阅读说明：本技术 一种电位法与复型法相结合的曲线裂纹扩展监测方法 (Curve crack propagation monitoring method combining potential method and replica method ) 是由 周斌 郭秩维 胡绪腾 宋迎东 于 2021-06-28 设计创作，主要内容包括：本发明公开了一种电位法与复型法相结合的曲线裂纹扩展监测方法,包括如下步骤：对含有不同裂纹长度的试样进行稳态电流传导有限元分析,得到电位公式；在试样上连接导线,连接电源电压表形成闭环电路,安装试样,进行试验；裂纹扩展过程中,根据电位公式和实时的电压变化,中途停机复型若干次；根据断口形貌判断裂纹扩展过程中的裂纹形状及裂纹尺寸的变化,重新计算得到电位公式；根据复型数据和从断口测量的裂纹长度数据修正上一步计算的电位公式；根据记录的电位数据结合修正的电位公式处理得到裂纹前沿任意一点的裂纹长度变化曲线,进而计算得到裂纹前沿任意一点的裂纹扩展速率。本发明解决了传统电位法由于外界干扰因素,可靠性较低的问题。(The invention discloses a curve crack propagation monitoring method combining a potential method and a replica method, which comprises the following steps: carrying out steady-state current conduction finite element analysis on samples containing different crack lengths to obtain a potential formula; connecting a lead on the sample, connecting a power supply voltmeter to form a closed-loop circuit, installing the sample, and performing a test; in the crack propagation process, stopping the machine for multiple times in midway according to a potential formula and real-time voltage change; judging the change of the crack shape and the crack size in the crack propagation process according to the fracture morphology, and recalculating to obtain a potential formula; correcting the potential formula calculated in the previous step according to the replica data and the crack length data measured from the fracture; and processing the recorded potential data by combining the corrected potential formula to obtain a crack length change curve of any point of the crack front, and further calculating the crack propagation rate of any point of the crack front. The invention solves the problem of low reliability of the traditional potential method due to external interference factors.)

1. A curve crack propagation monitoring method combining a potential method and a replica method is characterized by comprising the following steps:

step 1, carrying out steady-state current conduction finite element analysis on samples containing different crack lengths, calculating the relation between voltage and the crack length, and fitting to obtain a potential formula;

step 2, connecting a lead on the sample, connecting a power supply voltmeter to form a closed loop circuit, setting the magnitude of the power supply current, and performing a test;

step 3, in the crack propagation process, stopping and duplicating for a plurality of times in midway according to the potential formula and the real-time voltage change obtained in the step 1;

step 4, judging the change of the crack shape and the crack size in the crack propagation process according to the fracture morphology, and recalculating to obtain a potential formula;

step 5, correcting the potential formula calculated in the step 4 according to the replica data and the crack length data measured from the fracture;

and 6, processing the recorded potential data by combining the corrected potential formula to obtain a crack length change curve of any point of the crack front, and further calculating the crack propagation rate of any point of the crack front.

2. The method for monitoring the crack propagation of the curve by combining the potentiometric method and the replica method according to claim 1, wherein the step 1 is specifically as follows:

establishing a geometric model of a crack-containing sample, selecting a current input point, applying a constant current, setting a zero potential point, and performing steady-state current conduction calculation; after the calculation is finished, selecting a voltage output point and extracting a voltage value of the voltage output point; secondly, changing the size of the crack according to the change of the shape of the crack in the crack expansion process, keeping other conditions unchanged, calculating the voltage value again, and calculating the voltage values at a plurality of moments in the crack expansion process by analogy; then, the voltage value at a certain smaller crack length is selected as the reference voltage V_rNormalizing the voltage with respect to a reference voltage, normalizing the crack length with respect to the sample size, and fitting to obtain a potential formula with the normalized crack length as an independent variable and the normalized voltage as a dependent variable.

3. The method for monitoring the crack propagation of a curve combining a potentiometric method and a replica method as claimed in claim 2, wherein in the step 1, the current input point and the voltage output point are selected to maximize the sensitivity of the voltage to the crack length, and the selection is assisted by continuously performing finite element simulation.

4. The method for monitoring the crack propagation curve by combining the potentiometric method and the replica method according to claim 1, wherein the step 2 is specifically as follows:

connecting a wire on the sample according to the current input point and the voltage output point selected in the step 1, connecting a power supply voltmeter to form a closed loop circuit, setting the constant current of the power supply, setting test parameters, and starting a test; the connecting mode of the lead on the test sample is welding or riveting.

5. The method for monitoring the crack propagation curve by combining the potentiometric method and the replica method according to claim 1, wherein the step 3 is specifically as follows:

firstly selecting the crack lengths of a plurality of moments needing to be reshaped in the crack expanding process, then calculating the percentage of the crack expanding to the expected length and voltage increase according to the potential formula obtained in the step 1, and then stopping reshaping when the voltage increases the percentage according to real-time voltage data.

6. The method for monitoring the crack propagation curve by combining the potentiometric method and the replica method according to claim 1 or 5, wherein in the step 3, the replica step is:

(1) cleaning the surface of a crack gap of the sample by using an alcohol cotton ball before the copying;

(2) applying a static load which is not more than 80% of the maximum test load to completely open the crack, aligning the nozzle head of the assembled replica system to the crack surface, slowly injecting replica glue until the replica glue completely fills the gap or covers the crack, and lightly pressing the bottom of the replica glue by using a small paper sheet so as to ensure that the later taken replica bottom is flat; after 5 minutes, after the replica glue is completely solidified, taking down the replica glue by using tweezers;

(3) taking down the replica, properly storing the replica, attaching a corresponding replica number label, and recording the number of the cycle at the moment;

(4) when the replica is observed by a microscope, obvious crack appearance is observed, no bubbles or inclusions exist, and the replica is invalid and is reconstructed.

7. The method for monitoring the crack propagation curve by combining the potentiometric method and the replica method according to claim 1, wherein the step 5 is specifically as follows:

according to the color layering line of the fracture surface, taking any point at the front edge of the crack as a research object, and measuring the crack length of the point at a plurality of shutdown moments; then, discontinuous potential data which are recorded in the crack expansion process and caused by shutdown replica for several times are connected into a continuous voltage history curve to obtain voltage values when the crack is stopped for several times; and (4) correcting the potential formula obtained in the step (4) by using actually measured relation data of the crack length and the voltage, wherein the aim of correction is to make the curve of the potential formula most fit with the replica data point.

8. The method for monitoring the crack propagation curve by combining the potentiometric method and the replica method according to claim 1, wherein the step 6 specifically comprises:

converting the voltage value into the crack length according to the voltage history curve and by combining the potential formula obtained by correction in the step 5, thereby obtaining a crack length change curve of any point at the front edge of the crack, and obtaining the crack propagation rate at any time by deriving the curve; then, obtaining the relation between the stress intensity factor of any point at the front edge of the crack and the length of the crack through finite element calculation; and finally combining the two to obtain a crack propagation rate curve.

Technical Field

The invention belongs to the technical field of mechanical testing, and relates to a curve crack propagation monitoring method combining a potential method and a replica method.

Background

In engineering practice, the components often crack after being subjected to complex loads, and the initiation and propagation of cracks can lead to structural failure. According to the fracture mechanics theory, the crack size and the crack propagation rate of the service structure surface are important parameters for structure fatigue damage evaluation and residual fatigue life prediction. In order to avoid the premature fatigue fracture failure of the crack-containing structure in the service process, the length of the fatigue crack must be accurately measured in time, and the corresponding fatigue crack propagation rate is calculated, so as to determine the inspection period of the equipment and ensure the service performance and reliability of the equipment.

At present, various curve crack propagation monitoring methods are developed, and optical observation methods, replica methods and potential methods are commonly used, and all the three methods have defects. For an optical observation method, a recorded data point is limited, the measurement accuracy of the crack length is influenced by an observation device, and the measurement accuracy is influenced because the observation distance is long, so that the resolution of the crack tip is easily unclear. For the replica method, the test needs to be stopped for a plurality of times in the test process to measure the crack length, the measurement precision is good, but in the high-temperature crack propagation test, the test period is greatly increased because the temperature of the test sample is required to be reduced to the use temperature of the replica glue for replica. For the potential method, because external factors of interference potential are more, the potential formula of actual and simulated calculation inevitably has deviation, and therefore the calculated crack length is not an actual value.

Disclosure of Invention

The invention provides a curve crack propagation monitoring method combining a potential method and a replica method, which aims to solve the problem of low reliability of the traditional potential method due to external interference factors.

A curve crack propagation monitoring method combining a potential method and a replica method comprises the following steps:

step 1, carrying out steady-state current conduction finite element analysis on samples containing different crack lengths, calculating the relation between voltage and the crack length, and fitting to obtain a potential formula;

step 2, connecting a lead on the sample, connecting a power supply voltmeter to form a closed-loop circuit, setting the magnitude of the power supply current, installing the sample, and performing a test;

step 3, in the crack propagation process, stopping and duplicating for a plurality of times in midway according to the potential formula and the real-time voltage change obtained in the step 1;

step 4, judging the change of the crack shape and the crack size in the crack propagation process according to the fracture morphology, and recalculating to obtain a potential formula;

step 5, correcting the potential formula calculated in the step 4 according to the replica data and the crack length data measured from the fracture;

and 6, processing the recorded potential data by combining the corrected potential formula to obtain a crack length change curve of any point of the crack front, and further calculating the crack propagation rate of any point of the crack front.

The step 1 specifically comprises the following steps:

establishing a geometric model of a crack-containing sample, selecting a current input point, applying a constant current, setting a zero potential point, and performing steady-state current conduction calculation; after the calculation is finished, selecting a voltage output point and extracting a voltage value of the voltage output point; secondly, changing the size of the crack according to the change of the shape of the crack in the crack expansion process, keeping other conditions unchanged, calculating the voltage value again, and calculating the voltage values at a plurality of moments in the crack expansion process by analogy; then, the voltage value at a certain smaller crack length is selected as the reference voltage V_rNormalizing the voltage with respect to a reference voltage, normalizing the crack length with respect to the sample size, and fitting to obtain a potential formula with the normalized crack length as an independent variable and the normalized voltage as a dependent variable.

In the step 1, the current input point and the voltage output point are selected to maximize the sensitivity of the voltage to the crack length, and the selection is assisted by continuously performing finite element simulation.

The step 2 specifically comprises the following steps:

connecting a wire on the sample according to the current input point and the voltage output point selected in the step 1, connecting a power supply voltmeter to form a closed loop circuit, setting the constant current of the power supply, then installing the sample, setting test parameters, and starting a test; the connecting mode of the lead on the test sample is welding or riveting.

The step 3 specifically comprises the following steps:

firstly selecting the crack lengths of a plurality of moments needing to be reshaped in the crack expanding process, then calculating the percentage of the crack expanding to the expected length and voltage increase according to the potential formula obtained in the step 1, and then stopping reshaping when the voltage increases the percentage according to real-time voltage data.

In the step 3, the replication step is as follows:

(1) cleaning the surface of a crack gap of the sample by using an alcohol cotton ball before the copying;

(2) applying a static load which is not more than 80% of the maximum test load to completely open the crack, aligning the nozzle head of the assembled replica system to the crack surface, slowly injecting replica glue until the replica glue completely fills the gap or covers the crack, and lightly pressing the bottom of the replica glue by using a small paper sheet so as to ensure that the later taken replica bottom is flat; after 5 minutes, after the replica glue is completely solidified, taking down the replica glue by using tweezers;

(3) taking down the replica, properly storing the replica, attaching a corresponding replica number label, and recording the number of the cycle at the moment;

(4) when the replica is observed by a microscope, obvious crack appearance is observed, no bubbles or inclusions exist, and the replica is invalid and is reconstructed.

The step 5 specifically comprises the following steps:

according to the color layering line of the fracture surface, taking any point at the front edge of the crack as a research object, and measuring the crack length of the point at a plurality of shutdown moments; then, discontinuous potential data which are recorded in the crack expansion process and caused by shutdown replica for several times are connected into a continuous voltage history curve to obtain voltage values when the crack is stopped for several times; and (4) correcting the potential formula obtained in the step (4) by using actually measured relation data of the crack length and the voltage, wherein the aim of correction is to make the curve of the potential formula most fit with the replica data point.

The step 6 specifically comprises the following steps:

converting the voltage value into the crack length according to the voltage history curve and by combining the potential formula obtained by correction in the step 5, thereby obtaining a crack length change curve of any point at the front edge of the crack, and obtaining the crack propagation rate at any time by deriving the curve; then, obtaining the relation between the stress intensity factor of any point at the front edge of the crack and the length of the crack through finite element calculation; and finally combining the two to obtain a crack propagation rate curve.

Has the advantages that: the invention solves the problem of low reliability of the traditional potential method due to external interference factors, greatly shortens the test period compared with the traditional replica method, and can record the whole course data of crack propagation. In addition, due to the high-temperature color development effect, the change of the shape and the length of the crack can be directly observed through the fracture after the test is finished.

The invention stops the replica in the crack propagation process, corrects the potential formula of theoretical calculation by using the replica data and fracture data, makes up the defect that the potential method does not accord with the theoretical potential formula due to interference of various factors, and obtains reliable test data. The fracture can show the color layering phenomenon at high temperature due to midway stopping and replicating, so that the change of the shape and the size of the crack can be obviously seen, the crack length of any point at the front edge of the crack can be measured from the layering line of the fracture, the crack length and the replicating result are verified mutually, and the crack propagation rate of any point at the front edge of the crack can be finally obtained.

Drawings

FIG. 1 is a flow chart of the operation of the present invention;

FIG. 2 is a graph of curvilinear crack size definitions;

FIG. 3 is a sample drawing sheet example;

FIG. 4 is a schematic diagram of a potentiometric wire connection;

FIG. 5 is the result of simulation analysis of the steady-state current of the body with cracks;

FIG. 6 is a result of a preliminarily fitted potential curve;

FIG. 7 is a macroscopic image of a crack propagation fracture;

FIG. 8 is a crack length measured by replica method;

FIG. 9 is the crack length measured by fracture coloration effect;

FIG. 10 is a graph of the course of a potential change;

FIG. 11 is a potential curve corrected by actual crack length;

FIG. 12 is a surface point crack propagation process curve;

FIG. 13 is a surface point crack growth rate curve.

Detailed Description

The present invention will be described in further detail below by way of specific embodiments in conjunction with the accompanying drawings.

As shown in FIG. 1, the method for monitoring the crack propagation curve by combining the potentiometric method and the replica method comprises the following steps:

step 1, carrying out steady-state current conduction finite element analysis on samples containing different crack lengths, calculating the relation between voltage and the crack length, and fitting to obtain a potential formula. The method comprises the following specific steps:

establishing a geometric model of a crack-containing sample, selecting a current input point, applying a constant current, setting a zero potential point, and performing steady-state current conduction calculation; after the calculation is finished, selecting a voltage output point and extracting a voltage value of the voltage output point; secondly, changing the size of the crack by assuming the change of the shape of the crack in the expansion process according to experience, keeping other conditions unchanged, calculating the voltage value again, and calculating the voltage values at a plurality of moments in the crack expansion process by analogy; then, the voltage value at a certain smaller crack length is selected as the reference voltage V_rNormalizing the voltage with respect to a reference voltage, normalizing the crack length with respect to the sample size, and fitting to obtain a potential formula with the normalized crack length as an independent variable and the normalized voltage as a dependent variable. The current input point and the voltage output point are selected to enable the sensitivity of the voltage to the crack length to be maximum, and finite element simulation is continuously carried out to help selection.

In one embodiment, curvilinear crack sizeThe geometrical dimensions of the test piece are shown in fig. 3, and a semicircular surface defect is machined at the center of the notch face on one side of the test piece as shown in fig. 2. The wire connection diagram is shown in fig. 4, with current input terminals at both ends and voltage output terminals in the middle, and L1 and L2 represent distances between wire connection points, which are symmetrical about a middle section. Establishing a finite element geometric model with cracks of the middle gauge length section, setting the shape of the cracks to be semicircular and keeping the shape of the cracks unchanged in the expansion process, taking L1 as 24mm and L2 as 12mm, applying 5A current to one end of an input end, applying zero potential to one end of the input end, and performing steady-state current conduction analysis to obtain potential distribution as shown in figure 5. And finally, extracting the potential of the output end to obtain an output voltage value. Similarly, the output voltage under different crack lengths can be obtained by continuously changing the size of the crack. Selecting the voltage with crack radius of 0.35mm as reference voltage V_rThe fitted potential curve is shown in fig. 6, and the potential formula is as follows:

wherein, C₁＝1.3716，C₂＝-2.7116，C₃＝1.3400，C₄＝-1.9161，C₅＝0.9163。

And 2, connecting a lead on the sample, connecting a power supply voltmeter to form a closed-loop circuit, setting the magnitude of the power supply current, installing the sample, setting relevant parameters, and starting the test. The method comprises the following specific steps:

connecting wires on the sample according to the current input point and the voltage output point selected in the step 1, connecting a power supply voltmeter to form a closed-loop circuit, and setting the constant current of the power supply, wherein the constant current is generally within the range of 5-50A, and the typical output voltage is within the range of 0.1-50 mV. In order to obtain sufficient potential resolution, special attention should be paid to noise and null shift. Then, the test sample is installed, relevant test parameters are set, and the test is started. The connection mode of the lead on the test sample adopts welding or riveting, but the welding connection mode is preferred in order to reduce the risk of generating cracks at the riveting position in the test.

In one embodiment, the wires are connected by welding, the current is set to be 5A, the test load type is constant amplitude fatigue load, the crack type is central surface crack, the temperature is 600 ℃, the maximum load is 22.2kN, the load ratio is 0.05, the frequency is 10Hz, and the test is started after relevant parameters are set.

And 3, stopping and duplicating for a plurality of times in midway according to a potential formula obtained preliminarily and real-time voltage change in the crack propagation process. The method comprises the following specific steps:

firstly, selecting the crack lengths of a plurality of moments needing to be reshaped in the crack expanding process, then calculating the percentage of the crack expanding to the expected length and voltage increasing according to a potential formula obtained preliminarily, and then stopping reshaping when the voltage increases the percentage according to real-time voltage data. Wherein, the specific operation of the replica is as follows:

(1) before the compound, the surface of the gap is cleaned by an alcohol cotton ball.

(2) Applying a static load which is not more than 80% of the maximum test load to completely open the crack, aligning the nozzle head of the assembled replica system to the crack surface, slowly injecting replica glue until the replica glue completely fills the gap or covers the crack, and lightly pressing the bottom of the replica glue by using a small paper sheet so as to ensure that the taken-down replica bottom is flat and convenient to observe. After 5 minutes the replica gel was completely set, the replica gel was carefully removed with tweezers. To help identify the orientation of the replica, one of the corners may be clipped.

(3) And taking down the copy, properly storing the copy, attaching a corresponding copy number label to the copy, and recording the number of the cycles at the moment.

(4) When the replica is observed by a microscope, obvious crack appearance is observed, no bubbles or inclusions exist, and the replica is invalid and is reconstructed.

In one embodiment, the initial crack size 2a is approximately equal to 0.7mm, if the crack length is 1.7mm when the machine is stopped for the first time, the percentage of voltage increase needs to be calculated according to a potential formula and is 0.344%, according to real-time voltage data, the test is stopped when the voltage increases the percentage, the crack length is measured after the duplication, the crack length of the next duplication is selected, the machine is stopped according to the potential, and the like. The crack is required to be reshaped for a plurality of times in the crack propagation process, and the crack lengths are uniformly distributed as much as possible.

And 4, because the preliminary potential formula is obtained by assuming the shape of the crack, and the shape change of the actual crack propagation can be obtained according to the fracture morphology after the test is finished. And judging the change of the crack shape and the crack size in the crack propagation process according to the fracture morphology, and recalculating to obtain a potential formula.

In one embodiment, the specimen macro-fractures are shown in FIG. 7. Since the crack shape is roughly semicircular and is consistent with the previously assumed crack shape, the potential formula does not need to be recalculated.

And 5, correcting the potential formula calculated in the step 4 according to the replica data and the crack length data measured from the fracture.

The method comprises the following specific steps:

according to the color layering line of the fracture surface, taking any point of the crack front as a research object, and measuring the crack length of the point at a plurality of stopping moments. And then, discontinuous potential data which are recorded in the crack propagation process and caused by the halt copy for several times are connected into a continuous voltage history curve to obtain the voltage value when the machine is halted for three times. Due to the influence of factors such as wire connection and the like, the actual potential formula of the point always has deviation from the potential formula obtained in the step 4, so that the potential formula obtained in the step 4 is corrected by utilizing the relation data of the actually measured crack length and voltage, the specific correction method is determined according to the situation, and the aim is to make the potential formula curve and the replica data point most fit.

In one embodiment, the crack surface point is selected as the object of study, and the crack length at the point is measured by a replica or fracture, and the two can mutually verify the measurement accuracy, as shown in fig. 8 and 9. The potential data during crack propagation was trained to a continuous voltage history curve, as shown in fig. 10. Measuring the crack length of the surface point three times, and correcting the potential formula obtained in step 4, in this example, translating the theoretical potential curve to make the corrected curve most fit with the actually measured data, as shown in fig. 11, the corrected potential formula is as follows:

wherein, C_iThe value of (i ═ 1 to 5) is as above, and X is₀＝0.00124，Y₀＝-0.02。

And 6, processing according to the recorded potential data and the corrected potential formula to obtain a crack length change curve of any point of the crack front, and further calculating the crack propagation rate of any point of the crack front. The method comprises the following specific steps:

and converting the voltage value into the crack length according to the voltage history curve and a corrected potential formula, thereby obtaining a crack length change curve of any point of the front edge of the crack, and obtaining the crack propagation rate at any moment by differentiating the curve. And then, obtaining the relation between the stress intensity factor of any point of the crack front and the crack length through finite element calculation. And finally combining the two to obtain a crack propagation rate curve.

In one example, the resulting crack length variation curve is shown in FIG. 12 and the resulting crack propagation rate curve is shown in FIG. 13. Of course, if other points of the crack front are selected as the study object, the crack propagation rate curve can be obtained by the above steps.

The above description is only of the preferred embodiments of the present invention, and it should be noted that: it will be apparent to those skilled in the art that various modifications and adaptations can be made without departing from the principles of the invention and these are intended to be within the scope of the invention.