CN104406867A - Fatigue crack propagation test method based on replication and small time scale life forecast - Google Patents

Abstract

The invention discloses a fatigue crack growth test method based on a replica and small time scale life prediction. Step 1: According to the small time scale theory, five load cycles are selected on the sample from the position of the prefabricated notch to the critical growth position of the crack, and ten test points are symmetrically selected during the loading and unloading process in each load cycle ; Step 2, using the first replica method to obtain the microscopic appearance of each test point, and placing the sample after the replica under the scanning electron microscope for image acquisition of morphology observation; When the bottom is clear, the second replica of step three is required; in step three, the sample after the second replica is placed under a transmission electron microscope for morphology observation and image collection, and the fatigue crack growth increment and CTOD are measured according to the photos . This method is suitable for the fatigue crack growth test of metal components in plane strain state.

Description

Fatigue Crack Growth Test Method Based on Replica and Small Time Scale Life Prediction

technical field

The invention relates to a fatigue crack growth test method, in particular to a fatigue crack growth test method oriented to replica and small time scale life prediction, which is suitable for metal components in a plane strain state.

Background technique

Research on fatigue damage caused by fatigue cracks has always been a hot topic, and fatigue life prediction is one of the great challenges. The traditional fatigue damage methods are all analysis methods based on cyclic loads. The earliest method is based on the S-N curve. Due to the relatively large time scale of this method, it is generally used to describe the ultimate fatigue life of materials or structures. At present, the mainstream research method of fatigue life is based on the model of the relationship between the average fatigue crack growth rate per cycle and the stress intensity amplitude (Paris formula), which greatly reduces the space-time scale of the fatigue model, but its minimum time scale is still limited. In one load cycle, it cannot be applied to complex variable amplitude loads in engineering structures. In response to this problem, some scholars have proposed a small-time-scale fatigue crack growth analysis method, that is, using continuous time as the minimum measurement scale to study the instantaneous behavior of fatigue crack opening and closing and growth at any time within a load cycle.

The above-mentioned analysis of fatigue crack growth on a small time scale is based on the in-situ fatigue test, and the continuous change behavior of the crack tip is photographed through a scanning electron microscope, so as to measure the fatigue crack growth increment and CTOD (Crack Tip Opening Displacement, crack tip opening displacement). However, due to the limited maximum load that can be applied by the in-situ fatigue testing machine, and the large magnification of the scanning electron microscope results in a small range of observations, it is only suitable for thinner and smaller samples. However, this sample is in the The state of plane stress, but most of the components in the project are in the state of plane strain, which makes the research object not completely consistent with the actual structure of the project. Crack Growth Data.

Contents of the invention

In order to solve the inaccurate test accuracy and the inaccurate estimation of the remaining life of the tested product in the existing fatigue crack growth test method, the present invention uses digital information to characterize the crack growth behavior through the microscopic observation of the crack morphology; In small-time-scale fatigue life prediction, the size of the sample is limited due to the limitation of the experimental device. A fatigue crack growth test method based on replica and small-time-scale life prediction is proposed for metal components in a plane strain state. . This method uses small time scale theory and multiple image acquisition for analysis, but obtains the topography of the sample surface through the replica method, so as to measure the fatigue crack growth increment and CTOD, so as to predict the fatigue life and improve the quality of metal structural parts. safe use.

Technical scheme of the present invention is:

Step 1: According to the small time scale theory, five load cycles are selected on the sample from the position of the prefabricated notch to the critical extension position of the crack, and ten test points are symmetrically selected during the loading and unloading process in each load cycle ;

Step 2, using the first replica method to obtain the microscopic morphology of each test point, and placing the replicated sample under the scanning electron microscope; when the crack growth increment can be clearly observed under the scanning electron microscope , the image is collected; when the increment of crack growth cannot be seen clearly under the scanning electron microscope, the second replication of step 3 is required;

Step 3: Place the sample that needs to be replicated twice under a transmission electron microscope for morphology observation image acquisition; measure the fatigue crack growth increment and CTOD according to the morphology observation image.

The invention proposes a fatigue crack growth test method based on replica and small time scale life prediction, which is suitable for fatigue crack growth test of metal components in plane strain state.

Advantage and positive effect of the inventive method are:

(A) The analysis of fatigue crack growth on a small time scale takes continuous time as the minimum time scale, and it has received extensive attention as a new theoretical system. However, due to the limitation of the test device on the size of the sample, this analysis method can only be applied to the components in the plane stress state, while the components in engineering practice are mostly in the plane strain state. This limitation hinders the small time scale Theory cannot be fully applied in engineering. In the method of the present invention, the fatigue crack morphology of the sample in the plane strain state is transferred to the cellulose acetate film by the replica method. In this way, regardless of the shape and size of the sample, the morphology of the fatigue crack can be observed under the scanning electron microscope. The method of the invention fills in the difficult problem that fatigue crack analysis cannot be performed on components in a plane strain state under small time scales, and further improves the system of small time scale fatigue crack growth analysis methods.

(B) Although the life prediction model under the plane strain state can also be derived from the results under the plane stress state through a certain theory, its accuracy and effectiveness have yet to be verified. The method of the present invention is a prediction method for crack growth tests based entirely on samples in a plane strain state. The actual results can be obtained through these test data, compared and verified with theoretical derivations, and the theoretical prediction model is more accurate through improvement.

(C), the method of the present invention is universal to all metal materials such as aluminum alloys, titanium alloys, superalloys, and structural steels, and has a very wide range of applications.

(D), The structure in the actual engineering structure is mostly subjected to complex variable amplitude loads, but it is very difficult to define the load cycle sequence. The small time scale fatigue crack growth test method is to study the opening and closing behavior and fatigue crack growth behavior of fatigue cracks at different times in any load cycle, and establish a fatigue crack growth model based on small time scales, so as to realize the structure under complex load spectrum. fatigue life prediction.

Description of drawings

Fig. 1 is the standard drawing of the selected sample of the present invention.

Figure 2A is a constant amplitude loading spectrum of the present invention.

Figure 2B is the complex variable amplitude load spectrum of the present invention.

Fig. 2C is a schematic diagram of the selected measurement points in each load cycle of the present invention.

Fig. 3 is a replica topography photo of the present invention.

Fig. 4A is a schematic diagram of the sample surface replica position of the present invention.

Fig. 4B is a photo of the measurement of crack growth increment according to the present invention.

Fig. 5 is the relationship curve of crack growth increment-CTOD in the present invention.

Fig. 6 is a flow chart of measuring crack growth increment in the present invention.

Detailed ways

The present invention will be further described in detail below in conjunction with the accompanying drawings and embodiments.

Referring to Fig. 6, the present invention adopts replica and small time scale means to carry out fatigue crack growth test on aviation 7050 series aluminum alloy materials. The purpose of the test is to solve the inaccurate test accuracy, Inaccurate estimation of the remaining life of the tested product leads to inaccurate life prediction. The present invention uses digital information to characterize the crack growth behavior through microscopic observation of the crack morphology, thereby measuring the fatigue crack growth increment and CTOD to measure the fatigue life. The forecast improves the safe use of metal structural parts.

Preparation of samples

In the present invention, the sample adopts the standard SE (B) sample in GBT6398-2000 "Metallic Material Fatigue Crack Growth Rate Test Method".

For the convenience of description, the sample is shown in Figure 1, W represents the width of the sample, and the length is 4.2 times the width. In order to avoid errors caused by the single sample test, at least three samples are processed to perform the fatigue crack growth test of the method proposed in the present invention.

Prefabricate a notch on the standard SE(B) sample with a notch depth of 0.1W.

The fatigue crack growth test method based on replica and small time scale life prediction proposed by the present invention includes the following steps:

Step 1: Test point selection

On the standard SE(B) specimen, select five load cycles from the prefabricated notch length to the growth range of the crack critical size, and select ten test points symmetrically during the loading and/or unloading process in each load cycle; Five load cycles are shown in Figure 2A, N ₁ , N ₂ , N ₃ , N ₄ , N ₅ ; any appropriate load cycle is selected from among them for the complex variable amplitude load spectrum test, as shown in Figure 2B.

In the present invention, in each load cycle, if it is a constant-amplitude loading type, 10 test points i are selected during the loading process, that is, i=1, 2, 3, . . . , 10.

In the present invention, in each load cycle, if the crack tip closure phenomenon under constant amplitude load is to be studied or the complex variable amplitude load spectrum is to be studied, ten test points i need to be symmetrically selected in the loading process and the unloading process, such as Figure 2C.

During the test of the present invention, if it is found that there is no crack opening at the first few test points during loading, and only when the stress level reaches a certain value, the crack begins to open, then record the corresponding stress intensity of this point Factor K, then the test points before this point may not be replicated in the tests of several load cycles after that. The reason for this phenomenon is caused by the crack closure phenomenon. Since the crack growth increment and CTOD values are gradually increasing, in order to make the measurement results more uniform, the selected load cycles and test points should gradually become denser as possible.

Step 2: Obtain the microscopic morphology of each test point by the first replica

Step 21: When the gap is located at the ith (i= _{1, 2, 3, ..., 10) test point in the Nth (N=N 1} , N ₂ , N ₃ , N ₄ , N ₅ ) load cycle, Stop the cyclic loading and apply a static load to the specimen;

Step 22: Clean the surface of the notch with a cotton ball dipped in acetone, perform replica measurements in five load cycles of N ₁ , N ₂ , N ₃ , N ₄ , and N ₅ respectively, and drop 1-2 drops on the cellulose acetate film Acetone, and then lightly attach it to the surface of the cracked side of the sample respectively. After the film dries for 5 minutes, clamp the end of the film with tweezers, and slowly peel it off from the surface of the notch from top to bottom;

In the present invention, to help distinguish the orientation of the film, one of the corners of the film can be cut off.

Step 23: In order to ensure an effective replica result, perform replicas three times on the two side surfaces of the crack respectively, repeat step 22 for the replica method, and obtain a total of 6 pieces of cellulose acetate films for each test point;

Step 24: Paste the cellulose acetate film used for the replica flatly on the glass sheet with double-sided transparent tape, and paste the corresponding serial number of the replica. The serial number of the replica should be related to the number of load cycles and the stress in the load cycle corresponding to the level.

In the present invention, when performing step 24, care should be taken to ensure straightness, clear edges and corners, and no distortion to avoid damage.

Step 25: Put all the samples under the scanning electron microscope for observation (you can observe from the last replica sample to the first replica sample, and you can also observe in sequence), if the sample has a clear image under the scanning electron microscope, then The morphology image is collected directly, and the fatigue crack growth increment and CTOD are measured according to the morphology observation image; if the image of the sample observed under the scanning electron microscope is blurred, a second replica is performed.

Observe the microscopic morphology of the crack after the first replica sample, as shown in Figure 3. Due to the limited magnification of the scanning electron microscope, the magnification is usually between 5000 and 10000 times, and the minimum length that can be observed is generally 0.1 to 0.2 μm.

In the present invention, when the crack expansion is greater than 0.2 μm, the sample after the above-mentioned replica can be directly placed under the scanning electron microscope for observation; and when the crack expansion is less than 0.1 μm, it is difficult for the scanning electron microscope to observe the increase of the crack expansion. At this time, a second replica is required, and then placed under a transmission electron microscope for observation.

In the present invention, according to the crack length-cycle data obtained in the fatigue test of each sample after the first replica, estimate the crack growth of the sample at the i-th test point of the N cycle Increment and CTOD to determine whether the sample needs to be replicated twice. For samples whose crack growth increment and CTOD cannot be greater than 0.2 μm at the same time, a second replica is required, and then placed under a transmission electron microscope for observation.

Step 3: The second replica to obtain the microscopic morphology of each test point

The surface of the sample that cannot be observed under the scanning electron microscope after screening in step 25 is subjected to carbon spraying treatment, as shown in (a) in Figure 4A, attention should be made that the carbon film 1 should be evenly sprayed on the sample substrate 2, and the carbon film 1 should be controlled. The film thickness is within 20 μm (as shown in b in Figure 4A). According to the crack length corresponding to the cycle of the sample, roughly determine the position of the crack tip, cut the sample into small pieces of about 3 mm square, and place them in acetone reagent to dissolve the cellulose acetate film. When the carbon film floats for about 2 minutes, transfer the carbon film to another measuring cup filled with fresh acetone reagent, and rinse for another 5 minutes. Then use the sample trawl directly in the acetone measuring cup or place the carbon film on the surface of distilled water to expand and then fish it out. After drying, obtain the crack tip conversion sample 3 (c in Figure 4A), which can be observed under the transmission electron microscope and carried out. Photograph.

The fatigue crack growth increment and CTOD of each crack tip conversion sample were measured according to the photos taken under the transmission electron microscope, as shown in Fig. 4B. The relationship curve of the crack growth increment Δa-CTOD in each cycle is drawn, and the maximum stress intensity factor K _max value corresponding to the peak stress of the cycle is used to characterize this curve, as shown in Figure 5. In the figure, the crack growth increments in the four cycles are respectively: the maximum stress intensity factor value of the first cycle is marked as K max1 , the maximum stress intensity factor value of the second cycle is marked as K _max2 , and the maximum stress intensity factor value of the second cycle is marked as K _max2 . The maximum stress intensity factor value of the three cycles is denoted as K _max3 and the maximum stress intensity factor value of the fourth cycle is denoted as K _max4 .

The fatigue crack growth test method oriented to the replica and small time scale life prediction provided by the present invention aims at the sample in the plane strain state, and uses the replica technology to check the opening and closing of the fatigue crack of the material at any time in a load cycle and The value-added behavior is observed to provide a basis for the fatigue life prediction of materials. This method not only breaks through the shortcomings of the traditional fatigue life analysis method based on cyclic load analysis, but also solves the problem that the fatigue cracks cannot be observed and studied more essentially on a smaller time scale, and solves the problem that the current small time scale analysis method can only analyze The limitation of the sample in the plane stress state makes the actual damage analysis results closer to the engineering structure, improving the accuracy and reliability of the fatigue damage analysis.

Claims (4)

1., based on an investigating fatigue crack expansion method for replica and small time scales life prediction, the sample carrying out investigating fatigue crack expansion is standard SE (B) sample; It is characterized in that the step that investigating fatigue crack expansion comprises is:

Step one, theoretical according to small time scales on sample, choose five loading periods the Critical growth position from prefabricated gap position to crackle, in the compression and decompression process within each loading period, symmetry chooses ten test points respectively;

Step 2, adopts replication method for the first time to obtain the microscopic appearance of each test point, and under the sample after described replica is placed in scanning electron microscope; When Crack Extension increment clearly can carry out morphology observation under scanning electron microscope, then gather image, and according to morphology observation radiographic measurement crack Propagation increment and CTOD; When Crack Extension increment cannot be seen clearly under scanning electron microscope, the second time replica of step 3 need be carried out;

Step 3, carries out morphology observation image collection under being placed in transmission electron microscope by needing the sample after double replica; And according to morphology observation radiographic measurement crack Propagation increment and CTOD.

2. the investigating fatigue crack expansion method based on replica and small time scales life prediction according to claim 1, it is characterized in that step 2 first time replica treatment step be:

Step 21: when breach is positioned at any one loading period of N i-th test point, stops cyclic loading and applies static load to sample;

Step 22: with the cotton ball cleaning breach surface being moistened with acetone, carry out replica measurement five loading periods respectively, cellulose acetate film drips 1-2 and drips acetone, then it is attached to gently sample crackle side surface respectively.After the dry 5min of film, clamp the end of film with tweezers, peel off lentamente from breach surface from top to bottom;

Step 23: for ensureing to obtain effective replica result, carry out three replicas respectively at crackle two side surfaces respectively, replication method repeats step 22, and each test point obtains 6 acetate films altogether;

Step 24: be pasted onto the glass sheet posting two-sided scotch tape by smooth for the acetate film after replica, and stick corresponding replica sequence number, replica sequence number should be corresponding with the stress level in this loading period with number loading period;

Step 25: observe under all samples are placed in scanning electron microscope, if sample has image clearly under scanning electron microscope, then directly to gather its feature image, and according to morphology observation radiographic measurement crack Propagation increment and CTOD; If it is fuzzyyer that image ratio observed by sample under scanning electron microscope, then carry out the second time replica of step 3.

3. the investigating fatigue crack expansion method based on replica and small time scales life prediction according to claim 2, is characterized in that: first time the crackle microscopic appearance minimum length of replica sample be 0.1 ~ 0.2 μm.

4. the investigating fatigue crack expansion method based on replica and small time scales life prediction according to claim 1, is characterized in that: the metal component fatigue crack extend testing being applicable to be in plane strain state.