Method and system for evaluating damage tolerance of friction plug repair welding joint of carrier rocket storage tank

Abstract

The invention discloses a damage tolerance evaluation method and system for a friction plug repair welding joint of a carrier rocket storage tank, and the method comprises the steps: detecting an interface of the friction plug repair welding joint of the carrier rocket storage tank, and obtaining defect information, constitutive parameters of a material, and material attributes; establishing a finite element model, and accurately setting corresponding distribution interface defects; quantizing in-plane and out-plane restraint effects of the crack tip by adopting restraint parameters; processing and preparing a unilateral notch tensile sample, and measuring a local J-R curve of the tip of the precrack at the interface of the friction plug repair welding joint; correcting a real J-R curve of the friction plug repair welding joint of the rocket tank based on the influence of the distribution interface defect characteristics; then the tensile strength values of the friction plug repair welding joint in all the positions in the circumferential direction are corrected, and a corrected real J-R curve is obtained; and the limit crack size under the action of the corresponding internal pressure is obtained by utilizing the crack tip extension driving force curve, and then the critical J integral of the crack is calculated, so that the maximum crack size under the specific load or the maximum internal pressure under the crack with the specific size is obtained.

Description

Technical Field

[0001] The invention relates to the technical field of material mechanical property testing, and in particular to a damage tolerance assessment method and system for a friction plug repair welding joint of a launch vehicle tank. Background Art

[0002] The propellant tank is the backbone of a rocket, accounting for approximately 60% of its total weight. Bearing thousands of tons of payload in ultra-low temperatures, it has a crucial impact on rocket performance. Rocket tanks are typically large, thin-walled aluminum alloy welded components, with a thickness-to-diameter ratio of less than one thousandth. For example, the Long March 5 rocket tank has a diameter of 5 meters, a thin-section wall thickness of less than 5mm, and a total weld length exceeding 100 meters. With the growing demand for high-capacity space launch vehicles and the increasing size of launch vehicles, tank manufacturing is becoming increasingly challenging. Welding can soften the heat-affected zone (HAZ), introduce welding defects, and generate residual stress and deformation, making weld joints vulnerable areas. Therefore, weld joint quality is crucial to the tank's load-bearing capacity and reliability. Large / heavy rocket tanks are enormous, have weak rigidity, and demand high precision. Conventional fusion welding and friction stir welding (FSW) are difficult to achieve high-quality welds in critical areas. For example, the girth seam between the tank bottom and the barrel forging is extremely difficult to achieve due to the inherent rigidity required. Therefore, dual-shoulder FSW and pull-forged FSW are used for this purpose.

[0003] However, the existing friction plug welding technology for thick, high-strength aluminum alloys still has certain limitations, especially in the performance evaluation of friction plug welded joints. During the friction plug welding process of rocket tank bodies, the uneven distribution of the thermal field can easily lead to defects such as weak interface bonding and unwelded joints. The aggregation of interface oxides and Cu-rich phases can significantly reduce the tensile strength of the joints. In addition, rocket tanks are subject to complex loads during service, and their safety verification still relies on water pressure testing of full-scale tank structures. Traditional static strength assessments cannot reflect the damage evolution of defective joints, and a correlation model between defect size and critical failure load has not yet been established. Therefore, there is an urgent need to develop a damage tolerance assessment method for friction plug welded joints at the rocket tank structure level based on multi-scale defect characterization and crack propagation prediction to ensure the reliability of defective joints under extreme working conditions and fill the technical gap in the field of aerospace structure repair.

[0004] The existing damage tolerance assessment methods for friction plug welded joints at the rocket tank structure level have the following problems:

[0005] (1) Regarding the performance evaluation of friction plug welded joints at the rocket tank structural level, the existing method is mainly to conduct a hydrostatic test on the full-size tank structure containing friction plug welded joints. The rocket tank hydrostatic test (Hydrostatic Pressure Test) is an important structural strength verification test conducted on rocket fuel tanks in aerospace engineering to confirm that the tank will not rupture or permanently deform under conditions higher than the normal operating pressure, and to expose defects such as weak bonding and unwelded interfaces of the friction plug welded joints of the rocket tank. However, the cost of the rocket tank hydrostatic test is high, and it is impossible to achieve quantitative evaluation of the defects of the friction plug welded joints (size, orientation, position, etc.), and it has limited guiding significance for the optimization of process parameters;

[0006] (2) Currently, the tensile strength of friction plug welded joints with certain defects can be measured by conducting tensile tests on specimens processed and prepared to contain the entire or part of the friction plug welded joint. However, the friction plug welded joint has an annular characteristic. Under the same process parameters, the weakest position of the local performance of the friction plug welded joint may be located at any angle in the annular direction. Therefore, the tensile test results of a limited number of friction plug welded joints cannot be used to determine the damage tolerance of friction plug welded joints at the rocket tank structure level that are subjected to complex loads.

[0007] In summary, the existing evaluation technical means have certain limitations in the damage tolerance evaluation of friction plug welded joints of rocket fuel tanks. A damage tolerance assessment method based on the performance evaluation results of friction plug welded joints has been developed, which can be transplanted to the friction plug welded joints at the rocket tank structure level, so as to quantitatively, accurately and scientifically evaluate the damage tolerance of friction plug welded joints of rocket tank bodies. This is of great significance for ensuring the service safety of rocket tank structures containing friction plug welded joints. Summary of the Invention

[0008] The present invention aims to overcome the shortcomings of the prior art and provides a damage tolerance assessment method and system for launch vehicle tank friction plug repair joints. Based on nondestructive testing information of interface defects in tank friction plug repair joints and three-dimensional constraint theory, the method and system quantify the in-plane and out-of-plane constraint effects at the crack tip interface of the joint using constraint parameters that take into account the interface defect characteristics of the launch vehicle tank friction plug repair joint. The method and system then corrects the surface crack fracture toughness measurement results of the joint standard SENT specimen to the buried crack fracture toughness results of the tank friction plug repair joint interface. Based on the critical crack size under internal pressure, the critical J-integral of the interface crack of the launch vehicle tank friction plug repair joint is calculated, thereby calculating the maximum crack size under a specific load or the maximum internal pressure under a specific crack size.

[0009] To achieve the above objectives, the present invention provides, in a first aspect, a damage tolerance assessment method for a friction plug repair weld joint of a launch vehicle tank, comprising:

[0010] Step 1: Use non-destructive testing technology to detect defects / flaws at the interface of the friction plug welding joint of the launch vehicle tank, and obtain defect information, material constitutive parameters and material properties;

[0011] Step 2: Establish a finite element model of the launch vehicle tank friction plug repair weld joint. Based on the defect information, material constitutive parameters, and material properties obtained in step 1, accurately set the corresponding distributed interface defects in the finite element model.

[0012] Step 3: Based on the nanoindentation measurement results, the local constitutive relationship of the interface defect of the friction plug welding joint is established. The finite element numerical calculation method is used to calculate the in-plane and out-of-plane constraints of the crack tip of the friction plug welding joint under the service characteristic load;

[0013] Based on the three-dimensional constraint theory, the constraint parameter f is adopted to consider the interface defect characteristics of the friction plug welding joint of the launch vehicle tank. (t,w,θ) Quantify the above-mentioned in-plane and out-of-plane constraint effects at the crack tip; where t is the crack tip constraint parameter, w is the crack width constraint parameter, and θ is the crack angle constraint parameter;

[0014] Step 4: Prepare a single-edge notched tensile specimen and pre-crack it; use the straight line to replace the curve method to measure the crack propagation resistance curve (JR curve) at the interface of the friction plug welded joint where the pre-crack tip is located;

[0015] Step 5: Use the three-dimensional constraint parameter f that takes into account the interface defect characteristics of the launch vehicle tank friction plug repair welding joint (t,w,θ) , based on the three-dimensional restrained fracture mechanics theory, the real JR curve of the friction plug repair welding joint of the rocket tank is corrected to obtain the corrected real JR curve;

[0016] Step 6: Based on the micro-region constitutive relationship of the circumferential interface of the friction plug welded joint measured in Step 3, the finite element method is used to numerically simulate and calculate the driving force curves of crack tip propagation of different sizes under the condition of increasing internal pressure in the launch vehicle tank. When the crack tip propagation driving force curve corresponding to a certain internal pressure value is tangent to the corrected true JR curve obtained in Step 5, the crack size corresponding to the intersection point is the limit crack size under the corresponding internal pressure.

[0017] Step 7: Based on the critical crack size under internal pressure, calculate the critical J-integral of the interface crack of the friction plug welding joint of the launch vehicle tank, which is used to evaluate the remaining life of the structure under different load levels;

[0018] The maximum crack size under a specific load or the maximum internal pressure under a specific size crack is then calculated based on the critical J integral, which is used in the design and quality inspection of subsequent friction plug welding joints of launch vehicle fuel tanks.

[0019] Furthermore, the step 1 includes:

[0020] S101: Use an ultrasonic phased array flaw detector to conduct a full-coverage scan of the launch vehicle tank friction plug repair weld joints to obtain defect signals;

[0021] S102: reconstructing the acquired defect signal to obtain defect position, orientation, and size information of the weld head interface;

[0022] S103: For the aluminum alloy material used in the launch vehicle tank, a series of material mechanical property tests such as tensile tests, compression tests, and shear tests are conducted to obtain the constitutive relationship of the aluminum alloy material, namely the stress-strain curve and related constitutive parameters, which serve as input parameters for subsequent finite element simulation calculations. Specifically, the aluminum alloy friction plug repair joint used to determine the constitutive relationship of the interface region and the friction plug repair joint at the launch vehicle tank structure level use the same process parameters;

[0023] S104: For the launch vehicle tank friction plug repair welding joint area to be assessed for damage tolerance, nanoindentation technology is used to carry out testing. The measured micro-area material properties and constitutive relations are used to set the material properties for subsequent finite element simulation calculations.

[0024] Furthermore, the same process parameters were used for the aluminum alloy friction plug welding joints used to prepare the SENT specimens and the launch vehicle tank structure-level friction plug welding joints.

[0025] A second aspect of the present invention provides a damage tolerance assessment system for a friction plug repair welding joint of a launch vehicle tank, comprising:

[0026] The parameter acquisition module is used to detect defects / flaws at the interface of the friction plug welding joint of the launch vehicle tank using non-destructive testing technology to obtain defect information, material constitutive parameters and material properties;

[0027] The finite element model building module is used to build the finite element model and set the distributed interface defects based on the defect information, material constitutive parameters and material properties obtained by the parameter acquisition module;

[0028] A three-dimensional constraint parameter module is introduced to calculate the in-plane and out-of-plane constraint of the crack tip of the friction plug repair weld joint under the action of service characteristic loads; and based on the three-dimensional constraint theory, the constraint parameter f is adopted to take into account the interface defect characteristics of the friction plug repair weld joint of the launch vehicle tank. (t,w,θ) quantifying the in-plane and out-of-plane restraint effects at the crack tip;

[0029] The specimen preparation module is used to prepare single-edge notched tensile specimens of aluminum alloy friction plug repair joints using the same process parameters as those used for launch vehicle tank friction plug repair joints; and to measure the crack propagation resistance curve (JR curve) where the prefabricated crack tip is located at the interface of the friction plug repair joint;

[0030] The JR curve correction module is used to correct the JR curve obtained based on the sample preparation module, using the three-dimensional constraint parameter f that takes into account the interface defect characteristics of the friction plug welding joint of the launch vehicle tank into consideration. (t,w,θ) , based on the three-dimensional restrained fracture mechanics theory, the real JR curve of the friction plug repair welding joint of the rocket tank is corrected to obtain the corrected real JR curve;

[0031] The module for obtaining the ultimate crack size is used to calculate the driving force curves of crack tip extension of different sizes under the condition of increasing internal pressure of the launch vehicle tank through finite element method numerical simulation. When the crack tip extension driving force curve corresponding to a certain internal pressure value is tangent to the corrected true JR curve obtained by the JR curve correction module, the crack size corresponding to the intersection point is the ultimate crack size under the corresponding internal pressure.

[0032] The maximum crack size / maximum internal pressure module is used to calculate the critical J-integral of the interface crack of the friction plug repair joint of the launch vehicle tank based on the limit crack size obtained by the limit crack size acquisition module; and then calculate the maximum crack size under a specific load or the maximum internal pressure under a specific size crack based on the critical J-integral.

[0033] The beneficial effects of the present invention are:

[0034] The damage tolerance assessment method for a carrier rocket tank friction plug repair joint of the present invention is based on nondestructive testing information of interface defects of the carrier rocket tank friction plug repair joint and is based on three-dimensional constraint theory. The constraint parameter f(t, w, θ) that takes into account the interface defect characteristics of the carrier rocket tank friction plug repair joint is used to quantify the in-plane and out-of-plane constraint effects of the joint interface crack tip. The fracture toughness measurement results of the surface crack of the standard SENT specimen of the joint are then corrected to obtain the fracture toughness results of the buried crack at the interface of the carrier rocket tank friction plug repair joint. This achieves a scientific and accurate assessment of the damage tolerance of the carrier rocket tank friction plug repair joint based on the fracture mechanics framework.

[0035] Furthermore, the evaluation method uses the same process parameters as those used for friction plug welding joints on launch vehicle tanks to prepare single-edge notched tensile (SENT) specimens to determine JR curves; and uses three-dimensional restrained fracture mechanics theory to obtain modified JR curves. This allows for a more accurate reflection of the crack growth resistance of actual structures when defects are present.

[0036] The maximum crack size under a specific load or the maximum internal pressure under a specific size crack obtained by the evaluation method of the present invention provides guidance for the design and quality inspection of friction plug welding joints of launch vehicle fuel tanks, as well as for rocket manufacturing companies to determine the maintenance of reusable fuel tanks, and develops the application potential of friction plug welding technology in launch vehicle tanks. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is a flow chart of the damage tolerance assessment method for the friction plug repair welding joint of the launch vehicle tank according to the present invention;

[0038] Figure 2 is a schematic diagram of crack tip stress and geometric constraint in step 2 of the evaluation method;

[0039] Figure 3 1 is a schematic diagram of SENT sampling of the friction plug welding joint in step 4 of the evaluation method; wherein 10 is the friction plug welding joint; 11 is the distribution interface; 12 is the single-edge notch tensile specimen;

[0040] Figure 4 is a schematic diagram of a crack growth resistance curve (JR curve) obtained in step 4 of the evaluation method;

[0041] Figure 5 is a schematic diagram of the JR curve correction process in step five of the evaluation method;

[0042] Figure 6 This is a schematic diagram of solving the critical crack size of buried cracks in the friction plug repair welding joint of the rocket tank in step six of the evaluation method. DETAILED DESCRIPTION

[0043] In order to make the purpose, technical solutions, beneficial effects and significant improvements of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are clearly and completely described below in conjunction with the drawings provided in the examples of the present invention. Obviously, all the described embodiments are only partial embodiments of the present invention, rather than all embodiments; based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0044] In the description of this application, unless otherwise expressly specified or limited, the terms "first", "second", and "third" are used for descriptive purposes only and are not to be understood as indicating or implying relative importance; the term "plurality" refers to two or more; unless otherwise specified or explained, the terms "connected" and "fixed" should be understood in a broad sense. For example, "connected" can mean a fixed connection, a detachable connection, an integral connection, or an electrical connection; "connected" can mean a direct connection or an indirect connection through an intermediate medium. For those skilled in the art, the specific meanings of the above terms in this application can be understood according to the specific circumstances.

[0045] like Figure 1 As shown, a damage tolerance assessment method for a friction plug repair welding joint of a launch vehicle tank comprises:

[0046] Step 1: Detect the defects / flaws at the interface of the friction plug welding joint of the launch vehicle tank and obtain the defect information, material constitutive parameters and material properties. The specific steps include the following:

[0047] S101: Use an ultrasonic phased array flaw detector to conduct a full-coverage scan of the launch vehicle tank friction plug repair weld joints to obtain defect signals;

[0048] S102: reconstructing the acquired defect signal to obtain defect position, orientation, and size information of the weld head interface;

[0049] S103: For the aluminum alloy material used in the launch vehicle tank, a series of material mechanical property tests such as tensile tests, compression tests, and shear tests are conducted to obtain the constitutive relationship of the aluminum alloy material, namely the stress-strain curve and related constitutive parameters, which serve as input parameters for subsequent finite element simulation calculations. Specifically, the aluminum alloy friction plug repair joint used to determine the constitutive relationship of the interface region and the friction plug repair joint at the launch vehicle tank structure level use the same process parameters;

[0050] S104: For the launch vehicle tank friction plug repair welding joint area to be assessed for damage tolerance, nanoindentation technology is used to carry out testing. The measured micro-area material properties and constitutive relations are used to set the material properties for subsequent finite element simulation calculations.

[0051] Step 2: Create a finite element model

[0052] Establish a finite element model of a launch vehicle tank friction plug welded joint containing 11 structural defects of a certain shape, size, and distribution interface. Input the defect information obtained in step 1, the constitutive parameters and material properties of the material, and accurately set the corresponding interface defects in the finite element model.

[0053] Based on the microscopic mechanical properties data of the interface defect area obtained by nanoindentation measurement, the local constitutive relationship of the interface defect of the friction plug welding joint is set. The constitutive relationship (i.e., the stress-strain correspondence) is obtained by nanoindentation measurement; a typical service load, such as an internal pressure of 0.7 MPa, is applied, and the fine stress and strain field at the crack tip is calculated using the finite element numerical calculation method, such as Figure 2 As shown, σ 11 , σ 22 , σ 33 represent the normal stress components; σ 12 , σ 23 , σ 31 They represent the shear stress components respectively; r represents the radial distance from the crack tip to a certain point; θ is the polar angle, which is the angle between the crack propagation direction and the line connecting the radial segment and the crack tip, and is used to obtain key fracture mechanics parameter information such as in-plane and out-of-plane constraint.

[0054] Step 3: Introducing three-dimensional constraint parameters

[0055] Based on the three-dimensional constraint theory, the constraint parameter f is adopted to consider the interface defect characteristics of the friction plug welding joint of the launch vehicle tank. (t,w,θ) This method quantifies the in-plane and out-of-plane constraints at the crack tip, where t is the crack tip constraint parameter, w is the crack width constraint parameter, and θ is the crack angle constraint parameter. By introducing this three-dimensional constraint parameter, it is possible to more accurately describe the impact of complex defect morphologies on crack growth behavior in actual structures.

[0056] Step 4: Prepare the sample

[0057] like Figure 3 As shown in the figure, the same process parameters as those for the friction plug welding joint of the launch vehicle tank were used to prepare the aluminum alloy friction plug welding joint specimen; a single-side notch was machined on the specimen, and a crack was pre-made to prepare the single-side notch tensile (SENT) specimen12. Using the idea of ​​"replacing the curve with a straight line", the Figure 4 The crack propagation resistance curve (JR curve) shown is located at the local interface of the friction plug welding joint 10 where the prefabricated crack tip is located. The JR curve can reflect the material's ability to resist crack propagation and is key data for subsequent damage tolerance assessment.

[0058] Step 5: JR curve correction of friction plug welding joint based on three-dimensional constraint parameters at the crack tip

[0059] like Figure 5 As shown in the figure, the three-dimensional constraint parameter f is used to consider the influence of the defect characteristics of the distributed interface 11 of the friction plug welding joint of the launch vehicle tank. (t,w,θ), based on the three-dimensional constrained fracture mechanics theory, the real JR curve of the friction plug repair welding joint of the rocket tank is corrected and converted into a buried JR curve; the buried JR curve can more accurately reflect the ability to resist crack propagation when defects exist in the actual structure, and corresponds to the corrected JR curve of the friction plug repair welding joint mentioned in step six.

[0060] Step 6: Obtain the critical crack size

[0061] like Figure 6 As shown in the figure, the tangent method is used to accurately solve the damage tolerance of the friction plug repair joint of the aluminum alloy launch vehicle tank with a specific thickness and buried cracks. Based on the constitutive relationship of the circumferential interface micro-area of ​​the friction plug repair joint measured above, the finite element method is used to numerically simulate the driving force curves of crack tips of different sizes when the internal pressure of the launch vehicle tank increases from 0 MPa. When the crack tip propagation driving force curve corresponding to a certain internal pressure value (withstanding internal pressure) P is tangent to the JR curve of the friction plug repair joint of the structural tank, the crack size a corresponding to the intersection point is max This is the limit crack size under the corresponding internal pressure. This limit crack size is the maximum defect size that the structure can safely bear under the internal pressure load.

[0062] Step 7: Based on the critical crack size a under the action of internal pressure P max , calculate the critical J integral J of the interface crack of the friction plug welding joint of the launch vehicle tank mat This critical J-integral value can be used to evaluate the remaining life of the structure under different load levels, thereby guiding operation and maintenance decisions; and then calculate the maximum crack size under a specific load or the maximum internal pressure under a specific size crack, providing guidance for the design and quality inspection of friction plug repair joints of launch vehicle fuel tanks, as well as for rocket manufacturing companies to determine the maintenance of reusable fuel tanks, and exploring the application potential of friction plug repair welding technology in launch vehicle tanks.

[0063] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in the embodiments can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A damage tolerance assessment method for friction plug welding joints of launch vehicle tanks, characterized in that include: Step 1: Using non-destructive testing technology to detect defects / flaws at the interface of the friction plug welding joint (10) of the launch vehicle tank, and obtaining defect information, material constitutive parameters and material properties; Step 2: Establish a finite element model of the launch vehicle tank friction plug repair welding joint, and accurately set the corresponding distributed interface (11) defects in the finite element model based on the defect information, material constitutive parameters and material properties obtained in step 1; Step 3: Based on the nanoindentation measurement results, the local constitutive relationship of the interface defect of the friction plug welding joint is set, and the finite element numerical calculation method is used to calculate the in-plane and out-of-plane constraints of the crack tip of the friction plug welding joint (10) under the service characteristic load; Based on the three-dimensional constraint theory, the constraint parameter f is adopted to consider the interface defect characteristics of the friction plug welding joint of the launch vehicle tank. (t,w,θ) Quantify the above-mentioned in-plane and out-of-plane constraint effects at the crack tip; where t is the crack tip constraint parameter, w is the crack width constraint parameter, and θ is the crack angle constraint parameter; Step 4: Processing and preparing a single-edge notched tensile specimen (12) and pre-forming a crack; using a straight line to curve method to measure the crack propagation resistance JR curve at the local interface of the friction plug welding joint where the pre-formed crack tip is located; Step 5: Use the three-dimensional constraint parameter f that takes into account the defect characteristics of the distributed interface (11) of the friction plug welding joint of the launch vehicle tank (t,w,θ) , based on the three-dimensional restrained fracture mechanics theory, the real JR curve of the friction plug repair welding joint of the rocket tank is corrected to obtain the corrected real JR curve; Step 6: Based on the micro-region constitutive relationship of the circumferential interface of the friction plug welded joint measured in Step 3, the finite element method is used to numerically simulate and calculate the driving force curves of crack tip propagation of different sizes under the condition of increasing internal pressure in the launch vehicle tank. When the crack tip propagation driving force curve corresponding to a certain internal pressure value is tangent to the corrected true JR curve obtained in Step 5, the crack size corresponding to the intersection point is the limit crack size under the corresponding internal pressure. Step 7: Based on the critical crack size under internal pressure, calculate the critical J-integral of the interface crack of the friction plug welding joint 10 of the launch vehicle tank, which is used to evaluate the remaining life of the structure under different load levels; The maximum crack size under a specific load or the maximum internal pressure under a specific size crack is then calculated based on the critical J integral, which is used in the design and quality inspection of subsequent friction plug welding joints of launch vehicle fuel tanks.

2. The damage tolerance assessment method for the friction plug welding joint of a launch vehicle tank according to claim 1 is characterized in that: The step one comprises: S101: Use an ultrasonic phased array flaw detector to conduct a full-coverage scan of the launch vehicle tank friction plug repair weld joints to obtain defect signals; S102: reconstructing the acquired defect signal to obtain defect position, orientation, and size information of the weld head interface; S103: For the aluminum alloy material used in the launch vehicle tank, a series of material mechanical property tests such as tensile tests, compression tests, and shear tests are conducted to obtain the constitutive relationship of the aluminum alloy material, namely the stress-strain curve and related constitutive parameters, which serve as input parameters for subsequent finite element simulation calculations. Specifically, the aluminum alloy friction plug repair joint used to determine the constitutive relationship of the interface region and the friction plug repair joint at the launch vehicle tank structure level use the same process parameters; S104: For the launch vehicle tank friction plug repair welding joint area to be assessed for damage tolerance, nanoindentation technology is used to carry out testing. The measured micro-area material properties and constitutive relations are used to set the material properties for subsequent finite element simulation calculations.

3. The damage tolerance assessment method for the friction plug repair welding joint of a launch vehicle tank according to claim 1 is characterized in that: The aluminum alloy friction plug welding joints used to prepare the SENT specimens adopt the same process parameters as the launch vehicle tank structure-level friction plug welding joints.

4. A damage tolerance assessment system for friction plug welding joints of launch vehicle tanks, characterized in that: include: The parameter acquisition module is used to detect defects / flaws at the interface of the friction plug welding joint of the launch vehicle tank using non-destructive testing technology to obtain defect information, material constitutive parameters and material properties; A finite element model building module is used to build a finite element model and set a distribution interface (11) defect based on the defect information, material constitutive parameters and material properties obtained by the parameter acquisition module; A three-dimensional constraint parameter module is introduced to calculate the in-plane and out-of-plane constraint of the crack tip of the friction plug welding joint (10) under the action of service characteristic load; and based on the three-dimensional constraint theory, the constraint parameter f is adopted to take into account the interface defect characteristics of the friction plug welding joint of the launch vehicle tank. (t,w,θ) quantifying the in-plane and out-of-plane restraint effects at the crack tip; A sample preparation module is used to prepare a single-edge notch tensile specimen (12) of an aluminum alloy friction plug repair joint using the same process parameters as those of a launch vehicle tank friction plug repair joint; and to measure a crack propagation resistance JR curve where the prefabricated crack tip is located at a local interface of the friction plug repair joint (10); The JR curve correction module is used to correct the JR curve obtained by the sample preparation module, and adopts the three-dimensional constraint parameter f that takes into account the influence of the defect characteristics of the distributed interface (11) of the friction plug welding joint of the launch vehicle tank. (t,w,θ) , based on the three-dimensional restrained fracture mechanics theory, the real JR curve of the friction plug repair welding joint of the rocket tank is corrected to obtain the corrected real JR curve; The module for obtaining the ultimate crack size is used to calculate the driving force curves of crack tip extension of different sizes under the condition of increasing internal pressure of the launch vehicle tank through finite element method numerical simulation. When the crack tip extension driving force curve corresponding to a certain internal pressure value is tangent to the corrected true JR curve obtained by the JR curve correction module, the crack size corresponding to the intersection point is the ultimate crack size under the corresponding internal pressure. The maximum crack size / maximum internal pressure module is used to calculate the critical J integral of the interface crack of the friction plug welding joint (10) of the launch vehicle tank based on the limit crack size obtained by the limit crack size acquisition module; and then calculate the maximum crack size under a specific load or the maximum internal pressure under a specific size crack based on the critical J integral.

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