CN104913989A - Method for measuring fracture toughness crack stable-state extended length - Google Patents

Abstract

The invention discloses a method for measuring the steady-state extension length of fracture toughness cracks, which is mainly suitable for measuring the steady-state extension length of cracks in the process of ductile fracture toughness. The present invention mainly includes the following steps: (1) based on the image processing software, draw the fatigue crack steady-state growth trajectory; (2) calculate the graph area according to the number of pixels included in the drawn trajectory; (3) measure the time when the crack begins to expand Width, according to the area method to calculate the length of crack steady-state expansion (in pixels); (4) According to the scale on the image, calculate the pixel length corresponding to the actual unit length, and calculate the crack expansion length (in millimeters). The method for measuring the steady-state extension length of fracture toughness cracks of the present invention is more accurate than the method for calculating the arithmetic mean value of nine-segment crack lengths stipulated in the national standard (GB/T 21143-2007).

Description

A method for measuring the steady-state extension length of fracture toughness cracks

technical field

The invention belongs to the field of fracture toughness testing, and mainly relates to a method for measuring the steady-state extension length of a ductile fracture toughness crack, in particular to a method for testing the extension length of a crack with an irregular extension path.

Background technique

For engineering structures with welded joints, such as boilers, pressure vessels, nuclear reactors, metal pipes and other heterogeneous materials, it is inevitable that the macroscopic mechanical properties will be heterogeneous due to joint matching and local heating, which will cause special mechanical properties of the structure. There are significant differences in fracture characteristics compared with homogeneous materials, so understanding the safety margin of structures with defects is an important issue directly related to the safety and economy of the entire system.

The elastoplastic fracture toughness (ductile fracture toughness J _0.2BL ) is an important parameter in the fracture mechanics safety assessment of materials with defects. Through J _0.2BL and using the theory of elastic-plastic fracture mechanics, the relationship between stress, defects and material properties in pressure vessels can be established.

According to the national standard GB/T 21143-2007, the fracture toughness test method mainly adopts the multi-specimen method. By loading the three-point bending specimen with cracks on the static testing machine, the displacement of the load beam is controlled to ensure different fracture toughness. The crack propagation displacement is different in the elastic-plastic stage. Finally, according to different load beam displacement curves and the actual value Δa of crack steady-state growth under different loading displacements, the J value of different crack steady-state growth lengths is obtained, the J-Δa resistance curve is drawn, and the J _0.2BL value is obtained. Therefore, the determination accuracy of Δa directly determines the accuracy of J _0.2BL value. According to the national standard GB/T 21143-2007, the measurement method of Δa is to divide the section of the sample into 9 sections on average, measure the value of each section and then average them mathematically. This method has a large measurement error for the case of irregular crack growth.

Contents of the invention

The invention provides a method for measuring the steady-state extension length of fracture toughness cracks to accurately measure the steady-state extension length of cracks in fracture toughness. The technical principle of the present invention is to obtain the irregular shape area of the irregular crack propagation path, convert the irregular shape area into a rectangular area, and then obtain the steady-state average extension length of the crack according to the initial extension width.

Technical scheme of the present invention is as follows:

A method for measuring the steady-state extension length of fracture toughness cracks, which is based on Photoshop image processing software, depicts the crack steady-state extension track of the sample cross-section photo, then calculates the crack steady-state extension area, and then calculates the crack steady-state extension area according to the crack steady-state extension area Combined with the initial extension width of the crack, the steady-state extension length of the crack is obtained.

Preferably, the method for describing the crack steady-state propagation trajectory of the sample cross-section photo and finding the crack steady-state propagation area is: using the Photoshop magnetic lasso tool to describe the crack steady-state propagation trajectory, and according to the Photoshop histogram tool The pixel value of the steady-state extension area of the crack is obtained, and the steady-state extension area of the crack is calculated according to the pixel value.

Preferably, the method of calculating the crack steady-state extension length according to the crack steady-state extension area combined with the crack initial extension width is: firstly, according to the crack steady-state extension area and the crack initial extension width, the area Calculate the pixel value of the steady-state extension length of the crack by using the method, and then convert the actual crack extension length according to the pixel length corresponding to the unit length.

Preferably, the pixel length corresponding to the unit length is obtained by calculating the pixel length corresponding to the unit length according to the actual scale and the corresponding pixel value of the photo of the steady-state crack growth trajectory of the sample.

Preferably, the area method is based on obtaining the steady-state growth area of the irregular crack and dividing it by the width of the initial crack growth to obtain the steady-state growth length of the crack.

Preferably, the preparation method of the sample is: after the crack steady-state growth test, conduct the high-frequency fatigue test again, after the crack fatigue grows, break the sample, and take a picture of the fracture under a macroscopic microscope.

Preferably, the steady-state crack growth test is as follows: the sample is first subjected to fatigue crack prefabrication on a high-frequency fatigue testing machine, and then the crack steady-state growth test is performed on a static testing machine.

Preferably, the method for measuring the steady-state extension length of fracture toughness cracks is applicable to three-point bending specimens or compact tensile specimens.

Generally, the measurement method of the present invention mainly comprises the following steps: (1) based on the image processing software, the fatigue crack steady-state growth track is drawn; (2) according to the drawn track contains the number of pixels to obtain the graphic area; (3) measure Find the width when the crack begins to expand, and calculate the length of the crack's steady-state expansion (in pixels) according to the area of the above graph; (4) calculate the pixel length corresponding to the actual unit length according to the scale on the image, and calculate the crack expansion length (in millimeters).

Compared with the prior art, the beneficial effects of the present invention are as follows:

First, the method of the present invention calculates the average length of crack steady state expansion by measuring the crack steady state expansion area. The method of the present invention has less measurement error and high precision, and provides an effective and feasible new method for accurately calculating the J _0.2BL value. Measurement methods;

Second, the method for measuring the fracture toughness crack steady-state extension length of the present invention is more accurate than the national standard (GB/T 21143-2007) that adopts nine sections of crack lengths to obtain the arithmetic mean value.

Of course, any product implementing the present invention does not necessarily need to achieve all the above-mentioned advantages at the same time.

Description of drawings

Fig. 1 is a schematic diagram of a crack steady-state propagation trajectory drawn using the magnetic lasso tool in Photoshop software using a standard three-point bending sample in an embodiment of the present invention.

Detailed ways

The invention discloses a method for measuring the steady-state extension length of a fracture toughness crack, which is mainly suitable for more accurately measuring the steady-state extension of a crack when a three-point bending sample or a compact tensile sample is used to measure the fracture toughness of a material length. The main principle of the invention is to calculate the steady-state extension length of the crack according to the area of the steady-state extension of the irregular crack and the width of the initial extension of the crack.

The main steps of the test method of the present invention are as follows: After the fracture toughness test sample is completed through fatigue loading-static loading-fatigue loading test, the sample is interrupted, and then the fracture morphology is photographed with a macroscopic metallographic microscope and added. Scale; Based on the Photoshop image processing software, the steady-state expansion area of the crack under static loading is obtained, and finally the average length of the crack steady-state expansion is obtained.

A specific sample preparation process in the test method of the present invention can be:

First, the sample is prefabricated for fatigue cracks on a high-frequency fatigue testing machine, then the crack steady-state growth test is performed on a static testing machine, and finally the sample is subjected to secondary fatigue crack growth on a high-frequency fatigue testing machine, and then the sample is punched Fracture, take a picture of the fracture under a macro microscope, so that the crack can be clearly observed in the steady state, and the scale is added to the picture.

The steps of a specific embodiment of the method for measuring the fracture toughness crack steady-state extension length of the present invention mainly include the following steps, for example:

(1) The three-point bending specimen (SEB) or the compact tensile specimen (CT) for measuring the fracture toughness is prefabricated on the fatigue testing machine, and the prefabricated length of the crack is prefabricated according to the national standard GB/T 21143-2007;

(2) The sample with fatigue prefabricated cracks is subjected to a crack steady state growth test on a static testing machine;

(3) After the crack steady-state growth test, the sample is again subjected to a secondary fatigue crack growth test on a high-frequency fatigue testing machine under a small load in order to obtain a clear interface of different crack growth paths;

(4) Break the tested sample, use a macroscopic microscope to take pictures of the crack propagation fracture, and add a scale on the pictures obtained by taking pictures;

(5) Use Photoshop software to open the macro-fracture photos taken, and use the magnetic lasso tool in Photoshop to select the crack steady state expansion area;

(6) Use the histogram tool in Photoshop to obtain the total number of pixels in the selected area, convert the area of the irregular shape into a rectangular area, and obtain the number of pixels corresponding to the actual unit length according to the scale marked on the photo;

(7) According to the total pixel area and the number of crack starting pixels, divide the total number of pixels by the number of crack starting pixels to calculate the number of pixels for crack expansion, and then according to the number of pixels corresponding to the actual unit length obtained in step 6 , to find the actual length of the crack growth.

The present invention will be further described below in conjunction with specific embodiments.

Example 1

This embodiment provides the following method for measuring the steady-state extension length of fracture toughness cracks.

In this implementation case, the test material is 9Cr steel, and the multi-sample method is used to measure its fracture toughness based on three-point bending specimens.

The first step is to perform fatigue crack prefabrication on the three-point bending sample on a high-frequency fatigue testing machine, and stop prefabrication of cracks when the prefabricated length of the crack is controlled to be about 2.0mm;

The second step is to load the prefabricated fatigue cracked sample on the static tensile test machine in a steady state, and stop loading when the displacement reaches the required measurement value according to the load-displacement curve, and end the test;

The third step is to perform secondary fatigue crack prefabrication on the high-frequency fatigue testing machine after the static loading of the sample. At this time, it is necessary to control the maximum load of the prefabricated crack to be less than the maximum load of the elastic section of the load-displacement curve on the static testing machine; during this process, Fatigue vibration times until the material can no longer vibrate, so as to obtain a larger fatigue crack propagation distance;

In the fourth step, the sample after the secondary fatigue is directly broken, and the fracture is photographed under a macroscopic microscope, and the scale is added; at this time, the following four areas can be clearly observed in the fracture: the primary prefabricated fatigue crack area, Statically loaded crack steady-state growth zone, secondary fatigue crack growth zone and final transient fracture zone;

The fifth step, based on the Photoshop image processing software, select the macro-fracture photos taken, use the magnetic lasso tool to accurately describe the outline of the static-loaded crack steady-state growth area, and use the histogram tool to calculate the static-loaded crack steady-state growth The number of pixels in the area; then according to the number of pixel widths of the initial crack, the average pixel length (unit: pixel) of stable crack expansion is obtained;

The sixth step is to find the unit length corresponding to the actual number of pixels according to the ruler in the metallographic photo, combined with the resolution of the image, and compare it with the unit length corresponding to the number of pixels calculated according to the resolution, so as to obtain The actual length of the crack (in millimeters) is corrected proportionally to the length value obtained in the fifth step to obtain the average length of the actual crack propagation (in millimeters).

Fig. 1 is a schematic diagram of a crack steady-state propagation trajectory drawn using the magnetic lasso tool in Photoshop software using a standard three-point bending specimen in this embodiment. According to the GB/T 21143-2007 standard, it is necessary to calculate the expansion distance of the cracks in Figure 1 along the lines 1 to 9 after static loading, and then use the formula:

$\mathrm{<span\; class="notranslate">\; \&Delta;a</span>}<span\; class="notranslate">\; =</span>\frac{\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; 8</span>}}<span\; class="notranslate">\; [</span>\frac{<span\; class="notranslate">\; (</span>{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; +</span>{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; 9</span>}}<span\; class="notranslate">\; )</span>}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; +</span>\underset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 2</span>}}{\overset{\mathrm{<span\; class="notranslate">\; i</span>}<span\; class="notranslate">\; =</span>\mathrm{<span\; class="notranslate">\; 8</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}{\mathrm{<span\; class="notranslate">\; a</span>}}_{\mathrm{<span\; class="notranslate">\; i</span>}}<span\; class="notranslate">\; ]</span>$

Calculate the average extension length of the crack, and obtain the average extension length according to the national standard method, Δa=0.30mm; and the value of the average extension length of the crack obtained by adopting the method of the present invention is: Δa=0.35mm. The error between the two is 16.7%. It can be seen that the accuracy of this method is higher.

The above-mentioned embodiment is only an example for the method of measuring the steady-state growth length of a crack by using a three-point bending specimen. The essence of the present invention is based on the method of finding the average value of the crack steady state extension length based on the area. The above-mentioned embodiments should not be interpreted as limiting the present invention. Any other modifications that do not violate the spirit and principles of the present invention, Modifications, substitutions, combinations, and simplifications should all be equivalent replacement methods, and are all included within the protection scope of the present invention.

The preferred embodiments of the invention disclosed above are only to help illustrate the invention. The preferred embodiments are not exhaustive in all detail, nor are the inventions limited to specific embodiments described. Obviously, many modifications and variations can be made based on the contents of this specification. This description selects and specifically describes these embodiments in order to better explain the principle and practical application of the present invention, so that those skilled in the art can well understand and utilize the present invention. The invention is to be limited only by the claims, along with their full scope and equivalents.

Claims (10)

1. the method for a measurement of fracture toughness Crack Stable Growth length, it is characterized in that, based on Photoshop image processing software, depict the Crack Stable Growth track of sample section photo, then obtain Crack Stable Growth area, then obtain described Crack Stable Growth length according to Crack Stable Growth area in conjunction with crack initiation extension width.

2. the method for measurement of fracture toughness Crack Stable Growth length according to claim 1, it is characterized in that, the Crack Stable Growth track of described description sample section photo with the method obtaining Crack Stable Growth area is: utilize Photoshop Magnetic Lasso Tool, Crack Stable Growth track is depicted, and the pixel value of Crack Stable Growth area is drawn according to Photoshop histogram tools, obtain Crack Stable Growth area according to described calculated for pixel values.

3. the method for measurement of fracture toughness Crack Stable Growth length according to claim 1 and 2, it is characterized in that, describedly according to Crack Stable Growth area in conjunction with the method that crack initiation extension width obtains described Crack Stable Growth length be: first adopt area-method to calculate the pixel value of described Crack Stable Growth length according to described Crack Stable Growth area and described crack initiation extension width, then corresponding according to unit length length in pixels, converses actual crack extension length.

4. the method for measurement of fracture toughness Crack Stable Growth length according to claim 3, it is characterized in that, length in pixels corresponding to described unit length is obtained by following method: according to the actual scale of Crack Stable Growth track photo and the pixel value of correspondence thereof of sample, calculate the length in pixels that described unit length is corresponding.

5. the method for measurement of fracture toughness Crack Stable Growth length according to claim 3, it is characterized in that, described area-method is the area according to obtaining irregular Crack Stable Growth, this irregularly shaped area is scaled rectangular area, and this area obtains Crack Stable Growth length divided by the width that crack initiation is expanded.

6. the method for measurement of fracture toughness Crack Stable Growth length according to claim 1, it is characterized in that, the preparation method of described sample is: after Crack Stable Growth test, carry out secondary high frequency fatigue test, and the maximum load controlling precrack is less than the maximum load of load displacement curve stretch section on static test-machine; After Fatigue Propagation of Cracks, sample is broken, and under macroscopical microscope, take fracture overall picture photo.

7. the method for measurement of fracture toughness Crack Stable Growth length according to claim 6, it is characterized in that, described Crack Stable Growth test is: first sample is carried out fatigue crack prefabrication in HF fatigue testing machine, then on static test-machine, carry out Crack Stable Growth test.

8. the method for the measurement of fracture toughness Crack Stable Growth length according to claim 6 or 7, is characterized in that, in described secondary high frequency fatigue test process, Fatigue Vibration number of times is until material can not starting of oscillation.

9. the method for measurement of fracture toughness Crack Stable Growth length according to claim 1, is characterized in that, the method for described measurement of fracture toughness Crack Stable Growth length is applicable to For Three Points Bending Specimen or compact tensile specimen.

10. a method for measurement of fracture toughness Crack Stable Growth length, is characterized in that, according to the area of the irregular Crack Stable Growth obtained, the width divided by crack initiation expansion obtains Crack Stable Growth length.