CN1587988A - Sample for measuring fatigue crack expansion rate in corrosion liquid and test method - Google Patents

Abstract

A sample and test method for measuring the fatigue crack growth rate in corrosive liquid. The upper side of the sample away from the crack propagation surface adopts the method of integral design to set the clamping knife edge of the electronic extensometer; according to the data collected by the control system of the testing machine Based on a large amount of test data, the flexibility coefficient of the designed sample is solved, and the functional relationship between the normalized crack length and the flexibility coefficient is established to realize the fatigue crack growth rate (da/dN) test in the corrosive liquid. The length detection is automated, and the test method established based on it provides a guarantee for the smooth progress of this type of test. Compared with most domestic units currently measuring the fatigue crack growth rate (da/dN) in the corrosive solution and using the visual method to detect the crack length, the accuracy of measuring the crack length is significantly improved, and the labor intensity is also greatly reduced. Using the test results, the safety design and life assessment of ships serving in corrosive media can be more accurately carried out.

Description

Specimen and test method for measuring fatigue crack growth rate in corrosive solution

Technical field:

The invention relates to the technical field of metal material corrosion test methods, and relates to a sample and a test method for measuring the fatigue crack growth rate in corrosive liquid.

Background technique:

At present, in China's GB/T6398-2000 (metal material fatigue crack growth rate test method), the extensometer clamping knife edge of the standard compact tensile C(T) sample is on both sides of the crack growth plane. When the fatigue crack growth rate (da/dN) test is carried out in the corrosive solution, the fatigue crack length is measured by the flexibility method only on the horizontal fatigue testing machine; if this test is carried out on the vertical fatigue testing machine, the price is expensive High-quality extensometers (imported about 5,000 U.S. dollars, and domestic ones cost thousands of yuan) tend to reduce their linearity, sensitivity or be completely damaged due to erosion by corrosive liquids; while most domestic fatigue testing machines are vertical Therefore, it is almost impossible to measure the fatigue crack length by the compliance method when performing the fatigue crack growth rate (da/dN) test in the corrosive solution. In view of this situation, when most domestic units conduct such tests, they generally do not use extensometers to measure the fatigue crack length through flexibility techniques, but use optical microscopes to observe the crack growth by visual inspection. However, the fatigue cracks of metal materials are often relatively thin, especially the fatigue cracks of copper alloys, titanium alloys and other materials, which are not easy to observe clearly in the air, and because the corrosive liquid is often corroded by the suspended metal material samples The product becomes turbid and unclear, which makes it more difficult to observe the length of fatigue cracks by visual inspection, so it is difficult to ensure the accuracy of fatigue crack length observation by visual inspection, and the labor intensity is also very high.

At present, there are few domestic literatures related to the measurement of fatigue crack length in the field of test and testing, especially the research on flexibility technology and the automatic detection of crack length in corrosive environment using flexibility technology have not been seen.

Invention content:

In order to solve the above existing problems, the present invention provides a sample and test method for measuring the fatigue crack growth rate in corrosive liquid. And the damage will eventually make the test fail. Compared with the visual method of measuring the fatigue crack length, this method can significantly improve the accuracy of measuring the fatigue crack length, and also greatly reduce the labor intensity.

The present invention is realized by adopting the following technical solutions:

The sample for measuring the fatigue crack growth rate in the corrosive liquid is provided with a clamping knife edge of an electronic extensometer on the upper side of the sample away from the crack propagation surface; Using the test data of the imported equipment, the flexibility coefficient of the designed sample is calculated as follows:

Symbol of compliance coefficient C ₀ C ₁ C ₂ C ₃ C ₄ C ₅

Value of flexibility coefficient 0.9927 -4.8803 17.031 -189.39 781.63 -821.74

At the same time, the functional relationship between the normalized crack length and the flexibility coefficient is established to realize the automatic detection of the crack length when performing the fatigue crack growth rate (da/dN) test in the corrosive solution.

Implement the test method of the sample for measuring the fatigue crack growth rate in the corrosive solution according to the following steps:

(1) The initial crack of the sample is induced by wire electric discharge cutting;

(2) Input the calculated flexibility coefficient of the designed sample into the pre-programmed test program;

(3) Measure the thickness and width parameters of the sample, and input them into the pre-programmed program;

(4) Connect the corrosion circulation system, related sample fixtures and samples with the vertical fatigue testing machine;

(5) Add the corrosive solution to the circulation system and the corrosion box containing the sample. The corrosive solution in the circulation system should be sufficient, and the corrosive solution in the corrosion box should be guaranteed to soak the crack propagation surface from time to time;

(6) Install the electronic extensometer at the knife edge on the upper side of the sample;

(7) Set various parameters of prefabricated fatigue cracks, including stress intensity factor range ΔK, control mode, frequency, waveform and length of prefabricated fatigue cracks;

(8) Start the testing machine, start prefabricating fatigue cracks, observe the current crack extension length from time to time, pay attention to the entire corrosion cycle system, to prevent corrosion of the testing machine due to leakage of corrosive liquid;

(9) After the fatigue cracks are prefabricated, according to the current results, set the parameters of the load range, stress ratio, and frequency required for the test;

(10) Start the testing machine again and start the formal test;

(11) After the test, according to the collected test data, fit the fatigue crack growth rate curve and equation of the test material according to the expression form of the Paris (Paris) empirical equation, and finally issue the corresponding test report.

For the sample measuring the fatigue crack growth rate in the corrosive solution, the solution steps of the flexibility coefficient of the sample are as follows:

(1) Measure the thickness B, width and the effective crack length a caused by any method of the designed sample, and test the elastic modulus E of the test material in advance;

(2) Initiate cracks on the designed sample by wire electric discharge cutting, initiate once every a/W=0.025, and re-measure the induced crack length a, as the actual value used;

(3) Install the sample after the crack is induced on the testing machine, and install the electronic extensometer at the knife edge of the sample, and reset it;

(4) Slowly stretch the sample after crack initiation within the elastic range. At this time, it is necessary to collect the load P acting on the sample and the deformation V of the extensometer at a higher sampling rate;

(5) Solve the dimensionless flexibility value Ux of the sample according to the formula Ux=1/[(BEV _x /P) ^1/2 +1];

(6) After each crack is initiated, a normalized crack length a/W value is required to be solved;

(7) When the normalized crack length a/W=0.75, the test can be stopped;

(8) Carry out a quintic incremental polynomial regression on the obtained data set (a/W, Ux), and its regression coefficient can be used as the flexibility coefficient of the sample;

(9) Carry out the above steps for multiple samples, and six flexibility coefficients of each sample can be obtained respectively;

(10) The compliance coefficients of multiple samples are averaged to obtain the final six compliance coefficients C ₀ , C ₁ , C ₂ , C ₃ , C ₄ , and C ₅ .

Owing to adopting above-mentioned technical scheme, the present invention has following positive effect:

The sample designed by the present invention to measure the fatigue crack growth rate (da/dN) in the corrosive liquid (sea water, salt water, river water, etc.) mentions the clamping edge of the electronic extensometer to the upper side away from the crack propagation surface, so as to solve the problem When using the flexibility method to measure the fatigue crack length on the vertical fatigue testing machine, the expensive extensometer is often damaged by the corrosion of the corrosive liquid, which leads to the failure of the test. This sample can also be used on a horizontal fatigue testing machine.

The function equation of the normalized crack length and the flexibility coefficient was established by using the sample flexibility coefficient obtained from the solution, which theoretically laid the foundation for measuring the fatigue crack length through the flexibility technology, so that fatigue in the corrosive solution The detection of the crack length in the crack growth rate (da/dN) test has been automated, and compared with most domestic units currently measuring the fatigue crack growth rate (da/dN) in the corrosive solution and using the visual method to detect the crack length, the measurement has been significantly improved. The accuracy of the crack length also greatly reduces the labor intensity.

The successful design of the sample for measuring the fatigue crack growth rate in the corrosive solution, the correct calculation of the flexibility coefficient and the test method established accordingly provide a guarantee for the smooth progress of this type of test. Using the test results, the safety design and life assessment of ships serving in corrosive media can be more accurately carried out.

Description of drawings:

Figure 1 is a standard compact tensile C(T) sample diagram;

Fig. 2 is the sample diagram designed in the sample and test method for measuring the fatigue crack growth rate in corrosive liquid;

Fig. 3 is the fatigue crack growth rate (da/dN)-ΔK curve diagram of the standard compact tensile C (T) specimen;

Fig. 4 is a curve diagram of the fatigue crack growth rate (da/dN)-ΔK of the sample for measuring the fatigue crack growth rate in the corrosive solution.

Detailed ways:

1. Sample form for measuring fatigue crack growth rate (da/dN) in corrosive liquid

In the corrosive solution, the fatigue crack growth rate (da/dN) test is carried out on the standard compact tensile C(T) specimen, and the fatigue crack length is observed by visual inspection. On the one hand, the accuracy of the crack length read by the optical microscope cannot be guaranteed. On the other hand, the labor intensity is also quite large. As shown in Figure 1, the extensometer clamping knife edge (①) of the sample is on both sides of the symmetric crack propagation plane. When measuring the fatigue crack growth rate (da/dN) in the corrosive liquid (2 is the corrosive liquid level) on the vertical fatigue testing machine, the expensive extensometer is often eroded by the corrosive liquid. If the clamping edge of the extensometer is moved to the upper side of the crack propagation surface, firstly, the expensive extensometer can be guaranteed not to be corroded. In addition, if the flexibility coefficient of this sample can be solved, the fatigue crack length in the corrosive liquid can be measured by the flexibility technology, which can ensure the accuracy of the crack length measurement, and will greatly reduce the visual inspection method. A lot of hard work put in.

Therefore, on the basis of the standard compact tensile C(T) specimen, we designed a specimen for measuring the fatigue crack growth rate (da/dN) in the corrosive solution, as shown in Figure 2, the electronic extensometer The clamping knife edge (3) is designed on the upper side away from the crack propagation surface by the method of overall design, thus, on the one hand, it solves the problem that the expensive electronic extensometer is corroded by the corrosive liquid (4 is the corrosive liquid surface), and on the other hand On the one hand, it also lays the foundation for measuring the fatigue crack length with the flexibility technique. Based on a large amount of test data collected by the control system of the testing machine, such as the normalized crack length, the dimensionless flexibility of the sample, and the flexibility coefficient of unknown parameters, on the basis of strict reasoning and practice, we make full use of the multiple The function characteristics of the test software are used to solve the flexibility coefficient of the designed sample. The typical feature of the designed sample is: the electronic extensometer clamping knife edge used for indirect measurement of the fatigue crack length is designed on the upper side away from the crack propagation surface by the method of integral design, so that on the one hand, it can be guaranteed during the test The expensive extensometer is far away from the corrosive liquid surface, avoiding the possibility of reducing its linearity, sensitivity or complete damage due to the corrosion of the corrosive liquid during the measurement of the fatigue crack length. The middle extensometer will not fail the test due to falling off, and it is also convenient for loading and unloading operations. The successful design of the sample provides the possibility to automatically detect the crack length when performing the fatigue crack growth rate (da/dN) test in the corrosive solution.

2. The flexibility coefficient of the designed sample

After a lot of reasoning and experimental verification, the flexibility coefficient of the solved sample has high precision, strong reliability, and wide practicability: it can be applied to different metal materials, different sample sizes, and all samples with the same form as the designed sample , Proportional to the size of the sample. The function equation of the normalized crack length and flexibility coefficient was established by using the flexibility coefficients of the six samples solved, which theoretically laid the foundation for the measurement of the fatigue crack length by the flexibility technology, so as to be used in the corrosion solution It is possible to automatically measure the fatigue crack length in the fatigue crack growth rate (da/dN) test. Apply the necessary data collected by the control system of the testing machine such as the load acting on the sample and the opening displacement of the extensometer to the computer program written by the required theoretical formula, and the current crack extension length of the sample can be automatically calculated at any time , and can be displayed on the computer desktop from time to time. Finally, after calculation and processing, the fatigue crack growth rate of the test material in the corrosive solution can be obtained. The determination of the flexibility coefficient of the designed sample makes the detection of the crack length automatic when performing the fatigue crack growth rate (da/dN) test in the corrosive liquid, and at the same time, it also significantly improves the measurement accuracy of the crack length and greatly reduces the crack length. labor intensity.

The flexibility coefficient of the inventive design sample is as follows:

Symbol of compliance coefficient C ₀ C ₁ C ₂ C ₃ C ₄ C ₅

Value of flexibility coefficient 0.9927 -4.8803 17.031 -189.39 781.63 -821.74

3. The solution steps of the flexibility coefficient of the designed sample

(1) Measure the thickness B, width and the effective crack length a caused by any method of the designed sample, and test the elastic modulus E of the test material in advance;

(2) Initiate cracks on the designed sample by wire electric discharge cutting, initiate once every a/W=0.025, and re-measure the induced crack length a, as the actual value used;

(3) Install the sample after the crack is induced on the testing machine, and install the electronic extensometer at the knife edge of the sample, and reset it;

(4) Slowly stretch the sample after crack initiation within the elastic range. At this time, it is necessary to collect the load P acting on the sample and the deformation of the extensometer at a higher sampling rate (such as 100 data points/sec). volume V;

(5) Solve the dimensionless flexibility value Ux of the sample according to the formula Ux=1/[(BEVV _x /P) ^1/2 +1];

(6) After each crack is initiated, a normalized crack length a/W value is required to be solved;

(7) When the normalized crack length a/W=0.75 (the value generally will not exceed 0.70 during the formal test), the test can be stopped;

(8) Carry out a quintic incremental polynomial regression on the obtained data set (a/W, Ux), and its regression coefficient can be used as the flexibility coefficient of the sample;

(9) Carry out the above steps for multiple samples, and six flexibility coefficients of each sample can be obtained respectively;

(10) The compliance coefficients of multiple samples are averaged to obtain the final six compliance coefficients C ₀ , C ₁ , C ₂ , C ₃ , C ₄ , and C ₅ .

4. Test method

Using the designed sample and its flexibility coefficient, the fatigue crack growth rate (da/dN) test in the corrosive solution can be completed according to the following steps:

(1) The initial crack of the sample is induced by wire electric discharge cutting;

(2) Input the calculated flexibility coefficient of the designed sample into the pre-programmed test program;

(3) Measure parameters such as sample thickness and width, and input them into the pre-programmed program;

(4) Connect the corrosion circulation system, related sample fixtures and samples with the vertical fatigue testing machine;

(5) Add the corrosive solution to the circulation system and the corrosion box containing the sample. The corrosive solution in the circulation system should be sufficient, and the corrosive solution in the corrosion box should be guaranteed to soak the crack propagation surface from time to time;

(6) Install the electronic extensometer at the knife edge on the upper side of the sample;

(7) Set various parameters of prefabricated fatigue cracks, including stress intensity factor range ΔK, control mode, frequency, waveform and length of prefabricated fatigue cracks, etc.;

(8) Start the testing machine, start prefabricating fatigue cracks, observe the current crack extension length from time to time, pay attention to the entire corrosion cycle system, to prevent corrosion of the testing machine due to leakage of corrosive liquid;

(9) After the fatigue cracks are prefabricated, according to the current results, set the required load range, stress ratio, frequency and other parameters for the test;

(10) Start the testing machine again and start the formal test;

(11) After the test, according to the collected test data, fit the fatigue crack growth rate curve and equation of the test material according to the expression form of the Paris (Paris) empirical equation, and finally issue the corresponding test report.

It can be seen from the sample form of our invention that the sample can undoubtedly be used in the fatigue test in the corrosive solution to ensure that the electronic extensometer is not corroded by the corrosive solution. In order to verify the correctness and feasibility of the designed specimen flexibility coefficient and test method, we selected a structural steel material and processed it into a set of standard compact tensile specimens and a set of specimens designed by us. Since the standard compact tensile specimen on the vertical testing machine cannot measure the fatigue crack growth rate of the test material in the corrosive solution, and the fatigue crack growth rate of the metal material in the air and in the corrosive solution is often very different. Therefore, we believe that if the fatigue crack growth rate (da/dN) of the test material is tested in air using the compliance technique with standard compact tensile specimens, then with our inventively designed specimen and its coefficient of compliance Using the same test conditions to test, since the test materials are the same, if the consistency of the test results is very good, it can show that the sample and test method we invented are correct.

The test equipment is a vertical MTS810 electro-hydraulic servo fatigue testing machine.

The test was carried out using the steps of the above-mentioned test method.

The test conditions used are as follows:

Test load range ΔP = 12.0KN, stress ratio R = 0.1, waveform is sine wave, frequency f = 10Hz, test in air at room temperature.

The test results (including the da/dN-ΔK curve diagram and its equation) are shown in Figure 3 and Figure 4, as can be seen from Figure 3 and Figure 4, the distribution of the data sets (da/dN, ΔK) of the two samples is completely Same. Considering the uniformity of the structure of the test material and the processing error of each sample, the crack growth rate of the obtained test material in two different sample forms (corresponding to different flexibility coefficients) is consistent. This fully shows that the sample we designed and invented to measure the fatigue crack growth rate of metal materials in corrosive liquid is practical, the flexibility coefficient is reliable, and the established test method is reliable and feasible.

Claims (3)

1. A sample for measuring the fatigue crack growth rate in corrosive liquid, characterized in that: the sample is provided with a clamping device for an electronic extensometer on the upper side of the sample away from the crack propagation surface by adopting an overall design method Knife edge: According to a large amount of test data collected by the control system of the testing machine, using a number of test software of imported equipment, the flexibility coefficient of the designed sample is calculated as follows: Symbol of compliance coefficient C ₀ C ₁ C ₂ C ₃ C ₄ C ₅ Flexibility coefficient value 0.9927 -4.8803 17.031 -189.39 781.63 -821.74 At the same time, the functional relationship between the normalized crack length and the flexibility coefficient is established to realize the automatic detection of the crack length when performing the fatigue crack growth rate (da/dN) test in the corrosive solution. 2, implement the test method of the sample of fatigue crack growth rate in the measurement corrosion solution described in claim 1, it is characterized in that: this test method is carried out according to the following steps: (1) The initial crack of the sample is induced by wire electric discharge cutting; (2) Input the calculated flexibility coefficient of the designed sample into the pre-programmed test program; (3) Measure the thickness and width parameters of the sample, and input them into the pre-programmed program; (4) Connect the corrosion circulation system, related sample fixtures and samples with the vertical fatigue testing machine; (5) Add the corrosive solution to the circulation system and the corrosion box containing the sample. The corrosive solution in the circulation system should be sufficient, and the corrosive solution in the corrosion box should be guaranteed to soak the crack propagation surface from time to time; (6) Install the electronic extensometer at the knife edge on the upper side of the sample; (7) Set various parameters of prefabricated fatigue cracks, including stress intensity factor range ΔK, control mode, frequency, waveform and length of prefabricated fatigue cracks; (8) Start the testing machine, start prefabricating fatigue cracks, observe the current crack extension length from time to time, pay attention to the entire corrosion cycle system, to prevent corrosion of the testing machine due to leakage of corrosive liquid; (9) After the fatigue cracks are prefabricated, according to the current results, set the parameters of the load range, stress ratio, and frequency required for the test; (10) Start the testing machine again and start the formal test; (11) After the test, according to the collected test data, fit the fatigue crack growth rate curve and equation of the test material according to the expression form of the Paris (Paris) empirical equation, and finally issue the corresponding test report. 3. The sample for measuring the fatigue crack growth rate in corrosive liquid according to claim 1, characterized in that: the steps for solving the flexibility coefficient of the sample are as follows: (1) Measure the thickness B, width and the effective crack length a caused by any method of the designed sample, and test the elastic modulus E of the test material in advance; (2) Initiate cracks on the designed sample by wire electric discharge cutting, initiate once every a/W=0.025, and re-measure the induced crack length a, as the actual value used; (3) Install the sample after the crack is induced on the testing machine, and install the electronic extensometer at the knife edge of the sample, and reset it; (4) Slowly stretch the sample after crack initiation within the elastic range. At this time, it is necessary to collect the load P acting on the sample and the deformation V of the extensometer at a higher sampling rate; (5) Solve the dimensionless flexibility value Ux of the sample according to the formula Ux=1/[(BEV _x /P) ^1/2 +1]; (6) After each crack is initiated, a normalized crack length a/W value is required to be solved; (7) When the normalized crack length a/W=0.75, the test can be stopped; (8) Carry out a quintic incremental polynomial regression on the obtained data set (a/W, Ux), and its regression coefficient can be used as the flexibility coefficient of the sample; (9) Carry out the above steps for multiple samples, and six flexibility coefficients of each sample can be obtained respectively; (10) The compliance coefficients of multiple samples are averaged to obtain the final six compliance coefficients C ₀ , C ₁ , C ₂ , C ₃ , C ₄ , and C ₅ .

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