Variable cross-section compression creep sample and processing method thereof

Abstract

The invention discloses a variable-cross-section compression creep sample and a processing method, and belongs to the technical field of sample manufacturing of compression creep tests.The sample comprises a uniform-cross-section working section and a variable-cross-section transition section, the uniform-cross-section working section is a cylinder, and the variable-cross-section transition section is a variable-diameter rotary body with the axis of the sample as the rotation center; the diameter of the variable-diameter rotary body is gradually increased from the end close to the uniform-section working section to the contact end surface; the side contour line of the variable-diameter rotary body is a part of an elliptic arc of an ellipse, the long axis a of the ellipse is vertical or basically vertical to the axis of the sample, and the short axis b of the ellipse is parallel or basically parallel to the axis of the sample; and the ratio range of the long axis a to the short axis b of the ellipse is 1.5-2.5. The variable cross-section compression creep test sample can simultaneously solve the problems of uneven deformation and test bed damage.

Description

Technical Field

[0001] This application belongs to the field of specimen manufacturing technology for compression creep testing, and specifically relates to a variable cross-section compression creep specimen and its processing method. Background Technology

[0002] High-temperature compression creep testing is a key method for evaluating the long-term load-bearing capacity of materials under high-temperature environments. It is widely used in the performance characterization of key structural materials such as high-temperature alloys and heat-resistant steels in aerospace, energy, and petrochemical industries. Currently, the compression creep specimens commonly used in the industry (such as GB / T44030-2024 "Metallic Materials - Compression Test at Room Temperature") have a uniform cross-section structure, typically cylindrical or square prism-shaped. These specimens present two major problems during testing: First, uneven deformation leads to distorted test data. During compression loading, there is an unavoidable frictional constraint between the two end faces of the specimen and the compression table of the testing machine. This constraint hinders the lateral deformation of the specimen end faces. For specimens with uniform cross-sections, since their cross-sectional dimensions are consistent along the axial direction, the frictional constraint is transmitted along the axial direction of the specimen. This results in the working section of the specimen not producing uniform axial compression deformation, but rather exhibiting a "barrel-shaped distortion" where the middle region bulges outward. This distortion causes the stress distribution in the working section of the specimen to be non-uniform, meaning the stress in the middle region is lower than at the ends. This leads to deviations between the measured key parameters such as creep strain and creep strength and the actual material properties. Especially for high-temperature alloys with low plasticity and high strength, this deviation can reach 10%-15%, seriously affecting the reliability of the test results.

[0003] Secondly, the test bench is easily damaged, affecting testing accuracy. When performing compression tests on high-strength materials such as high-temperature alloys and heat-resistant steel, the contact area between the uniform cross-section specimen and the test bench is fixed, and the compressive stress on the test bench's bearing surface is roughly equal to the compressive stress borne by the specimen. When the specimen needs to withstand high stress exceeding its yield strength (e.g., the creep strength test of high-temperature alloys often requires loading to 60%-80% of the material's yield strength), the stress on the test bench's contact surface will also increase synchronously. If the strength of the test bench material is lower than that of the specimen (e.g., high-temperature alloys are commonly used in the compression bench of testing machines, and the yield strength of high-temperature alloys can reach over 1000 MPa), the test bench is prone to localized plastic deformation, forming permanent indentations. On the one hand, indentations lead to unevenness on the test bench surface, shortening its service life and increasing equipment maintenance costs; on the other hand, compression tests have extremely high requirements for the coaxiality of the specimen and the test bench (usually requiring a coaxiality error ≤0.05 mm). Unevenness of the test bench will exacerbate eccentric forces during loading, further amplifying the unevenness of deformation, forming a vicious cycle of "decreased testing accuracy - equipment damage - further decrease in accuracy."

[0004] In existing technologies, some solutions attempt to reduce friction between the specimen and the test bench by applying lubricating grease (such as the end-face lubrication measures mentioned in GB / T10128-2007 "Metallic Materials - Torsion Test at Room Temperature"). However, in high-temperature environments (such as above 600℃), the lubricating grease is prone to volatilization or carbonization, and the lubrication effect quickly fails, failing to fundamentally solve the problem. Another solution uses a spherical connector mounting cavity to compensate for coaxiality errors (such as CN223650301U, a type of hanging specimen connection fixture for permanent testing with automatic coaxiality adjustment). However, this solution can only alleviate the problem of eccentric loading and has no effect on improving specimen barrel distortion and test bench indentation damage.

[0005] Therefore, there is an urgent need in this field to provide a novel compression creep specimen and processing method that can simultaneously solve the problems of uneven deformation and test bench damage. Summary of the Invention

[0006] The main purpose of this application is to provide a novel variable cross-section compression creep specimen and its processing method, so as to simultaneously solve the two major technical problems of uneven deformation of the working section of the specimen and easy damage to the compression table of the testing machine, thereby improving the testing accuracy and protecting the testing equipment.

[0007] To achieve the above, one aspect of this application provides a variable cross-section compression creep specimen, comprising a uniform cross-section working section that is rotationally symmetrical along the specimen axis and has the same cross-section at any point along the axial direction, and a variable cross-section transition section coaxially symmetrically connected to both ends of the uniform cross-section working section; the uniform cross-section working section is a cylinder, and the variable cross-section transition section is a variable diameter rotating body with the specimen axis as the center of rotation; the diameter of the variable diameter rotating body increases from one end near the uniform cross-section working section to the diameter at the contact end face; the side profile of the variable diameter rotating body is a partial elliptical arc of an ellipse, the major axis a of the ellipse is perpendicular or substantially perpendicular to the specimen axis, and the minor axis b of the ellipse is parallel or substantially parallel to the specimen axis; the ratio of the major axis a to the minor axis b of the ellipse is in the range of 1.5 to 2.5 times.

[0008] Furthermore, the ratio of the length of the major axis a of the ellipse to the diameter d1 of the cylinder ranges from 0.75 to 1.25.

[0009] Furthermore, the length L1 of the cylinder is 3 to 10 times the diameter d1 of the cylinder.

[0010] Furthermore, the ratio of the length L2 of the variable cross-section transition section in the axial direction to the length of the minor axis b ranges from 0.25 to 0.95.

[0011] Furthermore, the elliptical arc of the variable cross-section transition section is tangentially connected to the cylindrical surface of the constant cross-section working section.

[0012] Another aspect of this application provides a method for processing the aforementioned variable cross-section compression creep specimen, comprising the following steps: Step S1, provide the bar stock; Step S2: Using the axis of the bar as the axis of the sample, turn a cylindrical working section with a constant cross-section. Step S3: Machining the variable cross-section transition sections at both ends of the constant cross-section working section. Controlling the turning tool to move along a predetermined elliptical trajectory, so that the side profile of the variable cross-section transition section forms an elliptical arc segment. The diameter increases from the end close to the constant cross-section working section to the contact end face. The major axis of the ellipse is perpendicular or approximately perpendicular to the sample axis, and the minor axis is parallel or approximately parallel to the sample axis. The ratio of the major axis a to the minor axis b of the ellipse is in the range of 1.5 to 2.5 times. Step S4: Perform fine machining and grinding on the contact surfaces at both ends of the variable cross-section transition section to ensure that the perpendicularity error between the contact surface and the sample axis and the surface roughness of the contact surface meet the preset requirements.

[0013] Furthermore, in step S3, the tool path of the turning tool is precisely controlled by the CNC machine tool so that the elliptical arc segment is tangentially connected to the cylindrical surface of the uniform cross-section working segment.

[0014] Furthermore, the bar is selected from nickel-based superalloy materials, titanium alloy materials, or steel; Furthermore, the perpendicularity between the contact end face and the sample axis is ≤0.01mm.

[0015] Furthermore, the surface roughness Ra of the contact end face is ≤0.4μm.

[0016] Compared with the prior art, this application has the following beneficial effects: This application, through the design of a variable cross-section transition segment with an elliptical trajectory, ensures that the diameter change rate of the transition segment is a continuous curve, avoiding local stress peaks that may occur during circular arc transitions. Furthermore, the limitation of the major-minor axis ratio of the ellipse to 1:1.5-1:2.5 further ensures the uniformity of stress distribution. The elliptical arc segment design of the variable cross-section compression creep specimen in this application simultaneously achieves the following three aspects: first, it makes the overall variable cross-section section relatively short, limiting its impact on the calculation of overall creep deformation; second, it achieves a larger specimen end face area with a smaller variable cross-section length, thereby minimizing the compressive stress on the test bench bearing surface; and third, it can guide the pressure from the test bench to the specimen body in a roughly uniform manner, without significant stress concentration.

[0017] In summary, the variable cross-section compression specimen of this application can completely overcome two shortcomings of the prior art: on the one hand, it significantly improves the accuracy of material compression testing, and on the other hand, it significantly extends the service life of the test bench.

[0018] Other features and effects of this application will be set forth in the following description and will be apparent in part from the description or may be learned by practicing this application. Attached Figure Description

[0019] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 A cross-sectional schematic diagram of a variable cross-section compression creep specimen according to an embodiment of this application is shown.

[0021] Figure 2 A schematic cross-sectional view of a cylindrical compression creep specimen according to the prior art is shown.

[0022] The above figures include the following reference numerals: 1. Constant cross-section working section; 2. Variable cross-section transition section; 3. Cylindrical specimen. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0024] To achieve the above objectives, a first aspect of the present application provides a variable cross-section compression creep specimen, comprising a uniform cross-section working section that is rotationally symmetrical along the specimen axis and has the same cross-section at any point along the axial direction, and a variable cross-section transition section coaxially symmetrically connected to both ends of the uniform cross-section working section; the uniform cross-section working section is a cylinder, and the variable cross-section transition section is a variable diameter rotating body with the specimen axis as the center of rotation; the diameter of the variable diameter rotating body increases from one end near the uniform cross-section working section to the diameter at the contact end face; the side profile of the variable diameter rotating body is a partial elliptical arc of an ellipse, the major axis a of the ellipse is perpendicular or substantially perpendicular to the specimen axis, and the minor axis b of the ellipse is parallel or substantially parallel to the specimen axis; the ratio of the major axis a to the minor axis b of the ellipse ranges from 1.5 to 2.5 times (e.g., including but not limited to 1.5 times, 1.6 times, 1.7 times, 1.8 times, 1.9 times, 2.0 times, 2.1 times, 2.2 times, 2.3 times, 2.4 times, and 2.5 times).

[0025] While existing technologies have proposed creep-resistant tensile arc deformation specimens, specimens with a constant cross-section are still commonly used for compression creep tests. Directly converting creep-resistant tensile arc deformation specimens to compression creep tests is impossible because: firstly, the variable cross-section portion of the creep-resistant tensile arc deformation specimen is entirely circular, resulting in an excessively long transition section that introduces very large and difficult-to-measure creep deformation; secondly, the outer part of the transition section is a long clamping section, making it impossible to measure the creep deformation of the cylindrical section.

[0026] This application designs the variable cross-section transition section as a variable diameter rotating body with an elliptical arc profile. This design effectively disperses the frictional constraints on the specimen end face, allowing the load to be uniformly transferred to the constant cross-section working section. This significantly suppresses "barrel distortion" and makes the deformation of the constant cross-section working section closer to the ideal uniform compression state, thereby greatly improving the measurement accuracy of parameters such as creep strain and creep strength. Furthermore, it allows for a larger specimen end face area with a smaller variable cross-section length, thus minimizing the compressive stress on the test bench bearing surface and keeping it below the yield strength of the test bench material. This effectively prevents plastic indentation on the test bench and extends the service life of the equipment.

[0027] In this application, the ratio of the major axis to the minor axis of the ellipse can be adjusted according to the strength of the test material and the test stress level: for materials with higher strength (such as single-crystal superalloys), the ratio can be adjusted to 2.2-2.5 times to further increase the contact area at both ends and reduce the stress on the test bench; for medium- and low-strength materials (such as heat-resistant steel), the ratio can be adjusted to 1.5-1.8 times to simplify the processing difficulty while ensuring the safety of the test bench. Compared with other curves, the elliptical trajectory of this application is easier to process and has stable mechanical properties, making it particularly suitable for high-precision creep testing scenarios such as aerospace materials.

[0028] In some preferred embodiments of this application, the ratio of the length of the major axis a of the ellipse to the diameter d1 of the cylinder ranges from 0.75 to 1.25 times (e.g., including but not limited to 0.75 times, 0.80 times, 0.85 times, 0.90 times, 0.95 times, 1.0 times, 1.15 times, 1.20 times, and 1.25 times). By reasonably limiting the range of the ratio of the length of the major axis a to the diameter d1 of the cylinder, the optimal effect of balancing minimizing the length of the transition section and uniformly transmitting pressure at both ends is achieved.

[0029] In some preferred embodiments of this application, the length L1 of the cylinder is 3 to 10 times the diameter d1 of the cylinder (e.g., including but not limited to 3 times, 4 times, 5 times, 6 times, 7 times, 8 times, 9 times, 10 times).

[0030] In some preferred embodiments of this application, the ratio of the axial length L2 of the variable cross-section transition section to the length of the minor axis b ranges from 0.25 to 0.95 (e.g., including but not limited to 0.25, 0.30, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 0.95). Reasonably limiting this range further ensures uniform downward pressure transmission and minimizes manufacturing costs.

[0031] In some preferred embodiments of this application, the elliptical arc of the variable cross-section transition section is tangentially connected to the cylindrical surface of the constant cross-section working section. This tangency allows the pressure applied via the two end faces to be uniformly transmitted to the cylinder through the transition section.

[0032] A second aspect of the embodiments of this application also provides a method for processing the aforementioned variable cross-section compression creep specimen, comprising the following steps: Step S1, provide the bar stock; Step S2: Using the axis of the bar as the axis of the sample, turn a cylindrical working section with a constant cross-section. Step S3: Machining the variable cross-section transition sections at both ends of the constant cross-section working section. Controlling the turning tool to move along a predetermined elliptical trajectory, so that the side profile of the variable cross-section transition section forms an elliptical arc segment. The diameter increases from the end close to the constant cross-section working section to the contact end face. The major axis of the ellipse is perpendicular or approximately perpendicular to the sample axis, and the minor axis is parallel or approximately parallel to the sample axis. The ratio of the major axis a to the minor axis b of the ellipse is in the range of 1.5 to 2.5 times. Step S4: Perform fine machining and grinding on the contact surfaces at both ends of the variable cross-section transition section to ensure that the perpendicularity error between the contact surface and the sample axis and the surface roughness of the contact surface meet the preset requirements.

[0033] In some preferred embodiments of this application, the tool path of the turning tool is precisely controlled by a CNC machine tool so that the elliptical arc segment is tangentially connected to the cylindrical surface of the uniform cross-section working segment.

[0034] In some preferred embodiments of this application, the bar material includes, but is not limited to, nickel-based superalloy materials, titanium alloy materials, or steel.

[0035] In some preferred embodiments of this application, the perpendicularity of the contact end face to the sample axis is ≤0.01mm, and the surface roughness Ra of the contact end face is ≤0.4μm.

[0036] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0037] Example A variable cross-section compression creep specimen, the structure of which is shown in [reference needed]. Figure 1 It includes a uniform cross-section working section 1 that is rotationally symmetrical along the axis of the specimen and has the same cross-section in any direction along the axis, and a variable cross-section transition section 2 that is coaxially symmetrically connected to both ends of the uniform cross-section working section.

[0038] The constant cross-section working section 1 is a cylinder with a diameter d1 = 10 mm and a length L1 = 48 mm. The variable cross-section transition section 2 is a rotating body with a variable diameter centered on the specimen axis. The maximum diameter of this rotating body is d2 = 14.68 mm (this maximum diameter refers to the end face diameter d2 of the rotating body), and the minimum diameter is d1 = 10 mm (i.e., the diameter of the cylinder). The side profile of the variable cross-section transition section 2 is a partial elliptical arc, which is tangentially connected to the cylindrical surface of the constant cross-section working section 1. The major axis a = 10 mm, the minor axis b = 5 mm, and the axial length L2 of the variable cross-section transition section 2 is 1.25 mm. In this embodiment, the total length of the variable cross-section compression creep specimen is L = 48 mm + (2 × 1.25 mm) = 50.5 mm. Since the stress in the variable cross-section section is small and the deformation is negligible, the converted nominal length is still 50 mm, which conforms to the proportion in the standard (GB / T 10128).

[0039] The processing method for the variable cross-section compression creep specimen in this embodiment includes the following steps: ① Select a φ12mm nickel-based superalloy (grade GH4169) bar, anneal it to eliminate processing stress and ensure subsequent processing accuracy to obtain nickel-based superalloy bar material; ② Using the axis of the nickel-based high-temperature alloy bar as the axis of the sample, a cylindrical working section 1 with a constant cross-section was machined. ③ The variable cross-section transition sections at both ends of the constant cross-section working section 1 are machined by turning. The diameter gradient trajectory of the variable cross-section transition section is precisely controlled according to the ellipse equation: the major axis of the ellipse is 10mm, the minor axis is 5mm, the major axis of the ellipse is perpendicular to the sample axis, the minor axis of the ellipse is parallel to the sample axis, and the distance between the major axis of the ellipse and the near end face of the sample is 1.25mm. During turning, the tool moves along the outer trajectory of this ellipse to achieve a smooth gradient from a diameter of 10mm to 14.68mm at the end face, and the two variable cross-section transition sections 2 are centrally symmetrical with the constant cross-section working section 1. ④ Turn the contact surfaces at both ends of the specimen and grind the end faces at both ends of the specimen to ensure that the perpendicularity of the end face to the specimen axis is ≤0.01mm and the surface roughness Ra is ≤0.4μm, so as to avoid eccentricity during loading and ensure good contact with the compression table of the testing machine.

[0040] High-temperature compression creep test comparison: The variable cross-section compression creep specimen prepared in this embodiment (see...) Figure 1) and existing conventional cylindrical specimen 3 (length 50mm, diameter 10mm, see Figure 2 The samples are installed between the compression platforms of the high-temperature creep testing machine. The assembly steps are the same, specifically: ① Clean the contact surfaces of the upper and lower compression platforms and wipe them with anhydrous ethanol to remove oil and oxide scale; ② Place the sample vertically in the center of the lower compression platform; ③ Slowly lower the upper compression platform until it lightly touches the end face of the sample, apply a pre-pressure of 500N to make the sample fit tightly with the compression platform, and let it stand for 5 minutes after pre-pressure to avoid loading fluctuations caused by gaps in the early stage of the test.

[0041] Test parameters were set as follows: the test temperature was 650℃ (the typical service temperature range of GH4169 alloy), and the applied stress was 600MPa (determined based on the endurance strength level of this alloy at 650℃ to avoid premature sample fracture or excessively long test periods). During the test, the temperature fluctuation inside the furnace was maintained at ≤±2℃ through the temperature control system of the testing machine. The axial deformation of the working section of the sample was monitored in real time, and the testing machine simultaneously recorded the applied force, axial displacement, and temperature data.

[0042] After the test, the deformation state, test bench damage, and test data of the two groups of samples were compared and analyzed. The results are as follows: Comparison of deformation uniformity: The conventional constant-section cylindrical specimen 3 exhibits significant barrel-shaped distortion in its working section due to frictional constraints at both ends, with the maximum bulge diameter reaching 11.9 mm and a deformation non-uniformity of 19%. In this embodiment, the variable-section compression creep specimen, due to the gradually increasing diameter of the variable-section transition section 2 at both ends, mitigates the influence of end-face friction on the constant-section working section. The constant-section working section 1 deforms uniformly, with diameter variations at all locations ≤0.2 mm, and a deformation non-uniformity of only 3.2%. Furthermore, the total specimen length is 50.5 mm, and the stress in the variable-section transition section is relatively low (stress calculation formula: ...). Where F is the applied force and A is the contact area; since A is larger, the stress is... (Even smaller), its deformation is only 0.03mm, which, after being converted to a nominal length of 50mm, is consistent with the standard length of conventional specimens, ensuring the effectiveness of deformation data comparison.

[0043] Damage comparison of the test bench: Existing conventional cylindrical specimen 3 with a constant cross-section has a small contact area with the test bench due to the compression end face having the same diameter as the working section (10mm). Under a loading stress of 600MPa, the contact stress on the test bench reaches 600MPa, resulting in an indentation of approximately 0.025mm deep on the contact surface. The variable cross-section compression creep specimen of this application increases the diameter of the compression end face to 14.68mm through a variable cross-section transition section, thus increasing the contact area compared to the constant cross-section specimen (14.68mm). 2 / 10 2 ≈2.15 times, according to Under the same loading force, the contact stress of the test bench decreased to about 278 MPa, which is lower than the yield strength of the test bench material. After the test, there were no obvious indentations on the surface of the compression bench, and the surface roughness remained in the initial state (Ra≤0.4μm), which significantly reduced the risk of damage to the test bench.

[0044] Comparative Example 1 A variable cross-section compression creep specimen differs from Example 1 only in that the ratio of the major axis a to the minor axis b of the ellipse is 0.75. This comparative example's variable cross-section compression creep specimen was tested according to the aforementioned high-temperature compression creep test. Because the ratio of the major axis a to the minor axis b of the ellipse was too small, the transition section was too long (3.8 mm), resulting in uneven creep deformation within the transition section. This ultimately led to poor test accuracy, with the measured creep strength deviating from the true value by 7.2%.

[0045] Comparative Example 2 A variable cross-section compression creep specimen differs from Example 1 only in that the ratio of the major axis a to the minor axis b of the ellipse is 3. This comparative example's variable cross-section compression creep specimen was tested according to the above-described high-temperature compression creep test. Because the ratio of the major axis a to the minor axis b of the ellipse is too large, the major axis a is too long, resulting in an excessively short transition section (0.7 mm). This makes it impossible to uniformly guide the pressure transmitted from the specimen end to the working section, leading to uneven deformation in the working section and noticeable curling at the ends. The measured creep strength deviates from the true value by 10.5%.

[0046] Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A variable cross-section compression creep specimen, characterized in that, It includes a uniform cross-section working section that is rotationally symmetrical along the sample axis and has the same cross-section at any point along the axial direction, and a variable cross-section transition section that is coaxially symmetrically connected to both ends of the uniform cross-section working section; the uniform cross-section working section is a cylinder, and the variable cross-section transition section is a variable diameter rotating body with the sample axis as the center of rotation; the diameter of the variable diameter rotating body increases from one end close to the uniform cross-section working section to the diameter at the contact end face; the side profile of the variable diameter rotating body is a partial elliptical arc of an ellipse, the major axis a of the ellipse is perpendicular or substantially perpendicular to the sample axis, and the minor axis b of the ellipse is parallel or substantially parallel to the sample axis; the ratio of the major axis a to the minor axis b of the ellipse is in the range of 1.5 to 2.5 times.

2. The variable cross-section compression creep specimen according to claim 1, characterized in that, The ratio of the length of the major axis a of the ellipse to the diameter d1 of the cylinder is in the range of 0.75 to 1.

25.

3. The variable cross-section compression creep specimen according to claim 1, characterized in that, The length L1 of the cylinder is 3 to 10 times the diameter d1 of the cylinder.

4. The variable cross-section compression creep specimen according to claim 1, characterized in that, The ratio of the length L2 of the variable cross-section transition section in the axial direction to the length of the minor axis b ranges from 0.25 to 0.

95.

5. The variable cross-section compression creep specimen according to claim 1, characterized in that, The elliptical arc of the variable cross-section transition section is tangentially connected to the cylindrical surface of the constant cross-section working section.

6. A method for processing a variable cross-section compression-creep specimen as described in any one of claims 1 to 5, characterized in that, Includes the following steps: Step S1, provide the bar stock; Step S2: Using the axis of the bar as the axis of the sample, turn a cylindrical working section with a constant cross-section. Step S3: Machining the variable cross-section transition sections at both ends of the constant cross-section working section. Controlling the turning tool to move along a predetermined elliptical trajectory, so that the side profile of the variable cross-section transition section forms an elliptical arc segment. The diameter increases from the end close to the constant cross-section working section to the contact end face. The major axis of the ellipse is perpendicular or approximately perpendicular to the sample axis, and the minor axis is parallel or approximately parallel to the sample axis. The ratio of the major axis a to the minor axis b of the ellipse is in the range of 1.5 to 2.5 times. Step S4: Perform fine machining and grinding on the contact surfaces at both ends of the variable cross-section transition section to ensure that the perpendicularity error between the contact surface and the sample axis and the surface roughness of the contact surface meet the preset requirements.

7. The processing method according to claim 6, characterized in that, In step S3, the tool path of the turning tool is precisely controlled by the CNC machine tool so that the elliptical arc segment is tangentially connected to the cylindrical surface of the uniform cross-section working segment.

8. The processing method according to claim 6, characterized in that, The bar is selected from nickel-based superalloy materials, titanium alloy materials, or steel.

9. The processing method according to claim 6, characterized in that, The perpendicularity between the contact end face and the sample axis is ≤0.01mm.

10. The processing method according to claim 6, characterized in that, The surface roughness Ra of the contact end face is ≤0.4μm.

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