughness test according to the present invention when the ratio of the crack length a to the test piece width W is 0.5IA numerical calculation result;
FIG. 10 shows a type II dimensionless stress intensity factor Y of a test piece for rock fracture toughness test according to the present invention when the ratio of the crack length a to the test piece width W is 0.5IIA numerical calculation result;
FIG. 11 shows the result of numerical calculation of T x when the ratio of the fracture length a to the test piece width W is 0.5 for the test piece for rock fracture toughness test according to the present invention;
FIG. 12 shows a type I dimensionless stress intensity factor Y of a test piece for rock fracture toughness test according to the present invention when the ratio of the crack length a to the test piece width W is 0.7IA numerical calculation result;
FIG. 13 shows a type II dimensionless stress intensity factor Y of a test piece for rock fracture toughness test according to the present invention when the ratio of the crack length a to the test piece width W is 0.7IIA numerical calculation result;
fig. 14 shows the results of numerical calculation of T x when the ratio of the fracture length a to the specimen width W is 0.7 for the specimen according to the invention used for the rock fracture toughness test.
Detailed description of the invention
The test piece for rock fracture toughness test and the rock fracture toughness test method according to the present invention are further described by the following examples.
In the following embodiments, the testing equipment for testing the fracture toughness of the rock is a microcomputer universal material testing machine (MTS 815 rock mechanical testing machine can also be used), and a three-point bending fixture of the equipment is used; the test is started after zero clearing operation is carried out on parameters such as displacement except load in a test program, and time t, load P, machine displacement and LVDT displacement are mainly collected in the test process. The test was carried out in 4 groups of four times each, and the experimental data were averaged.
In the following examples, the specimen length L was 200mm, the width W was 100mm, the thickness B was 80mm, the crack length a was 50mm, and the distance S between the left abutment and the center line was used160mm, and 10kN directly above the crack. Based on ABAQUS numerical simulation, the dimensionless stress intensity factor is calibrated according to a formula, and the result is as follows:
TABLE 2 a/W ═ 0.5, S10.6 value of/W
Example 1
According to the deep beam test piece for testing the rock fracture toughness, shown in fig. 1-4, a test material is sandstone, a test piece body 1 is cuboid, a central line opening along the thickness B direction of the lower end face of the body is formed in the body, extends towards the interior of the body and penetrates through cutting grooves 2 of the front end face and the rear end face of the body in the thickness direction, the length L of the test piece is 200mm, the width W of the test piece is 100mm, and the thickness B of the test piece is 80 mm; the length a of the notch is 50mm (the ratio of a/W is 0.5), and the crack inclination angle beta is 90 deg. The rock fracture test operation using the above test piece was as follows:
1) adjusting the distance S between the left support and the central line according to the three-point bending fixture graduated scale1And the distance S between the right support and the central line2So that S1Is 60mm, S2The distance between the front ends and the distance between the rear ends of the two adjusted supports are not more than 1 percent after the adjustment;
2) drawing three auxiliary lines in the vertical direction on the front end surface of the test piece, wherein one auxiliary line is positioned on the central line of the front end surface of the test piece, the other two auxiliary lines are distributed on two sides, and the distances between the left auxiliary line and the right auxiliary line and the central line are respectively 60mm and 6.5 mm;
3) after the auxiliary line is drawn, the test piece is horizontally pushed to an ideal position by virtue of a straight ruler with the width of 12mm, and centers of two support round rods (4) and (5) and a loading end round rod (3) of the fixture are calibrated by virtue of the auxiliary line, so that overlarge test error caused by the fact that the test piece is inclined is avoided, and the test piece is well placed;
4) the testing machine is manually controlled to adjust the small displacement, the top force application device is adjusted and moved to the position above the test piece quickly through the remote controller, and then the speed is adjusted to be low, so that the pressure head at the upper end is in slight contact with the test piece; moving the LVDT displacement sensor to a required position and installing the LVDT displacement sensor;
5) the LVDT displacement sensor is used for controlling loading, the loading rate is 0.05mm/min, the rock mechanical testing machine is operated to start the test, and the collected peak load PcrWas 17.0 kN.
From Table 2, the I-type dimensionless stress intensity factor YI0.00217, type II dimensionless stress intensity factor YIIWhen the angle β is 90 °, S is-0.4870251Is 60mm, S2At 6.5mm, a pure type II fracture and, therefore, according to the peak load P recorded in the testcrAnd YIIThe calculated sandstone II type fracture toughness is as follows:
example 2
According to the deep beam test piece for testing the rock fracture toughness, shown in fig. 1-4, a test material is sandstone, a test piece body 1 is cuboid, a central line opening along the thickness B direction of the lower end face of the body is formed in the body, extends towards the interior of the body and penetrates through cutting grooves 2 of the front end face and the rear end face of the body in the thickness direction, the length L of the test piece is 200mm, the width W of the test piece is 100mm, and the thickness B of the test piece is 80 mm; the length a of the notch is 50mm (the ratio of a/W is 0.5), and the crack inclination angle beta is 90 deg. The rock fracture test operation using the above test piece was as follows:
1) adjusting the distance S between the left support and the central line according to the three-point bending fixture graduated scale1And the distance S between the right support and the central line2So that S1Is 60mm, S2The distance between the front ends and the distance between the rear ends of the two adjusted supports are not more than 1 percent after adjustment, wherein the distance is 20 mm;
2) drawing three auxiliary lines in the vertical direction on the front end surface of the test piece, wherein one auxiliary line is positioned on the central line of the front end surface of the test piece, the other two auxiliary lines are distributed on two sides, and the distances between the left auxiliary line and the right auxiliary line and the central line are respectively 60mm and 20 mm;
3) after the auxiliary line is drawn, the test piece is horizontally pushed to an ideal position by virtue of a straight ruler with the width of 12mm, and centers of two support round rods (4) and (5) and a loading end round rod (3) of the fixture are calibrated by virtue of the auxiliary line, so that overlarge test error caused by the fact that the test piece is inclined is avoided, and the test piece is well placed;
4) the testing machine is manually controlled to adjust the small displacement, the top force application device is adjusted and moved to the position above the test piece quickly through the remote controller, and then the speed is adjusted to be low, so that the pressure head at the upper end is in slight contact with the test piece; moving the LVDT displacement sensor to a required position and installing the LVDT displacement sensor;
5) the LVDT displacement sensor is used for controlling loading, the loading rate is 0.05mm/min, the rock mechanical testing machine is operated to start the test, and the collected peak load PcrIt was 23.5 kN.
From Table 2, the I-type dimensionless stress intensity factor YI0.52048, type II dimensionless stress intensity factor YIIWhen the angle β is 90 °, S is-0.2876311Is 60mm, S220mm, I-II complex fracture. Thus, according to the peak load P recorded in the testcrAnd YI、YIIThe calculated sandstone I-II composite fracture toughness is as follows:
example 3
According to the deep beam test piece for testing the rock fracture toughness, shown in fig. 1-4, a test material is sandstone, a test piece body 1 is cuboid, a central line opening along the thickness B direction of the lower end face of the body is formed in the body, extends towards the interior of the body and penetrates through cutting grooves 2 of the front end face and the rear end face of the body in the thickness direction, the length L of the test piece is 200mm, the width W of the test piece is 100mm, and the thickness B of the test piece is 80 mm; the length a of the notch is 50mm (the ratio of a/W is 0.5), and the crack inclination angle beta is 90 deg. The rock fracture test operation using the above test piece was as follows:
1) adjusting the distance S between the left support and the central line according to the three-point bending fixture graduated scale1And the distance S between the right support and the central line2So that S1Is 60mm, S2The distance between the front ends and the distance between the rear ends of the two adjusted supports are not more than 1 percent after being adjusted to be 40 mm;
2) drawing three auxiliary lines in the vertical direction on the front end surface of the test piece, wherein one auxiliary line is positioned on the central line of the front end surface of the test piece, the other two auxiliary lines are distributed on two sides, and the distances from the left auxiliary line to the central line to the right auxiliary line are respectively 60mm and 40 mm;
3) after the auxiliary line is drawn, the test piece is horizontally pushed to an ideal position by virtue of a straight ruler with the width of 12mm, and centers of two support round rods (4) and (5) and a loading end round rod (3) of the fixture are calibrated by virtue of the auxiliary line, so that overlarge test error caused by the fact that the test piece is inclined is avoided, and the test piece is well placed;
4) the testing machine is manually controlled to adjust the small displacement, the top force application device is adjusted and moved to the position above the test piece quickly through the remote controller, and then the speed is adjusted to be low, so that the pressure head at the upper end is in slight contact with the test piece; moving the LVDT displacement sensor to a required position and installing the LVDT displacement sensor;
5) the LVDT displacement sensor is used for controlling loading, the loading rate is 0.05mm/min, the rock mechanical testing machine is operated to start the test, and the collected peak load PcrWas 14.0 kN.
From Table 2, the I-type dimensionless stress intensity factor YI1.031894 type II dimensionless stressIntensity factor YIIWhen the angle β is 90 °, S is-0.1006981Is 60mm, S240mm, I-II composite fracture, and therefore, according to the peak load P recorded in the testcrAnd YIIThe calculated sandstone I-II composite fracture toughness is as follows:
example 4
According to the deep beam test piece for testing the rock fracture toughness, shown in fig. 1-4, a test material is sandstone, a test piece body 1 is cuboid, a central line opening along the thickness B direction of the lower end face of the body is formed in the body, extends towards the interior of the body and penetrates through cutting grooves 2 of the front end face and the rear end face of the body in the thickness direction, the length L of the test piece is 200mm, the width W of the test piece is 100mm, and the thickness B of the test piece is 80 mm; the length a of the notch is 50mm (the ratio of a/W is 0.5), and the crack inclination angle beta is 90 deg. The rock fracture test operation using the above test piece was as follows:
1) adjusting the distance S between the left support and the central line according to the three-point bending fixture graduated scale1And the distance S between the right support and the central line2So that S1Is 60mm, S2The distance between the front ends and the distance between the rear ends of the two adjusted supports are not more than 1 percent after adjustment;
2) drawing three auxiliary lines in the vertical direction on the front end face of the test piece, wherein one auxiliary line is positioned on the central line of the front end face of the test piece, the other two auxiliary lines are symmetrically distributed on two sides, and the distance between every two adjacent auxiliary lines is 60 mm;
3) after the auxiliary line is drawn, the test piece is horizontally pushed to an ideal position by virtue of a straight ruler with the width of 12mm, and centers of two support round rods (4) and (5) and a loading end round rod (3) of the fixture are calibrated by virtue of the auxiliary line, so that overlarge test error caused by the fact that the test piece is inclined is avoided, and the test piece is well placed;
4) the testing machine is manually controlled to adjust the small displacement, the top force application device is adjusted and moved to the position above the test piece quickly through the remote controller, and then the speed is adjusted to be low, so that the pressure head at the upper end is in slight contact with the test piece; moving the LVDT displacement sensor to a required position and installing the LVDT displacement sensor;
5) the LVDT displacement sensor is used for controlling loading, the loading rate is 0.05mm/min, the rock mechanical testing machine is operated to start the test, and the collected peak load PcrWas 10.5 kN.
From Table 2, the I-type dimensionless stress intensity factor YI1.372582, type II dimensionless stress intensity factor YIIWhen the angle β is 0, the angle β is 90 ° and S is known as1Is 60mm, S260mm, pure type I fracture, according to the peak load P recorded in the testcrAnd YIThe calculated sandstone I-type fracture toughness is as follows:
Claims (7)
1. the method for testing the composite fracture toughness of the rock based on the asymmetric three-point bending loading of the NDB sample is characterized in that the NDB sample is used, the NDB sample is a cuboid, vertical cutting grooves which are opened from the center line position of the lower end face of the NDB sample and are parallel to the thickness B direction are formed in the NDB sample, the cutting grooves penetrate through the front end face and the rear end face in the thickness B direction, the length L of the NDB sample is 2 times of the width W of the NDB sample, the thickness B of the NDB sample is not less than 0.8W, the depth of the cutting grooves is a, and a is not less than 0.3W and not more than 0.7;
adopting asymmetric three-point bending loading to carry out pure I type, pure II type and I-II composite fracture toughness tests on rocks, wherein the distance between a three-point bending left support and the central plane of a cutting groove isS 1The distance between the right support and the central plane of the cutting groove isS 2And is andS 2≤S 1(ii) a Is less than or equal to 0.3a/WSelected within the range of less than or equal to 0.7aAnd is less than or equal to 0.4S 1/WDetermined in the range of less than or equal to 0.9S 1Regulating is madeS 2/WTake a value fromS 1/WStarting to decrease gradually until pure form II correspondences are obtainedS 2So as to obtain different load combination degrees; wherein the scale is adjusted by a three-point bending fixtureS 2The difference between the front end distance and the rear end distance of the two adjusted supports is not more than 1%, and the same supports are used in the same group of experimentsS 1；
The method specifically comprises the following steps:
comprehensively calibrating dimensionless stress intensity factor of the crack tip when the NDB sample is broken under the condition of asymmetric three-point bending loading by using finite element numerical software ABAQUS or ANSYSY I、Y IIAnd dimensionless nonsingular stressTAnd obtaining pure II type loading correspondencesS 2The method comprises the following steps:
(1) establishing a corresponding numerical calibration model: establishing a three-dimensional geometric model of the NDB sample by using finite element numerical software, assuming that a rock material is a linear elastic material and endowing the NDB sample with elastic parameters, then dividing a grid, constraining the vertical and horizontal displacement of a node at a left support, constraining the vertical displacement at a right support, and applying a reference node load to an upper pressure head loading pointPUnder loadPUnder the action of the stress intensity factors of type I and type II of the NDB sampleK I，K IIAnd is nonsingularTThe stress is expressed as:
reversely deducing the dimensionless stress intensity factor according to the formulas (1) to (3)Y I、Y IIAnd dimensionless nonsingular stressTIs as followsShown in the figure:
(2) based on the numerical calibration model in the step (1), the calculation is continuously reduced by adopting a dichotomyS 2Taking values; when the calculated load combination degreeM eIn the vicinity of 1, the error does not exceed 10-4Is considered to have reached a pure type II fracture, a standoff distance is obtained that achieves a pure type II loading, wherein the load composite is given by:
(3) calibrating the model based on the numerical value obtained in the step (1) and the numerical value obtained in the step (2)S 2Determining what is required for the testaAndS 1right support spacing fromS 1Gradually reducing to the set step pitchS 2Calculating dimensionless stress intensity factors one by oneY IAndY IIand dimensionless nonsingular stressT*；
(4) Drawing three auxiliary lines in the vertical direction on the front end face in the thickness direction of the NDB sample, wherein one auxiliary line is positioned on the central line of the front end face of the NDB sample, the other two auxiliary lines are distributed on two sides, and the distance between every two adjacent auxiliary lines is equal to that between every two adjacent auxiliary linesS 1AndS 2；
(5) the NDB sample is arranged on the test bed by means of the ruler and the auxiliary lines on the front end face of the NDB sample, one auxiliary line in the middle of the three auxiliary lines on the front end face of the NDB sample is aligned to the load loading pressure head, and the auxiliary lines on the two sides are aligned to the support fulcrum;
(6) adjusting the rock mechanics testing machine to enable the load loading pressure head to slightly contact with the NDB sample, finally checking the placing position of the NDB sample, and installing an LVDT displacement sensor after the NDB sample is confirmed to be correct;
(7) controlling loading by using an LVDT displacement sensor, and operating a rock mechanics testing machine to load the NDB sample until the NDB sample is damaged and completely loses the bearing capacity;
(8) according to peak load in experimentP cr And type I/II dimensionless stress intensity factorY I/Y IICalculating the composite fracture toughness of the rock according to the following formula:
at 0.4W≤S 1≤0.9WAnd 0.3W≤a≤0.7WVariation within the scopeS 1Andaand (5) repeating the steps (1) to (8).
2. The method of claim 1, wherein the loading rate of the NDB sample by the rock mechanics tester is less than 0.2 mm/min.
3. The method of claim 1 wherein the load ram is a round bar structured load ram and the left and right supports are round bar structured supports, the round bar structured load ram contour being parallel to the slot center plane.
4. The method according to any one of claims 1 to 3, wherein any two adjacent faces of the NDB sample are at an angle of 90 ° ± 0.5 °; the cross-sectional dimension deviation of any two positions in one direction of the NDB sample is not more than 0.1 mm.
5. According to any one of claims 1 to 3The method of claim, wherein the NDB sample has a lengthLNot less than 10 times the particle size of the rock and not less than 76 mm.
6. The method of any one of claims 1 to 3, wherein the NDB sample has a lengthLIs a widthW2 times of the error, and the error is not more than 0.04W.
7. The method of any one of claims 1 to 3, wherein the NDB sample has a thicknessBNot less than widthW0.8 times of that of the total amount of the components, and is not less than 30 mm; width of the cutting groovebLess than 1 mm.
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