Round thin plate test piece bending fatigue test device and test method

Abstract

The invention discloses a device and a method for testing the bending fatigue performance of a circular thin plate test piece. A device body is composed of a pressing head 2, a support 7 and a bottom plate 10. The testing method comprises the following steps: firstly, mounting the pressure head 2 and the bottom plate 10 on a loading rod and a load sensor of a material testing machine through a thread 1 and a thread 11 respectively, and embedding the support 7 into the positioning groove 9; a strain gauge is adhered to the center of the bottom of the test piece 4 and is horizontally placed on the top plate 5 of the support 7; the testing machine is controlled to repeatedly load the middle of the test piece 4 through the pressure head 2, the load and strain values in the loading process are collected until the test piece 4 is broken, and the loading frequency at the moment is the fatigue life of the test piece. The method has the beneficial effects that compared with a one-way tension state of the bottom of a test piece in a traditional fatigue test, the bottom of the test piece in the method is in a radial tension state which is consistent with a stress state of an actual pavement structure under the action of a wheel load, and the performance of a material in the actual pavement structure can be reflected more truly.

Description

Technical Field

[0001] The invention relates to the field of road engineering material performance testing and evaluation, and in particular to a circular thin plate specimen bending fatigue testing device and a testing method. Background Art

[0002] The crack resistance of pavement materials is an important parameter for pavement structure design, material design and performance evaluation. Among them, fatigue cracking is one of the most important forms of pavement failure. Two-point, three-point or four-point bending fatigue tests of beam specimens, splitting fatigue tests of cylindrical specimens, and bending fatigue tests of semicircular specimens are usually used at home and abroad to test the bending fatigue life of pavement materials. In these test methods, the failure zone of the specimen is only subjected to unidirectional tensile stress, and no force is applied in the thickness direction of the specimen. This stress state is completely different from the stress distribution state of bidirectional tension on the bottom horizontal plane of each structural layer of the pavement structure under the action of vehicle load. Since the pavement material is a non-uniform, anisotropic material composed of aggregate and binder, its mechanical properties are closely related to the stress state. Therefore, the fatigue life measured by the usual fatigue test method cannot truly reflect the true fatigue resistance of the pavement material in the actual pavement structure. According to the deformation mode and stress distribution state of the pavement structure under the action of tire load, the present invention proposes to use a bending fatigue test of a circular thin plate specimen to test the bending fatigue resistance of the pavement material, and invents a corresponding test device and test method, so that the measured fatigue life can more accurately characterize the fatigue resistance of the material in the actual pavement structure. Summary of the invention

[0003] The present invention provides a bending fatigue test device and a test method for pavement structure materials based on a circular thin plate specimen, which solves the problem that in the commonly used fatigue test method, the specimen is subjected to unidirectional force, which is inconsistent with the force mode and stress state of the actual pavement structure under the action of wheel load, thereby resulting in the test results being unable to reflect the actual performance of the material in the actual pavement structure.

[0004] The technical solution adopted in the present invention is: a bending test is performed on a circular thin plate specimen with a simply supported bottom periphery under a lateral load in a central local area to simulate the deformation mode and stress of the pavement structure layer material under wheel load (the asphalt pavement structure design theory regards the pavement structure layer as a structure with infinite size in the horizontal direction, and solves it as an axisymmetric problem. The stress mode of the circular thin plate specimen in the present invention under the central local load is also an axisymmetric problem).

[0005] The main body of the complete test device of the present invention is composed of a pressure head 2, a support 7 and a bottom plate 10; the thread 1 on the top of the pressure head 2 is used to connect the loading rod of the testing machine, and the rubber pad 3 at the bottom can evenly distribute the applied load in the middle area of ​​the specimen 4; the annular top plate 5 and baffle 6 on the upper part of the support 7 are used to support and restrain the specimen 4, and the circular threading hole 8 at the bottom is used to lead out the wires of the sensor adhered to the bottom of the specimen 4; the thread 11 at the lower part of the bottom plate 10 is used to connect the load sensor of the testing machine, and the annular positioning groove 9 thereon can accurately position the support 7 in the center of the bottom plate 10, so that the pressure head 2 always acts vertically on the center of the specimen 4.

[0006] The present invention provides a method for testing the bending fatigue performance of a circular thin plate specimen, comprising the following steps:

[0007] Step 1: forming a circular thin plate specimen 4, and measuring its diameter a and thickness h;

[0008] Step 2: Assemble the test device: The bottom plate 10 is mounted on the load sensor of the material testing machine through the thread 11, the pressure head 2 is connected to the loading rod of the testing machine through the thread 1, and the support 7 is embedded in the positioning groove 9 on the upper surface of the bottom plate 10;

[0009] Step 3: Install the specimen: Paste the strain gauge at the bottom center of the specimen 4, and then place it horizontally on the top plate 5. The wire of the strain gauge is led out through the threading hole 8 on the support 7 and connected to the data acquisition system of the testing machine;

[0010] Step 4: Perform fatigue loading test: start the material testing machine, apply vertical load to the middle of the specimen 4 cyclically through the indenter 2 according to the predetermined fatigue load size, load waveform and loading frequency, and collect the load and strain values ​​during the loading process until the specimen 4 is damaged;

[0011] Step 5: Test result analysis: The number of load cycles applied when specimen 4 is destroyed is its fatigue life; the fatigue stress value at the bottom of specimen 4 can be calculated according to the radius a and thickness h of specimen 4, the radius b at the bottom of indenter 2, the Poisson's ratio μ of the material, and the peak value Q of the applied load, according to the formula calculate. BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Figure 1 is a three-dimensional diagram of a testing device in an embodiment of the present invention;

[0013] Figure 2 A three-dimensional diagram of a pressing head in an embodiment of the present invention;

[0014] Figure 3 is a front view of a pressure head in an embodiment of the present invention;

[0015] Figure 4 A three-dimensional diagram of a cylinder in an embodiment of the present invention;

[0016] Figure 5 It is a front view of a cylinder in an embodiment of the present invention;

[0017] Figure 6 A three-dimensional diagram of a bottom plate in an embodiment of the present invention;

[0018] Figure 7 is a front view of a bottom plate in an embodiment of the present invention;

[0019] Figure 8 This is a schematic diagram of the installation of sensors on a test piece in an embodiment of the present invention;

[0020] In the figure: thread 1, pressure head 2, rubber pad 3, test piece 4, top plate 5, baffle 6, support 7, threading hole 8, positioning groove 9, bottom plate 10, thread 11. DETAILED DESCRIPTION

[0021] The following will be combined with the accompanying drawings of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0022] The dimensions of the main components of the whole set of devices made in this embodiment are as follows: the pressure head 2 is a steel rod with a radius of 10 mm and a length of 300 mm. A rubber pad 3 with a thickness of 5 mm is adhered to its lower end. The radius of the rubber pad is the same as that of the pressure head 2. Its function is to make the load applied by the pressure head 2 evenly distributed in the central area of ​​the upper surface of the specimen; the upper part of the pressure head 2 is connected to the loading rod of the testing machine by a thread 1; the support 7 is a steel cylinder with an inner diameter of 140 mm, an outer diameter of 150 mm, and a wall thickness of 5 mm, which serves as a top plate 5 supporting the specimen 4. A baffle 6 with an outer diameter of 157 mm and an inner diameter of 150 mm is arranged on the outside of the top plate. 2mm, and 5mm higher than the top plate, used to constrain the test piece 4 so that it will not change its position during the loading process; a circular opening with a diameter of 2cm is set at the lower part of the support 7 as a threading hole 8, used to pass the wire of the sensor 12 pasted on the test piece 4; the bottom plate 10 has a diameter of 160mm and a thickness of 40mm, used to support and constrain the support 7, and its upper surface has a 5.5mm wide and 5mm deep circular positioning groove 9, which is used to embed the support 7 therein, so as to position and constrain the support 7; the bottom of the bottom plate 10 is connected to the load sensor of the testing machine by using a screw 11 with a diameter of 30mm;

[0023] In this embodiment, cement-stabilized crushed stone mixture is used to make circular thin plate specimens 4 for bending fatigue test. The detailed process is as follows: according to the designed gradation, cement dosage and water dosage, after being mixed evenly, a cylindrical specimen with a diameter of 150mm and a height of 150mm is formed by compaction method; after curing for 28 days, 10mm of the upper and lower surfaces are sawed off, and the remaining middle section is sawed into circular thin plate specimens with a thickness of about 25mm to 35mm for standby use;

[0024] This embodiment carries out a thin plate bending fatigue test at room temperature on a cement stabilized crushed stone mixture specimen, and the test process is as follows:

[0025] Step 1: Take a prepared circular thin plate specimen and measure its thickness with a vernier caliper. The thickness can be measured in four cross-symmetrical directions. The average of the four thickness measurement values ​​of each specimen is taken as the thickness h of the specimen.

[0026] Step 2: Assemble the test device. First, install the base plate 10 on the load sensor of the universal testing machine MTS through the thread 11, connect the pressure head 2 to the loading rod of the MTS testing machine through the thread 1, and embed the support 7 into the positioning groove 9 on the surface of the base plate 10;

[0027] Step 3: Install the specimen. Paste the strain gauge at the bottom center of the circular thin plate specimen 4, and then place it horizontally on the top plate 5 of the support 7. The wire of the strain gauge is led out through the threading hole 8 on the support 7 and connected to the data acquisition system of the testing machine;

[0028] Step 4: Perform fatigue loading test. Start the MTS material testing machine and input the test loading parameters: According to the tensile stress σ=1.320MPa at the bottom of the control board, according to the formula The peak load F of the test was calculated to be 714.774N. To prevent the indenter of the testing machine from detaching from the upper surface of the specimen 4 when the unloading is 0, the minimum load of fatigue loading is set to 5N. The load waveform is a half-sine wave, and the loading frequency f is 10Hz. The fatigue test is performed by applying a load to the middle part of the specimen 4 through the indenter 2. The load and strain values ​​during the loading process are collected. When the test is loaded to 362831 cycles, the specimen 4 is damaged and the test is stopped.

[0029] Using the above test method, bending fatigue performance tests were applied to different specimens under three different peak loads, and the corresponding fatigue life was obtained, as shown in the table below.

[0030]

[0031] Based on the test results in the above table, we can analyze how the fatigue life of the material changes with the stress at the bottom of the slab, and establish a fatigue equation for use in pavement structure design.

[0032] Note: The above description is only a preferred embodiment of the present invention and is not intended to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention are included in the protection scope of the present invention.

Claims

1. A circular thin plate specimen bending fatigue performance test device and test method, characterized in that: The main body of the test device is composed of a pressure head 2, a support 7 and a base plate 10; the corresponding bending fatigue test method is to assemble the test device and connect it to the loading system of the testing machine, then install the specimen, and repeatedly apply vertical load to the specimen according to the set stress level and loading frequency until the specimen breaks. At this time, the number of repeated loading is the fatigue life of the plate specimen under the loading condition.

2. A circular thin plate specimen bending fatigue performance testing device according to claim 1, characterized in that: The thread 1 on the top of the pressure head 2 is used to connect to the loading rod of the testing machine, and the rubber pad 3 at the bottom can evenly distribute the applied load in the middle area of ​​the test piece 4.

3. The device for testing the bending fatigue performance of a circular thin plate specimen according to claim 1, characterized in that: The annular top plate 5 and baffle 6 on the upper part of the support 7 are used to support and constrain the test piece 4, and the circular threading hole 8 at the bottom is used to lead out the wires of the sensor adhered to the bottom of the test piece 4.

4. The device for testing the bending fatigue performance of a circular thin plate specimen according to claim 1, characterized in that: The thread 11 at the bottom of the bottom plate 10 is used to connect the load sensor of the testing machine. The annular positioning groove 9 thereon can accurately position the support 7 at the center of the bottom plate 10, so that the pressure head 2 always acts vertically on the center of the test piece 4.

5. A circular thin plate specimen bending fatigue test method according to claim 1, characterized in that: ① Assemble the test device: install the bottom plate 10 on the load sensor of the material testing machine through the thread 11, connect the pressure head 2 to the loading rod of the testing machine through the thread 1, and embed the support 7 into the positioning groove 9 on the upper surface of the bottom plate 10; ② Install the test piece: stick the strain gauge at the bottom center of the test piece 4, and then place it horizontally on the top plate 5, and lead the wire of the strain gauge through the threading hole 8 on the support 7; ③ Conduct fatigue loading test: Start the material testing machine, and apply a vertical load to the middle part of the test piece 4 through the pressure head 2 according to the predetermined fatigue load size, load waveform and loading frequency, and collect the load and strain values ​​during the loading process until the test piece 4 is destroyed. ④ Analysis of test results: The number of cycles of the load applied when the test piece 4 is destroyed is its fatigue life; the fatigue stress value at the bottom of the test piece 4 can be calculated according to the radius a and thickness h of the test piece 4, the radius b of the bottom of the pressure head 2, the Poisson's ratio μ of the material, and the peak value Q of the applied load, according to the formula calculate.

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