Fibre optic (FO) sensors are becoming increasingly popular for different applications in structural monitoring. Among their excellent properties, a strong interest for this type of sensors are represented by the possibility of embedding FOs inside composite components. In this case, one of the factors that significantly influence the efficiency of the whole Structural Health Monitoring (SHM) system is the interfacial adhesion between FO sensors and the host material. The main objective of this work is to investigate the interfacial adhesion between embedded fibre optic sensors and epoxy matrix to find the best type of optical fibre to be used in epoxy matrices to produce smart composites. Four types of optical fibres with different diameters and coatings (i.e. polyimide, polyacrylate and ormoceramic) were used. Pull-out tests were carried out and different methods were used to obtain the composite/optical fibre interfacial properties. Finally, an optical microscopy and Scanning Electron Microscopy (SEM) analysis were performed to characterize the fibre/matrix interfaces. It was found that the optical fibre that presented the highest energy required for interface rupture and, consequently, less invasiveness to the host material was the ormoceramic fibre with the smallest diameter.

The purpose of this work is to evaluate the electrode wear behaviour and the distribution of hardness in a joint made by the resistance spot welding process. The lifetime of electrodes was evaluated by examination of the electrode caps after more than 2000 tests with acceptable welds. A microstructural evaluation by optical microscopy (OM) and scanning electron microscopy (SEM) was carried out. The microstructure of the welded joint is primarily composed of ferrite grains. The microhardness testing results indicated a significant increase in hardness in the weld nugget, caused by the formation of martensite in the fusion zone. The hardness results showed lower values in all regions at the end of the useful lifetime of the electrode. The methodology used for selection of welding parameters has highlighted an optimal combination of 200 kgf electrode force and 9-cycle (cy) welding time, to increase the lifetime of the electrode and to achieve more weld points. Still, the cover electrode tends to form brass (Zn + Cu) on the indentation obtained by spot welding, subsequently reducing the electrode life. The results indicate the importance of combining and optimizing the welding parameters in order to reduce the wear of the electrode used for resistance spot welding.