Tensile testing apparatus and tensile testing method

Abstract

To provide a tensile testing apparatus and a tensile testing method capable of performing highly accurate measurement even on a specimen composed of granular materials such as soil. 【Solution means】A tensile testing apparatus comprising a pair of upper and lower holding means for holding the vicinity of the upper and lower ends of a three-dimensional specimen, and capable of applying a tensile load or a compressive load to the specimen held by the holding means, wherein the pair of upper and lower holding means has contact surfaces that respectively contact the upper and lower surfaces of the specimen, and a pair of upper and lower pressure bodies attached to the tensile testing apparatus main body, and in a state where the contact surfaces contact the specimen, a pair of upper and lower soft sleeves that respectively cover the upper and lower outer peripheral ends of about 1 / 12 to 1 / 4 of the total length from the upper and lower end faces of the specimen, and one or more air intake devices for inhaling the gas remaining inside the pair of upper and lower sleeves.

Description

Technical Field

[0001] The present invention relates to a tensile test apparatus and a tensile test method for testing the tensile strength of a material composed of granular substances such as soil in the field of civil engineering.

Background Art

[0002] Tension cracks occurring on the back of earth embankments and retaining walls are often caused by tensile stress acting on the ground. It is considered that this crack may cause significant problems in terms of geotechnical, geological, and environmental engineering. For example, if the size of this crack progresses with an earthquake or weathering, water penetration is promoted, which may lead to problems such as a decrease in the strength of the ground and a decrease in the stability of the structure. In addition, when cracks occur in the covering soil part of a nuclear waste disposal site applying a capillary barrier (water barrier performance due to the action of capillary force), there is a risk of inducing radioactive leakage. Therefore, when designing ground structures, it is considered that the need to examine the tensile stress acting in the ground and the tensile resistance (tensile strength) of the soil itself will increase more and more in the future. However, in order to determine the tensile strength of soil, unlike uniform and continuous hard materials such as concrete and iron, since granular soil particles are bonded by the adhesion between particles, mechanical fixation required in a tensile test cannot be achieved. Also, even if it can be fixed by other methods, detachment at the fixed end and breakage at unintended parts often occur. Therefore, in past research, it has remained at indirectly estimating the strength by completely different methods, forms, and special test apparatuses with no versatility, so the development of a simple and highly versatile test method is required.

[0003] Patent Document 1 describes an apparatus that covers and fixes the upper and lower parts of a specimen so as to be in close contact with independent outer shells, applies a dynamic tensile load in the vertical direction to the outer shells, and causes the specimen to be tensilely fractured.

[0004] In Patent Document 2, a tensile test is conducted using the apparatus for forming a specimen as it is. Since the forming apparatus that serves as the mold for the specimen can be divided at the top, bottom, and center, the form during the tensile test is similar to that in Patent Document 1.

[0005] As described above, in Patent Documents 1 and 2, a method is commonly used in which the upper and lower ends to the central part of the specimen are each covered with an independent outer shell (mold) and brought into close contact with the specimen, and tensile stress is applied to the upper and lower outer shells (molds). Since the adhesive surface between the specimen and the outer shell (mold) is large, stress transmission becomes complicated, and there is a possibility that local shear failure may occur and a pure tensile fracture surface may not be formed.

[0006] In Patent Document 3, a method of applying a load by vacuum-sucking both end faces of the specimen is similar to the present invention. However, with this method alone, the load on the specimen does not act evenly, and the tensile fracture surface concentrates near the ends. Therefore, in order to avoid this, a groove-shaped notch is provided in the central part of the specimen to forcibly induce the tensile fracture surface to that part, so there is a possibility that the ideal fracture behavior may not be captured. The notch in the central part of the specimen is also a method applied in Patent Document 2.

[0007] From the above, the conventional development apparatuses are implemented by a combination of a method using an outer shell (mold) for fixedly installing the specimen and a method of engraving a groove-shaped notch on the specimen to induce the tensile fracture surface to the central part of the specimen. Since both use special jigs or specimens with special shapes, they are methods with low versatility. There is a possibility that complex fracture forms and non-uniform stress transmission may intervene, so there is a drawback that it is difficult to simply evaluate the obtained test results.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Patent Document 3

[0009] In view of the above-described conventional drawbacks, an object of the present invention is to provide a tensile test apparatus and a tensile test method capable of performing highly accurate measurement even on a specimen composed of granular materials such as soil. MEANS FOR SOLVING THE PROBLEMS

[0010] In order to achieve the above object, a tensile test apparatus according to claim 1 of the present invention includes a pair of upper and lower holding means for holding the vicinity of upper and lower end portions of a three-dimensional specimen, and is a tensile test apparatus capable of applying a tensile load or a compressive load to the specimen held by the holding means. The pair of upper and lower holding means has contact surfaces that respectively contact the upper surface and the lower surface of the specimen, and a pair of upper and lower pressure bodies attached to the tensile test apparatus main body. In a state where the contact surfaces contact the specimen, a pair of upper and lower soft sleeves that respectively cover upper and lower outer peripheral end portions of about 1 / 12 to 1 / 4 of the total length from upper and lower end faces of the specimen, and a single or a plurality of air intake devices that intake gas remaining inside the pair of upper and lower sleeves.

[0011] The tensile test apparatus according to claim 1, wherein the sleeve of the tensile test apparatus according to claim 2 is formed of soft rubber.

[0012] The tensile test method of the tensile test apparatus according to claim 3 includes a contact step of bringing upper and lower pressure bodies into contact with the upper surface and the lower surface of a three-dimensional specimen, respectively, and upper and lower outer peripheral end portions of about 1 / 12 to 1 / 4 of the total length from upper and lower end faces of the specimen are respectively covered with soft upper and lower pair of A sleeve mounting step of covering with a sleeve, an air intake step of intake gas remaining inside the pair of upper and lower sleeves, a loading step of applying a load to the specimen, and a data acquisition step of acquiring data of the specimen during the loading step. In the loading step of the tensile test method according to claim 4, a uniaxial or triaxial load is applied to the specimen, which is characterized in that.

Advantages of the Invention

[0013] As is clear from the above description, the following effects can be obtained in the present invention. (1) In each of the inventions described in claims 1 to 4, since a soft sleeve that covers about 1 / 12 to 1 / 4 from the end of the specimen is provided, and an air intake device that can intake the gas inside this sleeve is provided, even when a simple tensile test or the like is performed with a vacuum pressure (negative pressure) acting on both end faces of the specimen and a tensile external force acting, it is possible to perform the tensile test without installing a stable and special outer shell (formwork). Therefore, since a standard specimen can be used without processing, it can be applied to various types of soil materials. (2) Also, since it is possible to use a standard specimen for testing without processing, and since it is a test that simply pulls and the maximum value of the tensile stress can be directly identified as the tensile strength, the measurement of data and the arrangement of results become extremely simple and clear. (3) The tensile strength of soil, which has been measured by various methods so far, will be measured by a unified test, and it is expected that the interpretation of the measurement data will progress and the overall evaluation of the ground physical properties will be improved. In addition, by increasing the measurement accuracy of the tensile strength of soil, which has not been actively evaluated in the analysis and design of the ground so far, it is expected that by introducing this into future analysis and design, a more reasonable design of structures and the construction of a high-precision simulation model can be realized.

Brief Description of the Drawings

[0014] Figs. 1 to 4 are explanatory diagrams showing a first embodiment of the present invention. Figs. 5 and 6 are explanatory diagrams showing a second embodiment of the present invention.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Mode for Carrying Out the Invention

[0015] Hereinafter, the present invention will be described in detail according to the mode for carrying out the present invention shown in the drawings.

[0016] In the first mode for carrying out the present invention shown in FIGS. 1 to 4, reference numeral 1 denotes a tensile test apparatus capable of applying a uniaxial to triaxial load mainly to a specimen 2.

[0017] As shown in FIG. 1, this tensile test apparatus 1 includes a pair of upper and lower holding means 3 for holding the upper and lower end portions of a three-dimensional shaped specimen 2 such as a cylinder, and applies a load to the specimen 2 held by the pair of upper and lower holding means 3, and can acquire the applied load and the change data due to the load of the specimen 2.

[0018] The specimen 2 is mainly formed by shaping particulate materials such as soil into a three-dimensional shape such as a cylindrical shape, and is formed to have the shape and dimensions of the specimen shown in the uniaxial compression test method of soil. Specifically, the shape is cylindrical, the diameter is 35 mm or 50 mm, and the height is formed to be 1.8 to 2.5 times the diameter.

[0019] The upper and lower pair of holding means 3 has contact surfaces 4 that respectively contact the upper and lower surfaces of the specimen 2, and a pair of upper and lower pressure bodies 5 that are attached to a tensile test apparatus main body (not shown), and in a state where the contact surfaces 4 are in contact with the specimen 2, a pair of upper and lower soft sleeves 6 that cover the upper and lower outer peripheral ends of about 1 / 12 to 1 / 4 of the total length from the upper and lower end faces of the specimen 2, and one or more air intake devices 7 that intake the gas remaining inside the pair of upper and lower sleeves 6 respectively.

[0020] The pressure body 5 of this holding means 3 is a substantially columnar member with a substantially horizontal contact surface 4 that contacts the specimen 2, and the surface on the opposite side of the surface where the contact surface 4 is formed is connected to a reaction force frame 8 or a loading piston 9 of a tensile test apparatus main body (not shown). In this embodiment, the upper holding means 3 is connected to the reaction force frame 8, and the lower holding means 3 is connected to the loading piston 9. Since a known tensile test apparatus main body can be used for the tensile test apparatus main body, illustration and description are omitted.

[0021] The sleeve 6 is mainly formed in a thin cylindrical shape using an elastic soft material such as rubber in this embodiment, and is formed in a shape that can cover about 1 / 12 to 1 / 4 from the outer peripheral portion of the pressure body 5 and the upper and lower ends of the specimen 2 without a gap. In this embodiment, it is formed in a thin cylindrical shape that can cover the outer peripheral portion of the pressure body 5 and the upper and lower ends of the specimen 2.

[0022] The air intake device 7 is composed of a vacuum pump 10, a vacuum regulator 11 and a vacuum gauge 12 connected to this vacuum pump 10, and an air intake path 13 that is connected to this vacuum regulator 11 and is provided so that the gas inside the sleeve 6 covering the specimen 2 can be intake in the vertical direction in this embodiment.

[0023] One or more valves 14 are provided in this air intake path 13. In this embodiment, valves 14 are provided one by one at the portion where the air intake path 13 branches vertically from the vacuum regulator 11 and at the portion where the air intake path 13 branches into a pair of upper and lower air intake paths 13.

[0024] When performing air intake with the vacuum pump 10 of this intake device 7, it is preferable to perform air intake so that the value of the vacuum gauge 12 becomes about -100 kPa.

[0025] In this way, by the sleeve 6, about 1 / 12 to 1 / 4 of the end of the specimen 2 is covered, and by sucking the gas inside the sleeve 6 in the vertical direction, the specimen 2 can be firmly held, and at the same time, leakage occurs through the gaps in the soil particle structure constituting the specimen 2, and it is possible to prevent the adhesion state of the loading surface from becoming non-uniform. By performing a tensile test in a state where the specimen 2 is held by the holding means 3 and air intake is performed by the intake device 7, it becomes possible to subject the standard specimen 2 to the test without processing, and since it is a test that simply pulls, and the maximum value of the tensile stress can be directly identified as the tensile strength, the measurement of data and the arrangement of results become extremely simple and clear.

[0026] Regarding the optimal coating length by the sleeve 6, the test results using a dummy of the specimen 2 without unevenness are shown in Fig. 3. In this experiment, for the specimen 2 with the coating length of the sleeve 6 changed respectively, a vacuum pressure of about -100 kPa was supplied, a load was applied to the specimen 2, and the strain was recorded. As is clear from this result, when the coating length is 0 mm, the vacuum pressure cannot be maintained at all. When the specimen 2 is coated by 5 mm (1 / 24 of the length of the specimen 2) from both ends of the specimen 2 respectively, the vacuum pressure supplied by the intake device 7 decreases. Therefore, even for a specimen without unevenness, a coating length of at least 10 mm (1 / 12 of the length of the specimen 2) is required. Since there are unevenness in the actual specimen 2, when conducting a proper tensile test, it is desirable that the coating length of the sleeve 6 be 1 / 6 or more of the total length of the specimen 2, and optimally, the coating length of the sleeve 6 be 1 / 4 of the total length of the specimen 2. On the other hand, when the portion covering the specimen 2 by the sleeve 6 exceeds 1 / 4 from the end, the contact surface between the specimen 2 and the sleeve becomes large, so the stress transmission becomes complicated, and there is a possibility that an appropriate tensile strength cannot be measured. By the way, when the coating length by the sleeve 6 is 5 mm (1 / 24 of the length of the specimen 2) from both ends of the specimen 2, a phenomenon that the axial force increases was also observed although not shown in the figure.

[0027] In such a state where the gas inside the sleeve 6 is inhaled by the intake device 7, a tensile load or a compressive load is applied to conduct a tensile test, and various data are acquired.

[0028] As shown in Fig. 2, the tensile test method 15 of the present invention using such a tensile test device 1 includes a contact step 16 of making the upper and lower ends of the specimen 2 substantially contact with the upper and lower pressure bodies 5 of the tensile test device 1, a sleeve mounting step 17 of covering at least a part of the pressure body 5 and about 1 / 6 to 1 / 4 of the upper and lower ends of the specimen 2 with a soft sleeve 6, an intake step 18 of inhaling the gas inside the sleeve 6, a loading step 19 of applying a load to the specimen 2, and a data acquisition step 20 of acquiring at least the data of the specimen 2 during the loading step 19.

[0029] In the contact step 16, the test specimen 2 is placed on the lower pressure body 5, and the upper pressure body 5 is lowered or the lower pressure body 5 is raised so that the upper surface of the test specimen 2 and the upper pressure body 5 come into contact. At this time, it is desirable to confirm that a load of about 5 to 10 N is applied to the load meter of the tensile test device 1, and to ensure that the upper pressure body 5 and the upper surface of the test specimen 2 are in contact. Note that before the contact step 16, zero adjustment of the load meter, monitoring of the load (data acquisition step 20), etc. are started.

[0030] After performing this contact step 16, a sleeve mounting step 17 is performed in which at least a part of the pressure body 5 and about 1 / 12 to 1 / 4 of the upper and lower ends of the test specimen 2 are covered with a soft sleeve 6. In the present embodiment, a cylindrical thin rubber material is used as the sleeve 6 so that the outer peripheral portion of the pressure body 5 and the vicinity of the end of the test specimen 2 can be covered.

[0031] Note that in order to perform the sleeve mounting step 17, before performing the contact step 16, preferably before connecting the pressure body 5 to the tensile test device main body, the sleeve 6 is mounted on the outer peripheral portions of the upper and lower pressure bodies 5, and the end of the sleeve 6 is wound toward the pressure body 5 side. When mounting the sleeve 6 on the pressure body 5, an O-ring or the like may be mounted and fixed near one end.

[0032] After performing the sleeve mounting step 17, an air intake step 18 is performed in which the gas (air) inside the sleeve is sucked. In the air intake step 18, a pair of air intake paths 13 are connected so as to penetrate the pressure body 5, and the vacuum pump 10 is operated to supply a vacuum pressure. At this time, the valve 14 in front of the pressure body 5 is kept closed.

[0033] The vacuum regulator 11 is operated with the valve 14 closed, the value of the vacuum gauge 12 is checked, and the vacuum pressure is adjusted. The vacuum pressure is preferably about -100 kPa. After adjusting the vacuum pressure in this way, the valve 14 of the air intake path 13 connected to the upper and lower pressure bodies 5 is opened, and the vacuum pressure is applied to the sleeves 6 on both end portions of the test specimen 2.

[0034] In Loading Step 19, a tensile load or a compressive load is applied to the specimen 2. Which load to apply can be changed according to the purpose of the test. In this embodiment, a tensile testing machine that can apply only a uniaxial (axial direction of the specimen 2) load is used.

[0035] In Data Acquisition Step 20, data such as stress and strain acting on the specimen 2 are acquired. Specifically, before performing Loading Step 19, a displacement gauge for vertical displacement is set and zero-adjusted, and monitoring is started by a PC. Then, recording of the vertical force and vertical displacement is started, and Loading Step 19 is started after the start of recording. In this embodiment, Loading Step 19 is terminated when the vertical stress reaches the residual state, and the vertical force and vertical displacement are recorded until the specimen 2 reaches a predetermined state.

[0036] An example of the results of tensile testing several specimens is shown in FIG. 4 as a graph of the relationship between strain and stress. In this tensile test, the specimen 2 was tensile fractured at a location not covered by the sleeve 6 while maintaining a state where the pressurizing body 5 and the end of the specimen 2 were in close contact. The tensile strength can be derived from the maximum value of this relationship graph and is represented by the value obtained by dividing the maximum tensile load by the cross-sectional area of the specimen.

[0037] [Different Modes for Carrying Out the Invention] Next, different modes for carrying out the present invention shown in FIGS. 5 and 6 will be described. In the description of these different modes for carrying out the present invention, the same reference numerals are given to the same components as those in the first mode for carrying out the present invention, and redundant descriptions are omitted.

[0038] In the second mode for carrying out the present invention shown in FIGS. 5 and 6, the main difference from the first mode for carrying out the present invention is that a tensile testing apparatus main body equipped with a triaxial pressure chamber 21 is used. Even in such a tensile testing apparatus 1A or a tensile testing method 15A in which Loading Step 19A for applying a load from three axes is performed using this tensile testing apparatus 1A, the same operational effects as those in the first mode for carrying out the present invention can be obtained, and data when a load is applied from three axes can also be acquired.

Explanation of Symbols

[0039] 1, 1A: Tensile test device, 2: Specimen 3: Holding means, 4: Contact surface 5: Pressing 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 mounting process, 18: Intake process 19, 19A: Loading process, 20: Data acquisition process 21: Triaxial pressure chamber

Claims

1. A tensile testing apparatus comprising a pair of upper and lower holding means for holding the vicinity of the upper and lower ends of a three-dimensional specimen, and capable of applying a tensile load or a compressive load to the specimen held by the holding means, The pair of upper and lower holding means have abutment surfaces that abut the upper and lower surfaces of the specimen, respectively, and are attached to the tensile testing apparatus main body. The tensile testing apparatus is composed of a pair of upper and lower pressurizing bodies, a pair of soft upper and lower sleeves that cover the upper and lower outer peripheral ends of the specimen, each extending from the upper and lower end faces to approximately 1 / 12 to 1 / 4 of the total length, when the abutment surfaces are in contact with the specimen, and one or more suction devices that suck in gas remaining inside the pair of upper and lower sleeves.

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

3. A tensile testing method comprising the steps of: a contact process in which the upper and lower pressurizing bodies of a tensile testing device are brought into contact with the upper and lower surfaces of a three-dimensional specimen, respectively; a sleeve attachment process 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 a pair of soft upper and lower sleeves; an intake process in which gas remaining inside the pair of upper and lower sleeves is sucked in; a loading process in which a load is applied to the specimen; and a data acquisition process in which data on the specimen during the loading process is acquired.

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

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