Titanium alloy fatigue crack tip plastic zone size measuring method based on DIC

Abstract

The invention discloses a DIC-based titanium alloy fatigue crack tip plastic zone size measurement method. The method comprises the following steps: 1, material constitutive calibration; 2, calibrating fatigue characteristics; 3, carrying out DIC full-field strain analysis; 4, intelligent judgment of a plastic zone; and 5, reconstructing the size of the plastic zone. According to the method, a DIC full-field strain analysis technology is deeply fused with static and dynamic constitutive characteristics of a material, in-situ, dynamic and separated quantitative characterization of a monotonic plastic zone and a cyclic plastic zone of a crack tip under a fatigue load is realized for the first time, and accurate data support is provided for fatigue life prediction and damage tolerance design of titanium alloy.

Description

Technical Field

[0001] This invention belongs to the field of material fatigue performance testing and fracture mechanics analysis technology, and in particular relates to a method for determining the size of the plastic zone at the tip of fatigue cracks in titanium alloys based on DIC. Background Technology

[0002] Titanium alloys, due to their superior comprehensive properties such as high specific strength, excellent corrosion resistance, and good high-temperature stability, have become core structural materials in key fields such as marine engineering and biomedicine. These applications place extremely stringent requirements on the lightweight, long service life, and high reliability of equipment. Against this backdrop, the fatigue failure problem of structural materials, especially the propagation behavior of fatigue cracks, directly determines the service life and operational safety of the entire component and even the entire equipment system.

[0003] During fatigue crack propagation, a microscopic region undergoing plastic deformation—the crack tip plastic zone—occurs at the crack tip due to high stress concentration. This region is the core area for energy dissipation, microstructural evolution, and crack driving force; its size, morphology, and distribution characteristics fundamentally control the crack propagation rate and path selection. Therefore, accurate measurement and characterization of the parameters of this plastic zone are crucial for a deep understanding of the fatigue fracture mechanism of materials, the establishment of accurate prediction models, and ultimately, the realization of damage-tolerant structural design.

[0004] However, for a long time, the measurement of this plastic zone has mainly relied on traditional metallographic sectioning methods and theoretical estimation models based on ideal assumptions (such as the Irwin model). These traditional methods have a series of inherent defects that are difficult to overcome. For example, metallographic methods require cutting, polishing, and etching of the sample, which completely destroys the sample. They cannot continuously track the dynamic evolution of the plastic zone under alternating loads in real time and in situ, and can only obtain a "snapshot" of a static moment in the load history. Traditional methods cannot effectively distinguish between different types of plastic zones: under fatigue loads, both monotonic plastic zones and cyclic plastic zones are generated simultaneously, and traditional methods are unable to accurately separate and quantitatively characterize them. Theoretical models are usually based on the small-scale yield assumption of continuum mechanics, and fail to fully consider the material's true cyclic constitutive response, anisotropy, and microstructure evolution, resulting in significant deviations between the predicted results and the actual situation, thus limiting their engineering guidance value.

[0005] These limitations severely restrict a deep understanding and technological breakthroughs in the fatigue fracture behavior of materials. Therefore, developing a new method for measuring the plastic zone at the crack tip that can achieve in-situ, dynamic, full-field, non-contact measurement and accurately reflect the constitutive properties of materials has become a major technical challenge that urgently needs to be addressed in the fields of fracture mechanics and materials testing. Summary of the Invention

[0006] The technical problem to be solved by this invention is to address the shortcomings of the prior art by providing a method for measuring the size of the plastic zone at the fatigue crack tip of titanium alloys based on DIC. By deeply integrating DIC full-field strain analysis technology with the static and dynamic constitutive properties of materials, this invention achieves for the first time in-situ, dynamic, and separate quantitative characterization of the monotonic plastic zone and cyclic plastic zone at the crack tip under fatigue load, providing accurate data support for fatigue life prediction and damage tolerance design of titanium alloys.

[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is: a method for determining the size of the plastic zone at the tip of fatigue cracks in titanium alloys based on DIC, the method comprising the following steps: Step 1: Constitutive calibration of materials: Obtain the true stress-strain curve of titanium alloy through static tensile test to determine the equivalent yield strain; Step 2, Fatigue Characteristic Calibration: Determine the equivalent yield cyclic strain under fatigue loading conditions based on the SN curve; Step 3: DIC full-field strain analysis: The evolution of the strain field at the crack tip under fatigue load is captured in real time using dual-wavelength speckle and sub-pixel displacement algorithms; Step 4: Intelligent determination of the plastic zone: The monotonic plastic zone and the cyclic plastic zone are distinguished by a strain threshold criterion fusion algorithm; Step 5: Reconstruct the size of the plastic zone: Calculate the radius of the equivalent circle of the plastic zone based on the DIC data.

[0008] Furthermore, in step one, based on the actual stress-strain curve and the engineering stress-strain curve, the material parameters of the titanium alloy are obtained. These material parameters include the elastic modulus, yield stress, and strain hardening index, according to the formula... Calculate its equivalent yield strain, and control the accuracy of the equivalent yield strain measurement within ±0.02%; among which, For equivalent yield strain, For elastic yield strain, For plastic yield strain, For yield stress, For elastic modulus, Intensity factor The strain hardening index is denoted as .

[0009] Furthermore, in step two, the stress level for subsequent fatigue crack propagation tests is determined based on the SN curve, i.e. Fatigue tests under strain control were conducted to determine the cyclic stress-strain curves of titanium alloy materials at a specific number of cycles, based on the formula... The equivalent yield cyclic strain was obtained, and the accuracy of the measurement of the equivalent yield cyclic strain was controlled within ±0.02%; where the specific number of cycles refers to the half-life of the titanium alloy material. For equivalent yield cyclic strain, For elastic yield cyclic strain, For plastic yield cyclic strain, For equivalent yield cyclic stress, Cyclic intensity factor It is the equivalent yield cyclic strain hardening index.

[0010] Furthermore, in step three, the dual-wavelength speckle pattern uses simultaneous illumination of 470nm blue light and 850nm infrared light; the sub-pixel displacement algorithm achieves a displacement resolution of 0.01 pixels; and the image acquisition frequency is 10 times the load frequency.

[0011] Furthermore, in step four, the strain threshold criterion fusion algorithm includes setting a monotonic plastic zone determination threshold based on static equivalent yield strain and setting a cyclic plastic zone determination threshold based on equivalent yield cyclic strain.

[0012] Furthermore, in step five, the morphology of the plastic region is binarized using Image-Pro Plus software; the formula for calculating the equivalent circle radius is then given. ,in, Let the radius be the equivalent circle radius of the plastic zone. This represents the actual area of ​​the plastic zone.

[0013] Compared with the prior art, the present invention has the following advantages: 1. This invention deeply integrates DIC full-field strain analysis technology with the static and dynamic constitutive properties of materials, and for the first time realizes in-situ, dynamic, and separate quantitative characterization of the monotonic plastic zone and cyclic plastic zone at the crack tip under fatigue load, providing accurate data support for fatigue life prediction and damage tolerance design of titanium alloys.

[0014] 2. This invention uses DIC technology to accurately measure the size of the plastic zone at the tip of fatigue cracks in titanium alloys without requiring destructive treatments such as cutting, polishing, or etching, thus maintaining the integrity of the sample and allowing continuous, in-situ monitoring of the entire fatigue crack propagation process of the same sample. This not only significantly saves time and cost, but more importantly, it provides continuous data on the dynamic evolution of the plastic zone during crack propagation.

[0015] 3. The DIC technology used in this invention can acquire displacement and strain information of tens of thousands of points within the entire field of view of the crack tip, rather than data of a single point; it can accurately capture the complex two-dimensional / three-dimensional distribution of the non-uniform and asymmetric plastic zone, avoiding huge errors caused by improper selection of measurement point positions, and fully revealing the true shape and size of the plastic zone.

[0016] 4. This invention requires only one DIC system (including camera, speckle and software) and fatigue testing machine, without the need for other special and expensive dedicated equipment. Moreover, the testing process can be carried out simultaneously with conventional fatigue testing. A single test can obtain multi-dimensional data such as crack length, propagation rate and plastic zone size, which greatly enriches the database for fatigue crack propagation research and provides a more solid and direct experimental basis for fatigue life prediction, safety assessment and damage tolerance design of titanium alloy structural components.

[0017] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0018] Figure 1 This is a flowchart of the method of the present invention.

[0019] Figure 2 This is the full-field strain diagram of the crack tip based on DIC in this invention.

[0020] Figure 3 This is a diagram showing the distribution of the plastic zone at the tip of fatigue cracks in the titanium alloy of this invention.

[0021] Explanation of reference numerals in the attached figures: 1—Plastic zone. Detailed Implementation

[0022] like Figures 1 to 3 The method shown is for determining the plastic zone size at the tip of fatigue cracks in titanium alloys based on DIC. This method includes the following steps: Step 1: Constitutive calibration of materials: Obtain the true stress-strain curve of titanium alloy through static tensile test to determine the equivalent yield strain; Step 2, Fatigue Characteristic Calibration: Determine the equivalent yield cyclic strain under fatigue loading conditions based on the SN curve; Step 3: DIC full-field strain analysis: The evolution of the strain field at the crack tip under fatigue load is captured in real time using dual-wavelength speckle and sub-pixel displacement algorithms; Step 4: Intelligent determination of the plastic zone: The monotonic plastic zone and the cyclic plastic zone are distinguished by a strain threshold criterion fusion algorithm; Step 5: Reconstruct the size of the plastic zone: Calculate the radius of the equivalent circle of the plastic zone based on the DIC data.

[0023] This invention deeply integrates DIC full-field strain analysis technology with the static and dynamic constitutive properties of materials, and for the first time realizes in-situ, dynamic, and separate quantitative characterization of the monotonic plastic zone and cyclic plastic zone at the crack tip under fatigue load, providing accurate data support for fatigue life prediction and damage tolerance design of titanium alloys.

[0024] This invention enables precise measurement of the plastic zone size at the tip of fatigue cracks in titanium alloys using DIC technology. This eliminates the need for destructive treatments such as cutting, polishing, and etching, preserving the integrity of the sample and allowing for continuous, in-situ monitoring of the entire fatigue crack propagation process for the same sample. This not only significantly saves time and cost but, more importantly, provides continuous data on the dynamic evolution of the plastic zone during crack propagation.

[0025] The DIC technology used in this invention can acquire displacement and strain information of tens of thousands of points within the entire field of view of the crack tip, rather than data of a single point; it can accurately capture the complex two-dimensional / three-dimensional distribution of the non-uniform and asymmetric plastic zone, avoiding huge errors caused by improper selection of measurement point positions, and fully revealing the true shape and size of the plastic zone.

[0026] This invention requires only one DIC system (including camera, speckle and software) and fatigue testing machine, without the need for other special and expensive dedicated equipment. The testing process can be carried out simultaneously with conventional fatigue testing. A single test can obtain multi-dimensional data such as crack length, propagation rate and plastic zone size, which greatly enriches the database for fatigue crack propagation research and provides a more solid and direct experimental basis for fatigue life prediction, safety assessment and damage tolerance design of titanium alloy structural components.

[0027] It should be noted that DIC stands for Digital Image Correlation, a non-contact, full-field deformation measurement technology that uses the similarity of multiple images to measure displacement and strain. It is characterized by its ease of operation and strong environmental adaptability.

[0028] In this embodiment, in step one, the material parameters of the titanium alloy are obtained based on the actual stress-strain curve and the engineering stress-strain curve. These material parameters include the elastic modulus, yield stress, and strain hardening index, according to the formula... Calculate its equivalent yield strain, and control the accuracy of the equivalent yield strain measurement within ±0.02%; among which, For equivalent yield strain, For elastic yield strain, For plastic yield strain, For yield stress, For elastic modulus, Intensity factor The strain hardening index is denoted as .

[0029] In this embodiment, in step two, the stress level for the subsequent fatigue crack propagation test is determined based on the SN curve, i.e. Fatigue tests under strain control were conducted to determine the cyclic stress-strain curves of titanium alloy materials at a specific number of cycles, based on the formula... The equivalent yield cyclic strain was obtained, and the accuracy of the measurement of the equivalent yield cyclic strain was controlled within ±0.02%; where the specific number of cycles refers to the half-life of the titanium alloy material. For equivalent yield cyclic strain, For elastic yield cyclic strain, For plastic yield cyclic strain, For equivalent yield cyclic stress, Cyclic intensity factor It is the equivalent yield cyclic strain hardening index.

[0030] In this embodiment, in step three, the dual-wavelength speckle pattern uses 470nm blue light and 850nm infrared light for simultaneous illumination; the sub-pixel displacement algorithm achieves a displacement resolution of 0.01 pixels; and the image acquisition frequency is 10 times the load frequency.

[0031] In this embodiment, in step four, the strain threshold criterion fusion algorithm includes setting a monotonic plastic zone determination threshold based on static equivalent yield strain and setting a cyclic plastic zone determination threshold based on equivalent yield cyclic strain.

[0032] In this embodiment, in step five, Image-Pro Plus software is used to binarize the morphology of the plastic region; then the formula for calculating the equivalent circle radius is: ,in, Let the radius be the equivalent circle radius of the plastic zone. This represents the actual area of ​​the plastic zone.

[0033] In this embodiment, step one is the foundation for all subsequent quantitative analyses, aiming to obtain the accurate static yield criterion of the tested titanium alloy; the process of determining the equivalent yield strain is as follows: Step 101: Experiment and Data Acquisition; Quasi-static uniaxial tensile tests were performed on a universal testing machine. To match the DIC measurement, a high-contrast speckle field was prepared on the surface of the specimen gauge length. During the test, a high-resolution CCD camera was used to simultaneously acquire a sequence of deformation images of the specimen surface, and the load-displacement data output by the testing machine was recorded. Step 102: Data Processing; The image sequence is processed using DIC analysis software to calculate the true strain distribution across the entire field, and the true stress-strain curve for the uniform deformation stage is extracted. Based on the Hollomon equation... Fit the straight line to obtain the intensity coefficient. and strain hardening index Based on the engineering stress-strain curve, the elastic modulus E and yield stress were obtained using the 0.2% offset method. ; Step 103: Determine the equivalent yield strain; according to the formula To obtain the equivalent yield strain This invention requires that the calibration accuracy of this value be controlled within ±0.02% to ensure the reliability of the subsequent plastic zone boundary criterion.

[0034] Step two aims to obtain the material's yield response under specific cyclic loading, which is used to define the cyclic plastic region; the process of determining the equivalent yield cyclic strain is as follows: Step 201: SN curve testing; Obtain the SN curve of the material through standard fatigue testing, and determine the stress level for subsequent fatigue crack propagation testing based on this curve. ; Step 202: Cyclic equivalent yield strain determination; using strain-controlled fatigue testing, the cyclic stress-strain curve of the material is determined at a specific number of cycles, thereby determining its equivalent strength coefficient under fatigue load. Sum of equivalent hardening index According to the formula To obtain the equivalent yield cyclic strain This parameter represents the threshold value at which a material undergoes macroscopic yielding under cyclic loading, and is a key criterion for distinguishing between the cyclic plastic region and the elastic region.

[0035] Step 3: Dynamic Acquisition and Analysis of DIC Full-Field Strain. This step is the core data acquisition stage, requiring high precision and strong anti-interference capability to capture the transient strain field at the crack tip. The process of dynamic acquisition and analysis of full-field strain is as follows: Step 301: Sample and speckle preparation; A standard fatigue crack propagation CT sample is processed, and a dual-wavelength anti-interference speckle pattern is prepared on the surface of its observation area. This speckle pattern can exhibit high signal-to-noise ratio contrast under illumination by both 470nm blue light and 850nm infrared light, effectively suppressing interference from ambient white light, sample surface reflection, etc., and ensuring image quality in complex experimental environments; Step 302: Experimental system setup; Mount the CT specimen on the MTS servo hydraulic fatigue testing machine. Position the three-dimensional DIC system (including two synchronized high-speed cameras, a dual-wavelength light source controller, and a synchronous triggering device) directly at the specimen observation area; Set the camera sampling frequency to 10 times the fatigue load frequency to ensure that the dynamic changes of the strain field within each load cycle can be clearly captured; Step 303: Data Acquisition; Start the fatigue test and DIC system, and ensure that the load signal and image acquisition are strictly synchronized through the synchronous trigger card; The system continuously records the binocular speckle image at each moment (corresponding to a specific load point) throughout the crack propagation process; Step 304: Strain field calculation; using DIC analysis software (MatchID Stereo), employing the fracture module and crack path detection crack opening data module, the full-field displacement data of each image relative to the reference image is calculated; then, through displacement field differentiation and Green-Lagrange strain tensor calculation, the full-field equivalent strain distribution cloud map is finally obtained.

[0036] Step 4: Intelligent Identification and Separation of Plastic Regions. This step is the core of the methodology of this invention, achieving a leap from "seeing" the strain field to "identifying" specific plastic regions. The process of intelligent identification and separation of plastic regions is as follows: Step 401: Identification of monotonic plastic zone; compare the maximum strain field in each load cycle (usually obtained at the maximum load point) with the equivalent yield strain calibrated in Step 1; preliminarily determine all regions with strain values ​​greater than or equal to the equivalent yield strain as monotonic plastic zones; Step 402: Identify the cyclic plastic zone; calculate the strain amplitude field over a complete load cycle. The region with strain amplitude greater than or equal to the equivalent yield cyclic strain determined in step two is initially identified as the cyclic plastic zone.

[0037] Step 5: Quantification and Reconstruction of Plastic Zone Dimensions. This step completes the final output from the two-dimensional contour to quantitative dimensional indicators. The process of quantifying and reconstructing the plastic zone dimensions is as follows: Step 501: Area Calculation and Equivalence; Import the plastic region contour identified in Step 4 into MATLAB analysis software to accurately calculate its projected area. To facilitate comparison with classical theories (such as the Irwin model), the area of ​​the irregular plastic region is equivalent to a circle of equal area, and its equivalent radius is calculated as the final quantitative index characterizing the size of the plastic region; Step 502: Dynamic output; Repeat the above steps for each or every N cycles of data collected during the fatigue test to output the curves of the dynamic evolution of the size of the monotonic plastic zone and the cyclic plastic zone with the number of cycles / crack length, thereby fully revealing the evolution law of the plastic zone size throughout the entire fatigue life.

[0038] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the present invention. Any simple modifications, alterations, or equivalent structural changes made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

Claims

1. A method for determining the size of the plastic zone at the tip of fatigue cracks in titanium alloys based on DIC, characterized in that, The method includes the following steps: Step 1: Constitutive calibration of materials: Obtain the true stress-strain curve of titanium alloy through static tensile test to determine the equivalent yield strain; Step 2, Fatigue Characteristic Calibration: Determine the equivalent yield cyclic strain under fatigue loading conditions based on the SN curve; Step 3: DIC full-field strain analysis: The evolution of the strain field at the crack tip under fatigue load is captured in real time using dual-wavelength speckle and sub-pixel displacement algorithms; Step 4: Intelligent determination of the plastic zone: The monotonic plastic zone and the cyclic plastic zone are distinguished by a strain threshold criterion fusion algorithm; Step 5: Reconstruct the size of the plastic zone: Calculate the radius of the equivalent circle of the plastic zone based on the DIC data.

2. The method for determining the size of the plastic zone at the tip of fatigue cracks in titanium alloys based on DIC according to claim 1, characterized in that: In step one, based on the actual stress-strain curve and the engineering stress-strain curve, the material parameters of the titanium alloy are obtained. These parameters include the elastic modulus, yield stress, and strain hardening index, according to the formula... Calculate its equivalent yield strain, and control the accuracy of the equivalent yield strain measurement within ±0.02%; among which, For equivalent yield strain, For elastic yield strain, For plastic yield strain, For yield stress, For elastic modulus, Intensity factor The strain hardening index is denoted as .

3. The method for determining the size of the plastic zone at the tip of fatigue cracks in titanium alloys based on DIC according to claim 1, characterized in that: In step two, the stress level for subsequent fatigue crack propagation tests is determined based on the SN curve. Fatigue tests under strain control were conducted to determine the cyclic stress-strain curves of titanium alloy materials at a specific number of cycles, based on the formula... The equivalent yield cyclic strain was obtained, and the accuracy of the measurement of the equivalent yield cyclic strain was controlled within ±0.02%; where the specific number of cycles refers to the half-life of the titanium alloy material. For equivalent yield cyclic strain, For elastic yield cyclic strain, For plastic yield cyclic strain, For equivalent yield cyclic stress, Cyclic intensity factor It is the equivalent yield cyclic strain hardening index.

4. The method for determining the size of the plastic zone at the tip of a fatigue crack in titanium alloy based on DIC according to claim 1, characterized in that: In step three, the dual-wavelength speckle pattern uses simultaneous illumination of 470nm blue light and 850nm infrared light; the sub-pixel displacement algorithm achieves a displacement resolution of 0.01 pixels; and the image acquisition frequency is 10 times the load frequency.

5. The method for determining the size of the plastic zone at the tip of fatigue cracks in titanium alloys based on DIC according to claim 1, characterized in that: In step four, the strain threshold criterion fusion algorithm includes setting a monotonic plastic zone determination threshold based on static equivalent yield strain and setting a cyclic plastic zone determination threshold based on equivalent yield cyclic strain.

6. The method for determining the size of the plastic zone at the tip of a fatigue crack in titanium alloy based on DIC according to claim 1, characterized in that: In step five, the morphology of the plastic region is binarized using Image-Pro Plus software; the formula for calculating the equivalent circle radius is then given. ,in, Let the radius be the equivalent circle radius of the plastic zone. This represents the actual area of ​​the plastic zone.

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