Axial controlled cycle fatigue test method for metal tubes

Abstract

The present invention belongs to the technical field of metal pipe performance testing, and specifically discloses a method for axial controlled cyclic fatigue testing of metal pipes. It is used to solve the problem that the coaxiality requirement of the specimen in the current axial controlled cyclic fatigue test is difficult to ensure. It includes the following steps: sampling from the metal pipe longitudinally to obtain a test section with a curved surface, and then processing the test section into a curved rectangular specimen for testing; taking two curved rectangular specimens of the same metal pipe to form a specimen group for paired testing, and the two curved rectangular specimens of the same specimen group are clamped back-to-back symmetrically or clamped oppositely symmetrically; applying an alternating load to the specimen group according to the conditions required by the fatigue test until one of the curved rectangular specimens in the specimen group fails, recording the number of cycles of the curved rectangular specimen, and discarding the other specimen in the specimen group. The present invention ensures that the geometric axis of the specimen group is coaxial with the loading direction of the testing machine, so that the load is evenly distributed on the specimen.

Description

Technical Field

[0001] The invention belongs to the technical field of metal pipe performance testing, and in particular relates to an axial controlled cycle fatigue test method for metal pipes. Background Art

[0002] At present, axial controlled cyclic fatigue test is a difficult point in controlling the mechanical properties of aluminum alloy pipes, mainly including the following four aspects: First, the specimen design is relatively difficult. Fatigue tests must emphasize the coaxiality of the loading load and the geometric center of the specimen. The cross-section of the working part of the commonly used fatigue specimen is circular or rectangular. When loading, the load center and the geometric center of the working section of the specimen are easy to coincide, and it is relatively easy to achieve the coaxiality problem required by the standard, but it is very difficult for pipes to achieve this. Second, since the symmetry center of the pipe is on the central axis of the specimen, only full-section loading can meet the coaxiality requirements of the specimen center and the loading load center. In fact, due to the deformation of the cross-section and shape of the pipe itself during the clamping process of the testing machine, the coaxiality requirements of the specimen are extremely difficult to guarantee. Third, since it must be a full-section specimen test, if the specimen size is large, the actual load can easily exceed the load limit of the testing machine, so there are difficulties that are difficult to solve when using full-section pipe specimens as fatigue specimens. Fourth, it is not feasible to use only a part of the pipe as a test specimen, because the pipe is cut into an arc-section test specimen. Since the cross-section of the test specimen is an arc, the single test specimen itself does not have a rotational central symmetry axis and cannot meet the fatigue test standard's requirements for the coaxiality of the loaded test specimen. Summary of the invention

[0003] The purpose of the present invention is to provide a method for axial controlled cyclic fatigue testing of metal pipes, which effectively solves the problem that the coaxiality requirement of the specimen in the current axial controlled cyclic fatigue test is difficult to ensure.

[0004] In order to solve the above technical problems, the technical solution adopted by the present invention is: a method for axial controlled cyclic fatigue testing of metal pipes, comprising the following steps: S1, sampling from the metal pipe longitudinally to obtain a test section with a curved surface, and then processing the test section into a curved rectangular specimen for testing in accordance with the rectangular specimen design principles and recommended illustrations in HB5287, wherein the clamping section of the curved rectangular specimen is not flattened.

[0005] S2. Take two curved rectangular specimens of the same metal pipe to form a specimen group for paired testing. The two curved rectangular specimens of the same specimen group are clamped symmetrically back to back or symmetrically opposite to each other.

[0006] S3. Apply an alternating load to the specimen group according to the conditions required by the fatigue test, with the loading axis being coaxial with the geometric axis of the specimen group, until one of the arc-surface rectangular specimens in the specimen group fails, and record the number of cycles of the arc-surface rectangular specimen to obtain the fatigue life of the metal pipe under the loading stress, and discard the other arc-surface rectangular specimen in the specimen group.

[0007] S4. Organize the test according to the test purpose. If it is high-cycle fatigue, refer to GB / T 3075 or GB / T 37616 for the test; if it is low-cycle fatigue, refer to GB / T 15248 for the test.

[0008] Furthermore, in step S1, the method for obtaining the test section is: first, a first longitudinal cutting is performed along a first cutting line parallel to a transverse symmetry line of the cross section of the metal pipe.

[0009] Secondly, a second cutting is performed along a second cutting line formed by connecting the center of the cross section of the metal pipe and an intersection point of the inner arc surface of the metal pipe and the first cutting line.

[0010] Finally, a third cutting is performed along a third cutting line formed by connecting the center of the cross section of the metal pipe and another intersection point of the inner arc surface of the metal pipe and the first cutting line.

[0011] Furthermore, in step S1, the arc-surface rectangular specimen includes a clamping section located at the left and right ends and a working section located between the two clamping sections. The cross-sectional area of ​​the working section is smaller than the cross-sectional area of ​​each clamping section. A transition arc is used to smoothly transition between the working section and the clamping section. All corners of the arc-surface rectangular specimen are chamfered.

[0012] Further, in step S2, the clamping structure for symmetrically clamping the two curved rectangular specimens in back-to-back orientation is as follows: the back-to-back curved surfaces of the two curved rectangular specimens are opposite to each other, a first clamping section gasket is placed at the clamping section position between the back-to-back curved surfaces of the two curved rectangular specimens, and a first working section gasket is placed at the working section position between the back-to-back curved surfaces of the two curved rectangular specimens, and both the first clamping section gasket and the first working section gasket are in contact with the back-to-back curved surfaces of the two curved rectangular specimens at corresponding positions.

[0013] The first clamping fixture is installed at the clamping section position on the inner side of the arc surface of the two arc-surface rectangular specimens. The side of the first clamping fixture that contacts the arc-surface rectangular specimen is an arc surface. The arc surface of the first clamping fixture fits the inner side of the arc surface of the arc-surface rectangular specimen, and the side of the first clamping fixture opposite to its arc surface is a plane.

[0014] Furthermore, in step S2, the clamping structure for symmetrically clamping the two arc-surface rectangular specimens is as follows: the arc-surface inner sides of the two arc-surface rectangular specimens are opposite to each other, a second clamping section gasket is placed at the clamping section position between the arc-surface inner sides of the two arc-surface rectangular specimens, and a second working section gasket is placed at the working section position between the arc-surface inner sides of the two arc-surface rectangular specimens, and both the second clamping section gasket and the second working section gasket are in contact with the arc-surface inner sides of the two arc-surface rectangular specimens at corresponding positions.

[0015] A second clamping fixture is installed at the clamping section position of the two curved rectangular specimens facing away from the curved surface. The side of the second clamping fixture that contacts the curved rectangular specimen is a curved surface. The curved surface of the second clamping fixture fits the curved surface facing away from the curved rectangular specimen, and the side of the second clamping fixture opposite to its curved surface is a flat surface.

[0016] Furthermore, the two clamping sections of the arc-surface rectangular specimen are both provided with the same unique identification.

[0017] Compared with the prior art, the beneficial technical effects of the present invention are as follows: (1) The present invention obtains a test section with a curved surface by sampling from the longitudinal direction of the pipe, and then processes the test section into a curved rectangular specimen for testing in accordance with the rectangular specimen design principles and recommended illustrations in HB5287. The clamping section of the curved rectangular specimen is not flattened and the cross-sectional area of ​​the working section is reduced, which effectively solves the problem that the loading load exceeds the load limit of the testing machine due to the excessive size of the pipe and solves the problem that the test is invalid due to the flattening of the clamping section.

[0018] (2) The present invention adopts paired testing to clamp two curved rectangular specimens symmetrically back to back or symmetrically towards each other to form a specimen group, which is beneficial to ensure that the geometric axis of the specimen group is coaxial with the loading direction of the testing machine, so that the load is evenly distributed on the curved rectangular specimen, which is beneficial to truly reflect the fatigue performance of the material and solve the problem that the axis of the rectangular fatigue specimen of the pipe is not coaxial with the axial load; in addition, since the curved rectangular specimen is sampled and processed from the pipe, the large-section pipe can be processed into a small specimen, which makes it easier to meet the load requirements of the testing machine and solves the problem that the loading load exceeds the load limit of the testing machine due to the excessive size of the pipe. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 This is a schematic diagram of the cutting method of the test section.

[0020] Figure 2 It is a schematic diagram of the structure of a curved rectangular specimen.

[0021] Figure 3 It is an exploded diagram of the clamping structure for back-symmetrical clamping of curved rectangular specimens.

[0022] Figure 4It is a schematic diagram of the clamping structure for back-to-back symmetrical clamping of curved rectangular specimens, where the direction indicated by the arrow is the direction of the loading axis.

[0023] Figure 5 It is an exploded diagram of the clamping structure for symmetrically clamping curved rectangular specimens.

[0024] Figure 6 It is a schematic diagram of the clamping structure for symmetrically clamping curved rectangular specimens.

[0025] Figure 7 is a SN curve diagram of Example 1, wherein black rectangular symbols represent test data points.

[0026] Explanation of the accompanying drawings: first cutting line-1; second cutting line-2; third cutting line-3; arc-surface rectangular specimen-4; clamping section-5; working section-6; transition arc-7; first clamping section gasket-8; first working section gasket-9; first clamping tool-10; second clamping section gasket-11; second working section gasket-12; second clamping tool-13. DETAILED DESCRIPTION

[0027] Example 1: The test objective of this example is to draw a longitudinal SN curve of a 7050-T7451 aluminum alloy pipe with a wall thickness of 3 mm. The SN curve is a curve obtained by fitting the median fatigue life at different stress levels and fitting the median fatigue strength at each specified life, that is, a stress-life relationship curve.

[0028] The test steps include: (1) In order to avoid the problem that the loading load exceeds the load limit of the testing machine due to the large size of the pipe, a longitudinal sample is taken from the 7050-T7451 aluminum alloy pipe to obtain a test section with a curved surface.

[0029] The method for obtaining the test section is as follows: Figure 1 As shown, first, a first longitudinal cut is made along a first cutting line 1 which is parallel to the transverse symmetry line of the cross section of the aluminum alloy tube, and the specific position of the first cutting line 1 is determined according to the processing size of the arc-surface rectangular specimen 4, i.e., the arc width, and the processing size of the arc-surface rectangular specimen 4 refers to the rectangular specimen design principle of HB5287; secondly, a second cut is made along a second cutting line 2 which is formed by connecting the center O of the cross section of the aluminum alloy tube and the intersection B of the inner arc surface of the aluminum alloy tube with the first cutting line 1; finally, a third cut is made along a third cutting line 3 which is formed by connecting the center O of the cross section of the aluminum alloy tube and the inner arc surface of the aluminum alloy tube with another intersection C of the first cutting line 1.

[0030] Then the test section was processed into a curved rectangular specimen 4 according to the rectangular specimen design principles and recommended illustrations in HB5287 for testing. Figure 2As shown, the arc-surface rectangular specimen 4 includes a clamping section 5 located at the left and right ends and a working section 6 located between the two clamping sections 5. A transition arc 7 is used to smoothly transition between the working section 6 and the clamping section 5. All corners of the arc-surface rectangular specimen 4 should be rounded.

[0031] In order to solve the problem that the clamping section 5 is flattened and the test is invalid, the clamping section 5 of the arc-surface rectangular specimen 4 is not flattened. The cross-sectional area of ​​the working section 6 is reduced and the cross-sectional area of ​​the working section 6 is smaller than the cross-sectional area of ​​each clamping section 5, so that under the same load, the stress on the working section 6 is greater than the stress on the clamping section 5.

[0032] In this embodiment, the arc thickness of the arc rectangular sample 4 is 3 mm, the width of the clamping section 5 is 22 mm, the width of the working section 6 is 12 mm, the length of the working section 6 is 25 mm, and the radius of the transition arc 7 is 76 mm.

[0033] (2) 30 groups of test specimens are prepared, each group of test specimens has two curved rectangular test specimens 4, and the two curved rectangular test specimens 4 in the same test specimen group are clamped symmetrically in the opposite direction (e.g. Figure 4 as shown) or symmetrically clamped (as shown Figure 6 as shown).

[0034] Since the two curved rectangular specimens 4 are symmetrically clamped during each test, the entire structure has a rotationally symmetrical center line, and the symmetrical clamping structure can ensure that the loading axis is coaxial with the geometric axis of the specimen group, thereby solving the problem that the axis of the rectangular fatigue specimen of the pipe is not coaxial with the axial load. In addition, since the specimen is sampled and processed from the tubular specimen, the large-section tubular specimen can be processed into a small specimen, thereby making it easier to meet the load requirements of the testing machine.

[0035] In this embodiment, 60 arc-surface rectangular specimens 4 are taken to form 30 specimen groups, of which 15 groups are used for testing fatigue limit by lifting method, and the other 15 groups are used for testing fatigue life in the short-life zone by group method. The two clamping sections 5 of each arc-surface rectangular specimen 4 are provided with the same unique identification, such as LH-1; the identifications of the two arc-surface rectangular specimens 4 of each specimen group should be distinguished, such as LH-1A and LH-1B respectively.

[0036] like Figure 3As shown, the clamping structure for the back-to-back symmetrical clamping of the two curved rectangular specimens 4 is as follows: the outer back-to-back curved surfaces of the two curved rectangular specimens 4 are opposite to each other, a first clamping section gasket 8 is placed at the clamping section 5 between the back-to-back curved surfaces of the two curved rectangular specimens 4, and a first working section gasket 9 is placed at the working section 6 between the back-to-back curved surfaces of the two curved rectangular specimens 4. The first clamping section gasket 8 and the first working section gasket 9 are both fitted with the back-to-back curved surfaces of the two curved rectangular specimens 4 at the corresponding positions; a first clamping fixture 10 is installed at the clamping section 5 on the inner side of the curved surface of the two curved rectangular specimens 4, a side of the first clamping fixture 10 in contact with the curved rectangular specimen 4 is a curved surface, the curved surface of the first clamping fixture 10 is fitted with the inner side of the curved surface of the curved rectangular specimen 4, and a side of the first clamping fixture 10 opposite to its curved surface is a plane. The structure after installation is as shown in FIG. Figure 4 As shown, the direction of the loading axis is Figure 4 In the direction indicated by the arrow.

[0037] like Figure 5 As shown, the clamping structure for symmetrically clamping two curved rectangular specimens 4 is as follows: the curved inner sides of the two curved rectangular specimens 4 face each other, a second clamping section gasket 11 is placed at the clamping section 5 between the curved inner sides of the two curved rectangular specimens 4, and a second working section gasket 12 is placed at the working section 6 between the curved inner sides of the two curved rectangular specimens 4. Both the second clamping section gasket 11 and the second working section gasket 12 fit with the curved inner sides of the two curved rectangular specimens 4 at corresponding positions; a second clamping fixture 13 is installed at the clamping section 5 facing away from the curved surface of the two curved rectangular specimens 4, a side of the second clamping fixture 13 in contact with the curved rectangular specimen 4 is a curved surface, the curved surface of the second clamping fixture 13 fits with the curved surface facing away from the curved rectangular specimen 4, and a side of the second clamping fixture 13 opposite to its curved surface is a plane. The structure after installation is as shown in FIG. Figure 6 shown.

[0038] There is a deviation between the geometric center line of the single-piece arc-surface rectangular specimen 4 and the loading axis. When loading in this case, the load distribution on the arc-surface rectangular specimen 4 will be obviously uneven. The farther the distance from the loading axis is, the lower the load level will be, and a stress gradient will be generated on the surface of the entire arc-surface rectangular specimen 4. The failure time of the position with high stress is earlier than the actual failure time, and the failure time of the position with low stress is delayed, which cannot truly reflect the fatigue performance of the material. In this embodiment, two arc-surface rectangular specimens 4 are tested in pairs. The geometric axis of the specimen group composed of the two arc-surface rectangular specimens 4 is coaxial with the loading axis, so that the stress distribution on the working surface is uniform, which can truly reflect the fatigue performance of the material. It is easier to complete the fatigue performance test by clamping the two arc-surface rectangular specimens 4 symmetrically in back to back than by clamping them symmetrically in opposite directions. In this embodiment, Figure 3The clamping structure shown in the figure clamps two curved rectangular specimens 4 symmetrically in a back-to-back manner. The structure after installation is as follows: Figure 4 As shown, the direction of the loading axis is Figure 4 In the direction indicated by the arrow.

[0039] (3) Apply an alternating load to the specimen group according to the conditions required by the fatigue test, with the loading axis being coaxial with the geometric axis of the specimen group, until one of the arc-surface rectangular specimens 4 in the specimen group fails, and record the number of cycles of the arc-surface rectangular specimen 4, that is, the fatigue life of the aluminum alloy pipe in the specimen group under the loading stress is obtained. The other arc-surface rectangular specimen 4 in the specimen group is used as an auxiliary specimen and should be discarded regardless of whether it fails or not.

[0040] (4) Organize the test according to the test purpose. If it is high cycle fatigue, refer to GB / T 3075 or GB / T 37616 for the test; if it is low cycle fatigue, refer to GB / T 15248 for the test.

[0041] In this embodiment, in order to obtain the SN curve, fatigue performance needs to be tested in the low-medium life region and the high-life region respectively. The group method is used to test the cycle number of the low-medium life stage, and the lifting method is used to test the cycle number of the high-life stage. ,frequency .

[0042] The lifting method was used to test the cycle number when the arc-surface rectangular specimen 4 in the fatigue limit zone broke. The initial stress level was 280MPa. If any arc-surface rectangular specimen 4 in a specimen group broke, the test of this group ended, and 12 groups of valid data were obtained. The median fatigue strength was calculated to be 266.7MPa, which is listed in Table 1.

[0043] Table 1 Data processing results of lifting method test .

[0044] Table 2 Data processing results of group method test .

[0045] The group method was used to test the cycle number of the arc-surface rectangular specimen 4 in the medium and short life zone when it broke. The stress levels were selected as 340MPa, 360MPa and 380MPa respectively. If any arc-surface rectangular specimen 4 in a specimen group broke, the test of this group ended, and 12 groups of valid data were obtained. The median fatigue life, standard deviation and coefficient of variation of the three stress levels were calculated by data processing and listed in Table 2. By looking up the coefficient of variation table, the appendix of GB / T 37616 shows that the coefficient of variation of the four specimens should not be greater than 0.0314, and it is determined that the number of data groups for each stress level meets the test requirements.

[0046] Plotting the effective test data of the lifting method and the grouping method on the same cycle number-stress level curve will give the SN curve required by the test objective, such as Figure 7 shown.

[0047] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for axial controlled cyclic fatigue testing of metal pipes, characterized in that: The following steps are involved: S1. Take samples longitudinally from the metal pipe to obtain a test section with a curved surface, and then process the test section into a curved rectangular specimen for testing according to the rectangular specimen design principles and recommended illustrations in HB5287. The clamping section of the curved rectangular specimen is not flattened; S2. Take two curved rectangular specimens of the same metal pipe to form a specimen group for paired testing. The two curved rectangular specimens of the same specimen group are clamped symmetrically in back-to-back direction or symmetrically in opposite directions. S3. Apply an alternating load to the sample group according to the conditions required by the fatigue test, with the loading axis being coaxial with the geometric axis of the sample group, until one of the arc-surface rectangular samples in the sample group fails, and record the number of cycles of the arc-surface rectangular sample, that is, obtain the fatigue life of the metal pipe under the loading stress, and discard the other arc-surface rectangular sample in the sample group; S4. Organize the test according to the test purpose. For high cycle fatigue, refer to GB / T 3075 or GB / T 37616 for testing; for low cycle fatigue, refer to GB / T 15248 for testing; In step S1, the arc-surface rectangular specimen includes a clamping section located at the left and right ends and a working section located between the two clamping sections, the cross-sectional area of ​​the working section is smaller than the cross-sectional area of ​​each clamping section, a transition arc is used to smoothly transition between the working section and the clamping section, and each corner of the arc-surface rectangular specimen is chamfered; In step S2, the clamping structure for symmetrically clamping two curved rectangular specimens in a back-to-back manner is as follows: the back-to-back curved surfaces of the two curved rectangular specimens are opposite to each other, a first clamping section gasket is placed at the clamping section position between the back-to-back curved surfaces of the two curved rectangular specimens, and a first working section gasket is placed at the working section position between the back-to-back curved surfaces of the two curved rectangular specimens, and both the first clamping section gasket and the first working section gasket are in contact with the back-to-back curved surfaces of the two curved rectangular specimens at corresponding positions; The first clamping fixture is installed at the clamping section position on the inner side of the arc surface of the two arc-surface rectangular specimens. The side of the first clamping fixture that contacts the arc-surface rectangular specimen is an arc surface. The arc surface of the first clamping fixture fits the inner side of the arc surface of the arc-surface rectangular specimen, and the side of the first clamping fixture opposite to its arc surface is a plane.

2. The metal tube axial controlled cycle fatigue test method according to claim 1, characterized in that: In step S1, the method for obtaining the test section is as follows: first, a first longitudinal cutting is performed along a first cutting line parallel to a transverse symmetry line of a cross section of the metal pipe; Secondly, a second cutting is performed along a second cutting line formed by connecting the center of the cross section of the metal pipe and an intersection point of the inner arc surface of the metal pipe and the first cutting line; Finally, a third cutting is performed along a third cutting line formed by connecting the center of the cross section of the metal pipe and another intersection point of the inner arc surface of the metal pipe and the first cutting line.

3. The metal tube axial controlled cycle fatigue test method according to claim 2, characterized in that: In step S2, the clamping structure for symmetrically clamping the two curved rectangular specimens is as follows: the curved inner sides of the two curved rectangular specimens are opposite to each other, a second clamping section gasket is placed at the clamping section position between the curved inner sides of the two curved rectangular specimens, and a second working section gasket is placed at the working section position between the curved inner sides of the two curved rectangular specimens, and both the second clamping section gasket and the second working section gasket are in contact with the curved inner sides of the two curved rectangular specimens at corresponding positions; A second clamping fixture is installed at the clamping section position of the two curved rectangular specimens facing away from the curved surface. The side of the second clamping fixture that contacts the curved rectangular specimen is a curved surface. The curved surface of the second clamping fixture fits the curved surface facing away from the curved rectangular specimen, and the side of the second clamping fixture opposite to its curved surface is a flat surface.

4. The metal tube axial controlled cycle fatigue test method according to claim 3, characterized in that: The same unique identification is provided on both clamping sections of the curved rectangular specimen.

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