A method for measuring residual stress of bending-molded tubes using X-rays

Abstract

The present invention provides a method for arranging points for measuring the residual stress of a bend-formed tube using X-rays, and relates to the technical field of residual stress detection. The method specifically includes: obtaining the material mechanical performance parameters of the bend-formed tube and evaluating the residual stress sensitivity of the bend-formed tube material; evaluating the safety level of the bend-formed tube by establishing a two-dimensional risk assessment matrix based on the working environment of the bend-formed tube; evaluating the time cost and economic cost of measuring the residual stress of the bend-formed tube using X-rays, and determining the number of detection points within the budget of the bend-formed tube based on the evaluation results; calculating the number of detection points of the bend-formed tube based on the residual stress sensitivity and safety level of the bend-formed tube; and arranging detection points on the bend-formed tube using a differentiated point density strategy based on the total number of detection points and the number of detection points within the budget. The present invention achieves targeted and efficient residual stress detection of bend-formed tubes by rationally arranging stress detection points of different densities.

Description

Technical Field

[0001] The present invention relates to the technical field of residual stress detection, and in particular to a point arrangement method for measuring the residual stress of a bending-molded tube by utilizing X-rays. Background Art

[0002] As a core process in tube bending, bending is widely used in high-precision applications such as automotive exhaust pipes and aerospace hydraulic piping. Plastic deformation forms complex spatial structures into the tube material, meeting lightweight and functional requirements. However, during the bending process, significant residual stresses are generated due to localized plastic flow and contact between the material and the die. These internal stresses can easily cause deformation, fatigue cracking, and even stress corrosion, directly impacting the service life and safety of the component. Especially in extreme operating conditions such as aerospace engine piping and nuclear power pipelines, precise detection and control of residual stresses are critical to ensuring system reliability.

[0003] Among the many residual stress detection methods, X-ray diffraction stands out as the preferred technique for residual stress detection in pipe bends due to its non-destructive, high-precision, and material-universal advantages. By analyzing the X-ray diffraction angle shift caused by lattice strain, this method can accurately quantify surface and near-surface stress distribution, providing data support for optimizing bending process parameters and suppressing stress concentration, thereby promoting the integration of "non-destructive quality inspection and intelligent control" in high-end manufacturing.

[0004] In residual stress testing for pipe bends, test point placement is a key component in ensuring the reliability and validity of test results. It serves as the hub connecting process defect identification, data quality assurance, and subsequent optimization. Its core value lies in minimizing risks, such as structural failure caused by excessive stress, safety hazards, and potential increased costs, at the lowest cost. Traditional test point placement methods rely primarily on the operator's engineering experience, with the operator determining the placement based on this experience. This approach is highly subjective and can easily overlook high-stress risk areas, such as mold contact points, leading to missed crack detection. Furthermore, this approach lacks a unified standard and systematic test point placement specifications. The resulting test point placement scheme, driven by empirical guidance, can easily create redundant test areas, significantly reducing test efficiency and increasing testing costs. Therefore, while traditional test point placement methods can meet basic testing needs, their subjectivity and inefficiency hinder the development of high-precision manufacturing. Human error can easily introduce measurement bias, leading to missed detection of peak stress areas. Summary of the Invention

[0005] In response to the above-mentioned shortcomings of the existing technology, the present invention simulates the stress distribution of the bent pipe through finite element software, and selects high stress areas in the process of X-ray measurement of the residual stress of the bent pipe for distribution according to the simulation results. A distribution method for measuring the residual stress of the bent pipe using X-rays is proposed to solve the problems of missed detection, low efficiency and high cost that are easily generated in the traditional distribution method of residual stress test of bent pipes. It has the advantages of precise targeting, reduced redundancy, improved efficiency and cost optimization.

[0006] The present invention proposes a method for measuring residual stress of a bending-molded tube using X-rays, which includes the following steps:

[0007] For any bending forming tube to be tested, obtain the material mechanical property parameters of the bending forming tube and evaluate the residual stress sensitivity of the bending forming tube material;

[0008] According to the working environment of the bending forming tube to be tested, the safety level of the bending forming tube is evaluated by establishing a two-dimensional risk assessment matrix;

[0009] Evaluate the time and economic costs of using X-rays to measure the residual stress of the bend-formed tube to be tested, and determine the number of inspection points within the budget based on the evaluation results;

[0010] Calculate the number of inspection points of the bend-formed tube to be tested based on the residual stress sensitivity and safety level of the bend-formed tube to be tested;

[0011] The total number of inspection points on the bend-formed tube to be tested is compared with the number of inspection points within the budget, and a differentiated point density strategy is adopted to arrange the inspection points on the bend-formed tube to be tested based on the comparison results.

[0012] Furthermore, for any bending forming tube to be tested, the specific contents of obtaining the material mechanical performance parameters of the bending forming tube and evaluating the residual stress sensitivity of the bending forming tube material are:

[0013] Obtain the yield strength of the bending forming tube material and tensile strength , and calculate the yield strength and tensile strength The ratio of

[0014] According to the yield strength and tensile strength The sensitivity level of the bending forming tube material is determined to be high sensitivity, medium sensitivity or low sensitivity by the ratio of

[0015] The sensitivity encryption coefficient of the bending forming tube is determined according to the sensitivity level of the bending forming tube material.

[0016] Furthermore, the method for determining the sensitivity level of the bending forming tube material is as follows: if the ratio of the material , then the sensitivity level of the material is high sensitivity; if the corresponding ratio of the material When the material's sensitivity level is medium sensitivity; if the material's corresponding ratio , the sensitivity level of the material is low sensitivity.

[0017] Furthermore, the specific content of evaluating the safety level of the bend-formed tube by establishing a two-dimensional risk assessment matrix based on the working environment of the bend-formed tube to be tested is:

[0018] According to the working environment of the bending forming tube to be tested, the failure probability of similar bending forming tubes in the working environment is calculated. , and based on the probability of failure in the past Determine the failure probability level of the tested bend-formed tube as very high, high, medium, low or very low;

[0019] Based on the working environment of the tested bending forming tube, the failure consequences of the bending forming tube are analyzed from three dimensions: personnel safety, environmental impact, and economic loss. The severity level of the consequences of the tested bending forming tube is then determined as catastrophic, severe, moderate, minor, or negligible.

[0020] A two-dimensional risk assessment matrix is ​​constructed with the failure probability level as the column vector and the consequence severity level as the row vector;

[0021] Based on the two-dimensional risk assessment matrix, the safety level of the tested bending forming tube is generated according to the failure probability level and consequence severity level of the tested bending forming tube: very high, high, medium or low;

[0022] Determine the safety point density coefficient of the bending forming tube to be tested according to the generated safety level;

[0023] If the failure possibility level of the tested bending-molded tube cannot be determined, the safety point density coefficient is determined directly according to the severity level of the consequences of the tested bending-molded tube.

[0024] Furthermore, the probability of failure occurring in the past The method for determining the failure probability level of the bending forming tube to be tested is: , then the failure probability level of the tested bending forming tube is extremely high; if , then the failure probability level of the tested bending forming tube is high; if , then the failure probability level of the tested bending forming tube is medium; if , then the failure probability level of the tested bending forming tube is low; if , the failure possibility level of the tested bending forming tube is extremely low.

[0025] Furthermore, the specific content of calculating the number of inspection points of the bend-formed tube to be tested based on the residual stress sensitivity, safety level and budgeted number of inspection points of the bend-formed tube to be tested is:

[0026] For any curved tube, the X-ray measurement inspection area of ​​the curved tube includes: an outer inspection area, an inner inspection area, and a clamping area. The minimum number of inspection points in the outer inspection area is set to 5, the minimum number of inspection points in the inner inspection area is set to 3, and the minimum number of inspection points in the clamping area is set to 3.

[0027] The number of detection points in the outer detection area is obtained by calculating the product of the minimum number of detection points in the outer detection area, the sensitivity density coefficient and the safety point density coefficient;

[0028] The number of detection points in the inner detection area is obtained by calculating the product of the minimum number of detection points in the inner detection area, the sensitivity density coefficient and the safety point density coefficient;

[0029] The number of detection points in the clamping area is determined, and the number of detection points in the outer detection area, the number of detection points in the inner detection area and the number of detection points in the clamping area are summed to obtain the total number of detection points of the bending forming tube to be tested.

[0030] Furthermore, the method for arranging the testing points on the bending forming tube to be tested by adopting a differentiated point density strategy according to the comparison results is as follows:

[0031] If the total number of test points is less than or equal to the number of test points within the budget, a differentiated point density strategy is used to arrange the test points on the bent and formed tube to be tested.

[0032] If the total number of inspection points is greater than the number of inspection points within the budget, the number of inner inspection points should be reduced according to the budget requirements, or the total number of inspection points that meets the budgeted number of inspection points should be determined by re-evaluating the residual stress sensitivity and safety level of the bending forming tube material, and a differentiated point density strategy should be adopted to arrange the inspection points on the bending forming tube to be tested.

[0033] Furthermore, the method of arranging the detection points on the bending forming tube to be tested by adopting the differentiated point density strategy is as follows:

[0034] According to the number of detection points in the outer detection area and the number of detection points in the inner detection area, the detection points are evenly distributed in the outer detection area and the inner detection area of ​​the bending forming tube to be tested;

[0035] After all the inspection points of the bend-formed tube to be tested are arranged, if there is a remaining budget, additional inspection points are added to the outer inspection area according to the remaining calculation, or reference inspection points are arranged on the central axis of the bend-formed tube to be tested to assist in analyzing the residual stress changes inside and outside the bend-formed tube to be tested.

[0036] The beneficial effects of adopting the above technical solution are:

[0037] The method presented in this paper uses finite element software to model the tube bending machine mold system and the bent tube, and simulates and analyzes the bending process, resulting in a stress field, high-stress areas, and maximum stress points. High-stress areas are primarily characterized by tensile stress in the outer stretching zone, compressive stress in the inner compression zone, and contact areas. Based on the simulation results, the method proposes rationally arranging stress detection points of varying densities within each high-stress area to conduct X-ray stress testing, taking into account the varying stress distribution gradients within these areas.

[0038] The method of the present invention establishes a residual stress distribution prediction model through numerical simulation technology. Combining stress amplitude sensitivity analysis with gradient field topology characteristics, it achieves adaptive optimization of X-ray detection point configuration, significantly improving the scientific and economic efficiency of the detection scheme. The method of the present invention effectively solves the problems of traditional point placement methods that rely on empirical points, resulting in data incompleteness and resource waste due to the omission of local high-stress areas or excessive redundant testing of low-risk areas. It achieves targeted and efficient residual stress detection in bent pipes. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a flow chart of a method for measuring residual stress of a bending-molded tube using X-rays in this embodiment;

[0040] Figure 2 Schematic diagram of the tube bending machine mold system and the bent tube during the entire bending process in this embodiment;

[0041] Figure 3 Schematic diagram of the initial states of the components and elbows in the mold system in this embodiment;

[0042] Figure 4 This is the stress distribution diagram of the elbow obtained by simulation in this embodiment;

[0043] Figure 5 This is the improved layout diagram in this implementation mode;

[0044] In the figure: 1-bending pipe; 2-pressure die; 3-anti-wrinkle die; 4-bending die; 5-clamping die. DETAILED DESCRIPTION

[0045] For ease of understanding of the present application, the specific embodiments of the present invention are further described in detail below in conjunction with the accompanying drawings and embodiments. The following embodiments are used to illustrate the present invention, but are not intended to limit the scope of the present invention. On the contrary, the purpose of providing these embodiments is to make the disclosure of the present application more thoroughly and comprehensively understood.

[0046] This embodiment proposes a point distribution method for measuring the residual stress of bent pipes produced by the bending forming process using X-rays. Since the generation mechanism of residual stress is roughly the same and the rules are similar, for bent pipes produced by the bending forming process, the bending forming process is first numerically simulated based on finite element analysis software, and potential high-stress areas are identified from the simulation results. The simulation shows that the contact area between the bent pipe and the clamping die also has large stress, which cannot be ignored. Secondly, based on the collaborative verification of the simulation results and the theoretical model, a differentiated point distribution strategy based on stress field characteristic partitioning is further proposed to meet the high efficiency required for X-ray detection of residual stress.

[0047] In this embodiment, a method for measuring the residual stress of a bending forming tube by using X-ray is provided. Figure 1 As shown, the method includes the following processes:

[0048] For any bending forming tube to be tested, the material mechanical property parameters of the bending forming tube are obtained and the residual stress sensitivity of the bending forming tube material is evaluated.

[0049] The specific contents of obtaining the material mechanical performance parameters of any bending forming tube to be tested and evaluating the residual stress sensitivity of the bending forming tube material are as follows:

[0050] Obtain the yield strength of the bending forming tube material and tensile strength , and calculate the yield strength and tensile strength ratio.

[0051] According to the yield strength and tensile strength The sensitivity level of the bending forming tube material is determined to be high sensitivity, medium sensitivity or low sensitivity based on the ratio of

[0052] The method for determining the sensitivity level of the bending forming tube material is: if the material corresponding ratio , then the sensitivity level of the material is high sensitivity; if the corresponding ratio of the material When the material's sensitivity level is medium sensitivity; if the material's corresponding ratio , the sensitivity level of the material is low sensitivity.

[0053] The sensitivity encryption coefficient of the bending forming tube is determined according to the sensitivity level of the bending forming tube material.

[0054] In this embodiment, for known materials, the yield strength and tensile strength of the material can be obtained directly by consulting data such as material manuals; for new materials, the yield strength and tensile strength of the material can be measured experimentally, such as through tensile testing. and tensile strength The residual stress sensitivity of the bending forming tube material to be tested is evaluated, and the sensitivity encryption coefficients are set according to the evaluated sensitivity levels: 2.0, 1.5, and 1.0.

[0055] According to the working environment of the tested bending forming tube, the safety level of the bending forming tube is evaluated by establishing a two-dimensional risk assessment matrix.

[0056] The specific contents of evaluating the safety level of the bend-formed tube by establishing a two-dimensional risk assessment matrix based on the working environment of the bend-formed tube to be tested are as follows:

[0057] According to the working environment of the bending forming tube to be tested, the failure probability of similar bending forming tubes in the working environment is calculated. , and based on the probability of failure in the past Determine the failure probability level of the tested bend-formed tube as very high, high, medium, low, or very low.

[0058] The probability of failure in the past The method for determining the failure probability level of the bending forming tube to be tested is: , then the failure probability level of the tested bending forming tube is extremely high; if , then the failure probability level of the tested bending forming tube is high; if , then the failure probability level of the tested bending forming tube is medium; if , then the failure probability level of the tested bending forming tube is low; if , the failure possibility level of the tested bending forming tube is extremely low.

[0059] In this embodiment, for the working environment of the bent-to-form pipe, the probability of past failures is calculated based on historical data or engineering experience and used to classify the failure probability: when the probability of past failure is greater than 50%, the corresponding failure probability level is extremely high, for example: frequent mechanical damage (such as third-party construction damage). When the probability of past failure is between 20% and 50%, the corresponding failure probability level is high, for example: components with a corrosion rate greater than 0.3 mm / year. When the probability of past failure is between 5% and 20%, the corresponding failure probability level is medium, for example: bent pipes with minor defects due to poor bending process. When the probability of past failure is between 1% and 5%, the corresponding failure probability level is low, for example: the risk of misoperation of newly commissioned equipment. When the probability of past failure is less than 1%, the corresponding failure probability level is extremely low, for example: extreme natural disasters calculated theoretically.

[0060] Based on the working environment of the tested bending forming tube, the failure consequences of the bending forming tube are analyzed from three dimensions: personnel safety, environmental impact, and economic loss. The severity level of the consequences of the tested bending forming tube is then determined as catastrophic, severe, moderate, minor, or negligible.

[0061] In this embodiment, the failure consequences of the bending forming pipe are quantified from three dimensions: personnel safety, environmental impact and economic loss, and the severity of the consequences is divided: if the failure of the bending forming pipe will cause death or environmental disaster, the severity level of the consequences of the bending forming pipe will be classified as catastrophic, such as the leakage of the oil and gas mixed pipeline causing an ecological disaster; if the failure of the bending forming pipe will cause major economic losses or serious injuries to personnel, the severity level of the consequences of the bending forming pipe will be classified as severe, such as the shutdown of the natural gas gathering and transportation system; if the failure of the bending forming pipe will cause moderate economic losses or lead to local shutdowns, the severity level of the consequences of the bending forming pipe will be classified as medium, such as the partial interruption of the water supply network; if the maintenance cost of the failure of the bending forming pipe is controllable, the severity level of the consequences of the bending forming pipe will be classified as minor, such as leakage of a low-pressure water supply pipeline; if the failure of the bending forming pipe has no substantial impact, the severity level of the consequences of the bending forming pipe will be classified as negligible, such as slight deformation of a non-pressure elbow.

[0062] A two-dimensional risk assessment matrix is ​​constructed with the failure probability level as the column vector and the consequence severity level as the row vector.

[0063] In this embodiment, combined with the reliability requirements of the working environment assessment for the elbow, a risk assessment matrix is ​​established for the working environment of the elbow based on the two dimensions of the possibility of occurrence of the failure mode and the severity of the consequences, to achieve a scientific classification of the equipment safety level, as shown in Table 1.

[0064] Table 1 Classification of reliability requirements for bending forming tubes:

[0065]

[0066] Based on the two-dimensional risk assessment matrix, the safety level of the tested bending forming tube is generated according to the failure possibility level and consequence severity level of the tested bending forming tube: very high, high, medium or low.

[0067] The safety point density coefficient of the bending forming tube to be tested is determined according to the generated safety level.

[0068] In this embodiment, the security point encryption coefficients are set to 2, 1.5, 1.25, and 1.0 according to the four security levels of very high, high, medium, and low, respectively.

[0069] If the failure possibility level of the tested bending-molded tube cannot be determined, the safety point density coefficient is determined directly according to the severity level of the consequences of the tested bending-molded tube.

[0070] In this embodiment, it should also be noted that if the failure possibility of the bent-formed tube to be tested cannot be evaluated, the safety point encryption coefficient is directly set according to the severity of the consequences of the bent-formed tube to be tested, and the safety point encryption coefficient is set in descending order of the severity level of the consequences as 2, 1.5, 1.25, 1.1, and 1.0 respectively.

[0071] Evaluate the time and economic costs of using X-rays to measure the residual stress of the bend-formed tube to be tested, and determine the number of inspection points within the budget based on the evaluation results.

[0072] In this implementation, the time and economic costs of X-ray measurement experiments are evaluated. For X-ray residual stress testing, manual testing efficiency is approximately 10 points per hour. The economic cost of testing is analyzed based on different testing conditions. If outsourcing testing is chosen, the specific testing fee should be consulted with the outsourcing agency, which typically ranges from 80 to 300 yuan per testing point. If using in-house X-ray testing equipment, depreciation costs should be calculated based on the instrument model. Considering both economic costs and construction time constraints, determine the number of testing points that meets budget requirements.

[0073] Calculate the total number of inspection points for the bend-formed tube to be tested based on the residual stress sensitivity, safety level, and budgeted number of inspection points.

[0074] In this embodiment, the pipe bender used in the bending process to produce bent pipes consists of a mold system, a power and transmission system, a control system, and auxiliary systems. Since the mold system is the only part of the pipe bender that directly contacts the bent pipe and affects its deformation and stress changes, the impact of other systems on the simulation can be demonstrated by setting the motion parameters of the mold system. Therefore, a model is developed for the mold system in the pipe bender, which includes a bending die, a clamping die, an anti-wrinkle die, a mandrel, and other auxiliary adjustment components that can be selected based on actual conditions.

[0075] Based on the finite element analysis platform, a multi-body dynamic model of the mold system in the pipe bending machine is constructed. The dynamic characteristics of the stress field evolution during the pipe bending process are revealed through numerical simulation technology. The mapping relationship between various process parameters and residual stress distribution is quantitatively characterized, providing guidance and basis for the selection of X-ray detection points for residual stress in the bent pipe.

[0076] In order to accurately simulate the working state of the mold system in the pipe bending machine and the coupling state that the bent pipe bears during the entire bending process, it is necessary to clarify how the various components of the mold system work together. Figure 2 As shown, the pipe bending machine mold system mainly consists of a bending die 4, a clamping die 5, an anti-wrinkle die 3, and a pressure die 2. During the pipe bending process, the bending die 4, the clamping die 5, the anti-wrinkle die 3, and the pressure die 2 each play a key role, working together to ensure a precise bending effect. The bending die 4 applies the main bending force according to the preset curvature, gradually shaping the pipe; the clamping die 5 firmly fixes the pipe to prevent displacement during the bending process; the anti-wrinkle die 3 effectively prevents wrinkles on the inside and outside of the pipe during bending, maintaining a smooth surface; the pressure die 2 assists in shaping the pipe by applying appropriate pressure, ensuring a stable shape after bending. The close cooperation of the four molds enables the pipe bending process to be completed efficiently and accurately.

[0077] First, geometric modeling is performed on each component of the mold system and the elbow 1, and corresponding material properties are assigned. In this embodiment, the elbow 1 adopts material parameters of a titanium alloy.

[0078] Then, the components are integrated according to the assembly requirements to construct the initial state of the components in the mold system and the elbow 1, such as Figure 2 In order to ensure the accuracy of numerical calculations while improving the simulation efficiency, the model was reasonably simplified and the solution algorithm was optimized. Figure 2 and Figure 3 Because the mold system can be approximated as a rigid body during simulation, the appearance of each part of the mold system does not need to be modeled according to the prototype. It is only necessary to ensure that the contact area assembly is consistent with the actual situation. This model simplification method also improves the efficiency of the solution calculation.

[0079] Secondly, the interaction between components, applied loads and boundary conditions were set according to the actual working conditions, the numerical simulation of the working state was realized, and reasonable simulation results were obtained in the process of repeatedly adjusting parameters.

[0080] Finally, the stress distribution of the elbow is obtained through simulation, such as Figure 4 As shown, the bent tube 1, produced using the bending process, lies in close contact with the bending die 4. Therefore, the outer side of the bent tube 1 experiences tensile strain during the bending process, causing the outer side to lengthen, thereby defining the tensile stress region of the bent tube as a high-tensile stress zone. Conversely, the inner side of the bent tube 1 experiences compressive strain, causing the tube to shorten, thereby defining the compressive stress region of the bent tube as a high-tensile stress zone. It is noteworthy that, in addition to the tensile and compressive regions of the bent tube, the contact area between the clamping die 5 and the bent tube 1 also exhibits significant high-tensile stress.

[0081] Under the action of bending moment, the material outside the neutral layer undergoes non-coordinated deformation due to its distance from the center of curvature. Its axial extension behavior is coupled with geometric constraints and plastic flow, resulting in a gradient distribution of the tensile stress field. According to beam bending theory, the axial strain borne by the outer fibers during bending reaches its maximum. The strain at a point in the outer region of the tube along the axial direction of the bend can be expressed as:

[0082] ;

[0083] in is the strain of a point in the outer area of ​​the pipe along the axial direction of the bend; is the distance from a point in the outer area of ​​the tube to the neutral layer; is the bending radius.

[0084] During the bending process, as the bending die drives the entire part to rotate, the bending radius becomes smaller. When the strain exceeds the elastic limit, there is , It represents the strain corresponding to when the stress reaches the yield stress, and the material enters the plastic stage. The stress is described by the power law hardening model as:

[0085] ;

[0086] in is the stress along the axial direction at a certain point of the elbow; is the strength coefficient; is the initial plastic strain; is the plastic strain; is the hardening index.

[0087] At the same time, the wall thickness of the elbow becomes thinner due to the Poisson effect during the tension process, which can be expressed as:

[0088] ;

[0089] in is the wall thickness variation; is Poisson's ratio; is the initial wall thickness.

[0090] Due to the section moment of inertia of the thin-walled ring and wall thickness As the stretching process progresses, the wall thickness decreases and the moment of inertia of the section decreases accordingly, further exacerbating the stress concentration. Therefore, in the stretching area, it is necessary to pay attention to stress control to reduce the possibility of cracks.

[0091] In the inner area of ​​the bend, due to the mold friction constraint, the plastic flow of the inner material is hindered, resulting in local thickening of the wall thickness and the formation of high residual compressive stress, the value of which can approach or exceed the yield strength of the material. According to the thin-walled tube buckling theory, the critical buckling stress is determined by the following formula:

[0092] ;

[0093] in is the critical buckling stress; is the elastic modulus; is the elbow diameter.

[0094] When the actual compressive stress When the residual stress is too high, it will cause local instability such as wrinkling or buckling, which will significantly reduce the load-bearing capacity and fatigue life of the elbow. Instability is more likely to occur in thin-walled pipes. Therefore, in order to prevent the occurrence of buckling and wrinkling defects, residual stress testing is also required on the inner area of ​​the elbow.

[0095] In addition, what is easy for operators to overlook is that stress concentration also occurs in the contact area between the elbow and the clamping die. The tensile stress concentration phenomenon at the contact interface can be attributed to the multi-physics field coupling mechanism: the geometric constraints of the clamping die cause strain localization, the friction shear stress and plastic flow produce non-uniform hardening, and process parameters such as clamping force and bending rate affect stress redistribution through dynamic load paths. The rigid constraints of the clamping die lead to geometric mismatch of the contact interface, and the sudden change of its edge curvature causes local stress concentration, while the friction shear stress The vector superposition with the main stress field of the elbow further intensifies the axial tensile stress amplitude. is the friction shear stress; is the friction coefficient; is the radial pressure.

[0096] The plastic flow of the material is limited by the axial constraint of the clamping die, which leads to a significant increase in the plastic strain gradient and provides flow stress through work hardening; at the same time, the Poisson effect causes the radial compressive strain to be converted into additional axial tensile strain. , and there are .in is Poisson's ratio; is the radial compressive strain. The bending-induced cross-sectional ovalization effect triggers stress redistribution through the conversion of circumferential strain into axial tensile component and neutral layer offset, causing the inner side of the contact area to bear higher axial load. In addition, process parameters such as high clamping force, overspeed bending and lubrication failure will amplify the stress concentration effect through dynamic hardening, inertial force coupling and boundary friction mechanism, and ultimately form an axial tensile stress peak with significant gradient characteristics. These factors may also cause excessive axial tensile stress. Therefore, for the stability and safety of the bent pipe, the detection of the contact area is equally important. However, the existing methods of measuring the residual stress of the bent pipe using X-rays often ignore the contact area.

[0097] In terms of point distribution strategy, given that the outer tensile area of ​​the bend has a higher risk of failure due to the thinning effect of the tube wall and the significant stress gradient distribution, this embodiment adopts a high-density point distribution scheme to meet the needs of capturing the gradient characteristics of the stress field; and the inner compression area of ​​the bend is usually easy to identify with the naked eye due to the shrinkage phenomenon, and is mainly concentrated near the starting point, so only precise point distribution is required, and the point distribution density is relatively low on the outside. In addition, the finite element analysis results show that in the contact area between the clamping die and the bend, the high tensile stress is mainly concentrated in the outer vertex and the upper part of the neutral axis on both sides. Only the three key areas of the outer tensile area, the inner compression area and the tightening area need to be tested. For key areas that cannot be confirmed by the naked eye or by the operator, it can be judged by selecting several points on the outer vertices of the two ends of the bend for stress detection. Usually, there is a higher axial tensile stress outside the clamping area. Therefore, the traditional method is optimized through a differentiated point distribution density strategy. The improved point distribution scheme in this embodiment is as follows. Figure 5 shown.

[0098] The specific content of calculating the total number of inspection points of the bend-formed tube to be tested based on the residual stress sensitivity, safety level and budgeted number of inspection points of the bend-formed tube to be tested is as follows:

[0099] For any bent and formed tube, the X-ray measurement detection area of ​​the bent and formed tube includes: an outer detection area, an inner detection area and a clamping area, and the minimum number of detection points in the outer detection area is set to 5, the minimum number of detection points in the inner detection area is set to 3, and the minimum number of detection points in the clamping area is set to 3.

[0100] The number of detection points in the outer detection area is obtained by calculating the product of the minimum number of detection points in the outer detection area, the sensitivity encryption coefficient and the safety point encryption coefficient.

[0101] The number of detection points in the inner detection area is obtained by calculating the product of the minimum number of detection points in the inner detection area, the sensitivity encryption coefficient and the safety point encryption coefficient.

[0102] In this embodiment, at least three points in the contact area must be satisfied, such as Figure 5 As shown, try to ensure that the ratio of the number of detection points in the inner detection area to the number of detection points in the outer detection area is between 1:1.5 and 1:3. If there is enough, you can also select a few points on the neutral axis for testing as a reference.

[0103] The number of detection points in the clamping area is determined, and the number of detection points in the outer detection area, the number of detection points in the inner detection area and the number of detection points in the clamping area are summed to obtain the total number of detection points of the bending forming tube to be tested.

[0104] In this embodiment, the minimum detection standard is set as Figure 5 As shown, there are at least 3 test points on the inside and at least 5 test points on the outside. The number of test points on the outside = 5 × sensitivity encryption coefficient × safety point encryption coefficient, the number of test points on the inside = 3 × sensitivity encryption coefficient × safety point encryption coefficient. The above values ​​are rounded up during the calculation process. For bends where the contact area cannot be confirmed, it is necessary to first take stress tests on several points outside the potential contact area. Usually, there is a higher axial tensile stress outside the clamping area. And the circle where the maximum compressive stress point is located is tested according to Figure 5 Two points on the axis in the arrangement shown.

[0105] The total number of inspection points on the bend-formed tube to be tested is compared with the number of inspection points within the budget, and a differentiated point density strategy is adopted to arrange the inspection points on the bend-formed tube to be tested based on the comparison results.

[0106] The method for arranging the test points on the bending forming tube to be tested by adopting a differentiated point density strategy according to the comparison results is as follows:

[0107] If the total number of inspection points is ≤ the number of inspection points within the budget, a differentiated point density strategy is adopted to arrange the inspection points on the bent and formed tube to be tested.

[0108] If the total number of inspection points is greater than the number of inspection points within the budget, the number of inner inspection points should be reduced according to the budget requirements, or the total number of inspection points that meets the budgeted number of inspection points should be determined by re-evaluating the residual stress sensitivity and safety level of the bending forming tube material, and a differentiated point density strategy should be adopted to arrange the inspection points on the bending forming tube to be tested.

[0109] The specific content of arranging the test points on the bending forming tube to be tested by adopting the differentiated point density strategy is as follows:

[0110] According to the number of detection points in the outer detection area and the number of detection points in the inner detection area, the detection points are evenly distributed in the outer detection area and the inner detection area of ​​the bending forming tube to be tested.

[0111] After all the inspection points of the bend-formed tube to be tested are arranged, if there is a remaining budget, additional inspection points are added to the outer inspection area according to the remaining calculation, or reference inspection points are arranged on the central axis of the bend-formed tube to be tested to assist in analyzing the residual stress changes inside and outside the bend-formed tube to be tested.

[0112] In this embodiment, if the total number of detection points is ≤ the number of detection points within the budget, differentiated point distribution is directly performed. According to the preset number of detection points on the inside and outside, uniform grid distribution is performed on the inside and outside areas of the arc structure, and the number of detection points on the outside can be increased according to the excess budget. This is because the outside is usually an area with greater stress, and increasing the detection points can monitor the stress distribution more comprehensively; or the points on the neutral axis are detected as a reference. This is because the stress on the neutral axis is usually smaller, which can be used as a comparison to help analyze the changes in stress on the inside and outside.

[0113] If the total number of inspection points is greater than the number of inspection points within the budget, the number of inner inspection points should be reduced according to the budget requirements, or the total number of inspection points that meets the budgeted number of inspection points should be determined by re-evaluating the residual stress sensitivity and safety level of the bending forming tube material, and then a differentiated point density strategy should be adopted to arrange the inspection points on the bending forming tube to be tested.

[0114] In this embodiment, if the total number of detection points is greater than the number of detection points within the budget, the number of inner detection points is reduced to meet the budget requirement. If the gap with the budget is too large, re-evaluation is required.

[0115] It should also be noted that for industry fields with standardized detection point configurations, the principle of priority application shall be followed; the innovative application scope of this implementation method specifically refers to non-standardized detection scenarios.

[0116] In summary, the core of the present invention is to use finite element software to perform precise numerical simulation of the bending process, and based on the simulation results and theoretical analysis, adopt a differentiated point density strategy in each functional area, and finally determine the point location for X-ray detection of residual stress in the bending pipe.

[0117] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope defined by the present invention.

Claims

1. A method for measuring residual stress of a bending-molded tube using X-rays, characterized in that: The method includes the following steps: For any bending forming tube to be tested, obtain the material mechanical property parameters of the bending forming tube and evaluate the residual stress sensitivity of the bending forming tube material; According to the working environment of the bending forming tube to be tested, the safety level of the bending forming tube is evaluated by establishing a two-dimensional risk assessment matrix; Evaluate the time and economic costs of using X-rays to measure the residual stress of the bend-formed tube to be tested, and determine the number of inspection points within the budget based on the evaluation results; Calculate the number of inspection points of the bend-formed tube to be tested based on the residual stress sensitivity and safety level of the bend-formed tube to be tested; The total number of inspection points on the bend-formed tube to be tested is compared with the number of inspection points within the budget, and a differentiated point density strategy is adopted to arrange the inspection points on the bend-formed tube to be tested based on the comparison results.

2. The method for measuring residual stress of a bending-molded tube using X-rays according to claim 1, characterized in that: The specific contents of obtaining the material mechanical performance parameters of any bending forming tube to be tested and evaluating the residual stress sensitivity of the bending forming tube material are as follows: Obtain the yield strength of the bending forming tube material and tensile strength , and calculate the yield strength and tensile strength The ratio of According to the yield strength and tensile strength The sensitivity level of the bending forming tube material is determined to be high sensitivity, medium sensitivity or low sensitivity by the ratio of The sensitivity encryption coefficient of the bending forming tube is determined according to the sensitivity level of the bending forming tube material.

3. The method for measuring residual stress of a bending-molded tube using X-rays according to claim 2, characterized in that: The method for determining the sensitivity level of the bending forming tube material is as follows: if the material corresponding ratio , then the sensitivity level of the material is high sensitivity; if the corresponding ratio of the material When the material's sensitivity level is medium sensitivity; if the material's corresponding ratio , the sensitivity level of the material is low sensitivity.

4. The method for measuring residual stress of a bending-molded tube using X-rays according to claim 3, characterized in that: The specific contents of evaluating the safety level of the bend-formed tube by establishing a two-dimensional risk assessment matrix based on the working environment of the bend-formed tube to be tested are as follows: According to the working environment of the bending forming tube to be tested, the failure probability of similar bending forming tubes in the working environment is calculated. , and based on the probability of failure in the past Determine the failure probability level of the tested bend-formed tube as very high, high, medium, low or very low; Based on the working environment of the tested bending forming tube, the failure consequences of the bending forming tube are analyzed from three dimensions: personnel safety, environmental impact, and economic loss. The severity level of the consequences of the tested bending forming tube is then determined as catastrophic, severe, moderate, minor, or negligible. A two-dimensional risk assessment matrix is ​​constructed with the failure probability level as the column vector and the consequence severity level as the row vector; Based on the two-dimensional risk assessment matrix, the safety level of the tested bending forming tube is generated according to the failure probability level and consequence severity level of the tested bending forming tube: very high, high, medium or low; Determine the safety point density coefficient of the bending forming tube to be tested according to the generated safety level; If the failure possibility level of the tested bending-molded tube cannot be determined, the safety point density coefficient is determined directly according to the severity level of the consequences of the tested bending-molded tube.

5. The method for measuring residual stress of a bending-molded tube using X-rays according to claim 4, characterized in that: The probability of failure in the past The method for determining the failure probability level of the bending forming tube to be tested is: , then the failure probability level of the tested bending forming tube is extremely high; if , then the failure probability level of the tested bending forming tube is high; if , then the failure probability level of the tested bending forming tube is medium; if , then the failure probability level of the tested bending forming tube is low; if , the failure possibility level of the tested bending forming tube is extremely low.

6. The method for measuring residual stress of a bending-molded tube using X-rays according to claim 5, characterized in that: The specific content of calculating the number of inspection points of the bend-formed tube to be tested based on the residual stress sensitivity, safety level and budgeted number of inspection points of the bend-formed tube to be tested is as follows: For any curved tube, the X-ray measurement inspection area of ​​the curved tube includes: an outer inspection area, an inner inspection area, and a clamping area. The minimum number of inspection points in the outer inspection area is set to 5, the minimum number of inspection points in the inner inspection area is set to 3, and the minimum number of inspection points in the clamping area is set to 3. The number of detection points in the outer detection area is obtained by calculating the product of the minimum number of detection points in the outer detection area, the sensitivity density coefficient and the safety point density coefficient; The number of detection points in the inner detection area is obtained by calculating the product of the minimum number of detection points in the inner detection area, the sensitivity density coefficient and the safety point density coefficient; The number of detection points in the clamping area is determined, and the number of detection points in the outer detection area, the number of detection points in the inner detection area and the number of detection points in the clamping area are summed to obtain the total number of detection points of the bending forming tube to be tested.

7. The method for measuring residual stress of a bending-molded tube using X-rays according to claim 6, characterized in that: The method for arranging the test points on the bending forming tube to be tested by adopting a differentiated point density strategy according to the comparison results is as follows: If the total number of test points is less than or equal to the number of test points within the budget, a differentiated point density strategy is used to arrange the test points on the bent and formed tube to be tested. If the total number of inspection points is greater than the number of inspection points within the budget, the number of inner inspection points should be reduced according to the budget requirements, or the total number of inspection points that meets the budgeted number of inspection points should be determined by re-evaluating the residual stress sensitivity and safety level of the bending forming tube material, and a differentiated point density strategy should be adopted to arrange the inspection points on the bending forming tube to be tested.

8. The method for measuring residual stress of a bending-molded tube using X-rays according to claim 7, characterized in that: The method for arranging the testing points on the bending forming tube to be tested by adopting the differentiated point density strategy is as follows: According to the number of detection points in the outer detection area and the number of detection points in the inner detection area, the detection points are evenly distributed in the outer detection area and the inner detection area of ​​the bending forming tube to be tested; After all the inspection points of the bend-formed tube to be tested are arranged, if there is a remaining budget, additional inspection points are added to the outer inspection area according to the remaining calculation, or reference inspection points are arranged on the central axis of the bend-formed tube to be tested to assist in analyzing the residual stress changes inside and outside the bend-formed tube to be tested.

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