**Preface to "Optical Sensors for Structural Health Monitoring"**

The intention to prevent the severe damage and eventual collapse of structures, and consequent human, material and economic losses, fostered the need for the preservation, maintenance and retrofitting of existing structures. Our built heritage comprises multiple structural systems with different materials, designed and constructed in different periods, and thus with different design criteria, detailing and construction techniques. Effective monitoring enables the reduction of maintenance costs (optimized maintenance), or even reducing the costs of eventual interventions needed to upgrade the level of structural safety, supporting retrofitting strategies based on rich information about the structural behavior. For this, accurate assessment of structural safety is needed, which may benefit from innovative monitoring approaches, equipment and sensors. Photonic technologies have become vitally important in developing monitoring solutions. In recent decades, there has been a growing interest in monitoring research, especially sensors, not only in electronics but also in fiber optics. These have proved to be a promising technology because they are durable, stable, insensitive to electromagnetic interference and generally have minimal aesthetic impact; as such, they are particularly interesting for long-term structural health surveillance. The fact that no power supply at the measurement site is required, their multiplexing capability (allowing dozens of sensors on the same fiber optic cable) and low attenuation of the optical fiber are additional advantages. Their application may also allow the implementation of a safer monitoring system (without risk of short circuiting). The results of the information captured with the monitoring strategy may be used for the calibration of structural numerical models, based on data obtained in real-time, and eventually, the implementation of early warning systems in the case of danger.

This volume is a collection of papers that originated as a Special Issue, focused on some recent advances related to optical sensors and structural health monitoring. Applications including corrosion, load, crack opening and strain or deflection monitoring in different types of structural systems, such as pipelines, aircrafts, concrete beams and bridges, are presented.

The authors of this book are grateful to all the contributing authors, journal editors, reviewers and the production team.

> **Paulo Antunes, Humberto Varum** *Editors*
