In-situ deformation monitoring of aerospace qualified composites with embedded improved draw tower fibre Bragg gratings

Aerospace certified fibre reinforced plastics (FRPs) are extreme performing construction materials, which today are increasingly applied in primary structures of the new generation aircrafts (e.g. Boeing 787, Airbus 350, Bombardier C-Series), such as the fuselage, the wings and the fin. An interesting aspect on the technological point of view of sensing is that airplane manufacturers such as Airbus and Boeing are looking at incorporating health-monitoring systems (such as optical fibre sensors, especially fibre Bragg gratings) that will allow the airplane to self-monitor and report maintenance requirements to ground-based computer systems. However, one has to realize that the mechanical behaviour of anisotropic FRPs is significantly different compared to conventional isotropic construction materials. In this dissertation, the author focuses on monitoring the strain and (permanent) deformation in carbon reinforced plastic laminates with embedded fibre Bragg gratings. The research is divided in two main parts. In the first part of this research, the existing fibre draw tower technology is utilized, to manufacture an improved version of the existing in-line high quality, draw tower fibre Bragg gratings (DTG®s). With respect to accurate measurements and structural integrity, the research focuses on reducing the total diameter of the optical fibre, so the incorporation in the reinforcement fibres is enhanced and the distortion in the composite is reduced. The author elaborates in detail the methods of strain and temperature calibrations and the different setups which are applied. Additionally, with respect to the high temperatures during the composite manufacturing process, the thermal stability of the DTG®s is studied at elevated temperatures (>300°C). In the second part, the author embeds the DTG®s in specific types of thermoset and thermoplastic carbon reinforced plastic laminates. The author applies the embedded DTG®s in several stages of the composite lifetime. Starting with the monitoring of the composite manufacturing process and ending with fatigue testing until failure of the composite laminates. During the different experiments, the sensors are subjected to high temperatures, high pressures, extreme longitudinal strains and transverse strains and in the mean time, they are employed to very accurately measure (multi-axial) strains inside composites at microstrain level (~10 6)

Optical fiber coating optimization tool for composite embedded health monitoring purposes through a novel transfer matrix method

This work presents a new methodology based on finite element analysis, allowing the user to quickly optimize the coating thickness for any type of load case within any type of lay-up (given certain boundary conditions on minimum layer thickness). The method finds the same optimal values as Dasgupta for axial loads and Hadjiprocopiou for transverse loads

Simultaneous interrogation of multiple fiber bragg grating sensors using an arrayed waveguide grating filter fabricated in SOI platform

A novel fiber Bragg grating (FBG) interrogator is demonstrated based on an optimized arrayed waveguide grating (AWG) filter. The AWG response is optimized to achieve large crosstalk between the output channels, which allows simultaneous detection of multiple FBG peaks, using centroid signal processing techniques, without constraints on the minimum FBG peak spectral width. The measured interrogator resolution is 2.5 pm, and the total measurement range is 50 nm. The device is fabricated in a silicon-on-insulator platform and has a footprint of only 2.2 x 1.5 mm. A novel approach to minimize the polarization dependence of the device is proposed and experimentally demonstrated

Crack monitoring in historical masonry with distributed strain and acoustic emission sensing techniques

The analysis of crack patterns and crack growth is one of the most important steps in the assessment of structural damage in historical masonry. In a search for integrated and accurate monitoring techniques for crack measurements in masonry, several novel techniques based on distributed strain monitoring and acoustic emission (AE) sensing have been investigated in an experimental test campaign. Aim of the test program was to develop integration procedures for the strain and AE sensors, analyse their use for crack monitoring specifically in historical masonry and assess their robustness and efficiency with respect to the experimentally observed crack pattern.This work is performed within the framework of the GEPATAR project (“GEotechnical and Patrimonial Archives Toolbox for ARchitectural conservation in Belgium” BR/132/A6/GEPATAR), which is financially supported by BRAIN-be, Belspo.Postprint (updated version

In-situ local strain measurement in textile composites with embedded optical fibre sensors

To understand the local strains inside a textile composite, numerical simulations are typically done on the scale of one repetitive unit cell of the weaving pattern. Periodic boundary conditions are applied to the edges of the unit cell and different load cases can then be applied to the unit cell of the textile composite. Most often, the periodic boundary conditions are applied on all faces of the unit cell, which implies the assumption that the material is repeating itself over an infinite distance in all three orthogonal directions. This assumption is more or less valid for the textile composite material in the midplane of thick laminates, where it is constrained by neighbouring material in all three directions. It is very difficult to validate such simulations, because local strain measurements inside a textile composite have rarely been done, and the interpretation is not straightforward. This paper shows the successful use of embedded optical fibre sensors to measure the local strains inside a satin weave carbon/PPS composite (typically used in aerospace applications). The length of the Bragg grating inside the optical fibre sensor has been chosen such that it is longer than the length of one unit cell of the satin weave architecture (7.4 mm). The read-outs of the optical fibre sensor give the minimum and maximum local strains that occur along the length of the Bragg grating