A Stepped Load Testing Method for Stress Corrosion Crack Propagation Threshold

Abstract

This invention discloses a method for testing the stress corrosion crack propagation threshold using a stepped loading method, comprising the following steps: Stepped loading setting: estimating the stress corrosion crack propagation threshold KTSCC of the sample under the test environment, calculating the corresponding load P0 based on the pre-prepared crack length, and setting the first-level constant load P1; determining the load step ΔP between different levels of constant load in the stepped loading; Testing and monitoring: slowly increasing the load to P1 according to the set displacement loading rate; when the load increases to P1, starting the timer and holding it for time dt; if crack propagation is detected within time dt, stopping the test; otherwise, increasing the load to P2 = P1 + ΔP, and when the load increases to P2, starting the timer and holding it for time dt, until crack propagation is detected within the action time of a certain level of constant load Pn, stopping the test; This invention effectively balances testing efficiency and accuracy through the "single sample, stepped constant load loading" method, avoiding the drawbacks of traditional methods that are time-consuming and material-intensive.

Description

Technical Field

[0001] This invention belongs to the field of materials testing and analysis technology, and in particular relates to a step-by-step loading test method for stress corrosion crack propagation threshold. Background Technology

[0002] The stress corrosion crack propagation threshold, also known as the critical stress intensity factor (K) for stress corrosion crack propagation. TSCC Kc is the maximum stress intensity factor value in a specific material-environment system that prevents stress corrosion crack propagation in a specimen. This parameter is a key indicator for evaluating the resistance of engineering materials (such as high-strength steel, aluminum alloys, and titanium alloys) to stress corrosion cracking (SCC) in corrosive environments, and is directly related to the safety design and life prediction of critical load-bearing structures in marine engineering, aerospace, and nuclear power equipment. Therefore, accurate and efficient determination of Kc is crucial. TSCC It is of vital importance to ensure structural integrity and prevent catastrophic accidents.

[0003] Currently, it can be used to determine K TSCC The load control methods mainly follow two major technical approaches, but both have limitations: 1. Gradual Loading or Slow Strain Rate Method: This method typically applies a continuously and slowly increasing load (or displacement) to a single pre-cracked specimen, and monitors the crack propagation behavior in real time using high-precision sensors (such as DC potential drop). The core idea is to inversely deduce a threshold by capturing the load conditions corresponding to crack initiation or reaching a certain small propagation rate. However, there is currently no good way to select a reasonable displacement rate under specific test conditions. For materials highly sensitive to loading rates, the cracking behavior under gradual loading is affected by the load or displacement growth rate. A rate that is too fast may mask the stress corrosion susceptibility under specific environments, while a rate that is too slow will prolong the test cycle, reduce efficiency, and increase testing costs.

[0004] 2. Constant Load Method: This method uses a set of pre-fabricated cracked specimens with identical geometric dimensions, and exposes them to constant loads at different levels. K is estimated by observing and statistically analyzing whether the crack propagates within a specific time period. TSCC The main drawback of this method is the high sample consumption; to obtain a single threshold data point, multiple samples typically need to be prepared and tested, resulting in high material and processing costs. In particular, for the complex and time-consuming stress corrosion threshold test, this significantly reduces testing efficiency and increases testing costs.

[0005] In summary, the core contradiction facing existing technologies lies in the fact that the pursuit of testing accuracy often comes at the cost of efficiency or economy, making it difficult to achieve a good balance between accuracy, efficiency, and reliability. Summary of the Invention

[0006] This invention addresses the problems existing in the prior art by proposing a stepped load testing method for stress corrosion crack propagation threshold. The aim is to solve the problem that the pursuit of testing accuracy in the prior art often comes at the cost of efficiency or economy, making it difficult to achieve a good balance between accuracy, efficiency and reliability.

[0007] To solve its technical problem, the present invention adopts the following technical solution: A method for testing the stress corrosion crack propagation threshold using a stepped loading method, characterized by the following steps: Step 1: Sample preparation: Select a sample with pre-existing fatigue cracks. Step 2, Environment Setup: Set the appropriate environmental parameters according to the experimental purpose, and prepare the environmental media and equipment that meet the experimental requirements; Step 3, Stepped loading settings: (3.1) Estimate the stress corrosion crack propagation threshold K of the specimen under the test environment. TSCC , calculate the corresponding load P0 based on the pre-made crack length, and set the first level constant load P1; (3.2) determine the load step ΔP between different levels of constant load in the stepped loading; Step 4, Testing and Monitoring: (4.1) Using a testing machine equipped with environmental requirements, after the test specimen is in full contact with the test environment, slowly increase the load to P1 according to the set displacement loading rate; (4.2) When the load increases to P1, start timing and hold for time dt. If crack propagation is detected within time dt, stop the test; otherwise, increase the load to P2 = P1 + ΔP. When the load increases to P2, start timing and hold for time dt until crack propagation is detected within the action time of a certain constant load Pn, then stop the test; (4.3) Use optical methods, resistance methods, etc. to monitor whether crack propagation occurs; n = 2, 3, 4, ...; The value of Pn is not affected by the estimated stress corrosion crack propagation threshold K of the specimen under the test environment. TSCC Under the constraint, the value of Pn can be greater than K. TSCC .

[0008] Step 5, Results and Judgment: (5.1) After the test is stopped, the sample is broken and the crack tip is marked; (5.2) Measure the fatigue pre-crack length and calculate the effective crack length; (5.3) Use the stress intensity factor value corresponding to the maximum first-order constant load that does not cause stress corrosion crack propagation as the stress corrosion crack propagation threshold K. TSCC .

[0009] Furthermore, the first-level constant load P1 is selected to include 0.7P0 ~ 0.9P0.

[0010] Furthermore, the load step ΔP is selected to include 0.05P0 ~ 0.2P0.

[0011] Furthermore, the duration of the constant load includes dt being a multiple of 20h.

[0012] Furthermore, the displacement loading rate includes no more than 10⁻⁶ m / s.

[0013] Furthermore, the method is applicable to complex stress conditions such as Type I cracks and Type I-II combined cracks, as well as stress corrosion crack propagation threshold testing of structural components.

[0014] Furthermore, if crack propagation is detected when the load is increased to P1, it indicates that the predicted K... TSCC If P0 is too large compared to the true value, P1 and ΔP should be reselected, and the test should be restarted with a new sample. Advantages and effects of the present invention

[0015] 1. To address the issues of the gradual loading method potentially masking the stress corrosion susceptibility of test specimens under specific environments due to excessively rapid load rates, the excessively slow load increase rate leading to long test cycles, and the constant load method requiring a large number of test specimens and increasing specimen processing costs, a new single-specimen, stepped constant load loading mode is adopted. K is obtained through a single specimen. TSCC The value retains the advantages of constant load control while greatly reducing sample consumption and processing costs.

[0016] 2. To address the issue of load step size relying on experience, thresholds can be obtained conveniently and accurately by defining load steps and initial ranges. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the dimensions of the Ti-6Al-4V ELI titanium alloy sample of the present invention (unit: mm). Figure 2 Fatigue predictions were measured at three locations: two sides of the sample and at three locations: 0.25 times, 0.5 times, and 0.75 times the thickness.

[0018] Figure 3 This is a flowchart of a step-by-step load testing method for stress corrosion crack propagation threshold according to the present invention. Detailed Implementation Innovation of this invention

[0019] 1. A new “stepped constant load” test mode was created: by using the method of “single sample, stepped constant load loading”, the test efficiency and accuracy are effectively balanced, avoiding the drawbacks of traditional methods that are time-consuming and material-intensive.

[0020] 2. A refined full-process parameter control system was established: By defining the load step ΔP and the initial load range P1, the loading process is made orderly and controllable. By limiting the loading rate, strain rate interference is eliminated or reduced, making the results more reliable.

[0021] 4. Expanded the scope of application of the method: The method can be extended to stress corrosion crack propagation threshold testing of composite cracks and components, which significantly improves the universality and engineering application value of the method. Summarize

[0022] This invention innovatively proposes a "single sample, stepped constant load" mode, supplemented by a clear parameter control strategy, which solves the problem in the prior art that the pursuit of test accuracy often comes at the cost of efficiency or economy, and provides a more universal stress corrosion crack propagation threshold test method that balances efficiency and accuracy. Design principle of the invention

[0023] 1. "Single-sample, stepped constant load loading" strategy: Compared to the discrete, statistical approach of multiple samples and single load points, this invention (single-sample, multiple load points) is continuous and tracking. This not only saves on sample costs, but more importantly, a single sample can typically obtain a K0 value under a specific corrosive environment. TSCC Numerical value.

[0024] 3. Comprehensive Management of Uncertainty: This invention employs a low displacement rate: avoiding or reducing the influence of strain rate, an active interference variable, making the dominance of constant load-environment-induced cracking (stress corrosion) more prominent and improving data accuracy. A fixed dt: providing a standardized duration for each load level, making crack propagation results comparable across different load levels and reducing uncertainty in result interpretation caused by inconsistent observation times. Single-sample testing avoids the uncertainty caused by the dispersion between multiple samples.

[0025] Based on the above principles, this invention designs a method for testing the stress corrosion crack propagation threshold using a stepped loading method, such as... Figures 1-3 As shown, its characteristics include the following steps: Step 1: Sample preparation: Select a sample with pre-existing fatigue cracks. Step 2, Environment Setup: Set the appropriate environmental parameters according to the experimental purpose, and prepare the environmental media and equipment that meet the experimental requirements; Step 3, Stepped loading settings: (3.1) Estimate the stress corrosion crack propagation threshold K of the specimen under the test environment. TSCC , calculate the corresponding load P0 based on the pre-made crack length, and set the first level constant load P1; (3.2) determine the load step ΔP between different levels of constant load in the stepped loading; Step 4, Testing and Monitoring: (4.1) Using a testing machine equipped with environmental requirements, after the test specimen is in full contact with the test environment, slowly increase the load to P1 according to the set displacement loading rate; (4.2) When the load increases to P1, start timing and hold for time dt. If crack propagation is detected within time dt, stop the test; otherwise, increase the load to P2 = P1 + ΔP. When the load increases to P2, start timing and hold for time dt until crack propagation is detected within the action time of a certain constant load Pn, then stop the test; (4.3) Use optical methods, resistance methods, etc. to monitor whether crack propagation occurs; n = 2, 3, 4, ...; The value of Pn is not affected by the estimated stress corrosion crack propagation threshold K of the specimen under the test environment. TSCC Under the constraint, the value of Pn can be greater than K. TSCC .

[0026] Step 5, Results and Judgment: (5.1) After the test is stopped, the sample is broken and the crack tip is marked; (5.2) Measure the fatigue pre-crack length and calculate the effective crack length; (5.3) Use the stress intensity factor value corresponding to the maximum first-order constant load that does not cause stress corrosion crack propagation as the stress corrosion crack propagation threshold K. TSCC .

[0027] Furthermore, the first-level constant load P1 is selected to include 0.7P0 ~ 0.9P0.

[0028] Furthermore, the load step ΔP is selected to include 0.05P0 ~ 0.2P0.

[0029] Furthermore, the duration of the constant load includes dt being a multiple of 20h.

[0030] Furthermore, the displacement loading rate includes no more than 10⁻⁶ m / s.

[0031] Furthermore, the method is applicable to complex stress conditions such as Type I cracks and Type I-II combined cracks, as well as stress corrosion crack propagation threshold testing of structural components.

[0032] Furthermore, if crack propagation is detected when the load is increased to P1, it indicates that the predicted K... TSCC If P0 is too large compared to the true value, P1 and ΔP should be reselected, and the test should be restarted with a new sample. Example 1

[0033] The following is a measurement Figure 1The Ti-6Al-4V ELI titanium alloy specimen shown has a stress corrosion crack propagation threshold K under simulated seawater conditions (yield strength 900 MPa). TSCC For example, this paper illustrates the application of this method in determining the stress corrosion crack propagation threshold of materials under specific corrosive environments. (Related...) Figure 1 For information on intermediate specimens, please refer to the national standard "Corrosion of metals and alloys - Stress corrosion testing - Part 6: Preparation and application of pre-cracked specimens under constant load or constant displacement".

[0034] (1) Sample preparation Processing Figure 1 The Ti-6Al-4V ELI titanium alloy specimen shown was subjected to pre-induced fatigue cracks. The fatigue stress ratio was 0.1, and the frequency was 10 Hz.

[0035] (2) Environmental settings Based on the experimental objectives, a simulated seawater environment was selected, and a suitable experimental system and environmental apparatus were prepared to meet the experimental requirements. The experimental temperature was room temperature.

[0036] (3) Step loading settings ① In order to more accurately estimate the stress corrosion crack propagation threshold K of the test specimen TSCC The fracture toughness K of three identical specimens in an air environment IC or K Q The K value was tested, and the values ​​of 3 samples were obtained. Q The average value is 108 MPa·m 1 / 2 The stress corrosion crack propagation threshold K of Ti-6Al-4V ELI titanium alloy specimens under simulated seawater conditions was estimated. TSCC 0.5K Q That is, 54 MPa·m 1 / 2 Based on the pre-fabricated crack length, the loads P0 corresponding to the three specimens were calculated to be 25.10 kN, 24.95 kN, and 24.95 kN, respectively. The first-level constant load P1 was selected as 0.8P0, meaning the P1 corresponding to the three specimens were 20.08 kN, 19.96 kN, and 19.96 kN, respectively. ② Here, K... TSCC A relatively rough test was conducted, and the load step ΔP was selected as 0.2P0, that is, the ΔP corresponding to the three samples were 5.02 kN, 4.99 kN and 4.99 kN respectively.

[0037] (4) Testing and monitoring Creep testing of Ti-6Al-4V ELI titanium alloy was conducted using a creep testing machine with a simulated seawater environment. TSCCTesting. After the test specimen came into contact with the test environment, the load was slowly increased to P1, with a displacement loading rate of 10⁻⁶ m / s. Timing began when the load reached P1 and was maintained for 20 hours. If crack propagation was detected within 20 hours, the test was stopped; otherwise, the load was increased to P2, and timing began again and was maintained for 20 hours, until crack propagation was detected within a certain constant load period, at which point the test was stopped. During the test, a 4K intelligent measuring camera YK800V (pixel size 2.0 μm × 2.0 μm) was used to monitor the propagation of stress corrosion cracks. A total of three Ti-6Al-4V ELI titanium alloy specimens were tested.

[0038] (5) Results and Judgments ① Stress corrosion crack propagation was observed in all three specimens under constant load P2, and continued loading under this load caused the specimens to break.

[0039] ② After the specimen fractures, the fatigue pre-crack length is measured at two sides of the specimen and at three locations: 0.25 times, 0.5 times, and 0.75 times the thickness. Figure 2 Table 1 shows the results. Upon inspection, the fatigue pre-crack and crack surface of the three specimens all met the following conditions: a) the difference between any two of the measured values ​​at 0.25 times, 0.5 times, and 0.75 times the specimen thickness did not exceed 2.5% of the specimen width; b) the difference between the maximum and minimum crack lengths did not exceed 5% of the specimen width; c) the plane containing any part of the fatigue crack surface deviated from the notch plane by no more than 10°; d) the fatigue crack was on one surface, and there was no multinucleation effect; e) the fatigue crack length could be accurately measured. Therefore, the average value of the fatigue pre-crack length measured at the two sides of the specimen and at the three locations of 0.25 times, 0.5 times, and 0.75 times the thickness was used to calculate K. TSCC The effective crack length. K TSCC This is the stress intensity factor value corresponding to the maximum first-order constant load that prevents stress corrosion crack propagation. Specifically, the stress corrosion crack propagation threshold values ​​for the three Ti-6Al-4V ELI titanium alloy samples under simulated seawater conditions are 54 MPa·m. 1 / 2 54 MPa·m 1 / 2 and 54 MPa·m 1 / 2 Considering the differences between the samples, statistical analysis was performed on the test results of the three samples. The average stress corrosion crack propagation threshold of the three Ti-6Al-4V ELI titanium alloy samples in simulated seawater environment was found to be 54 MPa·m. 1 / 2 .

[0040] a and b above: Check the uniformity of crack length. The length difference measured at different thickness locations should be very small (not exceeding 2.5% of the specimen width), and the overall maximum and minimum length difference should also be small (not exceeding 5% of the specimen width). c: Check the straightness of the crack surface. The crack propagation direction should not deviate significantly from the preset notch plane (deviation angle ≤ 10°). d: Check the singularity of the crack. The crack should propagate from a single initiation point (core), rather than from multiple merging initiation points, as this will affect the accuracy of stress intensity calculations. e: Confirm that the crack length can be accurately measured.

[0041] Conclusion: All samples met the above standards, therefore the measurement data are valid.

[0042] It should be noted that, under specific corrosive environments, K TSCC The value is usually closely related to the constant load time and load step, and should be considered in combination with test requirements and test conditions.

[0043] It should be emphasized that the above specific embodiments are merely explanations of the present invention and are not intended to limit the present invention. After reading this specification, those skilled in the art can make modifications to the above embodiments without contributing any inventive step, but as long as they are within the scope of the claims of the present invention, they are protected by patent law.

Claims

1. A method for testing the stress corrosion crack propagation threshold using a stepped loading method, characterized in that, Includes the following steps: Step 1: Sample preparation: Select a sample with pre-existing fatigue cracks. Step 2, Environment Setup: Set the appropriate environmental parameters according to the experimental purpose, and prepare the environmental media and equipment that meet the experimental requirements; Step 3, Stepped loading settings: (3.1) Estimate the stress corrosion crack propagation threshold K of the specimen under the test environment. TSCC , calculate the corresponding load P0 based on the pre-made crack length, and set the first level constant load P1; (3.2) determine the load step ΔP between different levels of constant load in the stepped loading; Step 4, Testing and Monitoring: (4.1) Using a testing machine equipped with environmental requirements, after the test specimen is in full contact with the test environment, slowly increase the load to P1 according to the set displacement loading rate; (4.2) When the load increases to P1, start timing and hold for time dt. If crack propagation is detected within time dt, stop the test; otherwise, increase the load to P2 = P1 + ΔP. When the load increases to P2, start timing and hold for time dt until crack propagation is detected within the action time of a certain constant load Pn, then stop the test; (4.3) Use optical methods, resistance methods, etc. to monitor whether crack propagation occurs; n = 2, 3, 4, ...; The value of Pn is not affected by the estimated stress corrosion crack propagation threshold K of the specimen under the test environment. TSCC Under the constraint, the value of Pn can be greater than K. TSCC ; Step 5, Results and Judgment: (5.1) After the test is stopped, the sample is broken and the crack tip is marked; (5.2) Determine the fatigue pre-crack length and calculate the effective crack length; (5.3) The stress intensity factor value corresponding to the maximum first-order constant load that does not cause stress corrosion crack propagation is taken as the stress corrosion crack propagation threshold K. TSCC .

2. The method for testing the stress corrosion crack propagation threshold using a stepped loading method according to claim 1, characterized in that: The first-level constant load P1 is selected to include 0.7P0 ~ 0.9P0.

3. The method for testing the stress corrosion crack propagation threshold using a stepped loading method according to claim 1, characterized in that: The load step ΔP is selected to include 0.05P0 ~ 0.2P0.

4. The method for testing the stress corrosion crack propagation threshold using a stepped loading method according to claim 1, characterized in that: The duration of the constant load includes dt being a multiple of 20h.

5. The method for testing the stress corrosion crack propagation threshold using a stepped loading method according to claim 1, characterized in that: The displacement loading rate includes no more than 10⁻⁶ m / s.

6. The method for testing the stress corrosion crack propagation threshold using a stepped loading method according to claim 1, characterized in that: The method is applicable to complex stress conditions such as type I cracks and type I-II combined cracks, as well as stress corrosion crack propagation threshold testing of structural components.

7. The method for testing the stress corrosion crack propagation threshold using a stepped loading method according to claim 1, characterized in that: If crack propagation is detected when the load is increased to P1, it indicates that the predicted K TSCC If P0 is too large compared to the true value, P1 and ΔP should be reselected, and the test should be restarted with a new sample.

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