A method for measuring internal crack propagation rate of visual ultra-high cycle fatigue

Abstract

The present application relates to a kind of visual determination method of ultra-high cycle fatigue internal crack propagation rate, belong to the technical field of fatigue internal crack propagation rate determination, the present application utilizes the heat dissipation behavior of local plastic deformation and cracking of metal material, the temperature field generated by energy dissipation is recorded using high-speed infrared camera to visualize the process of crack propagation, precise positioning of metal material fatigue crack initiation point and in-situ monitoring of crack propagation process are realized, and crack propagation rate is obtained by introducing propagation time and load cycle frequency to calculate.

Description

Technical Field

[0001] This invention relates to the technical field of fatigue internal crack propagation rate determination, and specifically to a visualized method for determining the internal crack propagation rate of ultra-high cycle fatigue. Background Technology

[0002] Structures and components in aerospace, defense, and automotive industries often endure long-cycle fatigue loads. Statistics on failure cases of all parts in jet aircraft show that fatigue failure accounts for up to 49% of all parts during service. For critical components in aero-engines, such as turbine blades and compressor disks, the fatigue cyclic loads they endure far exceed 10 cycles. 7 The cycle count is the standard for traditional high-cycle fatigue, and the US Engine Structural Integrity Guidelines stipulated as early as the last century that the service life of components used in aero engines should reach 10 years. 9 More than 10 weeks, these 10 7 ~10 9 It has entered the realm of ultra-high cycle fatigue, making reliability research on ultra-long life fatigue essential.

[0003] Numerous existing studies have shown that during ultra-high cycle fatigue, crack initiation and propagation occur within the material, with the initial initiation stage occurring only at the micrometer scale, making visualization and precise non-destructive testing difficult. Methods relying on large scientific facilities such as synchrotron radiation and neutron diffraction to scan and reconstruct the material's interior are also difficult to perform in situ, costly, inefficient, and impractical for engineering applications, as they cannot detect the dynamic process of crack propagation. This difficulty in accurately locating and monitoring internal cracks in long-life components significantly reduces structural reliability and may pose significant safety hazards to service components.

[0004] The ASTM-E647 standard recommends three methods for measuring fatigue crack size: visual inspection, compliance testing, and potential difference testing. However, the characteristics of internally initiated cracks in ultra-high cycle fatigue render these conventional crack length measurement methods unusable. The inventors previously proposed a method to infer crack propagation rate from fracture surface markings, which could obtain the propagation rate of internal cracks. However, this method only estimates the rate based on post-fracture morphology and cannot provide real-time damage information or detect the propagation behavior of small cracks in the early stages of fatigue damage. Therefore, there is an urgent need for a method for measuring crack propagation rate that is accurate, directly recorded, and convenient to use. Summary of the Invention

[0005] In view of the above problems, this invention proposes a method for visually measuring the internal crack propagation rate of ultra-high cycle fatigue. It utilizes the heat dissipation behavior of local plastic deformation and cracking in metallic materials, and applies a high-speed infrared camera to record the temperature field generated by energy dissipation, thereby visualizing the crack propagation process. This enables precise positioning of the fatigue crack initiation point in metallic materials and in-situ monitoring of the crack propagation process. The crack propagation rate is obtained by introducing propagation time and load cycle frequency.

[0006] This invention provides a method for visually determining the internal crack propagation rate during ultra-high cycle fatigue, comprising:

[0007] S1. Process the fatigue specimen to be tested; the fatigue specimen to be tested is a variable cross-section plate-shaped ultrasonic fatigue specimen.

[0008] Preferably, the shape and size of the fatigue test specimen in step S1 are designed based on the dynamic elastic modulus and density of the fatigue test specimen.

[0009] For example, the fatigue test specimen is processed using an electrical discharge wire cutting machine.

[0010] The dimensions of the variable cross-section plate-shaped ultrasonic fatigue specimen described in step S1 of this invention meet the requirements of ultrasonic resonance, and at the same time facilitate high-precision infrared monitoring of the initiation of fatigue cracks.

[0011] For example, Ti-6Al-4V alloy prepared by additive manufacturing was selected and placed in a heat treatment furnace for 2 hours at 600°C to obtain the fatigue sample to be tested.

[0012] The dynamic elastic modulus of the fatigue specimen under test is 96.455 GPa;

[0013] The density of the fatigue test specimen is 4.38 g / cm³. 3 .

[0014] For example, the ultrasonic resonance frequency of the ultrasonic fatigue testing machine is 20kHz;

[0015] S2. The surface of the fatigue test specimen is subjected to secondary grinding and coating treatment to obtain the treated specimen.

[0016] Preferably, the specific steps for obtaining the treated sample in step S2 include:

[0017] The fatigue test specimen is subjected to surface mechanical grinding and electrolytic polishing treatment, and a matte coating is deposited evenly and smoothly on the surface of the treated fatigue test specimen to obtain the treated specimen.

[0018] Furthermore, the infrared emissivity of the matte coating is 0.9 to 1;

[0019] For example, step S2 involves secondary polishing and coating deposition on the surface of the fatigue test specimen to obtain the treated specimen. The specific steps include:

[0020] Step S21: Apply a CNC grinding machine to perform the first polishing on the surface of the fatigue sample to be tested; the roughness of the CNC grinding machine is Ra<0.4;

[0021] Step S22: Use 3000-grit sandpaper and diamond polishing compound with a particle size of 0.25μm to perform a second polishing on the surface of the fatigue sample to be tested after the first polishing;

[0022] Step S23: Place the fatigue test sample after the second polishing into an alcohol bath containing polishing solution;

[0023] It is understood that the temperature of the alcohol bath is -19 to 21°C;

[0024] The polishing solution comprises 5.5-6.5% perchloric acid, 58.5-59.5% methanol, and 34.5-35.5% n-butanol;

[0025] Step S24: After the temperature of -19 to -21℃ has stabilized, connect the cleaned sample to the anode of the DC regulated power supply and connect a copper sheet to the cathode of the DC regulated power supply. Place them together in the polishing liquid and simultaneously place a magnetic rotor that continuously stirs the polishing liquid.

[0026] Step S25: Electrolytic polishing is performed for 30 seconds. After polishing, the sample is cleaned in pure alcohol and dried quickly to obtain the cleaned sample.

[0027] Step S26: Deposit a layer of matte water-based paint on the cleaned sample surface using an atomizing device to obtain the treated sample.

[0028] In this invention, the complex phase interface formed by the multiple resin components of the matte water-based paint creates a micro-rough surface that strongly diffuses light after the coating is cured. This increases the emissivity of the sample surface and avoids the reflection of infrared rays by the metal, thus preventing it from affecting the monitoring quality.

[0029] S3. Before the fatigue test, the emissivity of the coating deposited on the surface of the treated sample is measured to obtain the coating emissivity.

[0030] Preferably, in step S3, a thermocouple and a blackbody-calibrated infrared camera are used to measure the emissivity of the coating deposited on the surface of the treated sample to obtain the coating emissivity.

[0031] For example, the processed sample is placed in an induction coil, a type K thermocouple is selected for temperature measurement, and a spot welding machine is used to fix the thermocouple on the sample surface under a welding current of 60A.

[0032] Simultaneously, a high-precision infrared camera calibrated by a blackbody is used for synchronous temperature measurement.

[0033] Furthermore, when the temperature measured by the thermocouple reaches the test temperature of 200℃, the emissivity of the infrared camera temperature sensor is adjusted. When the emissivity is 0.95, the temperature measured by the infrared temperature sensor is the same as the temperature measured by the thermocouple, thus obtaining the accurate emissivity of the coated sample.

[0034] S4. Install the treated sample on the ultrasonic fatigue testing machine, set up the infrared monitoring device, and input the coating emissivity measured in step S3 into the infrared detection device;

[0035] It is understood that the infrared monitoring device is connected to a computer and uses a high frame rate mode to record video and images of the temperature field on the surface of the fatigue specimen changing over time during the fatigue loading process.

[0036] Preferably, the infrared monitoring device in step S4 is a high-speed, high-resolution infrared camera;

[0037] Furthermore, the lens of the infrared camera is aligned with the working plane of the fatigue test specimen, kept parallel to each other, and focused to allow clear observation of the temperature field distribution on the surface of the fatigue test specimen.

[0038] For example, the specimen is mounted on an ultrasonic fatigue testing machine, a high-resolution high-speed infrared camera is set up with its lens aligned with the working section of the specimen, and a high-temperature resistant macro lens with a pixel resolution of 640×480 is installed, which is kept parallel to the plane of the working section and at a distance of about 2cm.

[0039] Focus the infrared camera or autofocus until the initial temperature field on the sample surface can be clearly monitored, and input an infrared emissivity of 0.95;

[0040] Connect an infrared camera to a computer and use a high frame rate video mode of 30 frames per second to record video and images of the temperature field on the sample surface changing over time during the loading process.

[0041] S5. Set the stress ratio and stress amplitude, start the fatigue test, and simultaneously turn on the infrared monitoring device to record in real time and obtain the video file of the infrared monitoring.

[0042] The infrared monitoring video file includes the temperature field formed by local plastic deformation and heat dissipation of cracking of the processed sample, the temperature field changes, and the dynamic process of crack propagation.

[0043] The dynamic process of crack propagation is the dynamic process of crack propagation observed by locating the crack tip through temperature field changes.

[0044] S6. Based on the video file from infrared monitoring, obtain the length and time of crack propagation in the sample;

[0045] The length of the crack propagation in the sample is characterized as the crack propagation rate, expressed as:

[0046]

[0047] Among them, da i / dt is the crack propagation rate in the i-th frame, Δa i Let a be the length of the crack propagation in the i-th frame, Δt be the unit time, and a be the length of the crack propagation in the i-th frame. i Let be the crack length detected by infrared in the i-th frame.

[0048] Based on the crack propagation rate, the crack propagation rate da based on the number of fatigue cycles is obtained. i / dN is:

[0049]

[0050] Where N is the number of fatigue cycles and f is the frequency of the fatigue cycle;

[0051] The present invention further includes step S7, based on the crack propagation rate da according to the fatigue cycle number. i / dN and stress intensity factor ΔK i The relationship between crack propagation rate and stress intensity factor was established.

[0052] The stress intensity factor ΔK i The expression is:

[0053]

[0054] Where C is the crack location parameter, σ a This refers to fatigue cyclic stress.

[0055] Compared with the prior art, the present invention has at least the following beneficial effects:

[0056] (1) This invention enables visualization and in-situ monitoring of internal fatigue cracks that are difficult to detect in real time under ultra-long life conditions. It can not only accurately locate the initiation location of internal cracks, but also record the crack propagation process in detail. The data obtained is rich and accurate, and can detect the tiny crack changes and temperature field changes of a few micrometers in the early stage of crack initiation.

[0057] (2) The monitoring device of this invention does not contact the fatigue specimen, has no impact on the fatigue testing equipment, and is convenient to install and debug, saving time and economic costs. It has a fast response time and excellent recording accuracy, and the output data is reliable;

[0058] (3) The method of this invention has strong generalization ability and can be extended to crack propagation rate testing of various materials under various fatigue conditions, and is not limited to ultrasonic fatigue and internal crack monitoring of this invention. Surface crack monitoring, other types of specimens, and other fatigue test frequencies are also applicable to the method proposed in this invention. Attached Figure Description

[0059] The accompanying drawings are for illustrative purposes only and are not intended to limit the scope of the invention.

[0060] Figure 1 This is a flowchart illustrating the visualized internal crack propagation rate during ultra-high cycle fatigue, as disclosed in an embodiment of the present invention.

[0061] Figure 2 This is a schematic diagram of the ultrasonic fatigue specimen size and infrared monitoring area disclosed in the embodiments of the present invention;

[0062] Figure 3 This is a schematic diagram of the crack propagation behavior results obtained by infrared monitoring in an embodiment of the present invention;

[0063] Figure 4 This is a schematic diagram of the relationship between crack propagation rate and stress intensity factor disclosed in an embodiment of the present invention. Detailed Implementation

[0064] To better understand the above-described objectives, features, and advantages of the present invention, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments of the present invention and the features thereof can be combined with each other. Furthermore, the present invention can be implemented in other ways different from those described herein; therefore, the scope of protection of the present invention is not limited to the specific embodiments disclosed below.

[0065] A specific embodiment of the present invention, such as Figure 1-4 This invention provides a method for visually determining the internal crack propagation rate during ultra-high cycle fatigue. To illustrate the effectiveness of the method proposed in this invention, a specific embodiment is provided below for detailed explanation of the above-mentioned technical solution. The specific implementation steps are as follows:

[0066] S1. Process the fatigue specimen to be tested; the fatigue specimen to be tested is a variable cross-section plate-shaped ultrasonic fatigue specimen.

[0067] Preferably, the shape and size of the fatigue test specimen in step S1 are designed based on the dynamic elastic modulus and density of the fatigue test specimen.

[0068] For example, the fatigue test specimen is processed using an electrical discharge wire cutting machine.

[0069] The dimensions of the variable cross-section plate-shaped ultrasonic fatigue specimen described in step S1 of this invention meet the requirements of ultrasonic resonance, and at the same time facilitate high-precision infrared monitoring of the initiation of fatigue cracks.

[0070] For example, Ti-6Al-4V alloy prepared by additive manufacturing was selected and placed in a heat treatment furnace for 2 hours at 600°C to obtain the fatigue sample to be tested.

[0071] The dynamic elastic modulus of the fatigue specimen under test is 96.455 GPa;

[0072] The density of the fatigue test specimen is 4.38 g / cm³. 3 .

[0073] For example, the ultrasonic resonance frequency of the ultrasonic fatigue testing machine is 20kHz;

[0074] S2. The surface of the fatigue test specimen is subjected to secondary grinding and coating treatment to obtain the treated specimen.

[0075] Preferably, the specific steps for obtaining the treated sample in step S2 include:

[0076] The fatigue test specimen is subjected to surface mechanical grinding and electrolytic polishing treatment, and a matte coating is deposited evenly and smoothly on the surface of the treated fatigue test specimen to obtain the treated specimen.

[0077] Furthermore, the infrared emissivity of the matte coating is 0.9 to 1;

[0078] For example, step S2 involves secondary polishing and coating deposition on the surface of the fatigue test specimen to obtain the treated specimen. The specific steps include:

[0079] Step S21: Apply a CNC grinding machine to perform the first polishing on the surface of the fatigue sample to be tested; the roughness of the CNC grinding machine is Ra<0.4;

[0080] Step S22: Use 3000-grit sandpaper and diamond polishing compound with a particle size of 0.25μm to perform a second polishing on the surface of the fatigue sample to be tested after the first polishing;

[0081] Step S23: Place the fatigue test sample after the second polishing into an alcohol bath containing polishing solution;

[0082] It is understood that the temperature of the alcohol bath is -19 to 21°C;

[0083] The polishing solution comprises 5.5±6.5% perchloric acid, 58.5-59.5% methanol, and 34.5-35.5% n-butanol;

[0084] Step S24: After the temperature of -19 to -21℃ has stabilized, connect the cleaned sample to the anode of the DC regulated power supply and connect a copper sheet to the cathode of the DC regulated power supply. Place them together in the polishing liquid and simultaneously place a magnetic rotor that continuously stirs the polishing liquid.

[0085] Step S25: Electrolytic polishing is performed for 30 seconds. After polishing, the sample is cleaned in pure alcohol and dried quickly to obtain the cleaned sample.

[0086] Step S26: Deposit a layer of matte water-based paint on the cleaned sample surface using an atomizing device to obtain the treated sample.

[0087] In this invention, the complex phase interface formed by the multiple resin components of the matte water-based paint creates a micro-rough surface that strongly diffuses light after the coating is cured. This increases the emissivity of the sample surface and avoids the reflection of infrared rays by the metal, thus preventing it from affecting the monitoring quality.

[0088] S3. Before the fatigue test, the emissivity of the coating deposited on the surface of the treated sample is measured to obtain the coating emissivity.

[0089] Preferably, in step S3, a thermocouple and a blackbody-calibrated infrared camera are used to measure the emissivity of the coating deposited on the surface of the treated sample to obtain the coating emissivity.

[0090] For example, the processed sample is placed in an induction coil, a type K thermocouple is selected for temperature measurement, and a spot welding machine is used to fix the thermocouple on the sample surface under a welding current of 60A.

[0091] Simultaneously, a high-precision infrared camera calibrated by a blackbody is used for synchronous temperature measurement.

[0092] Furthermore, when the temperature measured by the thermocouple reaches the test temperature of 200℃, the emissivity of the infrared camera temperature sensor is adjusted. When the emissivity is 0.95, the temperature measured by the infrared temperature sensor is the same as the temperature measured by the thermocouple, thus obtaining the accurate emissivity of the coated sample.

[0093] S4. Install the treated sample on the ultrasonic fatigue testing machine, set up the infrared monitoring device, and input the coating emissivity measured in step S3 into the infrared detection device;

[0094] It is understood that the infrared monitoring device is connected to a computer and uses a high frame rate mode to record video and images of the temperature field on the surface of the fatigue specimen changing over time during the fatigue loading process.

[0095] Preferably, the infrared monitoring device in step S4 is a high-speed, high-resolution infrared camera;

[0096] Furthermore, the lens of the infrared camera is aligned with the working plane of the fatigue test specimen, kept parallel to each other, and focused to allow clear observation of the temperature field distribution on the surface of the fatigue test specimen.

[0097] For example, the specimen is mounted on an ultrasonic fatigue testing machine, a high-resolution high-speed infrared camera is set up with its lens aligned with the working section of the specimen, and a high-temperature resistant macro lens with a pixel resolution of 640×480 is installed, which is kept parallel to the plane of the working section and at a distance of about 2cm.

[0098] Focus the infrared camera or autofocus until the initial temperature field on the sample surface can be clearly monitored, and input an infrared emissivity of 0.95;

[0099] Connect an infrared camera to a computer and use a high frame rate video mode of 30 frames per second to record video and images of the temperature field on the sample surface changing over time during the loading process.

[0100] S5. Set the stress ratio and stress amplitude, start the fatigue test, and simultaneously turn on the infrared monitoring device to record in real time and obtain the video file of the infrared monitoring.

[0101] The infrared monitoring video file includes the temperature field formed by local plastic deformation and heat dissipation of cracking of the processed sample, the temperature field changes, and the dynamic process of crack propagation.

[0102] The dynamic process of crack propagation is the dynamic process of crack propagation observed by locating the crack tip through temperature field changes.

[0103] For example, a sinusoidal load spectrum with a stress ratio of -1 and a stress amplitude of 325MPa is set on the computer terminal controlling the ultrasonic fatigue testing machine to start the fatigue test; at the same time, a high-speed infrared camera is activated to synchronously record the heat dissipation temperature field on the sample surface due to local deformation and cracking inside the material, and the dynamic process of crack propagation is recorded by the change and movement of the temperature field representing the crack tip.

[0104] Figure 3 The image shows 24 frames of images of the temperature field changing with crack propagation during ultrasonic fatigue of a specimen with a stress amplitude of 325 MPa. The point where the temperature field peaks compared to the previous frame represents the plastic zone at the crack tip where heat dissipation is greatest, and the movement of the peak temperature point represents the propagation of the crack.

[0105] S6. Based on the video file from infrared monitoring, obtain the length and time of crack propagation in the sample;

[0106] The length of the crack propagation in the sample is characterized as the crack propagation rate, expressed as:

[0107]

[0108] Among them, da i / dt is the crack propagation rate in the i-th frame, Δa i Let a be the length of the crack propagation in the i-th frame, Δt be the unit time, and a be the length of the crack propagation in the i-th frame. i Let be the crack length detected by infrared in the i-th frame.

[0109] Based on the crack propagation rate, the crack propagation rate da based on the number of fatigue cycles is obtained. i / dN is:

[0110]

[0111] Where N is the number of fatigue cycles and f is the frequency of the fatigue cycle;

[0112] The present invention further includes step S7, based on the crack propagation rate da according to the fatigue cycle number. i / dN and stress intensity factor ΔK i The relationship between crack propagation rate and stress intensity factor was established.

[0113] The stress intensity factor ΔK i The expression is:

[0114]

[0115] Where C is the crack location parameter, σ a This refers to fatigue cyclic stress.

[0116] Table 1 Infrared Monitoring and Calculation Results

[0117]

[0118] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for determining the internal crack growth rate of visualized ultra-high cycle fatigue, characterized by, The method comprises the following steps: S1, processing a fatigue test sample; selecting a Ti-6Al-4V alloy prepared by additive manufacturing, placing it in a heat treatment furnace for 600℃ heat preservation for 2h to obtain a fatigue test sample; The fatigue test sample is a variable cross-section plate-shaped ultrasonic fatigue test sample; the shape and size of the variable cross-section plate-shaped ultrasonic fatigue test sample are designed according to the dynamic elastic modulus and density of the fatigue test sample, and meet the ultrasonic resonance; S2, surface mechanical polishing and electrolytic polishing treatment are performed on the fatigue test sample, a matte coating is uniformly deposited on the surface of the treated fatigue test sample, and a treated sample is obtained; the ultrasonic resonance frequency of the ultrasonic fatigue test machine is 20kHz; The infrared emissivity of the matte coating is 0.9-1; S3, before the fatigue test, the emissivity of the coating deposited on the surface of the treated sample is measured by using a thermocouple and an infrared camera calibrated by a black body, When the temperature measured by the thermocouple reaches the test temperature of 200℃, the emissivity of the infrared camera temperature sensor is adjusted, when the emissivity is 0.95, the temperature measured by the infrared temperature sensor is the same as the temperature measured by the thermocouple, and the accurate emissivity of the sample with the coating is obtained; S4, the treated sample is installed on the ultrasonic fatigue test machine, the infrared monitoring device is erected, and the coating emissivity obtained in step S3 is introduced into the infrared monitoring device; S5, set the stress ratio and stress amplitude, start the fatigue test, and simultaneously start the infrared monitoring device to record in real time, and obtain the infrared monitoring video file; The infrared monitoring video file includes the temperature field formed by the heat dissipation of the local plastic deformation and cracking of the treated sample, the temperature field change, and the dynamic process of crack propagation; S6, based on the infrared monitoring video file, the length and time of crack propagation of the treated sample are obtained; The length of crack propagation of the sample is characterized as the crack propagation rate; According to the crack propagation rate, the crack propagation rate based on the fatigue cycle number is obtained; The expression of the crack propagation rate based on the fatigue cycle number is: （2） wherein, is the number of cycles to failure, i is the length of the crack at the nth frame, f is the frequency of the fatigue cycles; is the length of the crack at the nth frame, i is the length of the crack propagation at the nth frame, is the unit of time; Step S7, according to the crack propagation rate based on the fatigue cycle number and the stress intensity factor, the relationship between the crack propagation rate and the stress intensity factor is established; The stress intensity factor The expression for the stress intensity factor is: （3） where C is a location parameter of the crack, is the fatigue cycle stress.

2. The method for measuring the internal crack propagation rate of ultrahigh cycle fatigue according to claim 1, wherein, Step S2, the surface of the fatigue test sample is polished twice and a coating is deposited, and the specific steps of the treated sample include: Step S21, a numerical control grinding machine is used to polish the surface of the fatigue test sample for the first time; Step S22, sandpaper and diamond polishing agent are used to polish the surface of the fatigue test sample for the second time; Step S23, the fatigue test sample polished for the second time is placed in an alcohol bath with polishing liquid; Step S24, after the temperature is constant, the cleaned sample is connected to the anode of a direct current stabilized power supply, a copper sheet is connected to the cathode of the direct current stabilized power supply, and they are placed in the polishing liquid together, and a magnetic rotor for continuously stirring the polishing liquid is also placed in the polishing liquid. Step S25, power on electrolytic polishing, after polishing, cleaning in pure alcohol and quick drying, get cleaned sample; Step S26, a layer of matte water-based paint is deposited on the surface of the cleaned sample to obtain a treated sample.

3. The method of claim 1, wherein the dynamic process of crack propagation is observed by temperature field change positioning crack tip. The dynamic process of crack propagation is observed by temperature field change positioning crack tip.

Images