Method for measuring cracks on a metal surface based on double-slit interference

Abstract

The metal surface crack measuring method based on double-slit interference discloses the parallel laser beam is incident to the half-transmission half-reflection mirror, is reflected vertically and is irradiated on the metal surface to be measured, and the reflected light beam of the metal surface to be measured is transmitted to the photosensitive screen with the distance L from the metal surface to be measured through the half-transfection half-reflection mirror; the metal surface to be measured is moved to make the incident light beam scan along the long side, when the incident light beam covers the crack, the crack will cut the incident light beam into two independent reflected lights, the two reflected lights are transmitted to the photosensitive screen and interference is occurred to form the interference fringes; the crack width is calculated by measuring the center distance between the adjacent interference fringes, combining the wavelength of the laser beam and the distance L. The application is based on the double-slit interference principle, the width of the crack is measured by the interference fringes formed by the double-slit light, the distance between the interference fringes can directly reflect the width of the crack, and the complex imaging or contact detection is not needed, so that the complexity of the measuring method is greatly reduced without reducing the detection precision.

Description

Technical Field

[0001] This invention belongs to the technical field of metal surface crack measurement methods, specifically relating to a metal surface crack measurement method based on double-slit interference. Background Technology

[0002] Metals are widely used in many critical fields such as aerospace, machinery manufacturing, construction, and precision instruments. Surface cracks in metals pose a significant threat to structural safety. For example, in aircraft, if cracks appear in the metal structure of the fuselage, they can rapidly propagate under the complex stress environment of high altitudes, leading to aircraft disintegration and endangering passenger lives. In machinery manufacturing, if metal components of equipment fail due to cracks, it can cause production line shutdowns and result in substantial economic losses. Accurate and effective testing methods can detect cracks in their early stages, providing a basis for repair or replacement, preventing safety accidents, and also helping to assess the remaining lifespan of metal materials, rationally schedule equipment maintenance cycles, improve production efficiency, and reduce operating costs—all crucial for ensuring the stable operation of industrial production.

[0003] There are many methods for measuring cracks on metal surfaces. When the cracks are large, direct observation and imaging methods can yield good results. However, when the crack width is several hundred to several tens of wavelengths or smaller, the detection methods become more complex, such as those using ESPI (Electronic Stimulation Laser), eddy current testing, high-frequency phased array ultrasound, and XCT. Reducing the complexity of the detection method without compromising accuracy not only improves the environmental usability of the measurement system but also lowers the cost of the measurement method. Summary of the Invention

[0004] The purpose of this invention is to provide a method for measuring cracks on metal surfaces based on double-slit interference, which solves the problem of high complexity in existing direct observation measurement methods.

[0005] The technical solution adopted in this invention is as follows: a method for measuring cracks on metal surfaces based on double-slit interference. First, a parallel laser beam is incident on a semi-transparent mirror. The incident beam is reflected and perpendicularly illuminates the metal surface to be measured. The reflected beam from the metal surface is then transmitted through the semi-transparent mirror to a CCD sensor screen at a distance L from the metal surface. Second, the metal surface is moved so that the incident beam scans along its long side. When the incident beam covers the crack, the crack cuts the incident beam into two independent reflected beams. When the two split reflected beams propagate to the CCD sensor screen, they interfere and superimpose to form alternating bright and dark interference fringes. Finally, the crack width is calculated by measuring the center distance between adjacent interference fringes on the CCD sensor screen, combined with the laser beam wavelength and the distance L.

[0006] The invention is further characterized in that,

[0007] The measurement platform includes a laser. A semi-transparent mirror is positioned at a 45° angle in front of the direct beam path of the laser. A displacement stage is positioned on the reflected beam path of the semi-transparent mirror. The metal to be measured, with its surface perpendicular to the reflected beam path of the semi-transparent mirror, is placed on the displacement stage. A CCD sensor, coaxial with the reflected beam path and parallel to the surface of the metal to be measured, is positioned on the back side of the semi-transparent mirror. An integrating sphere absorber, coaxial with the incident beam path of the laser, is positioned on the back side of the semi-transparent mirror. The distance L satisfies the requirement of equation (1):

[0008] L>D 2 / 2λ(1)

[0009] In equation (1), D is the width of the parallel laser beam and λ is the wavelength of the parallel laser beam.

[0010] The metal surface under test is moved by a displacement stage, causing the incident light beam reflected by a semi-transparent mirror to scan gradually along the long side of the metal. When alternating bright and dark interference fringes appear on the CCD photosensitive screen, it is determined that a crack exists at the current scanning position. The crack width d is calculated using equation (2):

[0011] d=λL / Δ x (2)

[0012] In equation (2), Δ x The distance between adjacent stripes on a CCD sensor screen is measured.

[0013] The beneficial effects of this invention are as follows: This invention is a method for measuring cracks on metal surfaces based on double-slit interference. Based on the principle of double-slit interference, the crack itself splits the incident light beam into two reflected beams, which is equivalent to transforming the crack on the metal surface into a special "double slit". The width of the crack is then measured by using the fringes formed by the double-slit light interference. The spacing of the interference fringes can directly reflect the crack width. There is no need for complex imaging or contact detection, thus greatly reducing the complexity of the measurement method without reducing the detection accuracy. Attached Figure Description

[0014] Figure 1 This is a schematic diagram illustrating the principle of the metal surface crack measurement method based on double-slit interference of the present invention.

[0015] In the figure, 1. Laser, 2. CCD sensor, 3. Absorber, 4. Semi-transparent mirror, 5. Metal under test, 6. Crack, 7. Initial incident beam, 8. Transmitted incident beam, 9. Reflected incident beam, 10. Transmitted reflected beam, 11. Left beam after splitting the reflected incident beam, 12. Right beam after splitting the reflected incident beam, 13. Displacement stage. Detailed Implementation

[0016] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0017] Example 1

[0018] This invention provides a method for measuring cracks on metal surfaces based on double-slit interference, such as... Figure 1 As shown, firstly, a parallel laser beam is incident on a semi-transparent mirror 4. The incident beam is reflected and perpendicularly illuminates the surface of the metal to be tested 5. The reflected beam from the surface of the metal to be tested 5 is then transmitted through the semi-transparent mirror 4 to a CCD photosensitive screen 2 at a distance L from the surface of the metal to be tested 5. Secondly, the surface of the metal to be tested 5 is moved so that the incident beam scans along its long side. When the incident beam covers the crack 6, the crack 6 cuts the incident beam into two independent reflected beams. When the two split reflected beams propagate to the CCD photosensitive screen 2, they interfere and superimpose to form alternating bright and dark interference fringes. Finally, the width of the crack 6 is calculated by measuring the center distance between adjacent interference fringes on the CCD photosensitive screen 2, combined with the laser beam wavelength and the distance L. The specific steps include:

[0019] Step 1: Set up the measurement platform.

[0020] Step 2: Set parameters. This includes the wavelength λ and width D of the parallel laser beam, and adjusting the distance L between the CCD photosensitive screen 2 and the surface of the metal to be tested 5.

[0021] Step 3: Start scanning. Move the surface of the metal to be tested 5 using the displacement stage 13 to gradually scan along the long side of the metal.

[0022] Step 4: Identify cracks. Observe the intensity distribution of the CCD sensor 2: if a rectangular uniform light spot is displayed, it is determined that there are no cracks; if stripes appear, it is determined that there is a crack 6 at the current scanning position.

[0023] Step 5: Calculate the crack width.

[0024] In the above-described manner, the present invention provides a method for measuring metal surface cracks based on double-slit interference. The surface of the metal under test 5 is smooth, and the reflection of the light beam is approximately specular. However, the depth of the crack 6 is much greater than the wavelength, and the inner wall of the crack 6 is rough and non-reflective. Due to factors such as scattering and absorption, the light incident into the crack 6 is difficult to be received by the CCD photosensitive screen 2. This means that the light incident into the crack 6 is almost completely absorbed by the crack 6, thus splitting the originally complete light beam into two independent small beams through the crack 6. This is equivalent to artificially creating two adjacent "slits". The two small beams of light separated by the crack 6 are reflected out by themselves, similar to light waves emitted from two points (two sides of the crack). During their propagation, they will overlap and interfere with each other. On the distant CCD photosensitive screen 2, the overlapping area of ​​these two beams of light will form alternating bright and dark interference fringes due to the superposition or cancellation of wave crests and troughs. The density of the fringes directly and uniquely reflects the width of the crack. That is, the denser the fringes, the wider the crack; the sparser the fringes, the narrower the crack. Therefore, by measuring the distance between the centers of adjacent stripes on the CCD photosensitive screen 2, the actual width of the crack 6 can be directly calculated without complex imaging or contact detection. The essence of this invention is to use the phenomenon of light interference to transform the invisible micro-crack width information into a clear, visible and measurable optical stripe pattern.

[0025] Example 2

[0026] This invention provides a method for measuring cracks on metal surfaces based on double-slit interference. Based on Example 1, the measurement platform built in step 1 preferably includes a laser 1. A semi-transparent mirror 4 is set at a 45° angle in front of the direct light path of the laser 1. A displacement stage 13 is set on the reflected light path of the semi-transparent mirror 4. The metal to be measured 5 with its surface perpendicular to the reflected light path of the semi-transparent mirror 4 is placed on the displacement stage 13. A CCD photosensitive screen 2 is set on the back side of the semi-transparent mirror 4, which is coaxial with the reflected light path and parallel to the surface of the metal to be measured 5. An integrating sphere absorber is set on the back side of the semi-transparent mirror 4, which is coaxial with the incident light path of the laser 1, to avoid stray light interference.

[0027] Example 3

[0028] This invention provides a method for measuring cracks on metal surfaces based on double-slit interference. Building upon Example 1, after the measurement platform is constructed, the wavelength λ and width D of the parallel beam emitted by laser 1 are set, and the distance L between the CCD photosensitive screen 2 and the surface of the metal 5 to be measured is adjusted to meet the requirements of equation (1):

[0029] L>D 2 / 2λ(1)

[0030] Example 4

[0031] This invention provides a method for measuring cracks on metal surfaces based on double-slit interference. Based on embodiment 1, step 3 is preferably as follows: turn on the laser 1, and incident a parallel initial incident beam 7 onto a semi-transparent mirror 4. The initial incident beam 7 is transmitted through the semi-transparent mirror 4 to obtain a transmitted incident beam 8, which is then completely absorbed by the absorber 3. The initial incident beam 7 is reflected by the semi-transparent mirror 4 to obtain a reflected incident beam 9, which is perpendicularly irradiated onto the surface of the metal to be tested 5. The metal to be tested is moved by the displacement stage 13 so that the reflected incident beam 9 is gradually scanned along the long side of the metal to be tested 5.

[0032] Example 5

[0033] This invention provides a method for measuring cracks on metal surfaces based on double-slit interference. Based on embodiment 1, step 4 is preferably as follows: the reflected light beam from the surface of the metal to be measured 5 is transmitted through a semi-transparent mirror 4 to obtain a transmitted reflected light beam 10, which then propagates to the CCD photosensitive screen 2. When the reflected incident light beam 9 covers the crack 6, the crack 6 cuts the reflected incident light beam 9 into two independent reflected light beams, namely the left beam 11 and the right beam 12 after the reflected incident light beam is split. When the two split reflected light beams propagate to the CCD photosensitive screen 2, they interfere and superimpose to form alternating bright and dark interference fringes. At this time, it is determined that there is a crack 6 at the current scanning position.

[0034] Example 6

[0035] This invention provides a method for measuring cracks on metal surfaces based on double-slit interference. Based on Example 1, step 5 is preferably: measuring the spacing Δ between adjacent fringes on the CCD sensor 2. x The current crack width d is calculated using equation (2):

[0036] d=λL / Δ x (2)

[0037] Example 7

[0038] This invention provides a method for measuring cracks on metal surfaces based on double-slit interference, such as... Figure 1 As shown, the initial incident beam 7 is a uniform rectangular spot, which can be approximated as a parallel light wave. The length and width of the spot are a and D, respectively. The length and width of the surface of the metal to be tested 5 are c and a, respectively. The long side of the beam is equal to the width of the surface of the metal to be tested 5, both being a. The long side of the reflected incident beam 9 follows the wide side of the surface of the metal to be tested 5, and the reflected incident beam 9 scans gradually from left to right along the long side of the metal to be tested 5.

[0039] When there is no crack in the beam during scanning, the CCD sensor 2 receives a uniform rectangular spot with a width of D. When there is a crack in the beam during scanning, the beam scans from left to right. The crack will split the reflected incident beam 9 into two parts. As scanning continues, the crack will split the reflected incident beam 9 into two beams of equal width. At this time, the width of the left beam 11 after the reflected incident beam is split is D1, and the width of the right beam 12 after the reflected incident beam is split is D2. Therefore, D1 = D2 = (Dd) / 2.

[0040] Calculate the angle formed by the crack width d and the distance L. : ≈tg =d / L. Due to diffraction, the light split into two parts produces a divergence angle of . The divergence of the beam is related to the width and wavelength of the beam itself; the divergence angle between the two parts of the light is... It can be calculated using the following formula: =2λ / D1.

[0041] Based on the principle of light transmission, it can be known that when '> When the two beams overlap, interference fringes are generated in the overlap region. '≤ There were no interference fringes.

[0042] when '> Sometimes, 2λ / D1 > d / L. Therefore, when L is chosen to be sufficiently large, satisfying the following relationship: L > dD1 / 2λ, the two beams of light can coherently superimpose on the photosensitive screen. Replacing D1 and d with D, the relationship becomes:

[0043] L>D 2 / 2λ(1)

[0044] The chosen L must satisfy L>dD1 / 2λ. When a crack exists, the intensity distribution on the CCD sensor 2 will coherently superimpose, resulting in interference fringes. Based on the principles of diffraction and interference, the intensity formula on the CCD sensor 2 is:

[0045]

[0046] Where I0 is the peak light intensity. and These are the parameters for single-slit diffraction and double-slit interference, respectively. , Since L>>D, ≈ .

[0047] The fringe spacing of the interference fringes can be calculated using the intensity formula. : The transformation yields:

[0048] d=λL / Δ x (2)

[0049] As can be seen from equation (2), by measuring the spacing Δ of the interference fringes on the CCD photosensitive screen 2 x Substituting into formula (2), the crack width d can be obtained. The wavelength is determined by the incident beam, and L is determined according to formula (1) to obtain a suitable value.

[0050] Because the number of interference fringes N: Therefore, information about the crack width can also be obtained by observing the number of stripes on the receiving surface; the more stripes there are, the wider the crack is.

[0051] Based on the measurement principle, the measurement should be performed according to the following steps:

[0052] 1) First, follow Figure 1 Set up a test platform on the left. Record the incident wavelength λ and the input light width D, and substitute the values ​​into equation (1) to determine the appropriate L.

[0053] 2) Turn on laser 1 and adjust. Figure 1 The components in the structure ensure that the initial incident beam 7, the reflected incident beam 9, and the transmitted incident beam 8 meet the requirements of the measurement principle.

[0054] 3) Move from left to right (E→C) so that the reflected incident beam 9 diffracts the leftmost side of the surface of the metal under test 5 and scans it to the right step by step.

[0055] 4) Observe the CCD photosensitive screen 2. When a uniform light spot appears, it is considered that there are no cracks on the surface; when uneven stripes begin to appear, it is considered that cracks exist in the initial scan.

[0056] 5) Continue moving the surface of the metal to be tested 5. The interference fringes of the CCD photosensitive screen 2 gradually become symmetrical and uniform. At this time, record the intensity distribution of the CCD photosensitive screen 2.

[0057] 6) The ratio of the double seam spacing to the seam width can be obtained by counting the number of stripes. .

[0058] 7) Measure the distance between the centers of two adjacent bright (or dark) stripes to obtain Δ. x Then, substitute the values ​​into equation (2) to obtain the corresponding crack width d1 and the number of cracks M=1.

[0059] 8) Continue moving the surface of the metal to be tested 5. The symmetrical and uniform stripes become asymmetrical. When the intensity distribution of the CCD photosensitive screen 2 becomes uniform and there are no more interference stripes, it indicates that the beam scanning area has completely transitioned from the crack to the crack-free area.

[0060] 9) Continue moving the surface of the metal to be tested 5 and repeat steps 4) to 8). Record the measured crack widths as d2, d3, etc. For each crack width value measured, the M number increases by 1.

Claims

1. A method for measuring cracks on metal surfaces based on double-slit interference, characterized in that, First, a parallel laser beam is incident on a semi-transparent mirror. The incident beam is reflected and perpendicularly illuminates the surface of the metal under test. The beam reflected from the surface of the metal under test is then transmitted through the semi-transparent mirror to a CCD sensor screen at a distance L from the surface of the metal under test. Second, the surface of the metal under test is moved so that the incident beam scans along its long side. When the incident beam covers the crack, the crack cuts the incident beam into two independent reflected beams. When the two split reflected beams propagate to the CCD sensor screen, they interfere and superimpose to form alternating bright and dark interference fringes. Finally, the crack width is calculated by measuring the center distance between adjacent interference fringes on the CCD sensor screen, combined with the laser beam wavelength and the distance L.

2. The method for measuring cracks on metal surfaces based on double-slit interference as described in claim 1, characterized in that, The measurement platform includes a laser (1), a semi-transparent mirror (4) is set at a 45° angle in front of the direct light path of the laser (1), a displacement stage (13) is set on the reflected light path of the semi-transparent mirror (4), the metal to be measured (5) is placed on the displacement stage (13) with its surface perpendicular to the reflected light path of the semi-transparent mirror (4), and a CCD photosensitive screen (2) is set on the back side of the semi-transparent mirror (4) that is coaxial with the reflected light path and parallel to the surface of the metal to be measured (5).

3. The method for measuring cracks on metal surfaces based on double-slit interference as described in claim 2, characterized in that, The back side of the semi-transparent mirror (4) is provided with an absorber (3) that is coaxial with the incident light path of the laser (1).

4. The method for measuring cracks on metal surfaces based on double-slit interference as described in claim 3, characterized in that, The light absorber (3) is an integrating sphere.

5. The method for measuring cracks on metal surfaces based on double-slit interference as described in claim 1, characterized in that, The distance L satisfies the requirement of equation (1): L>D 2 / 2λ(1) In equation (1), D is the width of the parallel laser beam and λ is the wavelength of the parallel laser beam.

6. The method for measuring cracks on metal surfaces based on double-slit interference as described in claim 1, characterized in that, The surface of the metal to be tested (5) is moved by a displacement stage (13) so that the incident beam reflected by the semi-transparent mirror (4) scans the long side of the metal step by step.

7. The method for measuring cracks on metal surfaces based on double-slit interference as described in claim 1, characterized in that, When alternating bright and dark interference fringes appear on the CCD photosensitive screen (2), it is determined that there is a crack (6) at the current scanning position.

8. The method for measuring cracks on metal surfaces based on double-slit interference as described in claim 1, characterized in that, The width d of the crack (6) is calculated using equation (2): d=λL / Δ x (2) In equation (2), λ is the wavelength of the parallel laser beam, and Δ x The distance between adjacent stripes on the CCD photosensitive screen (2) is measured.

Images