Tensile testing device and tensile testing method

Abstract

To provide a tensile testing device and a tensile testing method which can perform highly accurate measurement even for a test piece which is composed of a granular substance such as soil.SOLUTION: A tensile testing device includes pair of upper and lower holding means for holding a test piece of a three dimensional shape at the vicinity of upper and lower edges respectively, and can apply a tensile load or a compressive load to the test piece held by the holing means. The pair of upper and lower holding means are constituted of: a pair of upper and lower pressing bodies having contact surfaces respectively coming into contact with the upper and lower surfaces of the test piece and attached to the tensile testing device body; a pair of soft upper and lower sleeves respectively covering upper and lower circumferential edges about 1 / 12 to 1 / 4 the overall length from each of upper and lower edge faces of the test piece while the contact surfaces are in contact with the test piece; and one or a plurality of suction devices for drawing in gas remaining in the inside of each of the pair of upper and lower sleeves.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a tensile testing device and a tensile testing method for testing the tensile strength of materials made of granular substances such as soil in the field of civil engineering. [Background technology]

[0002] Tension cracks that occur behind embankments and retaining walls are often caused by tensile stress acting on the ground. These cracks are thought to pose significant problems in geotechnical, geological, and environmental engineering terms. For example, if these cracks grow larger due to earthquakes or weathering, they can promote water infiltration, potentially leading to problems such as a weakening of the ground and a decrease in the stability of structures. Furthermore, if cracks occur in the soil cover of a nuclear waste repository that uses a capillary barrier (water-blocking performance due to capillary forces), they could lead to radioactive leaks. Therefore, when designing geotechnical structures, it is expected that the need to compare the tensile stress acting within the ground with the tensile resistance (tensile strength) of the soil itself will become increasingly important. However, unlike uniform, continuous, hard materials such as concrete and steel, soil is a material in which granular soil particles are held together by inter-particle adhesive forces, which makes mechanical fixation, which is essential for tensile testing, impossible. Even if fixation can be achieved using other methods, adhesion at the fixed ends or unintended failure often occurs. Therefore, previous research has only been able to indirectly estimate strength using completely different methods and formats, and specialized testing equipment that is not versatile, so there is a need to develop a simple, versatile testing method.

[0003] Patent document 1 describes an apparatus in which a test specimen is covered and fixed tightly with separate outer shells on the top and bottom, and a dynamic tensile load is applied in the vertical direction to the shells, causing the test specimen to fracture in tension.

[0004] In Patent Document 2, a tensile test is performed using the same device that forms the specimen. The forming device, which serves as the mold for the specimen, can be separated at the top, bottom, and center, so the configuration during the tensile test is similar to that of Patent Document 1.

[0005] As mentioned above, Patent Documents 1 and 2 have in common a method in which the upper and lower ends of a specimen are covered with independent outer shells (forms) from the top to the center, and the outer shells (forms) are brought into close contact with the specimen, and then upper and lower tensile stresses are applied to the outer shells (forms).However, because the adhesive surface between the specimen and the outer shells (forms) is large, stress transmission becomes complicated, and there is a risk of localized shear failure occurring or a pure tensile failure surface not being formed.

[0006] In Patent Document 3, the method of applying a load by vacuum-adhering both end faces of the specimen is similar to that of the present invention, but this method alone does not apply the load evenly to the specimen, and the tensile fracture surface is concentrated near the end. To avoid this, a groove-shaped notch is provided in the center of the specimen to forcibly guide the tensile fracture surface to that location, which may not necessarily capture ideal fracture behavior. Note that the notch in the center of the specimen is a method also used in Patent Document 2.

[0007] As described above, previously developed equipment combines a method using an outer shell (formwork) to fix and install the test specimen, and a method of cutting groove-shaped notches into the test specimen and applying load to guide the tensile failure surface to the center of the test specimen. Both methods are not very versatile because they require special jigs and test specimens with special shapes, and there is a possibility of complex failure modes and uneven stress transmission, which makes it difficult to simply evaluate the test results obtained. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-225056 [Patent Document 2] Patent No. 6064936 [Patent Document 3] Japanese Patent Application Publication No. 6-331521 Summary of the Invention [Problem to be solved by the invention]

[0009] In view of the above-mentioned drawbacks of the conventional technology, the present invention aims to provide a tensile testing device and a tensile testing method that can perform highly accurate measurements even on test specimens made of granular materials such as soil. [Means for solving the problem]

[0010] In order to achieve the above object, the tensile testing apparatus described in claim 1 of the present invention is a tensile testing apparatus comprising a pair of upper and lower holding means for holding the upper and lower ends of a three-dimensional specimen, respectively, and capable of applying a tensile load or a compressive load to the specimen held by said holding means, wherein said pair of upper and lower holding means have contact surfaces that contact the upper and lower surfaces of the specimen, respectively, and are comprised of a pair of upper and lower pressure bodies attached to the tensile testing apparatus body, a pair of soft upper and lower sleeves that cover the upper and lower outer peripheral ends of the specimen from each of the upper and lower end faces to approximately 1 / 12 to 1 / 4 of the total length when said contact surfaces are in contact with the specimen, and one or more suction devices that suck in gas remaining inside each of said pair of upper and lower sleeves.

[0011] 3. The tensile testing device according to claim 1, wherein the sleeve of the tensile testing device according to claim 2 is made of soft rubber.

[0012] The tensile test method for the tensile tester according to claim 3 includes a contacting step of contacting upper and lower pressure bodies with the upper and lower surfaces of a three-dimensional specimen, respectively, and a soft pressure body for pressing the upper and lower outer peripheral end portions of the specimen at approximately 1 / 12 to 1 / 4 of the total length from each of the upper and lower end surfaces. A pair of upper and lower A tensile testing method comprising a sleeve mounting process for covering the specimen with a sleeve, an intake process for suctioning gas remaining inside each of the pair of upper and lower sleeves, a loading process for applying a load to the specimen, and a data acquisition process for acquiring data on the specimen during the loading process. The tensile testing method according to claim 4 is characterized in that the loading step includes applying a uniaxial or triaxial load to the test piece. [Effects of the Invention]

[0013] As is clear from the above description, the present invention provides the following effects. (1) In each of the inventions described in claims 1 to 4, a soft sleeve is provided that covers approximately 1 / 12 to 1 / 4 of the specimen from the end, and an intake device is provided that can intake the gas inside this sleeve. Therefore, even when a simple tensile test is performed with a vacuum pressure (negative pressure) applied to both end surfaces of the specimen and an external tensile force applied, the tensile test can be performed stably and without installing a special outer shell (formwork). Therefore, standard specimens can be used without processing, making it possible to apply the method to various types of soil materials. (2) In addition, standard specimens can be subjected to the test without any processing, and since the test is simply a matter of tension, and the maximum value of tensile stress can be identified as the tensile strength, data measurement and the organization of the results become extremely simple and clear. (3) Soil tensile strength, which has previously been measured using various methods, will now be measured using a unified test, which is expected to improve the interpretation of measurement data and the overall evaluation of ground properties. In addition, the improved accuracy of soil tensile strength measurements, which have not been actively evaluated in ground analysis and design, will be incorporated into future analysis and design, which is expected to lead to more reasonable structure designs and the construction of highly accurate simulation models. [Brief explanation of the drawings]

[0014] 1 to 4 are explanatory diagrams showing a first embodiment of the present invention. 5 and 6 are explanatory diagrams showing a second embodiment of the present invention. [Figure 1] FIG. 1 is a schematic diagram of a main part of a tensile testing device according to a first embodiment. [Figure 2]FIG. 2 is a process diagram of the tensile test method according to the first embodiment. [Figure 3] A reference diagram showing the relationship between the sleeve covering length and vacuum pressure. [Figure 4] Stress-strain diagram obtained during the data acquisition process. [Figure 5] FIG. 10 is a schematic diagram of a main part of a tensile testing device according to a second embodiment. [Figure 6] FIG. 10 is a process diagram of a tensile test method according to a second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0015] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail below with reference to the accompanying drawings showing preferred embodiments of the present invention.

[0016] In the first embodiment for carrying out the present invention shown in FIGS. 1 to 4, reference numeral 1 denotes a tensile testing device capable of applying a load to a test piece 2 mainly in one or more directions, from one axis to three axes.

[0017] As shown in Figure 1, this tensile testing device 1 is equipped with a pair of upper and lower holding means 3 that hold the upper and lower ends of a test piece 2 having a three-dimensional shape such as a cylinder, and is capable of applying a load to the test piece 2 held by the pair of upper and lower holding means 3, and obtaining data on the applied load and changes in the test piece 2 due to the load.

[0018] Specimen 2 is made of granular material, such as soil, formed into a three-dimensional shape such as a cylinder, and is formed to have the shape and dimensions of the specimen specified in the soil uniaxial compression test method. Specifically, it is formed to be cylindrical, with a diameter of 35 mm or 50 mm and a height of 1.8 to 2.5 times the diameter.

[0019] The pair of upper and lower holding means 3 have contact surfaces 4 that contact the upper and lower surfaces of the test specimen 2, respectively, and are composed of a pair of upper and lower pressure bodies 5 that are attached to the tensile testing device main body (not shown), a pair of soft upper and lower sleeves 6 that cover the upper and lower outer peripheral ends of the test specimen 2, respectively, from each of the upper and lower end faces to approximately 1 / 12 to 1 / 4 of the total length, when the contact surfaces 4 are in contact with the test specimen 2, and one or more suction devices 7 that suck in gas remaining inside the pair of upper and lower sleeves 6.

[0020] The pressure body 5 of this holding means 3 is a substantially cylindrical member formed with a substantially horizontal contact surface 4 that contacts the specimen 2, and the surface opposite to the surface on which the contact surface 4 is formed is connected to a reaction frame 8 and a loading piston 9 of the tensile testing apparatus main body (not shown). In this embodiment, the upper holding means 3 is connected to the reaction frame 8, and the lower holding means 3 is connected to the loading piston 9. Note that since a known tensile testing apparatus main body can be used as the tensile testing apparatus main body, illustrations and descriptions thereof are omitted.

[0021] In this embodiment, the sleeve 6 is formed into a thin cylindrical shape using a soft elastic material such as rubber, and is formed into a shape that can cover the outer periphery of the pressurizing body 5 and about 1 / 12 to 1 / 4 of the top and bottom ends of the specimen 2 without any gaps. In this embodiment, the sleeve 6 is formed into a thin cylindrical shape that can cover the outer periphery of the pressurizing body 5 and the top and bottom ends of the specimen 2.

[0022] In this embodiment, the suction device 7 is composed of a vacuum pump 10, a vacuum regulator 11 and a vacuum gauge 12 connected to the vacuum pump 10, and an suction path 13 connected to the vacuum regulator 11 and arranged to allow the gas inside the sleeve 6 covering the test piece 2 to be sucked in in the vertical direction.

[0023] This intake path 13 is provided with one or more valves 14, and in this embodiment, one valve 14 is provided at the point where the intake path 13 branches off from the vacuum regulator 11 into upper and lower parts, and at each point after the path branches off into a pair of upper and lower intake paths 13.

[0024] When suction is performed by the vacuum pump 10 of the suction device 7, it is advisable to suction so that the value on the vacuum gauge 12 becomes approximately -100 kPa.

[0025] In this way, by covering about 1 / 12 to 1 / 4 of the edge of the specimen 2 with the sleeve 6 and sucking the gas inside the sleeve 6 in the vertical direction, the specimen 2 can be held firmly and it is possible to prevent air leakage through the gaps in the soil particle structure that makes up the specimen 2, which could cause uneven adhesion on the loading surface. By conducting a tensile test while the specimen is held by the holding means 3 and air is sucked in by the air suction device 7, it is possible to test a standard specimen 2 without any processing, and because it is a simple test that only involves pulling, and the maximum value of the tensile stress can be identified as the tensile strength as is, data measurement and organizing the results become extremely simple and clear.

[0026] Regarding the optimal covering length of the sleeve 6, the results of a test using a dummy specimen 2 without any irregularities are shown in Figure 3. In this experiment, a vacuum pressure of approximately -100 kPa was applied to specimen 2 with various covering lengths of the sleeve 6, and strain was recorded under load. As is clear from these results, when the covering length was 0 mm, the vacuum pressure could not be maintained at all. Furthermore, when 5 mm (1 / 24 the length of specimen 2) was covered from both ends of specimen 2, the vacuum pressure supplied by the intake device 7 decreased. Therefore, even for specimens without any irregularities, a covering length of at least 10 mm (1 / 12 the length of specimen 2) is required. Because the actual specimen 2 has irregularities, in order to perform an appropriate tensile test, it is desirable for the covering length of the sleeve 6 to be at least 1 / 6 of the total length of specimen 2, and optimally, it is 1 / 4 of the total length of specimen 2. On the other hand, if the portion of the test piece 2 covered by the sleeve 6 exceeds 1 / 4 from the end, the contact surface between the test piece 2 and the sleeve becomes large, which complicates stress transmission and may prevent appropriate tensile strength measurement. Incidentally, when the covering length of the sleeve 6 is 5 mm from both ends of the test piece 2 (1 / 24 the length of the test piece 2), a phenomenon was observed in which the axial force increased, although this is not shown.

[0027] With the gas inside the sleeve 6 sucked in by the suction device 7, a tensile test is performed by applying a tensile load or a compressive load, and various data are obtained.

[0028] As shown in Figure 2, a tensile testing method 15 of the present invention using such a tensile testing apparatus 1 is composed of an abutment process 16 in which the upper and lower ends of a specimen 2 are approximately abutted against the upper and lower pressure bodies 5 of the tensile testing apparatus 1; a sleeve attachment process 17 in which at least a portion of the pressure bodies 5 and approximately 1 / 6 to 1 / 4 of the upper and lower ends of the specimen 2 are covered with a soft sleeve 6; an intake process 18 in which gas is sucked from inside the sleeve 6; a loading process 19 in which a load is applied to the specimen 2; and a data acquisition process 20 in which data on at least the specimen 2 is acquired during the loading process 19.

[0029] In the contact step 16, the specimen 2 is placed on the lower pressurizing member 5, and the upper pressurizing member 5 is lowered or raised so that the top surface of the specimen 2 and the upper pressurizing member 5 come into contact. At this time, it is desirable to confirm that a load of approximately 5 to 10 N has been applied to the load meter of the tensile testing device 1, and to ensure that the upper pressurizing member 5 and the top surface of the specimen 2 are in reliable contact. Before the contact step 16, zero adjustment of the load meter and load monitoring (data acquisition step 20) are started.

[0030] After the abutting step 16, a sleeve fitting step 17 is performed in which at least a part of the pressurizing body 5 and about 1 / 12 to 1 / 4 of the top and bottom ends of the specimen 2 are covered with a soft sleeve 6. In this embodiment, a cylindrical thin rubber material is used as the sleeve 6 so that the outer periphery of the pressurizing body 5 and the vicinity of the end of the specimen 2 can be covered.

[0031] In order to perform the sleeve mounting step 17, it is preferable to mount the sleeve 6 on the outer periphery of the upper and lower pressurizing bodies 5 and turn up the end of the sleeve 6 toward the pressurizing body 5 before performing the abutting step 16, preferably before connecting the pressurizing body 5 to the tensile testing device body. When mounting the sleeve 6 on the pressurizing body 5, it is preferable to mount an O-ring or the like near one end to fix it in place.

[0032] After the sleeve mounting process 17, an intake process 18 is performed to suck out the gas (air) inside the sleeve. In the intake process 18, a pair of intake paths 13 are connected so as to pass through the pressurizing body 5, and the vacuum pump 10 is operated to supply vacuum pressure. At this time, the valve 14 before the pressurizing body 5 is kept closed.

[0033] With the valve 14 closed, the vacuum regulator 11 is operated, the value on the vacuum gauge 12 is checked, and the vacuum pressure is adjusted. A vacuum pressure of approximately -100 kPa is desirable. After adjusting the vacuum pressure in this way, the valves 14 on the intake paths 13 connected to the upper and lower pressurizing bodies 5 are opened, and vacuum pressure is applied to the sleeves 6 on both end sides of the specimen 2.

[0034] In the loading step 19, a tensile load or a compressive load is applied to the specimen 2. The type of load to be applied can be changed depending on the purpose of the test. In this embodiment, a tensile testing machine that can only apply a uniaxial load (in the axial direction of the specimen 2) is used.

[0035] In the data acquisition process 20, data such as stress and strain acting on the specimen 2 is acquired. Specifically, before the loading process 19 is performed, a displacement meter for vertical displacement is set and zero-adjusted, and monitoring begins using a PC. Then, recording of the vertical force and vertical displacement begins, and after recording begins, the loading process 19 begins. In this embodiment, the loading process 19 ends when the vertical stress reaches a residual state, and recording of the vertical force and vertical displacement continues until the specimen 2 reaches a predetermined state.

[0036] Figure 4 shows an example of the strain-stress relationship graph from the results of tensile tests on several specimens. In this tensile test, specimen 2 failed in a tensile test at a location not covered by sleeve 6, while the pressure body 5 and the end of specimen 2 remained in close contact. The tensile strength can be derived from the maximum value of this relationship graph, and is expressed as the maximum tensile load divided by the cross-sectional area of ​​the specimen.

[0037] [Different modes for carrying out the invention] Next, different embodiments for carrying out the present invention will be described with reference to Figures 5 and 6. In describing these different embodiments for carrying out the present invention, the same components as those in the first embodiment for carrying out the present invention will be designated by the same reference numerals, and redundant description will be omitted.

[0038] The second embodiment of the present invention shown in FIGS. 5 and 6 differs from the first embodiment of the present invention mainly in that a tensile testing apparatus body equipped with a triaxial pressure chamber 21 is used. Such a tensile testing apparatus 1A and a tensile testing method 15A using this tensile testing apparatus 1A to perform a loading step 19A of applying loads in three directions can achieve the same effects as the first embodiment of the present invention, and can also obtain data when loads are applied in three directions. [Explanation of symbols]

[0039] 1, 1A: tensile test apparatus, 2: test piece, 3: Holding means, 4: Contact surface, 5: pressure body, 6: sleeve, 7: intake device; 8: reaction frame; 9: Loading piston, 10: Vacuum pump, 11: vacuum regulator; 12: vacuum gauge; 13: intake path; 14: valve; 15, 15A: Tensile test method, 16: Contact process, 17: Sleeve installation process, 18: Intake process, 19, 19A: Loading process, 20: Data acquisition process, 21: Triaxial pressure chamber.

Claims

1. A tensile testing device that includes a pair of upper and lower holding means that hold the vicinity of the upper and lower ends of a three-dimensional specimen, respectively, and that can apply a tensile load or a compressive load to the specimen held by the holding means, The pair of upper and lower holding means have contact surfaces that contact the upper and lower surfaces of the test piece, respectively, and are attached to the tensile testing apparatus main body; a pair of soft upper and lower sleeves that cover the upper and lower outer peripheral ends of the test piece, each extending from the upper and lower end faces to approximately 1 / 12 to 1 / 4 of the total length, when the contact surfaces are in contact with the test piece; and one or more intake devices that suck in gas remaining inside the pair of upper and lower sleeves.

2. 2. The tensile testing device according to claim 1, wherein the sleeve is made of soft rubber.

3. A tensile testing method comprising: a contacting step in which the upper and lower pressure bodies of a tensile testing device are brought into contact with the upper and lower surfaces of a three-dimensionally shaped specimen, respectively; a sleeve fitting step in which the upper and lower outer peripheral ends of the specimen, each extending from the upper and lower end faces thereof to approximately 1 / 12 to 1 / 4 of the total length, are covered with soft sleeves; an intake step in which gas remaining inside the pair of upper and lower sleeves is sucked in; a loading step in which a load is applied to the specimen; and a data acquisition step in which data on the specimen is acquired during the loading step.

4. 4. The tensile test method according to claim 3, wherein the loading step applies a uniaxial or triaxial load to the test piece.

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