Welding structure fatigue crack prediction method and application

Abstract

The invention provides a method for predicting fatigue cracks of a welding structure. The method comprises the following steps: S1, accurately acquiring the crack length and the maximum crack depth of a sample under different cycle times; and S2, obtaining a fatigue crack propagation model formula, namely, the unit is as follows. S3, if the crack depth is L1 when the fatigue test is circulated for N1 times, and the cycle index corresponding to the crack depth L2 is N2, N2 is calculated according to the following formula. According to the method, the maintenance interval can be scientifically prolonged or maintenance can be arranged in advance by establishing the crack propagation model and predicting the residual life of the structure, and if the residual life predicted by the model is far longer than the next planned maintenance period, the maintenance period is properly prolonged to save docking cost, labor loss and the like; if the residual life predicted by the model is lower than a safety threshold value, overhauling is arranged in advance, and hidden dangers are eliminated to avoid sudden disasters and accidents.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of fatigue life prediction, in particular to a welding structure fatigue crack prediction method and application. BACKGROUND

[0002] Welding structures are widely used in important equipment such as bridges, ships, pressure vessels, etc. The weld toe part is a high-risk area of fatigue crack initiation and propagation due to stress concentration. Accurate measurement of fatigue crack propagation is crucial for assessing the safety and remaining life of the structure.

[0003] The current common methods for measuring fatigue cracks include direct visual inspection, replica method, alternating potential drop method, etc. Among them, the microscope visual inspection or replica method can only measure the surface length, but cannot obtain the depth information of the crack. Although the alternating potential drop method can estimate the depth of the crack, the measurement accuracy is greatly affected by the material and temperature, and is not sensitive to small cracks. Usually, ultrasonic detection needs complex scanning to locate the crack, and is strongly dependent on the experience of the detection personnel, making it difficult to achieve accurate and repeatable measurement of small cracks. Since the existing technical means cannot achieve high repeatability and quantitative dynamic tracking of fatigue cracks, the accuracy of the obtained crack information is poor, which has no practical guiding significance for actual engineering.

[0004] Therefore, the present application is proposed. SUMMARY

[0005] The problem solved by the present application is that the prior art cannot accurately predict fatigue cracks.

[0006] To solve the above problems, the present application provides a welding structure fatigue crack prediction method, comprising: S1, accurately obtaining the crack length and maximum crack depth of the sample under different cycle numbers; S2, obtaining a fatigue crack propagation model formula, i.e. da / dN = 6.48 x 10 -14 ×(ΔK) 3.85 , where da / dN is in mm / cycle and ΔK is in MPa√mm. S3, if the crack depth is L1 when the fatigue test is cycled N1 times, and the crack depth corresponding to the cycle number N2 is L2, then N2 is calculated according to the following formula:

[0007] Preferably, step S1 comprises:

[0008] S11, fix the scanning device on the sample to be tested, adjust the position to align the acoustic beam center line of the ultrasonic phased array probe with the weld toe, take the probe wafer center position as the reference, use the scanning device to scan a certain distance along the weld toe and accurately record the position information, and obtain the scanning image before fatigue loading;

[0009] S12, fatigue experiments are carried out, and loading is paused after every ΔN cycles, the relative position of the ultrasonic phased array probe and the welded structure is ensured to be unchanged as in S11, the ultrasonic phased array probe is driven to scan in a direction parallel to the weld toe through the scanning device, and S-scan images of the weld toe at the current cycle number are acquired again;

[0010] S13, the plurality of S-scan images acquired in S12 are compared with the reference image acquired in S11 respectively, new fatigue cracks are identified from the diffraction signals, and crack depth and crack length are acquired.

[0011] Preferably, the scanning device comprises a transverse linear guide, the transverse linear guide is provided with a longitudinal linear guide, the end of the longitudinal linear guide is provided with a height adjusting device, the bottom of the height adjusting device is provided with a fixing clamp for holding the ultrasonic phased array probe; and the longitudinal linear guide is provided with a high-precision encoder for recording the position of the ultrasonic phased array probe in real time.

[0012] Preferably, the scanning device further comprises a suction cup base, the suction cup base is provided with a vertical rod arranged in a vertical manner, the upper end of the vertical rod is provided with a horizontal rod arranged in a horizontal manner, and the transverse linear guide is assembled at the end of the horizontal rod away from the vertical rod through a second fixing device.

[0013] Preferably, the sample is prepared by the following method: a ship high-strength steel welded butt plate with a size of 440mm*200mm*40mm is welded by manual welding, a crescent-shaped notch is machined in the center of the front weld along the width direction as a fatigue crack initiation source, the notch length is 15mm, the depth is 2mm, and the excess height of the back weld is removed by machining.

[0014] Preferably, the mean load of the fatigue experiment is-350 to-450kN, the amplitude load is 200 to 400kN, the span is 200 to 400mm, and the frequency is 2 to 5Hz. Preferably, the mean load of the fatigue experiment is-400kN, the amplitude load is 300kN, the span is 300mm, and the frequency is 3Hz.

[0015] The application further discloses application of the welded structure fatigue crack prediction method in a welded structure residual life prediction model.

[0016] Compared with the prior art, the welding structure fatigue crack prediction method and application has the following beneficial effects: 1) The application provides accurate prediction based on actual damage state, accurately predicts how long the structure can be used based on accurate prediction of the structure, and arranges maintenance or replacement at the most reasonable time point, so as to achieve the best balance between safety and economy; 2) The prediction method can quickly evaluate the risk degree of cracks in actual working conditions, and provide important decision basis for whether to immediately shut down, repair or continue to monitor and use; 3) The application predicts based on the actually detected cracks, thereby significantly reducing the excessive safety margin reserved due to inaccurate prediction, or fully developing the material potential under the same safety margin. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 A structural schematic diagram of the scanning device according to the embodiment of the application is shown in the figure;

[0018] Figure 2 Ultrasonic phased array S-scan images of a sample before and after fatigue loading in the embodiment of the application are shown in the figures;

[0019] Figure 3 Fatigue crack propagation curves under different cycle numbers in the embodiment of the application are shown in the figures.

[0020] REFERENCE SIGNS:

[0021] 001 - chuck base; 002 - vertical rod; 003 - transverse linear guide rail; 004 - longitudinal linear guide rail; 005 - first fixing device; 006 - fixing clamp; 007 - height adjusting device; 008 - second fixing device; 009 - high-precision encoder; 010 - cross bar. DETAILED DESCRIPTION

[0022] In order to make the above-mentioned purposes, features and advantages of the application more obvious and easy to understand, the specific embodiments of the application will be described in detail below with reference to the drawings. On the premise of not conflicting, the technical features of the embodiments of the application can be combined with each other.

[0023] Embodiment 1

[0024] A welding structure fatigue crack prediction method, comprising the following steps:

[0025] S1, accurately obtaining the crack length and maximum crack depth of the sample under different cycle numbers;

[0026] S2, inversely calculating the material fatigue crack propagation parameters and obtaining a fatigue crack propagation model formula, that is, da / dN = 6.48 x 10 -14 x (ΔK) 3.85 , wherein da / dN is in mm / cycle, and ΔK is in MPa√mm.

[0027] S3, if the crack depth is L1 when the fatigue test cycle is N1 times, and the crack depth is L2 corresponding to the cycle number N2, then N2 is calculated according to the following formula:

[0028] The welding structure fatigue crack prediction method described in the application can scientifically prolong the maintenance interval or arrange maintenance in advance, and realize the change from "planned maintenance" to "scientific prediction maintenance" of the ship welding structure and the like by periodically recording the crack condition of the structure and establishing a crack propagation model to predict the remaining life of the welding structure.

[0029] Specifically, the Paris fatigue crack propagation model is taken as the core, the fatigue crack data obtained are used to calculate the fatigue crack propagation rate and the stress intensity factor amplitude, and linear regression analysis is performed thereon to obtain the model formula by inverting the model parameters of the tested sample; for the set critical crack depth, the numerical integral of the model is performed from the last crack depth to predict the remaining life required for the crack to propagate from the current depth to the critical depth.

[0030] Preferably, step S1 comprises:

[0031] S11, fix the scanning device on the to-be-tested sample, adjust the position to align the acoustic beam center line of the ultrasonic phased array probe with the weld toe, use the scanning device to make the ultrasonic phased array probe scan a certain distance along the weld toe direction and accurately record the position information, obtain and save the scanning image before fatigue loading;

[0032] This setting can realize repeated positioning of the probe, eliminate positioning errors caused by human operation, make the detection data at different time points have very high comparability, and the crack measurement accuracy can reach millimeter level; the two-dimensional cross-sectional information of the crack is obtained at one time by using the S-scan imaging function of the ultrasonic phased array, and then the crack depth is obtained, and the crack length information is obtained based on the high-precision encoder; by integrating the ultrasonic phased array dynamic monitoring data with the fracture mechanics model, the fundamental change from measuring the crack size to predicting the remaining life of the structure is realized, and the technical level is significantly improved.

[0033] S12, carry out fatigue test by loading on the reverse side of the weld through three-point bending experiment, pause loading every ΔN cycles, ensure that the relative position of the ultrasonic phased array probe and the welding structure remains unchanged with S11 and drive the ultrasonic phased array probe to scan along the direction parallel to the weld toe through the scanning device, and obtain the S-scan image of the weld toe part under the current cycle number again;

[0034] S13, respectively, S12 obtains the S-scan image with S11 obtains the reference image, and identifies the new fatigue crack diffraction signal, and obtains the crack depth through the measurement tool in the software, and determines the start and end positions of the crack signal through the encoder to obtain the crack length.

[0035] The application seamlessly integrates the ultrasonic phased array technology into the fatigue test, obtains a series of "snapshots" by pausing the loading and scanning every ΔN cycles, and directly obtains the complete data of the crack depth changing with the cycle number N, thereby providing valuable data for studying the material fatigue performance and life prediction; the S-scan of the ultrasonic phased array can be regarded as a longitudinal section view, and the accurate depth of the crack can be directly read through the measurement tool in the software, which is a key parameter for evaluating the remaining life of the structure; and the scanning device is used to ensure that the path, speed and angle of the probe are completely consistent every time the probe is scanned, and the position information is accurately recorded, thereby eliminating the human operation error and ensuring that the images obtained at different periods have high comparability.

[0036] Preferably, the mean load of the fatigue test is 400 kN, the amplitude load is 300 kN, the span is 300 mm, and the frequency is 3 Hz.

[0037] Preferably, the specimen in step S1 is prepared by the following method: a ship high-strength steel welded butt plate with a size of 440 mm*200 mm*40 mm is welded by manual welding, a crescent-shaped notch is machined on the center of the front weld as a fatigue crack initiation source along the width direction, the notch length is 15 mm, the depth is 2 mm, and the reverse weld reinforcement is eliminated by machining.

[0038] By setting the crescent-shaped notch to form a stress concentration point, the initiation position of the fatigue crack is successfully "locked" in the preset area, and it is not necessary to find the crack in the entire weld or heat-affected zone, thereby greatly improving the observation pertinence and accuracy; in a real welded joint, the weld toe is a geometric shape mutation, and there is significant stress concentration, which is the most common initiation site of fatigue cracks, and the crescent-shaped notch can accurately simulate the "weld toe".

[0039] Preferably, the scanning device comprises a transverse linear guide rail 003, the transverse linear guide rail 003 is provided with a longitudinal linear guide rail 004, the end of the longitudinal linear guide rail 004 is provided with a height adjusting device 007, the bottom of the height adjusting device 007 is provided with a fixed clamp 006 for holding the ultrasonic phased array probe; the longitudinal linear guide rail 004 is provided with a high-precision encoder 009 for real-time recording the position of the ultrasonic phased array probe, as shown in Figure 1 .

[0040] The setting forms a precise rectangular coordinate system through the transverse linear guide rail 003 and the longitudinal linear guide rail 004, so that the ultrasonic phased array probe can be precisely moved in any path on the plane above the weld toe, and the positioning accuracy of the scanning path can reach millimeter or even sub-millimeter level; meanwhile, the height adjusting device 007 allows the distance between the ultrasonic phased array probe and the surface of the sample to be accurately adjusted, so as to ensure that the sound beam can cover the weld toe area at the best angle and focus, thereby obtaining the highest detection sensitivity and resolution; through the setting of the high-precision encoder, the position of each small interval of the ultrasonic phased array probe can be recorded in real time, and the position information is strictly synchronized with the S-scan image.

[0041] Preferably, the scanning device further comprises a suction cup base 001, a vertical rod 002 is arranged on the suction cup base 001 in a vertical manner, a horizontal rod 010 is arranged at the upper end of the vertical rod 002 in a horizontal manner, and the transverse linear guide rail 003 is assembled at the end of the horizontal rod 010 away from the vertical rod 002 through the second fixing device 008.

[0042] The setting lifts and extends the whole scanning device above the sample or workpiece, so that the ultrasonic phased array probe and all moving parts operate in the space above the surface of the workpiece, completely avoiding collision and interference with the weld reinforcement, spatter or other uneven surface features, and ensuring the continuity and safety of the scanning; the suction cup base 001 arranged can be firmly fixed on any smooth and flat workpiece surface in a few seconds through vacuum suction, and will not cause surface damage, and is suitable for the detection of finished products, in-service equipment or precision components with strict surface requirements; in addition, the "cantilever type" frame composed of the vertical rod 002 and the horizontal rod 010 can provide a stable installation reference for the transverse linear guide rail 003 and the longitudinal linear guide rail 004, prevent them from shaking during scanning, and thus ensure the accuracy of the positioning of the ultrasonic phased array probe and the stability of the scanning image.

[0043] Preferably, the height adjusting device 007 is fixed to a preset position of the longitudinal linear guide rail 004 through the first fixing device 005. It should be noted that the scanning device moves along the X-axis, Y-axis and Z-axis through the driving structure, and the specific structure and assembly relationship are prior art.

[0044] Embodiment 2

[0045] A welding structure fatigue crack prediction method, comprising the following steps:

[0046] S1, accurately obtaining the crack length and maximum crack depth of the sample under different cycle numbers, comprising:

[0047] S11. Fix the scanning device on the test sample, adjust the position so that the center line of the ultrasonic phased array probe beam is aligned with the weld toe, and use the scanning device to make the ultrasonic phased array probe scan a certain distance along the weld toe direction and accurately record the position information, and obtain and save the scan image before fatigue loading.

[0048] S12. A fatigue test was conducted on the reverse side of the weld using a three-point bending test. Loading was paused after every 3500 cycles to ensure the relative position of the ultrasonic phased array probe and the welded structure remained unchanged from S11. The ultrasonic phased array probe was then driven by a scanning device to scan along a direction parallel to the weld toe, and the S-scan image of the weld toe at the current cycle number was acquired again. (See...) Figure 2 ;

[0049] S13. Compare the S-scan images acquired in S12 with the reference images acquired in S11 to identify the diffraction signals of the newly added fatigue cracks. Obtain the crack depth using the measurement tools in the software. Simultaneously, determine the start and end positions of the crack signal using the encoder to obtain the crack length. The results are shown in Table 1. Figure 3 .

[0050] Table 1 Crack length and maximum crack depth at different number of cycles

[0051] Number of cycles Crack length 2c (mm) Crack depth a (mm) 3500 19 2.6 7000 31 9.2 10500 37 12.6 14000 56 16.6 17500 82 23.1 20000 113 27.6

[0052] S2. Invert the fatigue crack propagation parameters of the material and obtain the fatigue crack propagation model formula, i.e., da / dN = 6.48 × 10⁻⁶. -14 ×(ΔK) 3.85 Where da / dN is in mm / cycle and ΔK is in MPa√mm;

[0053] Preferably, based on the crack length 2c and crack depth a data before 17,500 cycles in Table 1, the fatigue crack propagation parameters of the material are inverted to obtain a fatigue crack propagation model, and the accuracy of the model is verified by 20,000 cycles of data.

[0054] Specifically, calculate the crack propagation rate da / dN and stress intensity factor amplitude ΔK for each data point. For a semi-elliptical crack on the sample surface, ΔK can be determined by the formula ΔK=F*Δσ*√(πa).

[0055] Where Δσ is the applied cyclic stress amplitude of 562 MPa, and F is the comprehensive correction factor, which is F = F e *F w *F g ;where F eThe crack shape factor, calculated from the crack geometry ratio—that is, the ratio of depth to half-length a / c—is used to correct for differences in stress intensity factors at different locations along the crack tip; F w The finite width factor is calculated using a trigonometric function formula, taking into account the constraint of the finite width and thickness of the specimen on the stress field at the crack tip; the free surface factor F g This is a standard value used to correct for the crack opening effect on the component surface.

[0056] Linear regression analysis (least squares method) was performed on the obtained series of da / dN and ΔK, and the model constant C = 6.48 × 10⁻⁶ was obtained through inversion. -14 With m = 3.85, the fatigue crack propagation model formula is obtained as da / dN = 6.48 × 10⁻⁶. -14 ×(ΔK) 3.85 Where da / dN is in mm / cycle and ΔK is in MPa√mm.

[0057] S3. If the crack depth is 23.1 mm after 17,500 fatigue test cycles, and the number of cycles corresponding to a crack depth of 27.6 mm is N2, then according to... Calculations show that N2 is 17500 + 2459 = 19959 cycles, while the predicted result differs from the actual number of cycles required for the crack to propagate to the same depth (2500 cycles) by 41 cycles, with an error of approximately 2%.

[0058] Compared with existing technologies, the fatigue crack prediction method for welded structures described in this invention has the following advantages:

[0059] 1) It achieves a leap from "periodic maintenance" to "condition-based maintenance": Traditional maintenance strategies are based on fixed time intervals, which may be too conservative (leading to waste) or too risky (leading to accidents). This application provides accurate prediction based on the actual damage state, and based on accurate prediction of "how long the structure can still be used", maintenance or replacement can be arranged at the most reasonable time, achieving the best balance between safety and economy.

[0060] 2) Provide quantitative basis for structural safety assessment: When a crack is detected in an in-service structure, the prediction method described in this application can quickly assess its degree of danger, providing a crucial basis for decision-making on whether to immediately shut down, repair, or continue to monitor its use.

[0061] 3) Significantly improves prediction accuracy and reduces safety risks: In fatigue life prediction, traditional prediction methods based on SN curves (stress-life method) may have large errors because they do not consider initial defects and damage accumulation. However, this application makes predictions based on actually detected cracks, thereby significantly reducing the excessive safety margin reserved due to inaccurate predictions, or fully utilizing the material's potential with the same safety margin.

[0062] While the present invention has been disclosed above, it is not limited thereto. Any person skilled in the art can make various modifications and alterations without departing from the spirit and scope of the invention; therefore, the scope of protection of the present invention should be determined by the scope defined in the claims.

Claims

1. A method for predicting fatigue cracks in welded structures, characterized in that, include: S1. Accurately obtain the crack length and maximum crack depth of the specimen under different cycle numbers; S2. Obtain the fatigue crack propagation model formula, i.e. ,in Units are , Units are S3. If the crack depth is L1 after N1 fatigue test cycles, and the number of cycles corresponding to a crack depth of L2 is N2, then N2 is calculated according to the following formula: .

2. The fatigue crack prediction method for welded structures according to claim 1, characterized in that, Step S1 includes: S11. Fix the scanning device on the sample to be tested, adjust the position so that the center line of the ultrasonic phased array probe beam is aligned with the weld toe, and use the scanning device to make the ultrasonic phased array probe scan a certain distance along the weld toe direction and accurately record the position information to obtain the scan image before fatigue loading. S12. Conduct fatigue tests and pause loading every ΔN cycles to ensure that the relative position of the ultrasonic phased array probe and the welded structure remains unchanged from S11. Drive the ultrasonic phased array probe along the direction parallel to the weld toe using a scanning device to obtain the S-scan image of the weld toe at the current cycle number. S13. Compare the multiple S-scan images obtained in S12 with the reference image obtained in S11 to identify the diffraction signal of the newly added fatigue crack and obtain the crack depth and crack length.

3. The fatigue crack prediction method for welded structures according to claim 1, characterized in that, The scanning device includes a transverse linear guide rail (003), a longitudinal linear guide rail (004) is provided on the transverse linear guide rail (003), a height adjustment device (007) is provided at the end of the longitudinal linear guide rail (004), and a fixing clamp (006) is provided at the bottom of the height adjustment device (007) for holding and fixing the ultrasonic phased array probe; a high-precision encoder (009) is provided on the longitudinal linear guide rail (004) for recording the position of the ultrasonic phased array probe in real time.

4. The fatigue crack prediction method for welded structures according to claim 3, characterized in that, The scanning device also includes a suction cup base (001), on which a vertically placed upright rod (002) is provided. A horizontally placed crossbar (010) is provided at the upper end of the upright rod (002). The horizontal linear guide rail (003) is assembled at the end of the crossbar (010) away from the upright rod (002) by a second fixing device (008).

5. The fatigue crack prediction method for welded structures according to claim 2, characterized in that, The sample was prepared by the following method: a marine high-strength steel welded butt plate with dimensions of 440mm*200mm*40mm was welded by hand. A crescent-shaped notch was machined along the width direction at the center of the front weld as a fatigue crack initiation source. The notch was 15mm long and 2mm deep. The excess height of the back weld was eliminated by machining.

6. The fatigue crack prediction method for welded structures according to claim 2, characterized in that, The fatigue test had a mean load of -350 to -450 kN, an amplitude load of 200 to 400 kN, a span of 200 to 400 mm, and a frequency of 2 to 5 Hz.

7. The application of the fatigue crack prediction method for welded structures according to any one of claims 1-6 in the remaining life prediction model of welded structures.

Images