A method and system for real-time monitoring of stress during a laser additive repair process of a long fatigue crack of a turbine casing

Abstract

This invention discloses a method and system for real-time stress monitoring during laser additive repair of fatigue long cracks in turbine casings. The method includes: preparing a high-temperature resistant speckle pattern in the pre-treated repair area; simultaneously acquiring a sequence of speckle images and a sequence of temperature field images during the laser repair process; calculating the strain field based on digital image correlation and calculating the real-time stress evolution using thermo-elastic-plastic stress-strain increment theory. The real-time stress value is compared with a dynamic threshold set based on the material's yield strength, and the laser power or scanning speed is controlled based on the feedback of the result to achieve closed-loop control of the repair stress. This invention achieves high-precision real-time online monitoring of stress under extreme repair environments, providing a basis for optimizing and intelligently controlling repair process parameters, effectively suppressing residual stress and the risk of secondary cracks, and significantly improving the repair quality and reliability of turbine casings.

Description

Technical Field

[0001] This invention belongs to the field of laser additive repair technology, specifically relating to a method and system for real-time stress monitoring during the laser additive repair process of fatigue long cracks in turbine casings. Background Technology

[0002] As a critical load-bearing component of aero-engines and gas turbines, the turbine casing operates under high temperature and high stress cyclic loading for extended periods, making it highly susceptible to fatigue crack initiation in stress concentration areas. Laser additive repair technology, due to its controllable heat input and excellent repair properties, has become an important repair method for such high-value components. However, laser additive repair is a rapid, non-equilibrium melting process, accompanied by severe temperature gradients and complex thermo-mechanical coupling effects, which can generate significant residual stress in the repair area and adjacent substrate. Excessive residual stress can not only lead to component deformation but also induce secondary cracks, directly threatening the operational safety of the engine.

[0003] Currently, the assessment of stress in repaired components mainly relies on post-repair testing methods, such as X-ray diffraction, ultrasonic testing, or blind hole testing. These methods cannot provide any stress data during the repair process, resulting in long process debugging cycles, poor repair quality stability, and the inability to intervene in the repair process in real time, representing a typical "open-loop" control. Digital image correlation (DIC), as a non-contact optical measurement technique, has been widely used in the testing of material mechanical properties. However, in the extreme environment of laser additive repair, challenges exist such as strong light interference from the molten pool, speckle failure caused by high temperatures, and complex coupling relationships between the deformation field and the temperature field, making it difficult to directly apply traditional DIC technology to the real-time and accurate monitoring and control of stress during the repair process.

[0004] Therefore, there is an urgent need for a method and system that can monitor the stress evolution of the turbine casing in real time, online, and with high precision throughout the entire laser additive repair process and achieve closed-loop control. Summary of the Invention

[0005] To address the technical problems of poor real-time performance, insufficient anti-interference capability, and inability to achieve closed-loop control in existing technologies, this invention provides a method and system for real-time stress monitoring during the laser additive repair process of long fatigue cracks in turbine casings, thereby enabling real-time monitoring and closed-loop control of stress during the laser repair process.

[0006] The technical solution adopted in this invention is: A method for real-time stress monitoring during laser additive repair of fatigue long cracks in turbine casings, characterized by comprising the following steps: S1: Pre-treatment of the area to be repaired includes crack detection, beveling and surface cleaning, and laser marking process to create micron-level high-temperature resistant speckle patterns on the substrate surface adjacent to the area to be repaired. S2: During the laser additive repair process, a sequence of speckle images containing the speckle pattern and a sequence of temperature field images of the repaired area are acquired simultaneously. S3: Based on the sequence of speckle images, the full-field strain distribution of the repair area is calculated using digital image correlation. S4: The strain field obtained in step S3 is spatiotemporally matched with the temperature field obtained in step S2. Based on the thermo-elastic-plastic stress-strain increment theory and combined with temperature-related material constitutive parameters, the real-time stress increment is calculated step by step and integrated to obtain real-time stress evolution data of the entire repair process. S5: Compare the real-time stress evolution data with the preset dynamic stress control threshold. When the real-time stress evolution data exceeds the upper limit threshold, a control signal is automatically issued to adjust the laser repair process parameters. When the real-time stress evolution data falls back to below the lower limit threshold and stabilizes, the original process parameters are restored, thereby realizing closed-loop control of stress in the repair process.

[0007] Furthermore, in step S3, the digital image correlation method uses a first-order or second-order shape function model to describe the coordinate mapping relationship of sub-regions between the reference image and the deformed image, and its general form is: (1) (2) in, and These are any points in the reference image sub-region. and Rigid displacement in the direction, , , , These are the first-order displacement gradient parameters. The coordinate offset relative to the center point of the reference sub-region is used to describe simple translation, rotation, scaling, or shear deformation of an object. , , , , , , , , It is a second-order displacement gradient parameter used to describe non-rigid deformations of an object, such as bending and torsion. Using the zero-normalized least squares difference as the correlation criterion, the Newton-Raphson iterative algorithm is used to optimize the shape function parameters to minimize the correlation coefficient, thereby obtaining a high-precision displacement field and displacement gradient parameters of each order.

[0008] Furthermore, in step S4, the step of real-time calculation of stress increment using the thermo-elastic-plastic stress-strain increment theory includes: Calculate the strain increment caused by temperature change based on the temperature field data. ; (3) in, The elasticity matrix is ​​temperature-dependent. Given the current stress state, This is the current coefficient of thermal expansion; The mechanical strain is obtained by subtracting the strain caused by temperature change from the total strain; Based on the stress state of the previous time step and the current strain increment and the material's yield strength at the current temperature. The loading and unloading status of materials is determined in the following form: (4) in, For equivalent stress, To be related to plastic deformation and temperature The relevant yield strength; When yield function and When this occurs, it is determined to be plastic loading; when or and When this occurs, it is determined to be either elastic loading or elastic unloading; Based on the determination of the loading or unloading state, the corresponding constitutive relation is selected to calculate the current stress increment; when it is determined to be elastic loading or elastic unloading, the current stress increment is directly solved according to the generalized Hooke's law. ,Right now: (5) in, This represents the current total strain increment; When plastic loading is determined, the plastic strain increment is calculated based on the flow law, and the stress increment is solved according to the temperature-dependent elastoplastic constitutive relation, i.e.: (6) (7) (8) in, The temperature-dependent elastoplastic matrix The H function represents the stress increment caused by temperature change, reflecting the new yield stress. The dependence of the total amount of plastic equivalent strain on tensile strain can be obtained from tensile tests at different temperatures. It represents the partial derivative of the H function with respect to the plastic strain at a given temperature; By accumulating and integrating the stress increments at each time step, the real-time stress evolution curves of each point in the monitoring area as a function of time are obtained.

[0009] Furthermore, in step S5, the dynamic stress control threshold includes an upper limit threshold. With lower threshold And satisfy: The upper limit threshold The lower limit threshold is 0.7 to 0.9 times the yield strength of the material at the real-time temperature. Take 0.5 to 0.7 times the yield strength of the material at the real-time temperature, and When the stress at the monitoring point reaches or exceeds When the stress at the monitoring point drops to a certain level, the system automatically reduces the laser power or increases the scanning speed; when the stress at the monitoring point drops back to a certain level... After the preset time has been stabilized, the system will revert to the original process parameters.

[0010] A second aspect of the present invention relates to a real-time stress monitoring system for laser additive repair of turbine casings for implementing the method described above, characterized in that it comprises: The laser additive repair unit (2) is used to perform additive repair on the fatigue crack area of ​​the turbine casing (1) to be repaired; The laser marking unit is used to prepare a high-temperature resistant speckle pattern on the substrate surface adjacent to the area to be repaired before repair. Optical imaging unit (3) is used to acquire a sequence of speckle images including the speckle pattern; Infrared temperature measurement unit (4) is used to acquire sequential temperature field images of the repair area; Synchronization control unit (5) is used to synchronously trigger the optical imaging unit and the infrared temperature measurement unit to acquire data. The data processing and feedback unit (6) is used to perform strain calculation, thermo-elastic-plastic stress solution and stress threshold determination, and output control signals according to the stress change results to realize dynamic adjustment of laser process parameters.

[0011] The technical concept of this invention is as follows: a speckle pattern is prepared directly on the turbine casing substrate using laser marking. During the repair process, strain and temperature data are collected simultaneously, and stress evolution is calculated in real time to obtain real-time stress evolution data throughout the repair process. Based on the real-time yield strength of the material, dynamic upper and lower stress thresholds are constructed, realizing real-time monitoring and closed-loop control of stress during the laser additive repair of the turbine casing. This helps to suppress stress peaks and residual stress levels, reduce the risk of secondary cracks, and improve repair quality and service reliability.

[0012] Compared with the prior art, the beneficial effects of the present invention are reflected in: 1. Good high-temperature stability: The speckle pattern is prepared directly on the turbine casing substrate by laser marking, avoiding the problem of coating peeling. The speckle pattern is stable in the high-temperature repair environment for a long time, which can significantly improve the success rate and accuracy of calculation.

[0013] 2. Achieved true real-time online monitoring: By synchronously collecting strain and temperature data during the repair process and calculating stress evolution in real time, the stress state during the repair process is "visualized", which greatly shortens the process debugging cycle.

[0014] 3. High measurement accuracy: The temperature field is accurately measured by an infrared thermal imager, and the thermal strain is accurately compensated and the plastic strain is analyzed by using the thermo-elastic-plastic theory. This avoids the huge error caused by simply treating the total strain as elastic strain, and significantly improves the accuracy of stress calculation.

[0015] 4. Strong anti-interference capability: By installing a narrowband filter with a specific wavelength on the high-speed camera, strong radiation interference from lasers and the molten pool is effectively suppressed, ensuring the quality of the speckle image. Combined with a high-temperature resistant speckle pattern preparation process, the accuracy of monitoring is guaranteed.

[0016] 5. A closed-loop control foundation has been established: Based on the real-time yield strength of the material, dynamic upper and lower stress thresholds are constructed, realizing real-time monitoring and closed-loop control of stress during the laser additive repair of the turbine casing. This helps to suppress stress peaks and residual stress levels, reduce the risk of secondary cracks, and improve repair quality and service reliability. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the real-time stress monitoring and calculation process of the present invention; Figure 2 This is a schematic diagram of the overall structure of the monitoring system of the present invention; Figure 3 This is the digital speckle generation process in the embodiment; Figure 4 The image shows the speckle pattern on the material surface after laser marking in this embodiment. Figure 5 This is a real-time stress cloud diagram of the laser additive repair process in the embodiment; Figure 6 These are the curves showing the real-time stress X-direction component change over time at each monitoring point in the embodiment; Figure 7 The curves showing the change of the real-time stress Y-direction component at each monitoring point over time are shown in the embodiment.

[0018] Explanation of reference numerals in the attached drawings: 1. Turbine casing to be repaired; 2. Laser additive repair unit; 3. Optical imaging unit; 4. Infrared temperature measurement unit; 5. Synchronization control unit; 6. Data processing and feedback unit. Detailed Implementation

[0019] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the scope of the present invention.

[0020] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0021] The present invention will now be described in detail with reference to the accompanying drawings and exemplary embodiments.

[0022] Example 1

[0023] Taking the nickel-based superalloy K4169 as an example, it is required to achieve real-time monitoring of the stress in the heat-affected zone during the laser additive repair process.

[0024] refer to Figure 1 , Figure 3 , Figure 4 , Figure 5 , Figure 6 and Figure 7 The present invention discloses a method for real-time stress monitoring during laser additive repair of fatigue long cracks in turbine casings, which specifically includes the following steps: S1. Non-destructive testing is performed on the fatigue cracks in the turbine casing to determine the crack length and depth. Grooving or beveling is carried out around the crack to ensure the quality of laser additive forming and fusion. The surface of the repair area is cleaned and repaired using processes such as grinding and sandblasting. Random speckle patterns are prepared on the substrate area on both sides of the crack using laser marking equipment. The speckle feature size and spacing are preferably on the order of tens of micrometers to balance image contrast and related calculation accuracy.

[0025] S2. Arrange the laser additive repair head, high-speed camera, and infrared thermal imager around the turbine casing repair area. Install a narrow-band filter on the high-speed camera to suppress laser reflection and strong light interference from the molten pool. Send TTL trigger signals to the high-speed camera and infrared thermal imager through the synchronization control unit to synchronously acquire speckle image sequences and temperature field image sequences at 1 ms time intervals to achieve image time axis alignment.

[0026] S3. After distortion correction and grayscale normalization preprocessing of the acquired speckle image, sub-region matching and shape function parameter solving are performed using digital image correlation method to obtain the full-field displacement and strain distribution of the repair area; non-uniform response correction and spatial calibration are performed on the infrared thermal image, and the coordinate correspondence between the optical image and the infrared image is realized through calibration plate or feature points, so as to obtain the full-field temperature field matching the strain field.

[0027] After determining the shape function and correlation function, correlation calculations need to be performed on the reference image sub-region and the deformed image. First, a reference sub-region is selected, and the initial correlation deformation parameters are substituted into the shape function to obtain the gray values ​​of all corresponding points within the reference sub-region. Then, the correlation coefficient value is calculated. If the obtained correlation coefficient value reaches a threshold, the deformation parameters can be determined at this time; otherwise, the calculation is repeated until the threshold is reached.

[0028] S4. Calculate real-time stress based on the thermo-elastic-plastic stress-strain increment theory, determine the loading or unloading state, and integrate to obtain the stress evolution curve.

[0029] (1) Calculate their increments based on the input strain and temperature data and proceed to step (2).

[0030] (2) Determine whether the strain increment and temperature increment are less than the processing threshold. If they are, proceed directly to step (3); otherwise, process them equally and recursively call step (3).

[0031] (3) Based on the given strain increment and temperature increment, predict their loading mode and determine whether the prediction is correct. If the prediction is correct, proceed to step (4); otherwise, perform elastic unloading and determine whether a new yield will occur after unloading. If not, proceed to step (4); otherwise, return to step (2).

[0032] When a material is under elastic loading or elastic unloading, the generalized Hooke's law applies:

[0033] When entering the plastic loading state, the elastic-plastic matrix is ​​modified using an isotropic hardening model based on the Mises yield criterion and flow law. The stress increment is obtained as follows:

[0034] Integrating the obtained stress increment over time yields the real-time stress evolution curve. .

[0035] (4) Return the calculated stress increment and integrate it to obtain the current stress state. Output the current stress value and return to step (1).

[0036] (5) Repeat steps (1) to (4) until the program ends.

[0037] S5. When the real-time stress at each monitoring point reaches 0.8 times the yield strength at that temperature (dynamic threshold), the system sends a control signal to automatically reduce the laser power by 10%. Once the stress drops back to 0.6 times the yield strength and stabilizes for 1 second, the original power is restored. The data processing and feedback unit displays the stress cloud map, the stress along the laser scanning direction, and the stress perpendicular to the scanning direction at each monitoring point as a function of time on the monitoring interface in real time. It also automatically stores the original images and calculation results for post-process analysis and optimization.

[0038] Figure 6 and Figure 7 The stress component curves along and perpendicular to the laser scanning direction at each monitoring point, representing the changes in stress state along two typical directions in laser additive manufacturing under the influence of laser movement, reflect the changes in stress state at each monitoring point. The stress component along the scanning direction primarily reflects the stress changes caused by the temperature gradient between the high-temperature region being scanned by the laser, the relatively high-temperature region already scanned by the laser, and the low-temperature region. The stress component perpendicular to the scanning direction primarily reflects the stress changes caused by the temperature gradient between the high-temperature and low-temperature regions.

[0039] When the stress component is negative, it indicates that the laser-induced zone is far from the point, and the monitoring point is under compressive stress due to the pressure from the laser-heated zone. When the stress component rises to a positive value, it indicates that the monitoring point has entered the laser-induced zone, and the point is under tensile stress due to thermal expansion. The stress continues to rise during the laser heating process until it reaches its peak value. After the laser stops heating, the stress begins to decrease until it reaches a negative value, i.e., the compressive stress state.

[0040] Example 2

[0041] The real-time stress monitoring system for laser additive repair of turbine casings based on the method described in Example 1 includes: Laser additive repair unit (2): used for layer-by-layer cladding repair of fatigue crack areas in the turbine casing (1) to be repaired. The laser additive repair unit includes a fiber laser, a scanning control module, and a protective gas delivery device. The laser power range is 800-1200 W, the spot diameter is 0.8-1.2 mm, the scanning speed is 3-8 mm / s, and the protective gas is high-purity argon.

[0042] Laser marking unit: Used to prepare high-temperature resistant speckle patterns on the surface of the repair area. A 30 W galvanometer fiber laser marking machine is used with a scanning interval of 30 μm to form a high-contrast speckle pattern with an average diameter of about 60 μm and random distribution.

[0043] Optical imaging unit (3): used to acquire sequential images containing speckle patterns. A high-speed camera (e.g., Phantom V2640) is used at a frame rate of 1000 fps, equipped with a 1064 nm narrowband filter to isolate laser reflection interference.

[0044] Infrared temperature measurement unit (4): used to acquire sequential temperature field images of the repair area. A mid-wave infrared thermal imager (FLIR A615) was used, with a temperature measurement range of 0 to 2000 ℃, a frame rate of 500 fps, and a resolution of 640×480 pix.

[0045] Synchronization control unit (5): Used to simultaneously trigger the optical imaging unit and the infrared temperature measurement unit to acquire data. An NI-PXI hardware control card is used, and time synchronization is achieved through TTL pulse signals.

[0046] Data processing and feedback unit (6): This is an industrial computer equipped with dedicated algorithms, used to perform image registration, strain calculation, thermo-elastic-plastic stress solution, and process feedback control. This unit communicates with the laser additive repair equipment and can automatically adjust the laser power or scanning speed according to stress changes.

[0047] refer to Figure 2 In actual operation, the laser marking unit first prepares a speckle pattern on the surface of the area to be repaired in the turbine casing. Then, the laser additive repair unit, optical imaging unit, and infrared thermography unit are arranged around the turbine casing repair area. The high-speed camera and infrared thermal imager are fixed approximately 300 mm above the workpiece and supported by independent brackets. The synchronous control unit is connected to each acquisition device to ensure synchronous sampling. All data is fed into the data processing and feedback unit for real-time analysis and display, ultimately generating stress cloud maps, stress-time curves, and dynamic control signals.

[0048] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," "counterclockwise," "axial," "radial," and "circumferential" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing this invention and simplifying the description, and are not intended to indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention.

[0049] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Thus, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0050] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection, an electrical connection, or a connection that allows communication between them; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components, unless otherwise explicitly limited. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0051] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0052] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0053] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for real-time stress monitoring during laser additive repair of fatigue long cracks in turbine casings, characterized in that, Specifically, the following steps are included: S1: Pre-treatment of the area to be repaired includes crack detection, beveling and surface cleaning, and laser marking process to create micron-level high-temperature resistant speckle patterns on the substrate surface adjacent to the area to be repaired. S2: During the laser additive repair process, a sequence of speckle images containing the speckle pattern and a sequence of temperature field images of the repaired area are acquired simultaneously. S3: Based on the sequence of speckle images, the full-field strain distribution of the repair area is calculated using digital image correlation. S4: The strain field obtained in step S3 is spatiotemporally matched with the temperature field obtained in step S2. Based on the thermo-elastic-plastic stress-strain increment theory and combined with temperature-related material constitutive parameters, the real-time stress increment is calculated step by step and integrated to obtain real-time stress evolution data of the entire repair process. S5: Compare the real-time stress evolution data with the preset dynamic stress control threshold. When the real-time stress evolution data exceeds the upper limit threshold, a control signal is automatically issued to adjust the laser repair process parameters. When the real-time stress evolution data falls back to below the lower limit threshold and stabilizes, the original process parameters are restored, thereby realizing closed-loop control of stress in the repair process.

2. The method according to claim 1, characterized in that, In step S3, the digital image correlation method uses a first- or second-order shape function model to describe the coordinate mapping relationship between sub-regions of the reference image and the deformed image. Its general form is: （1） （2） in, and These are any points in the reference image sub-region. and Rigid displacement in the direction, , , , These are the first-order displacement gradient parameters. The coordinate offset relative to the center point of the reference sub-region is used to describe simple translation, rotation, scaling, or shear deformation of an object. , , , , , , It is a second-order displacement gradient parameter used to describe non-rigid deformations of an object, such as bending and torsion. Using the zero-normalized least squares difference as the correlation criterion, the Newton-Raphson iterative algorithm is used to optimize the shape function parameters to minimize the correlation coefficient, thereby obtaining a high-precision displacement field and displacement gradient parameters of each order.

3. The method according to claim 1, characterized in that, In step S4, the real-time calculation of stress increment using the thermo-elastic-plastic stress-strain increment theory includes: Calculate the strain increment caused by temperature change based on the temperature field data. ; （3） in, The elasticity matrix is ​​temperature-dependent. Given the current stress state, This is the current coefficient of thermal expansion; The mechanical strain is obtained by subtracting the strain caused by temperature change from the total strain; Based on the stress state of the previous time step and the current strain increment and the material's yield strength at the current temperature. The loading and unloading status of materials is determined in the following form: （4） in, For equivalent stress, To be related to plastic deformation and temperature The relevant yield strength; When yield function and When this occurs, it is determined to be plastic loading; when or and When this occurs, it is determined to be either elastic loading or elastic unloading; Based on the determination of the loading or unloading state, the corresponding constitutive relation is selected to calculate the current stress increment; when it is determined to be elastic loading or elastic unloading, the current stress increment is directly solved according to the generalized Hooke's law. ,Right now: （5） in, This represents the current total strain increment; When plastic loading is determined, the plastic strain increment is calculated based on the flow law, and the stress increment is solved according to the temperature-dependent elastoplastic constitutive relation, i.e.: （6） （7） （8） in, The temperature-dependent elastoplastic matrix The H function represents the stress increment caused by temperature change, reflecting the new yield stress. The dependence of the total amount of plastic equivalent strain on tensile strain can be obtained from tensile tests at different temperatures. It represents the partial derivative of the H function with respect to the plastic strain at a given temperature; By accumulating and integrating the stress increments at each time step, the real-time stress evolution curves of each point in the monitoring area as a function of time are obtained.

4. The method according to claim 1, characterized in that, In step S5, the dynamic stress control threshold includes an upper limit threshold. With lower threshold And satisfy: The upper limit threshold The lower limit threshold is 0.7 to 0.9 times the yield strength of the material at the real-time temperature. Take 0.5 to 0.7 times the yield strength of the material at the real-time temperature, and When the stress at the monitoring point reaches or exceeds When the stress at the monitoring point drops to a certain level, the system automatically reduces the laser power or increases the scanning speed; when the stress at the monitoring point drops back to a certain level... After the preset time has been stabilized, the system will revert to the original process parameters.

5. A real-time stress monitoring system for laser additive repair of turbine casings for implementing the method described in any one of claims 1 to 4, characterized in that, include: The laser additive repair unit (2) is used to perform additive repair on the fatigue crack area of ​​the turbine casing (1) to be repaired; The laser marking unit is used to prepare a high-temperature resistant speckle pattern on the substrate surface adjacent to the area to be repaired before repair. Optical imaging unit (3) is used to acquire a sequence of speckle images including the speckle pattern; Infrared temperature measurement unit (4) is used to acquire sequential temperature field images of the repair area; Synchronization control unit (5) is used to synchronously trigger the optical imaging unit and the infrared temperature measurement unit to acquire data. The data processing and feedback unit (6) is used to perform strain calculation, thermo-elastic-plastic stress solution and stress threshold determination, and output control signals according to the stress change results to realize dynamic adjustment of laser process parameters.

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