Nanoparticle- (NP-) doped optical fibres show the potential to increase the signal-to-noise ratio and thus the sensitivity of optical fibre strain detection for structural health monitoring. In this paper, our previous experimental/simulation study is extended to a design study for strain monitoring. 100 nm spherical gold NPs were randomly seeded in the optical fibre core to increase the intensity of backscattered light. Backscattered light spectra were obtained in different wavelength ranges around the infrared C-band and for different gauge lengths. Spectral shift values were obtained by cross-correlation of the spectra before and after strain change. The results showed that the strain accuracy has a positive correlation with the relative spectral sensitivity and that the strain precision decreases with increasing noise. Based on the simulated results, a formula for the sensitivity of the NP-doped optical fibre sensor was obtained using an aerospace case study to provide realistic strain values. An improved method is proposed to increase the accuracy of strain detection based on increasing the relative spectral sensitivity, and the results showed that the error was reduced by about 50%, but at the expense of a reduced strain measurement range and more sensitivity to noise. These results contribute to the better application of NP-doped optical fibres for strain monitoring.

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Embedding fiber optic sensors (FOSs) within parts for strain measurement is attracting widespread interest due to its great potential in the field of structural health monitoring (SHM). This work proposes a novel method of embedding FOSs using capillaries within solid structures and investigates fiber positions and orientation uncertainties within capillaries of different sizes and their influences on strain measurement accuracies. To investigate how the fiber positions and orientation variations influence strain measurement accuracy, both analytical and numerical models are utilized to predict strain distributions along embedded fibers at different positions and with different orientations within the specimen. To verify the predictions, a group of specimens made of Aluminum 6082 was prepared, and the specimens in each group had capillaries of 2 mm, 4 mm, and 6 mm diameters, respectively. Fibers were embedded within each specimen using the capillaries. Four-point bending static tests were conducted for each specimen with embedded FOSs, performing in situ strain measurement. Subsequently, the specimens were partitioned into several pieces, and the cross sections were observed to know the real positions of the embedded fiber. Finally, the strain predictions at the real locations of the fiber were compared with the measured strain from the embedded FOSs. The predicted strain distributions as a function of the fiber positions alone and as a function of both the fiber positions and orientations were compared to assess the influence of fiber orientation change. The results from a combination of analytical, numerical, and experimental techniques suggest that the fiber position from the capillary center is the main factor that can influence strain measurement accuracies of embedded FOSs, and potential fiber misalignments within the capillary had a negligible influence. The fiber position-induced measured error increases from 10.5% to 18.5% as the capillary diameter increases from 2 mm to 6 mm. A 2 mm capillary diameter is able to lead to the lowest measurement error in this study and maintains ease of embedding. In addition, it is found that the measured strain always lies within a strain window defined by the strain distribution along capillary boundaries when there are no cracks. This can be further studied for crack detection.@en

Additively manufacturing can bring opportunities and risk factors to the aerospace industry. On one hand, additive manufacturing allows the manufacturing of structures with geometries that are difficult or impossible to fabricatewith conventional machining procedures. This geometry flexibility may lead to components with a greater strength-to-weight ratio, which can enhance the aircraft’s fuel efficiency. On the other hand, possible defects in the additively manufactured parts can lead to reduced strength and increased fatigue susceptibility. In addition, it is very difficult to apply traditional nondestructive testing techniques to additively manufactured specimens with complex geometry due to limited accessibility.....@en

As an initial step of research 'Embedded fiber optic sensor (FOS) within additive layer manufactured part for structural health monitoring', this work investigates strain measurement accuracy of the embedded FOS. There are different factors that could influence strain measurement accuracy, and this study singles out fiber position variation within capillaries by using milled Aluminum 6082. To be more specific, three Aluminum 6082 parts are milled, each of which has a straight capillary inside, with diameters 2mm, 4mm and 6mm respectively. Optical fibers are placed into capillaries centers manually and adhesives are filled in between to bond the fiber and specimens. Subsequently, strain measurement accuracy by embedded FOSs is studied both analytically and experimentally. An analytical model is used to calculate theoretical strain of specimens with embedded FOSs. Static four-point bending tests are performed on the three Aluminum 6082 parts with embedded FOSs under 8KN. Based on the analytical model, theoretical strain distribution along the capillary center is plotted and two strain distribution extremes corresponding to fibers on upper and lower edges of capillaries respectively are plotted as well. Comparison of results from both the model and test shows that test results lie within the two strain distribution extremes, and yet discrepancy exists between the test measurement and theoretical strain distribution, which increases when capillaries holding the embedded FOS gets larger.@en