Multi-damage detection for composite materials has been a hot issue in the field of structural health monitoring. In order to further study the problem by means of FBG sensors, an optical fiber sensing system has been built to enable the detection of delamination damage in composite laminates. The system includes sensing strain, data processing and damage localization algorithms. A model of a carbon fiber/epoxy laminate is created using the finite element analysis. The stress cloud chart of the laminate when subjected to a hammering load is calculated and analyzed and it is concluded that stress waves propagate along 45 degree direction of carbon fiber and stress is concentrated at the delamination and then decreased obviously. The Fourier transform and wavelet transform are used to analyse the key level signal of the wavelet transform and to determine the area of delamination damage, and the experiment is checked by ultrasonic C-scan. The experimental results show that by building a network of fibre Bragg gratings(FBG) sensing systems, the hammering test can detect multi-delamination damages of different sizes in composite laminates, the smallest of which is 10mm in diameter, providing a basis for subsequent composite structures health monitoring in complex environments.
Composite joint plays a significant role in the composite sandwich structural connection. However, its complex structure and environmental conditions can lead to difficulties in real-time monitoring. Regarding this, a threedimensional finite element simulation model has been developed for the prediction, propagation and improvement of adhesion failure in an adhesively bonded laminated structure. In this study, a tensile fracture monitoring test under fiber Bragg grating-based monitoring was designed using the Fiber Bragg Grating (FBG) sensors. The simulation, based on the cohesive zone method with Hashin, Chang-Chang, Hou criteria and the mixed criteria, also indicates that debonding of the filler area was the main failure behavior of this composite joint. Using the finite element simulation model based on mixed criterion, the predicted initial and final failure loads have a good agreement with experiments, and the effects of the FBG sensors layout were investigated and discussed on Acquisition and processing of failure signals of composite joint. The findings in this work are helpful to improve mechanical behaviour of composite joint and improve the FBG monitoring to meet the high criteria of the aerospace industry.
Honeycomb structure with high stiffness and light weight is expected to be more applied in the field of morphing wing. We propose a surface reconstruction algorithm based on FBG sensors to reconstruct the surface deformation of honeycomb structure real-timely and rapidly. When flexible honeycomb cores are driven by SMA actuators, the surface curvature monitored by FBG sensing array can be inferred from the changes of central wavelength. According to the surface reconstruction algorithm we proposed, the surface shape can be reconstructed. Composite single-row honeycomb structure specimen consisting of 8 cores, whose cell walls length and thickness is 8mm and 2mm respectively, is bended by electrified SMA actuators into the new steady shape. The experiment shows that the reconstructed surface shape has great agreement with the visual recording surface shape and the error is 5.76% on average.
An analysis method of spectrum was proposed to assess the damage of honeycomb structures. The non-homogeneous strain fields of honeycomb cell walls were obtained by finite element analysis when tension was applied on the honeycomb structures. Spectrum changes of FBG sensors stuck on the cell walls were monitored and analyzed. Our analysis revealed that spectral bandwidth was broadened from 1nm to 3.5nm and spectrum was split to 12 peaks. The changes of light intensity of secondary peak corresponded to inflection points of load-displacement curves. The regular variations of spectrum were able to indicate progressive damage of honeycomb structures.
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