In this article, a mid-infrared all-fiber light-induced thermoelastic spectroscopy (LITES) sensor based on a hollow-core anti-resonant fiber (HC-ARF) was reported for the first time. The HC-ARF was applied as a light transmission medium and gas chamber. The constructed all-fiber structure has merits of low loss, easy optical alignment, good system stability, reduced sensor size and cost. The mid-infrared transmission structure can be utilized to target the strongest gas absorption lines. The reversely-tapered SM1950 fiber and the HC-ARF were spatially butt-coupled with a V-shaped groove between the two fibers to facilitate gas entry. Carbon monoxide (CO) with an absorption line at 4291.50 cm^-1 (2.33 µm) was chosen as the target gas to verify the sensing performance. The experimental results showed that the all-fiber LITES sensor based on HC-ARF had an excellent linear response to CO concentration. Allan deviation analysis indicated that the system had excellent long-term stability. A minimum detection limit (MDL) of 3.85 ppm can be obtained when the average time was 100 s.

The increase in the number of space debris is a serious threat to the safe operation of in-orbit spacecraft. The propagation law of the impact signal in the stiffened panel of the spacecraft’s sealed bulkhead is very complicated, and there is less research on the impact source location in the high-stiffened panel. In this paper, an adaptive energy compensation threshold filtering (AECTF) method based on acoustic emission is proposed, which can realize large-scale, fast and accurate locating of the impact source on the stiffened panel with less resource consumption. The influence law of the stiffeners on the lamb wave is analyzed by finite element simulation, and the Lamb wave energy factor curve is obtained. The correctness of the simulation is verified by the locating experiment on the impact point. The results show that the proposed AECTF method has better adaptability and can correctly locate the impact points in complicated locations. By selecting the appropriate frequency band to filter the signal, the locating accuracy and stability can be improved. When the frequency band is 100–200 kHz, the locating result is optimal, the average absolute error is 7.0 mm, the average relative error is 0.86%, and the error standard deviation is 3.5 mm. This study will generate fresh insight into the impact location technology of high-stiffened panel and provide a reference for the in-orbit spacecraft health monitoring system.

The analysis of acoustic signals to localize acoustic sources (AE) has emerged as a kind of favored approach. Despite recent advances, the task of localizing acoustic emission sources such as impacts in the discontinuous structure with a hole still poses a considerable challenge. A localizing method based on the gradient vector descent method is presented in this paper, which is applied to a discontinuous structure with a hole. When an impact event is observed, this method will first calculate the time difference-of-arrival values of different sensors and set the most likely location of impact as origin coordinates. Then the tracks of acoustic signals on the surface of the structure are calculated by vector analysis, which is used to compute the difference of path for different sensors. The calculated parameters will be used in an objective function, then coordinates of impact will be corrected by the gradient vector descent strategy and difference-of-path values will be recalculated to complete iteration until satisfying the cut-off condition. In addition, simulation and Laser Doppler experiments verify the correctness of the propagation path. The experiment results of extensive Pencil lead break (PLB) tests on the structure with a hole further indicate that the proposed method can achieve higher localizing accuracy (the average error is 1.034cm) than traditional methods with a relatively low computational cost.

Composites play an important role in the development of modern science and technology, but the special anisotropy leads to difficulties in impact localization. To achieve impact localization on composite materials, this paper proposes a time difference matrix matching method with edge accuracy optimization. The method uses the propagation characteristics of Lamb wave in composite materials to solve the direction dependence of wave velocity. To overcome the anisotropy, the time difference feature vector of each element of the composite material is obtained by grid division, and the impact location is found according to the similarity. In addition, edge accuracy optimization is proposed to further improve localization accuracy. Using 49 points in the 800mm×800mm composite area for 147 impact tests, the result shows that the average error can reach 0.532cm, and the variance is 0.217cm ² .

Composite materials are widely used, but they are often subjected to impacts from foreign objects, causing structural damage. To ensure the safety of use, it is necessary to locate the impact point. This paper investigates impact sensing and localization technology for composite plates and proposes a method of acoustic source localization for CFRP composite plates based on wave velocity-direction function fitting. This method divides the grid of composite plates, constructs the theoretical time difference matrix of the grid points, and compares it with the actual time difference to form an error matching matrix to localize the impact source. In this paper, finite element simulation combined with a lead-break experiment is used to explore the wave velocity-angle function relationship of Lamb waves in composite materials. The simulation experiment is used to verify the feasibility of the localization method, and the lead-break experimental system is built to locate the actual impact source. The results show that the acoustic emission time-difference approximation method can effectively solve the problem of impact source localization in composite structures, and the average localization error is 1.44 cm and the maximum localization error is 3.35 cm in 49 experimental points with good stability and accuracy.

The stiffened plate has complex influences on the propagation characteristics of Lamb waves, which causes difficulties for the impact location. In order to realize the impact location in stiffened plate, this paper proposes a transmission coefficient weighted impact intensity (TCWII) method. The method uses cross-correlation curves to calculate impact intensity maps and reduce the location error by superposing the maps of different filter bands. The transmission coefficient weights for different bands are applied to compensate for the effect of the stiffeners on the signal propagation. By finding the maximum impact intensity point in the transmission coefficient weighted impact intensity map, the impact point can be located. To evaluate the performance of the TCWII method, 130 sets of experiments have been taken. The location result shows the TCWII method can achieve good performance in stiffened plate with an average error of 2.29 cm in 50cm $\times50$ cm testing range. The comparation with the traditional TDOA method shows the TCWII has improvement on accuracy in stiffened plates.