Design, manufacturing and testing of smart beams with EFPI strain sensor for damage detection
This thesis aimed at the development of a fibre optic strain sensor-based damage detection and evaluation system (FODDAS) based on the composite beams. EFPI strain sensors were used with their integrity being assessed. Their performance, either bonded on the surfaces or embedded was examined extensively. They were shown to be adequate and reliable for strain measurements. Through-the-width damages were simulated by artificially-embedded delaminations, which were located at several through-the-thickness locations, each with two different sizes. The overall design considerations were guided by ply stresses and strains which were estimated by using the modified classical lamination theory (CLT). Considerable efforts were devoted to assessing the through-the-thickness mechanical behaviours of the beams containing optical fibres in three-point bending and short beam shear (SBS). They involved various optical fibre orientations with respect to 00 plies / longitudinal axis and at various through-the-thickness locations, each with different number of optical fibres. The understanding of these behaviours paved the way for the evaluation of the beam-based FODDAS. Smart preconditioned beams were subjected to the quasi -static loads whose magnitudes and locations were required to be well controlled. The viability and effectiveness of the beam-based FODDAS was evaluated in terms of strength and strain obtained by the embedded sensor as well as the surface-bonded strain gauges via the cross comparison of ten cases. For the strength, each beam was incrementally loaded up to the ultimate failure either in three-point bending or SBS. After each increment, the beam was unloaded and inspected for damage. For the given locations of EFPI-SS and artificial delamination as well as the sizes of the latter, it was found that the embedded EFPI-SSs were capable of picking up the stiffness degradation when the 10- mm as well as the 20-mm delamination was located at the 29-30 ply interface in the tensile region of a 32-ply quasi-isotropic carbon/epoxy smart composite beam. It was speculated from single tests results that the propagation of the embedded delamination of the sufficient size was able not only to be detected but also to be monitored by the sensors.