A method for preparing in-situ crack propagation test specimens of welded joints
By using laser-assisted preparation of in-situ crack propagation test specimens of welded joints, the problem of the inability of traditional test methods to decouple the microstructure of welded joints was solved, enabling precise research and model establishment of crack propagation properties of welded joints.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- BEIHANG UNIV
- Filing Date
- 2026-03-18
- Publication Date
- 2026-07-17
AI Technical Summary
Existing technologies are insufficient to accurately study the crack propagation properties of welded joints. Traditional macroscopic mechanical tests cannot decouple the microstructure characteristics of welded joints, and pre-crack methods cannot accurately simulate the actual damage process.
A laser-assisted three-dimensional coordinate calibration and precision loading system was used to prepare in-situ crack propagation test specimens for welded joints. By controlling the length and speed of the generated pre-crack, the crack propagation process was accurately simulated, and dynamic monitoring was carried out in conjunction with high-resolution characterization equipment.
It has achieved accurate monitoring and multi-dimensional characterization of crack propagation damage mechanisms in different regions of welded joints, established a quantitative model of microstructure and crack propagation resistance, and supported the development of damage tolerance design theory.
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Figure CN122409268A_ABST
Abstract
Description
Technical Field
[0001] This invention belongs to the field of aero-engine technology, and in particular relates to a method for preparing an in-situ crack propagation test specimen for a welded joint. Background Technology
[0002] Improving the thrust-to-weight ratio and power-to-weight ratio is one of the main design goals of advanced aero-engines. Welding processes enable aero-engine components to operate safely, reliably, and sustainably in harsh environments with high temperatures, high pressures, corrosion, and complex mechanical loads. Furthermore, they effectively meet the weight reduction requirements of aero-engine structures, with welded structures widely replacing bolt / tenon connections for critical components such as engine disks / shafts and casings. In addition, the application of welded structures simplifies manufacturing processes and improves structural reliability. For materials with good weldability, welded structures often exhibit mechanical properties similar to the base material.
[0003] Under the influence of high-density heat sources such as electric arcs or lasers, materials undergo rapid heating-cooling cycles, forming significant local temperature gradients and leading to inhomogeneities in the microstructure of welded joints. Welded joints are generally divided into a molten zone, a heat-affected zone, and a base metal zone. The grain size, morphology, and texture of these different zones differ significantly, making the analysis of the mechanical behavior of welded joints based on their microstructure extremely complex. Furthermore, the inhomogeneous structural characteristics of welded joints often lead to stress concentration and crack nucleation, thereby affecting the overall structural strength and service life.
[0004] The conventional macroscopic mechanical crack propagation test, widely used in current engineering fields, essentially achieves quantitative characterization of the overall mechanical parameters of materials by preparing macroscopic standard specimens. When the research object involves welded joints, the test design usually adopts a technical solution that integrates the weld structure into the geometry of the standard specimen, aiming to evaluate the approximate mechanical property effect of the weld microstructure on the overall performance of the component through macroscopic mechanical response.
[0005] However, this traditional research method has theoretical limitations when dealing with the unique microstructure of welded joints. At the testing mechanism level, the acquisition of macroscopic mechanical parameters is based on the assumption of uniform deformation across the entire sample. However, welded joints, as typical heterogeneous multiphase structures, encompass multiple characteristic regions with significant performance gradients, including the fusion zone, coarse-grained zone, fine-grained zone, and the base material. The data obtained by traditional testing methods are essentially an averaged representation of the synergistic effects of these heterogeneous structures, failing to decouple the constitutive response of specific microstructures. The test results of such multi-regional composite specimens essentially reflect the macroscopic equivalent properties exhibited after the mixing of heterogeneous structures, rather than the intrinsic mechanical properties of the pure weld structure. More importantly, the strain monitoring accuracy of macroscopic-scale tests is insufficient to capture the local deformation mechanisms of welded joints at the microscale, leading to the inability to effectively observe the evolution of key damage modes such as interface failure and microcrack initiation.
[0006] In-situ mechanical testing, as a core method for cross-scale material behavior research, essentially involves the synchronous integration of a precision loading system and high-resolution characterization equipment. Under controlled load spectra, it enables the simultaneous in-situ verification of the dynamic evolution of microstructure and mechanical response, thereby constructing a correlation model between external mechanical loading and material microstructure evolution. Compared to the limitations of the homogeneous assumptions in traditional macroscopic mechanical testing, in-situ testing, with its dual advantages of micro-region localized loading and nanoscale resolution observation, can directly capture the microscopic damage mechanisms of materials under different loading conditions. Especially for gradient material systems such as welded joints, which include fusion zones, heat-affected zones, and base materials, it can accurately analyze the synergistic / competitive effects of multi-region microstructures during load transfer, and quantitatively establish the mapping relationship between microscopic features such as columnar crystal orientation, residual stress distribution, and interface segregation and macroscopic mechanical properties.
[0007] For the fabrication of in-situ crack propagation test specimens, wire cutting and other methods are commonly used to pre-fabricate cracks on the completed specimens. However, this macroscopic fabrication method is limited by the diameter of the wire cutting wire, and can only produce cracks with straight paths, relatively large widths (minimum 0.1 mm), and rough semi-circular bases. This differs significantly from the tortuous paths and microscopic long cracks formed by the connection of small cracks from grain boundary cracking or intragranular slip during actual material damage. Therefore, to accurately study the crack propagation properties of welded joints, it is necessary to develop new methods for pre-fabricated crack preparation. Summary of the Invention
[0008] This invention proposes a method for preparing in-situ crack propagation test specimens for welded joints, thereby solving the aforementioned technical problems.
[0009] The first aspect of this invention provides a method for preparing an in-situ crack propagation test specimen for a welded joint, the method comprising: Step S1: Determine the spatial distribution and direction of the weld in the blank, and use a laser to perform three-dimensional coordinate calibration on the blank so that the loading direction of the CT sample machine for processing the in-situ crack propagation test specimen of the welded joint is orthogonal to the weld axis, and the fusion line center of the weld structure is located at the tip of the processing notch of the CT sample machine, thus preparing the CT sample. Step S2: Processing pre-cracks on the CT specimen, including: determining the initial stress intensity factor based on the fracture toughness of the base material of the CT specimen; determining the critical threshold of the maximum cyclic load of the CT specimen; and controlling the length and speed of pre-crack generation on the CT specimen based on the critical threshold of the maximum cyclic load and the current stress intensity factor. Step S3: Based on the pre-crack length of the in-situ crack propagation test specimen of the welded joint, an in-situ specimen is cut from the CT specimen; wherein, when cutting the in-situ specimen, the sampling path is perpendicular to the main crack propagation direction of the CT specimen; the pre-crack length of the in-situ crack propagation test specimen of the welded joint is determined based on the intended use. Step S4: Perform fine machining and disturbed polishing on the surface of the in-situ sample to form an in-situ crack propagation test specimen for the welded joint.
[0010] Preferably, in step S2, controlling the length and rate of pre-crack formation on the CT specimen based on the critical threshold of the maximum cyclic load and the current stress intensity factor includes: Step S21: Assign the current stress intensity factor value to the initial stress intensity factor value, assign the maximum load value to the critical threshold of the maximum cyclic load, and initialize the crack length increment to 0; determine the stress ratio based on the current stress intensity factor amplitude, and determine the crack growth rate on the CT specimen according to the stress ratio and the maximum load value; Step S22: If the length of the crack on the CT sample does not reach the preset length, proceed to step S23; otherwise, proceed to step S3. Step S23: When the crack length increment on the CT sample has not reached the preset step size, continue to grow the crack according to the growth rate and monitor the crack length increment in real time until the crack length increment reaches the preset step size. Step S24: Based on the current crack length CT sample width The current stress intensity factor is calculated; the maximum load value is then assigned to the current maximum load value. (1-k), set the crack length increment to 0, determine the stress ratio based on the current stress intensity factor amplitude, determine the crack growth rate on the CT specimen according to the stress ratio and the maximum load value, and proceed to step S22.
[0011] Preferably, the initial stress intensity factor is an empirical value; The current formula for calculating the stress intensity factor is: in, The current stress intensity factor, The load acting on the CT specimen. For CT sample thickness, For CT sample width, The geometric shape factor of the CT sample. This represents the current crack length.
[0012] Preferably, the preset step size is 0.2-0.5 mm, and k≤10%.
[0013] Preferably, during the pre-cracking process of CT specimen processing, multi-stage mechanical control is implemented in accordance with the ASTM E1820 standard specification.
[0014] Preferably, in step S3, the sampling position of the in-situ sample in the CT sample is determined based on the pre-crack length in the in-situ sample.
[0015] The present invention has the following technical effects: (1) The in-situ crack propagation test specimen of the welded joint prepared by the present invention can establish a correlation model between the crack propagation damage mechanism and the microstructure in different regions of the welded structure.
[0016] (2) The in-situ crack propagation test specimen of the welded joint prepared by the present invention facilitates real-time monitoring and multi-dimensional characterization of the pre-crack propagation process in the molten region of the welded joint. It can realize dynamic monitoring of the entire crack propagation process. This multi-scale coupling analysis method can not only accurately determine the crack propagation mode (intergranular / transgranular / mixed type), but more importantly, it can establish a quantitative model between microstructure parameters (such as grain size, precipitate spacing, orientation difference angle) and crack propagation resistance (such as crack closure effect, roughness-induced shielding effect), providing experimental support for the development of damage tolerance design theory based on micromechanism.
[0017] (3) The in-situ crack propagation test specimen of the welded joint prepared by the present invention can accurately distinguish the transgranular fracture tendency of columnar crystals in the fusion zone, the strain localization characteristics of columnar crystals in the heat-affected zone, and the coordinated deformation capacity of equiaxed crystals in the base material. This analysis of the partitioned damage mechanism based on microstructure characteristics provides an irreplaceable experimental basis for establishing a multi-scale constitutive model that considers the interfacial strength gradient and residual stress field, marking the transformation of material failure analysis from phenomenon description to mechanism prediction. Attached Figure Description
[0018] Figure 1 This is a schematic diagram of the process for testing crack propagation in welded joints in the prior art. Figure 2A schematic flowchart illustrating the method for preparing in-situ crack propagation test specimens for welded joints provided by the present invention; Figures 3(A)-3(D) These are schematic diagrams showing the dimensions of the in-situ crack propagation specimens and the specific processing and sampling methods for the welded joints of the present invention. Detailed Implementation
[0019] To make the objectives, technical solutions, and advantages of the embodiments of this disclosure clearer, the technical solutions of the embodiments of this disclosure will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this disclosure, and not all embodiments. Based on the embodiments of this disclosure, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this disclosure.
[0020] Terminology Explanation: SEM: Scanning Electron Microscope, is a commonly used microscopic analysis instrument that uses various physical signals excited by a finely focused electron beam scanning the sample surface to modulate images.
[0021] EBSD: Electron Backscattered Diffraction, is a technique based on scanning electron microscopy that can analyze, visualize, and quantify the microstructure of a sample.
[0022] DIC (Digital Image Correlation) is an optical measurement method used to analyze the deformation of an object's surface. It achieves measurement by tracking changes in a subset of pixels (i.e., small areas of grayscale patterns) during deformation. This technique, with its high precision, non-contact operation, and full-field measurement capabilities, has been widely used in engineering and scientific research.
[0023] TEM: Transmission Electron Microscope, is a type of microscope that uses an accelerated electron beam to image a very thin sample. It produces images by causing electrons to change direction through collisions with atoms in the sample.
[0024] OFDR: Optical Frequency Domain Reflectometry. This technology is a measurement technique that locates scattered signals by measuring the frequency of the Rayleigh scattering signal generated by the modulated probe light. It has extremely high spatial resolution and sensing accuracy.
[0025] like Figure 1As shown, a method for preparing an in-situ crack propagation test specimen for a welded joint is provided, the method comprising: Step S1: Determine the spatial distribution and direction of the weld in the blank, and use a laser to perform three-dimensional coordinate calibration on the blank so that the loading direction of the CT sample machine for processing the in-situ crack propagation test specimen of the welded joint is orthogonal to the weld axis, and the fusion line center of the weld structure is located at the tip of the processing notch of the CT sample machine, thus preparing the CT sample. Step S2: Processing pre-cracks on the CT specimen, including: determining the initial stress intensity factor based on the fracture toughness of the base material of the CT specimen; determining the critical threshold of the maximum cyclic load of the CT specimen; and controlling the length and speed of pre-crack generation on the CT specimen based on the critical threshold of the maximum cyclic load and the current stress intensity factor. Step S3: Based on the pre-crack length of the in-situ crack propagation test specimen of the welded joint, an in-situ specimen is cut from the CT specimen; wherein, when cutting the in-situ specimen, the sampling path is perpendicular to the main crack propagation direction of the CT specimen; the pre-crack length of the in-situ crack propagation test specimen of the welded joint is determined based on the intended use. Step S4: Perform fine machining and disturbed polishing on the surface of the in-situ sample to form an in-situ crack propagation test specimen for the welded joint.
[0026] For welds, inertial friction welding exhibits significant unsteady thermodynamic field characteristics during the energy loading stage. The dynamic plastic flow of the rotating material under the constraint of angular momentum conservation leads to periodic shear-adhesion alternation at the weld interface. This transient thermomechanical coupling effect causes the macroscopic morphology of the weld to deviate from the ideal vertical axis, and the weld surface exhibits a wavy streamline distribution at the microscale. The periodicity of its crests and troughs is strongly correlated with the rotational speed decay curve. In contrast, electron beam welding achieves precise energy deposition through an electron beam with excellent collimation. Its molten pool solidification dynamics are controlled by the geometric parameters of the electron beam scanning path and the anisotropy of material thermal diffusion, ultimately forming a trapezoidal molten zone with self-supporting characteristics in the direction of beam action.
[0027] In this invention, step S1 involves using laser-assisted positioning technology to calibrate the three-dimensional coordinates of the blank based on the spatial distribution characteristics of the weld. During the wire-cutting of the specimen, the weld position and direction are clearly defined, ensuring that the weld axis is orthogonal to the loading direction of the specimen, and that the center of the fusion line is strictly located on the geometric symmetry plane of the CT specimen, i.e., at the tip of the notch in the CT specimen. This embodiment avoids stress distribution distortion caused by the microstructure deviating from the principal strain zone, and eliminates anisotropic interference caused by the angle between the weld direction and the load axis. This ensures that the weld structure is the primary object of study during crack propagation without introducing anisotropic elements.
[0028] For machined notches, the machined notches in CT specimens typically employ a V-shaped or U-shaped symmetrical groove structure, with the opening angle strictly controlled within 60°±2° and the root curvature radius not exceeding 0.25 mm, to ensure a highly concentrated stress field at the crack tip. The specimen thickness-to-width ratio follows the specification 0.5≤B / W≤1.25 (ASTM E399), and the notch length is set to 45%-55% of the specimen width W. This design ensures that the crack propagation path precisely coincides with the load axis, satisfying the plane strain dominance condition. The notch surface is machined with wire EDM or diamond tools to achieve a low roughness of Ra≤1.6 micrometers, forming a sharp initial crack lead edge in conjunction with subsequent fatigue pre-cracking processes. Its core function is to generate a quantifiable stress intensity factor field through geometric constraints, providing a standardized test benchmark for studying the correlation between crack propagation rate and stress intensity factor amplitude. Simultaneously, it guides the crack to propagate stably along a predetermined direction, avoiding crack deflection or bifurcation caused by specimen geometric asymmetry, thereby ensuring the repeatability of fracture toughness test results and the comparability of cross-laboratory data.
[0029] For pre-cracks, small-sized cracks are pre-fabricated at the tip of the machined notch to construct a standardized crack tip morphology that conforms to the assumptions of fracture mechanics theory, ensuring the reliability of experimental data and the applicability of theory.
[0030] Further, in step S2, based on the critical threshold of the maximum cyclic load and the current stress intensity factor, the length and rate of pre-crack formation on the CT specimen are controlled, including: Step S21: Assign the current stress intensity factor value to the initial stress intensity factor value, assign the maximum load value to the critical threshold of the maximum cyclic load, and initialize the crack length increment to 0; determine the stress ratio based on the current stress intensity factor amplitude, and determine the crack growth rate on the CT specimen according to the stress ratio and the maximum load value; Step S22: If the length of the crack on the CT sample does not reach the preset length, proceed to step S23; otherwise, proceed to step S3. Step S23: When the crack length increment on the CT sample has not reached the preset step size, continue to grow the crack according to the growth rate and monitor the crack length increment in real time until the crack length increment reaches the preset step size. Step S24: Based on the current crack length CT sample width The current stress intensity factor is calculated; the maximum load value is then assigned to the current maximum load value. (1-k), set the crack length increment to 0, determine the stress ratio based on the current stress intensity factor amplitude, determine the crack growth rate on the CT specimen according to the stress ratio and the maximum load value, and proceed to step S22.
[0031] Furthermore, the initial stress intensity factor is an empirical value; The current formula for calculating the stress intensity factor is: in, The current stress intensity factor, The load acting on the CT specimen. For CT sample thickness, For CT sample width, The geometric shape factor of the CT sample. This represents the current crack length.
[0032] The preset step size is 0.2-0.5 mm, and k≤10%.
[0033] In this invention, the initial stress intensity factor is an empirical value, that is, a range of values obtained empirically, which needs to be greater than the material's threshold stress intensity factor K. th Less than 0.8 times the material's fracture toughness K IC .
[0034] In step S2, during the pre-crack process of the CT specimen, multi-stage mechanical control is implemented according to the ASTM E1820 standard. First, the CT specimen is clamped within the loading frame of the universal testing machine using a high-precision clamping system, ensuring strict alignment between the specimen's centerline and the loading axis to eliminate additional bending moments caused by eccentric loading. Based on measured values of the fracture toughness of the base material or reference values from literature, the initial stress intensity factor for the pre-crack stage is calculated using linear elastic fracture mechanics theory to determine the critical threshold of the maximum cyclic load Pmax.
[0035] In the crack propagation control stage, the K-reduction method is used to achieve a gradient reduction of the crack propagation driving force. Specifically, by monitoring the crack length increment in real time, when the crack propagation reaches a preset step size (typically 0.2-0.5 mm), the stress intensity factor is recalculated using a standardized formula based on the ratio of the current crack length a1 to the specimen width W. The load amplitude is then reduced in a stepwise manner, with each reduction not exceeding 10% of the maximum load in the previous stage. This reduction strategy effectively suppresses the nonlinear growth of the plastic zone size at the crack tip by gradually reducing the ΔK amplitude, while maintaining the quasi-static stability of crack propagation. The entire pre-cracking process requires real-time calibration of the crack length using a high-resolution microscope or compliance method. Engineering specifications require a pre-crack length in the range of 2.5-3.0 mm. In this invention's experiment, the pre-crack in the CT specimen does not participate in the subsequent crack propagation test; therefore, it can be appropriately increased to approximately 10 mm based on the subsequent in-situ specimen sampling plan, while still meeting the quality control standard that the crack front straightness deviation is ≤5% of the specimen thickness.
[0036] In this invention, step S3 involves determining the sampling position of the in-situ sample within the CT sample based on the pre-crack length in the in-situ sample. During sampling, it is also necessary to ensure that the test surface of the in-situ sample is parallel to the CT sample, and that the weld and crack are located in the middle of the parallel section of the in-situ part. Generally, the thickness of the CT part is around 10 mm, and multiple in-situ test pieces can be processed in the thickness direction.
[0037] The width of the parallel section of the in-situ specimen is approximately 3 mm, and the length of the internal pre-crack should not exceed 1 mm. This maintains the driving force threshold for stable crack propagation and effectively inhibits crack bifurcation or secondary crack initiation. After determining the pre-crack length of the in-situ component (0.5 mm is recommended), its position in the CT part blank can be determined, and in-situ specimen sampling and processing can begin. During wire EDM processing, a laser-assisted positioning device must be used to ensure that the sampling path is perpendicular to the main crack propagation direction of the CT specimen. After processing, the in-situ specimen must be inspected for parallelism deviation and surface roughness using a white light interferometer to ensure the axisymmetric distribution characteristics of the stress field during subsequent in-situ loading.
[0038] In this invention, during machining processes such as cutting, the weld microstructure becomes very difficult to discern due to scratches on the sample surface, thus affecting subsequent processing of the weld microstructure. Therefore, it is necessary to perform fine machining on the sample surface to clearly identify the weld location, thereby enabling clear statistical analysis of the microstructural characteristics of different regions of the weld. Therefore, in step S4, for in-situ samples containing pre-existing cracks, surface treatment is first performed using an electrolytic vibration composite polishing process: polishing is performed using a nanodiamond suspension under the drive of an asymmetric dual-frequency vibration platform, resulting in a surface roughness Ra ≤ 15 nm for the in-situ sample. After surface preparation, crystallographic characterization of the plastic region at the crack tip is performed using an electron backscatter diffraction system mounted on a field emission scanning electron microscope. The detection area is centered at the bottom of the field of view with the crack tip as the center point, covering three times the radius of the maximum plastic region along the propagation direction, and laterally covering 20 characteristic grain size ranges. A high-resolution scanning mode with a step size of 0.5 μm was used to acquire parameters such as grain orientation difference distribution, grain boundary feature distribution, and spatial distribution of the Schmid factor, providing key input parameters for establishing a crack propagation prediction model based on crystal plasticity theory. Vibration polishing was performed on the in-situ specimen, and EBSD detection was conducted on the crack tip region to obtain information on grain orientation and size distribution in this study area, which will be used for subsequent crack propagation path analysis based on microstructure.
[0039] The foregoing has only described certain exemplary embodiments of the present invention by way of illustration. Undoubtedly, those skilled in the art can modify the described embodiments in various ways without departing from the spirit and scope of the present invention. Therefore, the foregoing drawings and descriptions are illustrative in nature and should not be construed as limiting the scope of protection of the claims of the present invention.
Claims
1. A method for preparing an in-situ crack propagation test specimen for a welded joint, characterized in that, The method includes the following steps: Step S1: Determine the spatial distribution and direction of the weld in the blank, and use a laser to perform three-dimensional coordinate calibration on the blank so that the loading direction of the CT sample machine for processing the in-situ crack propagation test specimen of the welded joint is orthogonal to the weld axis, and the fusion line center of the weld structure is located at the tip of the processing notch of the CT sample machine, thus preparing the CT sample. Step S2: Processing pre-cracks on the CT specimen, including: determining the initial stress intensity factor based on the fracture toughness of the base material of the CT specimen; determining the critical threshold of the maximum cyclic load of the CT specimen; and controlling the length and speed of pre-crack generation on the CT specimen based on the critical threshold of the maximum cyclic load and the current stress intensity factor. Step S3: Based on the pre-crack length of the in-situ crack propagation test specimen of the welded joint, an in-situ specimen is cut from the CT specimen; wherein, when cutting the in-situ specimen, the sampling path is perpendicular to the main crack propagation direction of the CT specimen; the pre-crack length of the in-situ crack propagation test specimen of the welded joint is determined based on the intended use. Step S4: Perform fine machining and disturbed polishing on the surface of the in-situ sample to form an in-situ crack propagation test specimen for the welded joint.
2. The method as described in claim 1, characterized in that, In step S2, based on the critical threshold of the maximum cyclic load and the current stress intensity factor, the length and rate of pre-crack formation are controlled on the CT specimen, including: Step S21: Assign the current stress intensity factor value to the initial stress intensity factor value, assign the maximum load value to the critical threshold of the maximum cyclic load, and initialize the crack length increment to 0; determine the stress ratio based on the current stress intensity factor amplitude, and determine the crack growth rate on the CT specimen according to the stress ratio and the maximum load value; Step S22: If the length of the crack on the CT sample does not reach the preset length, proceed to step S23; otherwise, proceed to step S3. Step S23: When the crack length increment on the CT sample has not reached the preset step size, continue to grow the crack according to the growth rate and monitor the crack length increment in real time until the crack length increment reaches the preset step size. Step S24: Based on the current crack length CT sample width The current stress intensity factor is calculated; the maximum load value is then assigned to the current maximum load value. (1-k), set the crack length increment to 0, determine the stress ratio based on the current stress intensity factor amplitude, determine the crack growth rate on the CT specimen according to the stress ratio and the maximum load value, and proceed to step S22.
3. The method as described in claim 2, characterized in that, The initial stress intensity factor is an empirical value; The current formula for calculating the stress intensity factor is: in, The current stress intensity factor, The load acting on the CT specimen. For CT sample thickness, For CT sample width, The geometric shape factor of the CT sample. This represents the current crack length.
4. The method as described in claim 3, characterized in that, The preset step size is 0.2-0.5 mm, and k≤10%.
5. The method as described in claim 1, characterized in that, During the pre-cracking process of CT specimens, multi-stage mechanical control was implemented in accordance with the ASTM E1820 standard specification.
6. The method as described in claim 1, characterized in that, In step S3, the sampling location of the in-situ sample in the CT sample is determined based on the pre-crack length in the in-situ sample.