System and method for calculating stress intensity factor at crack tip of solid material

Abstract

This invention discloses a system and method for calculating the stress intensity factor at the crack tip of solid materials, belonging to the field of materials testing and analysis technology. The calculation system includes a pulsed laser generator, a first beam splitter, a second beam splitter, a first optical path, a second optical path, a load loading device, an image acquisition unit, and an image processing unit. A transparent, homogeneous material sample is placed at the end of the second optical path. The pulsed laser is split into two beams, which pass through the first and second optical paths respectively to form a reference beam and an object beam. The reference beam and the object beam interfere on the second beam splitter. The image acquisition unit acquires interference fringe images, and the image processing unit calculates the stress field and crack tip stress intensity factor of the material sample under load based on the interference fringe images. This invention achieves the direct quantification of the crack tip stress intensity factor.

Description

Technical Field

[0001] This invention relates to the field of materials testing and analysis technology, and in particular to a stress intensity factor calculation system and method based on ultrafast laser holography technology. Background Technology

[0002] Rapid fracture processes in solid materials (such as brittle solid materials like rock and concrete) are widespread in various practical engineering fields, seriously threatening the reliability and safety of engineering structures. The stress intensity factor at the crack tip, as a crucial indicator for determining whether a solid material has failed, is key to reflecting the safety of an engineering structure. Fracture occurs suddenly, and the crack propagation process in solid materials under actual load is extremely rapid, making it difficult to clearly and quickly capture the evolution of the crack tip. Therefore, it is impossible to directly visualize and quantify the rapidly changing stress intensity factor at the crack tip. Summary of the Invention

[0003] The purpose of this invention is to provide a system and method for calculating the stress intensity factor at the crack tip of a solid material, in order to solve the problem that the rapid change of the stress intensity factor at the crack tip cannot be directly quantified due to the difficulty in clearly and quickly capturing the evolution process of the crack tip.

[0004] In a first aspect, the present invention provides a system for calculating the stress intensity factor at the crack tip of a solid material. The system includes a pulsed laser generator, a first beam splitter, a second beam splitter, a first optical path, a second optical path, a load loading device, an image acquisition unit, and an image processing unit. A transparent and homogeneous material sample is provided at the end of the second optical path, and the load loading device is used to load the material sample.

[0005] The pulsed laser generated by the pulsed laser generator is split into two beams by the first beam splitter. The first laser beam passes through the first optical path to form a reference beam, and the second laser beam passes through the second optical path to illuminate the loaded material sample to form an object beam. The reference beam and the object beam interfere with each other at the second beam splitter.

[0006] The image acquisition unit is used to acquire interference fringe images within the second beam splitter; the image processing unit is used to calculate the stress field and crack tip stress intensity factor of the material sample under load based on the interference fringe images.

[0007] Furthermore, the first beam splitter divides the pulsed laser into two beams according to a certain intensity ratio, wherein the intensity ratio of the first laser beam to the second laser beam is 1:4 to 1:10.

[0008] Furthermore, both the first and second optical paths include a reflector, a beam expander, a collimating lens, a polarizer, and a quarter-wave plate arranged sequentially along the optical path.

[0009] Furthermore, the light coverage area of ​​the second laser beam after being diffused by the beam expander in the second optical path and reaching the collimating lens is equal to the light transmission area of ​​the collimating lens.

[0010] Furthermore, the image acquisition unit employs an ultrafast camera, the pulsed laser generator employs a monochromatic ultrashort pulse laser, the frame rate of the ultrafast camera is consistent with the frequency of the pulsed laser output by the monochromatic ultrashort pulse laser, and the exposure time of the ultrafast camera is consistent with the picosecond-level half-width at half-maximum of the pulsed laser.

[0011] Secondly, the present invention also provides a method for calculating the stress intensity factor using the stress intensity factor calculation system described above, the method comprising the following steps:

[0012] While loading the material sample using a load-loading device, a pulsed laser generator produces pulsed laser light.

[0013] The pulsed laser is split into two beams. The first laser beam passes through a first optical path to form a reference beam, and the second laser beam passes through a second optical path to illuminate the loaded material sample, forming an object beam. The reference beam and the object beam interfere with each other at the second beam splitter.

[0014] Acquire the interference fringe image when the reference light and the object light interfere;

[0015] The stress field and crack tip stress intensity factor of the material sample under load are calculated based on the interference fringe image.

[0016] Furthermore, the material sample is a transparent, homogeneous three-point bending specimen containing pre-existing cracks, prepared using 3D printing technology; the specific preparation process of the three-point bending specimen is as follows: making a sample model, spraying polymer, and laser curing to form a three-dimensional integrated polymer material model.

[0017] Furthermore, the specific formula for calculating the stress intensity factor at the crack tip is as follows:

[0018]

[0019]

[0020] Among them, K I Here, d is the stress intensity factor at the crack tip of a type I fracture, r is the polar radius with the crack tip as the origin, and d is the stress intensity factor at the crack tip. x d is the perpendicular distance between the measured location and the crack tip in the x-direction. y denoted as , where is the perpendicular distance between the measured location and the crack tip along the y-direction; x is the crack propagation direction; y is the normal direction of the crack plane; h is the thickness of the material sample; and n is the crack thickness. pf is the order of the isopleths of the stripes. p This represents the material stripe value.

[0021] The beneficial effects of this invention are:

[0022] 1. This invention utilizes a first beam splitter to divide a pulsed laser into two beams, which, after passing through a first optical path and a second optical path, form a reference beam and an object beam, respectively. An interference fringe image is obtained showing the interference between the reference beam and the object beam on the second beam splitter. Based on the interference fringe image, the distribution and evolution of interference fringes in the entire field and at the crack tip during the failure and fracture of the tested material sample can be obtained. Based on the interference fringes, the stress field generated by the tested material sample under load and the stress intensity factor at the crack tip can be calculated, thus realizing the direct quantification of the stress intensity factor at the crack tip.

[0023] 2. This invention combines an ultrafast camera with a monochromatic ultrashort pulse laser. The time resolution of the monochromatic ultrashort pulse laser reaches the picosecond level, which can provide sufficient exposure for the ultrafast camera within the picosecond time range. This allows the ultrafast camera to quickly record the interference fringe image generated by the load, and then calculate the stress intensity factor at the crack tip. This solves the problem of the inability to capture the rapid crack propagation in time, and enables rapid, real-time, and non-destructive quantification of the stress intensity factor at the crack tip during dynamic fracture. Attached Figure Description

[0024] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only one embodiment of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 This is a schematic diagram of the stress intensity factor calculation system in Embodiment 1 of the present invention;

[0026] Figure 2 This is a flowchart of the stress intensity factor calculation method in Embodiment 2 of the present invention;

[0027] Figure 3 This is a dimension diagram of the material sample in Embodiment 2 of the present invention;

[0028] Figure 4 This is a schematic diagram of stress components and shear stress in Embodiment 2 of the present invention.

[0029] Explanation of reference numerals in the attached figures: 1-First beam splitter, 2-Reflector, 3-Beam expander, 4-Collimator, 5-Polarizer, 6-Quarter-wave plate, 7-Material sample, 8-Second beam splitter, 9-Ultrafast camera, 10-Image processing unit. Detailed Implementation

[0030] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] The technical solutions of this application will be described in detail below with specific embodiments. The following specific embodiments can be combined with each other, and the same or similar concepts or processes may not be described again in some embodiments.

[0032] Example 1

[0033] like Figure 1 As shown, an embodiment of the present invention provides a system for calculating the stress intensity factor at the crack tip of a solid material, comprising a pulsed laser generator, a first beam splitter 1, a second beam splitter 8, a first optical path, a second optical path, a load loading device, an image acquisition unit, and an image processing unit 10. A transparent, homogeneous material sample 7 is placed at the end of the second optical path, and the load loading device is used to load the material sample 7. The pulsed laser generated by the pulsed laser generator is split into two beams by the first beam splitter 1. The first laser beam passes through the first optical path to form a reference beam, and the second laser beam passes through the second optical path to illuminate the loaded material sample 7, forming an object beam. The reference beam and the object beam interfere within the second beam splitter 8. The image acquisition unit is used to acquire interference fringe images on the second beam splitter 8; the image processing unit 10 is used to calculate the stress field and crack tip stress intensity factor of the material sample 7 due to loading based on the interference fringe images.

[0034] In one specific embodiment of the present invention, the first beam splitter 1 and the second beam splitter 8 are both beam splitters. The first beam splitter 1 splits the pulsed laser into two beams, namely the first laser beam and the second laser beam, according to a certain light intensity ratio. The light intensity ratio of the first laser beam to the second laser beam is 1:4 to 1:10. The second laser beam irradiates the material sample 7 to be tested to form the object beam. The second laser beam serves as the reference beam and subsequently interferes with the object beam in the second beam splitter 8. The optical path lengths of the object beam and the reference beam are substantially equal.

[0035] An excessively high intensity ratio results in a weak reference beam, while an excessively low intensity ratio results in an overly strong reference beam; both can lead to indistinguishable interference fringes. In this embodiment, the intensity ratio of the first laser beam to the second laser beam is 1:4 to 1:10, which enables the formation of clear interference fringes.

[0036] In one specific embodiment of the present invention, the first optical path and the second optical path are the same and both specifically include a reflector 2, a beam expander 3, a collimating lens 4, a polarizer 5, and a quarter-wave plate 6 arranged sequentially along the optical path.

[0037] In this embodiment, the collimating lens 4 is a high-quality lens with a large light-transmitting diameter and a short focal length, wherein the light-transmitting diameter is greater than the length of the material sample 7, so that a clear and high-quality laser source with less loss can be obtained.

[0038] In this embodiment, the beam expander 3 is a microscope objective with a magnification of 15 to 100, and the height of the beam expander 3 matches the height of the collimating lens 4, that is, the focal height of the beam expander 3 is consistent with the focal height of the collimating lens 4. Different samples have different interference fringe densities. For samples that are sensitive to stress optics, selecting a microscope objective with a higher magnification can enable the image acquisition unit to better capture the distribution and evolution of interference fringes.

[0039] By adjusting the distance between the collimating lens 4 and the beam expander 3, the light coverage area of ​​the laser beam reaching the collimating lens 4 after being diffused by the beam expander is equal to the light transmission area of ​​the collimating lens 4, so that the laser beam irradiating the material sample 7 completely envelops the sample.

[0040] In one specific embodiment of the present invention, the image acquisition unit employs an ultrafast camera 9, and the pulsed laser generator employs a monochromatic ultrashort pulse laser with good coherence, small divergence angle, and concentrated energy. The frame rate of the ultrafast camera 9 is consistent with the frequency of the pulsed laser output by the monochromatic ultrashort pulse laser, and the exposure time of the ultrafast camera 9 is consistent with the picosecond-level full width at half maximum (FWHM) of the pulsed laser.

[0041] Currently, most solutions to solid material fracture problems rely on numerical simulation methods. Commonly used experimental methods, such as digital speckle image processing, strain gauges (for measuring fracture strain), and caustics, all suffer from low time resolution, which cannot match the material fracture velocity. This results in large errors in the experimental data and makes it difficult to capture the crack propagation process in a timely manner. In contrast, the time resolution of a monochromatic ultrashort pulse laser reaches the picosecond level. The energy of the emitted laser source is concentrated and the divergence angle is small. Within the picosecond time range, it provides sufficient exposure for the ultrafast camera 9, enabling the ultrafast camera 9 to acquire the crack propagation and evolution process of the tested sample during fracture.

[0042] Traditional laser holography is limited by harsh experimental conditions and the processing techniques of holographic films, making it difficult to apply widely. Digital holography can acquire the stress distribution and interference fringe evolution at the crack tip of the tested sample in real time, but it is limited by the time resolution and pixel resolution of CCD image sensors. The required parameters are difficult to determine quickly, resulting in the inability to judge the fracture state in time. This invention uses laser holography, a monochromatic ultrashort pulse laser, and an ultrafast camera to effectively solve the problem of rapid crack propagation that cannot be captured in time. It can quickly, in real time, and non-destructively quantify the stress intensity factor at the crack tip during dynamic fracture.

[0043] When the pulsed laser reaches the ultrafast camera 9, its light intensity is not lower than the minimum illumination required by the ultrafast camera 9 to ensure the clarity of the interference fringe image.

[0044] Example 2

[0045] like Figure 2 As shown, the present invention also provides a method for calculating the stress intensity factor using the stress intensity factor calculation system described above. The method includes the following steps: Step 1: While loading the material sample using a load loading device, a pulsed laser generator generates a pulsed laser; Step 2: The pulsed laser is split into two beams. The first laser beam passes through a first optical path to form a reference beam, and the second laser beam passes through a second optical path to irradiate the loaded material sample, forming an object beam; the reference beam and the object beam interfere on a second beam splitter; Step 3: Obtain an interference fringe image when the reference beam and the object beam interfere; Step 4: Calculate the stress field and crack tip stress intensity factor of the material sample due to the load based on the interference fringe image.

[0046] like Figure 3 As shown, the material sample is a transparent, homogeneous three-point bending specimen containing a pre-existing crack, prepared using 3D printing technology to ensure that the failure of the tested sample occurs at the pre-existing crack. In this embodiment, the pre-existing crack is a through crack, the sample length is 80 mm, the sample width is 20 mm, the sample thickness is 6 mm, the pre-existing crack width is 1 mm, the pre-existing crack length is 5 mm, the distance between the upper loading point and the sample edge is 40 mm, and the distance between the lower loading point and the sample edge is 15 mm. The specific preparation process of the three-point bending specimen is as follows: making a sample model, spraying polymer (i.e., 3D printing material), and laser curing to form a three-dimensional integrated polymer material model. The three-point bending specimen satisfies the photoelastic principle, that is, it is a transparent, homogeneous, isotropic solid material with birefringent optical properties.

[0047] The material sample was observed using traditional photoelasticity to ensure that it contained no interference fringes, thus minimizing the influence of residual stress caused by the manufacturing process. The study focused on the dynamic evolution of stress caused by load changes and the initiation and propagation of cracks in order to determine the stress intensity factor at the crack tip.

[0048] Loading devices are used to apply loads to the sample being tested. Loading methods include three-point bending, uniaxial compression, and drop hammer impact.

[0049] In step 4, the stress field and crack tip stress intensity factor generated by the load on the material sample are calculated based on the interference fringe image.

[0050] According to the theory of linear elastic fracture mechanics, the stress field of a type I crack is as follows:

[0051]

[0052] In equation (1), x is the crack propagation direction, and y is the normal direction of the crack plane; σ x and σ y τ represents the stress components along the crack propagation direction and the crack plane normal direction, respectively. xy For shear stress; r is the polar radius with the crack tip as the origin, and θ is the polar angle with the crack tip as the origin; K I For the stress intensity factor at the tip of a type I fracture crack, such as Figure 4 As shown. Figure 4 The rectangular coordinate system in the diagram has the crack tip as the origin and the sample thickness direction as the Z-axis. The relationship between polar coordinates and rectangular coordinates can be expressed as:

[0053]

[0054]

[0055] Where, d x d is the perpendicular distance between the measured location and the crack tip in the x-direction. y This is the vertical distance between the measured location and the crack tip in the y-direction.

[0056] According to the requirements of the Westergaard stress function, the distance between the point of interest in the stress distribution and the crack tip should not be too small or too large, which is applicable to the area studied in this invention.

[0057] According to mechanics of materials, the principal stresses are:

[0058]

[0059]

[0060] Where σ1 and σ2 are the two principal stresses, the principal stresses being the normal stress at a point within the sample when the shear stress on a micro-area element with normal vector n = (1, 2, 3) is zero, and the direction of n is the principal stress direction at that point; since the tested sample underwent Type I fracture, the isopleths are relatively dense in the direction perpendicular to the crack plane, therefore, [the following is selected] Calculations show that the principal stresses can be expressed as:

[0061]

[0062] Interference fringes recorded by transmission-type digital holographic photoelasticity are used to visualize and quantify the dynamic stress field during sample fracturing. Under plane stress conditions, the stress component in the z-direction is zero. Combining this with the principles of digital photoelasticity, we can obtain:

[0063]

[0064] In the formula, h is the sample thickness, and n p f is the order of the isopleths of the stripes. p This represents the material striation value. Therefore, the stress intensity factor at the tip of a type I fracture crack is:

[0065]

[0066] This invention can obtain the picosecond-level crack tip evolution process through clear optical interference fringe images, thereby calculating the stress field distribution and stress intensity factor during the crack tip fracture process, effectively solving the problem of the inability to quantify the rapid fracture process of materials.

[0067] Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention. Clearly, those skilled in the art can make various alterations and modifications to the invention without departing from its spirit and scope. Thus, if these modifications and modifications of the invention fall within the scope of the claims and their equivalents, the invention is also intended to include these modifications and modifications.

Claims

1. A system for calculating the stress intensity factor at the crack tip of a solid material, characterized in that, The computing system includes a pulsed laser generator, a first beam splitter, a second beam splitter, a first optical path, a second optical path, a load loading device, an image acquisition unit, and an image processing unit; a transparent and homogeneous material sample is provided at the end of the second optical path, and the load loading device is used to load the material sample; The pulsed laser generated by the pulsed laser generator is split into two beams by the first beam splitter. The first laser beam passes through the first optical path to form a reference beam, and the second laser beam passes through the second optical path to illuminate the loaded material sample to form an object beam. The reference beam and the object beam interfere with each other at the second beam splitter. The optical path lengths of the object beam and the reference beam are substantially equal. The image acquisition unit is used to acquire interference fringe images within the second beam splitter; the image processing unit is used to calculate the stress field and crack tip stress intensity factor of the material sample under load based on the interference fringe images; the image acquisition unit uses an ultrafast camera, the pulsed laser generator uses a monochromatic ultrashort pulse laser, the frame rate of the ultrafast camera is consistent with the frequency of the pulsed laser output by the monochromatic ultrashort pulse laser, and the exposure time of the ultrafast camera is consistent with the picosecond-level half-width at half-maximum of the pulsed laser; The first beam splitter divides the pulsed laser into two beams according to a certain intensity ratio, and the intensity ratio of the first laser beam to the second laser beam is 1:4 to 1:

10.

2. The stress intensity factor calculation system for crack tips in solid materials according to claim 1, characterized in that, Both the first and second optical paths include a reflector, a beam expander, a collimator, a polarizer, and a quarter-wave plate arranged sequentially along the optical path.

3. The stress intensity factor calculation system for crack tips in solid materials according to claim 2, characterized in that, The area of ​​light covering the collimating lens after the second laser beam is diffused by the beam expander in the second optical path is equal to the light transmission area of ​​the collimating lens.

4. A method for calculating stress intensity factor using the stress intensity factor calculation system at the crack tip of a solid material as described in any one of claims 1 to 3, characterized in that, The method includes the following steps: While loading the material sample using a load-loading device, a pulsed laser generator produces pulsed laser light. The pulsed laser is split into two beams. The first laser beam passes through a first optical path to form a reference beam, and the second laser beam passes through a second optical path to illuminate the loaded material sample to form an object beam. The reference beam and the object beam interfere with each other at the second beam splitter. Acquire the interference fringe image when the reference light and the object light interfere; The stress field and crack tip stress intensity factor of the material sample under load are calculated based on the interference fringe image.

5. The method for calculating the stress intensity factor according to claim 4, characterized in that, The material sample is a transparent, homogeneous three-point bending specimen containing pre-existing cracks, prepared using 3D printing technology. The specific preparation process of the three-point bending specimen is as follows: making a sample model, spraying polymer, and laser curing to form a three-dimensional integrated polymer material model.

6. The method for calculating the stress intensity factor according to claim 4 or 5, characterized in that, The specific formula for calculating the stress intensity factor at the crack tip is as follows: Among them, K I Here, d represents the stress intensity factor at the crack tip of a type I fracture, r is the polar radius with the crack tip as the origin, and d represents the stress intensity factor at the crack tip. x d is the perpendicular distance between the measured location and the crack tip in the x-direction. y denoted as , where is the perpendicular distance between the measured location and the crack tip along the y-direction; x is the crack propagation direction; y is the normal direction of the crack plane; h is the thickness of the material sample; and n is the crack thickness. p f is the order of the isopleths of the stripes. p This represents the material stripe value.

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