Method for positioning and detecting in-plane crack defects of CFRP composite material based on rectangular differential probe

Abstract

The invention discloses a method for positioning and detecting in-plane crack defects of a CFRP composite material based on a rectangular differential probe, and relates to the technical field of engineering nondestructive testing. Comprising the steps that a to-be-detected damage-free CFRP laminated board workpiece is flatly placed on a table top and fixed, and a rectangular differential probe is placed at a preset position; a sine excitation current signal is applied to enable the eddy current probe to rotate around a central rotating shaft for one time until the probe rotates for one circle, a differential signal and a voltage signal of each rotating position are recorded, and the part with the numerical value smaller than 0.1 in the normalized differential signal is combined with the voltage change at the corresponding moment to obtain the fiber orientation and the up-and-down position relation of the CFRP laminated board. And arranging a CFRP laminated board containing longitudinal surface cracks, determining fiber orientation, performing C scanning, and accurately positioning crack defects. According to the method, the technical limitation that the unidirectional / orthogonal CFRP in-plane cracks are difficult to detect at high precision in an existing eddy current method is broken through, and rapid and accurate positioning of the defects is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of engineering nondestructive testing, and in particular to a method for detecting in-plane crack defects of CFRP composite materials based on rectangular differential probe positioning. Background Art

[0002] With the continuous development of industry and materials manufacturing technology, the new materials industry has received great attention and strong support from the government. Carbon fiber reinforced polymer (CFRP), as a perfect combination of carbon fiber reinforcement and resin matrix, has lightweight properties such as high specific strength and high specific stiffness. In recent years, it has been widely used in many fields such as aerospace, electric power, and automobiles.

[0003] However, the performance of composite structures can be compromised by the external forces generated by long-term impact or alternating loads. Whether defects originating during the manufacturing process or damage incurred during use, simple visual inspection cannot identify them. Therefore, timely flaw detection of CFRP is crucial for ensuring its safe, reliable, and high-performance application. Eddy current nondestructive testing technology, with its low cost, rapid testing speed, and portability, has demonstrated significant application value in CFRP defect detection.

[0004] Crack damage is one of the most common types of damage in CFRP. Eddy current methods have been used to detect this type of damage, demonstrating good sensitivity and accuracy in surface crack detection. However, due to the skin effect of eddy current methods and the unique anisotropic conductivity of CFRP, existing absolute and TR probes often provide poor detection results when detecting in-plane (subsurface) cracks in CFRP. The resulting crack defect image is significantly larger than the actual defect length and width, resulting in low sensitivity.

[0005] Therefore, it is an urgent problem for those skilled in the art to provide a method for detecting in-plane crack defects of CFRP composite materials based on rectangular differential probe positioning to solve the difficulties existing in the prior art. Summary of the Invention

[0006] In view of this, the present invention provides a method for detecting in-plane crack defects in CFRP composite materials based on rectangular differential probe positioning, which breaks through the technical limitation of the existing eddy current method that is difficult to detect unidirectional / orthogonal in-plane cracks in CFRP with high precision, and realizes rapid and accurate positioning of defects.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A method for detecting in-plane crack defects in CFRP composite materials based on rectangular differential probe positioning includes the following steps:

[0009] Place the undamaged CFRP laminate workpiece to be tested flat on the marble platform and fix it, and place the rectangular differential probe at the preset position;

[0010] A magnitude of A is applied to the excitation coil. jl The eddy current probe is rotated once around the central rotation axis by Δθ until the probe returns to the initial rotation position after one rotation, and the differential signal V at each rotation position is recorded. c and the voltage signal of the receiving coil;

[0011] A normalized differential signal polar coordinate graph is constructed based on the differential signal, and a voltage change line graph is constructed based on the voltage signal. The fiber orientation layer of the CFRP is directly obtained based on the portion with a larger value in the normalized differential signal polar coordinate graph. The fiber orientation of the CFRP laminate and the upper and lower positional relationship of each fiber orientation layer are obtained by combining the portion with a value less than 0.1 in the normalized differential signal polar coordinate graph with the voltage change line graph trend at the corresponding moment.

[0012] A CFRP laminate with a longitudinal crack is set up. The probe distance is kept constant. The probe is scanned in the C direction after the fiber orientation of the layer containing the longitudinal crack. The crack defect is accurately located based on the obtained process point signal changes and the corresponding probe position relationship.

[0013] Optionally, the excitation coil and the receiving coil are rectangular coils, and the receiving coil includes a first receiving coil and a second receiving coil, which have the same specifications, and the corresponding receiving signals are V js1 and V js2 .

[0014] Optionally, the receiving signals V of the first receiving coil and the second receiving coil js1 and V js2 The expression is:

[0015]

[0016] Among them, Φ1 and Φ2 are the magnetic flux passing through the first receiving coil and the second receiving coil respectively, M1 and M2 are the number of coil turns, B1 and B2 are the magnetic induction intensities passing through the receiving coils, S1 and S2 are the effective cross-sectional areas of the vertical magnetic field of the two receiving coils, and the magnetic induction intensities B1 and B2 passing through the receiving coils are the magnetic field B generated by the excitation source and the eddy current. s With B e Working together, the corresponding expression is:

[0017] B1=B S1 -B e1 , B2=B S2 -B e2 .

[0018] Optional, differential signal V c The initial signal of the receiving coil is obtained by demodulating it with a lock-in amplifier. The corresponding expression is:

[0019] V c =V js1 -V js2 .

[0020] Optionally, determining the fiber orientation includes: when the symmetry axis of the probe is consistent with the fiber orientation direction of the unidirectional / orthogonal plate, the magnetic induction intensities B1 and B2 of the receiving coils are the same, so the voltage signals are the same and the differential signal is zero.

[0021] Optionally, determining the fiber orientation of a portion of the polar coordinate diagram of the normalized differential signal having a value less than 0.1 includes:

[0022] A voltage drop in a single receiving coil indicates that the eddy current effect is enhanced and the receiving coil is close to the fiber orientation. A value close to 0 indicates that the receiving voltages of the two receiving coils are almost the same and the differential signal is close to 0.

[0023] Optionally, locating the crack defect location includes:

[0024] The absolute value of the detected differential signal will show a trend of first increasing and then decreasing, and its inflection point locates the crack defect position.

[0025] It can be seen from the above technical solution that, compared with the existing technology, the present invention provides a method for detecting in-plane crack defects in CFRP composite materials based on rectangular differential probe positioning, which has the following beneficial effects: the present invention suppresses the lift-off effect by proposing a differential receiving coil structure, and can quickly determine the CFRP fiber orientation by proposing a rotating probe to observe the differential signal changes. The sensitivity of in-plane longitudinal crack detection is greatly improved by designing a rectangular coil and a differential receiving coil structure, and the fast and accurate positioning of defects is achieved in conjunction with the signal change law, breaking through the technical limitation of the existing eddy current method that is difficult to detect unidirectional / orthogonal CFRP in-plane cracks with high precision. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] 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 or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0027] Figure 1 A block diagram of a method for detecting in-plane crack defects in CFRP composite materials based on rectangular differential probe positioning disclosed in the present invention;

[0028] Figure 2This is a schematic diagram of the placement of the eddy current probe and the CFRP laminate in the initial state disclosed by the present invention;

[0029] Figure 3 A top view of the placement of the eddy current probe and the CFRP laminate in the initial state disclosed in the present invention;

[0030] Figure 4 This is a schematic diagram of the probe rotation disclosed in the present invention;

[0031] Figure 5 A detailed structural diagram of a CFRP laminate disclosed in an embodiment of the present invention;

[0032] Figure 6 A polar coordinate diagram of a normalized differential signal obtained during the rotation of the probe disclosed in an embodiment of the present invention;

[0033] Figure 7 Schematic diagram of the specifications and locations of the preset CFRP in-plane crack defects disclosed in the embodiment of the present invention;

[0034] Figure 8 A fitting curve diagram of the voltage change of a single receiving coil during the rotation of the probe disclosed in an embodiment of the present invention;

[0035] Figure 9 A schematic diagram of defect C scan disclosed in an embodiment of the present invention;

[0036] Figure 10 This is a line graph of the differential signal change obtained by scanning along the defect width direction when x=1 and -1 mm according to an embodiment of the present invention;

[0037] Figure 11 This is a line graph of the differential signal change obtained by scanning along the defect length direction when y=1 and -1 mm according to an embodiment of the present invention;

[0038] Among them, 1 is the excitation coil, 2 is the first receiving coil, 3 is the second receiving coil, 4 is the 8-layer CFRP laminate to be tested, 41 is the 1st and 2nd layers of 90° fiber orientation boards, 42 is the 3rd and 4th layers of 0° fiber orientation boards, 43 is the 5th and 6th layers of 90° fiber orientation boards, and 44 is the 7th and 8th layers of 0° fiber orientation boards. DETAILED DESCRIPTION

[0039] 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.

[0040] Reference Figure 1As shown, the present invention discloses a method for detecting in-plane crack defects of CFRP composite materials based on rectangular differential probe positioning, comprising the following steps:

[0041] Place the undamaged CFRP laminate workpiece to be tested flat on the marble platform and fix it, and place the rectangular differential probe at the preset position Δh. Figure 2 and Figure 3 As shown;

[0042] Apply an amplitude of A to the excitation coil 1. jl The sine excitation current signal is then used to make the eddy current probe rotate once Δθ around the central rotation axis. The rotation direction is as follows: Figure 4 As shown, until the probe rotates one circle back to the initial rotation position, record the differential signal V at each rotation position c and the voltage signal of the receiving coil;

[0043] A normalized differential signal polar coordinate graph is constructed based on the differential signal, and a voltage change line graph is constructed based on the voltage signal. The fiber orientation layer of the CFRP is directly obtained based on the portion with a larger value in the normalized differential signal polar coordinate graph. The fiber orientation of the CFRP laminate and the upper and lower positional relationship of each fiber orientation layer are obtained by combining the portion with a value less than 0.1 in the normalized differential signal polar coordinate graph with the voltage change line graph trend at the corresponding moment.

[0044] A CFRP laminate with a longitudinal crack is set up. The probe distance is kept constant. The probe is scanned in the C direction after the fiber orientation of the layer containing the longitudinal crack. The crack defect is accurately located based on the obtained process point signal changes and the corresponding probe position relationship.

[0045] Furthermore, Δh=0.2-1mm, the overall thickness H of the CFRP laminate is 0.8-2mm, the number of layers N is 8-16, the single rotation angle Δθ is 15°, and A jl 0.5-2A, longitudinal crack specification length a = 5-20mm, width b = 0.2-1mm, thickness c = 0.3mm,

[0046] Furthermore, the excitation coil 1 and the receiving coil are rectangular coils, and the receiving coil includes a first receiving coil 2 and a second receiving coil 3, which have the same specifications, and the corresponding receiving signals are V js1 and V js2 .

[0047] Furthermore, the receiving signals V of the first receiving coil 2 and the second receiving coil 3 are js1 and V js2 The expression is:

[0048]

[0049] Among them, Φ1 and Φ2 are the magnetic flux passing through the first receiving coil 2 and the second receiving coil 3 respectively, M1 and M2 are the number of coil turns, B1 and B2 are the magnetic induction intensities passing through the receiving coils, S1 and S2 are the effective cross-sectional areas of the vertical magnetic field of the two receiving coils, and the magnetic induction intensities B1 and B2 passing through the receiving coils are composed of the magnetic field B generated by the excitation source and the eddy current. s With B e Working together, the corresponding expression is:

[0050] B1=B S1 -B e1 , B2=B S2 -B e2 .

[0051] Specifically, since the specifications of the first receiving coil 2 and the second receiving coil 3 are exactly the same, M1 = M2 and S1 = S2.

[0052] Furthermore, the differential voltage signal V c The initial signal of the receiving coil is obtained by demodulating it with a lock-in amplifier. The corresponding expression is:

[0053] V c =V js1 -V js2 .

[0054] Furthermore, determining the fiber orientation includes: when the probe's symmetry axis is consistent with the unidirectional / orthogonal plate fiber orientation direction, the receiving coil magnetic induction intensities B1 and B2 are the same, so the voltage signals are the same and the differential signal is zero.

[0055] Furthermore, judging the fiber orientation of the portion of the polar coordinate diagram of the normalized differential signal having a value less than 0.1 includes:

[0056] A voltage drop in a single receiving coil indicates that the eddy current effect is enhanced and the receiving coil is close to the fiber orientation. A value close to 0 indicates that the receiving voltages of the two receiving coils are almost the same and the differential signal is close to 0.

[0057] Specifically, the precise determination of uncertain fiber orientation based on the trend of the voltage curve obtained during the rotation of a single receiving coil is based on the electromagnetic induction principle of eddy current testing. Simply stated, when a single receiving coil approaches a fiber orientation, the enhanced eddy current effect (electromagnetic induction) causes an increase in the secondary magnetic field, resulting in a decrease in the receiving coil voltage. However, when moving away from the fiber orientation, the voltage increases due to a weakening of the secondary magnetic field. The vertical positional relationship of each fiber orientation layer is determined based on the voltage of the single receiving coil at each fiber orientation. A lower voltage indicates a stronger eddy current effect, and the fiber orientation layer is closer to the probe.

[0058] Furthermore, locating the crack defect position includes:

[0059] The absolute value of the detected differential signal will show a trend of first increasing and then decreasing, and its inflection point locates the crack defect position.

[0060] Specifically, the probe's symmetry axis is rotated to the fiber orientation direction of the layer where the crack is located to maintain high signal sensitivity. The differential receiving coil structure will cause the excitation coil 1 to start to cause a significant difference in the secondary magnetic field at the two receiving coils when it approaches the defect in the width direction, until the probe's symmetry axis coincides with the defect's width symmetry axis, and the differential signal is zero. Then the excitation coil 1 gradually moves away from the defect. The differential signal in this process will have the same change trend as the process from the excitation coil 1 reaching the defect until the probe's symmetry axis coincides with the defect's width symmetry axis, but the values ​​are opposite. The width-direction point differential signal V caused by the excitation coil 1 reaching the defect in the width direction until it completely moves out of the defect c The trend is to first increase significantly, then decrease to zero (at which point the two axes coincide), then decrease again, and then increase significantly again. This segment of the line graph is antisymmetric about the node where the two axes coincide (i.e., the value is zero). During multiple length-direction scans, if the excitation coil 1 can enter the defect at a certain point and the two axes do not coincide, the absolute value of the differential signal will first increase and then decrease from the time the excitation coil 1 reaches the defect to the time it completely exits the defect. Based on these signal changes and the corresponding coil position, the in-plane defect position can be accurately located.

[0061] In a specific embodiment, Δh=0.5 mm, and the CFRP laminate is laid as follows Figure 5 As shown, it is [90°2, 0°2]2, the total thickness H is 1.2mm, and the number of layers N is 8. The excitation coil 1 and the two receiving coils have the same specifications, with a length and width of 2mm and a thickness of 0.8mm. The probe single rotation angle Δθ is 15°, and the excitation A is applied. jl 1A. Longitudinal crack specifications are as follows Figure 7 As shown, the length a = 8 mm, the width b = 1 mm, the thickness c = 0.3 mm, the position is at the center of the CFRP plate, and the depth is located in the 3rd and 4th layers of the plate (that is, in the plane). The specific test steps are as follows:

[0062] Place the undamaged CFRP laminate workpiece to be tested flat on a marble platform and secure it. Place the designed eddy current probe in the initial position, where the center of the probe coincides with the center of the CFRP plate, and lift it to a position 0.5 mm above the upper surface of the CFRP laminate.

[0063] Apply a sinusoidal excitation current signal with an amplitude of 1A to the excitation coil 1, and then Figure 5As shown, the eddy current probe is rotated 15° around the central rotation axis until the probe completes a full rotation and returns to the initial rotation position. The differential signal V at each rotation position is recorded. c And the signal of a single receiving coil, the voltage of the first receiving coil 2 is V js1 , the voltage of the second receiving coil 3 is V js2 The obtained normalized differential signal polar coordinate image is as follows: Figure 6 As shown in the figure, it can be seen that when the rotation angle is 0°, 180°, 90°, and 270°, the differential signal is almost less than 0.1, and the values ​​are also very small at 45°, 135°, 225°, and 315°. In other words, 0° and 90° must be the fiber orientation of CFRP, while 45° and -45° may be the fiber orientation of CFRP.

[0064] Reference Figure 8 As shown in the fitted curve of the voltage change of a single receiving coil during the rotation of the probe, it is observed that the voltage change trend of the second receiving coil 3 increases when the probe symmetry axis is from 45° to 60° (at this time, the second receiving coil 3 is gradually approaching the 45° direction), indicating that the secondary magnetic field is weakened for the second receiving coil 3 during this period, and it is gradually moving away from the fiber orientation, which means that the 45° direction is not the fiber orientation. Similarly, the voltage change trend of the second receiving coil 3 increases when the probe symmetry axis is from 135° to 150° (at this time, the second receiving coil 3 is gradually approaching the 135° direction), indicating that -45° is not the fiber orientation either. It can be determined that the fiber orientation of the measured CFRP is 0° and 90°. Subsequently, the voltages of any single coil when the probe symmetry axis is 90° and 0° are 7.19×10 -3 V and 5.81×10 -3 V, indicating that the 90° fiber layer is on the top and the 0° fiber layer is on the bottom;

[0065] Keeping the probe distance unchanged, replace the CFRP laminate with the same specifications that has a 10×1×0.3mm longitudinal in-plane crack. According to the layer where the defect is located (i.e., layers 3 and 4, where the fiber orientation is determined to be 0°), rotate the probe symmetry axis to the 0° direction to maintain high signal sensitivity. Figure 9 The defect C scan experiment is carried out as shown in the figure, where the midpoint of the defect is the zero point of the coordinate axis. In this embodiment, the scan is from (-9, -4.5) to (9, 4.5). Due to the excessive amount of data for all point positions, two groups of representative differential signals V are selected in the width direction. c , a line graph of changes is drawn as Figure 10As shown in the figure, it can be observed that the differential signal has an obvious increase from near 0 when the y-axis point is -1.5, indicating that the excitation coil 1 has just entered the defect at this time. The differential signal from -1.5 to 0 increases first and then decreases to near 0, and then the voltage from 0 to 1.5 shows an antisymmetric trend with the -1.5 to 0 section, which is consistent with the theoretical derivation, indicating that when the y-axis point is 1.5, the excitation coil 1 has just walked out of the defect. Based on this, the width of the crack defect is calculated to be 1mm. Two groups of representative differential signals V are selected in the length direction. c , a line graph of changes is drawn as Figure 11 As shown, it can be seen that for both sets of data, the absolute value of the differential signal increases significantly when the x-point is -3, and decreases significantly when the x-point is 7. These two points correspond to the excitation coil 1 just entering the defect and just exiting the defect, respectively. The calculated length of the crack defect is 10 mm.

[0066] In this embodiment, the proposed rectangular differential probe method was successfully used to accurately locate the in-plane longitudinal crack defect with a length of 10 mm and a width of 1 mm in the 3rd and 4th layers of CFRP.

[0067] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for detecting in-plane crack defects in CFRP composite materials based on rectangular differential probe positioning, characterized in that: The following steps are involved: Place the undamaged CFRP laminate workpiece to be tested flat on the marble platform and fix it, and place the rectangular differential probe at the preset position; Apply a voltage of amplitude A to the excitation coil (1). jl The eddy current probe is rotated once around the central rotation axis by Δθ until the probe returns to the initial rotation position after one rotation, and the differential signal V at each rotation position is recorded. c and the voltage signal of the receiving coil; A normalized differential signal polar coordinate diagram is constructed based on the differential signal, a voltage change line graph is constructed based on the voltage signal, and the fiber orientation layer of the CFRP is directly obtained based on the larger value part of the normalized differential signal polar coordinate diagram; The fiber orientation of the CFRP laminate and the upper and lower position relationship of each fiber orientation layer are obtained by combining the voltage change line graph trend at the corresponding moment with the value less than 0.1 in the polar coordinate diagram of the normalized differential signal; A CFRP laminate with a longitudinal crack is set up. The probe distance is kept constant. The probe is scanned in the C direction after the fiber orientation of the layer containing the longitudinal crack. The crack defect is accurately located based on the obtained process point signal changes and the corresponding probe position relationship.

2. The method for detecting in-plane crack defects of CFRP composite materials based on rectangular differential probe positioning according to claim 1 is characterized in that: The exciting coil (1) and the receiving coil are rectangular coils. The receiving coil includes a first receiving coil (2) and a second receiving coil (3). The specifications are exactly the same. The corresponding receiving signals are V js1 and V js2 .

3. The method for detecting in-plane crack defects of CFRP composite materials based on rectangular differential probe positioning according to claim 2 is characterized in that: The receiving signal V of the first receiving coil (2) and the second receiving coil (3) js1 and V js2 The expression is: Wherein, Φ1 and Φ2 are the magnetic fluxes passing through the first receiving coil (2) and the second receiving coil (3), respectively; M1 and M2 are the number of coil turns; B1 and B2 are the magnetic induction intensities passing through the receiving coils; S1 and S2 are the effective cross-sectional areas of the vertical magnetic fields of the two receiving coils; the magnetic induction intensities B1 and B2 passing through the receiving coils are jointly affected by the magnetic fields Bs and Be generated by the excitation source and the eddy currents, and the corresponding expressions are: B1=B S1 -B e1 ,B2=B S2 -B e2 。 4. The method for detecting in-plane crack defects of CFRP composite materials based on rectangular differential probe positioning according to claim 3 is characterized in that: Differential signal V c The initial signal of the receiving coil is obtained by demodulating it with a lock-in amplifier. The corresponding expression is: V c =V js1 -V js2 。 5. The method for detecting in-plane crack defects of CFRP composite materials based on rectangular differential probe positioning according to claim 4 is characterized in that: The fiber orientation is determined as follows: when the probe's axis of symmetry is consistent with the fiber orientation direction of the unidirectional / orthogonal plate, the magnetic induction intensities B1 and B2 of the receiving coils are the same, so the voltage signals are the same and the differential signal is zero.

6. The method for detecting in-plane crack defects of CFRP composite materials based on rectangular differential probe positioning according to claim 4 is characterized in that: The fiber orientation is determined for the portion of the polar coordinate diagram of the normalized differential signal with a value less than 0.1, including: A voltage drop in a single receiving coil indicates that the eddy current effect is enhanced and the receiving coil is close to the fiber orientation. A value close to 0 indicates that the receiving voltages of the two receiving coils are almost the same and the differential signal is close to 0.

7. The method for detecting in-plane crack defects of CFRP composite materials based on rectangular differential probe positioning according to claim 1 is characterized in that: Locating crack defect locations includes: The absolute value of the detected differential signal will show a trend of first increasing and then decreasing, and its inflection point locates the crack defect position.

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