Laser ultrasonic sensitive mode selection method for defects with different buried depths in spacecraft grid rib structure

Abstract

The present invention relates to a method for selecting laser ultrasonic sensitive modes for defects of different buried depths in a spacecraft grid rib structure, including: 1) establishing a three-dimensional finite element simulation model of a spacecraft grid rib structure; 2) performing simulation settings on the spacecraft grid rib structure; 3) simulation calculation; 4) performing integral calculation on the wave packet in the time domain to obtain energy; 5) constructing a laser ultrasonic signal detection system based on defects of different buried depths in the spacecraft grid rib structure; 6) performing tests on the spacecraft grid rib structure and comparing the simulation results to determine the selection of laser ultrasonic sensitive modes for defects of different buried depths in the spacecraft grid rib structure. The present invention highlights useful signal information, removes noise interference, and reduces the difficulty of signal analysis; by combining simulation and experiment, a laser ultrasonic sensitive mode that meets the requirements is obtained through conditional judgment, and is verified by experiment to ensure the reliability and accuracy of the method.

Description

Technical Field

[0001] The invention belongs to the technical field of laser ultrasonic nondestructive testing, and in particular relates to a laser ultrasonic sensitive mode selection method for defects with different buried depths in a spacecraft grid rib structure. Background Art

[0002] As an emerging non-destructive testing method, laser ultrasonic technology is widely used in various defect detection due to its flexible laser sound source, convenient operation, non-contact detection and other characteristics. In addition, since the ultrasonic pulse width excited by the pulsed laser is very narrow, multiple modes of ultrasonic waves can be excited at one time, such as surface waves, lamb waves, longitudinal waves, etc., which can realize rapid detection of the object being tested.

[0003] As manned or unmanned spacecraft, spacecraft has a series of complex structural and functional requirements. The spacecraft cabin has complex structural features such as thin-wall reinforcement and local variable thickness, which makes the defects very diverse. The grid and ribs in the spacecraft grid rib structure are different in structural design and function. In the spacecraft grid rib structure, the grid is a space frame structure composed of a series of intersecting transverse and longitudinal components. These components are generally straight segments or curved segments, which are fixed together by welding, connection or bolts to form a stable grid structure. The function of the grid is to provide the rigidity and stability of the spacecraft as a whole, bear the load and maintain the shape of the cabin. The grid structure can effectively disperse the load and has high strength and stiffness. Rib is a component in the spacecraft grid rib structure. It is usually located at the edge or inside of the grid structure and extends along the transverse and longitudinal directions of the grid. The function of the rib is to increase the stiffness and strength of the grid structure and prevent the grid structure from buckling or deformation when subjected to force. Rib generally has higher strength and stiffness and can withstand larger axial and bending forces. Generally speaking, the thickness is thicker at the reinforcement, while the grid is more thin-walled.

[0004] Therefore, due to the complex characteristics of spacecraft grid rib defects, it is difficult to characterize defects at different burial depths using a single mode of ultrasonic wave. Laser ultrasonic wave can excite multiple modes of ultrasonic wave at one time. Therefore, it is very important to select the appropriate laser ultrasonic sensitive mode for the detection of defects at different burial depths of spacecraft grid rib structures.

[0005] A search of published patent documents did not find any published patent documents identical to the present patent application. Summary of the invention

[0006] The purpose of the present invention is to overcome the shortcomings of the prior art and provide a laser ultrasonic sensitive mode selection method for defects of different buried depths in a spacecraft grid rib structure, so as to detect defects of different buried depths in a spacecraft grid rib structure by extracting sensitive modes.

[0007] The present invention solves the technical problem by the following technical solutions:

[0008] 1. A laser ultrasonic sensitive mode selection method for defects of different buried depths in a spacecraft grid rib structure, characterized in that the steps of the method are:

[0009] S1. Taking the spacecraft grid rib structure as the experimental object, a three-dimensional finite element simulation model of the spacecraft grid rib structure is established using simulation software according to its material properties and dimensions;

[0010] S2. The above-mentioned spacecraft grid rib structure is equivalent to a combination of a grid structure and a ribbed structure, where the connection between the ribbed root and the grid is considered to be a grid structure, and simulation settings are performed, specifically:

[0011] a) For the reinforced structure, multiple defect models with the same size and different burial depths and damage-free models are established. The defects are set at different burial depths inside the reinforcement. The center of the reinforced structure is selected as the excitation point. The optimal excitation laser radius and laser rise time and other parameters are selected for excitation. A receiving point is set on the opposite side of the excitation point to receive the signal.

[0012] b) For the grid structure, multiple defect models with the same size and different burial depths and damage-free models are established. The defects are set up at different burial depths inside the grid. The center of the grid structure is selected as the excitation point. The optimal excitation laser radius and laser rise time and other parameters are selected for excitation. A receiving point is set on the opposite side of the excitation point to receive the signal.

[0013] S3, perform simulation calculation, set the preselected surface wave time domain segment, and extract the surface wave displacement of the receiving point in the case of defects at different burial depths and without damage through a bandpass filter;

[0014] S4, integrating the wave packet in the time domain to obtain the energy, specifically:

[0015] a) For reinforced structures, the ratio k1 of the energy of the receiving point at different burial depths and that at no damage in this time domain is calculated. When the ratio k1 is less than the set threshold TH, the surface wave is selected as the sensitive mode; if not, the longitudinal wave is selected as the sensitive mode;

[0016] b) For the grid structure, find the lamb wave with the main frequency component corresponding to the dispersion curve, determine the lamb wave mode, extract and integrate the energy ratio k2 of the receiving point under different burial depths and the undamaged state in the lamb wave time domain segment and the energy ratio k1 of the receiving point under different burial depths and the undamaged state in the surface wave time domain segment, and select the modal ultrasonic wave with the smaller ratio as the sensitive mode;

[0017] S5. Construct a laser ultrasonic signal detection system based on defects of different buried depths of a spacecraft grid rib structure, the system comprising a spacecraft, a mobile turntable, an AGV platform vehicle, a laser and an optical system, a laser vibrometer, a robotic arm, and a computer. The laser and the optical system constitute an excitation unit, and the laser vibrometer serves as a receiving unit. The excitation unit and the receiving unit are supported by the robotic arm and placed on the AGV platform vehicle with the computer for longitudinal movement, and the spacecraft is placed on the mobile turntable and rotates laterally.

[0018] S6. Use a pulse signal as an excitation signal to test the spacecraft grid rib structure. Use the method of S4 to obtain different modal acoustic wave signals and perform energy calculations, and compare them with the simulation results. If the results are consistent, the simulation realizes the selection of laser ultrasonic sensitive modes for defects of different buried depths of spacecraft grid ribs. If they are inconsistent, adjust the simulation model parameters according to the test results to determine the selection of laser ultrasonic sensitive modes for defects of different buried depths of spacecraft grid rib structure.

[0019] Moreover, the expression for calculating the energy of the reinforced structure in S4 a) is:

[0020] E0=∫(|x0(t) 2 |)dt

[0021] E1=∫(|x1(t) 2 |)dt

[0022] in:

[0023] E0 is the energy of the receiving point in the reinforced structure without loss;

[0024] E1 is the receiving point energy in the lossy case of the reinforced structure;

[0025] x0(t) is the time domain signal of the receiving point in the reinforced structure without loss;

[0026] x1(t) is the time domain signal of the receiving point in the case of loss in the reinforced structure;

[0027] The expression of energy ratio is:

[0028]

[0029] Where: k1 is the ratio of the energy of the receiving point at different burial depths in the reinforced structure to that at the undamaged state

[0030] The expression for calculating the grid structure energy in b) of S4 is:

[0031] E a =∫(|x a (t) 2 |)dt

[0032] E b =∫(|x b (t) 2 |)dt

[0033] in:

[0034] E a The energy of the receiving point in the grid structure without loss;

[0035] E b The receiving point energy in the case of loss in the grid structure;

[0036] x a (t) is the time domain signal of the receiving point in the grid structure without loss;

[0037] x b (t) is the time domain signal of the receiving point in the case of loss in the grid structure;

[0038] The expression of energy ratio is:

[0039]

[0040] Where: k2 is the ratio of the energy of the receiving point at different burial depths in the grid structure to that at the undamaged state.

[0041] The advantages and beneficial effects of the present invention are:

[0042] 1. The present invention provides a laser ultrasonic sensitive mode selection method for defects of different buried depths in a spacecraft grid rib structure, which highlights useful signal information, removes noise interference, and reduces the difficulty of signal analysis;

[0043] 2. The present invention provides a laser ultrasonic sensitive mode selection method for defects of different buried depths in a spacecraft grid rib structure. By combining simulation and experiment, the laser ultrasonic sensitive mode that meets the requirements is obtained through conditional judgment, and is verified by experiment to ensure the reliability and accuracy of the method.

[0044] 3. The present invention provides a method for selecting laser ultrasonic sensitive modes for defects of different buried depths in a spacecraft grid rib structure, which comprehensively considers factors such as signal stability and time domain characteristics to determine the laser ultrasonic sensitive mode. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 is a flow chart of the present invention;

[0046] Figure 2 It is a schematic diagram of the reinforcement structure of the three-dimensional finite element simulation model of the present invention;

[0047] Figure 3 A schematic diagram of a grid structure of a three-dimensional finite element simulation model of the present invention;

[0048] Figure 4 It is a schematic diagram of the laser ultrasonic signal detection system of the present invention. DETAILED DESCRIPTION

[0049] The present invention is further described in detail below through specific examples. The following examples are only illustrative and not restrictive, and the protection scope of the present invention cannot be limited thereto.

[0050] like Figure 1 As shown, a laser ultrasonic sensitive mode selection method for defects of different buried depths in a spacecraft grid rib structure is innovative in that the steps of the method are:

[0051] 1. Taking the spacecraft grid rib structure as the experimental object, considering its material properties such as elastic modulus, Poisson's ratio and density and other parameters into the simulation model, according to its actual geometric size, the COMSOL 3D simulation software is used to establish a 3D finite element simulation model. Based on the minimum wavelength in different ultrasonic modes excited by laser ultrasonic, the COMSOL calculation grid size is set to be less than 1 / 4 of the minimum wavelength, and the minimum calculation time step is also guaranteed to be less than 1 / 180 of the modal period, so as to improve the calculation accuracy of the model.

[0052] The parameters of the spacecraft bulkhead grid ribs are set as follows: elastic modulus 68 GPa, Poisson's ratio 0.33, density 2.68 g / mm 3

[0053] The simulated pulse laser rise time is 8ns, the laser radius is 0.5mm, and the excitation signal expression is:

[0054] I=I0f(r)g(t)

[0055]

[0056]

[0057] Where: I is the excitation signal;

[0058] I0 is the peak power density;

[0059] a0 is the spot radius of laser irradiation;

[0060] t0 is the pulse laser rise time.

[0061] In the established finite element simulation model, the basis for mesh division is that the mesh size needs to meet the physical characteristics of laser ultrasonic propagation and the accuracy requirements of displacement. In the simulation of laser ultrasonic, the mesh size should meet

[0062]

[0063] Where: min The minimum wavelength of the excited ultrasonic wave;

[0064] At the same time, during the action of laser ultrasound, the laser rise time is in the nanosecond level. In order to ensure the time step calculation and provide sufficient time resolution, the time inequality needs to be satisfied:

[0065]

[0066] Where: f max The maximum frequency of ultrasonic excitation.

[0067] According to the laser ultrasonic thermoelastic effect, and the absorption of laser energy by spacecraft materials mainly occurs in the surface area, the laser can be regarded as a boundary heat source. The laser light source beam is generally a Gaussian beam. The appropriate laser radius, laser power and other parameters are continuously adjusted to determine the optimal laser excitation conditions and improve the signal-to-noise ratio of the laser ultrasonic signal under the thermoelastic effect.

[0068] 2. The spacecraft grid rib structure is equivalent to a combination of a grid structure and a reinforced structure, where the reinforcement root and the grid connection are considered to be a grid structure, and the simulation settings are carried out as follows:

[0069] a) If defects of different buried depths in the reinforced structure are detected, five defect models with the same buried depth and size and a damage-free model are established. A rectangular defect is used to simulate the crack damage in the reinforced structure. The center of the reinforced structure is selected as the excitation point. The optimal excitation laser radius of 0.5 mm and the laser rise time of 8 ns are selected for excitation. The defect is set at a distance of 1 mm from the excitation point in the positive x direction. A receiving point is set at a distance of 2 mm from the excitation point in the positive x direction to receive the signal. A bandpass filter is used to filter the received signal to reduce noise and interference, such as Figure 2 As shown;

[0070] b) If defects with different buried depths in the grid structure are to be detected, five defect models with the same buried depth and size and a damage-free model are also established. A rectangular defect is used to simulate the crack damage in the reinforced structure. The center of the grid is selected as the excitation point. The optimal excitation laser radius of 0.5 mm and the laser rise time of 8 ns are selected for excitation. The defect is set at a distance of 1 cm from the excitation point in the positive x direction. A receiving point is set at a distance of 2 cm from the excitation point in the positive x direction to receive the signal. A bandpass filter is used to filter the received signal to reduce noise and interference, such as Figure 3 As shown;

[0071] 3. Considering the coupling effects of various physical fields existing in the grid ribs of spacecraft in actual situations, a more comprehensive simulation model is developed to couple the physical fields of solid mechanics and solid heat transfer. At the same time, structural acoustics is considered, and mechanical vibration is combined with sound wave propagation for simulation calculation. The preselected surface wave time domain segment is set according to the surface wave velocity, and the surface wave displacement of the receiving point in the case of defects at different burial depths and without damage is extracted through a bandpass filter;

[0072] 4. Integrate the signal in the time domain to obtain the energy.

[0073] The energy ratio of defects with different burial depths in the reinforced structure is k1, and the energy ratio of defects with different burial depths in the grid structure is k2;

[0074] a) If the defects at different buried depths in the reinforced structure are to be detected, the ratio k1 of the receiving point energy at different buried depths and that at no damage in this domain is calculated. According to preliminary experiments and experience, the TH standard value is set to 0.3. When the ratio k1 is less than the set threshold TH of 0.3, it is considered that the surface wave and the defect have obvious effects, and the defect is considered to be a surface or sub-surface structural defect. Then the surface wave is selected as the sensitive mode to characterize the defect at this buried depth; if not, the longitudinal wave is selected as the sensitive mode for characterization;

[0075] b) If the defects at different buried depths in the grid structure are to be detected, the lamb wave signal is firstly subjected to spectrum analysis according to the dispersion curve of the lamb wave in the spacecraft material properties, and the signal is transferred to the frequency domain by Fourier transform to find the lamb wave with the most important frequency component corresponding to the dispersion curve, and the mode is determined. The lamb wave of the mode is extracted by bandpass filter and the ratio k2 of the energy of the receiving point at different buried depths and the energy at no damage in the time domain section of the lamb wave of the mode and the ratio k1 of the energy of the receiving point at different buried depths and the energy at no damage in the time domain section of the surface wave are calculated. If the ratio k1 of the energy in the time domain section of the surface wave is small, it is considered that the surface wave and the defect have obvious effects, and the defect is identified as a surface or sub-surface structural defect, and the surface wave is selected as the sensitive mode to characterize the defect at this buried depth; if not, the lamb wave of the mode is selected as the sensitive mode for characterization;

[0076] The expression for energy calculation of reinforced structure is:

[0077] E0=∫(|x0(t) 2 |)dt

[0078] E1=∫(|x1(t) 2 |)dt

[0079] in:

[0080] E0 is the energy of the receiving point in the reinforced structure without loss;

[0081] E1 is the receiving point energy in the lossy case of the reinforced structure;

[0082] x0(t) is the time domain signal of the receiving point in the reinforced structure without loss;

[0083] x1(t) is the time domain signal of the receiving point in the case of loss in the reinforced structure;

[0084] The expression of energy ratio is:

[0085]

[0086] Where: k1 is the ratio of the energy of the receiving point at different burial depths in the reinforced structure to that at the undamaged state;

[0087] The expression for calculating the grid structure energy in b) of S4 is:

[0088] E a =∫(|x a (t) 2 |)dt

[0089] E b =∫(|x b (t) 2 |)dt

[0090] in:

[0091] E a The energy of the receiving point in the grid structure without loss;

[0092] E b The receiving point energy in the case of loss in the grid structure;

[0093] x a (t) is the time domain signal of the receiving point in the grid structure without loss;

[0094] x b (t) is the time domain signal of the receiving point in the case of loss in the grid structure;

[0095] The expression of energy ratio is:

[0096]

[0097] Where: k2 is the ratio of the energy of the receiving point at different burial depths in the grid structure to that at the undamaged state.

[0098] 5. Construct a laser ultrasonic signal detection system based on different buried depth defects of spacecraft grid rib structure, such as Figure 4As shown, the system includes a spacecraft, a mobile turntable, an AGV platform vehicle, a laser and an optical system, a laser vibrometer, a robotic arm, and a computer. The laser and the optical system constitute an excitation unit, and the laser vibrometer serves as a receiving unit. The excitation unit and the receiving unit are supported by the robotic arm and placed on the AGV platform vehicle with the computer for longitudinal movement. The spacecraft is set on the mobile turntable and rotates horizontally to ensure that all parts of the spacecraft can be detected. In order to reduce the interference of the unstable time domain segment, the computer controls the laser Doppler vibrometer to receive the signal repeatedly and collect it for multiple times and average it 100 times, and the received signal is filtered by a bandpass filter. The excitation receiving device is integrated and clamped by the robotic arm and cooperates with the motorized mechanism AGV platform vehicle, turntable, etc. The laser and the laser Doppler vibrometer are fixed on the AGV platform vehicle, and the two are adjusted to maintain a relatively appropriate position. The longitudinal scanning is realized by the lifting and displacement stage, and the space capsule is located on the rotating turntable to realize the lateral scanning.

[0099] 6. The excitation signal generated by the laser is the simulation excitation signal, which is used to detect the simulated spacecraft grid rib structure with preset defects of different buried depths. The laser and the optical system cooperate to make the laser excitation radius the same as the simulation excitation radius, which is 0.5 mm. In order to improve the signal-to-noise ratio of the received signal, reflective paper is pasted in the receiving area. The same method as above is used for signal acquisition, calculation and processing, and compared with the simulation results. The threshold TH and the parameters and conditions of the simulation model are adjusted according to the experimental results to make the simulation consistent with the experimental results, and finally realize the selection of laser ultrasonic sensitive mode for defects of different buried depths in the spacecraft grid rib structure.

[0100] Although the embodiments and drawings of the present invention are disclosed for illustrative purposes, those skilled in the art will appreciate that various substitutions, changes and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the scope of the present invention is not limited to the contents disclosed in the embodiments and drawings.

Claims

1. A laser ultrasonic sensitive mode selection method for defects of different buried depths in a spacecraft grid rib structure, characterized by: The steps of the method are: S1. Taking the spacecraft grid rib structure as the experimental object, a three-dimensional finite element simulation model of the spacecraft grid rib structure is established using simulation software according to its material properties and dimensions; S2. The above-mentioned spacecraft grid rib structure is equivalent to a combination of a grid structure and a ribbed structure, where the connection between the ribbed root and the grid is considered to be a grid structure, and simulation settings are performed, specifically: a) For the reinforced structure, multiple defect models with the same size and different burial depths and damage-free models are established. The defects are set at different burial depths inside the reinforcement. The center of the reinforced structure is selected as the excitation point. The optimal excitation laser radius and laser rise time are selected, and a receiving point is set on the opposite side of the excitation point to receive the signal; b) For the grid structure, multiple defect models with the same size and different burial depths and damage-free models are established. The defects are set up at different burial depths inside the grid. The center of the grid structure is selected as the excitation point. The optimal excitation laser radius and laser rise time are selected, and a receiving point is set on the opposite side of the excitation point to receive the signal; S3, perform simulation calculation, set the preselected surface wave time domain segment, and extract the surface wave displacement of the receiving point in the case of defects at different burial depths and without damage through a bandpass filter; S4, integrating the wave packet in the time domain to obtain the energy, specifically: a) For reinforced structures, the ratio k1 of the energy of the receiving point at different burial depths and that at no damage in this time domain is calculated. When the ratio k1 is less than the set threshold TH, the surface wave is selected as the sensitive mode; if not, the longitudinal wave is selected as the sensitive mode; b) For the grid structure, find the lamb wave with the main frequency component corresponding to the dispersion curve, determine the lamb wave mode, extract and integrate the energy ratio k2 of the receiving point under different burial depths and the undamaged state in the lamb wave time domain segment and the energy ratio k1 of the receiving point under different burial depths and the undamaged state in the surface wave time domain segment, and select the modal ultrasonic wave with the smaller ratio as the sensitive mode; S5. Construct a laser ultrasonic signal detection system based on defects of different buried depths of a spacecraft grid rib structure, the system comprising a spacecraft, a mobile turntable, an AGV platform vehicle, a laser and an optical system, a laser vibrometer, a robotic arm, and a computer. The laser and the optical system constitute an excitation unit, and the laser vibrometer serves as a receiving unit. The excitation unit and the receiving unit are supported by the robotic arm and placed on the AGV platform vehicle with the computer for longitudinal movement, and the spacecraft is placed on the mobile turntable and rotates laterally. S6. Use a pulse signal as an excitation signal to test the spacecraft grid rib structure. Use the method of S4 to obtain different modal acoustic wave signals and perform energy calculations, and compare them with the simulation results. If the results are consistent, the simulation realizes the selection of laser ultrasonic sensitive modes for defects of different buried depths of spacecraft grid ribs. If they are inconsistent, adjust the simulation model parameters according to the test results to determine the selection of laser ultrasonic sensitive modes for defects of different buried depths of spacecraft grid rib structure.

2. The laser ultrasonic sensitive mode selection method for defects of different buried depths in a spacecraft grid rib structure according to claim 1 is characterized by: The expression for calculating the energy of the reinforced structure in S4 a) is: E0=∫(|x0(t) 2 |)dt E1=∫(|x1(t) 2 |)dt in: E0 is the energy of the receiving point in the reinforced structure without loss; E1 is the receiving point energy in the lossy case of the reinforced structure; x0(t) is the time domain signal of the receiving point in the reinforced structure without loss; x1(t) is the time domain signal of the receiving point in the case of loss in the reinforced structure; The expression of energy ratio is: Where: k1 is the ratio of the energy of the receiving point at different burial depths in the reinforced structure to that at the undamaged state The expression for calculating the grid structure energy in b) of S4 is: E a =∫(|x a (t) 2 |)dt E b =∫(|x b (t) 2 |)dt in: E a The energy of the receiving point in the grid structure without loss; E b The receiving point energy in the case of loss in the grid structure; x a (t) is the time domain signal of the receiving point in the grid structure without loss; x b (t) is the time domain signal of the receiving point in the case of loss in the grid structure; The expression of energy ratio is: Where: k2 is the ratio of the energy of the receiving point at different burial depths in the grid structure to that at the undamaged state.

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