Objective The current methods for structural health monitoring are suitable for real-time monitoring of aircraft structures because they have the characteristics of the complicated measurement process, high quality,and complicated operations. Therefore, the aircraft structure state detection technology based on fiber grating sensors is becoming increasingly important. Aiming at structural deformation problems caused by strain field changes and environmental loads, the overall strain field change monitoring of the free-form surface structure is performed, and the fiber Bragg grating (FBG) array is used to measure and obtain the curvature information and strain of the structure, using structural health monitoring and three-dimensional (3D) surface reconstruction in the aircraft as the research background. To achieve the goal of real-time perception of the state of the curved surface structure, a three-dimensional reconstruction algorithm is built to reconstruct the surface structure in three dimensions.
Methods The finite element model of the free-form surface structure is established using ANSYS simulation software to understand the degree of deformation of the specific surface of the test piece under different pressures. The static stress simulation software performs a numerical simulation of the strain field distribution under different loads of 0, 20, 40, 60, 80, and 100 N, which is gradually increasing. The simulation results show the strain distribution trend of the surface strain field of the free-form surface structure. The maximum deformation is 0.243 mm at 100 N, and the position of the FBG array is determined by the stress field distribution characteristics. A test system for surface position offset and three-dimensional stress field distribution is built, a free-form surface structure state sensing system based on the fiber grating sensor network is designed, and a three-dimensional reconstruction algorithm based on FBG array test data is proposed.
Results and Discussions When the load is applied, we measure the center wavelength shift of the fiber grating at the positions of the six fibers, record the change in physical value at 0, 20, 40, 60, 80, and 100 N, and observe microstrain under the condition of no plastic deformation. Under different pressures, the microstrain με changes at different positions of the workpiece. The strain should have a linear functional relationship with the FBG center wavelength offset, and the three-dimensional reconstruction algorithm in section 2.2 demonstrates that the curvature is positively correlated with the fiber center wavelength offset (Fig. 6). Because the wavelength shift of the pressure points s1, s2, s3, s5, and s8 has the same changing trend when different loads are applied, we choose FBG measurement data at point s2 to compare with ANSYS simulation values and Handyscan measurement data. Similarly, data comparison experiments at s7 and s9 are conducted (Fig. 6). When the pressure of the pressure gauge is fixed at 100 N, the maximum change in the strain of the FBG strain gauge group monitored under the same force is 28.5 με -1, and the simulated value of ANSYS has the largest strain under the same force. The amount of change is 28.3 με -1. When compared to the simulated strain data, the maximum absolute error between the experimentally measured strain and the simulated value is 0.7 με -1, and the relative error is less than 2.81% (Fig. 7). It demonstrates that the FBG method of measurement has high precision and it is close to the theoretical value (Fig. 7). The test data obtained by the FBG array is close to the data measured by the Handyscan scanner using the two measurement methods. Since the Handyscan scans the center of the positioning punctuation, it deviates from the actual position, and there will be many burrs when scanning the surface. Therefore, the Handyscan measures the coordinate points before and after the force is applied to the workpiece to be tested. There is an error between the data and the ANSYS simulation value. The absolute error of Handyscan measurement data and actual measurement data is kept within 0.026 mm. The relative error of each measurement point data of the FBG reconstructed from the deformed surface and the Handyscan measurement data is kept within 6.67%, and the average relative error is less than 4.53% (Tables 1--3). Therefore, the spatial offset value in the force-bearing area reversed by the strain sensor is close to the Handyscan measurement result, which is consistent with the expected effect of the experiment. The 3D reconstruction surface obtained by the 3D reconstruction algorithm is approximately close to the surface scanned by the Handyscan. The more the pasted FBG sensors are, the closer the reconstruction effect to the actual situation is and the smaller the deviation between the reconstructed surface and the ideal surface is. It provides strong evidence that the FBG array can be used to monitor the curved structure’s state change in real-time (Figs. 8 and 9).
Conclusions This paper uses the spacecraft surface structure as the research object, uses linear interpolation to continuously process the space curvature, proposes a 3D reconstruction algorithm based on the FBG array test data, and finally builds a set of surface position offsets and a 3D stress field distribution test system. This paper presents the FBG sensor network distribution design that conforms to the complex surface structure, combined with the free-form surface strain detection test, optimizes the test data corresponding to the variable sensitive position, and provides new theories and data for aircraft structural health monitoring support layout design. The free-form surface structure state sensing system and 3D surface reconstruction algorithm based on the fiber grating sensor network investigated in this paper can more accurately reconstruct and reproduce complex free-form surfaces, which not only validates the feasibility of related research ideas but also helps to further realize the spacecraft real-time perception and reconstruction of typical structural forms,therefore providing useful ideas and technical reserves.
