Ultra-high cycle tensile fatigue specimen with cutting plane and its design method and test method

Abstract

The present invention discloses an ultra-high cycle tensile fatigue specimen with a cutting plane and a design method thereof, as well as a method for fatigue testing the specimen, the design method comprising: designing a dog-bone-shaped specimen with a central symmetrical structure and a natural frequency close to the design frequency, the dog-bone-shaped specimen including a cylindrical test section, a variable cross-section section and a cylindrical resonance section from the middle to both sides, the cross-sectional radius of the test section being smaller than the cross-sectional radius of the resonance section; cutting out opposite U-shaped grooves on both sides of the test section, the bottom surface of the U-shaped groove being an observation plane parallel to the axial direction, and the side surface being an inclined side surface at an angle of θ; using finite element software to analyze and obtain the axial vibration mode natural frequency of the specimen with a cutting plane and reversely correcting the resonance length so that its natural frequency is close to the design frequency. The specimen with a cutting plane obtained by the above-mentioned design method obtains a flat observation surface on the premise that its natural frequency meets the requirements, so as to facilitate the observation of the failure mechanism of the material.

Description

Technical Field

[0001] The present invention belongs to the technical field of material performance testing, and more specifically, relates to an ultra-high cycle tensile fatigue specimen with a cutting plane and a design method and a testing method thereof. Background Art

[0002] As the performance of aircraft engines continues to improve, the service life and safety and reliability standards of engines are also getting higher and higher. Key components such as engine blades and blisks will be subjected to high-frequency aerodynamic loads during operation, and the actual fatigue life will exceed 10 8 In order to ensure the safe operation of the engine, in addition to theoretically ensuring that the fatigue life of key components meets the design requirements, we also need to conduct ultra-high cycle fatigue tests on typical engine materials to eliminate possible fatigue failures.

[0003] Since ultra-high cycle fatigue tests often reach 10 9 For the above cycle, the traditional 100Hz fatigue test machine would take more than three months to test, so some scholars have proposed to use high-frequency loading to shorten the ultra-high cycle fatigue test cycle. At present, the more common method at home and abroad is to use 20kHz frequency ultrasonic vibration to conduct experiments. This experimental method can complete 10 9 The first cycle takes less than 14 hours. The ultrasonic testing machine uses ultrasonic vibration excitation to make the specimen resonate. Therefore, the design of the ultra-high cycle fatigue test specimen needs to meet the resonance condition. The shape and size of the specimen determine the displacement, strain and stress distribution of the specimen during the vibration process. Therefore, compared with other material mechanics experiments, the design of the specimen largely determines the effectiveness of the ultra-high cycle fatigue test.

[0004] At present, many research works have been carried out in the field of ultra-high cycle fatigue test specimen design at home and abroad. For example, the invention patent CN201510828131.3 designs an ultrasonic fatigue specimen under tension and compression loads of a plate-shaped metal material with a uniform cross-section (referred to as a plate-shaped specimen), and the invention patent CN201520691782.8 designs an ultrasonic fatigue specimen under tension and compression loads of a circular arc-shaped metal material with a uniform cross-section (referred to as a dog-bone-shaped specimen), which realizes the ultra-high cycle tension and compression fatigue performance test of metal materials under different specimen shapes. Whether it is a plate or an arc, the middle test section is a uniform cross-sectional structure. Compared with the plate-shaped specimen, the dog-bone specimen has the advantages of a large vibration displacement stress coefficient, a simple appearance and easy processing, so it is more widely used. However, the traditional dog-bone-shaped specimen is in the shape of an arc, and it is impossible to effectively characterize the microscopic fatigue failure mechanism of the material through microscopic technology, and it is difficult to accurately describe the typical microscopic fatigue characteristics. Summary of the invention

[0005] In view of the above defects or improvement needs of the prior art, the present invention provides an ultra-high cycle tensile fatigue specimen with a cutting plane and a design method and a testing method thereof, the purpose of which is to enable the specimen to resonate on an ultrasonic fatigue testing machine and facilitate the characterization of the microscopic fatigue failure mechanism of the material.

[0006] To achieve the above object, according to one aspect of the present invention, a design method for an ultra-high cycle tensile fatigue specimen with a cutting plane is provided, comprising:

[0007] A dog-bone specimen with a centrally symmetrical structure and a natural frequency close to the design frequency f is preliminarily designed. The dog-bone specimen includes a cylindrical test section, a variable cross-section section and a cylindrical resonance section from the middle to both sides. The cross-sectional radius R1 of the test section is smaller than the cross-sectional radius R2 of the resonance section.

[0008] Relative U-shaped grooves are cut inward on both sides of the test section to form a test piece with a cutting plane. The bottom surface of the U-shaped groove is an observation plane parallel to the axial direction, and the side surface is an inclined side surface with a tangent line at the opening at an angle of θ. The test piece with a cutting plane maintains a central symmetrical structure;

[0009] The axial vibration mode natural frequency f1 of the specimen with the cutting plane is obtained by finite element software analysis, and the resonance length L2 is corrected according to the axial vibration mode natural frequency f1 so that the axial vibration mode natural frequency f1 of the specimen with the cutting plane is close to the design frequency f.

[0010] In one embodiment, when the dog-bone type specimen is preliminarily designed, the length 2L0 and the cross-sectional radius R1 of the test section of the dog-bone type specimen, the length L1 and the arc radius R0 of each variable cross-sectional section, the cross-sectional radius R2 and the resonant length L2 of each resonant section, the expected design frequency f, and the resonant length L2 are determined to meet the following requirements:

[0011]

[0012] Among them, the intermediate parameter k = ω / c, ω is the circular frequency of the specimen vibration, ω = 2πf, c is the propagation speed of the elastic wave in the material, E d and ρ are the elastic modulus and material density of the specimen material, respectively. The intermediate parameters α is the catenary parameter of the variable cross-section segment, α=cosh -1 (R2 / R1) / L1.

[0013] In one embodiment, the design frequency f is in the range of [F-500Hz, F+500Hz], where F is the operating frequency of the fatigue testing machine.

[0014] In one embodiment, the resonant length L2 is corrected so that the axial vibration mode natural frequency f1 of the specimen with the cut plane is close to the design frequency f, including: correcting the resonant length L2 so that the axial vibration mode natural frequency f1 of the specimen with the cut plane is within the expected range [f-20Hz, f+20Hz].

[0015] In one of the embodiments, when the axial vibration mode natural frequency f1 is low, the resonant length L2 of the resonance section is gradually reduced so that the axial vibration mode natural frequency f1 gradually increases and approaches the design frequency f; when the axial vibration mode natural frequency f1 is high, the resonant length L2 of the resonance section is gradually increased so that the axial vibration mode natural frequency f1 gradually decreases and approaches the design frequency f.

[0016] In one embodiment, when the axial vibration mode natural frequency f1 is outside the range of [f-450Hz, f+450Hz], the adjustment step size of each resonant length is 0.05mm to 0.5mm; when the axial vibration mode natural frequency f1 is within the range of [f-450Hz, f+450Hz], the adjustment step size of each resonant length is 0.01mm to 0.05mm.

[0017] In one embodiment, the ratio of the bottom area of ​​the U-shaped groove to the longitudinal cross-sectional area of ​​the test section is In the range of , the cutting angle θ is in the range of [10°, 30°], and the cutting depth is in the range of

[0018] According to another aspect of the present invention, there is provided an ultra-high cycle tensile fatigue specimen with a cutting plane, wherein the specimen is a dog-bone-type specimen having a centrally symmetrical structure and an axial vibration mode natural frequency close to the design frequency f, wherein the dog-bone-type specimen comprises a test section, a variable cross-section section and a cylindrical resonance section from the middle to both sides in sequence, wherein the test section is formed by cutting relative U-shaped grooves inwardly on both sides of the cylinder, wherein the bottom surface of the U-shaped groove is an observation plane parallel to the axial direction, and the side surface is an inclined side surface whose tangent at the opening is at an angle θ, and the cylindrical cross-sectional radius R1 of the test section is smaller than the cross-sectional radius R2 of the resonance section.

[0019] In one embodiment, the ratio of the bottom area of ​​the U-shaped groove to the longitudinal cross-sectional area of ​​the test section is In the range of , the cutting angle θ is in the range of [10°, 30°], and the cutting depth is in the range of

[0020] According to another aspect of the present invention, there is provided a method for performing ultra-high cycle tensile fatigue testing on a material, comprising:

[0021] Using an ultrasonic fatigue testing machine to resonantly load the ultra-high cycle tensile fatigue specimen with a cutting plane described in any one of the above items until the specimen breaks;

[0022] The bottom surface of the groove was used as the observation surface to carry out microscopic quantitative characterization of the specimen and analyze the failure mechanism of the specimen.

[0023] In general, the above technical solutions conceived by the present invention can achieve the following beneficial effects compared with the prior art:

[0024] The design method mentioned in this application first preliminarily designs a dog-bone specimen of equal cross-section. The dog-bone specimen is a traditional dog-bone-shaped specimen. The natural frequency of the preliminarily designed dog-bone-shaped specimen is close to the design frequency f, wherein the design frequency f is the frequency that can make the specimen resonate on the ultrasonic fatigue testing machine. The traditional method is to directly use the above-mentioned dog-bone-shaped specimen to perform ultrasonic ultra-high cycle tensile fatigue testing. In this application, further processing is performed based on the traditional dog-bone-shaped specimen, and a U-shaped groove with an inclined side is cut out in the middle test section. The test section after cutting is no longer a structure of equal cross-section. In traditional concepts, the test section of the specimen must be kept as an equal cross-section in order to ensure that the stiffness distribution of the specimen is uniform and obtain better test data. Therefore, in traditional technology, whether it is a plate-shaped specimen or a dog-bone-shaped specimen, the middle test section is designed to be equal cross-section. In this application, the conventional design is broken, and a U-shaped groove is cut in the test section of the dog-bone specimen. Although the test section after cutting is no longer a uniform cross-section structure, it has been verified through experiments that the cutting size can be controlled and the effect of cutting on the stiffness of the test section can be ignored. After cutting, the natural frequency of the specimen with the cutting plane is analyzed by finite element analysis, and the resonant length is adjusted by feedback to make the natural frequency of the specimen with the cutting plane close to the design frequency, so that the specimen with the cutting plane can resonate on the ultrasonic fatigue testing machine. It has been verified through experiments that the specimen with the cutting plane can not only be used for ultrasonic ultra-high cycle fatigue testing, but also presents the following advantages:

[0025] 1. The stress amplification factor of the specimen with the cutting plane is further increased, which is conducive to the subsequent ultra-high cycle fatigue test;

[0026] 2. When using a specimen with a cutting plane for ultrasonic ultra-high cycle tension and compression fatigue testing, the fracture is likely to occur in the cutting plane section;

[0027] 3. Compared with arc surface, cutting plane is easier to quantitatively analyze the microscopic characteristics of the specimen. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A flowchart of the steps of a design method of an ultra-high cycle tensile fatigue test piece with a cutting plane according to an embodiment;

[0029] Figure 2 A schematic diagram of a coordinate system established for a dog-bone type test piece according to an embodiment;

[0030] Figure 3is a three-dimensional diagram of a test piece with a cutting plane according to an embodiment;

[0031] FIG4( a) is a side view of a test piece with a cutting plane according to an embodiment;

[0032] FIG4( b ) is a side view of a test piece with a cutting plane according to another embodiment;

[0033] Figure 5 A top view of a cutting plane specimen of an embodiment;

[0034] Figure 6 A side view of a cutting plane specimen with size markings according to an embodiment;

[0035] Figure 7 A top view of a cutting plane specimen with size markings according to an embodiment. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0037] like Figure 1 The figure is a flow chart of a design method of a ultra-high cycle tensile fatigue specimen with a cutting plane in one embodiment, which includes the following steps:

[0038] Step S100: Preliminary design of a dog-bone specimen with a centrally symmetrical structure and a natural frequency close to the design frequency f, wherein the dog-bone specimen includes a cylindrical test section, a variable cross-section section and a cylindrical resonance section from the middle to both sides, and the cross-sectional radius R1 of the test section is smaller than the cross-sectional radius R2 of the resonance section.

[0039] To conduct ultrasonic ultra-high cycle fatigue testing on a material, it is necessary to use the material to make a test piece, and use an ultrasonic fatigue testing machine to resonantly load the test piece. Therefore, before designing the test piece, it is necessary to first clarify the design frequency that the test piece is expected to achieve based on the operating frequency of the testing machine to ensure that the test piece resonates with the testing machine.

[0040] Here, a dog-bone type specimen is obtained, and the natural frequency of the dog-bone type specimen is close to the design frequency f, and can resonate with the ultrasonic fatigue testing machine. Assuming that the working frequency of the ultrasonic fatigue testing machine is F, the determined design frequency f is in the range of [F-500Hz, F+500Hz]. For example, F=20kHz, then the design frequency f can be selected as 20kHz-0.5kHz, 20kHz-0.3kHz, 20kHz, 20kHz+0.3kHz, 20kHz+0.5kHz, etc. If the natural frequency of the specimen is close to the design frequency f, that is, the natural frequency is equal to the design frequency f or the deviation from the design frequency f is in a tolerable range, it can resonate with the ultrasonic fatigue testing machine. Among them, the degree of approximation (deviation range) varies according to the actual test environment and conditions. In this embodiment, the tolerable deviation between the natural frequency and the design frequency f is 20Hz, that is, the natural frequency is within the expected range [f-20Hz, f+20Hz], which is close to the design frequency f.

[0041] Specifically, the morphology of the dog-bone specimen obtained by the preliminary design refers to the traditional dog-bone specimen, such as Figure 2 As shown, the middle position of the dog-bone type specimen is a cylindrical test section, and the two sides are cylindrical resonance sections, and the test section and the resonance section are transitionally connected by a variable cross-section section. Among them, the length of the test section is 2L0, the cross-sectional radius is R1, the length of each variable cross-sectional section is L1, the arc radius is R0, and the cross-sectional radius of each resonance section is R2 and the resonance length is L2. The design method of the preliminary designed dog-bone type specimen is not limited here, and the traditional dog-bone type specimen design method can be used, as long as the natural frequency of the specimen is close to the design frequency f.

[0042] In one embodiment, in order to make the natural frequency of the preliminarily designed dog-bone specimen as close to the design frequency f as possible, the resonant length L2 of each resonant section is ensured to satisfy:

[0043]

[0044] Among them, the intermediate parameter k = ω / c, ω is the circular frequency of the specimen vibration, ω = 2πf, c is the propagation speed of the elastic wave in the material, E d and ρ are the elastic modulus and material density of the specimen material, respectively. The intermediate parameters α is the catenary parameter of the variable cross-section segment, α=cosh -1 (R2 / R1) / L1.

[0045] The following describes the derivation process of obtaining the calculation formula for the above-mentioned resonant length L2.

[0046] According to the wave propagation theory in elastic bodies, the material satisfies the ideal elastic condition. Under the one-dimensional vibration state, assuming that the origin of the coordinate is the axial center of the specimen and the axial direction of the specimen is taken as the x-axis, the longitudinal wave equation of the specimen is:

[0047]

[0048]

[0049] Where ρ is the material density; x is the displacement along the axial direction; t is the time; S(x) is the cross-sectional area at x; u(x,t) is the longitudinal displacement of the specimen at x at time t; f(x,t) is the force acting on the x section; E d is the dynamic elastic modulus of the material.

[0050] If the test meets the resonance condition of ultrasonic vibration of fatigue testing machine, u(x,t) can be set as:

[0051] u(x, t) = U(x)sin(ωt) (3)

[0052] Where U(x) is the specimen amplitude, ω is the circular frequency of the specimen vibration, and equation (1) can be simplified as:

[0053]

[0054] Where p(x) = S′(x) / S(x), the propagation speed of elastic waves in the material is ω=2πf,k=ω / c. In the following calculations, let U0(x), U1(x), U2(x) be the amplitudes of the test section, variable section section, and resonant section of the specimen, respectively, and let U0′(x), U2′(x), U2′(x) be the strains of the test section, variable section section, and resonant section of the specimen, respectively.

[0055] Since the specimen is symmetrical, only the right half can be calculated. The area distribution of each section along the axial direction of the right half of the specimen can be expressed as:

[0056]

[0057] In order to obtain an analytical solution, the catenary is used instead of the arc in the calculation of the time-varying cross-section segment, and the error can be controlled within 3%. The catenary parameter α = cosh -1 (R2 / R1) / L1, when designing the specimen, the arc radius satisfies R0=(L1 2 +(R2-R1) 2 ) / 2(R2-R1), the total length of the right half specimen L=L0+L1+L2.

[0058] Substitute equation (5) into the wave equation and combine it with the boundary conditions of the resonance of the ultrasonic fatigue specimen. Assuming that the maximum amplitude of the specimen is A0, then:

[0059]

[0060] The continuity conditions for the displacement amplitude and force amplitude at L0 are:

[0061]

[0062] The continuity conditions for the displacement amplitude and force amplitude at L0+L1 are:

[0063]

[0064] The displacement distribution function in the specimen can be solved as:

[0065]

[0066] In which, let β = (α 2 -k 2 ) 0.5 , F s is the unknown coefficient, which represents the stress magnification factor of the specimen. The strain distribution function of the specimen can be obtained by derivation of formula (9) with respect to x:

[0067]

[0068] Substituting equations (9) and (10) into equation (8), we can obtain the formula for the resonant length L2 of the dog-bone specimen and the stress amplification factor F: s :

[0069]

[0070]

[0071] After the preliminary design of the uniform cross-section dog-bone specimen, proceed to:

[0072] Step S200: Relative U-shaped grooves are cut inward on both sides of the test section to form a specimen with a cutting plane. The bottom surface of the U-shaped groove is an observation plane parallel to the axial direction, and the side surface is an inclined side surface with a tangent line at the opening at an angle θ. The specimen with a cutting plane maintains a central symmetrical structure.

[0073] On the upper and lower sides of the test section of the dog-bone specimen, U-shaped grooves are cut inward at an angle of θ to obtain the following Figure 3 to Figure 5 The specimen with cutting plane shown, where Figure 3 FIG4 is a stereoscopic view of the specimen with a cutting plane, FIG4(a) and FIG4(b) are side views of the specimen with a cutting plane, Figure 5It is a top view of the specimen with a cutting plane. The bottom surface of the U-shaped groove is an observation plane with a length a and a width b parallel to the axial direction (X axis), and the side surface is an inclined side surface with a tangent at the opening at an angle of θ. In one embodiment, as shown in FIG4(a), the inclined side surface can be an inclined plane with an angle of θ. In another embodiment, as shown in FIG4(b), the inclined side surface can be an arc-shaped side surface with a tangent at the opening at an angle of θ and a radius of R3. In one embodiment, in order to reduce the influence of the groove on the performance of the specimen and enable the specimen to be normally used for fatigue testing, the cutting size of the groove is limited, and the ratio of the bottom surface area of ​​the U-shaped groove to the longitudinal cross-sectional area of ​​the test section is limited to 60°. Within the range, you can choose The range of cutting angle θ is [10°, 30°], and you can choose 10°, 15°, 20°, 25°, 30°. The range of cutting depth h is Specific options The size of the cutting plane should be moderate to ensure that the stiffness change of the test section is small and the cutting plane area is large for easy observation. In other embodiments, it can also be adjusted to other sizes according to the test as long as resonance can eventually occur.

[0074] Step S300: using finite element software to analyze and obtain the axial vibration mode natural frequency f1 of the specimen with the cutting plane, and correcting the resonant length according to the axial vibration mode natural frequency f1 so that the axial vibration mode natural frequency f1 of the specimen with the cutting plane is close to the design frequency f.

[0075] Since the dog-bone specimen was cut, its size and shape changed, and the natural frequency of the obtained specimen with a cut plane changed. Therefore, the natural frequency needs to be corrected to make it close to the design frequency f and meet the conditions for resonance with the fatigue testing machine.

[0076] In this step, the ultrasonic axial fatigue specimen model is first established in the modeling software, and the finite element software is imported for modal analysis to obtain the natural frequency f1 corresponding to the axial vibration mode of the specimen, and the geometric dimensions of the ultrasonic axial fatigue specimen are corrected according to the modal analysis results. Specifically, when the natural frequency f1 is low, the resonant length L2 is gradually reduced so that the axial vibration mode natural frequency f1 gradually increases and approaches the design frequency f; when the axial vibration mode natural frequency f1 is high, the resonant length L2 is gradually increased so that the axial vibration mode natural frequency f1 gradually decreases and approaches the design frequency f.

[0077] Take the expected range of approaching the design frequency f as [f-20Hz, f+20Hz] as an example. If the natural frequency f1 corresponding to the axial vibration mode of the specimen is less than the design natural frequency f-20Hz, then reduce the specimen resonant length L2 within the range of 0.01mm to 0.5mm, increase the natural frequency corresponding to the axial vibration mode of the specimen, and then modify the model size to meet the resonance condition, perform modal analysis again through the finite element software, and repeat the steps until the natural frequency f1 corresponding to the axial vibration mode of the specimen obtained by the modal analysis is within the range of [f-20Hz, f+20Hz], and finally determine the geometric dimensions of the specimen to complete the specimen design. If the natural frequency f1 corresponding to the axial vibration mode of the specimen is greater than the designed natural frequency f+20Hz, increase the resonant length L2 of the specimen in the range of 0.01mm to 0.5mm, reduce the natural frequency corresponding to the axial vibration mode of the specimen, and then modify the model size to meet the resonance conditions. Perform modal analysis again using the finite element software, and repeat the steps until the natural frequency f1 corresponding to the axial vibration mode of the specimen obtained by the modal analysis is within the range of [f-20Hz, f+20Hz]. Finally, determine the geometric dimensions of the specimen and complete the specimen design.

[0078] Furthermore, in order to take into account both the adjustment speed and the adjustment accuracy, the adjustment step is set according to different situations. Specifically, when the axial vibration mode natural frequency f1 is outside the range of [f-450Hz, f+450Hz], the adjustment step of each resonance length is 0.05mm to 0.5mm, and when the axial vibration mode natural frequency f1 is within the range of [f-450Hz, f+450Hz], the adjustment step of each resonance length is 0.01mm to 0.05mm.

[0079] Through the above design method, the geometric dimensions of the specimen with cutting plane can be determined and the specimen design can be completed.

[0080] The above process is described in detail below by taking nickel-based alloy as an example.

[0081] Step 1: Determine the elastic modulus E of the nickel-based alloy material d =199.9GPa and density ρ = 8240kg / m 3 .

[0082] Step 2: Determine the design frequency f according to the operating frequency of the fatigue testing machine, and preliminarily design a dog-bone specimen with a fixed frequency close to the design frequency f.

[0083] At the same time, in order to ensure that the arc of the variable cross-section segment is tangent to the test section, the radius of the variable cross-section segment must satisfy: R0 = (L1 2 +(R2-R1) 2) / 2(R2-R1). In this embodiment, the working frequency of the fatigue testing machine is 20kHz. Therefore, the design frequency of the specimen determined in this example is f=20kHz, the arc radius of the variable cross-section section is R0=15mm, the cross-sectional radius of the test section is R1=1.5mm, the cross-sectional radius of the resonance section is R2=4.5mm, the test section is L0=5mm, the length of the variable cross-section section is L1=9mm, and the resonance length L2 is determined by the formula The calculated value of L2 = 14.2 mm. Based on the above dimensions, a dog-bone specimen can be preliminarily designed.

[0084] Step 3: Cut out an observation plane with a length a and a width b symmetrically at an angle θ in the test section of the specimen, as shown in Figure 6 and Figure 7 As shown, the angle determined in this example is θ=15°, a=6mm, b=2mm, and the cutting depth h is 0.38mm. Figure 6 and Figure 7 The m5=0.8 in it indicates the screw hole size.

[0085] Step 4: Obtain the axial vibration mode natural frequency f1 of the specimen with the cutting plane through the finite element analysis software, and feedback adjust the resonant length until its axial vibration mode natural frequency f1 falls within the range of [20kHz--20Hz, 20kHz+20Hz], thereby completing the design of the entire specimen.

[0086] At present, there are still many defects in the research on the microscopic quantitative characterization of ultra-high cycle fatigue mechanism of metal materials. Most of the specimens used in the existing ultra-high cycle fatigue test are arc-shaped, and their surfaces are curved, which is not convenient for microscopic quantitative characterization and analysis of ultra-high cycle fatigue. After ultra-high cycle failure, the fatigue failure mechanism of the material cannot be effectively observed, and it is difficult to accurately describe the microscopic fatigue characteristics of the material. In this application, a design method for an ultrasonic fatigue specimen with a smooth observation surface is proposed, and a plane is symmetrically cut out of the test section of the specimen to minimize the cross-sectional area of ​​the cut plane section, and equal maximum tensile and compressive stresses are obtained in the cut plane section, so as to facilitate the observation of the failure mechanism of the material. At the same time, in order to solve the error generated in the process of adjusting the specimen size, the natural frequency of the tensile and compressive vibration mode of the actually manufactured specimen may not meet the frequency requirements of the ultrasonic fatigue testing machine, and the resonant loading cannot be achieved. The present invention uses the finite element correction method of geometric dimensions to correct the resonant length to ensure that the specimen can resonate on the testing machine.

[0087] Correspondingly, the present application also relates to an ultra-high cycle tensile fatigue test piece with a cutting plane, such as Figure 3As shown in the figure, the specimen is a dog-bone specimen with a centrally symmetrical structure and an axial vibration mode natural frequency close to the design frequency f. The dog-bone specimen includes a test section, a variable cross-section section and a cylindrical resonance section from the middle to the sides. The test section is formed by cutting out opposite U-shaped grooves on both sides of the cylinder. The cylindrical cross-section radius R1 of the test section is smaller than the cross-section radius R2 of the resonance section. Specifically, the ratio of the bottom area of ​​the U-shaped groove to the longitudinal cross-section area of ​​the test section is In the range of , the cutting angle θ is in the range of [10°, 30°], and the cutting depth is in the range of In a specific embodiment, the ultra-high cycle tensile fatigue specimen with a cutting plane is a specimen designed using the design method of the ultra-high cycle tensile fatigue specimen with a cutting plane.

[0088] Accordingly, the present application also relates to a method for performing ultra-high cycle tensile fatigue testing on a material, which comprises:

[0089] An ultrasonic fatigue testing machine is used to perform resonant loading on the ultra-high cycle tensile fatigue specimen with a cutting plane until the specimen breaks. The bottom surface of the groove is used as the observation surface to perform microscopic quantitative characterization of the specimen and analyze the failure mechanism of the specimen.

[0090] It will be easily understood by those skilled in the art that the above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A design method for an ultra-high cycle tensile fatigue specimen with a cutting plane, characterized in that: include: A dog-bone specimen with a centrally symmetrical structure and a natural frequency close to the design frequency f is preliminarily designed. The dog-bone specimen includes a cylindrical test section, a variable cross-section section and a cylindrical resonance section from the middle to both sides. The cross-sectional radius R1 of the test section is smaller than the cross-sectional radius R2 of the resonance section. Relative U-shaped grooves are cut inward on both sides of the test section to form a test piece with a cutting plane. The bottom surface of the U-shaped groove is an observation plane parallel to the axial direction, and the side surface is an inclined side surface with a tangent line at the opening at an angle of θ. The test piece with a cutting plane maintains a central symmetrical structure; The axial vibration mode natural frequency f1 of the specimen with the cutting plane is obtained by analyzing with finite element software, and the resonance length L2 is corrected according to the axial vibration mode natural frequency f1 so that the axial vibration mode natural frequency f1 of the specimen with the cutting plane is close to the design frequency f; Among them, when preliminarily designing the dog-bone specimen, determine the length 2L0 and cross-sectional radius R1 of the test section of the dog-bone specimen, the length L1 and arc radius R0 of each variable cross-sectional section, the cross-sectional radius R2 and resonant length L2 of each resonant section, the expected design frequency f, and the resonant length L2 to meet the following requirements: Among them, the intermediate parameter k = ω / c, ω is the circular frequency of the specimen vibration, ω = 2πf, c is the propagation speed of the elastic wave in the material, E d and ρ are the elastic modulus and material density of the specimen material, respectively. The intermediate parameters α is the catenary parameter of the variable cross-section segment, α=cosh -1 (R2 / R1) / L1; The design frequency f is in the range of [F-500Hz, F+500Hz], where F is the operating frequency of the fatigue testing machine.

2. The design method according to claim 1, characterized in that: The resonant length L2 is corrected so that the axial vibration mode natural frequency f1 of the specimen with the cut plane approaches the design frequency f, including: the resonant length L2 is corrected so that the axial vibration mode natural frequency f1 of the specimen with the cut plane is within the expected range [f-20Hz, f+20Hz].

3. The design method according to claim 1, characterized in that: When the axial vibration mode natural frequency f1 is too low, the resonance length L2 of the resonance section is gradually reduced so that the axial vibration mode natural frequency f1 gradually increases and approaches the design frequency f; when the axial vibration mode natural frequency f1 is too high, the resonance length L2 of the resonance section is gradually increased so that the axial vibration mode natural frequency f1 gradually decreases and approaches the design frequency f.

4. The design method according to claim 3, characterized in that: When the axial vibration mode natural frequency f1 is outside the range of [f-450Hz, f+450Hz], the adjustment step size of the resonance length is 0.05mm~0.5mm each time; when the axial vibration mode natural frequency f1 is within the range of [f-450Hz, f+450Hz], the adjustment step size of the resonance length is 0.01mm~0.05mm each time.

5. The design method according to claim 1, characterized in that: The ratio of the bottom area of ​​the U-shaped groove to the longitudinal cross-sectional area of ​​the test section is In the range of , the cutting angle θ is in the range of [10°, 30°], and the cutting depth is in the range of 6. An ultra-high cycle tensile fatigue specimen with a cutting plane designed based on the design method according to any one of claims 1 to 5, characterized in that: The specimen is a dog-bone-type specimen with a centrally symmetrical structure and an axial vibration mode natural frequency close to the design frequency f. The dog-bone-type specimen includes a test section, a variable cross-section section and a cylindrical resonance section from the middle to both sides. The test section is formed by cutting relative U-shaped grooves inward on both sides of the cylinder. The bottom surface of the U-shaped groove is an observation plane parallel to the axial direction, and the side surface is an inclined side surface with a tangent at the opening at an angle θ. The cylindrical cross-sectional radius R1 of the test section is smaller than the cross-sectional radius R2 of the resonance section.

7. The ultra-high cycle tensile fatigue test piece with a cutting plane according to claim 6, characterized in that: The ratio of the bottom area of ​​the U-shaped groove to the longitudinal section area of ​​the test section is In the range of , the cutting angle θ is in the range of [10°, 30°], and the cutting depth is in the range of

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