A digital image-based method for measuring crack tip closure effect

Abstract

The present invention discloses a digital image-based crack tip closure effect measurement method, which includes: preparing speckle patterns on the surface of a processed specimen; mounting the specimen on a fatigue testing machine, adjusting the position of a CCD camera, and selecting a suitable calibration plate for camera calibration; starting a crack growth rate test while simultaneously opening Vic‑Snap software to capture speckle images; after the crack growth rate test is completed, analyzing the captured images to obtain full-field strain data of the specimen, deriving the effective stress intensity factor range under different crack lengths, and ultimately obtaining an effective crack growth rate equation under the test conditions. The digital image-based crack tip closure effect measurement method provided by the present invention obtains changes in the plastic strain field at the fatigue crack tip using an optical extensometer, determines the load at which the crack closes by capturing digital images within a single cycle, and ultimately obtains an effective crack growth rate curve.

Description

Technical Field

[0001] The invention relates to the technical field of fatigue crack growth rate testing, and in particular to a method for measuring crack tip closure effect based on digital images. Background Art

[0002] In the development of fracture mechanics, the three regions of the fatigue crack growth curve are usually characterized by the near-threshold region, the stable crack growth region and the unstable crack growth region. The crack growth rate in the near-threshold region is not easy to measure. The fatigue crack growth rate decreases rapidly with the decrease of the stress intensity factor range ΔK. The near-threshold region ΔK is usually used to measure the fatigue crack growth rate. th To characterize the stress intensity factor range of this area. ΔK th It is the design basis of some important structural materials. If in actual application, the stress intensity factor range ΔK is less than ΔK th , the crack will not expand, or the crack expansion cannot be monitored, so the fatigue crack expansion threshold ΔK th It is very important for engineering design applications. However, studies have shown that as the stress ratio decreases, the crack closure phenomenon becomes more and more obvious as the stress intensity factor decreases in this region.

[0003] As early as the 1970s, foreign scholar Elber W. The Significance of Fatigue Crack Closure [J]. Astm Stp, 1971, 486, first proposed the effective stress intensity factor range ΔK in his paper. eff , ΔK eff =K max -K cl , K cl is the stress intensity factor corresponding to crack closure, K max is the maximum stress intensity factor under cyclic loading. Since then, the phenomenon of crack closure in the near-threshold region has been widely recognized, and obtaining the effective crack growth rate curve equation is of great significance for the basic design of structural materials.

[0004] Determining the relationship between crack growth rate and effective stress intensity factor range through fatigue crack growth rate testing is crucial for evaluating the safety performance of metallic structural components. Current methods for characterizing and measuring crack tip closure using digital image correlation present a challenge in accurately determining the cyclic load at which crack closure occurs. Summary of the Invention

[0005] The purpose of the present invention is to provide a digital image-based crack tip closure effect measurement method. The changes in the plastic strain field at the fatigue crack tip are obtained by an optical extensometer. By collecting digital images within a single cycle, the cyclic load when the crack closes is determined, and finally an effective crack growth rate curve is obtained.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] A method for measuring crack tip closure effect based on digital images, the method comprising the following steps:

[0008] S1, processing the sample and preparing speckles on the processed sample surface;

[0009] S2, mount the specimen on the fatigue testing machine, adjust the position of the CCD camera, adjust the focus, and finally select a suitable calibration plate to calibrate the camera;

[0010] S3, conduct crack growth rate test, input image acquisition time interval based on Vic-Snap acquisition software, and acquire speckle images of the specimen;

[0011] S4, after the crack growth rate test is completed, the speckle image is imported, the experimental data is processed to obtain the full-field strain data of the experiment, and the full-field strain data of the target area is output;

[0012] S5, selecting and collecting a series of digital images within a certain cycle, adding a virtual extensometer to the crack tip area in the image, and measuring the displacement field change trend of the pixel points on both sides of the crack;

[0013] S6, repeat the process of S5, analyze the digital images within the cycles corresponding to different crack lengths, find the inflection point in the load-displacement curve corresponding to each cycle, find the load corresponding to the point, calculate the stress intensity factor corresponding to when the crack closes, and finally calculate the effective stress intensity factor range.

[0014] Furthermore, in step S1, processing the sample and preparing speckles on the surface of the processed sample specifically includes:

[0015] For the metal materials studied, refer to the American standard ASTM E647-15 ε1 , processing standard compact tensile specimens, the specimen width is 50mm, the thickness is 12.5mm, the initial notch length of the specimen is 10mm, and the notch height is 2mm;

[0016] To prepare speckles on the surface of the sample, first spray the sample surface with white matte paint with a moderate thickness, and then apply black matte paint on the primer to prepare speckles.

[0017] Furthermore, in step S2, the sample is mounted on the testing machine, the position of the CCD camera is adjusted, the focus is adjusted, and finally a suitable calibration plate is selected to perform camera calibration, which specifically includes:

[0018] Mount the sample on the testing machine and adjust the equipment, mainly adjusting the position of the CCD camera so that the sample surface is perpendicular to the CCD camera to avoid off-plane displacement;

[0019] Adjust the focus, turn on the light source, adjust the aperture of the CCD camera to the optimal position, and adjust the image to the clearest level according to the exposure time on the Vic-Snap acquisition software;

[0020] Select a suitable calibration plate and place it on the position of the sample to ensure that the calibration plate is clear and bright as a whole without overexposure, darkening or blurring.

[0021] Furthermore, in step S6, the calculation expression of the effective stress intensity factor range is as follows:

[0022]

[0023] α=a / W

[0024] Where, P max is the maximum stress, N; P0 is the stress under crack closure effect, N; B is the thickness of the specimen, mm; W is the width of the specimen, mm; a is the crack length, mm.

[0025] Furthermore, the step S6 further includes: for the crack lengths under different cycle times, respectively calculating the effective stress intensity factor ranges under different crack lengths, and finally obtaining da / dN and ΔK eff The relationship equation is the effective crack growth rate equation.

[0026] According to specific embodiments provided by the present invention, the present invention discloses the following technical effects: The digital image-based crack tip closure effect measurement method provided by the present invention uses an optical extensometer to obtain changes in the plastic strain field at the fatigue crack tip. By collecting digital images within a single cycle, the load at which crack closure occurs is determined, and ultimately the effective crack growth rate curve equation is obtained. The present invention uses cloud maps of the strain field on both sides of the crack tip to more vividly characterize the crack closure phenomenon, visualize abstract problems, and accurately calculate the effective stress intensity factor range corresponding to crack closure. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0028] Figure 1 Schematic diagram of the process of the crack tip closure effect measurement method based on digital images of the present invention;

[0029] Figure 2a This is a front view of the sample of the present invention;

[0030] Figure 2b It is a side view of the sample of the present invention;

[0031] Figure 3 Adding a virtual extensometer diagram to the full-field strain diagram and crack tip of the target area of ​​the specimen of the present invention;

[0032] Figure 4 This is a diagram showing the displacement field changes on both sides of the crack tip of the present invention. DETAILED DESCRIPTION

[0033] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0034] The purpose of the present invention is to provide a digital image-based crack tip closure effect measurement method. The changes in the plastic strain field at the fatigue crack tip are obtained by an optical extensometer. By collecting digital images within a single cycle, the cyclic load when the crack closes is determined, and finally an effective crack growth rate curve is obtained.

[0035] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0036] like Figure 1 As shown, the method for measuring crack tip closure effect based on digital images provided by the present invention includes the following steps:

[0037] S1, processing the sample and preparing speckles on the processed sample surface; specifically including:

[0038] For the metal materials studied, refer to the American standard ASTM E647-15 ε1 , processing standard compact tensile specimens, such as Figure 2a and 2b The specimen shape is shown in the figure, with a width of 50 mm and a thickness of 12.5 mm. The initial cut length of the specimen is 10 mm and the cut height is 2 mm. Cold working and turning are not allowed during processing. The specimen surface should not have scratches, damage or rust, and the surface roughness of the specimen should not be higher than 0.8.

[0039] To create speckles on the sample surface, first spray the sample surface with white matte paint to a moderate thickness, then apply black matte paint on the primer to create speckles; the size of the black spots is usually about 3 pixels;

[0040] S2, mount the specimen on the fatigue testing machine, adjust the position of the CCD camera, adjust the focus, and finally select a suitable calibration plate to calibrate the camera; specifically, it includes:

[0041] Mount the sample on the testing machine and adjust the equipment, mainly adjusting the position of the CCD camera so that the sample surface is perpendicular to the CCD camera to avoid off-plane displacement;

[0042] Adjust the focus, turn on the light source, adjust the aperture of the CCD camera to the optimal position, and adjust the image to the clearest level according to the exposure time on the Vic-Snap acquisition software;

[0043] Select a suitable calibration plate and place it on the sample to ensure that the plate is clear and bright without overexposure, darkening or blurring.

[0044] S3, conduct crack growth rate test, input image acquisition time interval based on Vic-Snap acquisition software, and acquire speckle images of the specimen;

[0045] S4, after the crack growth rate test is completed, the speckle image is imported, the experimental data is processed to obtain the full-field strain data of the experiment, and the full-field strain data of the target area is output;

[0046] S5, select and collect a series of digital images within a certain cycle, and add a virtual extensometer to the crack tip area in the image, such as Figure 3 As shown in the figure, the displacement field change trend of the pixel points on both sides of the crack is measured; the displacement field change on both sides of the crack tip is as follows Figure 4 As shown;

[0047] S6, repeat the process of S5, analyze the digital images within the cycles corresponding to different crack lengths, find the inflection point in the load-displacement curve corresponding to each cycle, find the load corresponding to the point, calculate the stress intensity factor corresponding to when the crack closes, and finally calculate the effective stress intensity factor range.

[0048] The calculation expression of the effective stress intensity factor range is as follows:

[0049]

[0050] α=a / W

[0051] Where, P max is the maximum stress, N; P0 is the stress under crack closure effect, N; B is the thickness of the specimen, mm; W is the width of the specimen, mm; a is the crack length, mm.

[0052] The step S6 further includes: for the crack lengths under different cycle times, respectively calculating the effective stress intensity factor range corresponding to the crack lengths under different cycle times, and obtaining da / dN and ΔK eff The relationship equation is the effective crack growth rate equation:

[0053] da / dN=cΔK eff m

[0054] Where da / dN is the crack growth rate, mm / cycle; ΔK eff is the effective stress intensity factor range, c and m are both constants.

[0055] In summary, the digital image-based crack tip closure effect measurement method provided by the present invention obtains the changes in the plastic strain field at the fatigue crack tip through an optical extensometer, determines the load when the crack closes by collecting digital images within a single cycle, and finally obtains the effective crack growth rate curve; equivalent to the flexibility method, the present invention uses a cloud map of the strain field on both sides of the crack tip to more vividly characterize the crack closure phenomenon, visualize abstract problems, and accurately calculate the effective stress intensity factor range corresponding to the occurrence of crack closure.

[0056] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The above examples are only intended to help understand the method and core concept of the present invention. At the same time, those skilled in the art will find that the specific implementation methods and application scopes may vary based on the concept of the present invention. In summary, the contents of this specification should not be construed as limiting the present invention.

Claims

1. A method for measuring crack tip closure effect based on digital images, characterized in that: The following steps are involved: S1, processing the sample and preparing speckles on the processed sample surface; S2, mount the specimen on the fatigue testing machine, adjust the position of the CCD camera, adjust the focus, and finally select a suitable calibration plate to calibrate the camera; S3, conduct crack growth rate test, input image acquisition time interval based on Vic-Snap acquisition software, and acquire speckle images of the specimen; S4, after the crack growth rate test is completed, the speckle image is imported, the experimental data is processed to obtain the full-field strain data of the experiment, and the full-field strain data of the target area is output; S5, selecting and collecting a series of digital images within a certain cycle, adding a virtual extensometer to the crack tip area in the image, and measuring the displacement field change trend of the pixel points on both sides of the crack; S6, repeat the process of S5, analyze the digital images within the cycles corresponding to different crack lengths, find the inflection point in the load-displacement curve corresponding to each cycle, find the load corresponding to the point, calculate the stress intensity factor corresponding to the crack closure, and finally calculate the effective stress intensity factor range; The calculation expression of the effective stress intensity factor range is as follows: ； ； Where, P max is the maximum stress, N; P 0 is the stress under crack closure effect, N; B is the thickness of the specimen, mm; W is the width of the specimen, mm; a is the crack length, mm; The step S6 further includes: calculating the effective stress intensity factor range under different crack lengths at different cycle times, and finally obtaining da / dN and Δ Keff The relationship equation is the effective crack growth rate equation.

2. The method for measuring crack tip closure effect based on digital images according to claim 1, characterized in that: In step S1, processing the sample and preparing speckles on the surface of the processed sample specifically includes: For the metal materials studied, refer to the American standard ASTM E647-15 ε1 , processing standard compact tensile specimens, the specimen width is 50 mm, the thickness is 12.5 mm, the initial notch length of the specimen is 10 mm, and the notch height is 2 mm; To prepare speckles on the surface of the sample, first spray the sample surface with white matte paint with a moderate thickness, and then apply black matte paint on the primer to prepare speckles.

3. The method for measuring crack tip closure effect based on digital images according to claim 1, characterized in that: In step S2, the sample is mounted on the testing machine, the position of the CCD camera is adjusted, the focus is adjusted, and finally a suitable calibration plate is selected to perform camera calibration, which specifically includes: Mount the sample on the testing machine and adjust the equipment, mainly adjusting the position of the CCD camera so that the sample surface is perpendicular to the CCD camera to avoid off-plane displacement; Adjust the focus, turn on the light source, adjust the aperture of the CCD camera to the optimal position, and adjust the image to the clearest level according to the exposure time on the Vic-Snap acquisition software; Select a suitable calibration plate and place it on the position of the sample to ensure that the calibration plate is clear and bright as a whole without overexposure, darkening or blurring.

Images