A method for predicting stress corrosion initiation life coupling environmental and process effects

Abstract

The application belongs to the field of structure analysis and optimization design, and provides a stress corrosion initiation life prediction method coupled with environmental and process effects, comprising: H-K-T curve construction; multi-scale digital mockup generation considering assembly stress; R angle and assembly stress distribution of hole edge of stress corrosion key area are updated based on mockup and detection data inversion; key part stress intensity factor Kt calculation superimposed with assembly stress deviation; ground and air load spectrum compilation; stress corrosion crack initiation life estimation; the application breaks the limitation of traditional method only considering single load or environment, greatly improves the accuracy of stress corrosion initiation life prediction, and provides a more actual service working condition basis for life evaluation of key structure of aircraft.

Description

Technical Field

[0001] This invention belongs to the field of structural analysis and optimization design, specifically relating to a method for predicting the initiation life of stress corrosion coupled with environmental and process influences. Background Technology

[0002] Stress corrosion refers to the low-stress brittle fracture phenomenon that occurs in metallic materials under the combined action of static stress and a specific corrosive environment. This fracture often occurs at stress levels far below the material's yield strength, and initially there is no obvious macroscopic deformation. It has extremely strong concealment and suddenness, and once it occurs, it can cause catastrophic damage to critical structures.

[0003] Initiation life refers to the time or number of cycles that a metallic material undergoes from an initial state without visible cracks to the initiation of the first detectable macroscopic stress corrosion crack on its surface under the combined action of a specific corrosive environment and static stress. It is a key initial indicator for assessing the risk of structural stress corrosion failure and directly determines the basis for calculating subsequent crack propagation life and the accuracy of assessing the remaining life of the structure.

[0004] In the field of aircraft airframe structure analysis and optimization design, there is a dual impact of assembly stress and corrosive environment. After long-term use, stress corrosion cracks are prone to occur. At present, there is no engineering-available method for predicting the initiation life of stress corrosion that combines the effects of coupled environment and process. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a method for predicting the initiation life of stress corrosion coupled with environmental and process influences, thereby resolving the issues in the prior art. The technical solution adopted by this invention is as follows: A method for predicting the initiation lifetime of stress corrosion coupled with environmental and process influences includes the following steps: Step 1, H-Kt-T curve construction: Stress corrosion tests are conducted under different corrosion environments to obtain Kt-T curves for different corrosion environments, and a structured H-Kt-T data curve library is established; where H is the corrosion environment, Kt is the stress intensity factor, and T is the initiation lifetime; Step 2, Multi-scale digital prototype generation considering assembly stress: For key parts of stress corrosion, construct multi-level, multi-scale digital analysis prototypes, taking into account the residual stress distribution, deformation deviation and assembly stress of the parts during construction. Step 3: Based on the digital analysis prototype and test data, the assembly stress of the R-angle and hole edge of the key parts of stress corrosion is inverted and updated; Step 4, calculate the stress intensity factor Kt of the key parts of superimposed assembly stress deviation: introduce the assembly stress of Step 3 into the digital analysis prototype, apply flight load under envelope matching, and obtain the stress intensity factor Kt of the R angle and hole edge of the key parts of stress corrosion. Step 5, Ground-to-air load spectrum compilation: Based on the flight and parking envelopes, compile the ground-to-air load spectrum for stress corrosion lifetime analysis; Step 6, Stress corrosion crack initiation life estimation: The stress corrosion crack initiation life is obtained by using the stress intensity factor Kt and linear damage theory.

[0006] Furthermore, in step 1, the H-Kt-T curves of typical structural features in different corrosion environments are obtained, including the hole edge and R-angle features of key stress corrosion sites.

[0007] Furthermore, in step 1, when obtaining Kt-T curves for different corrosion environments and establishing a structured H-Kt-T data curve library, the following is included: Based on the experimental data obtained from stress corrosion tests, for each corrosion environment, a nonlinear fitting was performed on the initiation life T corresponding to different stress intensity factors Kt to establish a fitting model of Kt and T, expressed as: Where T is the initiation lifetime, Kt is the stress intensity factor, and A and n are fitting constants related to the corrosion environment and material. By fitting the model, Kt-T curves for different corrosion environments are generated, and an H-Kt-T data curve library is constructed.

[0008] Furthermore, in step 2, the multi-level, multi-scale digital analysis prototype includes three levels: the whole system level, the component level, and the stress corrosion critical part level. The criteria for determining the stress corrosion critical part level include: hole edge and R-corner characteristics; stress level under flight load exceeding the threshold; and direct exposure to the corrosive environment.

[0009] Furthermore, in step 4, the obtained stress intensity factor Kt includes the ground stress intensity factor corresponding to the ground parking state. Stress intensity factor corresponding to flight state .

[0010] Furthermore, in step 5, the ground-to-air load spectrum is based on a ground-to-air cycle and includes a ground parking state spectrum block and an air flight state spectrum block. Ground parking status spectrum: corresponding to the aircraft ground parking stage, the load is assembly stress and residual stress, records the total ground parking time in a single ground-to-air cycle, and matches the corrosive environment corresponding to ground parking; Airborne flight state spectrum block: corresponding to the aircraft's airborne flight phase, the loads are assembly stress, residual stress and flight working stress, recording the total airborne flight time in a single ground-to-air cycle, matching the corrosive environment corresponding to airborne flight; Furthermore, step 6 includes: Step 6.1, Single-cycle damage calculation: Obtain the priming lifetime of the ground-parked state and the air-flying state respectively, and then obtain the total damage of a single ground-to-air cycle; For ground-based parking, the corresponding ground corrosion environment should be considered. Ground stress intensity factor The initiation lifetime under this operating condition can be obtained by retrieving interpolation values ​​from the H-Kt-T curve library. ; For in-flight conditions, the corresponding airborne corrosion environment should be considered. Air stress intensity factor The stress corrosion crack initiation life under this working condition was obtained by retrieving interpolation values ​​from the H-Kt-T curve library. ; The formula for calculating the total damage in a single ground-to-air cycle is: Step 6.2, Calculation of crack initiation life throughout the entire life cycle: When the cumulative damage reaches 1, the structure is determined to have initiated stress corrosion cracks. The total number of cycles for stress corrosion crack initiation is calculated. for: Combine the total duration of a single loop The number of flight cycles per year is used to calculate the primordial lifespan in flight hours and calendar years.

[0011] The present invention has the following beneficial effects: By constructing an H-Kt-T data curve library, the present invention establishes a quantitative correlation between different corrosion environments, stress intensity factors and initiation life, and introduces process factors such as assembly stress and residual stress, breaking the limitation of traditional methods that only consider load or environment, and greatly improving the accuracy of stress corrosion initiation life prediction, providing a basis that is more in line with actual service conditions for the life assessment of key aircraft structures. Attached Figure Description

[0012] Figure 1 For flowcharts; Figure 2 A schematic diagram showing the deformation and residual stress of the part. Figure 3 This is a schematic diagram of the assembly stress at the radius (R) and the edge of the hole. Detailed Implementation

[0013] The following will be described in conjunction with embodiments of the present invention. Figures 1-3 The technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0014] like Figure 1 This invention proposes a method for predicting the initiation life of stress corrosion coupled with environmental and process influences, used for multi-scale analysis of aircraft airframe structures, including the following steps: Step 1, H-Kt-T curve construction: Stress corrosion tests are conducted under different corrosion environments to obtain Kt-T curves for different corrosion environments, and a structured H-Kt-T data curve library is established; where H is the corrosion environment, Kt is the stress intensity factor, and T is the initiation lifetime; Step 2, Multi-scale digital prototype generation considering assembly stress: For key parts of stress corrosion, construct multi-level, multi-scale digital analysis prototypes, taking into account the residual stress distribution, deformation deviation and assembly stress of the parts during construction. Step 3: Based on the digital analysis prototype and test data, the assembly stress of the R-angle and hole edge of the key parts of stress corrosion is inverted and updated; Step 4, calculate the stress intensity factor Kt of the key parts of superimposed assembly stress deviation: introduce the assembly stress of Step 3 into the digital analysis prototype, apply flight load under envelope matching, and obtain the stress intensity factor Kt of the R angle and hole edge of the key parts of stress corrosion. Step 5, Ground-to-air load spectrum compilation: Based on the flight and parking envelopes, compile the ground-to-air load spectrum for stress corrosion lifetime analysis; Step 6, Stress corrosion crack initiation life estimation: The stress corrosion crack initiation life is obtained by using the stress intensity factor Kt and linear damage theory.

[0015] Furthermore, in step 1, the H-Kt-T curves of typical structural features in different corrosion environments are obtained, including the hole edge and R-angle features of key stress corrosion sites.

[0016] Furthermore, in step 1, multiple independent corrosion environments are quantitatively defined for the entire service scenarios of the target aircraft, including but not limited to neutral salt spray environment, humid heat cycle environment, marine atmospheric environment, industrial acidic atmospheric environment, and dry-wet alternating environment; each environment corresponds to a unique structured parameter, including environmental code, corrosive medium composition and concentration, temperature, relative humidity, pressure, etc.

[0017] For the hole edge and R-corner characteristics of the critical stress corrosion zone, standard stress corrosion specimens with corresponding structures are designed, including tensile specimens with a central circular hole (matching hole edge characteristics) and notched tensile specimens with different fillet radii R (matching R-corner characteristics); the theoretical stress concentration factor of each group of specimens is calculated in advance, covering the common Kt value range of 1.5~6.0 for aircraft structures.

[0018] According to the GB / T15970 series of standards "Corrosion of metals and alloys - Stress corrosion testing", the constant load endurance tensile method was used for the test. For each corrosion environment Hi and each Kt level specimen, a constant tensile load was applied and the specimen was placed in the corresponding corrosion environment throughout the test. The crack initiation state of the specimen was continuously monitored by an in-situ crack monitoring system (DC potential method and ultrasonic microscopy). The duration of continuous loading when a detectable macroscopic crack (crack length ≥ 0.2 mm) appeared on the specimen was taken as the stress corrosion crack initiation life under that working condition.

[0019] When acquiring Kt-T curves for different corrosion environments and establishing a structured H-Kt-T data curve library, the following steps are included: Based on the experimental data obtained from stress corrosion tests, for each corrosion environment, a nonlinear fitting was performed on the initiation life T corresponding to different stress intensity factors Kt to establish a fitting model of Kt and T, expressed as: Where T is the initiation lifetime, Kt is the stress intensity factor, and A and n are fitting constants related to the corrosion environment and material. By fitting a model, Kt-T curves for different corrosion environments are generated, and an H-Kt-T data curve library is constructed. The data curve library fields include: corrosion environment code, environmental parameters, structural feature type, Kt value, experimental initiation life T, fitted curve equation, goodness of fit, material and heat treatment state, enabling rapid retrieval, interpolation and retrieval of operating parameters.

[0020] Furthermore, step 2 includes: Key area identification and multi-level, multi-scale model partitioning: Based on the overall load transfer path, corrosion environment exposure level, and structural feature type, a criterion for determining key areas of stress corrosion is established. The criteria for key parts of stress corrosion include: stress concentration features such as hole edges and radius corners; high stress levels under flight loads; direct exposure to a corrosive environment; and susceptibility to additional assembly stress during assembly. After identifying key areas based on the criteria, the model is partitioned into three levels and multiple scales. Full-system level: 1:1 full-size model, used for load boundary transfer and mapping across the entire machine; Component level: Models of large components such as fuselage, wings, and tail, used for modeling assembly relationships and contact boundaries; Stress corrosion critical component level: Sub-millimeter precision mesh model for refined stress concentration analysis at hole edges and radius corners.

[0021] For critical components susceptible to stress corrosion, simulation calculations are performed along the entire manufacturing process to obtain the residual stress distribution across the entire part. For sheet metal forming parts, elastoplastic finite element simulation is used to simulate the entire forming process and obtain residual stress. For machined parts, the thermo-mechanical coupling effect of the cutting process is simulated to obtain residual stress on the machined surface. For heat-treated parts, the microstructure transformation and stress evolution during quenching and aging are simulated to obtain residual stress. The residual stress field is used as the initial stress condition and mapped onto the part mesh of a multi-scale digital prototype. Then, based on the actual dimensional data of the part measured by coordinate measuring machine (CMM) or the deformation prediction results from the manufacturing process simulation, the ideal CAD model is geometrically corrected. Dimensional deviations and geometric tolerances of the part are introduced to construct a digital twin model with actual deformation, eliminating stress calculation errors caused by deviations between the ideal geometry and the actual part. Finally, based on the assembly process flow, finite element contact simulation was used to simulate the entire assembly process, including core processes such as bolt pre-tightening, riveting, interference fit, and fixture positioning deviation, to calculate the additional assembly stress generated during the assembly process; the assembly stress field was superimposed with the residual stress field of the parts as the initial stress state of the digital prototype, and the construction of the multi-scale digital prototype was completed.

[0022] Furthermore, step 3 includes: For critical stress corrosion areas such as the radius (R-angle) and hole edges, non-destructive testing methods were used to obtain measured stress data: X-ray diffraction was used to detect the total stress, including residual stress and assembly stress, on the surface of the hole edges and R-angles, with measuring points evenly distributed along the hole perimeter and R-angle contour, with a spacing of no more than 1 mm; the blind hole method was used to release stress at key measuring points and obtain the stress gradient distribution in the depth direction; ultrasonic phased array was used to detect the contact pressure of the assembly mating surfaces to supplement the assembly stress boundary conditions; and process characteristic parameters such as the actual heat treatment state of the parts, the actual preload of bolts, assembly clearance, and surface roughness were simultaneously collected as inputs for inversion optimization.

[0023] A stress field inversion proxy model for a digital prototype is constructed. Process characteristic parameters such as bolt preload, assembly gap, residual stress amplitude, and contact surface friction coefficient are used as input variables, and the measured stress values ​​at key measurement points are used as optimization objectives. The input-output proxy model is constructed using the response surface methodology, and the optimal combination of input parameters is solved by a genetic algorithm to ensure that the relative error between the calculated stress value and the measured value of the digital prototype is less than 5%.

[0024] The optimal process parameters obtained by inversion are substituted into the multi-scale digital prototype to recalculate the stress distribution across the entire field. The assembly stress fields of the R-angle and hole edge in the critical stress corrosion zone are updated, and the initial residual stress of the part is superimposed to obtain the total stress distribution in the critical area. The updated stress field is digitally characterized to output the stress distribution cloud map around the hole, the R-angle stress gradient curve, the stress concentration factor distribution, and the assembly stress ratio statistics, generating a stress field file that can be directly called.

[0025] Furthermore, step 4 includes: Based on the aircraft flight envelope, the ultimate load and operational load of typical flight conditions such as cruise, takeoff and landing, maneuvering, and gusts are extracted. Through full-aircraft load transfer calculation, the flight load is accurately mapped to the stress corrosion key parts of the multi-scale digital prototype. Load boundary conditions and constraints are applied to ensure that the load transfer path is the same as the actual service state.

[0026] The assembly stress field updated in step 3 is used as the initial stress condition and linearly superimposed with the working stress generated by the flight load. Elastic-plastic finite element analysis is carried out to calculate the full-field stress distribution in the key area, and extract the maximum principal stress distribution at the hole edge and the critical point of the R-corner, as well as the stress gradient in the stress concentration area.

[0027] For the critical crack initiation points at the hole edge and radius (R-angle), the stress intensity factor Kt is calculated using the finite element method of fracture mechanics: For the hole edge feature, the weighted function method is used to calculate the stress intensity factor at the crack tip around the hole, considering the effects of hole diameter, wall thickness, and spacing between adjacent holes; for the radius (R-angle) feature, the 1 / 4-node singular element method is used, setting fracture mechanics singular elements at the critical R-angle point to calculate the Kt value at the crack tip. The obtained stress intensity factor Kt includes the ground stress intensity factor corresponding to the ground parking state. Stress intensity factor corresponding to flight state .

[0028] Furthermore, step 5 includes: Based on actual aircraft usage scenarios, a complete ground-to-air cycle mission profile is broken down. A single ground-to-air cycle includes: the ground parking phase (parking on the tarmac, ground preparation), the takeoff and climb phase, the cruise flight phase, the maneuvering flight phase, the descent and landing phase, and the ground parking phase. The duration, load level, and corrosive environment of each phase are clearly defined. Considering the time-correlation characteristics of stress corrosion lifetime analysis, based on the ground-to-air cycle, it is divided into ground parking state spectrum blocks and air flight state spectrum blocks. Ground parking status spectrum: corresponding to the aircraft ground parking stage, the load is assembly stress and residual stress, records the total ground parking time in a single ground-to-air cycle, and matches the corrosive environment corresponding to ground parking; Airborne flight state spectrum block: corresponding to the aircraft's airborne flight phase, with loads including assembly stress, residual stress, and flight working stress, recording the total airborne flight time within a single ground-to-air cycle, and matching the corresponding corrosion environment for airborne flight.

[0029] Based on the annual number of aircraft flights, annual flight hours, and annual parking time, the average value of a single ground-to-air cycle is calculated. , The load spectrum was processed using the rainflow counting method to remove minor loads that contributed very little to damage, retaining the main load cycles that played a dominant role in crack initiation. Finally, a normalized ground-to-air load spectrum was compiled, with the spectrum file containing the ground time of each cycle. Time in the air Ground / air load level, corresponding corrosion environment type, and number of cycles throughout the entire life cycle.

[0030] Furthermore, step 6 includes: Step 6.1, Calculation of damage in a single cycle: Based on Miner’s linear cumulative damage theory, it is assumed that the damage of each ground-to-air cycle is independent and can be linearly superimposed. For a single ground-to-air cycle, the priming lifetime of the ground parking state and the air flight state are obtained respectively, and then the total damage of a single ground-to-air cycle is obtained. For ground-based parking, the corresponding ground corrosion environment should be considered. Ground stress intensity factor The initiation lifetime under this operating condition can be obtained by retrieving interpolation values ​​from the H-Kt-T curve library. ; For in-flight conditions, the corresponding airborne corrosion environment should be considered. Air stress intensity factor The stress corrosion crack initiation life under this working condition was obtained by retrieving interpolation values ​​from the H-Kt-T curve library. ; The formula for calculating the total damage in a single ground-to-air cycle is: Step 6.2, Calculation of crack initiation life throughout the entire life cycle: Based on the Miner criterion, when the cumulative damage reaches 1, the structure is determined to have initiated stress corrosion cracks. The total number of cycles for stress corrosion crack initiation is... for: Combine the total duration of a single loop The number of flight cycles per year is used to calculate the primordial lifespan, expressed in flight hours and calendar years. The calculation formula is as follows: Initiation lifespan in flight hours: in, Flight hour life refers to the total number of flight hours from when the structure is put into use until crack initiation. Germination lifespan in calendar years: or: in, Calendar lifespan refers to the total calendar years from when the structure is put into use until cracks begin to appear. This refers to the number of flight cycles per year, that is, the number of ground-to-air cycles completed each year.

[0031] The above embodiments are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Any modifications, alterations, alterations, or substitutions made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for predicting the initiation life of stress corrosion coupled with environmental and process influences, characterized in that, Includes the following steps: Step 1, H-Kt-T curve construction: Stress corrosion tests are conducted under different corrosion environments to obtain Kt-T curves for different corrosion environments, and a structured H-Kt-T data curve library is established; where H is the corrosion environment, Kt is the stress intensity factor, and T is the initiation lifetime; Step 2, Multi-scale digital prototype generation considering assembly stress: For key parts of stress corrosion, construct multi-level, multi-scale digital analysis prototypes, taking into account the residual stress distribution, deformation deviation and assembly stress of the parts during construction. Step 3: Based on the digital analysis prototype and test data, the assembly stress of the R-angle and hole edge of the key parts of stress corrosion is inverted and updated; Step 4, calculate the stress intensity factor Kt of the key parts of superimposed assembly stress deviation: introduce the assembly stress of Step 3 into the digital analysis prototype, apply flight load under envelope matching, and obtain the stress intensity factor Kt of the R angle and hole edge of the key parts of stress corrosion. Step 5, Ground-to-air load spectrum compilation: Based on the flight and parking envelopes, compile the ground-to-air load spectrum for stress corrosion lifetime analysis; Step 6, Stress corrosion crack initiation life estimation: The stress corrosion crack initiation life is obtained by using the stress intensity factor Kt and linear damage theory.

2. The method for predicting the initiation life of stress corrosion coupled with environmental and process influences according to claim 1, characterized in that, In step 1, the H-Kt-T curves of typical structural features in different corrosion environments are obtained. The typical structural features include the hole edge and R-angle features of key stress corrosion sites.

3. The method for predicting the initiation life of stress corrosion coupled with environmental and process influences according to claim 1, characterized in that, In step 1, when obtaining Kt-T curves for different corrosion environments and establishing a structured H-Kt-T data curve library, the following steps are included: Based on the experimental data obtained from stress corrosion tests, for each corrosion environment, a nonlinear fitting was performed on the initiation life T corresponding to different stress intensity factors Kt to establish a fitting model of Kt and T, expressed as: Where T is the initiation lifetime, Kt is the stress intensity factor, and A and n are fitting constants related to the corrosion environment and material. By fitting the model, Kt-T curves for different corrosion environments are generated, and an H-Kt-T data curve library is constructed.

4. The method for predicting the initiation life of stress corrosion coupled with environmental and process influences according to claim 1, characterized in that, In step 2, the multi-level, multi-scale digital analysis prototype includes three levels: the whole system level, the component level, and the stress corrosion critical part level. Criteria for determining critical stress corrosion include: features such as hole edges and radius (R-angle); stress levels exceeding a threshold under flight loads; and direct exposure to a corrosive environment.

5. The method for predicting the initiation life of stress corrosion coupled with environmental and process influences according to claim 1, characterized in that, In step 4, the obtained stress intensity factor Kt includes the ground stress intensity factor corresponding to the ground parking state. Stress intensity factor corresponding to flight state .

6. The method for predicting the initiation life of stress corrosion coupled with environmental and process influences according to claim 5, characterized in that, In step 5, the ground-to-air load spectrum is based on a ground-to-air cycle and includes a ground parking state spectrum block and an air flight state spectrum block. Ground parking status spectrum: corresponding to the aircraft ground parking stage, the load is assembly stress and residual stress, records the total ground parking time in a single ground-to-air cycle, and matches the corrosive environment corresponding to ground parking; Airborne flight state spectrum block: corresponding to the aircraft's airborne flight phase, with loads including assembly stress, residual stress, and flight working stress, recording the total airborne flight time within a single ground-to-air cycle, and matching the corresponding corrosion environment for airborne flight.

7. The method for predicting the initiation life of stress corrosion coupled with environmental and process influences according to claim 6, characterized in that, Step 6 includes: Step 6.1, Single-cycle damage calculation: Obtain the priming lifetime of the ground-parked state and the air-flying state respectively, and then obtain the total damage of a single ground-to-air cycle; For ground-based parking, the corresponding ground corrosion environment should be considered. Ground stress intensity factor The initiation lifetime under this operating condition can be obtained by retrieving interpolation values ​​from the H-Kt-T curve library. ; For in-flight conditions, the corresponding airborne corrosion environment should be considered. Air stress intensity factor The stress corrosion crack initiation life under this working condition was obtained by retrieving interpolation values ​​from the H-Kt-T curve library. ; The formula for calculating the total damage in a single ground-to-air cycle is: Step 6.2, Calculation of crack initiation life throughout the entire life cycle: When the cumulative damage reaches 1, the structure is determined to have initiated stress corrosion cracks. The total number of cycles for stress corrosion crack initiation is calculated. for: Combine the total duration of a single loop The number of flight cycles per year is used to calculate the primordial lifespan in flight hours and calendar years.

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