Method for enhancing fatigue microcrack display contrast of metal material

Abstract

The invention provides a method for enhancing fatigue microcrack display contrast of a metal material. The method comprises the following steps: step 1, preparing a sample; step 2, surface treatment; and step 3, fatigue test. The invention provides a method for effectively enhancing the fatigue microcrack display contrast of a metal material aiming at a fatigue crack growth rate test. In the existing fatigue micro-crack measurement method, most of the technical schemes adopt high-definition microscopes / cameras, cracks are found from the surface of a material through visual observation, and due to the fact that surface machining textures or scratches generally exist on the surface of the material, the technical defects that the cracks are difficult to judge, time and labor are wasted, and even misjudgment is generated are easily caused.

Description

Technical Field

[0001] The invention relates to the technical field of fatigue performance detection of metal materials, and in particular to a method for enhancing the display contrast of fatigue microcracks of metal materials. Background Art

[0002] Fatigue crack growth rate test is a test method that studies the fatigue fracture characteristics of materials by measuring the length of fatigue cracks generated by the specimen under the action of set alternating loads in real time. When conducting fatigue crack growth rate test, in order to ensure the accuracy of the calculation results, it is necessary to accurately measure the crack length during fatigue crack growth. In the indirect measurement method, the flexibility method and the potential method are based on the changes in other physical parameters caused by crack growth, and indirectly calculate the average crack growth size, crack intensity factor range ΔK and crack growth rate. However, the operation procedures of the two methods are relatively complicated and are more suitable for high temperature environment tests. At the same time, the measurement is sensitive to the crack length and too many variables are introduced during the calculation, which will cause a large deviation between the calculated value and the true value. In the direct measurement method, a high-definition microscope / camera is used to directly read the image scale by visual inspection to obtain the crack growth size. The operation procedure is simple and convenient, and has been widely used in room temperature environment testing. However, there are generally surface processing textures or scratches on the surface of metal materials. At this time, the crack morphology is similar to the texture and scratches, which often leads to technical defects such as difficulty in identifying cracks and time-consuming and labor-intensive, and sometimes even misjudgment. Therefore, a method is needed to enhance the contrast of fatigue microcrack display to avoid the influence of surface texture or scratches and meet the needs of precise measurement in the laboratory.

[0003] In terms of fatigue crack tracking and monitoring, some patents have adopted relevant technical means to achieve the above goals. Patent CN104181225 introduces a fatigue crack tracking and monitoring device for steel bridge decks and a monitoring method thereof. This method sprays contrast enhancer and black water magnetic suspension on the monitored steel bridge deck, and uses magnetic particle flaw detection to display the shape and position of fatigue cracks; Patent 108760546 introduces a fatigue crack growth rate measurement method based on infrared thermal imaging technology. This method sprays black matte paint on samples that meet the required geometric dimensions and surface roughness. The purpose is to increase the emissivity of the sample surface. It is suitable for infrared thermal imaging to record the temperature field evolution process of the fatigue crack tip area in real time. The patent CN110295340 introduces a crack monitoring sensor based on plasma spraying and its preparation and monitoring method. The method uses plasma spraying to integrate the insulating base layer, damage sensor element and packaging protection layer on the surface of the target monitoring structure, and determines the crack length of the detection structure by measuring the potential difference at both ends of the crack surface of the monitoring structure; the patent CN110926972 introduces a micro-fatigue surface crack detection method and system. The method uses binary silicone rubber to coat the contact parts, and determines the cracks in the contact parts by observing the coated thin film. In summary, the above patents all use relevant technical means and matching detection methods to improve the crack display contrast, thereby achieving accurate measurement of crack size. Summary of the invention

[0004] The purpose of the present invention is: In view of this, the present invention provides a method for enhancing the display contrast of fatigue microcracks in metal materials to solve the above technical problems.

[0005] The technical solution of the present invention is:

[0006] A method for enhancing the contrast of fatigue microcracks in metal materials is provided, the method comprising the following steps:

[0007] Step 1: Sample preparation

[0008] Machining of plate specimens to produce cracks and confirming that the crack tip meets the test requirements

[0009] Grind and polish the surface of the plate specimen to make the surface roughness meet the requirements of the fatigue crack growth rate test;

[0010] Step 2: Surface treatment

[0011] Spray black light-absorbing paint on the surface of the sample to be tested to form a uniform black light-absorbing coating to absorb the visible light emitted by the light source; achieving a strong light-absorbing effect.

[0012] Step 3: Fatigue test

[0013] The sample is mounted on a fatigue testing machine using a fixture adapted to the sample, the light source is adjusted to a corresponding position, and the lens focal length of the high-definition camera is adjusted until the crack tip profile is clearly observed;

[0014] Step 4: performing a fatigue crack growth rate test on the sample.

[0015] Furthermore, the light-absorbing coating is a strong light-absorbing coating.

[0016] Furthermore, the machined crack is located in the middle of the plate-shaped specimen.

[0017] Furthermore, the grinding and polishing are performed in a direction perpendicular to the crack propagation path.

[0018] Furthermore, the thickness of the black light-absorbing coating is 10-20 μm. When the crack does not expand, the strong light-absorbing coating absorbs visible light or ultraviolet light emitted by the strong light source, and the image displays a black background.

[0019] Furthermore, the light absorbing coating contains a fluorescent agent, and the light source is ultraviolet light.

[0020] When the crack expands, the strong light-absorbing coating is torn along with the crack, and visible light is diffusely reflected at the crack and captured by a high-definition camera, forming a bright line that contrasts sharply with the background and overlaps with the crack.

[0021] Furthermore, the light source is a strong light source.

[0022] Furthermore, the light source is directed directly or obliquely onto the sample surface.

[0023] The advantages of the present invention are as follows: the present invention proposes a method for effectively enhancing the contrast of fatigue microcracks in metal materials for fatigue crack growth rate tests. In the existing fatigue microcrack measurement methods, most of them use the technical solution of high-definition microscopes / cameras, relying on visual observation to find cracks from the surface of the material. Since there are generally surface processing textures or scratches on the surface of the material, the above-mentioned influences can easily lead to technical defects such as difficulty in judging cracks and time-consuming and labor-intensive processes, and even misjudgment. The present method adopts a method of spraying a thin and uniform black strong light-absorbing coating or a fluorescent coating + a black strong light-absorbing coating on the surface of the material. When the crack grows, the coating tears with the crack, and a strong light source is used to generate a bright line with a strong contrast to the background and overlapping with the crack position. The crack position and size are judged based on this, and the measurement accuracy is improved to accurately obtain crack growth rate data. DETAILED DESCRIPTION

[0024] In fact, many different examples can be described and these examples should not be construed as limited to the examples set forth herein. On the contrary, these examples are described so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art.

[0025] A method for enhancing the contrast of fatigue microcracks in metal materials is provided, the method comprising the following steps:

[0026] Step 1: Sample preparation

[0027] Machining of plate specimens to produce cracks and confirming that the crack tip meets the test requirements

[0028] Grind and polish the surface of the plate specimen to make the surface roughness meet the requirements of the fatigue crack growth rate test;

[0029] Step 2: Surface treatment

[0030] Spray black light-absorbing paint on the surface of the sample to be tested to form a uniform black light-absorbing coating to absorb the visible light emitted by the light source; achieving a strong light-absorbing effect.

[0031] Step 3: Fatigue test

[0032] The sample is mounted on a fatigue testing machine using a fixture adapted to the sample, the light source is adjusted to a corresponding position, and the lens focal length of the high-definition camera is adjusted until the crack tip profile is clearly observed;

[0033] Step 4: performing a fatigue crack growth rate test on the sample.

[0034] The light-absorbing coating is a strong light-absorbing coating.

[0035] The machined crack is located in the middle of the plate-shaped specimen.

[0036] The grinding and polishing are carried out in a direction perpendicular to the crack propagation path.

[0037] The thickness of the black light-absorbing coating is 10-20 μm. When the crack does not expand, the strong light-absorbing coating absorbs visible light or ultraviolet light emitted by the strong light source, and the image displays a black background.

[0038] The light absorbing coating contains a fluorescent agent, and the light source is ultraviolet light.

[0039] When the crack expands, the strong light-absorbing coating is torn along with the crack, and visible light is diffusely reflected at the crack and captured by a high-definition camera, forming a bright line that contrasts sharply with the background and overlaps with the crack.

[0040] The light source is a strong light source.

[0041] The light source is directly or obliquely irradiated on the sample surface.

[0042] The parameters of the following embodiment 1 may also be preferred:

[0043] Before the fatigue crack growth rate test is carried out using the method of this embodiment, the 7075 aluminum alloy is processed into a plate specimen with a length of 300.04 mm, a width of 100.06 mm, and a thickness of 4.76 mm according to ASTM647, and an artificial crack with a cut radius of 3.06 mm is processed in the center of the specimen.

[0044] The surface roughness of the sample is Ra<1.6μm;

[0045] The test specimen is mounted on the fatigue testing machine using a suitable fixture, the strong light source is adjusted to a suitable position, and the focus of the high-definition camera lens is adjusted until the outline of the artificial crack tip is clearly observed in the collected image. At this time, the background except for the artificial crack is black.

[0046] Fatigue loading uses a stress ratio of R = 0.1, and the prefabrication process uses a load reduction method. The pre-cracking process ends when the pre-crack size is set to 3mm. When the crack expands during cyclic loading, the strong light-absorbing coating will tear as the crack tip expands. At this time, the visible light generated by the strong light source is diffusely reflected at the crack and captured by the high-definition camera, forming a bright line with a strong contrast with the background and overlapping with the crack. The image is collected by the high-definition camera and the crack extension size is monitored and measured in real time. After the pre-crack, the horizontal load loading mode is adopted, and the crack extension size is monitored and measured using the same method as above, and the crack extension rate is calculated in real time.

[0047] The parameters of the following embodiment 2 can also be preferred:

[0048] Before the fatigue crack growth rate test is carried out using the method of this embodiment, the TC4 titanium alloy is processed into a plate specimen with a length of 300.14 mm, a width of 100.16 mm, and a thickness of 4.74 mm according to ASTM647, and an artificial crack with a cut radius of 3.02 mm is processed in the center of the specimen.

[0049] The surface roughness of the sample is Ra<1.6μm;

[0050] The test specimen is mounted on the fatigue testing machine using a suitable fixture, the strong light source is adjusted to a suitable position, and the focus of the high-definition camera lens is adjusted until the outline of the artificial crack tip is clearly observed in the collected image. At this time, the background except for the artificial crack is black.

[0051] Fatigue loading uses a stress ratio of R = 0.1, and the prefabrication process uses a load reduction method. The pre-cracking process ends when the pre-crack size is set to 3mm. When the crack expands during cyclic loading, the fluorescent coating and the strong light-absorbing coating will tear as the crack tip expands. The ultraviolet light generated by the strong light source produces fluorescence at the crack, forming a bright line with a strong contrast to the background and overlapping with the crack. The image is collected by a high-definition camera to monitor and measure the crack expansion size in real time. After the pre-crack, the horizontal load loading mode is adopted, and the crack expansion size is monitored and measured using the same method as above, and the crack expansion rate is calculated in real time.

[0052] The description of different advantageous arrangements has been presented for purposes of illustration and description, but the description is not intended to be exclusive or limited to examples of the disclosed form. Many modifications and variations will be apparent to those of ordinary skill in the art. Additionally, different advantageous examples may describe different advantages over other advantageous examples. The selected example or examples are chosen and described in order to best illustrate the principles of the examples, the practical application, and to enable those of ordinary skill in the art to understand the disclosure with various examples with various modifications suitable for the particular use contemplated.

Claims

1. A method for enhancing the contrast of fatigue microcracks in metal materials, characterized in that: The steps include: Step 1: Sample preparation Machining of plate specimens to produce cracks and confirming that the crack tip meets the test requirements Grind and polish the surface of the plate specimen to make the surface roughness meet the requirements of the fatigue crack growth rate test; Step 2: Surface treatment Spray black light-absorbing paint on the surface of the sample to be tested to form a uniform black light-absorbing coating to absorb the visible light emitted by the light source; Achieve strong light absorption effect. Step 3: Fatigue test The sample is mounted on a fatigue testing machine using a fixture adapted to the sample, the light source is adjusted to a corresponding position, and the lens focal length of the high-definition camera is adjusted until the crack tip profile is clearly observed; Step 4: performing a fatigue crack growth rate test on the sample.

2. A method for enhancing the contrast of fatigue microcracks in metal materials according to claim 1, characterized in that: The light-absorbing coating is a strong light-absorbing coating.

3. The method for enhancing the contrast of fatigue microcracks in metal materials according to claim 1, characterized in that: The machined crack is located in the middle of the plate-shaped specimen.

4. The method for enhancing the contrast of fatigue microcracks in metal materials according to claim 1, characterized in that: The grinding and polishing are carried out in a direction perpendicular to the crack propagation path.

5. The method for enhancing the contrast of fatigue microcracks in metal materials according to claim 1, characterized in that: The thickness of the black light-absorbing coating is 10-20 μm. When the crack does not expand, the strong light-absorbing coating absorbs visible light or ultraviolet light emitted by the strong light source, and the image displays a black background.

6. The method for enhancing the contrast of fatigue microcracks in metal materials according to claim 1, characterized in that: The light-absorbing coating contains a fluorescent agent, and the light source is ultraviolet light.

7. The method for enhancing the contrast of fatigue microcracks in metal materials according to claim 1, characterized in that: The light source is a strong light source.

8. The method for enhancing the contrast of fatigue microcracks in metal materials according to claim 1, characterized in that: The light source is directed directly or obliquely to the sample surface.